JP4725415B2 - Hot-pressed steel sheet, hot-pressed steel sheet member, and production method thereof - Google Patents
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Description
本発明は、自動車のボデー構造部品、足回り部品等を始めとする機械構造部品の製造に使用するのに適した熱間プレス用鋼板と熱間プレス鋼板部材ならびにそれらの製造方法に関する。 The present invention relates to a hot-pressed steel sheet and hot-pressed steel sheet member suitable for use in the manufacture of machine structural parts such as automobile body structural parts and undercarriage parts, and methods for producing the same.
近年、自動車の燃費向上のため、使用する鋼材の高強度化を図り、自動車の重量を減ずる努力が進んでいる。その結果、自動車に広く使用される鋼板においては、鋼板強度の増加に伴って、プレス成形性が低下し、複雑な形状を製造することが困難になってきている。具体的には、鋼板の延性が低下し、加工度が高い部位で破断が生じる、スプリングバックや壁反りが大きくなり寸法精度が劣化する、といった問題が発生する。このような理由から、高強度、特に引張強さが７８０ＭＰａ級以上の鋼板を用いて、プレス成形で部品を製造することは容易ではない。プレス成形ではなくロール成形によれば、高強度の鋼板の加工が可能であるが、ロール成形は長手方向に一様な断面を有する部品にしか適用できない。 In recent years, efforts have been made to reduce the weight of automobiles by increasing the strength of steel materials used to improve automobile fuel efficiency. As a result, in steel plates widely used in automobiles, as the strength of the steel plates increases, press formability decreases, making it difficult to manufacture complex shapes. Specifically, the ductility of the steel sheet is reduced, and a problem occurs in that fracture occurs at a site with a high degree of processing, springback and wall warpage increase, and dimensional accuracy deteriorates. For these reasons, it is not easy to produce a part by press molding using a steel plate having a high strength, particularly a tensile strength of 780 MPa or more. According to roll forming instead of press forming, it is possible to process a high-strength steel sheet, but roll forming is applicable only to parts having a uniform cross section in the longitudinal direction.
そのため、英国特許第１４９０５３５号公報に開示されているように、加熱した鋼板をプレスし成形する熱間プレス法とよばれる成形方法が開発された。熱間プレス法では、加熱された高温の鋼板が軟質、高延性になっているため、複雑な形状を寸法精度よく成形することが可能である。さらに、鋼板をオーステナイト域に加熱してからプレス成形し、金型内で急冷（焼入れ）することにより、マルテンサイト変態による鋼板の高強度化を成形と同時に達成できるという利点がある。 Therefore, as disclosed in British Patent No. 1490535, a forming method called a hot press method in which a heated steel sheet is pressed and formed has been developed. In the hot pressing method, since a heated high-temperature steel sheet is soft and highly ductile, it is possible to form a complicated shape with high dimensional accuracy. Further, there is an advantage that high strength of the steel sheet by martensitic transformation can be achieved at the same time as forming by press forming after heating the steel sheet to the austenite region and quenching (quenching) in the mold.
特開平１０−９６０３１号公報には、室温で予め所定の形状に成形してから、金型に入れたままオーステナイト域に加熱し、金型内で急冷することで鋼板の高強度化を成形と同時に達成する予成形プレスクエンチ法が開示されている。 In JP-A-10-96031, after forming into a predetermined shape at room temperature in advance, heating to the austenite region while being put in the mold, and quenching in the mold, the steel sheet is made to have high strength. A preformed press quench method is disclosed which is accomplished simultaneously.
このような熱間プレス法や予成形プレスクエンチ法は、プレス成形部材の高強度化を成形と同時に確保できる優れた成形方法であり、現在、バンパーの補強材やセンターピラーの補強材等に代表される自動車用補強部材等の製造に多用されるようになっている。
ところが、熱間プレス法で製造される鋼板部材の高強度化が進み、引張強さ（以下、ＴＳとも表記する）が１０８０ＭＰａ以上になってきたことから、靱性の問題が顕在化するようになってきた。周知のように、強度と靱性は両立が難しい。しかし、熱間プレス法により製造される引張強さが１０８０ＭＰａ以上の鋼板部材について、従来技術では靭性改善に関して詳細な検討が行われていないのが実状である。 However, since the strength of steel plate members manufactured by the hot press method has been increased, and the tensile strength (hereinafter also referred to as TS) has become 1080 MPa or more, the problem of toughness becomes apparent. I came. As is well known, it is difficult to achieve both strength and toughness. However, as for the steel sheet member with a tensile strength of 1080 MPa or more manufactured by the hot pressing method, the actual state is that detailed examination is not made on the toughness improvement in the prior art.
さらに、熱間プレス法により製造された鋼板部材は局所的な強度（硬度）のばらつきを生じる場合がある。そのような場合には、部材の靭性も悪化し、十分な性能が発揮できなくなる。そのため、熱間プレス後の局所的な強度（硬度）ばらつきが少ない熱間プレス用の鋼板が求められる。 Furthermore, the steel plate member manufactured by the hot press method may cause local variations in strength (hardness). In such a case, the toughness of the member also deteriorates and sufficient performance cannot be exhibited. Therefore, a steel sheet for hot pressing with little local strength (hardness) variation after hot pressing is required.
熱間プレス用鋼板は、熱間プレス後の強度（焼入れ強度）を確保するためＣを多量に含有するという特徴があり、鋼板内の局所的なＣ含有量のばらつきが局所的な製品強度のばらつきを生じさせる。また、熱間プレス用鋼板の平坦度が劣る場合には、金型との接触が不均一となり、局所的な冷却速度のばらつきを生じる場合があり、この局所的な冷却速度のばらつきも、局所的な製品強度のばらつきを生じさせる。 The steel sheet for hot pressing has a feature that it contains a large amount of C in order to ensure the strength (quenching strength) after hot pressing, and the local variation in the C content in the steel sheet has a local product strength. Causes variation. In addition, when the flatness of the steel sheet for hot pressing is inferior, the contact with the mold becomes non-uniform, and there may be a variation in local cooling rate. Cause variations in product strength.
本発明の目的は、上記現状に鑑み、自動車や各種の産業機械に用いられる、引張強さが１０８０ＭＰａ以上となる高強度の熱間プレス鋼板部材であって、上述した靱性の低下や硬度のばらつきが抑制された熱間プレス鋼板部材、およびそのような熱間プレス鋼板部材を製造するための熱間プレス用鋼板、ならびにそれらの製造方法を提供することである。 An object of the present invention is a high-strength hot-pressed steel sheet member having a tensile strength of 1080 MPa or more, which is used for automobiles and various industrial machines in view of the above-described situation, and has the above-described decrease in toughness and variation in hardness. It is providing the hot-pressed steel plate member by which hot-rolled steel plate was suppressed, the hot-pressed steel plate for manufacturing such a hot-pressed steel plate member, and the manufacturing method thereof.
本発明者らは、上記課題を解決すべく詳細な検討を行った結果、熱間プレス用鋼板中の介在物を低減し、さらに熱間プレス用鋼板内の中心偏析を抑制することにより、熱間プレス鋼板部材の靭性を大幅に改善できること、さらには、熱間プレス用鋼板における鋼組織を規定し、鋼板内に存在するＣの分布、いわゆる鋼組織内のセメンタイトの分布を均一化させることにより、一層靭性改善を図ることができるとの新知見を得た。 As a result of detailed investigations to solve the above problems, the present inventors have reduced the inclusions in the hot-press steel sheet, and further suppressed the center segregation in the hot-press steel sheet. By greatly improving the toughness of hot pressed steel sheet members, and further by defining the steel structure in the steel sheet for hot pressing, and by making the distribution of C existing in the steel sheet, the distribution of cementite in the so-called steel structure uniform New knowledge that toughness can be further improved.
上記新知見に基づく本発明は次の通りである。
(１) 質量％で、Ｃ：０.０９〜０.６０％、Ｓｉ：２.０％以下、Ｍｎ：０.５〜３.５％、Ｐ：０.１０％以下、Ｓ：０.０５％以下、Ａｌ：０.００５〜２.０％、およびＮ：０.０１％以下を含有し、残部がＦｅおよび不純物からなる化学組成を有し、清浄度が０.０８％以下、下記式(１)で規定されるＰの偏析度αが１.６以下、下記式(２)で規定されるＳの偏析度βが１.６以下であり、かつ熱間プレス後の引張強さが１０８０ＭＰａ以上であることを特徴とする熱間プレス用鋼板：
α＝［板厚中心部での最大Ｐ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｐ濃度（質量％）］ ・・・ (１)
β＝［板厚中心部での最大Ｓ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｓ濃度（質量％）］ ・・・ (２)。
The present invention based on the above new findings is as follows.
(1) By mass%, C: 0.09 to 0.60%, Si: 2.0% or less, Mn: 0.5 to 3.5%, P: 0.10% or less, S: 0.05 %, Al: 0.005 to 2.0%, and N: 0.01% or less, with the balance being a chemical composition of Fe and impurities, cleanliness of 0.08% or less, The segregation degree α of P defined by (1) is 1.6 or less, the segregation degree β of S defined by the following formula (2) is 1.6 or less, and the tensile strength after hot pressing is Steel sheet for hot pressing characterized by being 1080 MPa or more:
α = [maximum P concentration (mass%) at the thickness center portion] / [average P concentration (mass%) at the 1/4 depth position of the thickness from the surface] (1)
β = [maximum S concentration (mass%) at the thickness center portion] / [average S concentration (mass%) at the 1/4 depth position of the thickness from the surface] (2).
(２) 上記熱間プレス用鋼板において、前記化学組成が、Ｆｅの一部に代えて、質量％で、Ｔｉ：０.２％以下、Ｎｂ：０.２％以下、およびＶ：１.０％以下からなる群から選ばれた１種または２種以上を含有する。 (2) In the steel sheet for hot pressing, the chemical composition is in mass%, Ti: 0.2% or less, Nb: 0.2% or less, and V: 1.0 instead of a part of Fe. 1 type or 2 types or more selected from the group which consists of% or less.
(３) 上記熱間プレス用鋼板において、前記化学組成が、Ｆｅの一部に代えて、質量％で、Ｃｒ：１.０％以下、Ｍｏ：１.０％以下、Ｃｕ：１.０％以下、Ｎｉ：１.０％以下、およびＢ：０.０１％以下からなる群から選ばれた１種または２種以上を含有する。 (3) In the above steel sheet for hot pressing, the chemical composition is in mass% instead of part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% Hereinafter, it contains one or more selected from the group consisting of Ni: 1.0% or less and B: 0.01% or less.
(４) 上記熱間プレス用鋼板において、前記化学組成が、Ｆｅの一部に代えて、質量％で、Ｃａ：０.０１％以下、Ｍｇ：０.０１％以下、およびＲＥＭ：０.１％以下からなる群から選ばれた１種または２種以上を含有する。 (4) In the steel sheet for hot pressing, the chemical composition is in mass%, Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.1 instead of a part of Fe. 1 type or 2 types or more selected from the group which consists of% or less.
