WO2014030701A1 - Method for continuous casting of steel, and method for manufacturing bar steel - Google Patents

Abstract

This method for continuous casting is provided with an extraction step for extracting a cast slab in a solid-liquid coexisting state from a cylindrical casting mold, a first rolling reduction step for performing rolling reduction of the cast slab in the solid-liquid coexisting state in a first rolling reduction direction perpendicular to the longitudinal direction of the cast slab after the extraction step, and a second rolling reduction step for molding the cast slab, in a completely solidified state in which the temperature of a center part thereof is higher than the temperature of a surface part thereof after the first rolling reduction step, into a rounded rectangle having rounded corner parts as viewed in cross-section perpendicular to the longitudinal direction of the cast slab, while performing rolling reduction alternately in the first rolling reduction direction and a second rolling reduction direction, the second rolling reduction direction being a rolling reduction direction orthogonal to both the first rolling reduction direction and the longitudinal direction of the cast slab.

Description

鋼の連続鋳造方法及び条鋼の製造方法Method for continuous casting of steel and method for manufacturing strip steel

　本発明は、中心偏析、中心ポロシティ、及び内部割れの少ない内部品質の優れた鋳片を得るための連続鋳造方法、及び分塊圧延工程を省略可能な条鋼の製造方法に関する。
　本願は、２０１２年８月２２日に、日本に出願された特願２０１２－１８３１７９号に基づき優先権を主張し、その内容をここに援用する。 The present invention relates to a continuous casting method for obtaining a cast slab having excellent center quality, center segregation, center porosity, and few internal cracks, and a method of manufacturing a bar steel that can omit the ingot rolling step.
This application claims priority based on Japanese Patent Application No. 2012-183179 for which it applied to Japan on August 22, 2012, and uses the content here.

　一般に、高級（高品質）条鋼は、大断面の矩形鋳型を有するブルーム連続鋳造機で鋳片を鋳造し、この鋳片を分塊圧延し、その後、鋼片圧延を行うことによって製造される。この高級条鋼には、棒鋼や線材などが含まれる。これら条鋼では、連続鋳造鋳片の凝固末期に形成される中心偏析や中心ポロシティによって材料特性が悪化する。そのため、中心偏析及び中心ポロシティ等の欠陥を発生させないように鋳片を鋳造することが重要となる。この中心偏析及び中心ポロシティを低減させる代表的な方法として、連続鋳造機内における鋳片の未凝固軽圧下法及び完全凝固後圧下法が挙げられる。 Generally, high-grade (high quality) strip steel is manufactured by casting a slab with a bloom continuous casting machine having a rectangular mold with a large cross section, rolling the slab in pieces, and then rolling the slab. This high-grade steel bar includes steel bars and wire rods. In these strips, the material properties are deteriorated by central segregation and central porosity formed at the end of solidification of the continuous cast slab. Therefore, it is important to cast the slab so as not to generate defects such as center segregation and center porosity. Typical methods for reducing the center segregation and the center porosity include a non-solid light reduction method and a complete post-solid reduction method for a slab in a continuous casting machine.

　例えば特許文献１には、直径３４０ｍｍ以下の丸鋳片に対し、軸芯部の固相率が０．３～０．７の状態にて、１組のロールで１パスにより０．１～３．０％の圧下を加える技術が提案されている。しかしながら、この技術は直径３４０ｍｍ以下の断面サイズの小さな鋳片に適用される技術である。これに対して鋳片の断面サイズが大きくなると凝固収縮量も大きくなるため、１組のロールでの１パスによる未凝固圧下で上記効果を得るためには鋳片への圧下量を増大させる必要がある。すなわち、大断面である矩形鋳片の中心偏析及び中心ポロシティを１パスで消失させるためには、連続鋳造機内で大きな圧下量が必要となる。しかし、このような場合、未凝固圧下時に鋳片で内部割れが発生し、条鋼の材料特性が損なわれるという問題がある。このように、鋳片の中心偏析及び中心ポロシティの低減と、鋳片の内部割れの発生防止とを両立することが困難である。 For example, in Patent Document 1, for a round slab having a diameter of 340 mm or less, 0.1 to 3 is obtained by one pass with one set of rolls in a state where the solid phase ratio of the shaft core is 0.3 to 0.7. A technique for applying a reduction of 0.0% has been proposed. However, this technique is applied to a small slab having a cross-sectional size of 340 mm or less in diameter. On the other hand, as the cross-sectional size of the slab increases, the amount of solidification shrinkage also increases. Therefore, in order to obtain the above effect under unsolidified pressure by one pass with one set of rolls, it is necessary to increase the amount of reduction on the slab. There is. That is, in order to eliminate the center segregation and center porosity of the rectangular slab having a large cross section in one pass, a large reduction amount is required in the continuous casting machine. However, in such a case, there is a problem in that internal cracks occur in the slab at the time of unsolidified reduction, and the material properties of the bar steel are impaired. Thus, it is difficult to achieve both the center segregation and center porosity of the slab and the prevention of the occurrence of internal cracks in the slab.

　また、特許文献２には、例えば直径１８０ｍｍの丸鋳片に対し、製造条件を制御することで鋳片内部の等軸晶率を３５％以上とし、鋳片の中心固相率が０．２５～０．３５又は０．６０～０．９０の範囲となる位置で、１対のフラットロールにより２．０～３．５％の圧下を加える技術が提案されている。しかし、この技術では、鋳片の断面サイズが大きい場合、鋳片の断面における等軸晶率を３５％以上とするために鋳造速度を必要以上に低下させるといった鋳造条件の制約が必要となる。そのため十分な生産能力を確保することが困難となる。また、鋳片内部の等軸晶率は鋼組成にも影響を受けることが明らかなので、この技術を適用できる鋼種には限りがある。 In Patent Document 2, for example, for a round slab having a diameter of 180 mm, the production conditions are controlled so that the equiaxed crystal ratio in the slab is 35% or more, and the central solid fraction of the slab is 0.25. A technique for applying a reduction of 2.0 to 3.5% by a pair of flat rolls at a position in the range of ˜0.35 or 0.60 to 0.90 has been proposed. However, in this technique, when the cross-sectional size of the slab is large, it is necessary to restrict the casting conditions such that the casting speed is reduced more than necessary in order to make the equiaxed crystal ratio in the cross-section of the slab 35% or more. Therefore, it becomes difficult to ensure sufficient production capacity. Moreover, since it is clear that the equiaxed crystal ratio inside the slab is also affected by the steel composition, there are limits to the types of steel to which this technique can be applied.

　高級条鋼の材料特性を向上させるためには、鋳片の中心偏析及び中心ポロシティの低減と、鋳片の内部割れの発生防止とを両立することが重要である。しかし、鋳片の生産能力向上も含めて、中心偏析、中心ポロシティ、及び内部割れの少ない内部品質の優れた鋳片を得る連続鋳造技術の開発には至っていない。 In order to improve the material properties of high-grade steel bars, it is important to satisfy both the center segregation and center porosity of the slab and the prevention of internal cracks in the slab. However, continuous casting technology has not yet been developed for obtaining a slab having excellent internal quality with little center segregation, center porosity, and internal cracks, including improvement in slab production capacity.

日本国特開平９－９９３４９号公報Japanese Unexamined Patent Publication No. 9-99349 日本国特開平１１－３０９５５３号公報Japanese Unexamined Patent Publication No. 11-309553

　本発明は上記の現状に鑑みてなされたものである。本発明では、条鋼として使用される幅広い鋼種に対して適用可能であるとともに、中心偏析及び中心ポロシティの低減と鋳片の内部割れの防止とを両立可能な鋳片の連続鋳造方法、及び鋼片圧延前の分塊圧延工程を省略して生産性を向上させることが可能な条鋼の製造方法を提供することを課題とする。 The present invention has been made in view of the above situation. The present invention is applicable to a wide range of steel types used as strip steel, and is a continuous casting method of a slab capable of achieving both center segregation and reduction of center porosity and prevention of internal cracking of the slab, and steel slab. It is an object of the present invention to provide a method for manufacturing a bar steel that can improve productivity by omitting a block rolling process before rolling.

　本発明の要旨は、次のとおりである。
　（Ａ）本発明の一態様に係る連続鋳造方法は、円筒状の鋳型から固液共存状態の鋳片を引き抜く、引き抜き工程と；前記引き抜き工程後に、前記固液共存状態の前記鋳片に対して、前記鋳片の長手方向に垂直な第１圧下方向への圧下を施す、第１の圧下工程と；前記鋳片の前記長手方向及び前記第１圧下方向の双方と直交する圧下方向を第２圧下方向とするとき、前記第１の圧下工程後に、完全凝固状態でありかつ中心部の温度が表面部の温度よりも高い状態にある前記鋳片に対して、前記第１圧下方向及び前記第２圧下方向への圧下を交互に施しながら、前記鋳片の前記長手方向に垂直な断面で見た場合の角部に丸みが有る角丸矩形に成形する、第２の圧下工程と；を備える。 The gist of the present invention is as follows.
(A) A continuous casting method according to an aspect of the present invention includes a drawing step of drawing a slab in a solid-liquid coexistence state from a cylindrical mold; and after the drawing step; A first reduction step of applying a reduction in a first reduction direction perpendicular to the longitudinal direction of the slab; and a reduction direction orthogonal to both the longitudinal direction and the first reduction direction of the slab. When the second reduction direction is set, the first reduction direction and the first reduction step are performed after the first reduction step with respect to the slab in a completely solidified state and in a state where the temperature of the central portion is higher than the temperature of the surface portion. Forming a rounded rectangle with rounded corners when viewed in a cross section perpendicular to the longitudinal direction of the slab while alternately reducing in the second reduction direction; and a second reduction step; Prepare.

　（Ｂ）上記（Ａ）に記載の連続鋳造方法では、前記鋳型の内径が４００ｍｍ以上６００ｍｍ以下であり；前記鋳片の引き抜き速度が０．３５ｍ／分以上０．６５ｍ／分以下であり；前記引き抜き工程後かつ前記第１の圧下工程前における前記鋳片の前記中心部の固相率が、０．３以上０．８以下であり；前記第１の圧下工程における前記第１圧下方向への圧下の１回当たりの圧下率が、０．３％以上７．０％以下であり；前記第１の圧下工程後かつ前記第２の圧下工程前における前記鋳片の前記中心部の固相率が０．８超であり、なおかつ前記鋳片の前記中心部の前記温度が前記表面部の前記温度よりも１５０℃以上高く；前記第２の圧下工程における、前記第１圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であり、なおかつ前記第２圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であり；前記第２の圧下工程後の前記鋳片の前記長手方向に垂直な前記断面の形状が、長辺が２３５ｍｍ以上２７０ｍｍ以下であり、前記角部の曲率半径が５ｍｍ以上５０ｍｍ以下であってもよい。 (B) In the continuous casting method according to (A), the inner diameter of the mold is 400 mm or more and 600 mm or less; the drawing speed of the slab is 0.35 m / min or more and 0.65 m / min or less; The solid phase ratio of the central portion of the slab after the drawing step and before the first reduction step is 0.3 or more and 0.8 or less; in the first reduction direction in the first reduction step The rolling reduction per rolling is 0.3% or more and 7.0% or less; the solid phase ratio of the central portion of the slab after the first rolling process and before the second rolling process Is more than 0.8, and the temperature of the central portion of the slab is 150 ° C. or more higher than the temperature of the surface portion; in the second reduction step, the reduction in the first reduction direction The rolling reduction per time is 1.5% or more and 7.0% or less In addition, the reduction ratio per one time of the reduction in the second reduction direction is 1.5% or more and 7.0% or less; the cross section perpendicular to the longitudinal direction of the slab after the second reduction step The long side may be 235 mm or more and 270 mm or less, and the radius of curvature of the corner may be 5 mm or more and 50 mm or less.

