KR100352596B1 - Manufacturing method of 60KGF / mm2 structural steel - Google Patents
Manufacturing method of 60KGF / mm2 structural steel Download PDFInfo
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- KR100352596B1 KR100352596B1 KR1019980052500A KR19980052500A KR100352596B1 KR 100352596 B1 KR100352596 B1 KR 100352596B1 KR 1019980052500 A KR1019980052500 A KR 1019980052500A KR 19980052500 A KR19980052500 A KR 19980052500A KR 100352596 B1 KR100352596 B1 KR 100352596B1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Heat Treatment Of Steel (AREA)
Abstract
본 발명은 60kgf/㎟급 구조용강의 제조방법에 관한 것이며, 그 목적하는 바는 여러번에 걸쳐 압하를 행하는 제어압연등과 같은 압연방법이 아닌 다른 방법의 압연을 통하여 값비싼 합금원소의 첨가 없이 저탄소강에서 페라이트의 입도를 더욱 작게 제어함으로서, 용접성이 기존강재와 동등이상이면서도 항복강도 44kgf/㎟이상, 인장강도 58kgf/㎟이상 및 연신율 20%이상의 우수한 물성을 갖는 구조용강을 제조하는 방법을 제공하고자 하는데 있다.The present invention relates to a manufacturing method of 60kgf / mm2 structural steel, the object of which is to low carbon steel without the addition of expensive alloying elements through the rolling of a method other than a rolling method such as control rolling, such as rolling down several times By controlling the particle size of ferrite to be smaller than, in order to provide a method for producing structural steel having excellent physical properties of weldability equal to or higher than the existing steel, yield strength 44kgf / ㎠ or more, tensile strength 58kgf / ㎠ or more and elongation 20% or more have.
상기 목적을 달성하기 위한 본 발명은 구조용강을 제조하는 방법에 있어서, 중량%로, C:0.05-0.2%, Si:0.5%이하, Mn:0.9-1.8%, 기타 불가피한 불순물 및 Fe로 이루어진 주괴를 1100-1300℃의 온도범위에서 가열한 후, 오스테나이트 재결정역에서 열간압연을 50%이상의 압하를 가하고, Ar3-Ae3사이의 온도에서 50%이상 압연을 행한 후 서냉하여 페라이트를 30%이상 생성시킨 후 다시 50%이상의 압하로 압연하여 변형된 페라이트를 재결정시키고, 이후 5-15℃/s로 상온까지 냉각하는 60kgf/㎟급 구조용강의 제조방법에 관한 것을 그 요지로 한다.In order to achieve the above object, the present invention provides a method for manufacturing structural steel, in weight percent, C: 0.05-0.2%, Si: 0.5% or less, Mn: 0.9-1.8%, other inevitable impurities and ingots made of Fe. Is heated in the temperature range of 1100-1300 ° C, hot rolling is applied at 50% or more in the austenitic recrystallization zone, 50% or more rolling at the temperature between Ar 3 -Ae 3 After the above-mentioned production, the sheet is rolled to 50% or more to recrystallize the deformed ferrite, and then a summary of a method for producing 60 kgf / mm 2 structural structural steel cooled to room temperature at 5-15 ° C./s.
Description
본 발명은 구조용강을 제조하는 방법에 관한 것으로, 보다 상세하게는 미세한 페라이트 조직을 포함하는 60kgf/㎟급 구조용강의 제조방법에 관한 것이다.The present invention relates to a method for manufacturing structural steel, and more particularly, to a method for manufacturing a 60kgf / mm2 structural steel containing a fine ferrite structure.
