JP2013163827A - High-strength cold-rolled steel sheet, high-strength galvanized steel sheet, and high-strength alloying hot-dip galvanized steel sheet, excellent in bendability, and method for manufacturing the same - Google Patents
High-strength cold-rolled steel sheet, high-strength galvanized steel sheet, and high-strength alloying hot-dip galvanized steel sheet, excellent in bendability, and method for manufacturing the same Download PDFInfo
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本発明は、高強度冷延鋼板、高強度亜鉛めっき鋼板及び高強度合金化溶融亜鉛めっき鋼板に関し、優れた曲げ性を有する高強度鋼板およびその製造方法に関するものである。 The present invention relates to a high-strength cold-rolled steel sheet, a high-strength galvanized steel sheet, and a high-strength galvannealed steel sheet, and relates to a high-strength steel sheet having excellent bendability and a method for producing the same.
近年、自動車などに用いられるめっき鋼板の高強度化に対する要求が高まってきており、引張最大応力900MPa以上の高強度鋼板も用いられるようになってきている。このような高強度鋼板を用いて自動車の車両や部材を形成する方法としては、プレス加工などの曲げ加工が挙げられる。通常、曲げ性は、鋼板の強度を高くするほど悪くなる。このため、高強度鋼板に曲げ加工を行うと、変形部の鋼板内部に亀裂(クラック)が発生したり、鋼板表面でネッキングが発生したりという問題があった。 In recent years, demands for higher strength of plated steel sheets used in automobiles and the like have increased, and high-strength steel sheets having a tensile maximum stress of 900 MPa or more are also being used. Examples of a method for forming a vehicle or a member of an automobile using such a high-strength steel plate include bending work such as press work. Usually, the bendability becomes worse as the strength of the steel plate is increased. For this reason, when bending was performed on a high-strength steel plate, there was a problem that a crack (crack) occurred inside the steel plate of the deformed portion or necking occurred on the steel plate surface.
高強度鋼板の曲げ性を支配する因子としては、(a)ネッキングの起こり難さ、(b)鋼板内部での割れ(ボイド)の発生し難さ、が重要であることが知られている(例えば、非特許文献1)。これは、伸びが低い鋼板では、曲げ加工中にネッキングが起こり、変形が局在化することで、曲げ加工性が劣化することが知られていた。
一方では、フェライト及びマルテンサイトよりなる鋼では、マルテンサイトの割れや界面でのボイド形成が原因で、曲げ性が劣位であることが知られていた。この結果、高強度化は伸びの劣化を齎すので曲げ性が悪い。加えて、高強度化は、マルテンサイト体積分率の増加を伴う場合があるので、高強度化は曲げ性の劣化を引き起こしやすい。
As factors governing the bendability of high-strength steel sheets, it is known that (a) the difficulty of necking and (b) the difficulty of occurrence of cracks (voids) inside the steel sheet are important ( For example, Non-Patent Document 1). It has been known that, in a steel sheet having a low elongation, necking occurs during the bending process and the deformation is localized, so that the bending processability deteriorates.
On the other hand, it has been known that a steel made of ferrite and martensite has inferior bendability due to martensite cracking and void formation at the interface. As a result, increasing the strength leads to deterioration of elongation, so the bendability is poor. In addition, since the increase in strength may be accompanied by an increase in the martensite volume fraction, the increase in strength tends to cause deterioration in bendability.
鋼板の曲げ性を向上させる技術として、特許文献1には、鋼板の化学組成が、質量%で、C:0.02%を超え0.20%以下、Si:0.01〜2.0%、Mn:0.1〜3.0%、P:0.003〜0.10%、S:0.020%以下、Al:0.001〜1.0%、N:0.0004〜0.015%、Ti:0.03〜0.2%を含有し、残部がFeおよび不純物であるとともに、前記鋼板の金属組織がフェライトを面積率で30〜95%含有し、残部の第2相がマルテンサイト、ベイナイト、パーライト、セメンタイトおよび残留オーステナイトのうちの1種または2種以上からなり、かつマルテンサイトを含有するときのマルテンサイトの面積率は0〜50%であり、前記鋼板が粒径2〜30nmのTi系炭窒化析出物を平均粒子間距離30〜300nmで含有し、かつ粒径3μm以上の晶出系TiNを平均粒子間距離50〜500μmで含有する高張力溶融亜鉛めっき鋼板が提案されている。しかしながら、析出強化を利用しているため、強度−伸びバランスに劣るという課題を有していた。 As a technique for improving the bendability of a steel sheet, Patent Document 1 discloses that the chemical composition of the steel sheet is, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01 to 2.0%. Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.0004-0. 015%, Ti: 0.03 to 0.2%, the balance being Fe and impurities, the metallographic structure of the steel sheet contains 30 to 95% ferrite by area ratio, the remaining second phase is It consists of one or more of martensite, bainite, pearlite, cementite and retained austenite, and when martensite is contained, the area ratio of martensite is 0 to 50%, and the steel sheet has a particle size of 2 ~ 30nm Ti carbonitride precipitate average particle Distance contained in 30 to 300 nm, and high-tensile galvanized steel sheet containing particle diameter 3μm or more crystallization-based TiN with an average distance between particles 50~500μm have been proposed. However, since precipitation strengthening is used, there is a problem that the strength-elongation balance is inferior.