　（Ｃ）本発明の一態様に係る条鋼の製造方法は、上記（Ａ）または（Ｂ）に記載の連続鋳造方法によって前記鋳片を得る連続鋳造工程と；前記連続鋳造工程後に、前記鋳片を圧延する圧延工程と；を備える。 (C) A method for manufacturing a strip according to an aspect of the present invention includes a continuous casting step of obtaining the slab by the continuous casting method according to (A) or (B); and after the continuous casting step, the slab A rolling step of rolling

　本発明の上記態様によれば、鋼種によることなく、大断面を有する丸鋳片に複数回の鋳片厚み方向（第１圧下方向）及び鋳片幅方向（第２圧下方向）からの未凝固圧下及び完全凝固後圧下を加える。その結果、鋳片の内部割れを生じさせることなく、且つ、中心偏析及び中心ポロシティを低減させつつ、分塊圧延後に相当するサイズまで鋳片の断面サイズを縮小することができる。すなわち、本発明の上記態様によれば、条鋼として使用される幅広い鋼種に対して適用可能であるとともに、中心偏析及び中心ポロシティの低減と鋳片の内部割れの防止とを両立可能な鋳片の連続鋳造方法、及び鋼片圧延前の分塊圧延工程を省略して生産性を向上させることが可能な条鋼の製造方法を提供することができる。 According to the above aspect of the present invention, unsolidified from the slab thickness direction (first reduction direction) and the slab width direction (second reduction direction) to the round slab having a large cross section without depending on the steel type. Apply reduction and reduction after complete coagulation. As a result, the cross-sectional size of the slab can be reduced to a corresponding size after the partial rolling without causing internal cracking of the slab and reducing the center segregation and the center porosity. That is, according to the above aspect of the present invention, the slab can be applied to a wide range of steel types used as strip steel, and can achieve both center segregation and reduction of center porosity and prevention of internal cracks in the slab. It is possible to provide a method for manufacturing a bar steel capable of improving productivity by omitting the continuous casting method and the block rolling process before rolling the steel slab.

本発明の一実施形態に係る連続鋳造方法を説明するための概略図である。It is the schematic for demonstrating the continuous casting method which concerns on one Embodiment of this invention. 同実施形態に係る連続鋳造方法における鋳片の圧下形態を説明するための概略図である。It is the schematic for demonstrating the rolling form of the slab in the continuous casting method which concerns on the same embodiment. 同実施形態に係る連続鋳造方法における鋳片の鋳造速度と中心偏析との関係を示す図である。It is a figure which shows the relationship between the casting speed and center segregation of the slab in the continuous casting method which concerns on the same embodiment. 同実施形態に係る連続鋳造方法における鋳片の中心部および表面部の温度差と超音波探傷検査結果との関係を示す図である。It is a figure which shows the relationship between the temperature difference of the center part of the slab and the surface part, and ultrasonic flaw detection test result in the continuous casting method which concerns on the embodiment. 同実施形態に係る連続鋳造方法における第２の圧下工程での累積圧下率と超音波探傷検査結果との関係を示す図である。It is a figure which shows the relationship between the cumulative reduction rate in the 2nd reduction process in the continuous casting method which concerns on the embodiment, and an ultrasonic flaw detection test result.

　以下、本発明の好適な実施形態について図面を参照しながら詳細に説明する。しかし、本発明は以下の実施形態の構成のみに限定されるものではなく、本発明の趣旨を逸脱しない範囲で種々の変更が可能である。また、以下の説明で用いる図面は、本発明の特徴をわかりやすくするために、便宜上、要部となる部分を拡大して示している場合があり、各構成要素の寸法比率などが実際と同じであるとは限らない。 Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the configurations of the following embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, in the drawings used in the following description, in order to make the features of the present invention easier to understand, there is a case where a main part is shown in an enlarged manner for convenience, and the dimensional ratio of each component is the same as the actual one. Not necessarily.

　なお、以下の説明において、鋳片の厚み方向とは、ロール軸方向が連続鋳造機の据付面と平行でかつ鋳片の搬送方向と垂直になるように配置された水平ロールによる圧下方向（第１圧下方向）を意味する。また、鋳片の幅方向とは、ロール軸方向が連続鋳造機の据付面と垂直になるように配置された垂直ロールによる圧下方向（第２圧下方向）を意味する。すなわち、鋳片の長手方向に垂直な断面で見た場合に、鋳片の長手方向と、鋳片の厚み方向（第１圧下方向）と、鋳片の幅方向（第２圧下方向）とが直交する。 In the following description, the thickness direction of the cast slab refers to the roll-down direction (horizontal roll) arranged so that the roll axis direction is parallel to the installation surface of the continuous casting machine and perpendicular to the transport direction of the cast slab. 1 down direction). Further, the width direction of the cast slab means a reduction direction (second reduction direction) by a vertical roll disposed so that the roll axis direction is perpendicular to the installation surface of the continuous casting machine. That is, when viewed in a cross section perpendicular to the longitudinal direction of the slab, the longitudinal direction of the slab, the thickness direction of the slab (first reduction direction), and the width direction of the slab (second reduction direction) Orthogonal.

　また、鋳片の中心部とは、次の条件をみたす領域として定義される。鋳片の長手方向に垂直な断面で見た場合に、上記中心部の重心がこの断面の重心と一致し、上記中心部の輪郭の形状が上記断面の縮小した輪郭の形状と一致し、そして、上記中心部の面積が上記断面の面積の５０％となる領域。また、鋳片の表面部とは、鋳片の周面から深さ方向に鋳片直径に対して表面から５％である領域として定義される。また、以下の説明で特に断らない限り、鋳片の断面とは鋳片の長手方向に垂直な断面を意味する。 Also, the center part of the slab is defined as an area that satisfies the following conditions. When viewed in a cross section perpendicular to the longitudinal direction of the slab, the center of gravity of the central portion matches the center of gravity of the cross section, the shape of the contour of the central portion matches the shape of the reduced contour of the cross section, and A region where the area of the central portion is 50% of the area of the cross section. Further, the surface portion of the slab is defined as a region that is 5% from the surface with respect to the slab diameter in the depth direction from the peripheral surface of the slab. Further, unless otherwise specified in the following description, the cross section of the slab means a cross section perpendicular to the longitudinal direction of the slab.

　また、固相と液相とが共存状態にあるときの固相の体積比を固相率とするとき、鋳片の中心部で固相率が０．３～０．８（３０体積％～８０体積％）である場合を「未凝固（固液共存）」状態と定義し、鋳片の中心部で固相率が０．８（８０体積％）を超えている場合を「完全凝固」状態と定義する。また、連続鋳造機で、鋳片の中心部の固相率が０．３～０．８となる領域を「未凝固圧下帯」と定義し、鋳片の中心部の固相率が０．８を超える領域を「完全凝固圧下帯」と定義する。 Further, when the solid phase volume ratio when the solid phase and the liquid phase coexist is defined as the solid phase ratio, the solid phase ratio at the center of the slab is 0.3 to 0.8 (30% by volume to 30% by volume). 80% by volume) is defined as an “unsolidified (solid-liquid coexistence)” state, and when the solid phase ratio exceeds 0.8 (80% by volume) at the center of the slab, “completely solidified” Defined as state. Further, in a continuous casting machine, a region where the solid fraction at the center of the slab is 0.3 to 0.8 is defined as “unsolidified reduction zone”, and the solid fraction at the center of the slab is 0. A region exceeding 8 is defined as a “complete coagulation reduction zone”.

　なお、固相率は、例えば、次のように求めればよい。固相及び液相の体積比は合金状態図から類推が可能である。具体的には、鋼組成と温度とが決まれば、合金状態図から固相率を一義的に求めることが可能である。よって、鋼組成と、鋳片の中心部の温度とから、合金状態図を用いて、鋳片の中心部の固相率を求めればよい。なお、合金状態図として、熱力学計算システムに基づいた計算状態図を用いてもよい。 In addition, what is necessary is just to obtain | require the solid-phase rate as follows, for example. The volume ratio between the solid phase and the liquid phase can be inferred from the alloy phase diagram. Specifically, if the steel composition and temperature are determined, the solid phase ratio can be uniquely determined from the alloy phase diagram. Therefore, what is necessary is just to obtain | require the solid-phase rate of the center part of a slab from an alloy phase diagram from steel composition and the temperature of the center part of a slab. A calculation state diagram based on a thermodynamic calculation system may be used as the alloy state diagram.

　以下、本発明の一実施形態に係る鋼の連続鋳造方法を知見するに至った経緯について説明する。 Hereinafter, the background of finding out the continuous casting method of steel according to one embodiment of the present invention will be described.

　中心偏析及び中心ポロシティの少ない内部品質の優れた高級条鋼用の鋳片を鋳造するためには、例えば、大断面を有する鋳片を連続鋳造することが有効である。すなわち、鋳片の断面サイズを大きくすることで単位時間当たりの鋳造量が増加するため、生産性に支障を生じさせない範囲内で鋳造速度を低下することが可能となる。これによって、鋳片の断面における等軸晶率が相対的に増加し、また、取鍋、タンディッシュ、鋳型間での溶鋼中介在物の浮上分離の除去効率も向上する。 In order to cast a slab for high-grade steel bar with excellent internal quality with little center segregation and center porosity, for example, it is effective to continuously cast a slab having a large cross section. That is, since the amount of casting per unit time increases by increasing the cross-sectional size of the slab, it is possible to reduce the casting speed within a range that does not hinder productivity. Thereby, the equiaxed crystal ratio in the cross section of the slab is relatively increased, and the removal efficiency of floating separation of inclusions in the molten steel between the ladle, tundish, and mold is improved.

　しかしながら、断面サイズが大きい鋳片は、断面サイズが小さい鋳片に比べて凝固収縮量が大きくなる。従って、断面サイズが大きい鋳片の中心偏析を低減するために未凝固圧下を実施する場合、鋳片への圧下量を増加させる必要がある。このため、未凝固圧下時に鋳片の内部割れが発生しやすくなる。 However, a slab having a large cross-sectional size has a larger amount of solidification shrinkage than a slab having a small cross-sectional size. Therefore, in order to reduce the center segregation of a slab having a large cross-sectional size, when performing unsolidified reduction, it is necessary to increase the amount of reduction to the slab. For this reason, it becomes easy to generate | occur | produce the internal crack of a slab at the time of unsolidified pressure reduction.

　上記問題を解決するため、本発明者らが鋭意研究を重ねた結果、次の知見を見出した。 In order to solve the above problems, the present inventors have conducted extensive research and found the following knowledge.