구조용강으로 사용되는 것은 일반적으로 인장강도 40-50kgf/㎟급으로서, 제어압연(controlled rolling)이나 재결정제어압연(recrystallization controlled rolling, 이하, "RCR"이라고도 한다) 등과 같이 열간압연 후에 공냉하거나 경우에 따라서는 열간압연 후 가속냉각하는 방법으로 제조된다. 상기 RCR기술은 Ti, V, N 등의 합금원소로 설계한 강에서 슬라브 재가열과 고온압연시의 결정립 성장의 억제, 압연 중의 정적재결정에 의한 결정립 미세화, γ→α변태시 탄질화물 석출에 의한 페라이트 결정립 미세화 및 석출강화를 이용하여 FRT(압연 종료온도)를 900-1050℃로 높히고도 강도와 충격인성을 확보할 수 있는 기술이다. 또한, 강도를 증가시키기 위하여 합금원소를 첨가하거나 열처리(quenching and tempering)를 실시하는 것이 보통이다.Structural steels are generally used in the form of tensile strengths of 40-50 kgf / mm2, which are either air cooled or hot rolled after controlled rolling or recrystallization controlled rolling (hereinafter also referred to as "RCR"). Therefore, it is manufactured by the method of accelerated cooling after hot rolling. The RCR technology is designed to suppress slab reheating and grain growth during high temperature rolling in steels designed with alloying elements such as Ti, V, and N, refine grains by static recrystallization during rolling, and ferrite by precipitation of carbonitride during γ → α transformation. It is a technology that can increase the FRT (rolling end temperature) to 900-1050 ℃ and secure strength and impact toughness by using grain refinement and precipitation strengthening. It is also common to add alloying elements or to conduct heat treatment (quenching and tempering) to increase the strength.
하지만, 이러한 방법은 공정이 복잡하거나 합금원소의 첨가에 의한 제조원가 상승의 문제점을 갖는다. 이에, 공정의 단순화를 위하여 많은 노력이 있어 왔으며, 최근에는 압연을 종료한 후 750-600℃까지 4℃/s 이상으로 냉각하는 가속냉각법이 소개되기도 하였으나, 이 방법에서의 화학성분에는 값이 비싼 합금원소가 함유되어 있어 합금 원단위가 비싸고 또한 제강 및 연속주조시 어려운 문제가 발생할 소지가 많고 통상의 압연으로 열연을 행하기 때문에 압연회수의 증가로 생산성이 떨어지는 단점이 있다.However, this method has a problem of complicated manufacturing process or production cost increase due to the addition of alloying elements. Therefore, much effort has been made to simplify the process, and recently, an accelerated cooling method of cooling to 4 ° C./s or more up to 750-600 ° C. after rolling is introduced has been introduced, but the chemical composition of this method is expensive. Since the alloy element is contained, the alloy unit is expensive, and there are many problems that occur during steelmaking and continuous casting, and since hot rolling is performed by ordinary rolling, productivity is reduced due to the increase in the number of rolling.
또한, 합금의 첨가로 인해 용접성을 저해시키는 경우도 많이 있다.In addition, the weldability is often impaired due to the addition of the alloy.
이에, 본 발명은 상기와 같은 단점을 수반하지 않는 구조용강 제조방법을 제공하기 위한 것으로, 그 목적하는 바는 여러번에 걸쳐 압하를 행하는 제어압연등과 같은 압연방법이 아닌 다른 방법의 압연을 통하여 값비싼 합금원소의 첨가 없이 저탄소강에서 페라이트의 입도를 더욱 작게 제어함으로서, 용접성이 기존강재와 동등이상이면서도 항복강도 44kgf/㎟이상, 인장강도 58kgf/㎟이상 및 연신율 20%이상의 우수한 물성을 갖는 구조용강을 제조하는 방법을 제공하고자 하는데 있다.Accordingly, the present invention is to provide a method for producing structural steel that does not involve the above disadvantages, the object of the present invention is to provide a value through the rolling of a method other than a rolling method such as a control rolling, such as rolling down several times By controlling the size of ferrite smaller in low carbon steel without the addition of expensive alloying elements, the structural steel has excellent physical properties with weldability equal to or higher than the existing steel, yield strength of 44kgf / mm2, tensile strength of 58kgf / mm2, and elongation of 20% It is to provide a method of manufacturing.
도 1은 본 발명에 있어 제조 프로세스의 일예를 보이는 그래프1 is a graph showing an example of a manufacturing process in the present invention
도 2(a)(b)(c)는 광학현미경에 의한 미세조직 사진Figure 2 (a) (b) (c) is a microstructure photograph by an optical microscope
일반적으로 합금의 첨가는 용접성의 저하를 초래하게 되는데 이러한 단점을 극복할 수 있는 방법 증의 하나가 페라이트의 입도를 미세화시키는 것이다. 제어압연을 통하여도 폐라이트 결정립의 크기는 10μm정도를 얻을 수 있는데, 본 발명자들은 다른 압연 방법을 통하여 합금원소의 첨가없이도 저탄소강에서 페라이트의 입도를 더욱 작게 할 수 있음을 알아내었다.In general, the addition of the alloy leads to a decrease in weldability. One of the ways of overcoming this disadvantage is to refine the size of the ferrite. Even through controlled rolling, the size of the waste light grains can be obtained about 10 μm, and the present inventors found out that the grain size of the ferrite can be further reduced in low carbon steel without the addition of alloying elements through other rolling methods.