また、特許文献2には、曲げ性に優れる溶融亜鉛めっき鋼板として、質量%で、C:0.03〜0.11%、Si:0.005〜0.5%、Mn:2.0〜4.0%、P:0.1%以下、S:0.01%以下、sol.Al:0.01〜1.0%、N:0.01%以下を含有し、さらに、Ti:0.50%以下およびNb:0.50%以下の1種または2種をTi+Nb/2≧0.03(式中のTiおよびNbは各元素の含有量(単位:質量%)を示す)を満足する範囲で含有し、残部がFeおよび不純物からなる化学組成を有し、表面から1/20t深さ位置(t:鋼板の板厚)における圧延方向に展伸したMn濃化部の板幅方向の平均間隔が300μm以下であり、フェライトの面積率が60%以上であり、フェライトの平均粒径が1.0〜6.0μmであり、フェライト中に粒径1〜10nmの析出物を100個/μm2以上含有する鋼組織を有し、引張強度が540MPa以上である溶融亜鉛めっき鋼板が記載されている。しかしながら、主相をフェライトとし、残留オーステナイト体積分率を3%未満と制限しているため、900MPa以上の高強度鋼板への適用は難しいという課題を有していた。 In Patent Document 2, as a hot-dip galvanized steel sheet having excellent bendability, C: 0.03 to 0.11%, Si: 0.005 to 0.5%, Mn: 2.0 to 2.0% by mass 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0%, N: 0.01% or less, and Ti: 0.50% or less and Nb: 0.50% or less of one or two of Ti + Nb / 2 ≧ 0.03 (Ti and Nb in the formula indicate the content (unit: mass%) of each element), with the remainder having a chemical composition composed of Fe and impurities, The average interval in the plate width direction of the Mn-concentrated portion expanded in the rolling direction at the 20t depth position (t: plate thickness of the steel plate) is 300 μm or less, the area ratio of ferrite is 60% or more, and the average of ferrite A hot-dip galvanized steel sheet having a steel structure having a grain size of 1.0 to 6.0 μm, containing precipitates having a grain size of 1 to 10 nm in ferrite at 100 pieces / μm 2 or more, and having a tensile strength of 540 MPa or more. Have been described. However, since the main phase is ferrite and the retained austenite volume fraction is limited to less than 3%, there is a problem that it is difficult to apply to a high strength steel plate of 900 MPa or more.
また、特許文献3には、延性と曲げ性を両立させた溶融めっき鋼板として、質量%で、C:0.08〜0.25%、Si:0.7%以下、Mn:1.0〜2.6%、Al:1.5%以下、P:0.03%以下、S:0.02%以下およびN:0.01%以下を含有し、かつ、SiとAlとの関係が1.0%≦Si+Al≦1.8%を満足し、残部Feおよび不純物からなる化学組成を有し、TS≧590(TS:引張強度(MPa))、TS×El≧17500(El:全伸び(%))、およびρ≦1.5×t(ρ:限界曲げ半径(mm)、t:板厚(mm))を満たす機械特性を有する冷延鋼板の表面に亜鉛を含むめっき層を備えるものが記載されている。しかしながら、鋼板強度が900MPa未満と、更なる高強度鋼板への適用は難しかった。 Moreover, in patent document 3, as a hot-dip galvanized steel plate which made ductility and bendability compatible, by mass%, C: 0.08-0.25%, Si: 0.7% or less, Mn: 1.0- 2.6%, Al: 1.5% or less, P: 0.03% or less, S: 0.02% or less and N: 0.01% or less, and the relationship between Si and Al is 1 0.0% ≦ Si + Al ≦ 1.8%, having a chemical composition consisting of the balance Fe and impurities, TS ≧ 590 (TS: tensile strength (MPa)), TS × El ≧ 17500 (El: total elongation ( %)), And ρ ≦ 1.5 × t (ρ: critical bending radius (mm), t: plate thickness (mm)), a cold-rolled steel sheet having a plated layer containing zinc on the surface thereof Is described. However, since the steel plate strength is less than 900 MPa, it has been difficult to apply to a further high strength steel plate.