　（１）矩形鋳片の圧下時には、圧下ロールとの接触面全体に圧下応力がかかり、圧下ロールとの非接触面全体に渡るバルジング変形が生じることで鋳片の中心部への圧下浸透度（鋳片の中心部への集中した圧下が可能かどうかの度合い）が低下する。そのため、中心偏析の抑制及び中心ポロシティの圧着のためには大きな圧下量が必要となる。一方、断面形状が円形の丸鋳片であれば、鋳片の圧下時に圧下ロールに接触する鋳片の円弧面に集中して圧下応力が働くため、小さな圧下量でも鋳片の中心部への圧下浸透度を高めることが可能である。
　（２）鋳片の中心部と表面部とには温度差が存在し、連続鋳造機内では、鋳片の表面部と比較して中心部が高温状態にある。そのため、鋳片の中心部の変形抵抗は、鋳片の表面部の変形抵抗と比較して小さく、小さな圧下量であっても中心部への圧下浸透度を高めることが可能である。特に、鋳型から引き抜かれた鋳片に対し、鋳片厚み方向への未凝固圧下を行い、且つ、鋳片厚み方向及び幅方向からの複数回の完全凝固後圧下を繰り返し行うことで、良好な圧下浸透度で徐々に鋳片を圧下することができる。この場合、中心偏析及び中心ポロシティの低減と、内部割れの防止とを両立しながら鋳片を圧下することができる。
　（３）一般的な分塊圧延では、分塊圧延前に鋳片加熱工程で鋳片を十分に加熱するため、分塊圧延時の鋳片の中心部温度と表面部温度との温度差が、連続鋳造機内における鋳片内の温度差と比較して小さい。そのため、鋳片の中心部への圧下浸透度が必然的に小さくなり、中心ポロシティが十分に圧着されない場合がある。一方、連続鋳造機で鋳片を圧下する場合、上述の通り鋳片の中心部と表面部との温度差によって圧下浸透度が良好となる。それゆえ、連続鋳造機によって鋳片を分塊圧延後に相当するサイズまで圧下することで、中心偏析及び中心ポロシティの低減と内部割れの防止とを両立しながら、条鋼を製造するための鋼片圧延に直接適用することが可能なサイズの鋳片を得ることができる。言い換えれば、本実施形態に係る連続鋳造機によって鋳造された鋳片は、分塊圧延を行うことなく、直接、鋼片圧延に供して条鋼を製造することができる。 (1) When the rectangular slab is rolled, a rolling stress is applied to the entire contact surface with the rolling roll, and bulging deformation occurs across the entire non-contact surface with the rolling roll, thereby reducing the rolling penetration into the center of the slab ( The degree of whether or not concentrated reduction at the center of the slab is possible is reduced. Therefore, a large amount of reduction is required to suppress center segregation and to press the center porosity. On the other hand, if the cross-sectional shape is a round slab, the slab concentrates on the arc surface of the slab that is in contact with the reduction roll when the slab is being reduced, so that the reduction stress acts on the center of the slab even with a small reduction amount. It is possible to increase the osmotic pressure.
(2) There is a temperature difference between the center portion and the surface portion of the slab, and in the continuous casting machine, the center portion is at a higher temperature than the surface portion of the slab. Therefore, the deformation resistance of the center part of the slab is smaller than the deformation resistance of the surface part of the slab, and it is possible to increase the degree of reduction penetration into the center part even with a small amount of reduction. In particular, by performing unsolidified reduction in the slab thickness direction on the slab drawn from the mold, and repeatedly performing multiple post-solidification reductions from the slab thickness direction and width direction, The slab can be gradually reduced by the reduction permeation degree. In this case, the slab can be reduced while satisfying both the reduction of center segregation and center porosity and the prevention of internal cracks.
(3) In general segment rolling, the slab is sufficiently heated in the slab heating step before segment rolling, so the temperature difference between the center temperature and the surface temperature of the slab during segment rolling is The temperature difference in the slab in the continuous casting machine is small. For this reason, the reduction permeation into the center of the slab inevitably decreases, and the center porosity may not be sufficiently crimped. On the other hand, when the slab is squeezed by a continuous casting machine, the reduction permeation degree becomes good due to the temperature difference between the center part and the surface part of the slab as described above. Therefore, by rolling the slab to a corresponding size after partial rolling with a continuous casting machine, slab rolling for producing strip steel while achieving both center segregation and reduction of center porosity and prevention of internal cracks. It is possible to obtain a slab of a size that can be applied directly to In other words, the slab cast by the continuous casting machine according to the present embodiment can be directly subjected to steel slab rolling to produce strip steel without performing segment rolling.

　以下、本実施形態に係る連続鋳造方法の各工程について説明する。 Hereinafter, each process of the continuous casting method according to the present embodiment will be described.

＜連続鋳造方法＞
　図１に、本実施形態に係る連続鋳造方法を行うための連続鋳造機１０を概略的に示す。また、図２に、本実施形態に係る連続鋳造方法における鋳片の圧下形態を概略的に示す。図１、２に示すように、本実施形態に係る連続鋳造方法は：タンディッシュ１から鋳型２へと溶鋼を供給し、円筒状（引き抜き方向に垂直な断面で見た場合にその断面形状が円形）である上記鋳型２から、未凝固（固液共存）状態の鋳片３を引き抜く引き抜き工程と；上記引き抜き工程後に、鋳型２から引き抜かれて連続鋳造機ロール（サポートロール）４を経た上記固液共存状態の上記鋳片３（３ａ）に対して、ロール軸方向が連続鋳造機１０の据付面７と平行でかつ上記鋳片３の搬送方向と垂直になるように配置された水平ロール（第１圧下ロール）５を用いて、鋳片３の長手方向に垂直な第１圧下方向への圧下を施す第１の圧下工程と；鋳片３の長手方向に垂直な断面で見た場合に鋳片３の長手方向及び上記第１圧下方向の双方と直交する圧下方向を第２圧下方向とするとき、上記第１の圧下工程後に、完全凝固状態でありかつ中心部の温度が表面部の温度よりも高い状態にある上記鋳片３（３ｂ、３ｃ）に対して、水平ロール（第１圧下ロール）６ａによる上記第１圧下方向への圧下と、垂直ロール（第２圧下ロール）６ｂによる上記第２圧下方向への圧下とを交互に施しながら、鋳片３の長手方向に垂直な断面で見た場合の角部に丸みが有る角丸矩形に鋳片３を成形する第２の圧下工程と；を備える。 <Continuous casting method>
FIG. 1 schematically shows a continuous casting machine 10 for performing the continuous casting method according to the present embodiment. Moreover, in FIG. 2, the rolling form of the slab in the continuous casting method which concerns on this embodiment is shown roughly. As shown in FIGS. 1 and 2, the continuous casting method according to the present embodiment is as follows: molten steel is supplied from the tundish 1 to the mold 2, and the shape of the cross section is cylindrical (when viewed in a cross section perpendicular to the drawing direction). A drawing step of drawing the unsolidified (solid-liquid coexisting) slab 3 from the casting mold 2 which is circular); after the drawing step, the drawing piece 2 is drawn from the casting mold 2 and passed through a continuous casting machine roll (support roll) 4 A horizontal roll arranged so that the roll axis direction is parallel to the installation surface 7 of the continuous casting machine 10 and perpendicular to the conveying direction of the slab 3 with respect to the slab 3 (3a) coexisting with the solid and liquid. (First reduction roll) First reduction step of applying a reduction in a first reduction direction perpendicular to the longitudinal direction of the slab 3 using the first reduction roll; when viewed in a cross section perpendicular to the longitudinal direction of the slab 3 Perpendicular to both the longitudinal direction of the slab 3 and the first reduction direction The slab 3 (3b, 3c) is in a completely solidified state and in a state where the temperature of the central portion is higher than the temperature of the surface portion after the first reduction step when the reduction direction is the second reduction direction. In contrast, the horizontal roll (first reduction roll) 6a is alternately subjected to the reduction in the first reduction direction and the vertical roll (second reduction roll) 6b in the second reduction direction. A second reduction step of forming the cast piece 3 into a rounded rectangular shape with rounded corners when viewed in a cross section perpendicular to the longitudinal direction of the piece 3.

（引き抜き工程）
　引き抜き工程は、タンディッシュ１から鋳型２に供給された溶鋼の鋳型２との接触面を凝固させ、そして固液共存状態の鋳片３を鋳型２の底から連続的に引き抜く工程である。鋳型２から引き抜かれた鋳片３は、連続鋳造機ロール４によって支持されて、形状を保ちながら次工程に搬送される。 (Drawing process)
The drawing process is a process in which the contact surface of the molten steel supplied from the tundish 1 to the mold 2 with the mold 2 is solidified, and the slab 3 coexisting with the solid and liquid is continuously drawn from the bottom of the mold 2. The slab 3 drawn out from the mold 2 is supported by the continuous casting machine roll 4 and conveyed to the next process while maintaining its shape.

　丸鋳片３を得るための鋳型２の断面サイズに関しては、引き抜き方向に垂直な断面で見た場合に鋳型２の内径が４００ｍｍ以上であることが好ましく、４００ｍｍ以上６００ｍｍ以下であることがさらに好ましく、４００ｍｍ以上４６０ｍｍ以下であることが最も好ましい。鋳型２の断面サイズが小さすぎると、メニスカスから鋳片３が完全凝固状態となる位置までの距離が短くなるため、鋳片３（３ａ）に十分な未凝固圧下を付与するために特殊な鋳片圧下装置が必要となり設備コストが増大する虞がある。一方、鋳型２の断面サイズが大きすぎると、鋳片３の完全凝固状態となる位置が連続鋳造機１０の機長を超えるため、後述する完全凝固後圧下（第２の圧下工程）を十分に実施するために連続鋳造機１０の機長を延長しなければならない虞がある。 Regarding the cross-sectional size of the mold 2 for obtaining the round slab 3, the inner diameter of the mold 2 is preferably 400 mm or more, and more preferably 400 mm or more and 600 mm or less when viewed in a cross section perpendicular to the drawing direction. It is most preferable that it is 400 mm or more and 460 mm or less. If the cross-sectional size of the mold 2 is too small, the distance from the meniscus to the position at which the slab 3 is in a completely solidified state is shortened. Therefore, a special casting is required to impart sufficient unsolidified reduction to the slab 3 (3a). There is a risk that the equipment cost will increase because a single reduction device is required. On the other hand, if the cross-sectional size of the mold 2 is too large, the position where the slab 3 is in a completely solidified state exceeds the length of the continuous casting machine 10, so the complete solidification reduction (second reduction process) described later is sufficiently performed. Therefore, there is a possibility that the length of the continuous casting machine 10 must be extended.

　鋳片３の鋳造速度（引き抜き速度）は、特に限定されないが、０．３５ｍ／分以上０．６５ｍ／分以下であることが好ましく、０．４０ｍ／分以上０．６０ｍ／分以下であることがさらに好ましい。鋳造速度が遅すぎる場合は、鋳片３が連続鋳造機の未凝固圧下帯（水平ロール５）へ到達するまでに鋳片内部が完全凝固してしまい、鋳片３の未凝固圧下が不可能となって、中心偏析の抑制効果が得られなくなる虞がある。加えて、鋳造速度が遅すぎる場合は、完全凝固圧下帯（第２の圧下工程）で、鋳片３の中心部と鋳片３の表面部との温度差が小さくなり、鋳片３の中心部と鋳片３の表面部との変形抵抗差が小さくなる虞がある。そのため、完全凝固後圧下による鋳片３の中心部への圧下浸透度が低下し、中心ポロシティが十分に圧着されない虞がある。一方、鋳片３の鋳造速度（引き抜き速度）が速すぎると、鋳片３の完全凝固状態となる位置が、連続鋳造機１０の機長を超えるため、中心偏析の抑制効果が得られなくなる虞がある。加えて、完全凝固後圧下（第２の圧下工程）を行うことが不可能となって、中心ポロシティの圧着効果を十分に得ることができない場合がある。 The casting speed (drawing speed) of the slab 3 is not particularly limited, but is preferably 0.35 m / min or more and 0.65 m / min or less, and 0.40 m / min or more and 0.60 m / min or less. Is more preferable. When the casting speed is too slow, the inside of the slab is completely solidified before the slab 3 reaches the unsolidified reduction zone (horizontal roll 5) of the continuous casting machine, and the unsolidified reduction of the slab 3 is impossible. As a result, the effect of suppressing the center segregation may not be obtained. In addition, if the casting speed is too slow, the temperature difference between the center portion of the slab 3 and the surface portion of the slab 3 becomes small in the complete solidification reduction zone (second reduction step), and the center of the slab 3 There is a possibility that the deformation resistance difference between the portion and the surface portion of the slab 3 becomes small. For this reason, there is a possibility that the degree of reduction penetration into the center portion of the slab 3 due to reduction after complete solidification is lowered, and the center porosity is not sufficiently pressed. On the other hand, if the casting speed (drawing speed) of the slab 3 is too high, the position at which the slab 3 is completely solidified exceeds the length of the continuous casting machine 10, and therefore there is a possibility that the effect of suppressing center segregation cannot be obtained. is there. In addition, it may become impossible to perform the reduction after the complete solidification (second reduction step), and there may be a case where the pressure bonding effect of the center porosity cannot be sufficiently obtained.