상기한 바와 같은 관점으로 부터 출발한 본 발명은 구조용강을 제조하는 방법에 있어서, 중량%로, C:0.05-0.2%, Si:0.5%이하, Mn:0.9-1.8%, 기타 불가피한 불순물 및 Fe로 이루어진 주괴를 1100-1300℃의 온도범위에서 가열한 후, 오스테나이트 재결정역에서 열간압연을 50%이상의 압하를 가하고, Ar3-Ae3사이의 온도에서 50%이상 압연을 행한 후 서냉하여 페라이트를 30%이상 생성시킨 후 다시 50%이상의 압하로 압연하여 변형된 페라이트를 재결정시키고, 이후 5-15℃/s로 상온까지 냉각하는 것을 특징으로 하는 60kgf/㎟급 구조용강의 제조방법에 관한 것이다.Starting from the above point of view, the present invention is a method for producing structural steel, in weight%, C: 0.05-0.2%, Si: 0.5% or less, Mn: 0.9-1.8%, other unavoidable impurities and Fe After heating the ingot consisting of 1100-1300 ℃ in the temperature range, hot rolling is applied at least 50% of the hot rolling in the austenite recrystallization zone, rolling at least 50% at a temperature between Ar 3 -Ae 3 and then slowly cooled to ferrite After the production of more than 30% and rolling again to 50% or more reduction to recrystallize the modified ferrite, and then to a manufacturing method of 60kgf / mm2 structural steel characterized in that cooled to room temperature at 5-15 ℃ / s.
이하, 본 발명을 상세히 설명한다.Hereinafter, the present invention will be described in detail.
본 발명에서는 C:0.05-0.2%, Si:0.5%이하 및 Mn:0.9-1.8%를 함유한 강성분조성으로 이루어진다.In the present invention, the composition consists of a steel component containing C: 0.05-0.2%, Si: 0.5% or less and Mn: 0.9-1.8%.
상기 탄소(C)는 함량이 0.05%보다 적을 경우 제2상 조직의 분율이 저하하여 강도가 저하되고, 0.2%보다 많을 경우에는 강도는 증가하나 연신율을 해치고 용접성에도 나쁘다. 따라서, 본 발명에서는 C의 함량을 0.05-0.2%로 한정하는 것이 바람직하다.When the carbon (C) content is less than 0.05%, the fraction of the second phase tissue is lowered and the strength is lowered. When the carbon (C) is more than 0.2%, the strength is increased but the elongation is deteriorated and the weldability is bad. Therefore, in the present invention, it is preferable to limit the content of C to 0.05-0.2%.
상기 규소(Si)는 제강시 탈산제로 첨가되며 고용강화효과도 있으나, 충격천이온도를 높이고, 0.5%를 초과하여 첨가되면 용접성이 저하되며 강판표면에 산화피막이 심하게 형성된다. 따라서, 본 발명에서는 Si의 함량을 0.5%이하로 한정하는 것이 바람직하다.The silicon (Si) is added as a deoxidizer during steelmaking and has a solid solution effect, but when the impact transition temperature is increased and added in excess of 0.5%, weldability is degraded and an oxide film is severely formed on the surface of the steel sheet. Therefore, in the present invention, it is preferable to limit the content of Si to 0.5% or less.
상기 망간(Mn)은 그 함량이 0.9%미만이 되면 강의 경화능을 저하시켜 열간압연후의 냉각시 제2상 조직인 베이나이트를 형성하기 어려워 강도확보가 업렵고, 1.8%를 초과하면 용접성이 저하한다. 따라서, 본 발명에서는 Mn의 함량을 0.9-1.8%로 한정하는 것이 바람직하다.When the content of manganese (Mn) is less than 0.9%, the hardenability of the steel is lowered, and it is difficult to form bainite as a second phase structure upon cooling after hot rolling. . Therefore, in the present invention, it is preferable to limit the content of Mn to 0.9-1.8%.