特許文献4には、良好な延性と曲げ性とを具備する冷延鋼板として、質量%で、C:05 .08〜0.20%、Si:1.0%以下、Mn:1.8〜3.0%、P:0.1%以下、S:0.01%以下、sol.Al:0.005〜0.5%、N:0.01%以下およびTi:0.02〜0.2%を含有し、残部Feおよび不純物からなる化学組成を有し、体積%で、フェライト:10%以上、ベイナイト:20〜70%、残留オーステナイト:3〜20%およびマルテンサイト:0〜20%からなるとともに、前記フェライトの平均粒径が10μm以下、前記ベイナイトの平均粒径が10μm以下、前記残留オーステナイトの平均粒径が3μm以下および前記マルテンサイトの平均粒径が3μm以下である鋼組織を有し、引張強度(TS)が780MPa以上、引張強度(TS)と全伸び(El)との積(TS×El値)が14000MPa・%以上、かつ曲げ試験における最小曲げ半径が1.5t以下(t:板厚)である機械特性を有し、板厚が2.0mm以上である冷延鋼板が記載され、冷延鋼板の表面にめっきを施すことが記載されている。しかしながら、更なる強度と曲げバランスの向上が必要とされていた。 In Patent Document 4, as a cold-rolled steel sheet having good ductility and bendability, C: 05. 08 to 0.20%, Si: 1.0% or less, Mn: 1.8 to 3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.005 to 0.5%, N: 0.01% or less, and Ti: 0.02 to 0.2%, having a chemical composition consisting of the balance Fe and impurities, and in volume%, ferrite : 10% or more, bainite: 20 to 70%, residual austenite: 3 to 20% and martensite: 0 to 20%, the ferrite has an average particle size of 10 μm or less, and the bainite has an average particle size of 10 μm or less. And having a steel structure having an average particle size of the retained austenite of 3 μm or less and an average particle size of the martensite of 3 μm or less, a tensile strength (TS) of 780 MPa or more, a tensile strength (TS) and a total elongation (El). Product (TS × El value) is 14000 MPa ·% or more, and the minimum bending radius in the bending test is 1.5 t or less (t: plate thickness), and the plate thickness is 2.0. A cold-rolled steel sheet that is at least mm is described, and it is described that the surface of the cold-rolled steel sheet is plated. However, further improvements in strength and bending balance have been required.
特許文献5には、曲げ性に優れる合金化溶融亜鉛めっき鋼板として、質量%で、C:0.03〜0.12%、Si:0.02〜0.50%、Mn:2.0〜4.0%、P:0.1%以下、S:0.01%以下、sol.Al:0.01〜1.0%およびN:0.01%以下を含有し、さらに、Ti:0.50%以下およびNb:0.50%以下の1種または2種をTi+Nb/2≧0.03を満足する範囲で含有し、残部がFeおよび不純物からなる化学組成を有するとともに、フェライトの面積率が60%以上であり、フェライトの平均粒径が1.0〜6.0μmである鋼組織を有し、合金化溶融亜鉛めっき層は、質量%で、Fe:8〜15%およびAl:0.08〜0.50%を含有し、残部がZnおよび不純物からなり、前記合金化溶融亜鉛めっき鋼板は、引張強度が540MPa以上であり、曲げ性に優れる合金化溶融亜鉛めっき鋼板が記載されている。
しかしながら、Cの添加量を0.12%以下と低い範囲に限定しているため、780MPa以下の鋼板には適用できるものの、更なる高強度鋼板への適用は難しいという課題を有していた。また、残留オーステナイトの面積率を3%未満としているため、延性が低いという課題も有していた。
In Patent Document 5, as an alloyed hot-dip galvanized steel sheet having excellent bendability, C: 0.03-0.12%, Si: 0.02-0.50%, Mn: 2.0- 4.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.01 to 1.0% and N: 0.01% or less, and Ti: N0: 0.50% or less and Nb: 0.50% or less, Ti + Nb / 2 ≧ 0.03 is contained, and the balance has a chemical composition composed of Fe and impurities, the area ratio of ferrite is 60% or more, and the average grain size of ferrite is 1.0 to 6.0 μm. The alloyed hot-dip galvanized layer has a steel structure and contains, by mass%, Fe: 8 to 15% and Al: 0.08 to 0.50%, with the balance being Zn and impurities, The hot-dip galvanized steel sheet has a tensile strength of 540 MPa or more, and an alloyed hot-dip galvanized steel sheet having excellent bendability is described.
However, since the amount of addition of C is limited to a low range of 0.12% or less, although it can be applied to a steel plate of 780 MPa or less, it has a problem that it is difficult to apply to a further high strength steel plate. In addition, since the area ratio of retained austenite is less than 3%, there is a problem that ductility is low.
特許文献6には、加工性に優れた高強度溶融亜鉛めっき鋼板として、質量%で、C:0.03〜0.17%、Si:0.01〜0.75%、Mn:1.5〜2.5%、P:0.080%以下、S:0.010%以下、sol.Al:0.01〜1.20%、Cr:0.3〜1.3%を含有し、残部がFeおよび不可避不純物からなり、鋼組織が、体積率で30〜70%のフェライト、3%未満の残留オーステナイト、および残部のマルテンサイトからなり、マルテンサイトのうちの20%以上が焼戻しマルテンサイトである下地鋼板上に溶融亜鉛めっき層を有するものが記載されている。しかしながら、残留オーステナイトの体積率を3%未満と制限しているため、曲げ性に優れるものの均一伸びが低いという課題を有していた。その結果、曲げ加工においても、厚い板を曲げ加工する場合、鋼板表面にネッキングに起因した割れが生じる懸念がある。 In Patent Document 6, as a high-strength hot-dip galvanized steel sheet having excellent workability, C: 0.03 to 0.17%, Si: 0.01 to 0.75%, Mn: 1.5 -2.5%, P: 0.080% or less, S: 0.010% or less, sol. Al: 0.01 to 1.20%, Cr: 0.3 to 1.3%, the balance is Fe and inevitable impurities, the steel structure is 30 to 70% ferrite by volume, 3% There is a description of a steel sheet having a hot dip galvanized layer on a base steel sheet that is composed of less retained austenite and the remaining martensite, and 20% or more of the martensite is tempered martensite. However, since the volume ratio of retained austenite is limited to less than 3%, it has a problem that it has excellent bendability but low uniform elongation. As a result, also in bending, when a thick plate is bent, there is a concern that cracks due to necking may occur on the surface of the steel plate.