　図３に、引き抜き後の断面形状が円形でありその直径が４５０ｍｍである鋳片３を用いて調査した、鋳片３の引き抜き速度と中心偏析との関係を図示する。図３中で、縦軸は炭素濃度偏析度であり、横軸は鋳造速度（引き抜き速度）である。ここで、炭素濃度偏析度とは、鋳片３の中心部の炭素濃度測定値を、タンディッシュ１から鋳型２に供給された溶鋼の炭素濃度測定値で除した値である。鋳片３の中心部の炭素濃度は、例えば、φ５ｍｍドリルを用いて鋳片３の中心部から切粉を採取して化学分析することで得ればよい。この図３に示すように、鋳片３の引き抜き速度が０．３５ｍ／分以上０．６５ｍ／分以下であるときに、中心偏析の抑制効果が好ましく得られる。 FIG. 3 illustrates the relationship between the drawing speed of the slab 3 and the center segregation, which was investigated using the slab 3 having a circular cross-sectional shape after drawing and a diameter of 450 mm. In FIG. 3, the vertical axis represents the carbon concentration segregation degree, and the horizontal axis represents the casting speed (drawing speed). Here, the carbon concentration segregation degree is a value obtained by dividing the carbon concentration measurement value at the center of the slab 3 by the carbon concentration measurement value of the molten steel supplied from the tundish 1 to the mold 2. The carbon concentration in the center portion of the slab 3 may be obtained by, for example, collecting chips from the center portion of the slab 3 using a φ5 mm drill and performing chemical analysis. As shown in FIG. 3, when the drawing speed of the slab 3 is 0.35 m / min or more and 0.65 m / min or less, the effect of suppressing the center segregation is preferably obtained.

（第１の圧下工程）
　第１の圧下工程は、引き抜き工程後に、連続鋳造機１０の未凝固圧下帯で、円筒状の鋳型２から引き抜かれた未凝固状態（固液共存状態）の鋳片３（３ａ）に対して、水平ロール（第１圧下ロール）５を用いた鋳片厚み方向（第１圧下方向）からの圧下を施す工程である。未凝固状態でありかつ断面形状が円形である鋳片３（３ａ）を圧下することで、水平ロール５に接触する鋳片３の円弧面に集中して圧下応力が働くため、小さな圧下量でも鋳片３の中心部への圧下浸透度を高めることできる。加えて、未凝固状態の鋳片３の中心部の変形抵抗は、鋳片３の表面部の変形抵抗と比較して小さいため、小さな圧下量であっても中心部への圧下浸透度を高めることが可能である。すなわち、第１の圧下工程により、良好な圧下浸透度で鋳片３を圧下することができ、中心偏析及び中心ポロシティの低減と内部割れの防止とを両立しながら鋳片３を圧下することができる。 (First reduction step)
In the first reduction step, the slab 3 (3a) in an unsolidified state (solid-liquid coexistence state) drawn from the cylindrical mold 2 in the unsolidified reduction zone of the continuous casting machine 10 after the drawing step. In this step, rolling is performed from the slab thickness direction (first rolling direction) using a horizontal roll (first rolling roll) 5. By rolling down the slab 3 (3a) that is in an unsolidified state and having a circular cross-sectional shape, a rolling stress is concentrated on the arc surface of the slab 3 that contacts the horizontal roll 5, so even a small rolling amount can be obtained. The reduction permeation into the center of the slab 3 can be increased. In addition, since the deformation resistance of the center portion of the slab 3 in an unsolidified state is smaller than the deformation resistance of the surface portion of the slab 3, the degree of reduction penetration into the center portion is increased even with a small amount of reduction. It is possible. That is, the slab 3 can be squeezed with a good sag permeability by the first reduction process, and the slab 3 can be squeezed while reducing both center segregation and central porosity and preventing internal cracks. it can.

　第１の圧下工程では、各水平ロール５による鋳片３への圧下率が、各水平ロール５に進入直前である鋳片３の厚み（第１圧下方向の厚み）に対して、０．３％以上７．０％以下であることが好ましい。すなわち、第１の圧下工程における第１圧下方向への圧下の１回当たりの圧下率が、０．３％以上７．０％以下であることが好ましい。本実施形態では、第１の圧下工程で、軽圧下から大圧下のいずれの圧下を採用することも可能である。本実施形態において大圧下を適用する場合は、鋳片３の中心部の固相率が０．７付近の領域にて、鋳片３の中心部の未凝固部（固液共存部）の直径と等しい量の圧下を与えることで内部割れの残存を防止することができる。第１の圧下工程で、各水平ロール５の鋳片３への圧下率が小さすぎると、中心偏析の抑制効果が十分に得られない虞がある。また、各水平ロール５の鋳片３への圧下率が大きすぎると、鋳片３の表面に割れが生じる虞がある。尚、内部割れの発生を好ましく抑制するために第１の圧下工程での圧下を軽圧下に限定する場合には、各水平ロール５の圧下率を０．３％以上２．２％以下とすることが好ましい。 In the first reduction step, the reduction ratio of the horizontal rolls 5 to the slab 3 is 0.3 with respect to the thickness of the slab 3 immediately before entering each horizontal roll 5 (the thickness in the first reduction direction). % Or more and 7.0% or less is preferable. That is, it is preferable that the rolling reduction per rolling in the first rolling direction in the first rolling process is 0.3% or more and 7.0% or less. In the present embodiment, it is possible to employ any reduction from light pressure to large pressure in the first reduction process. When applying a large reduction in this embodiment, the diameter of the unsolidified portion (solid-liquid coexisting portion) at the center of the slab 3 in the region where the solid phase ratio at the center of the slab 3 is near 0.7. It is possible to prevent the remaining of internal cracks by applying an equal amount of reduction. If the reduction ratio of the horizontal rolls 5 to the slab 3 is too small in the first reduction step, the effect of suppressing the center segregation may not be sufficiently obtained. Further, if the rolling reduction ratio of each horizontal roll 5 to the slab 3 is too large, the surface of the slab 3 may be cracked. In order to preferably suppress the occurrence of internal cracks, when the reduction in the first reduction step is limited to light reduction, the reduction rate of each horizontal roll 5 is set to 0.3% or more and 2.2% or less. It is preferable.

　また、第１の圧下工程では、各水平ロール５による鋳片３への累積圧下率が、３．６％以上１０％以下であることが好ましい。各水平ロール５による累積圧下率が、３．６％以上１０％以下であるときに、鋳片３の中心偏析及び中心ポロシティの低減と、鋳片３の内部割れの発生防止とを好ましく両立することができる。なお、図１に示す連続鋳造機１０では、６対の水平ロ－ル５を用いている。 Moreover, in the first reduction step, it is preferable that the cumulative reduction ratio of the horizontal rolls 5 to the slab 3 is 3.6% or more and 10% or less. When the cumulative rolling reduction ratio by each horizontal roll 5 is 3.6% or more and 10% or less, both the center segregation and center porosity of the slab 3 and the prevention of internal cracks in the slab 3 are preferably achieved. be able to. In the continuous casting machine 10 shown in FIG. 1, six pairs of horizontal rolls 5 are used.

　第１の圧下工程に供される前に中心部が未凝固状態（中心部の固相率が０．３以上０．８以下）であった鋳片３（３ａ）は、上記条件にて第１の圧下工程に供された後、中心部が完全凝固（中心部の固相率が０．８を超える）状態となる。本実施形態では第２の圧下工程で、中心部が完全凝固状態であり、中心部の温度が表面部の温度よりも高い状態にある鋳片３（３ｂ、３ｃ）に対して、鋳片厚み方向（第１圧下方向）への圧下及び鋳片幅方向（第２圧下方向）への圧下を交互に行う。 The slab 3 (3a) in which the central portion was in an unsolidified state (the solid fraction of the central portion was 0.3 or more and 0.8 or less) before being subjected to the first reduction step was measured under the above conditions. After being subjected to the reduction step 1, the central portion is completely solidified (the solid phase ratio of the central portion exceeds 0.8). In this embodiment, in the second reduction step, the slab thickness is compared with the slab 3 (3b, 3c) in which the center is in a completely solidified state and the temperature of the center is higher than the temperature of the surface. The reduction in the direction (first reduction direction) and the reduction in the slab width direction (second reduction direction) are alternately performed.

（第２の圧下工程）
　第２の圧下工程は、第１の圧下工程の後に、連続鋳造機１０の完全凝固圧下帯で、完全凝固状態でありかつ中心部の温度が表面部の温度よりも高い状態にある鋳片３（３ｂ、３ｃ）に対して、水平ロール（第１圧下ロール）６ａを用いた鋳片厚み方向（第１圧下方向）からの圧下と、垂直ロール（第２圧下ロール）６ｂを用いた鋳片幅方向（第２圧下方向）からの圧下とを交互に施す工程である。本実施形態では、水平ロール６ａを用いた鋳片厚み方向からの圧下と垂直ロール６ｂを用いた鋳片幅方向からの圧下とを交互に施すことで、鋳片３（３ｂ、３ｃ）の内部割れを防ぎながら、中心偏析や中心ポロシティを低減することができる。加えて、連続鋳造機１０の出口（下流側機端）で、鋳片３の長手方向に垂直な断面で見た場合に、断面サイズが縮小された、角部を有しない矩形鋳片（角部に丸みを有する角丸矩形）を得ることができる。 (Second reduction step)
The second reduction step is a completely solidified reduction zone of the continuous casting machine 10 after the first reduction step, in a completely solidified state and in a state where the temperature of the central portion is higher than the temperature of the surface portion. (3b, 3c), slab using a horizontal roll (first reduction roll) 6a using a slab thickness direction (first reduction direction) and a vertical roll (second reduction roll) 6b This is a step of alternately performing reduction from the width direction (second reduction direction). In the present embodiment, the reduction in the slab thickness direction using the horizontal roll 6a and the reduction in the slab width direction using the vertical roll 6b are alternately performed, whereby the inside of the slab 3 (3b, 3c). Center segregation and center porosity can be reduced while preventing cracking. In addition, when viewed in a cross section perpendicular to the longitudinal direction of the slab 3 at the outlet (downstream machine end) of the continuous casting machine 10, a rectangular slab having a reduced corner size (corner) A rounded rectangle with rounded corners).