(５) 上記熱間プレス用鋼板において、面積％で、ポリゴナルフェライトが３０％以上、マルテンサイトが３０％以下である鋼組織を有し、前記ポリゴナルフェライトの平均結晶粒径が２５μｍ以下、前記ポリゴナルフェライトに占める、粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合が０.６以上である。 (5) The steel sheet for hot pressing has a steel structure in which the area% is polygonal ferrite is 30% or more and martensite is 30% or less, and the average crystal grain size of the polygonal ferrite is 25 μm or less, The number ratio of polygonal ferrite containing 5 or more cementite particles having a particle size of 0.2 μm or more in the polygonal ferrite is 0.6 or more.
(６) 表面にめっき層を備える上記熱間プレス用鋼板。
(７) 上記化学組成を有する溶鋼を、溶鋼の加熱温度を液相線温度から５℃以上高い温度かつ単位時間当たりの溶鋼鋳込み量を６ｔｏｎ／分以下とし、さらに鋳片が完全凝固する前に中心偏析低減処理を施す連続鋳造法によりスラブとなし、前記スラブを圧延することを特徴とする熱間プレス用鋼板の製造方法。
(6) The steel sheet for hot pressing provided with a plating layer on the surface.
(7) The molten steel having the above chemical composition is heated at a temperature higher than the liquidus temperature by 5 ° C or more and the casting amount of molten steel per unit time is 6 ton / min or less, and before the slab is completely solidified. A method for producing a steel sheet for hot pressing, characterized in that a slab is formed by a continuous casting method for performing a center segregation reduction treatment, and the slab is rolled.
(８) 上記スラブに熱間圧延を施し、熱間圧延完了後に１００℃／秒以下の平均冷却速度で冷却し、５００℃以上の温度で巻き取って熱間圧延鋼板となし、前記熱間圧延鋼板をスケール除去処理後、２０％以上の圧下率で冷間圧延を施して冷間圧延鋼板となし、前記冷間圧延鋼板に（Ａｃ１点−１００℃）以上、Ａｃ３点未満の温度範囲で５秒間以上、５００秒間以下保持する焼鈍を施し、次いで１℃／秒以上の平均冷却速度で３５０〜７００℃の温度域まで冷却することを特徴とする、熱間プレス用鋼板の製造方法。 (8) The slab is hot-rolled, cooled at an average cooling rate of 100 ° C./second or less after completion of the hot rolling, and taken up at a temperature of 500 ° C. or more to form a hot-rolled steel sheet. After removing the scale from the steel sheet, the steel sheet is cold-rolled at a rolling reduction of 20% or more to form a cold-rolled steel sheet, and the cold-rolled steel sheet has a temperature range of (Ac 1 point-100 ° C.) or more and less than Ac 3 points. A method for producing a steel sheet for hot pressing, comprising: annealing for 5 seconds to 500 seconds, and then cooling to a temperature range of 350 to 700 ° C. at an average cooling rate of 1 ° C./second or more.
(９) 上記製造方法にて得られた熱間プレス用鋼板の表面にめっき処理を施すことを特徴とする熱間プレス用鋼板の製造方法。
(１０) 上記化学組成を有し、清浄度が０.０８％以下、上記式(１)で規定されるＰの偏析度αが１.６以下、上記式(２)で規定されるＳの偏析度βが１.６以下であり、かつ引張強さが１０８０ＭＰａ以上であることを特徴とする熱間プレス鋼板部材。
(9) A method for producing a steel sheet for hot pressing, wherein the surface of the steel sheet for hot pressing obtained by the above production method is plated.
(10) having the above chemical composition, a cleanliness of 0.08% or less, a segregation degree α of P defined by the above formula (1) of 1.6 or less, and an S of S defined by the above formula (2) A hot-pressed steel sheet member having a segregation degree β of 1.6 or less and a tensile strength of 1080 MPa or more.
(１１) 表面にめっき層を備える、上記熱間プレス鋼板部材。
(１２) 上記熱間プレス用鋼板を、Ａｃ３点超、１１００℃以下に加熱したのちに、Ａｒ３点以上の温度でプレスを開始し、１０℃／秒以上の冷却速度で３５０℃以下の温度域まで冷却する熱間プレスを施すことを特徴とする、熱間プレス鋼板部材の製造方法。
(11) The hot-pressed steel sheet member having a plating layer on the surface.
(12) After heating the steel sheet for hot pressing above Ac 3 points to 1100 ° C. or lower, pressing starts at a temperature of Ar 3 points or higher, and 350 ° C. or lower at a cooling rate of 10 ° C./second or higher. A method for producing a hot-pressed steel sheet member, characterized by performing a hot press for cooling to a temperature range.
本発明の熱間プレス用鋼板は、熱間プレスを施すことにより、引張強さ１０８０ＭＰａ以上の高強度で靱性にも優れた熱間プレス鋼板部材とすることができるので、熱間プレス鋼板部材の素材として最適である。また、本発明の熱間プレス鋼板部材は自動車や各種の産業機械に用いられる構造部材の素材、特に自動車のメンバーや足廻り部品に代表される構造部材の素材として特に最適である。また安価に製造できるので、産業上格段の効果を奏する。 The hot-press steel sheet of the present invention can be made into a hot-press steel sheet member having high tensile strength of 1080 MPa or more and excellent toughness by performing hot press. Ideal as a material. The hot-pressed steel sheet member of the present invention is particularly suitable as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts. Moreover, since it can be manufactured at a low cost, it has a remarkable industrial effect.
＜鋼組成＞
本発明の熱間プレス用鋼板の鋼組成を上記のように特定した理由は次の通りである。なお、以下の説明において、鋼の化学組成（各元素の含有量）に関する％はすべて質量％である。
<Steel composition>
The reason why the steel composition of the steel sheet for hot pressing according to the present invention is specified as described above is as follows. In the following description, all percentages relating to the chemical composition of steel (content of each element) are mass%.
Ｃ：０.０９〜０.６０％
熱間プレスは、材料を加熱することで軟質化させ、成形を容易にすることが一つの特色であるが、あわせて、プレス金型等で急冷することで鋼を焼入れし、成形と同時に成形品を高強度化できるという別の特色があり、成形時に焼入れを行うのが普通である。
C: 0.09 to 0.60%
Hot press is one of the features of softening the material by heating and facilitating molding. In addition, steel is quenched by quenching with a press die, etc., and molded simultaneously with molding. Another feature is that the strength of the product can be increased, and quenching is usually performed during molding.
鋼の焼入れ後の強度は主に炭素（Ｃ）含有量によって決まるため、求める強度に応じてＣ含有量を設定する。本発明においては、熱間プレス後に１０８０ＭＰａ以上の引張強さを確保するために、Ｃ含有量を０.０９％以上とする。より高強度の成形品が必要な場合にはＣ含有量を０.２０％超にすることが望ましい。Ｃを過剰に含有する場合には、溶接性が低下する恐れがあるため、Ｃ含有量は０.６０％以下とし、更に好ましくは０.４０％以下である。 Since the strength after quenching of steel is mainly determined by the carbon (C) content, the C content is set according to the required strength. In the present invention, in order to secure a tensile strength of 1080 MPa or more after hot pressing, the C content is set to 0.09% or more. When a higher strength molded product is required, the C content is desirably over 0.20%. When C is contained excessively, the weldability may be lowered, so the C content is 0.60% or less, and more preferably 0.40% or less.
Ｓｉ：２.０％以下
Ｓｉは、鋼板の焼入れ性を高め、かつ焼入れ後の強度を安定して確保するのに有効な元素である。しかし、Ｓｉ含有量が２.０％超になると、熱間圧延時ならびに熱間プレス時の加熱中にＳｉスケールが多く発生し、スケール疵を生じ易くなる。そのため、Ｓｉ含有量の上限を２.０％とする。Ｓｉ含有量の下限は特に規定しないが、焼入れ性向上効果を確実に発揮させるには０.０２％以上含有させることが望ましい。コストと焼入れ性向上のバランスの観点から、Ｓｉ含有量を０.１〜１.０％とすることが更に望ましい。
Si: 2.0% or less Si is an element effective in enhancing the hardenability of the steel sheet and stably securing the strength after quenching. However, when the Si content exceeds 2.0%, a large amount of Si scale is generated during heating during hot rolling and during hot pressing, and scale flaws are likely to occur. Therefore, the upper limit of Si content is set to 2.0%. Although the lower limit of the Si content is not particularly defined, it is desirable to contain 0.02% or more in order to reliably exhibit the effect of improving hardenability. From the viewpoint of balance between cost and hardenability improvement, the Si content is more preferably 0.1 to 1.0%.
Ｍｎ：０.５〜３.５％
Ｍｎは、鋼板の焼入れ性を高め、かつ焼入れ後の強度を安定して確保するのに有効な元素である。Ｍｎ含有量が０.５％未満では、その効果は十分ではない。一方、Ｍｎ含有量が３.５％を超えると、その効果は飽和し、無駄なコストが嵩む上、焼入れ部の靭性劣化を招く。したがって、Ｍｎ含有量を０.５〜３.５％とする。コストと焼入れ性向上のバランスの観点から、Ｍｎ含有量を０.８〜２.５％とすることが望ましい。
Mn: 0.5 to 3.5%
Mn is an element that is effective for enhancing the hardenability of the steel sheet and stably securing the strength after quenching. If the Mn content is less than 0.5%, the effect is not sufficient. On the other hand, if the Mn content exceeds 3.5%, the effect is saturated, the useless cost increases, and the toughness of the quenched portion is deteriorated. Therefore, the Mn content is set to 0.5 to 3.5%. From the viewpoint of a balance between cost and hardenability improvement, the Mn content is desirably 0.8 to 2.5%.
Ｐ：０.１０％以下
Ｐは、不純物の１種として含有されるが、Ｐの含有量が０.１０％超では、鋼板板厚中心部以外にも過度のＰの偏析が発生するため、靭性が著しく劣化する。したがって、Ｐ含有量を０.１０％以下とする。
P: 0.10% or less P is contained as one type of impurity, but if the P content exceeds 0.10%, excessive segregation of P occurs in addition to the steel plate thickness center portion. Toughness deteriorates significantly. Therefore, the P content is 0.10% or less.
Ｐ含有量は少なければ少ないほど好ましいので、その下限は特に規定しない。しかし、Ｐ含有量を０.００３％より少なくするのは製造コストの著しい上昇を招くので、この観点からは、Ｐ含有量の下限を０.００３％とすることが望ましい。コストと靭性改善のバランスの観点からは、Ｐ含有量を０.００５〜０.０５％とすることが更に望ましい。 The lower the P content, the better. Therefore, the lower limit is not specified. However, if the P content is less than 0.003%, the production cost is significantly increased. From this viewpoint, the lower limit of the P content is preferably 0.003%. From the viewpoint of balance between cost and toughness improvement, the P content is more preferably 0.005 to 0.05%.