特許文献7には、曲げ加工性に優れた超高強度冷延鋼板として、wt%で、C:0.12〜0.30%、Si:1.2%以下、Mn:1〜3%、P:0.020%以下、S:0.010%以下、sol.Al:0.01〜0.06%を含有し、残部がFeおよび不可避不純物よりなる鋼であって、表層部にC:0.1wt%以下の軟質層を片面で3〜15vol%両面に有し、残部が10vol%未満の残留オーステナイトと低温変態相あるいはさらにフェライトとの複合組織からなるものが記載されている。しかしながら、鋼板表層の軟質層を形成するために、熱間圧延後と冷間圧延後に合計2回の脱炭焼鈍を行わねばならず製造性に劣るという課題を有していた。 In Patent Document 7, as an ultra-high-strength cold-rolled steel sheet having excellent bending workability, wt%, C: 0.12 to 0.30%, Si: 1.2% or less, Mn: 1 to 3%, P: 0.020% or less, S: 0.010% or less, sol. Al: 0.01 to 0.06% steel, the balance being Fe and inevitable impurities, C: 0.1 wt% or less soft layer on one side and 3 to 15 vol% on both sides In addition, it is described that the balance is a composite structure of residual austenite with a content of less than 10 vol% and a low-temperature transformation phase or further ferrite. However, in order to form the soft layer of the steel sheet surface layer, the decarburization annealing must be performed twice in total after hot rolling and after cold rolling, resulting in poor productivity.
以上のように、従来の技術では、特に高強度鋼板に曲げ加工を行った場合に、十分な曲げ性が得られないため、より一層曲げ性を向上させることが要求されている。このような現状に鑑み、本発明は、優れた曲げ性を有する高強度鋼板およびその製造方法を提供するものである。 As described above, according to the conventional technique, particularly when a high-strength steel sheet is bent, sufficient bendability cannot be obtained. Therefore, it is required to further improve bendability. In view of such a current situation, the present invention provides a high-strength steel sheet having excellent bendability and a method for producing the same.
本発明者らは、曲げ加工を行うことにより変形部に発生する鋼板内部の亀裂、鋼板表面のネッキングを防止することにより、優れた曲げ性の得られる引張最大強度900MPa以上の高強度鋼板を得るために鋭意検討を重ねた。
その結果、本発明者らは、所定の化学成分を有し、鋼板組織を所定の組織へと制御した上で、脱炭処理を施すことで鋼板表層を軟化することが出来、引張最大強度900MPa以上高強度鋼板であっても、あたかも、低強度の鋼板であるかのような優れた曲げ性を得ることが出来ることを明らかにした。この効果は、鋼板表層の硬度と1/4厚の硬度の比「(表層の硬度)/(1/4厚の硬度)」が0.35〜0.90とすることで得られる。
The present inventors obtain a high-strength steel sheet having a tensile maximum strength of 900 MPa or more that can obtain excellent bendability by preventing cracks in the steel sheet generated in the deformed portion by bending and necking of the steel sheet surface. In order to achieve this goal, we made extensive studies.
As a result, the present inventors have a predetermined chemical composition, can control the steel sheet structure to a predetermined structure, and can soften the steel sheet surface layer by performing a decarburization process, and have a maximum tensile strength of 900 MPa. As described above, it was clarified that even a high-strength steel plate can obtain excellent bendability as if it were a low-strength steel plate. This effect is obtained when the ratio of the hardness of the steel sheet surface layer to the hardness of 1/4 thickness “(surface layer hardness) / (1/4 thickness hardness)” is 0.35 to 0.90.
加えて、鋼板組織が、体積分率で3〜20%の残留オーステナイトを含有することで、ネッキング起因の割れも抑制でき、更なる曲げ性の向上が得られる。特に、曲げ加工は、表層ほど歪が大きくなることから、表層と鋼板内部の硬さを表記範囲内にすることで大きな曲げ性の改善効果が得られる。
本発明は、かかる知見に基づいて完成させたものであり、その要旨とするところは以下の通りである。
In addition, when the steel sheet structure contains 3-20% retained austenite in volume fraction, cracking due to necking can be suppressed, and further improvement in bendability can be obtained. In particular, since bending becomes larger as the surface layer is bent, a large bendability improvement effect can be obtained by setting the hardness of the surface layer and the inside of the steel sheet within the notation range.
The present invention has been completed on the basis of such knowledge, and the gist thereof is as follows.