　鋳片３の中心部の温度が表面部の温度よりも高い状態にある場合、鋳片３の中心部の変形抵抗が鋳片３の表面部の変形抵抗と比較して小さいため、小さな圧下量であっても中心部への圧下浸透度を高めることが可能である。本実施形態では、鋳片３の中心部の温度が表面部の温度よりも１５０℃以上高いことが好ましく、２００℃以上高いことがさらに好ましい。また、鋳片３の中心部の温度の上限は、特に限定されることはないが、鋳片３の鋼組成によって決まる液相線温度としてもよい。 When the temperature of the center part of the slab 3 is higher than the temperature of the surface part, the deformation resistance of the center part of the slab 3 is smaller than the deformation resistance of the surface part of the slab 3, so a small amount of reduction Even so, it is possible to increase the degree of reduction in the central portion. In this embodiment, it is preferable that the temperature of the center part of the slab 3 is 150 degreeC or more higher than the temperature of a surface part, and it is further more preferable that it is 200 degreeC or more higher. Moreover, the upper limit of the temperature of the center part of the slab 3 is not particularly limited, but may be a liquidus temperature determined by the steel composition of the slab 3.

　なお、鋳片３の中心部及び表面部の温度は、例えば、次のように求めればよい。鋳片３の中心部の温度を実測により求めることは容易でないので、熱伝導解析を行う冷却シミュレーション（伝熱計算モデル）によって上記温度を求めてもよい。具体的には、溶鋼温度、引き抜き速度、鋳片３の断面サイズ、鋳片３と連続鋳造機１０との熱交換熱量、鋳片３の放熱量、鋳片３の加工発熱量などの各製造条件に基づいて、鋳片３の表面部の温度と中心部の温度とを冷却シミュレーションによって求めればよい。または、上記の各製造条件における鋳片３の周面温度（表面温度）と、表面部の温度と、中心部の温度との関係を事前に冷却シミュレーションによって求めておき、そして鋳片３の周面温度（表面温度）を実測することで、その製造条件におけるその時点での鋳片３の表面部の温度と中心部の温度とを類推してもよい。この場合、より正確に鋳片３の表面部の温度と中心部の温度とを求めることができる。 In addition, what is necessary is just to obtain | require the temperature of the center part and surface part of the slab 3, for example as follows. Since it is not easy to obtain the temperature of the center portion of the slab 3 by actual measurement, the temperature may be obtained by a cooling simulation (heat transfer calculation model) for performing heat conduction analysis. Specifically, each production such as molten steel temperature, drawing speed, cross-sectional size of the slab 3, heat exchange heat amount between the slab 3 and the continuous casting machine 10, heat release amount of the slab 3, heat generation amount of the slab 3 Based on the conditions, the temperature of the surface portion of the slab 3 and the temperature of the center portion may be obtained by a cooling simulation. Alternatively, the relationship between the peripheral surface temperature (surface temperature) of the slab 3, the surface temperature, and the center temperature of the slab 3 in each of the above manufacturing conditions is obtained in advance by cooling simulation, and the slab 3 periphery By actually measuring the surface temperature (surface temperature), the temperature of the surface portion and the temperature of the center portion of the slab 3 at that time in the manufacturing conditions may be inferred. In this case, the temperature of the surface part of the slab 3 and the temperature of the center part can be obtained more accurately.

　図４に、鋳片３を完全凝固後に累積圧下率２０％で圧下を加えた際の中心部と表面部との温度差と、この鋳片を鋼片圧延した後の超音波探傷検査（ＵＳＴ：Ｕｌｔｒａ　Ｓｏｎｉｃ　Ｔｅｓｔ）による合格率との関係を示す。図４中で、縦軸は鋼片での超音波探傷検査（ＵＳＴ）による合格率であり、横軸は鋳片３の中心部と表面部との温度差である。この図４に示すように、鋳片３の中心部の温度が表面部の温度よりも１５０℃以上高いときに、超音波探傷検査による合格率が高いことから、中心部ポロシティ圧着の効果が好ましく得られていることが分かる。 FIG. 4 shows the temperature difference between the center portion and the surface portion when the slab 3 is completely solidified and then rolled at a cumulative reduction ratio of 20%, and the ultrasonic flaw inspection (UST) after rolling the slab into a slab. : Indicates the relationship with the pass rate by Ultra Sonic Test). In FIG. 4, the vertical axis represents the acceptance rate by ultrasonic flaw detection (UST) on a steel piece, and the horizontal axis represents the temperature difference between the center portion and the surface portion of the slab 3. As shown in FIG. 4, when the temperature of the center portion of the slab 3 is higher by 150 ° C. or more than the temperature of the surface portion, the pass rate by the ultrasonic flaw detection inspection is high, so the effect of the center portion porosity crimping is preferable. You can see that it is obtained.

　第２の圧下工程では、各水平ロール６ａによる鋳片３（３ｂ）への圧下率が、各水平ロール６ａに進入直前である鋳片３（３ｂ）の厚み（第１圧下方向の厚み）に対して、１．５％以上７．０％以下であることが好ましい。すなわち、第１圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であることが好ましい。また、各垂直ロール６ｂによる鋳片３（３ｃ）への圧下率が、各垂直ロール６ｂに進入直前である鋳片３（３ｃ）の幅（第２圧下方向の厚み）に対して、１．５％以上７．０％以下であることが好ましい。すなわち、第２圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であることが好ましい。本実施形態では、第２の圧下工程で、第１の圧下工程と同様に、軽圧下から大圧下のいずれの圧下を採用することも可能である。本実施形態の第２の圧下工程では完全凝固後状態の鋳片へ与える圧下であるため、大圧下による圧下を行っても内部割れが発生しにくい。第２の圧下工程で、各水平ロール６ａによる鋳片３（３ｂ）への圧下率及び各垂直ロール６ｂによる鋳片３（３ｃ）への圧下率が小さすぎると、中心ポロシティ残存の抑制効果が十分に得られない虞がある。また、各水平ロール６ａによる鋳片３（３ｂ）への圧下率及び各垂直ロール６ｂによる鋳片３（３ｃ）への圧下率が大きすぎると、圧下ロールとの非接触面全体に渡るバルジング変形が生じて、圧下ロールと接触していない鋳片３の表面に割れが生じる虞がある。尚、内部割れの発生を好ましく抑制しながら上記効果を好ましく得るために、第２の圧下工程での圧下を軽圧下に限定する場合には、各水平ロール６ａ及び各垂直ロール６ｂの圧下率を１．５％以上３．３％以下とすることが好ましい。 In the second reduction step, the reduction ratio of the horizontal rolls 6a to the slab 3 (3b) is equal to the thickness of the slab 3 (3b) immediately before entering each horizontal roll 6a (the thickness in the first reduction direction). On the other hand, it is preferably 1.5% or more and 7.0% or less. That is, it is preferable that the rolling reduction rate per one rolling in the first rolling direction is 1.5% or more and 7.0% or less. Further, the reduction ratio of the vertical rolls 6b to the slab 3 (3c) is 1. In comparison with the width (thickness in the second reduction direction) of the slab 3 (3c) immediately before entering the vertical rolls 6b. It is preferably 5% or more and 7.0% or less. That is, it is preferable that the rolling reduction per one time in the second rolling-down direction is 1.5% or more and 7.0% or less. In the present embodiment, in the second reduction step, any reduction from light pressure to large pressure can be employed as in the first reduction step. In the second reduction process of the present embodiment, since the reduction is applied to the slab in a completely solidified state, internal cracks are unlikely to occur even when the reduction is performed by a large reduction. In the second reduction step, if the reduction rate to the slab 3 (3b) by each horizontal roll 6a and the reduction rate to the slab 3 (3c) by each vertical roll 6b are too small, the effect of suppressing the remaining central porosity is obtained. There is a possibility that it cannot be obtained sufficiently. Further, if the rolling reduction rate to the slab 3 (3b) by each horizontal roll 6a and the rolling reduction rate to the slab 3 (3c) by each vertical roll 6b are too large, the bulging deformation over the entire non-contact surface with the rolling roll. May occur, and the surface of the slab 3 that is not in contact with the reduction roll may be cracked. In order to obtain the above-mentioned effect preferably while suppressing the occurrence of internal cracks, when the reduction in the second reduction step is limited to light reduction, the reduction rate of each horizontal roll 6a and each vertical roll 6b is set as follows. It is preferable to be 1.5% or more and 3.3% or less.

　図５に、第２の圧下工程での累積圧下率と、鋼片圧延後の超音波探傷検査での合格率との関係を示す。図５中で、縦軸は鋼片圧延後の超音波探傷検査（ＵＳＴ）での合格率であり、横軸は各ロールによる圧下率の累積値である。 FIG. 5 shows the relationship between the cumulative rolling reduction rate in the second rolling reduction step and the acceptance rate in the ultrasonic flaw detection after rolling the steel slab. In FIG. 5, the vertical axis represents the acceptance rate in ultrasonic flaw inspection (UST) after rolling the steel slab, and the horizontal axis represents the cumulative value of rolling reduction by each roll.

　第２の圧下工程による累積圧下率が、７５％以上であるときに、鋳片３の中心偏析及び中心ポロシティの低減を好ましく得ることができる。なお、図１に示す連続鋳造機１０では、７対の水平ロ－ル６ａ及び７対の垂直ロ－ル６ｂを用いている。 When the cumulative reduction ratio in the second reduction step is 75% or more, the center segregation of the slab 3 and the reduction of the center porosity can be preferably obtained. In the continuous casting machine 10 shown in FIG. 1, seven pairs of horizontal rolls 6a and seven pairs of vertical rolls 6b are used.

　第２の圧下工程後の鋳片３の形状が、長手方向に垂直な断面で見た場合に、角部に丸みを有する角丸矩形であることが好ましい。第２の圧下工程後の鋳片３の形状が角部に丸みを有することで、鋼片圧延時に角部を起点とするクラックの生成が好ましく抑制される。 The shape of the slab 3 after the second reduction step is preferably a rounded rectangle having rounded corners when viewed in a cross section perpendicular to the longitudinal direction. Since the shape of the slab 3 after the second reduction step has roundness at the corners, generation of cracks starting from the corners at the time of steel slab rolling is preferably suppressed.

　また、第２の圧下工程後の鋳片３は、長手方向に垂直な断面で見た場合に、角部の曲率半径が５ｍｍ以上である角丸矩形であることが好ましい。角部の曲率半径が５ｍｍ以上ときに、鋼片圧延時に角部を起点とするクラックの生成がさらに好ましく抑制される。また、角部の曲率半径の上限は、特に限定されることはないが、５０ｍｍ以下であることが好ましい。 Further, the slab 3 after the second reduction step is preferably a rounded rectangle having a radius of curvature of 5 mm or more when viewed in a cross section perpendicular to the longitudinal direction. When the curvature radius of the corner portion is 5 mm or more, the generation of cracks starting from the corner portion during steel slab rolling is further preferably suppressed. The upper limit of the radius of curvature of the corner is not particularly limited, but is preferably 50 mm or less.