Ｓ：０.０５％以下
Ｓは、鋼板の板厚中心部に偏析し、靭性を低下させる硫化物を生成させるため、可能な限り低減する必要のある不純物である。Ｓの含有量が０.０５％超では、鋼板板厚中心部以外にも過度のＳの偏析が発生するため、靭性が著しく劣化する。したがって、Ｓ含有量を０.０５％以下とする。
S: 0.05% or less S is an impurity that needs to be reduced as much as possible in order to generate sulfide that segregates in the center of the plate thickness of the steel sheet and lowers toughness. If the S content exceeds 0.05%, excessive segregation of S occurs in addition to the central portion of the steel plate thickness, so that the toughness is remarkably deteriorated. Therefore, the S content is set to 0.05% or less.
Ｓ含有量は少なければ少ないほど好ましいので、その下限は特に規定しない。しかし、Ｓ含有量を０.０００３％より少なくするのは製造コストの著しい上昇を招くので、この観点から、Ｓ含有量の下限を０.０００３％とすることが望ましい。コストと靭性改善のバランスの観点からＳ含有量を０.０００５〜０.０１％とすることが更に望ましい。 The lower the S content, the better. The lower limit is not particularly specified. However, if the S content is less than 0.0003%, the production cost is significantly increased. From this viewpoint, the lower limit of the S content is preferably 0.0003%. From the viewpoint of a balance between cost and toughness improvement, the S content is more preferably 0.0005 to 0.01%.
Ａｌ：０.００５〜２.０％
Ａｌは、鋼の脱酸剤として用いられる。その効果を得るには０.００５％以上のＡｌの含有が必要である。一方、Ａｌを２.０％を超えて含有させても、脱酸性が飽和し、製造コストが上昇する。したがって、Ａｌ含有量を０.００５〜２.０％とする。脱酸性とコストのバランスの観点よりＡｌ含有量を０.０１〜０.０８％とすることが好ましい。
Al: 0.005 to 2.0%
Al is used as a deoxidizer for steel. In order to obtain the effect, it is necessary to contain 0.005% or more of Al. On the other hand, even if Al is contained in excess of 2.0%, deacidification is saturated and the production cost increases. Therefore, the Al content is set to 0.005 to 2.0%. From the viewpoint of balance between deacidification and cost, the Al content is preferably set to 0.01 to 0.08%.
Ｎ：０.０１％以下
Ｎは、不純物として含有され、ＡｌやＴｉやＮｂ等と結合して窒化物を形成する。窒化物は靭性を劣化させる傾向を有するので、Ｎ含有量はできるだけ低減するのが望ましい。本発明においては、Ｎ含有量が０.０１％以下の含有量であれば実害はない。したがって、Ｎ含有量を０.０１％以下とする。Ｎ含有量は少なければ少ないほど好ましいので下限は特に規定しない。しかし、Ｎ含有量を０.０００５％より少なくするのは製造コストの著しい上昇を招くので、この観点からＮ含有量の下限を０.０００５％とすることが望ましい。コストと靭性改善のバランスの観点より、Ｎ含有量を０.０００５〜０.００５０％とすることが更に望ましい。
N: 0.01% or less N is contained as an impurity and combines with Al, Ti, Nb or the like to form a nitride. Since nitrides tend to degrade toughness, it is desirable to reduce the N content as much as possible. In the present invention, there is no actual harm as long as the N content is 0.01% or less. Therefore, the N content is set to 0.01% or less. Since the N content is preferably as small as possible, the lower limit is not particularly defined. However, if the N content is less than 0.0005%, the manufacturing cost is significantly increased. From this viewpoint, the lower limit of the N content is preferably set to 0.0005%. From the viewpoint of balance between cost and toughness improvement, the N content is more preferably 0.0005 to 0.0050%.
Ｔｉ：０.２％以下、Ｎｂ：０.２％以下、Ｖ：１.０％以下からなる群から選ばれた１種または２種以上
これらの元素は、鋼板の焼入れ性を高めかつ、焼入れ後の強度を安定して確保するのに有効な元素であるので、必要に応じて１種または２種以上を含有させることができる。しかし、過剰に含有させても上記効果は飽和して、無駄なコストが嵩むので、Ｔｉ、Ｎｂ、Ｖの含有量は、それぞれ０.２％以下、０.２％以下、１.０％以下とする。上記効果を確実に得るための望ましいＴｉ、Ｎｂ、Ｖの含有量は、それぞれ０.０１％以上、０.００５％以上、０.００５％以上である。
One or more selected from the group consisting of Ti: 0.2% or less, Nb: 0.2% or less, and V: 1.0% or less. These elements enhance the hardenability of the steel sheet and harden it. Since it is an element effective for ensuring the subsequent strength in a stable manner, one or more kinds can be contained as required. However, the above effect is saturated even if it is contained excessively, and wasteful costs increase, so the contents of Ti, Nb, and V are 0.2% or less, 0.2% or less, and 1.0% or less, respectively. And Desirable Ti, Nb, and V contents for reliably obtaining the above effects are 0.01% or more, 0.005% or more, and 0.005% or more, respectively.
Ｃｒ：１.０％以下、Ｍｏ：１.０％以下、Ｃｕ：１.０％以下、Ｎｉ：１.０％以下およびＢ：０.０１％以下からなる群から選ばれた１種または２種以上
これらの元素も、鋼板の焼入れ性を高め、かつ焼入れ後の強度を安定して確保するのに有効な元素であるので、必要に応じて１種または２種以上を含有させることができる。しかし、過剰に含有させても上記効果は飽和し、無駄コストが嵩むので、Ｃｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｂの含有量は、それぞれ１.０％以下、１.０％以下、１.０％以下、１.０％以下、０.０１％以下とする。上記効果を確実に得るための望ましいＣｒ、Ｍｏ、Ｃｕ、Ｎｉ、Ｂの含有量は、それぞれ０.０２％以上、０.０２％以上、０.０２％以上、０.０２％以上、０.０００２％以上である。
One or two selected from the group consisting of Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B: 0.01% or less Species or more These elements are also effective elements for enhancing the hardenability of the steel sheet and stably securing the strength after quenching, so that one or more kinds can be contained as required. . However, the above effect is saturated even if it is contained excessively, and wasteful costs increase, so the contents of Cr, Mo, Cu, Ni, and B are 1.0% or less, 1.0% or less, 1.0, respectively. % Or less, 1.0% or less, or 0.01% or less. Desirable Cr, Mo, Cu, Ni, and B contents for reliably obtaining the above effects are 0.02% or more, 0.02% or more, 0.02% or more, 0.02% or more, and 0.0%, respectively. 0002% or more.
Ｃａ：０.０１％以下、Ｍｇ：０.０１％以下およびＲＥＭ：０.１％以下からなる群から選ばれた１種または２種以上
これらの元素は、鋼中の介在物の形態を微細化し、介在物による熱間プレス時の割れを防止する効果を有するので、必要に応じて１種または２種以上を含有させることができる。しかし、過剰に含有させても上記効果は飽和し、無駄なコストが嵩むので、Ｃａ、Ｍｇ、ＲＥＭの含有量は、それぞれ０.０１％以下、０.０１％以下、０.１％以下とする。上記効果を確実に得るための望ましいＣａ、Ｍｇ、ＲＥＭの含有量は、それぞれ０.０００５％以上、０.０００５％以上、０.０００５％以上である。
One or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.1% or less. These elements finely form the inclusions in the steel. Since it has the effect of preventing cracking during hot pressing due to inclusions, it can contain one or more as required. However, the above effect is saturated even if it is contained excessively, and wasteful costs increase, so the contents of Ca, Mg, and REM are 0.01% or less, 0.01% or less, and 0.1% or less, respectively. To do. Desirable Ca, Mg, and REM contents for reliably obtaining the above effects are 0.0005% or more, 0.0005% or more, and 0.0005% or more, respectively.
＜鋼板中の介在物と偏析＞
熱間プレス後の強度（ＴＳ）が１０８０ＭＰａ以上となる高強度の熱間プレス鋼板部材の靭性を改善するには、熱間プレスに供する熱間プレス用鋼板の清浄度を０.０８％以下、下記式(１)で規定されるＰの偏析度αを１.６以下、下記式(２)で規定されるＳの偏析度βを１.６以下とすることが必要である。
<Inclusions and segregation in steel sheet>
In order to improve the toughness of a high strength hot-pressed steel sheet member having a strength (TS) after hot pressing of 1080 MPa or more, the cleanliness of the hot-press steel sheet to be subjected to hot pressing is 0.08% or less, It is necessary that the segregation degree α of P defined by the following formula (1) is 1.6 or less and the segregation degree β of S defined by the following formula (2) is 1.6 or less.
α＝［板厚中心部での最大Ｐ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｐ濃度（質量％）］ ・・・ (１)
β＝［板厚中心部での最大Ｓ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｓ濃度（質量％）］ ・・・ (２)
清浄度は、鋼板中に含まれるＡ系、Ｂ系、Ｃ系介在物量（ＪＩＳ Ｇ０５５５）の算術計算での総和で定義される。Ａ系、Ｂ系、Ｃ系の介在物は破壊の起点となり、介在物が増加すると亀裂伝播が容易に起こるため靭性が劣化する。ＴＳが１０８０ＭＰａ以上となる熱間プレス鋼板部材の靭性を改善するには、清浄度を０.０８％以下とする必要がある。清浄度が０.０８％超の場合、介在物の量が多いため、実用上十分な靭性を確保することが困難となる。靱性をより一層改善するには清浄度を０.０４％以下とすることが好ましい。
α = [maximum P concentration (mass%) at the thickness center portion] / [average P concentration (mass%) at the 1/4 depth position of the thickness from the surface] (1)
β = [maximum S concentration (mass%) at the thickness center portion] / [average S concentration (mass%) at the 1/4 depth position of the thickness from the surface] (2)
The degree of cleanliness is defined as the sum of arithmetic amounts of A, B and C inclusions (JIS G0555) contained in the steel sheet. Inclusions of A-type, B-type, and C-type serve as starting points of fracture, and when the inclusions increase, crack propagation easily occurs and the toughness deteriorates. In order to improve the toughness of a hot-pressed steel sheet member having a TS of 1080 MPa or more, the cleanliness needs to be 0.08% or less. When the degree of cleanliness exceeds 0.08%, the amount of inclusions is large, and it becomes difficult to ensure practically sufficient toughness. In order to further improve toughness, the cleanliness is preferably 0.04% or less.