(1)質量%で、
C:0.075〜0.300%、
Si:0.30〜2.50%、
Mn:1.30〜3.50%、
P:0.001〜0.050%、
S:0.0001〜0.0100%、
Al:0.001〜1.500%、
N:0.0001〜0.0100%、
O:0.0001〜0.0100%
を含有し、残部が鉄および不可避的不純物からなる引張最大強度900MPa以上の高強度鋼板であり、その組織が、体積分率で、主相としてベイナイトを50%以上含有し、3〜30%の残留オーステナイトを含有するものであり、鋼板表層の硬度(Hvs)と鋼板の1/4厚の硬度(Hvb)の比(Hvs/Hvb)が0.35〜0.90であることを特徴とする曲げ性に優れた高強度冷延鋼板。
(1) In mass%,
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0100%,
Al: 0.001-1.500%,
N: 0.0001 to 0.0100%,
O: 0.0001 to 0.0100%
Is a high-strength steel plate having a tensile maximum strength of 900 MPa or more consisting of iron and inevitable impurities, and the structure contains 50% or more of bainite as a main phase in a volume fraction of 3 to 30%. Residual austenite is contained, and the ratio (Hvs / Hvb) of the hardness (Hvs) of the steel sheet surface layer to the 1/4 thickness (Hvb) of the steel sheet is 0.35 to 0.90. High-strength cold-rolled steel sheet with excellent bendability.
(2)前記鋼板組織が、フレッシュマルテンサイトを15%以下に制限されるものであることを特徴とする前記(1)に記載の曲げ性に優れた高強度冷延鋼板。
(3)前記母材鋼板がさらに、質量%で、Ti:0.005〜0.150%、Nb:0.005〜0.150%、V:0.005〜0.150%の1種または2種以上を含有することを特徴とする前記(1)または(2)に記載の曲げ性に優れた高強度冷延鋼板。
(4)前記母材鋼板がさらに、質量%で、B:0.0001〜0.0100%、Cr:0.01〜2.00%、Ni:0.01〜2.00%、Cu:0.01〜2.00%、Mo:0.01〜1.00%、W:0.01〜1.00%の1種または2種以上を含有することを特徴とする前記(1)〜(3)のいずれかに記載の曲げ性に優れた高強度冷延鋼板。
(5)前記母材鋼板がさらに、Ca、Ce、Mg、Zr,Hf、REMの1種または2種以上を合計で0.0001〜0.5000質量%含有することを特徴とする前記(1)〜(4)のいずれかに記載の曲げ性に優れた高強度冷延鋼板。
(2) The high-strength cold-rolled steel sheet with excellent bendability according to (1), wherein the steel sheet structure is such that fresh martensite is limited to 15% or less.
(3) The base steel plate is further in mass%, Ti: 0.005 to 0.150%, Nb: 0.005 to 0.150%, V: 0.005 to 0.150%, or one type The high-strength cold-rolled steel sheet having excellent bendability as described in (1) or (2) above, comprising two or more kinds.
(4) The base material steel plate is further mass%, B: 0.0001 to 0.0100%, Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0. .01-2.00%, Mo: 0.01-1.00%, W: 0.01-1.00% 1 type or 2 types or more are contained, (1)-( A high-strength cold-rolled steel sheet excellent in bendability as described in any one of 3).
(5) The base steel plate further contains 0.0001 to 0.5000 mass% of one or more of Ca, Ce, Mg, Zr, Hf, and REM in total (1 A high-strength cold-rolled steel sheet excellent in bendability according to any one of (1) to (4).
(6)前記(1)〜(5)いずれかに記載の冷延鋼板の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜が形成されていることを特徴とする曲げ性に優れた高強度冷延鋼板。
(7)前記(1)〜(5)いずれかに記載の冷延鋼板の表面に、電気亜鉛めっき層を有することを特徴とする曲げ性に優れた高強度亜鉛めっき鋼板。
(8)前記(1)〜(5)いずれかに記載の冷延鋼板の表面に、溶融亜鉛めっき層を有することを特徴とする曲げ性に優れた高強度亜鉛めっき鋼板。
(9)前記(1)〜(5)いずれかに記載の冷延鋼板の表面に、合金化溶融亜鉛めっき層を有することを特徴とする曲げ性に優れた高強度亜鉛めっき鋼板。
(10)前記亜鉛めっき層の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜が形成されていることを特徴とする前記(7)〜(9)いずれかに記載の曲げ性に優れた高強度亜鉛めっき鋼板。
(6) A bendability characterized in that a film composed of phosphorus oxide and / or composite oxide containing phosphorus is formed on the surface of the cold-rolled steel sheet according to any one of (1) to (5). High strength cold-rolled steel sheet with excellent resistance.
(7) A high-strength galvanized steel sheet excellent in bendability characterized by having an electrogalvanized layer on the surface of the cold rolled steel sheet according to any one of (1) to (5).
(8) A high-strength galvanized steel sheet excellent in bendability characterized by having a hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of (1) to (5).
(9) A high-strength galvanized steel sheet excellent in bendability characterized by having an alloyed hot-dip galvanized layer on the surface of the cold-rolled steel sheet according to any one of (1) to (5).