　また、第２の圧下工程後の鋳片３は、長手方向に垂直な断面で見た場合に、第１の圧下工程前の鋳片３の上記断面（鋳型２の断面サイズに相当）と比較して、面積％で、５８％以下であることが好ましく、４４％以下であることがさらに好ましい。具体的には、第２の圧下工程後の鋳片３の長辺が、２３５ｍｍ以上２７０ｍｍ以下であることが好ましい。連続鋳造機１０内で鋳片３の長辺を２３５ｍｍ未満まで圧下を行う場合、鋳片３の表面部の温度低下に伴う表面部の延性低下に起因する表面割れが発生する虞がある。一方、鋳片３の長辺が２７０ｍｍを超過する場合、後述する条鋼の製造方法で、鋼片圧延時のロールミル負荷が過大となり、大規模な圧延ロール装置が必要となる虞がある。加えて、分塊圧延工程の省略が不可能となる虞がある。 Further, the slab 3 after the second reduction process is compared with the above-described cross section (corresponding to the cross-sectional size of the mold 2) of the slab 3 before the first reduction process when viewed in a cross section perpendicular to the longitudinal direction. Thus, the area% is preferably 58% or less, and more preferably 44% or less. Specifically, it is preferable that the long side of the slab 3 after the second reduction step is 235 mm or more and 270 mm or less. When the long side of the slab 3 is reduced to less than 235 mm in the continuous casting machine 10, there is a risk of surface cracking due to a decrease in ductility of the surface portion accompanying a temperature decrease of the surface portion of the slab 3. On the other hand, when the long side of the slab 3 exceeds 270 mm, the roll mill load at the time of rolling the steel slab becomes excessive in the method of manufacturing the steel bar described later, and there is a possibility that a large-scale rolling roll device is required. In addition, there is a possibility that omission of the ingot rolling process may be impossible.

　尚、本実施形態に係る連続鋳造方法では、上記引き抜き工程後で上記第１の圧工程前に、冷却工程として、鋳片３に対して、冷却水による２次冷却を行ってもよい。２次冷却条件に関しては、２次冷却比水量が０．１０Ｌ／ｋｇ－ｓｔｅｅｌから０．５５Ｌ／ｋｇ－ｓｔｅｅｌの範囲であることが望ましい。２次冷却比水量が０．１０Ｌ／ｋｇ－ｓｔｅｅｌ未満の場合は、２次冷却水量が小さくなりすぎて、冷却スプレー形状の維持が困難である。一方、２次冷却比水量が０．５５Ｌ／ｋｇ－ｓｔｅｅｌを超える場合は、鋳片３に対する冷却強度が局所的に過大となって、冷却時及び復熱時の熱振幅が大きくなり、その結果、鋳片３に表面割れが発生する虞がある。 In the continuous casting method according to this embodiment, secondary cooling with cooling water may be performed on the slab 3 as a cooling step after the drawing step and before the first pressure step. Regarding the secondary cooling condition, it is desirable that the secondary cooling specific water amount is in the range of 0.10 L / kg-steel to 0.55 L / kg-steel. When the secondary cooling specific water amount is less than 0.10 L / kg-steel, the secondary cooling water amount becomes too small and it is difficult to maintain the cooling spray shape. On the other hand, when the secondary cooling specific water amount exceeds 0.55 L / kg-steel, the cooling strength with respect to the slab 3 is locally excessive, and the thermal amplitude during cooling and recuperation increases, and as a result There is a risk of surface cracks occurring in the slab 3.

　本実施形態に係る連続鋳造方法は、炭素鋼或いは合金鋼といった種々の鋼種に対する連続鋳造方法として適用可能である。本実施形態に係る連続鋳造方法により得られる鋳片３は、断面の形状が角部のない略矩形状（角部に丸みを有する角丸矩形）の鋳片であり、中心偏析及び中心ポロシティが少なくかつ内部割れも抑制された内部品質の優れた鋳片３である。また、本実施形態に係る連続鋳造方法によって、鋳片３の長手方向に垂直な断面を、分塊圧延後に相当するサイズまで縮小することができる。すなわち、以下に説明するように、本実施形態に係る連続鋳造方法により得られた鋳片３を用いて条鋼を製造する場合、分塊圧延工程を省略することが可能となる。 The continuous casting method according to the present embodiment is applicable as a continuous casting method for various steel types such as carbon steel or alloy steel. The slab 3 obtained by the continuous casting method according to the present embodiment is a slab having a substantially rectangular shape (corner rounded rectangle with rounded corners) having a cross-sectional shape, and has a center segregation and a center porosity. The slab 3 is excellent in internal quality with few internal cracks. Moreover, the cross section perpendicular | vertical to the longitudinal direction of the slab 3 can be reduced by the continuous casting method which concerns on this embodiment to the size which corresponds after partial rolling. That is, as will be described below, when the strip is manufactured using the slab 3 obtained by the continuous casting method according to the present embodiment, the block rolling process can be omitted.

　また、上記した連続鋳造方法では、一例として、第１圧下方向への圧下を施す第１圧下ロール５（６ａ）を連続鋳造機１０の据付面７に対して水平に配置し、第２圧下方向への圧下を施す第２圧下ロール６ｂを連続鋳造機１０の据付面７に対して垂直に配置した。しかし、鋳片３の長手方向に垂直な断面で見た場合に、第１圧下方向と第２圧下方向とが直交するのであれば、第１圧下ロール５（６ａ）及び第２圧下ロール６ｂの連続鋳造機１０に対する配置は、特に限定されない。ただ、図１に示す連続鋳造機１０のように、鋳片３の搬送方向（進行方向）が一部で湾曲する場合には、第１圧下方向への圧下を施す第１圧下ロール５（６ａ）を、ロール軸方向が連続鋳造機１０の据付面７と平行でかつ鋳片３の搬送方向と垂直になるように配置し、第２圧下方向への圧下を施す第２圧下ロール６ｂを連続鋳造機１０の据付面７に対して垂直に配置することが好ましい。 In the continuous casting method described above, as an example, the first reduction roll 5 (6a) that performs the reduction in the first reduction direction is disposed horizontally with respect to the installation surface 7 of the continuous casting machine 10, and the second reduction direction. The second reduction roll 6 b that applies the reduction to the vertical position was disposed perpendicular to the installation surface 7 of the continuous casting machine 10. However, when viewed in a cross section perpendicular to the longitudinal direction of the slab 3, if the first reduction direction and the second reduction direction are orthogonal, the first reduction roll 5 (6a) and the second reduction roll 6b The arrangement with respect to the continuous casting machine 10 is not particularly limited. However, like the continuous casting machine 10 shown in FIG. 1, when the conveyance direction (traveling direction) of the slab 3 is partially curved, the first reduction roll 5 (6a) that performs reduction in the first reduction direction. ) Is arranged so that the roll axis direction is parallel to the installation surface 7 of the continuous casting machine 10 and perpendicular to the conveying direction of the cast slab 3, and the second reduction roll 6b that performs the reduction in the second reduction direction is continuous. Preferably, the casting machine 10 is disposed perpendicular to the installation surface 7.

　以上説明の本実施形態に係る連続鋳造方法について以下にまとめる。
　本実施形態に係る連続鋳造方法は、円筒状の鋳型２から固液共存状態の鋳片３を引き抜く引き抜き工程と、上記引き抜き工程後に、上記固液共存状態の上記鋳片３に対して、上記鋳片３の長手方向に垂直な第１圧下方向への圧下を施す第１の圧下工程と、上記鋳片３の上記長手方向及び上記第１圧下方向の双方と直交する圧下方向を第２圧下方向とするとき、上記第１の圧下工程後に、完全凝固状態でありかつ中心部の温度が表面部の温度よりも高い状態にある上記鋳片３に対して、上記第１圧下方向及び上記第２圧下方向への圧下を交互に施しながら、上記鋳片３の上記長手方向に垂直な断面で見た場合の角部に丸みが有る角丸矩形に成形する第２の圧下工程と、を備える。 The continuous casting method according to this embodiment described above will be summarized below.
In the continuous casting method according to the present embodiment, the drawing step of drawing the solid-liquid coexisting slab 3 from the cylindrical mold 2 and the slab 3 in the solid-liquid coexisting state after the drawing step are described above. A first reduction step of applying a reduction in a first reduction direction perpendicular to the longitudinal direction of the slab 3 and a second reduction in a reduction direction orthogonal to both the longitudinal direction and the first reduction direction of the slab 3 Direction, after the first reduction step, the first squeezing direction and the first reduction direction with respect to the slab 3 that is in a completely solidified state and in which the temperature of the central portion is higher than the temperature of the surface portion. A second reduction step of forming into a rounded rectangle with rounded corners when viewed in a cross section perpendicular to the longitudinal direction of the slab 3 while alternately reducing in the two reduction directions. .

　そして、本実施形態に係る連続鋳造方法は、上記鋳型２の内径が４００ｍｍ以上６００ｍｍ以下であることが好ましく、上記鋳片３の引き抜き速度が０．３５ｍ／分以上０．６５ｍ／分以下であることが好ましく、上記引き抜き工程後かつ上記第１の圧下工程前における上記鋳片３の上記中心部の固相率が、０．３以上０．８以下であることが好ましく、上記第１の圧下工程における上記第１圧下方向への圧下の１回当たりの圧下率が、０．３％以上７．０％以下であることが好ましく、上記第１の圧下工程後かつ上記第２の圧下工程前における上記鋳片３の上記中心部の固相率が０．８超であり、なおかつ上記鋳片３の上記中心部の上記温度が上記表面部の上記温度よりも１５０℃以上高いことが好ましく、上記第２の圧下工程における、上記第１圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であり、なおかつ上記第２圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であることが好ましく、上記第２の圧下工程後の上記鋳片３の上記長手方向に垂直な上記断面の形状が、長辺が２３５ｍｍ以上２７０ｍｍ以下であり、上記角部の曲率半径が５ｍｍ以上５０ｍｍ以下であることが好ましい。 In the continuous casting method according to this embodiment, the inner diameter of the mold 2 is preferably 400 mm or more and 600 mm or less, and the drawing speed of the cast piece 3 is 0.35 m / min or more and 0.65 m / min or less. It is preferable that the solid phase ratio of the central portion of the slab 3 after the drawing step and before the first reduction step is 0.3 or more and 0.8 or less, and the first reduction It is preferable that a reduction rate per one time of the reduction in the first reduction direction in the process is 0.3% or more and 7.0% or less, and after the first reduction process and before the second reduction process. It is preferable that the solid phase rate of the central part of the slab 3 is more than 0.8, and that the temperature of the central part of the slab 3 is higher by 150 ° C. than the temperature of the surface part, In the second reduction step The reduction rate per one time of reduction in the first reduction direction is 1.5% to 7.0%, and the reduction rate per reduction in the second reduction direction is 1.5%. Preferably, it is 7.0% or less, and the shape of the cross section perpendicular to the longitudinal direction of the slab 3 after the second reduction step has a long side of 235 mm or more and 270 mm or less, and the corner portion. The curvature radius is preferably 5 mm or more and 50 mm or less.

　以下、本発明の一実施形態に係る条鋼の製造方法について説明する。 Hereinafter, the manufacturing method of the strip according to one embodiment of the present invention will be described.

＜条鋼の製造方法＞
　本実施形態に係る条鋼の製造方法は、上記した鋼の連続鋳造方法によって、形状が角部に丸みを有する角丸矩形であり内部品質にも優れる鋳片を得る連続鋳造工程と、この連続鋳造工程後に、分塊圧延を施さずに棒鋼又は線材である条鋼を得るために、上記鋳片を圧延する圧延工程（鋼片圧延工程）と、を備えることを特徴とする。 <Manufacturing method of steel bars>
The manufacturing method of the strip according to the present embodiment includes a continuous casting process for obtaining a cast piece having a rounded rectangular shape with rounded corners and excellent internal quality by the steel continuous casting method described above, and this continuous casting. A rolling step (steel slab rolling step) for rolling the cast slab in order to obtain bar steel, which is a bar steel or a wire rod, without performing ingot rolling after the step.