また、鋼板の板厚断面中心部には、ＰおよびＳが中心偏析して濃化し、ＰおよびＳの粒界偏析または燐化物や硫化物の生成により靭性が悪化する。そのため、靭性を改善するためには、Ｐの偏析度αとＳの偏析度βをいずれも１.６以下とすることが必要である。Ｐの偏析度αまたはＳの偏析度βが１.６超となると、板厚中心部にＰまたはＳが多く存在するため、靭性が悪化する。靱性の一層の改善のためには、Ｐの偏析度αまたはＳの偏析度βの一方を１.２以下とすることが好ましく、Ｐの偏析度αとＳの偏析度βをいずれも１.２以下とすることがより好ましい。 Further, P and S are segregated and concentrated at the center of the plate thickness cross section of the steel sheet, and the toughness deteriorates due to the grain boundary segregation of P and S or the formation of phosphides and sulfides. Therefore, in order to improve toughness, it is necessary that both the segregation degree α of P and the segregation degree β of S be 1.6 or less. When the degree of segregation α of P or the degree of segregation β of S exceeds 1.6, toughness deteriorates because a large amount of P or S is present in the central portion of the plate thickness. In order to further improve toughness, it is preferable to set one of P segregation degree α and S segregation degree β to 1.2 or less, and P segregation degree α and S segregation degree β are both 1. More preferably, it is 2 or less.
この鋼板中の介在物と偏析は、主に鋼板組成、特に不純物含有量と、連続鋳造の条件により制御され、熱間圧延および熱間プレスの前後では実質的に変化しない。従って、熱間プレス用鋼板の介在物および偏析状況が本発明を満たしていれば、それから熱間プレスにより製造された熱間プレス鋼板部材の介在物および偏析状況も同様に本発明を満たす。 Inclusions and segregation in the steel sheet are mainly controlled by the steel sheet composition, particularly the impurity content, and the conditions of continuous casting, and do not substantially change before and after hot rolling and hot pressing. Therefore, if the inclusions and segregation status of the hot-press steel sheet satisfy the present invention, the inclusions and segregation status of the hot-pressed steel sheet member manufactured by hot pressing from the steel sheet also satisfy the present invention.
＜熱間プレス用鋼板の金属組織＞
靭性を更に向上させるには、熱間プレス用鋼板の金属組織を、面積％で、ポリゴナルフェライトが３０％以上、マルテンサイトが３０％以下である組織とし、かつ前記ポリゴナルフェライトの平均結晶粒径を２５μｍ以下とし、前記ポリゴナルフェライトに占める粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合を０.６以上とすることが好ましい。
<Metal structure of hot-press steel sheet>
In order to further improve the toughness, the metal structure of the steel sheet for hot pressing is a structure in which area%, polygonal ferrite is 30% or more, martensite is 30% or less, and the average crystal grain of the polygonal ferrite The diameter is preferably 25 μm or less, and the number ratio of polygonal ferrite containing 5 or more cementite particles having a particle size of 0.2 μm or more in the polygonal ferrite is preferably 0.6 or more.
金属組織に占めるポリゴナルフェライトの割合が３０面積％未満の場合には、鋼板が硬質となり、鋼板の平坦矯正が困難となる場合がある。鋼板の平坦が悪いと、熱間プレスに時にＡｃ３点以上に加熱しても、鋼板の平坦は修正されない。鋼板の平坦が悪い状態で熱間プレスされると、金型との接触状態が不均一になり、冷却速度（従って、焼入れ状況）が異なる部位が生じる。その結果、熱間プレス鋼板部材の硬度がばらついて、靭性の劣化を招く。したがって、ポリゴナルフェライトの面積率を３０％以上とすることが好ましい。マルテンサイトが３０面積％超である場合にも同様の問題が生じる場合があるので、マルテンサイトの面積率を３０％以下とすることが好ましい。 When the proportion of polygonal ferrite in the metal structure is less than 30% by area, the steel plate becomes hard and it may be difficult to correct the flatness of the steel plate. If the flatness of the steel plate is poor, the flatness of the steel plate is not corrected even if it is heated to Ac 3 or more points during hot pressing. When hot pressing is performed in a state where the flatness of the steel sheet is poor, the contact state with the mold becomes non-uniform, and parts with different cooling rates (and hence quenching conditions) are generated. As a result, the hardness of the hot-pressed steel sheet member varies, leading to deterioration of toughness. Therefore, the area ratio of polygonal ferrite is preferably 30% or more. Since the same problem may arise also when a martensite exceeds 30 area%, it is preferable to make the area ratio of a martensite 30% or less.
また、Ｃを多く含有するパーライト組織、ベイナイト組織、マルテンサイト組織、オーステナイト組織などは、ポリゴナルフェライトの粒界に生成するため、ポリゴナルフェライトが粗大になると鋼組織中のＣの分散が粗くなる。熱間プレスに際してはＡｃ３点以上に加熱するが、その時間は通常短時間であるため、鋼組織中のＣの分散が粗いとオーステナイト中のＣ濃度が不均一となり、熱間プレス鋼板部材の硬度ばらつきが生じ易くなる。したがって、ポリゴナルフェライトの平均結晶粒径を２５μｍ以下とすることが好ましい。この観点からのポリゴナルフェライトの平均結晶粒径の下限は特に規定する必要はないが、ポリゴナルフェライトの粒径が小さいと、鋼板が硬質となり、上記ポリゴナルフェライト面積率の下限限定理由やマルテンサイト面積率の上限限定理由と同様の問題が生じる場合があるので、ポリゴナルフェライトの平均結晶粒径は２μｍ以上とすることが好ましい。 Moreover, since the pearlite structure, bainite structure, martensite structure, austenite structure, etc. containing a large amount of C are formed at the grain boundaries of polygonal ferrite, the dispersion of C in the steel structure becomes coarse when the polygonal ferrite becomes coarse. . During hot pressing, Ac is heated to 3 or more points, but since the time is usually short, if the C dispersion in the steel structure is coarse, the C concentration in the austenite becomes non-uniform. Hardness variation tends to occur. Therefore, the average crystal grain size of polygonal ferrite is preferably 25 μm or less. From this viewpoint, the lower limit of the average crystal grain size of polygonal ferrite need not be specified. However, if the grain size of polygonal ferrite is small, the steel sheet becomes hard, and the reason for limiting the lower limit of the polygonal ferrite area ratio is not limited. Since the same problem as the reason for limiting the upper limit of the site area ratio may occur, the average crystal grain size of polygonal ferrite is preferably 2 μm or more.
さらに、前記ポリゴナルフェライトに占める粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合を０.６以上とすることにより、熱間プレスに際してＡｃ３点以上に加熱した時に、より短時間でオーステナイト中のＣ濃度を均一化させることができる。その結果、熱間プレス鋼板部材における硬度のばらつきをさらに抑制することが可能となる。 Furthermore, the number ratio of polygonal ferrite containing 5 or more cementite particles having a particle size of 0.2 μm or more in the polygonal ferrite is set to 0.6 or more, thereby increasing the Ac to 3 or more points during hot pressing. When heated, the C concentration in the austenite can be made uniform in a shorter time. As a result, it is possible to further suppress the hardness variation in the hot pressed steel sheet member.
前記個数割合が０.６未満の場合、セメンタイトを粒内にもつポリゴナルフェライトの割合が少ないため、熱間プレスに際しての加熱時におけるオーステナイト中のＣ濃度を均一化する効果が小さい。したがって、前記個数割合を０.６以上にすることが好ましい。 When the number ratio is less than 0.6, since the ratio of polygonal ferrite having cementite in the grains is small, the effect of uniformizing the C concentration in austenite during heating during hot pressing is small. Therefore, the number ratio is preferably 0.6 or more.
ポリゴナルフェライト粒内のセメンタイトは、ポリゴナルフェライト粒内におけるカーボンの拡散の観点から粒径０.２μｍ以上のものを５個以上含有することが望ましい。このような観点からは、ポリゴナルフェライト粒内のセメンタイトの個数の上限は特に規定する必要はないが、ポリゴナルフェライト粒内に多くセメンタイトが存在すると鋼板が硬質となり、上記ポリゴナルフェライト面積率の下限限定理由やマルテンサイト面積率の上限限定理由と同様の問題が生じる場合があるので、セメンタイト個数の上限は２００個程度が望ましい。特に望ましくは、ポリゴナルフェライト粒内に粒径０.２μｍ以上のセメンタイトが５〜８０個存在する場合である。 The cementite in the polygonal ferrite grains preferably contains five or more particles having a particle diameter of 0.2 μm or more from the viewpoint of carbon diffusion in the polygonal ferrite grains. From this point of view, the upper limit of the number of cementite in the polygonal ferrite grains need not be specified in particular, but if there is a lot of cementite in the polygonal ferrite grains, the steel plate becomes hard, and the above-mentioned polygonal ferrite area ratio Since the same problem as the reason for limiting the lower limit and the reason for limiting the upper limit of the martensite area ratio may occur, the upper limit of the number of cementite is preferably about 200. It is particularly desirable when 5 to 80 cementites having a particle size of 0.2 μm or more are present in the polygonal ferrite grains.
なお、ここでいうセメンタイトは、ＴｉＣ、ＮｂＣ、ＶＣなどの析出強化型微細炭化物を含まない。析出強化型微細炭化物はその大きさが数〜数十ｎｍと非常に小さく、Ａｃ３点以上の加熱によりごく短時間で固溶してしまうので、これらの影響を考慮する必要は実質上ないからである。 In addition, the cementite here does not contain precipitation strengthening type fine carbides such as TiC, NbC, and VC. Precipitation strengthened fine carbides are very small in size of several to several tens of nanometers and are dissolved in a very short time by heating at three or more points of Ac, so there is virtually no need to consider these effects. It is.
ポリゴナルフェライトおよびマルテンサイト以外の残部組織としては、圧延未再結晶フェライト、ベイニティックフェライトやベイナイト、パーライト、残留オーステナイトなどが例示される。前記残部組織は特に規定しないが、パーライトであることが望ましい。残部組織をパーライトもしくはパーライト面積率が５０％以上のパーライト主体の組織とすることにより、熱間プレス用鋼板をより軟質化することが可能となり、鋼板の平坦矯正やブランク加工などを容易にすることができる。 Examples of the remaining structure other than polygonal ferrite and martensite include rolled non-recrystallized ferrite, bainitic ferrite, bainite, pearlite, and retained austenite. The remaining structure is not particularly defined, but is preferably pearlite. By making the remaining structure pearlite or a pearlite-based structure with a pearlite area ratio of 50% or more, it becomes possible to soften the steel sheet for hot pressing, and to facilitate flattening of the steel sheet, blanking, etc. Can do.
＜熱間プレス用鋼板の製造方法＞
(１)連続鋳造
鋼板の清浄度を０.０８％以下にするには、溶鋼を連続鋳造する際に、溶鋼の加熱温度をその鋼の液相線温度から５℃以上高い温度とし、かつ、単位時間当たりの溶鋼鋳込み量を６ｔｏｎ／分以下に抑えることが有効である。このように、比較的高温の溶鋼をある上限以下の鋳込み量で連続鋳造することにより、鋼板の清浄度、言い換えればスラブ段階での介在物を効果的に減少させることができる。
<Method for producing steel sheet for hot pressing>
(1) Continuous casting To reduce the cleanliness of the steel sheet to 0.08% or less, when continuously casting the molten steel, the heating temperature of the molten steel is set to a temperature higher than the liquidus temperature of the steel by 5 ° C or more, and It is effective to suppress the molten steel casting amount per unit time to 6 ton / min or less. Thus, by continuously casting a relatively high temperature molten steel at a casting amount below a certain upper limit, the cleanliness of the steel sheet, in other words, inclusions at the slab stage can be effectively reduced.