(10) The bending according to any one of (7) to (9), wherein a film made of a phosphorus oxide and / or a composite oxide containing phosphorus is formed on the surface of the galvanized layer. High strength galvanized steel sheet with excellent properties.
(11)前記(1)、(3)〜(5)いずれかに記載の化学成分を有するスラブを1050℃以上に加熱し、Ar3変態点以上の仕上げ熱延温度で熱間圧延を完了し、750℃以下の温度域にて巻き取る熱間圧延後に、30〜80%の圧下率で冷間圧延した後、(Ac3変態点−20)℃〜(Ac3変態点+100)℃の温度域で、かつlog(水分圧/水素分圧)が−3.0〜0.0の雰囲気で20秒〜600秒焼鈍を行った後、700〜500℃間を0.5〜500℃/秒の冷却速度にて、300〜500℃に冷却し、続いて300〜500℃の温度範囲で10〜1000秒間の保持を行うことを特徴とする前記(1)〜(5)いずれかに記載の曲げ性に優れた高強度冷延鋼板の製造方法。 (11) The slab having the chemical component according to any one of (1) and (3) to (5) is heated to 1050 ° C. or higher, and hot rolling is completed at a finish hot rolling temperature not lower than the Ar3 transformation point, After hot rolling to be wound in a temperature range of 750 ° C. or lower, after cold rolling at a reduction rate of 30 to 80%, in a temperature range of (Ac3 transformation point−20) ° C. to (Ac3 transformation point + 100) ° C., And after performing annealing for 20 seconds to 600 seconds in an atmosphere having a log (moisture pressure / hydrogen partial pressure) of −3.0 to 0.0, a cooling rate of 0.5 to 500 ° C./second is applied between 700 to 500 ° C. The bendability according to any one of (1) to (5) above, wherein the bendability is cooled to 300 to 500 ° C. and subsequently held for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. An excellent method for producing high-strength cold-rolled steel sheets.
(12)前記(11)に記載の製造方法で製造した冷延鋼板の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜を付与する工程を行うことを特徴とする前記(6)に記載の曲げ性に優れた高強度冷延鋼板の製造方法。 (12) The step of applying a film made of a complex oxide containing phosphorus oxide and / or phosphorus to the surface of the cold-rolled steel sheet produced by the production method according to (11) above ( The manufacturing method of the high intensity | strength cold-rolled steel plate excellent in the bendability as described in 6).
(13)前記(11)に記載の製造方法で製造した冷延鋼板の表面に電気亜鉛めっきを施すことを特徴とする前記(7)に記載の曲げ性に優れた高強度亜鉛めっき鋼板の製造方法。
(14)前記(11)に記載の製造方法にしたがってスラブを熱間圧延し、冷間圧延し、焼鈍した後の冷却に続いて300〜500℃の温度範囲で、10〜1000秒間の保持を行った後、さらに、(亜鉛めっき浴温度―40)℃〜(亜鉛めっき浴温度+50)℃に加熱あるいは、冷却した後、亜鉛めっき浴に浸漬し、冷却することを特徴とする請求項8に記載の曲げ性に優れた高強度亜鉛めっき鋼板の製造方法。
(15)前記(11)に記載の製造方法にしたがってスラブを熱間圧延し、冷間圧延し、焼鈍した後の冷却に続いて300〜500℃の温度範囲で、10〜1000秒間の保持を行った後、さらに、(亜鉛めっき浴温度―40)℃〜(亜鉛めっき浴温度+50)℃に加熱あるいは、冷却した後、亜鉛めっき浴に浸漬し、さらに460℃以上の温度で合金化処理を施した後、冷却することを特徴とする請求項9に記載の曲げ性に優れた高強度亜鉛めっき鋼板の製造方法。
(13) Manufacture of a high strength galvanized steel sheet having excellent bendability as described in (7) above, wherein the surface of the cold rolled steel sheet manufactured by the manufacturing method as described in (11) is subjected to electrogalvanization. Method.
(14) The slab is hot-rolled in accordance with the manufacturing method described in (11), cold-rolled, and then cooled after being annealed, and maintained for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. 9. The method according to claim 8, further comprising heating or cooling to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C. and then immersing in the zinc plating bath and cooling. The manufacturing method of the high intensity | strength galvanized steel plate excellent in the bendability of description.
(15) The slab is hot-rolled according to the production method described in (11), cold-rolled, and then cooled after annealing, and then held for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. Then, after heating or cooling to (zinc plating bath temperature −40) ° C. to (zinc plating bath temperature +50) ° C., it is immersed in a zinc plating bath and further alloyed at a temperature of 460 ° C. or higher. The method for producing a high-strength galvanized steel sheet having excellent bendability according to claim 9, wherein the steel sheet is cooled after being applied.