　このように、本実施形態に係る条鋼の製造方法では、従来必要であった分塊圧延工程を省略することができ、生産性を向上させることが可能である。加えて、得られる条鋼の品質にも優れる。すなわち、上記した鋼の連続鋳造方法により、中心偏析や中心ポロシティの低減が適切に図られると同時に、内部割れも適切に防止しながら、第１の圧下工程及び第２の圧下工程を行って角丸矩形である鋳片を得ているため、鋼片圧延工程に供して得られる条鋼の内部品質も優れたものとなる。このように本実施形態に係る条鋼の製造方法は、高級（高品質）条鋼の製造にも十分に適用可能である。 Thus, in the manufacturing method of the strip according to the present embodiment, it is possible to omit the batch rolling process that has been conventionally required, and it is possible to improve productivity. In addition, the quality of the obtained strip is excellent. That is, the above-described continuous casting method of steel appropriately reduces the center segregation and the center porosity, and at the same time, appropriately prevents the internal cracks and performs the first and second reduction steps. Since a slab having a round rectangle is obtained, the internal quality of the strip obtained by subjecting it to the steel slab rolling process is also excellent. Thus, the manufacturing method of the strip according to the present embodiment is sufficiently applicable to the manufacture of high-grade (high quality) strip.

　実施例により本発明の一態様の効果を更に具体的に説明する。しかし、実施例での条件は、本発明の実施可能性及び効果を確認するために採用した一条件例であり、本発明は、この一条件例に限定されない。本発明は、本発明の要旨を逸脱せず、本発明の目的を達成する限り、種々の条件を採用し得る。 The effects of one embodiment of the present invention will be described more specifically with reference to examples. However, the conditions in the examples are one example of conditions used to confirm the feasibility and effects of the present invention, and the present invention is not limited to these one example conditions. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

　下記の表１に実施例で用いた鋳造鋼種を示す。鋼種はＡ、Ｂの２水準とした。 Table 1 below shows the cast steel types used in the examples. The steel grades were two levels A and B.

　表１に示す各鋼を、引き抜き方向に垂直な断面で見た場合に内径が４５０ｍｍとなる鋳型を用いて引き抜いた（引き抜き工程）。引き抜かれた鋳片を必要に応じて２次冷却比水量０．１５～０．２０Ｌ／ｋｇ－ｓｔｅｅｌで冷却した。鋳片の鋳造速度（引き抜き速度）は０．３０～０．６０ｍ／分であった。上記の鋳込み条件下では、未凝固圧下帯がメニスカスを基準として鋳片の搬送方向に１７～３２ｍの領域、完全凝固圧下帯がメニスカスを基準として鋳片の搬送方向に３２超～４５ｍの領域にあった。 Each steel shown in Table 1 was drawn using a mold having an inner diameter of 450 mm when viewed in a cross section perpendicular to the drawing direction (drawing step). The drawn slab was cooled at a secondary cooling specific water amount of 0.15 to 0.20 L / kg-steel as necessary. The casting speed (drawing speed) of the slab was 0.30 to 0.60 m / min. Under the casting conditions described above, the unsolidified reduction zone is in the region of 17 to 32 m in the direction of slab conveyance with respect to the meniscus, and the complete solidification reduction zone is in the region of over 32 to 45 m in the direction of conveyance of the slab with reference to the meniscus. there were.

　下記の表２に、本発明例及び比較例のそれぞれについて、連続鋳造条件を示す。尚、本発明例では、未凝固圧下帯で６対の水平ロ－ルを用いて鋳片圧下（第１の圧下工程）を行った。また、完全凝固圧下帯で７対の水平ロ－ル及び７対の垂直ロ－ルを用いて鋳片圧下（第２の圧下工程）を行った。すなわち、断面の形状が円形である鋳片から断面の形状が角丸矩形となるように、鋳片厚み方向（第１圧下方向）への圧下と鋳片幅方向（第２圧下方向）への圧下とを交互に７回ずつ行った。尚、表２に示すように、各本発明例及び各比較例について、鋳造速度（引き抜き速度）を変更している。鋳造速度が０．５０ｍ／分及び０．６０ｍ／分である場合、上記の未凝固圧下帯内で６対の水平ロ－ルによる圧下が可能であった。しかし、鋳造速度が０．３０ｍ／分である試験番号６及び８の場合、鋳造速度が遅すぎるため、未凝固圧下帯内で未凝固圧下ができなかった。 Table 2 below shows continuous casting conditions for each of the inventive examples and the comparative examples. In the present invention example, the slab reduction (first reduction step) was performed using 6 pairs of horizontal rolls in the unsolidified reduction zone. Moreover, slab reduction (second reduction step) was performed using 7 pairs of horizontal rolls and 7 pairs of vertical rolls in the complete solidification reduction zone. That is, from the slab having a circular cross-sectional shape to the slab thickness direction (first reduction direction) and the slab width direction (second reduction direction) so that the cross-sectional shape becomes a rounded rectangle. The pressing and rolling were alternately performed 7 times. As shown in Table 2, the casting speed (drawing speed) is changed for each of the inventive examples and the comparative examples. When the casting speed was 0.50 m / min and 0.60 m / min, 6 pairs of horizontal rolls could be used in the above-mentioned unsolidified reduction zone. However, in the case of Test Nos. 6 and 8 where the casting speed was 0.30 m / min, the casting speed was too slow, so that the unsolidified reduction could not be performed in the unsolidified reduction zone.

　表２中で、試験番号１～４が本発明例である。試験番号１～４では、未凝固圧下帯で、中心部における固相比率（中心部の固相率）が０．３０以上０．８０以下である鋳片に対して、各水平ロールに進入直前である鋳片の厚み（第１圧下方向の厚み）を基準として、圧下率が０．６％または１．６％の圧下を各水平ロールで与えた。また、完全凝固圧下帯で、中心部の固相率が０．８０超であり、かつ鋳片の中心部の温度が表面部の温度を基準として１５０℃以上（鋳片内外温度差）である鋳片に対して、各水平ロールに進入直前である鋳片の厚み（第１圧下方向の厚み）を基準として圧下率が５．７％の圧下を各水平ロールで与え、また各垂直ロールに進入直前である鋳片の幅（第２圧下方向の厚み）を基準として圧下率が５．７％の圧下を各垂直ロールで与えた。水平ロール及び垂直ロールによる圧下は交互に施した。鋳片の最終形状は、長辺が２７０ｍｍであり、角部の曲率半径が１０ｍｍであった。 In Table 2, test numbers 1 to 4 are examples of the present invention. In Test Nos. 1 to 4, immediately before entering each horizontal roll for a slab having a solid phase ratio (solid phase ratio in the central portion) of 0.30 or more and 0.80 or less in the unsolidified reduction zone. On the basis of the thickness of the slab (thickness in the first rolling direction), a rolling reduction of 0.6% or 1.6% was given by each horizontal roll. Further, in the fully solidified reduction zone, the solid fraction in the center is more than 0.80, and the temperature of the center of the slab is 150 ° C. or more (the temperature difference between the inside and outside of the slab) with respect to the surface temperature. For each slab, each horizontal roll gives a reduction of 5.7% on the basis of the slab thickness (thickness in the first reduction direction) immediately before entering each horizontal roll. Each vertical roll gave a reduction with a reduction ratio of 5.7% on the basis of the width of the slab immediately before entering (thickness in the second reduction direction). Rolling by the horizontal roll and the vertical roll was alternately performed. The final shape of the slab had a long side of 270 mm and a corner radius of curvature of 10 mm.

　試験番号５～８は比較例である。試験番号５及び７では、２次冷却比水量及び鋳造速度の条件が本発明例と同一である。しかし、未凝固圧下の圧下率が本発明例の同じ鋼種と比較して小さく、かつ完全凝固後圧下を鋳片厚み方向（第１圧下方向）のみで実施し、鋳片幅方向（第２圧下方向）では行わなかった例である。試験番号６及び８では、２次冷却比水量の条件が本発明例と同一である。しかし、鋳造速度が遅すぎるために未凝固圧下帯内で未凝固圧下が実施されず、かつ完全凝固後圧下を鋳片厚み方向（第１圧下方向）のみで実施し、鋳片幅方向（第２圧下方向）では行わなかった例である。 Test numbers 5 to 8 are comparative examples. In Test Nos. 5 and 7, the conditions for the secondary cooling specific water amount and the casting speed are the same as those of the example of the present invention. However, the reduction rate of unsolidified reduction is smaller than that of the same steel type of the present invention example, and the reduction after complete solidification is performed only in the slab thickness direction (first reduction direction), and the slab width direction (second reduction) (Direction) is an example that was not performed. In Test Nos. 6 and 8, the conditions for the secondary cooling specific water amount are the same as those of the present invention example. However, since the casting speed is too slow, unsolidified reduction is not performed in the unsolidified reduction zone, and complete solidification reduction is performed only in the slab thickness direction (first reduction direction), and the slab width direction (first This is an example that was not performed in the direction of 2 pressures.

　なお、鋳片の中心部温度と表面部温度との差である鋳片内外温度差は、伝熱計算モデルおよび接触式熱電対による鋳片表面温度測定方法によって測定した。 Note that the temperature difference between the inside and outside of the slab, which is the difference between the center temperature and the surface temperature of the slab, was measured by a slab surface temperature measurement method using a heat transfer calculation model and a contact thermocouple.

　なお、鋳片の中心部の固相率は、合金状態図を用いて伝熱計算モデルによって算出した。 The solid phase ratio at the center of the slab was calculated by a heat transfer calculation model using an alloy phase diagram.

　得られた各鋳片に対して、中心部の中心偏析、中心ポロシティ、及び内部割れに関する品質評価を行った。また、得られた各鋳片を用いて鋼片圧延により条鋼を製造し、この条鋼のＣｒ炭化物生成の有無を評価した。また、得られた条鋼を用いて引抜加工を実施し、引抜加工後にシェブロンクラック発生の有無に関する品質評価を行った。 Each of the obtained slabs was subjected to quality evaluation regarding center segregation at the center, center porosity, and internal cracks. Moreover, using the obtained slabs, steel bars were manufactured by rolling steel slabs, and the presence or absence of Cr carbide generation in these steel bars was evaluated. In addition, a drawing process was performed using the obtained bar steel, and a quality evaluation on the presence or absence of chevron cracking was performed after the drawing process.

　鋳片の中心偏析は、次のように評価した。第２の圧下工程後の鋳片の長手方向に垂直な断面の中心部からφ５ｍｍドリルを用いて切粉を採取して炭素濃度値を測定した。また、ヒート鋳造時に取鍋内から採取した溶鋼サンプルの炭素濃度値を測定した。そして、この溶鋼の炭素濃度値に対する、鋳片の切粉の炭素濃度値の割合を炭素濃度偏析度として評価した。炭素濃度偏析度は１．１０以下である場合を合格とした。表２中では、炭素濃度偏析度が１．１０以下である場合を１と記し、炭素濃度偏析度が１．１０超１．１５以下である場合を２と記し、炭素濃度偏析度が１．１５超である場合を３と記した。 The center segregation of the slab was evaluated as follows. Chips were collected using a φ5 mm drill from the center of the cross section perpendicular to the longitudinal direction of the slab after the second reduction step, and the carbon concentration value was measured. Moreover, the carbon concentration value of the molten steel sample extract | collected from the ladle at the time of heat casting was measured. And the ratio of the carbon concentration value of the slab chips to the carbon concentration value of the molten steel was evaluated as the carbon concentration segregation degree. The case where the carbon concentration segregation degree was 1.10 or less was regarded as acceptable. In Table 2, the case where the carbon concentration segregation degree is 1.10 or less is described as 1, the case where the carbon concentration segregation degree is more than 1.10 and 1.15 or less is described as 2, and the carbon concentration segregation degree is 1. The case of over 15 was marked as 3.