連続鋳造時に溶鋼の単位時間当たりの鋳込み量が６ｔｏｎ／分を超えると、鋳型内での溶鋼流動が速いために、凝固シェルに介在物が捕捉されやすくなり、スラブ中の介在物が増加する。また、溶鋼加熱温度が液相線温度から５℃未満であると、溶鋼の粘度が高くなり、連続鋳造機内にて介在物が浮上しにくく、介在物が増加し、清浄度が悪化し易い。 When the casting amount per unit time of the molten steel exceeds 6 ton / min during continuous casting, the molten steel flow in the mold is fast, so that inclusions are easily trapped in the solidified shell and inclusions in the slab increase. Moreover, when the molten steel heating temperature is less than 5 ° C. from the liquidus temperature, the viscosity of the molten steel becomes high, and inclusions hardly float in the continuous casting machine, inclusions increase, and cleanliness tends to deteriorate.
溶鋼の液相線温度からの溶鋼加熱温度を５℃以上、かつ単位時間当たりの溶鋼鋳込み量を６ｔｏｎ／分以下として鋳造することにより、介在物がスラブ内に持ち込まれにくくなり、０.０８％以下という鋼板清浄度を容易に達成できる。さらに好ましくは、溶鋼を連続鋳造する際、溶鋼の液相線温度からの溶鋼加熱温度を８℃以上、かつ単位時間当たりの溶鋼鋳込み量を５ｔｏｎ／分以下にすることであり、これにより清浄度を０.０４％以下とすることが容易にできる。 By casting the molten steel from the liquidus temperature of the molten steel at 5 ° C or more and the molten steel casting amount per unit time at 6 ton / min or less, inclusions are less likely to be brought into the slab, and 0.08% The following steel plate cleanliness can be easily achieved. More preferably, when continuously casting the molten steel, the molten steel heating temperature from the liquidus temperature of the molten steel is 8 ° C. or more, and the molten steel casting amount per unit time is 5 ton / min or less. Can be easily made 0.04% or less.
また、鋼板板厚中心部、言い換えればスラブ厚中心部では、Ｍｎ、Ｐ、Ｓなどが濃化するが、その中でもＰおよびＳの中心偏析は熱間プレス鋼板部材の靭性を著しく悪化させる。そのため、鋼板板厚断面中心部におけるＰの偏析度αを１.６以下およびＳの偏析度βを１.６以下にする。 Further, Mn, P, S and the like are concentrated in the steel plate thickness center portion, in other words, in the slab thickness center portion, and among these, the center segregation of P and S significantly deteriorates the toughness of the hot pressed steel plate member. Therefore, the segregation degree α of P is 1.6 or less and the segregation degree β of S is 1.6 or less.
これを達成するため、鋳片が完全凝固する前の未凝固層において、電磁攪拌や未凝固層圧下などにより完全凝固前にＰやＳが濃化した溶鋼を排出させる、中心偏析低減処理を実施する。中心偏析低減処理を実施しない場合、スラブ厚中心部にＰおよびＳが濃化し、Ｐの偏析度αが１.６超ならびにＳの偏析度βが１.６超になって熱間プレス後部材の靭性が悪化するようになる。 To achieve this, center segregation reduction processing is performed in the unsolidified layer before the slab is completely solidified, in which molten steel enriched in P and S is discharged before complete solidification by electromagnetic stirring or unsolidified layer pressure. To do. When the center segregation reduction treatment is not performed, P and S are concentrated at the center of the slab thickness, P segregation degree α exceeds 1.6 and S segregation degree β exceeds 1.6, and the member after hot pressing The toughness of the steel becomes worse.
(２)圧延および焼鈍
上記介在物ならびにＰおよびＳの中心偏析を低減したスラブを圧延することで本発明の熱間プレス用鋼板を容易に得ることができる。スラブの圧延は、熱間圧延により行うことが好ましい。熱間圧延の圧延完了温度は７００℃以上とすることが好ましい。圧延完了温度が７００℃未満の場合、温度が低すぎるため、圧延中に鋼板の破断や鋼板の表面割れを起こすことがある。
(2) Rolling and annealing The steel sheet for hot pressing of the present invention can be easily obtained by rolling the slab in which the inclusions and the central segregation of P and S are reduced. The slab is preferably rolled by hot rolling. The rolling completion temperature of hot rolling is preferably 700 ° C. or higher. When the rolling completion temperature is less than 700 ° C., the temperature is too low, and the steel plate may be broken or the steel plate may be cracked during rolling.
熱間圧延を施した鋼板は、必要な強度および靱性を確保するため、さらに次のような工程で冷間圧延を施して、熱間プレス用鋼板を製造することが好ましい。なお、最終的に得られる本発明の熱間プレス用鋼板の板厚は特に制限されないが、通常０.４〜８.０ｍｍの範囲内である。 In order to ensure the required strength and toughness, the hot-rolled steel sheet is preferably subjected to cold rolling in the following steps to produce a hot-press steel sheet. In addition, the plate | board thickness of the steel plate for hot presses of this invention finally obtained is although it does not restrict | limit, Usually, it exists in the range of 0.4-8.0 mm.
すなわち、熱間圧延完了後、１００℃／秒以下の平均冷却速度で冷却して５００℃以上の温度で巻き取って熱間圧延鋼板となし、この熱間圧延鋼板に、酸洗などのスケール除去処理を施した後、２０％以上の圧下率で冷間圧延を施して冷間圧延鋼板となし、得られた冷間圧延鋼板に、（Ａｃ１点−１００℃）以上、Ａｃ３点未満の温度範囲で５秒間以上、５００秒間以下保持する焼鈍を施し、次いで１℃／秒以上の平均冷却速度で３５０〜７００℃の温度域まで冷却して、熱間プレス用鋼板を製造する。 That is, after completion of hot rolling, it is cooled at an average cooling rate of 100 ° C./second or less and wound at a temperature of 500 ° C. or more to form a hot rolled steel plate, and scale removal such as pickling is performed on this hot rolled steel plate. After the treatment, cold rolling is performed at a reduction rate of 20% or more to form a cold rolled steel sheet, and the obtained cold rolled steel sheet is (Ac 1 point-100 ° C.) or more and less than Ac 3 points. A steel sheet for hot pressing is manufactured by annealing at a temperature range of 5 seconds to 500 seconds and then cooling to a temperature range of 350 to 700 ° C. at an average cooling rate of 1 ° C./second or more.
それにより、ポリゴナルフェライトの面積率が３０％以上、そのポリゴナルフェライトの平均結晶粒径が２５μm以下、そのポリゴナルフェライトに占める粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合が０.６以上の鋼組織を持ち、熱間プレス鋼板部材の硬度ばらつきを抑制することが可能な、熱間プレス用鋼板を容易に得ることができる。 As a result, the area ratio of polygonal ferrite is 30% or more, the average crystal grain size of the polygonal ferrite is 25 μm or less, and 5 or more cementite particles having a grain size of 0.2 μm or more in the polygonal ferrite are contained in the grain. A steel sheet for hot pressing that has a steel structure in which the number ratio of polygonal ferrite is 0.6 or more and can suppress the hardness variation of the hot pressed steel sheet member can be easily obtained.
熱間圧延完了温度は、上述したように７００℃以上とする。さらに好ましくは、Ａｒ３点以上である。Ａｒ３点未満のフェライト域圧延であると、圧延時の荷重変動が大きくなり、操業トラブルの危険性があるからである。 As described above, the hot rolling completion temperature is set to 700 ° C. or higher. More preferably, it is Ar 3 points or more. This is because if the ferrite region rolling is less than Ar 3 points, the load fluctuation during rolling becomes large and there is a risk of operation trouble.
熱間圧延完了後は、得られた熱間圧延鋼板に平均冷却速度１００℃／秒以下の冷却を施し、５００℃以上の温度で巻き取ることで、まず、熱間圧延鋼板の鋼組織をフェライトとパーライト、フェライトと球状化セメンタイト、またはフェライトとパーライトと球状化セメンタイトを含む組織にする。 After the hot rolling is completed, the obtained hot rolled steel sheet is cooled at an average cooling rate of 100 ° C./second or less and wound at a temperature of 500 ° C. or higher. And pearlite, ferrite and spheroidized cementite, or ferrite, pearlite and spheroidized cementite.
このようにして得られた熱間圧延鋼板に、圧下率２０％以上の冷間圧延を施すことにより、パーライト中のラメラ状のセメンタイトや球状化セメンタイトが、圧延未再結晶フェライト中に分散する。 By subjecting the hot-rolled steel sheet thus obtained to cold rolling at a reduction rate of 20% or more, lamellar cementite and spheroidized cementite in pearlite are dispersed in the rolled unrecrystallized ferrite.
このようにして得られた冷間圧延鋼板に、（Ａｃ１点−１００℃）〜Ａｃ３点未満の温度範囲で５秒間以上の焼鈍を施すことにより、圧延未再結晶フェライトが再結晶して最終的にポリゴナルフェライトとなり、そのポリゴナルフェライト粒内には冷間圧延によって分散されたセメンタイトが残存するようになる。その結果、ポリゴナルフェライトに占める粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合が０.６以上とすることが容易にできる。 The cold-rolled steel sheet thus obtained is annealed for 5 seconds or more in a temperature range from (Ac 1 point to 100 ° C.) to less than Ac 3 points, whereby the unrecrystallized rolled ferrite recrystallizes. Finally, it becomes polygonal ferrite, and cementite dispersed by cold rolling remains in the polygonal ferrite grains. As a result, the number ratio of polygonal ferrite containing 5 or more cementite particles having a particle size of 0.2 μm or more in the polygonal ferrite can be easily adjusted to 0.6 or more.
一方、熱間圧延後の平均冷却速度が１００℃／秒超または巻き取り温度が５００℃未満であると、熱間圧延鋼板中にベイナイト組織またはマルテンサイト組織が多く生成するため、その後の冷間圧延−焼鈍を経て得られる鋼組織において、ポリゴナルフェライトに占める粒径０.２μｍ以上のセメンタイトを粒内に５個以上含有するポリゴナルフェライトの個数割合が低下し、前記個数割合が０.６未満となってしまう場合がある。同様に冷間圧延時の圧下率が２０％未満であると、熱間圧延鋼板に生成しているパーライトや球状化セメンタイトが充分に分散せず、前記個数割合を０.６以上とすることが困難となる。 On the other hand, when the average cooling rate after hot rolling exceeds 100 ° C./second or the coiling temperature is less than 500 ° C., a lot of bainite structure or martensite structure is generated in the hot rolled steel sheet. In the steel structure obtained through rolling-annealing, the number ratio of polygonal ferrite containing 5 or more cementites having a particle size of 0.2 μm or more in the polygonal ferrite is decreased, and the number ratio is 0.6. It may be less than. Similarly, when the rolling reduction during cold rolling is less than 20%, the pearlite and spheroidized cementite produced in the hot rolled steel sheet are not sufficiently dispersed, and the number ratio may be 0.6 or more. It becomes difficult.