(16)前記(14)または(15)に記載の製造方法で製造した亜鉛めっき鋼板の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜を付与する工程を行うことを特徴とする前記(10)に記載の曲げ性に優れた高強度亜鉛めっき鋼板の製造方法。 (16) A step of applying a film made of a composite oxide containing phosphorus oxide and / or phosphorus to the surface of the galvanized steel sheet manufactured by the manufacturing method according to (14) or (15) is performed. The manufacturing method of the high intensity | strength galvanized steel plate excellent in the bendability as described in said (10).
本発明によれば、優れた曲げ性を有する引張最大強度900MPa以上の高強度冷延鋼板または亜鉛めっき鋼板、およびそれらの製造方法を提供できる。 According to the present invention, it is possible to provide a high-strength cold-rolled steel sheet or galvanized steel sheet having excellent bendability and a tensile maximum strength of 900 MPa or more, and a method for producing them.
本発明は、鋼板の化学成分を特定のものとし、かつ鋼板組織を特定の組織へと制御した上で、脱炭処理を施して鋼板表層を軟化することで、引張最大強度が900MPa以上とした上で、さらに曲げ性に優れた高強度鋼板としたものである。 In the present invention, the chemical composition of the steel sheet is specified, and the steel sheet structure is controlled to a specific structure, and then the decarburization treatment is performed to soften the steel sheet surface layer so that the maximum tensile strength is 900 MPa or more. Above, it is set as the high strength steel plate which was further excellent in bendability.
本発明で曲げ性に優れた鋼板とは、90度V曲げ試験にて、最小限界曲げ半径が1.0R以下のものを曲げ性に優れた鋼板と定義した。なお、本発明の鋼板は、曲げ加工時のネッキング抑制のために用いている残留オーステナイトは、引張試験やプレス加工時にもネッキング抑制を齎すことから、本発明の鋼は、伸びも良好であった。 In the present invention, the steel sheet excellent in bendability was defined as a steel sheet excellent in bendability in the 90-degree V-bending test having a minimum limit bending radius of 1.0 R or less. In addition, the steel sheet of the present invention has good elongation because the retained austenite used for the suppression of necking at the time of bending works to suppress necking at the time of tensile tests and press processing. .
(鋼の化学組成)
まず、本発明の冷延鋼板または亜鉛めっき鋼板を構成する鋼の化学成分(組成)について説明する。なお、以下の説明における%は、質量%を表す。
(Chemical composition of steel)
First, the chemical composition (composition) of steel constituting the cold-rolled steel sheet or galvanized steel sheet of the present invention will be described. In addition,% in the following description represents the mass%.
「C:0.075〜0.300%」
Cは、母材鋼板の強度を高めるために含有される。しかし、Cの含有量が0.300%を超えると溶接性が不十分となる。溶接性の観点から、Cの含有量は0.280%以下であることが好ましく、0.260%以下であることがより好ましい。一方、Cの含有量が0.075%未満であると強度が低下し、900MPa以上の引張最大強度を確保することが出来ない。強度を高めるため、Cの含有量は0.090%以上であることが好ましく、0.100%以上であることがより好ましい。
"C: 0.075-0.300%"
C is contained in order to increase the strength of the base steel sheet. However, if the C content exceeds 0.300%, weldability becomes insufficient. From the viewpoint of weldability, the C content is preferably 0.280% or less, and more preferably 0.260% or less. On the other hand, if the C content is less than 0.075%, the strength is lowered, and the maximum tensile strength of 900 MPa or more cannot be ensured. In order to increase the strength, the C content is preferably 0.090% or more, and more preferably 0.100% or more.
「Si:0.30〜2.50%」
Siは、脱炭反応を促進させ、鋼板表層の軟化を招くことから最も重要な添加元素である。このことから、0.30以上添加する必要がある。しかし、Siの含有量が2.50%を超えると母材鋼板が脆化し、延性が劣化する。延性の観点から、Siの含有量は2.20%以下であることが好ましく、2.00%以下であることがより好ましい。一方、Siの含有量が0.30%未満では粗大な鉄系炭化物が多量に生成し、残留オーステナイトを3〜30%とすることが出来ず、伸びが低下してしまう。この観点から、Siの下限値は0.50%以上であることが好ましく、0.70%以上がより好ましい。加えて、Siは、母材鋼板における鉄系炭化物の粗大化を抑制し、強度と成形性を高めるために必要な元素である。また、固溶強化元素として、鋼板の高強度化に寄与するため添加する必要がある。
“Si: 0.30 to 2.50%”
Si is the most important additive element because it promotes the decarburization reaction and softens the surface layer of the steel sheet. For this reason, it is necessary to add 0.30 or more. However, if the Si content exceeds 2.50%, the base steel sheet becomes brittle and the ductility deteriorates. From the viewpoint of ductility, the Si content is preferably 2.20% or less, and more preferably 2.00% or less. On the other hand, if the Si content is less than 0.30%, a large amount of coarse iron-based carbides are generated, the residual austenite cannot be made 3 to 30%, and the elongation is lowered. In this respect, the lower limit value of Si is preferably 0.50% or more, and more preferably 0.70% or more. In addition, Si is an element necessary for suppressing the coarsening of iron-based carbides in the base steel sheet and increasing the strength and formability. Moreover, it is necessary to add as a solid solution strengthening element in order to contribute to the strengthening of a steel plate.