또한, 본 발명에서는 상기한 바와같이 조성되는 주괴를 1100-1300℃의 온도범위에서 가열한 후, 오스테나이트 재결정역(1000-850℃정도)에서 열간압연을 50%이상의 압하를 가하고, Ar3-Ae3사이의 온도에서 50%이상 압연을 행한 후 서냉하여 페라이트를 30%이상 생성시킨 후 다시 50%이상의 압하로 압연하여 변형된 페라이트를 재결정시키고, 이후 5-15℃/s로 상온까지 냉각한다.In the present invention, the ingot formed as described above is heated at a temperature in the range of 1100-1300 ° C., and then hot rolling is applied to the austenitic recrystallization zone (about 1000-850 ° C.) by 50% or more, and Ar 3 − After rolling at least 50% at a temperature between Ae 3 and slow cooling to produce ferrite at least 30%, and again rolling at 50% or more to recrystallize the deformed ferrite, and then cooled to room temperature at 5-15 ° C./s. .
상기 재가열 온도에서 가열온도가 1300℃를 초과하는 경우에는 오스테나이트 입자도 너무 조대화되고 강중에 델타-페라이트가 일부 생성되어 강판의 성질을 열화시키는 문제가 있고, 가열온도가 1100℃미만으로 너무 낮으면 용질원자가 완전히 오스테나이트에 고용되지 않아서 강도 확보가 어렵고, 재결정역 압연온도를 맞추기 어렵다. 따라서, 재가열시 주괴가열온도는 1100-1300℃로 하는 것이 바람직하다.At the reheating temperature, when the heating temperature exceeds 1300 ° C., the austenite particles are too coarse and some delta-ferrite is formed in the steel, thereby deteriorating the properties of the steel sheet, and the heating temperature is too low at less than 1100 ° C. If the solute is not completely dissolved in austenite, it is difficult to secure the strength and it is difficult to match the recrystallization rolling temperature. Therefore, it is preferable that the ingot heating temperature at reheating be 1100-1300 ° C.
상기 재결정역 압연에서 압연온도가 오스테나이트 재결정역(1000℃정도)을 넘으면 오스테나이트 미세화 효과가 적고, 오스테나이트 재결정역(850℃정도)미만에서는 미재결정역이므로 조대한 오스테나이트의 변형에 의해 다음공정이 이루어지게 되어 그 때의 페라이트 미세화 효과는 작다.In the recrystallization zone rolling, when the rolling temperature exceeds the austenite recrystallization zone (about 1000 ° C.), the austenite refining effect is small. If the austenite recrystallization zone (about 850 ° C.) is less than the recrystallization zone, it is caused by deformation of coarse austenite. The process is made and the ferrite refinement effect at that time is small.
이때, 상기 열간압연은 압하량을 50%이상으로 주어 재결정에 의해 오스테나이트를 미세화시키는 것이 바람직하다.At this time, it is preferable that the hot rolling is made to reduce the amount of austenite by recrystallization given a reduction amount of 50% or more.
상기 Ar3-Ae3사이(미재결정역)에서의 압연은 오스테나이트에서 페라이트로 변태가 되기 직전에 압연을 행하여 변형대 생성을 극대화시키고자 하는 것으로, 첫번째 압연은 압하량을 50%이상으로 주어야 하고 압연후 페라이트를 30%이상 생성할 때까지 서냉하고, 다시 두 번째 압연을 50%이상의 압하율로 하여 페라이트를 재결정시킨다. 이때, 상기 첫 번째 압연은 열을 발생하게 되며, 상기 서냉에 의해 페라이트를 생성시킨후, 다시 Ar3-Ae3사이에서 두 번째 압연을 행하는 것이다.The rolling between Ar 3 -Ae 3 (unrecrystallized region) is intended to maximize the generation of strain bands by rolling just before transformation from austenite to ferrite, and the first rolling should give a rolling reduction of 50% or more. After cooling, the mixture was slowly cooled until 30% or more of ferrite was formed, and the second rolling was recrystallized with a rolling reduction of 50% or more. At this time, the first rolling generates heat, and after the ferrite is generated by the slow cooling, the second rolling is performed again between Ar 3 -Ae 3 .