　また、鋳片の中心ポロシティは、第２の圧下工程後の鋳片を用いて超音波探傷検査を行って調査した。そして、内部欠陥が０．３ｍｍ以下である場合を合格とした。表２中では、内部欠陥が０．３ｍｍ以下である場合を１と記し、内部欠陥が０．３ｍｍ超０．９ｍｍ以下である場合を２と記し、内部欠陥が０．９ｍｍ超である場合を３と記した。 Also, the central porosity of the slab was investigated by performing an ultrasonic flaw inspection using the slab after the second reduction process. And the case where an internal defect was 0.3 mm or less was set as the pass. In Table 2, the case where the internal defect is 0.3 mm or less is described as 1, the case where the internal defect is more than 0.3 mm and 0.9 mm or less is described as 2, and the case where the internal defect is more than 0.9 mm 3 was written.

　また、鋳片の内部割れは、次のように評価した。第２の圧下工程後の鋳片を用いて、鋳片の芯部を含むように鋳片の長手方向および鋳造方向に垂直な鋳片幅方向に沿って切断し、得られた断面（縦断面および横断面）をサルファプリントし、そして目視による内部割れの有無を判定した。 Moreover, the internal crack of the slab was evaluated as follows. Using the slab after the second reduction step, a cross section (longitudinal section) obtained by cutting along the slab width direction perpendicular to the longitudinal direction of the slab and the casting direction so as to include the core of the slab And the cross-section) were sulfaprinted, and the presence or absence of visual internal cracks was determined.

　一般に条鋼のＣｒ炭化物は中心偏析度が高い場合に、鋳片内部の濃化溶鋼残存部位で生成する。この条鋼のＣｒ炭化物生成の有無は、鋼片圧延後の条鋼の長手方向に平行な断面を顕微鏡で観察して調査した。そして、この断面にＣｒ炭化物が認められないものを合格とした。 Generally, the Cr carbide of the bar steel is generated at the remaining portion of the concentrated molten steel inside the slab when the central segregation degree is high. The presence or absence of Cr carbide formation in this strip was investigated by observing a cross section parallel to the longitudinal direction of the strip after rolling the steel slab with a microscope. And the thing in which Cr carbide | carbonized_material is not recognized by this cross section was set as the pass.

　引抜加工後のシェブロンクラックは、引抜加工後のサンプルの引張試験を行って調査した。そして、引張破断面がＶ偏析線に沿っていないものを合格とした。 The chevron crack after the drawing process was investigated by conducting a tensile test of the sample after the drawing process. And the thing whose tensile fracture surface does not follow a V segregation line was set as the pass.

　中心偏析及び中心ポロシティについては、上記の通り３段階評価とした。そして、１は合格、２は使用用途を限定すれば使用できるレベル、３は不合格とした。 As for the central segregation and central porosity, as described above, a three-level evaluation was performed. And, 1 is acceptable, 2 is a level that can be used if the usage is limited, and 3 is unacceptable.

　評価結果を表２に示す。表２から明らかなように、比較例の鋳片よりも本発明例の鋳片は、中心偏析及び中心ポロシティに関する評価が１であり、内部品質が良好であった。一方、比較例は、各評価項目ともに本発明例と比較して、内部品質が低位であった。すなわち、中心偏析の抑制が不足であり、かつ中心ポロシティの圧着不足であると推定される。これは、未凝固状態の鋳片への圧下量の不足、鋳片内外温度差が小さかったことによる鋳片の中心部への圧下浸透度の低下、及び完全凝固状態の鋳片への圧下量の不足に起因すると考えられる。 Evaluation results are shown in Table 2. As is clear from Table 2, the slab of the present invention was 1 in terms of center segregation and center porosity, and the internal quality was better than that of the slab of the comparative example. On the other hand, in the comparative example, the internal quality of each evaluation item was lower than that in the inventive example. That is, it is estimated that the center segregation is insufficiently suppressed and the center porosity is insufficiently crimped. This is because the amount of rolling down to the unsolidified slab is insufficient, the reduction of the rolling penetration into the center of the slab due to the small temperature difference between the inside and outside of the slab, and the amount of rolling down to the fully solidified slab. This is thought to be due to the lack of.

　また、本発明例では、連続鋳造機の出口（下流側機端）で、分塊圧延後に相当するサイズまで鋳片サイズを縮小することができ、条鋼を製造する際の分塊圧延工程の省略が可能であった。それに対し、比較例では、鋳片幅方向（第２圧下方向）からの完全凝固後圧下を適用しなかったため、鋳片サイズを十分に縮小することができず、条鋼を製造する際に分塊圧延工程の省略を行うことができなかった。 Further, in the present invention example, the slab size can be reduced to a size corresponding to after the bulk rolling at the outlet (downstream side machine end) of the continuous casting machine, and the block rolling process when manufacturing the bar steel is omitted. Was possible. On the other hand, in the comparative example, since the reduction after complete solidification from the slab width direction (second squeezing direction) was not applied, the slab size could not be reduced sufficiently, and when the strip was manufactured, The rolling process could not be omitted.

　本発明の上記態様によれば、条鋼として使用される幅広い鋼種に対して適用可能であるとともに、中心偏析及び中心ポロシティの低減と鋳片の内部割れの防止とを両立可能な鋳片の連続鋳造方法、及び鋼片圧延前の分塊圧延工程を省略して生産性を向上させることが可能な条鋼の製造方法を提供することができる。そのため、産業上の利用可能性が高い。 According to the above aspect of the present invention, continuous casting of a slab that is applicable to a wide range of steel types used as a bar steel and that can achieve both center segregation and reduction of center porosity and prevention of internal cracks in the slab. The method and the manufacturing method of the bar steel which can abbreviate | omit the ingot rolling process before slab rolling and can improve productivity can be provided. Therefore, industrial applicability is high.

　１：タンディッシュ
　２：鋳型
　３：鋳片
　３ａ：中心部が未凝固状態の鋳片
　３ｂ、３ｃ：完全凝固状態の鋳片
　４：連続鋳造機ロール（サポートロール）
　５：未凝固圧下帯の圧下ロール（水平ロール、第１圧下ロール）
　６：完全凝固圧下帯の圧下ロール
　６ａ：水平ロール（第１圧下ロール）
　６ｂ：垂直ロール（第２圧下ロール）
　７：連続鋳造機の据付面
　１０：連続鋳造機 1: Tundish 2: Mold 3: Cast slab 3a: Slab with unsolidified center 3b, 3c: Slab with fully solidified 4: Continuous caster roll (support roll)
5: Reduction roll of unsolidified reduction zone (horizontal roll, first reduction roll)
6: Rolling roll in a completely solidified rolling zone 6a: Horizontal roll (first rolling roll)
6b: Vertical roll (second reduction roll)
7: Installation surface of continuous casting machine 10: Continuous casting machine

Claims (3)

1. 　円筒状の鋳型から固液共存状態の鋳片を引き抜く、引き抜き工程と；
   　前記引き抜き工程後に、前記固液共存状態の前記鋳片に対して、前記鋳片の長手方向に垂直な第１圧下方向への圧下を施す、第１の圧下工程と；
   　前記鋳片の前記長手方向及び前記第１圧下方向の双方と直交する圧下方向を第２圧下方向とするとき、前記第１の圧下工程後に、完全凝固状態でありかつ中心部の温度が表面部の温度よりも高い状態にある前記鋳片に対して、前記第１圧下方向及び前記第２圧下方向への圧下を交互に施しながら、前記鋳片の前記長手方向に垂直な断面で見た場合の角部に丸みが有る角丸矩形に成形する、第２の圧下工程と；を備えることを特徴とする、連続鋳造方法。 A drawing process of drawing a solid-liquid coexisting slab from a cylindrical mold;
   A first reduction step of applying a reduction in a first reduction direction perpendicular to the longitudinal direction of the slab to the slab in the solid-liquid coexistence state after the drawing step;
   When the reduction direction perpendicular to both the longitudinal direction and the first reduction direction of the slab is the second reduction direction, after the first reduction step, the slab is in a completely solidified state and the temperature at the center is the surface portion. When the slab in a state higher than the temperature of the slab is viewed in a cross section perpendicular to the longitudinal direction of the slab while alternately performing the reduction in the first reduction direction and the second reduction direction And a second reduction step of forming a rounded rectangle with rounded corners. A continuous casting method, comprising:
2. 　前記鋳型の内径が４００ｍｍ以上６００ｍｍ以下であり；
   　前記鋳片の引き抜き速度が０．３５ｍ／分以上０．６５ｍ／分以下であり；
   　前記引き抜き工程後かつ前記第１の圧下工程前における前記鋳片の前記中心部の固相率が、０．３以上０．８以下であり；
   　前記第１の圧下工程における前記第１圧下方向への圧下の１回当たりの圧下率が、０．３％以上７．０％以下であり；
   　前記第１の圧下工程後かつ前記第２の圧下工程前における前記鋳片の前記中心部の固相率が０．８超であり、なおかつ前記鋳片の前記中心部の前記温度が前記表面部の前記温度よりも１５０℃以上高く；
   　前記第２の圧下工程における、前記第１圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であり、なおかつ前記第２圧下方向への圧下の１回当たりの圧下率が１．５％以上７．０％以下であり；
   　前記第２の圧下工程後の前記鋳片の前記長手方向に垂直な前記断面の形状が、長辺が２３５ｍｍ以上２７０ｍｍ以下であり、前記角部の曲率半径が５ｍｍ以上５０ｍｍ以下である；
   ことを特徴とする、請求項１に記載の連続鋳造方法。 The inner diameter of the mold is 400 mm or more and 600 mm or less;
   The drawing speed of the slab is 0.35 m / min or more and 0.65 m / min or less;
   The solid phase ratio of the central portion of the slab after the drawing step and before the first reduction step is 0.3 or more and 0.8 or less;
   A reduction rate per one time of reduction in the first reduction direction in the first reduction step is 0.3% or more and 7.0% or less;
   The solid phase ratio of the central portion of the slab after the first reduction step and before the second reduction step is more than 0.8, and the temperature of the central portion of the slab is the surface portion. 150 ° C. higher than the above temperature;
   In the second reduction step, a reduction rate per one time of reduction in the first reduction direction is 1.5% or more and 7.0% or less, and further, one reduction in the second reduction direction. The rolling reduction is 1.5% or more and 7.0% or less;
   The shape of the cross section perpendicular to the longitudinal direction of the slab after the second reduction step has a long side of 235 mm or more and 270 mm or less, and a curvature radius of the corner portion of 5 mm or more and 50 mm or less;
   The continuous casting method according to claim 1, wherein:
3. 　請求項１又は２に記載の連続鋳造方法によって前記鋳片を得る連続鋳造工程と；
   　前記連続鋳造工程後に、前記鋳片を圧延する圧延工程と；を備える
   ことを特徴とする、条鋼の製造方法。 A continuous casting step of obtaining the slab by the continuous casting method according to claim 1;
   And a rolling step of rolling the slab after the continuous casting step.

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