また、焼鈍温度がＡｃ３点以上であると、オーステナイト単相となってフェライト中に存在していたセメンタイトがすべて再固溶してしまい、前記個数割合が０.６未満になる。また、焼鈍温度が、（Ａｃ１点−１００℃）未満または焼鈍時間が５秒間未満では、圧延未再結晶フェライトのポリゴナルフェライトへの再結晶が不十分となり、最終的なポリゴナルフェライトの面積率が３０％未満となる。 Further, when the annealing temperature is Ac 3 point or higher, all cementite that is in the austenite single phase and is present in the ferrite is re-dissolved, and the number ratio is less than 0.6. Further, when the annealing temperature is less than (Ac 1 point-100 ° C.) or the annealing time is less than 5 seconds, recrystallization of the rolled non-recrystallized ferrite into polygonal ferrite becomes insufficient, and the area of the final polygonal ferrite is reduced. The rate is less than 30%.
焼鈍時間が５００秒間超、焼鈍後の平均冷却速度が１℃／秒未満または焼鈍後の冷却停止温度が７００℃超では、ポリゴナルフェライトの再結晶が過剰に促進され、ポリゴナルフェライトの平均結晶粒径が２５μｍ超となる場合がある。焼鈍後の平均冷却速度の上限は、特に規定しないが、冷却速度が２００℃／秒超になるとポリゴナルフェライトの平均結晶粒径が小さくなって鋼板が硬質となり、上記ポリゴナルフェライト面積率の下限限定理由やマルテンサイト面積率の上限限定理由と同様の問題が生じる場合があるので、上限を２００℃／秒とすることが好ましい。 When the annealing time exceeds 500 seconds, the average cooling rate after annealing is less than 1 ° C./second, or the cooling stop temperature after annealing exceeds 700 ° C., recrystallization of polygonal ferrite is excessively promoted, and the average crystal of polygonal ferrite The particle size may exceed 25 μm. The upper limit of the average cooling rate after annealing is not particularly specified, but when the cooling rate exceeds 200 ° C./second, the average crystal grain size of polygonal ferrite becomes small and the steel plate becomes hard, and the lower limit of the above-mentioned polygonal ferrite area ratio Since the same problem as the reason for limitation and the reason for limiting the upper limit of the martensite area ratio may occur, the upper limit is preferably set to 200 ° C./second.
焼鈍後の冷却の停止温度は３５０℃以上とすることで、マルテンサイトの生成を抑制して、マルテンサイトの面積率を３０％以下とすることができる。冷却停止温度が３５０℃未満であると、マルテンサイトの面積率が３０％超となる場合がある。 By setting the cooling stop temperature after annealing to 350 ° C. or higher, the martensite formation can be suppressed and the martensite area ratio can be 30% or less. If the cooling stop temperature is lower than 350 ° C., the martensite area ratio may exceed 30%.
(３)めっき
本発明の熱間プレス用鋼板および熱間プレス鋼板部材は、熱間プレスの加熱工程における表面酸化抑制や熱間プレス後の熱間プレス鋼板部材の耐食性向上を目的として、鋼板表面にめっき層を備えてもよい。
(3) Plating The steel sheet for hot pressing and the hot pressed steel sheet member of the present invention are steel sheet surfaces for the purpose of suppressing surface oxidation in the heating process of the hot press and improving the corrosion resistance of the hot pressed steel sheet member after hot pressing. May be provided with a plating layer.
めっきの種類は、前記目的に適うものであればよく特に限定する必要はない。アルミニウム系めっき鋼板 (例、溶融アルミニウムめっき鋼板、溶融５５％Ａｌ−Ｚｎ合金めっき鋼板)、亜鉛系めっき鋼板 (例、電気もしくは溶融亜鉛めっき鋼板、溶融５％Ａｌ-Ｚｎめっき鋼板、合金化溶融亜鉛めっき鋼板、電気Ｎｉ−Ｚｎ合金めっき鋼板) 等が例示される。また、めっき方法についても特に限定する必要はなく、溶融めっき、電気めっき、化学めっき、蒸着めっき等の周知の方法を適用することができる。めっき付着量は一般的な範囲内であればよく、通常は片面当たり３〜５００ｇ／ｍ2の範囲内である。 The type of plating is not particularly limited as long as it meets the above purpose. Aluminum-based plated steel sheet (eg, hot-dip aluminum-plated steel sheet, hot-dip 55% Al-Zn alloy-plated steel sheet), zinc-based steel sheet (eg, electric or hot-dip galvanized steel sheet, hot-dip 5% Al-Zn-plated steel sheet, alloyed hot-dip zinc Examples thereof include plated steel sheets and electric Ni—Zn alloy plated steel sheets). Moreover, it is not necessary to specifically limit the plating method, and well-known methods such as hot dipping, electroplating, chemical plating, and vapor deposition plating can be applied. The plating adhesion amount may be within a general range, and is usually within a range of 3 to 500 g / m 2 per side.
生産性の観点から、溶融めっきを適用することが好ましく、熱間プレスにおける加工性の観点からは溶融亜鉛めっきとすることが好ましく、耐食性の観点からはさらに合金化処理を施した方が好ましい。溶融めっき浴温度は、特に規定しないが、生産性の観点からめっきする金属の融点以上、（融点＋２００℃）以下の温度とすることが望ましい。 From the viewpoint of productivity, it is preferable to apply hot dip plating, from the viewpoint of workability in hot pressing, preferably hot dip galvanizing, and from the viewpoint of corrosion resistance, it is preferable to further perform alloying treatment. The temperature of the hot dipping bath is not particularly defined, but it is desirable to set the temperature to be not lower than the melting point of the metal to be plated and not higher than (melting point + 200 ° C.) from the viewpoint of productivity.
溶融亜鉛めっき後に合金化処理を施す場合には、鋼板表面温度が ４７０℃〜（Ａｃ１点＋５０℃）となる合金化処理温度域で１〜３０秒間保持して行うことが望ましい。合金化処理温度が４７０℃未満では、温度が低すぎるため、合金化処理に時間がかかり生産性が劣化する。生産性の観点から合金化処理温度は５００℃以上が望ましい。一方、合金化処理温度が（Ａｃ１点＋５０℃）超であると、鋼組織が変化して所望の鋼組織が得られない場合がある。合金化処理時間を１〜３０秒間することで、めっき皮膜中のＦｅ濃度を５％〜２５％とすることができる。 When the alloying treatment is performed after the hot dip galvanization, it is desirable that the steel sheet surface temperature is maintained for 1 to 30 seconds in an alloying treatment temperature range in which the steel sheet surface temperature is 470 ° C. to (Ac 1 point + 50 ° C.). If the alloying treatment temperature is less than 470 ° C., the temperature is too low, so the alloying treatment takes time and the productivity deteriorates. From the viewpoint of productivity, the alloying temperature is preferably 500 ° C. or higher. On the other hand, if the alloying treatment temperature is higher than (Ac 1 point + 50 ° C.), the steel structure may change and a desired steel structure may not be obtained. By making the alloying treatment time 1 to 30 seconds, the Fe concentration in the plating film can be 5% to 25%.
＜熱間プレス＞
鋼板の加熱温度はＡｃ３点超とする必要がある。加熱温度がＡｃ３点℃以下では、熱間プレス前にオーステナイト単相状態とはならず、鋼板中にフェライトまたは、パーライト、ベイナイトが残存してしまう。その結果、熱間プレス後にマルテンサイト単相組織とはならず、所望の硬度が得られない場合がある。また、熱間プレス鋼板部材の硬度ばらつきも大きくなってしまう。加熱温度の上限は１１００℃とする。加熱温度が１１００℃超であるとオーステナイトが、粗大化し、部材の靭性が劣化する場合がある。
<Hot press>
The heating temperature of the steel sheet needs to exceed Ac 3 points. When the heating temperature is Ac 3 points or lower, the austenite single phase state is not obtained before hot pressing, and ferrite, pearlite, or bainite remains in the steel sheet. As a result, a martensite single phase structure is not obtained after hot pressing, and a desired hardness may not be obtained. Moreover, the hardness variation of a hot press steel plate member will also become large. The upper limit of the heating temperature is 1100 ° C. When the heating temperature is higher than 1100 ° C., austenite becomes coarse and the toughness of the member may deteriorate.
加熱時間は、１〜１０分間が望ましい。加熱時間が１分未満であると、加熱してもオーステナイト単相化が不十分となる場合がある。１０分超では、オーステナイトが粗大化し、熱間プレス鋼板部材の靭性が劣化してしまう場合がある。 The heating time is desirably 1 to 10 minutes. When the heating time is less than 1 minute, austenite single phase may be insufficient even when heated. If it exceeds 10 minutes, austenite may become coarse and the toughness of the hot-pressed steel sheet member may deteriorate.
熱間プレスの開始温度は、Ａｒ３点以上とする。Ａｒ３点未満の温度であると、フェライト変態が始まるために、その後に強制冷却してもマルテンサイト単相組織にならない。熱間プレス後の冷却速度は１０℃／秒以上の急冷が望ましく、２０℃／秒以上で行うことがさらに望ましい。冷却速度の上限は特に規定しない。この冷却速度で冷却開始した後、一気に鋼板部材温度を３５０℃以下の温度域に下げる。冷却終了温度は、好ましくは１００℃以下、更に好ましくは室温である。熱間プレス後に一気に３５０℃以下に材料温度を下げることで、硬度ばらつきの少ない単一マルテンサイト組織となった熱間プレス鋼板部材を得ることができる。 The starting temperature of hot pressing is at least 3 points for Ar. If the temperature is lower than Ar 3 point, ferrite transformation starts, so even if it is forcibly cooled after that, it does not become a martensite single phase structure. The cooling rate after hot pressing is preferably 10 ° C./second or more, more preferably 20 ° C./second or more. There is no particular upper limit on the cooling rate. After starting cooling at this cooling rate, the steel plate member temperature is lowered to a temperature range of 350 ° C. or less at once. The cooling end temperature is preferably 100 ° C. or lower, more preferably room temperature. By reducing the material temperature to 350 ° C. or less at once after hot pressing, a hot pressed steel sheet member having a single martensite structure with little hardness variation can be obtained.