상기 페라이트를 재결정시킨 후의 냉각속도는 5℃/s미만으로 되면 생성되는 페라이트의 입도가 조대해지고, 15℃/s를 초과하여 너무 빠르면 마르텐사이트나 하부 베이나이트의 생성으로 연속항복(continuous yielding)이 발생하여 항복강도가 저하하고 또한 연성이 저하된다. 따라서, 상기 냉각속도는 5-15℃/s로 하는 것이 바람직하다.When the cooling rate after recrystallization of the ferrite is less than 5 ° C / s, the grain size of the ferrite produced is coarse, if it exceeds 15 ° C / s too fast, the continuous yielding is produced by the production of martensite or lower bainite The yield strength is lowered and the ductility is lowered. Therefore, the cooling rate is preferably 5-15 ° C / s.
이하, 실시예를 통하여 본 발명을 보다 상세히 설명한다.Hereinafter, the present invention will be described in more detail with reference to Examples.
실시예Example
하기 표1과 같은 성분의 저탄소강 주괴들을 준비하였다.To prepare low carbon steel ingots of the components shown in Table 1 below.
먼저, 상기 표1의 발명강과 같은 성분의 저탄소강을 1200℃로 가열하여, 도 1과 같은 압연 프로세스로 열간압연한 다음 상온까지 10℃/s의 냉각속도로 냉각하여 시편을 제조하였다.First, a low carbon steel of the same composition as the steel of Table 1 was heated to 1200 ℃, hot-rolled by a rolling process as shown in Figure 1 and then cooled to a cooling rate of 10 ℃ / s to a room temperature to prepare a specimen.
또한, 상기 표1의 비교강과 같은 성분의 AH32강을 이용하여, 일반적인 제어압연 및 가속냉각에 의하여 시편을 제조하였다. 즉, 하기 표2에서 비교예 1은 RCR법으로 제조된 것으로, 1250℃에서 2시간 재가열한 후 6-7패스로 제어압연을 행한 후 FRT 950℃에서 5℃/s로 가속냉각하여 제조한 것이며, 비교예 2는 일반압연 법으로 제조된 것으로 1250℃에서 2시간 재가열한 후 11패스로 매 패스당 압하율을 20%미만이 되게 압연을 행한 후 FRT 950℃에서 5℃/s로 가속냉각하여 제조된 것이다.In addition, a specimen was prepared by general controlled rolling and accelerated cooling using AH32 steel having the same component as that of the comparative steel of Table 1. That is, in Table 2, Comparative Example 1 was prepared by RCR method, and after reheating at 1250 ° C. for 2 hours, performing control rolling in a 6-7 pass, and then cooling it by accelerated cooling at 5 ° C./s at 950 ° C. at FRT. , Comparative Example 2 is manufactured by the general rolling method and after re-heating at 1250 ℃ for 2 hours, the rolling rate is reduced to less than 20% in each pass in 11 passes and then accelerated cooling at 5 ℃ / s at FRT 950 ℃ It is manufactured.
상기와 같이 제조된 시편들에 대한 항복강도, 인장강도, 연신율을 측정하여 그 측정결과를 하기 표2에 나타내었다.Yield strength, tensile strength, and elongation of the specimens prepared as described above were measured, and the measurement results are shown in Table 2 below.
상기 표2에서 알 수 있는 바와같이, 발명예는 항복강도 51kgf/㎟, 인장강도 61kgf/㎟, 연신율 28%이며, 페라이트 결정립 4μm이었다.As can be seen in Table 2, the invention example was yield strength 51kgf / mm 2, tensile strength 61kgf / mm 2, elongation 28%, was a ferrite grain 4μm.
이에 반하여, 비교예 1은 항복강도 37kgf/㎟, 인장강도 49kgf/㎟, 연신율은 30%이었으며, 비교예 2는 항복강도 35kgf/㎟, 인장강도 48kgf/㎟, 연신율 32%이었다.On the contrary, in Comparative Example 1, the yield strength was 37 kgf / mm 2, the tensile strength was 49 kgf / mm 2, and the elongation was 30%. In Comparative Example 2, the yield strength was 35 kgf / mm 2, the tensile strength was 48 kgf / mm 2, and the elongation was 32%.
이상과 같은 결과를 보다 세부적으로 분석함으로서 다음과 같은 결과를 얻을 수 있었다.By analyzing the above results in more detail, the following results were obtained.
즉, 발명예의 경우 오스테나이트의 재결정역에서의 압연에 의하여 오스테나이트 결정이 재가열온도에서 보다 1/10로 작아지고(250μm→25μm), 비교예의 경우는 재결정역에서의 오스테나이트 재결정 입도가 1/5 정도로 작다(250μm→50μm). 이는 페라이트가 생성될 수 있는 유효입계면적이 그 만큼 발명예의 경우가 더 크다는 것을 말한다.That is, in the case of the invention example, the austenite crystal is reduced by 1/10 (250 μm → 25 μm) at the recrystallization zone by rolling in the recrystallization zone of austenite, and in the case of the comparative example, the austenite recrystallization grain size in the recrystallization zone is 1 / It is as small as 5 (250 μm → 50 μm). This means that the effective grain area where ferrite can be produced is larger in the case of the invention example.
발명예의 경우 도 1에서와 같은 750℃에서의 연속압연은 먼저 1-스텝(step)압연의 경우, 앞서 작아진 오스테나이트를 50%의 변형에 의해 조직내의 변형대를 증가시켜서 페라이트의 핵생성 장소를 증대시키고 그후 서냉에 의해 페라이트를 재결정시켰다. 이어서, -10℃/s의 냉각속도로 상온까지 냉각하여 페라이트가 조대하지 않고 연속항복을 일으키는 마르텐사이트나 베이나이트의생성을 억제하고 미세한 페라이트와 펄라이트를 얻었다.In the case of the invention, continuous rolling at 750 ° C. as shown in FIG. 1 first increases the deformation zone in the tissue by 50% deformation of austenite, which has been reduced in the case of 1-step rolling, and places the nucleation of ferrite. The ferrite was then recrystallized by slow cooling. Subsequently, the mixture was cooled to room temperature at a cooling rate of −10 ° C./s to suppress the formation of martensite or bainite which caused continuous yield without ferrite coarsening, thereby obtaining fine ferrite and pearlite.
도 2는 얻어진 시편의 광학현미경 미세조직 사진으로 (a)는 발명예, (b)는 비교예 1, (c)는 비교예 2를 보인다. 페라이트 결정립의 크기는 발명예의 경우 4μm이고, 비교예 1의 경우는 10.0μm, 비교예 2는 10.5μm로서 발명예의 경우가 가장 작았다.2 is an optical microscope microstructure photograph of the obtained specimen (a) is an invention example, (b) is a comparative example 1, (c) shows a comparative example 2. The size of the ferrite grains was 4 μm in the case of the invention example, 10.0 μm in the comparative example 1, and 10.5 μm in the comparative example 2, which was the smallest in the case of the invention example.
이러한 효과로 인해 Ar3-Ae3영역에서 2-패스(pass)압연에 의해 제조한 발명예의 경우가 다패스로 압연한 비교예의 경우 보다 페라이트가 미세한 조직을 갖게 되어 합금원소 Ti가 첨가되지 않았음에도 불구하고 비교예 보다 훨씬 우수한 기계적성질을 나타내었다. 이와같이, 페라이트 변태 직전에 압하를 하고 다시 연속적인 압연을 행하여 미세한 페라이트를 얻을 수 있었다.Due to this effect, the invention example manufactured by two-pass rolling in the Ar 3 -Ae 3 region had a finer structure than the comparative example rolled in the multi-pass, so that the alloy element Ti was not added. Nevertheless, it showed much better mechanical properties than the comparative example. In this way, it was pressed down immediately before the ferrite transformation and continuous rolling again to obtain fine ferrite.
상술한 바와같이 본 발명에 의하면, 강도향상을 위한 합금원소의 첨가없이, 오스테나이트 재결정역(약 1000-850℃)단계에서 큰 압하를 가하여 재결정에 의해 오스테나이트를 미세화 시키고, Ar3-Ae3사이의 온도(약 850-700℃)에서 연속적으로 2회의 큰 압하를 가한 후 가속냉각함으로서, 페라이트의 재결정에 의하여 미세한 페라이트를 얻어 합금원소를 첨가하지 않는 동시에 열처리를 생략하고서도 고강도화, 즉 인장강도 60kgf/㎟급 구조용강을 제조할 수 있는 효과가 제공된다.As described above, according to the present invention, austenite is refined by recrystallization by applying a large reduction in the austenite recrystallization zone (about 1000-850 ° C.) without adding an alloying element for improving the strength, and Ar 3 -Ae 3 Accelerated cooling after applying two large reductions in succession at a temperature between about 850-700 ° C, resulting in fine ferrites by recrystallization of ferrite, resulting in high strength without the addition of alloying elements and without the heat treatment, ie tensile strength of 60kgf The effect of producing / mm2 structural steel is provided.
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