表１に示す化学成分を有する鋼を試験用転炉で溶製し、試験用連続鋳造機にて連続鋳造を実施した。連続鋳造時には、単位時間あたりの鋳込み量ならびに溶鋼加熱温度（液相線温度から加熱温度）を変更させた。また、スラブ最終凝固部において、連続鋳造機内の上下対のロール間隔を狭める未凝固層圧下により、完全凝固前にＰ、Ｓの濃化溶鋼を吐き出させる中心偏析低減処理を実施した。比較として、中心偏析低減処理をしないスラブも鋳造した。その後、試験用圧延機にて、熱間圧延を実施し、板厚２.０〜５.８ｍｍの熱間圧延鋼板を得た。熱間圧延鋼板のうち、板厚２.０ｍｍ超のものについては、酸洗を実施後、冷間圧延を実施し、板厚２.０ｍｍの冷間圧延鋼板とした。 Steels having the chemical components shown in Table 1 were melted in a test converter, and continuous casting was performed with a test continuous casting machine. During continuous casting, the casting amount per unit time and the molten steel heating temperature (from the liquidus temperature to the heating temperature) were changed. In the final solidification part of the slab, a central segregation reduction process for discharging P and S concentrated molten steel before complete solidification was carried out under unsolidified layer pressure which narrows the gap between the upper and lower pairs in the continuous casting machine. As a comparison, a slab without a center segregation reduction treatment was also cast. Then, hot rolling was implemented with the rolling mill for a test, and the hot rolled steel plate with a plate thickness of 2.0-5.8 mm was obtained. Among the hot-rolled steel sheets, those with a thickness of more than 2.0 mm were pickled and then cold-rolled to obtain cold-rolled steel sheets having a thickness of 2.0 mm.
一部の鋼板については、冷間圧延後、焼鈍試験装置にて焼鈍を実施した。さらに、得られた熱間圧延鋼板、冷間圧延鋼板の焼鈍板について、溶融めっき試験ラインにて溶融めっきを施した。一部のものについて合金化処理も行った。めっきは、亜鉛めっきならびにアルミニウムめっきを実施した。スラブの鋳造条件と熱間圧延鋼板の製造条件を表２に示す。冷間圧延ならびに冷間圧延後の焼鈍、めっきおよび合金化処理の条件と製品鋼板の種類を表３に示す。 About some steel plates, it annealed with the annealing test apparatus after cold rolling. Further, the obtained hot rolled steel sheet and cold rolled steel sheet were subjected to hot dip plating in a hot dip test line. Alloying treatment was also performed on some of them. For plating, zinc plating and aluminum plating were performed. Table 2 shows slab casting conditions and hot-rolled steel sheet manufacturing conditions. Table 3 shows the conditions of cold rolling and annealing, plating and alloying treatment after cold rolling, and the types of product steel plates.
得られた熱間圧延鋼板、冷間圧延鋼板、溶融めっき鋼板、または合金化溶融めっき鋼板について、鋼板清浄度、板厚中心部のＰおよびＳの偏析度αおよびβ、鋼組織、めっき目付量および合金化度、ならびに引張特性を次のようにして調査した。めっき鋼板または合金化溶融めっき鋼板の場合、めっき目付量および合金化度以外の調査項目も、めっき後に調査した。これらの試験結果は表４にまとめて示す。 About the obtained hot rolled steel sheet, cold rolled steel sheet, hot dip galvanized steel sheet, or alloyed hot dip galvanized steel sheet, steel sheet cleanliness, P and S segregation degrees α and β at the center of the plate thickness, steel structure, plating weight per unit area In addition, the degree of alloying and tensile properties were investigated as follows. In the case of a plated steel plate or an alloyed hot-dip plated steel plate, survey items other than the coating weight and the degree of alloying were also investigated after plating. These test results are summarized in Table 4.
＜鋼板清浄度＞
鋼板の清浄度は、ＪＩＳ Ｇ０５５５に準拠して調査した。各鋼板について５個所から供試材を切り出した。各供試材の板厚１／８ｔ、１／４ｔ、１／２ｔ、３／４ｔ、７／８ｔの各位置について、点算法にて清浄度を調査した。各板厚における清浄度の値が最も大きい（清浄度が最も低い）数値を、その供試材の清浄度とした。清浄度は、Ａ系、Ｂ系、Ｃ系介在物の総和とした。
<Steel plate cleanliness>
The cleanliness of the steel sheet was investigated according to JIS G0555. Test materials were cut out from five locations for each steel plate. The cleanliness was investigated by a point calculation method at each position of the plate thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, and 7 / 8t of each test material. The value with the largest cleanliness value (lowest cleanliness) at each plate thickness was defined as the cleanliness of the specimen. The cleanliness was the sum of inclusions of A, B, and C inclusions.
＜板厚中心部の偏析度＞
鋼板の偏析調査は、ＥＰＭＡによるＰとＳの成分面分析により行った。各鋼板について５個所から供試材を切り出した。板厚１／４ｔ、１／２ｔの各位置において調査し、各位置において５００倍の倍率にて１０視野ずつ調査した。
<Segregation degree at the center of the plate thickness>
The segregation investigation of the steel sheet was performed by component surface analysis of P and S by EPMA. Test materials were cut out from five locations for each steel plate. Investigation was made at each position of the plate thickness of 1/4 t and 1/2 t, and 10 visual fields were examined at each magnification at a magnification of 500 times.
Ｐの偏析度αおよびＳの偏析度βを、下記式(１)および(２)に従って算出し、５個の供試材の算術計算により偏析度を求めた。
α＝［板厚中心部での最大Ｐ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｐ濃度（質量％）］ ・・・ (１)
β＝［板厚中心部での最大Ｓ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｓ濃度（質量％）］ ・・・ (２)
＜鋼組織＞
鋼板の圧延方向に平行な断面について、走査型電子顕微鏡を用いて、鋼組織を観察した。測定は、板厚１／８ｔ、１／４ｔ、１／２ｔ、３／４ｔ、７／８ｔの各位置について倍率２０００倍で実施し、各供試材について２０視野ずつ測定した。得られた画像をもとに各組織の面積率、ポリゴナルフェライトの平均結晶粒径、ポリゴナルフェライト粒内における粒径０.２μｍ以上のセメンタイトの個数を画像処理にて調査し、ポリゴナルフェライトの面積率、マルテンサイトの面積率、パーライトの面積率、ベイナイトの面積率、オーステナイトの面積率、ポリゴナルフェライトの平均結晶粒径、ポリゴナルフェライトの全個数（α）に占める粒内に粒径０.２μｍ以上のセメンタイトを５個以上含有しているポリゴナルフェライトの個数（αC）の割合（αC／α）を算術計算にて求めた。ポリゴナルフェライトの平均結晶粒径は、ＪＩＳ Ｇ０５５２に準拠して測定した。
The segregation degree α of P and the segregation degree β of S were calculated according to the following formulas (1) and (2), and the segregation degree was obtained by arithmetic calculation of five specimens.
α = [maximum P concentration (mass%) at the thickness center portion] / [average P concentration (mass%) at the 1/4 depth position of the thickness from the surface] (1)
β = [maximum S concentration (mass%) at the thickness center portion] / [average S concentration (mass%) at the 1/4 depth position of the thickness from the surface] (2)
<Steel structure>
About the cross section parallel to the rolling direction of a steel plate, the steel structure was observed using the scanning electron microscope. The measurement was performed at a magnification of 2000 for each position of the plate thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, and 7 / 8t, and 20 fields of view were measured for each specimen. Based on the obtained images, the area ratio of each structure, the average crystal grain size of polygonal ferrite, and the number of cementites having a grain size of 0.2 μm or more in the polygonal ferrite grains are investigated by image processing. Area ratio, martensite area ratio, pearlite area ratio, bainite area ratio, austenite area ratio, average grain size of polygonal ferrite, grain size in the total number (α) of polygonal ferrite The ratio (α C / α) of the number (α C ) of polygonal ferrite containing 5 or more cementites of 0.2 μm or more was obtained by arithmetic calculation. The average crystal grain size of polygonal ferrite was measured according to JIS G0552.
＜めっきの目付量および合金化度＞
鋼板から２５ｍｍφの試料片を採取し、０.５Ｖｏｌ％インヒビター（商品名：朝日化学製「イビット７１０Ｎ」）を含有した１０％ＨＣｌ水溶液でめっき層を溶解し、得られた溶液をＩＣＰ法で分析することによりめっき層の組成分析を行った。得られた各成分の濃度からめっきの目付量および合金化度を算出した。
<Amount of plating and degree of alloying>
A 25 mmφ sample piece was taken from the steel plate, the plating layer was dissolved with a 10% HCl aqueous solution containing 0.5 Vol% inhibitor (trade name: “Ibit 710N” manufactured by Asahi Chemical), and the obtained solution was analyzed by the ICP method. As a result, the composition of the plating layer was analyzed. The basis weight of plating and the degree of alloying were calculated from the concentration of each component obtained.
＜引張り特性＞
得られた鋼板の引張り特性を引張り試験により評価した。各鋼板の圧延直角方向からＪＩＳ ５号引張試験を採取した。試験方法はＪＩＳ Ｚ２２４１に準じた。降伏点ＹＰ、引張強さＴＳ、伸びＥｌを測定した。
<Tensile properties>
The tensile properties of the obtained steel sheet were evaluated by a tensile test. A JIS No. 5 tensile test was taken from the direction perpendicular to the rolling of each steel plate. The test method conformed to JIS Z2241. Yield point YP, tensile strength TS, and elongation El were measured.
その後、熱間プレス試験装置を用いて、得られた熱間圧延鋼板、冷間圧延鋼板、溶融めっき鋼板、または合金化溶融めっき鋼板に対して、図１に示す条件にて熱間プレス（ハット成形）を実施した。熱間プレスは、鋼板を加熱炉内で鋼板表面温度９００℃に到達させ、その温度にて４分間保持し、加熱炉より取り出し、すぐさま冷却装置付きの金型にて熱間プレスを実施し、成形と同時に焼入れ処理を実施した。 Thereafter, using a hot press test apparatus, the obtained hot-rolled steel sheet, cold-rolled steel sheet, hot-dip galvanized steel sheet, or alloyed hot-dip galvanized steel sheet was hot-pressed (hat) under the conditions shown in FIG. Molding). In the hot press, the steel plate reaches a steel plate surface temperature of 900 ° C. in a heating furnace, is held at that temperature for 4 minutes, is taken out of the heating furnace, and immediately hot-pressed with a mold with a cooling device, A quenching process was performed simultaneously with the molding.
熱間プレス用の試験片サイズは板厚２.０ｍｍ×幅１２０ｍｍ×長さ３２０ｍｍとし、熱間プレス成形条件は成形高さ７０ｍｍとした。熱間プレスの開始温度は約９００℃であった。金型に設けた冷却装置は水冷式であり、冷却速度は約５０℃／秒であり、冷却終了温度は室温であった。 The size of the test piece for hot pressing was plate thickness 2.0 mm × width 120 mm × length 320 mm, and the hot press forming condition was forming height 70 mm. The starting temperature of hot pressing was about 900 ° C. The cooling device provided in the mold was water-cooled, the cooling rate was about 50 ° C./second, and the cooling end temperature was room temperature.
得られた熱間プレス成形品（鋼板部材）に対して、硬度のばらつきと靱性と次のようにして調査した。試験結果は表５にまとめて示す。
＜熱間プレス鋼板部材の硬度のばらつき＞
硬度はビッカース硬度計で測定した。測定荷重は９８ｋＮであった。熱間プレスにより得られた部材から、図２に示すようにマイクロカッターで切断して試料を採取した。切断個所は図中の(1)、(2)、(3)の３箇所とし、各位置から硬度測定用試料を５個採取した。各試料の板厚断面１／４ｔ位置においてｎ＝３の断面硬度を測定した。測定方法はＪＩＳ Ｚ２２４４に準拠した。各部材についてこうして得られた４５のビッカース硬度値（Ｈｖ）の最大値ならびに最小値から、ビッカース硬度の差（ΔＨｖ）を求めた。
The obtained hot press-formed product (steel plate member) was examined for hardness variation and toughness as follows. The test results are summarized in Table 5.
<Hardness variation of hot-pressed steel sheet member>
The hardness was measured with a Vickers hardness meter. The measurement load was 98 kN. A sample was collected from the member obtained by hot pressing by cutting with a microcutter as shown in FIG. There were three cutting points (1), (2), and (3) in the figure, and five samples for hardness measurement were taken from each position. The cross-sectional hardness of n = 3 was measured at the position of 1/4 t of the plate thickness cross section of each sample. The measuring method was based on JIS Z2244. The difference (ΔHv) in Vickers hardness was determined from the maximum value and the minimum value of 45 Vickers hardness values (Hv) thus obtained for each member.
＜熱間プレス後部材の靭性＞
得られた熱間プレス鋼板部材の靭性を、部材から切り出したシャルピー試験片に対してシャルピー衝撃試験を実施することにより調査した。試験片の切り出し位置は図２に示す通りである。得られた試験片５枚を重ね合わせてビス止めをし、合計板厚１０ｍｍの試験片とした。試験片形状は、ＪＩＳ Ｚ２２０２に記載されているＶノッチシャルピー試験片とした。試験方法は、ＪＩＳ Ｚ２２４２に記載されている方法に準じ、−１２０℃温度における吸収エネルギーを調査した。
<Toughness of members after hot pressing>
The toughness of the obtained hot-pressed steel sheet member was investigated by performing a Charpy impact test on the Charpy test piece cut out from the member. The cut-out position of the test piece is as shown in FIG. Five test pieces obtained were overlapped and screwed to obtain a test piece having a total thickness of 10 mm. The specimen shape was a V-notch Charpy specimen described in JIS Z2202. The test method investigated the absorbed energy at -120 degreeC temperature according to the method described in JISZ2242.
＜熱間プレス後部材の引張り特性＞
得られた熱間プレス鋼板部材からＪＩＳ １３Ｂ号引張試験を採取した。試験片の切り出し位置は図２に示す通りである。切断個所は図中の(4)である。試験方法はＪＩＳ Ｚ２２４１に準じた。引張強さＴＳを測定した。
<Tensile properties of hot-pressed parts>
A JIS No. 13B tensile test was collected from the obtained hot-pressed steel sheet member. The cut-out position of the test piece is as shown in FIG. The cutting point is (4) in the figure. The test method conformed to JIS Z2241. Tensile strength TS was measured.
上の表１〜５からわかるように、本発明に従った鋼組成および製造条件で製造された供試材Ｎｏ.１〜２５は、熱間プレス鋼板部材のシャルピー衝撃試験において−１２０℃での吸収エネルギーが４２〜７９Ｊ／ｃｍ2と十分な高さであり、靭性に優れていた。発明例のうち、Ｎｏ.４〜５、７〜１４、２２は、鋼板断面における鋼組織において、ポリゴナルフェライトの面積率が３０％以上であり、そのポリゴナルフェライトの平均結晶粒径が２５μｍ以下、ポリゴナルフェライトが０.６≦αC／αを満たし、残部組織においてマルテンサイトが３０％以下であるため、熱間プレス鋼板部材硬度のばらつきが１０Ｈｖ以下と小さくなった。そのため熱間プレス鋼板部材のシャルピー衝撃試験において−１２０℃での吸収エネルギーが６０〜７９Ｊ／ｃｍ2と特に高く、靭性が一層優れていた。 As can be seen from Tables 1 to 5 above, specimens Nos. 1 to 25 produced with the steel composition and production conditions according to the present invention were measured at −120 ° C. in a Charpy impact test of a hot-pressed steel sheet member. The absorbed energy was 42-79 J / cm 2 and a sufficient height, and the toughness was excellent. Among the inventive examples, Nos. 4 to 5, 7 to 14 and 22 have an area ratio of polygonal ferrite of 30% or more in the steel structure in the steel sheet cross section, and the average crystal grain size of the polygonal ferrite is 25 μm or less. Since the polygonal ferrite satisfies 0.6 ≦ α C / α and the martensite is 30% or less in the remaining structure, the variation in the hardness of the hot pressed steel sheet member is reduced to 10 Hv or less. Therefore, in the Charpy impact test of the hot pressed steel sheet member, the absorbed energy at −120 ° C. was particularly high as 60 to 79 J / cm 2 and the toughness was further excellent.
供試材Ｎｏ.２６〜３０の比較例を見ると、Ｎｏ.２６は、連続鋳造の際の溶鋼加熱温度が液相線温度から２℃と低かったため、鋼板の清浄度が０.１％と悪化した。そのため、熱間プレス鋼板部材のシャルピー衝撃試験における−１２０℃での吸収エネルギーは２０Ｊ／ｃｍ2と低く、靱性が不十分であった。 When the comparative example of test material No. 26-30 is seen, since the molten steel heating temperature in the case of continuous casting was as low as 2 degreeC from liquidus temperature, the cleanliness of a steel plate is 0.1%. It got worse. Therefore, the absorbed energy at −120 ° C. in the Charpy impact test of the hot pressed steel sheet member was as low as 20 J / cm 2 and the toughness was insufficient.
Ｎｏ.２７は、連続鋳造の際の単位時間当たりの溶鋼鋳込み量が６.５ｔｏｎ／分と本発明における上限を超えたため、鋼板の清浄度が０.１２％と悪化した。そのため、熱間プレス後部材のシャルピー衝撃試験における−１２０℃での吸収エネルギーは２１Ｊ／ｃｍ2と低く、靱性が低かった。 In No. 27, the amount of molten steel cast per unit time during continuous casting was 6.5 ton / min, which exceeded the upper limit in the present invention, and the cleanliness of the steel sheet deteriorated to 0.12%. Therefore, the absorbed energy at −120 ° C. in the Charpy impact test of the member after hot pressing was as low as 21 J / cm 2 and the toughness was low.
Ｎｏ.２８は、連続鋳造の際に中心偏析低減処理を実施しなかったため、Ｐの偏析度αが１.７、Ｓの偏析度βが１.７と、いずれも本発明における上限を超えた。熱間プレス鋼板部材のシャルピー衝撃試験における−１２０℃での吸収エネルギーは１４Ｊ／ｃｍ2と非常に低く、靱性が悪化した。硬度のばらつきは３６Ｈｖと大きかった。 No. 28 was not subjected to the center segregation reduction treatment during continuous casting, so the P segregation degree α was 1.7 and the S segregation degree β was 1.7, both exceeding the upper limit in the present invention. . The absorbed energy at −120 ° C. in the Charpy impact test of the hot-pressed steel sheet member was as extremely low as 14 J / cm 2 and the toughness was deteriorated. The variation in hardness was as large as 36 Hv.
Ｎｏ.２９は、連続鋳造の際に中心偏析低減処理を実施しなかったため、Ｐの偏析度αが１.８と本発明における上限を超えた。熱間プレス鋼板部材のシャルピー衝撃試験における−１２０℃での吸収エネルギーは２２Ｊ／ｃｍ2と、靱性が不十分であった。 No. 29 did not perform the center segregation reduction treatment during continuous casting, and therefore the P segregation degree α exceeded the upper limit in the present invention, which was 1.8. The absorbed energy at −120 ° C. in a Charpy impact test of a hot-pressed steel sheet member was 22 J / cm 2 and the toughness was insufficient.
Ｎｏ.３０は、連続鋳造の際に中心偏析低減処理を実施しなかったため、Ｓの偏析度αが１.８と本発明における上限を超えた。熱間プレス鋼板部材のシャルピー衝撃試験における−１２０℃での吸収エネルギーは１５Ｊ／ｃｍ2と非常に低く、靱性が悪化した。硬度のばらつきは３６Ｈｖと大きかった。 No. 30 did not perform the center segregation reduction treatment during continuous casting, so the segregation degree α of S exceeded 1.8, the upper limit in the present invention. The absorbed energy at −120 ° C. in the Charpy impact test of the hot-pressed steel sheet member was very low at 15 J / cm 2 and the toughness deteriorated. The variation in hardness was as large as 36 Hv.
Claims (12)
α＝［板厚中心部での最大Ｐ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｐ濃度（質量％）］ ・・・ (１)
β＝［板厚中心部での最大Ｓ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｓ濃度（質量％）］ ・・・ (２) In mass%, C: 0.09 to 0.60%, Si: 2.0% or less, Mn: 0.5 to 3.5%, P: 0.10% or less, S: 0.05% or less, Al: 0.005 to 2.0%, and N: 0.01% or less, with the balance being a chemical composition consisting of Fe and impurities, cleanliness of 0.08% or less, the following formula (1) The segregation degree α of P defined by the formula (1) is 1.6 or less, the segregation degree β of S defined by the following formula (2) is 1.6 or less, and the strength after hot pressing is 1080 MPa or more. A steel sheet for hot pressing characterized by
α = [maximum P concentration (mass%) at the thickness center portion] / [average P concentration (mass%) at the 1/4 depth position of the thickness from the surface] (1)
β = [maximum S concentration (mass%) at the thickness center portion] / [average S concentration (mass%) at the 1/4 depth position of the thickness from the surface] (2)
α＝［板厚中心部での最大Ｐ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｐ濃度（質量％）］ ・・・ (１)
β＝［板厚中心部での最大Ｓ濃度（質量％）］／［表面から板厚の１／４深さ位置での平均Ｓ濃度（質量％）］ ・・・ (２) The chemical composition according to claim 1, the cleanliness is 0.08% or less, the segregation degree α of P defined by the following formula (1) is 1.6 or less, and the following formula (2 A hot-pressed steel sheet member having a segregation degree β of S defined by) of 1.6 or less and a tensile strength of 1080 MPa or more.
α = [maximum P concentration (mass%) at the thickness center portion] / [average P concentration (mass%) at the 1/4 depth position of the thickness from the surface] (1)
β = [maximum S concentration (mass%) at the thickness center portion] / [average S concentration (mass%) at the 1/4 depth position of the thickness from the surface] (2)
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