「Mn:1.30〜3.50%」
Mnは、母材鋼板の強度を高めるために含有される。しかし、Mnの含有量が3.50%を超えると母材鋼板の板厚中央部に粗大なMn濃化部が生じ、脆化が起こりやすくなり、鋳造したスラブが割れるなどのトラブルが起こりやすい。また、Mnの含有量が3.50%を超えると溶接性も劣化する。したがって、Mnの含有量は、3.50%以下とする必要がある。溶接性の観点から、Mnの含有量は3.20%以下であることが好ましく、3.00%以下であることがより好ましい。一方、Mnの含有量が1.30%未満であると、焼鈍後の冷却中に軟質な組織が多量に形成されるため、900MPa以上の引張最大強度を確保することが難しくなる。このことから、Mnの含有量を1.30%以上とする必要がある。Mnの含有量は、さらに強度を高めるために、1.50%以上であることが好ましく、1.70%以上であることがより好ましい。
“Mn: 1.30 to 3.50%”
Mn is contained to increase the strength of the base steel sheet. However, if the Mn content exceeds 3.50%, a coarse Mn-concentrated portion is generated at the center of the thickness of the base steel sheet, and brittleness is likely to occur, and troubles such as cracking of the cast slab are likely to occur. . Further, when the Mn content exceeds 3.50%, the weldability is also deteriorated. Therefore, the Mn content needs to be 3.50% or less. From the viewpoint of weldability, the Mn content is preferably 3.20% or less, and more preferably 3.00% or less. On the other hand, when the content of Mn is less than 1.30%, a large amount of soft structure is formed during cooling after annealing, so that it is difficult to secure a maximum tensile strength of 900 MPa or more. For this reason, the Mn content needs to be 1.30% or more. In order to further increase the strength, the Mn content is preferably 1.50% or more, and more preferably 1.70% or more.
「P:0.001〜0.050%」
Pは母材鋼板の板厚中央部に偏析する傾向があり、溶接部を脆化させる。Pの含有量が0.050%を超えると溶接部が大幅に脆化するため、Pの含有量を0.050%以下に限定した。Pの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Pの含有量を0.001%未満とすることは製造コストの大幅な増加を伴うことから、0.001%を下限値とする。
“P: 0.001 to 0.050%”
P tends to segregate in the central part of the thickness of the base steel sheet, and causes the weld to become brittle. When the P content exceeds 0.050%, the welded portion is significantly embrittled, so the P content is limited to 0.050% or less. Although the lower limit of the content of P is not particularly defined, the effect of the present invention is exhibited. However, since the content of P is less than 0.001% is accompanied by a significant increase in production cost, 0.001 % Is the lower limit.
「S:0.0001〜0.0100%」
Sは、溶接性ならびに鋳造時および熱延時の製造性に悪影響を及ぼす。このことから、Sの含有量の上限値を0.0100%以下とした。また、SはMnと結びついて粗大なMnSを形成して延性や伸びフランジ性を低下させるため、0.0050%以下とすることが好ましく、0.0025%以下とすることがより好ましい。Sの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Sの含有量を0.0001%未満とすることは製造コストの大幅な増加を伴うため、0.0001%を下限値とする。
“S: 0.0001 to 0.0100%”
S adversely affects weldability and manufacturability during casting and hot rolling. Therefore, the upper limit value of the S content is set to 0.0100% or less. Further, since S is combined with Mn to form coarse MnS to lower the ductility and stretch flangeability, it is preferably 0.0050% or less, and more preferably 0.0025% or less. The lower limit of the content of S is not particularly defined, and the effect of the present invention is exhibited. However, if the content of S is less than 0.0001%, a significant increase in production cost is caused, so 0.0001% Is the lower limit.
「Al:0.001〜1.500%」
Alは、脱炭反応を促進させ、鋼板表層の軟化を招くことから重要な添加元素である。このことから、添加しても良い。しかし、Alの含有量が1.500%を超えると溶接性が悪化するため、Alの含有量の上限を1.500%とする。この観点から、Alの含有量は1.200%以下とすることが好ましく、0.900%以下とすることがより好ましい。また、Alは脱酸材としても有効な元素であるが、Alの含有量が0.001%未満では脱酸材としての効果が十分に得られないことから、Alの含有量の下限を0.001%以上とする。脱酸の効果を十分に得るにはAl量は0.003%以上とすることが好ましい。
"Al: 0.001-1.500%"
Al is an important additive element because it accelerates the decarburization reaction and softens the steel sheet surface layer. Therefore, it may be added. However, if the Al content exceeds 1.500%, weldability deteriorates, so the upper limit of the Al content is set to 1.500%. From this viewpoint, the Al content is preferably 1.200% or less, and more preferably 0.900% or less. Further, Al is an element effective as a deoxidizing material, but if the Al content is less than 0.001%, the effect as the deoxidizing material cannot be obtained sufficiently, so the lower limit of the Al content is 0. 0.001% or more. In order to obtain a sufficient deoxidation effect, the Al content is preferably 0.003% or more.
「N:0.0001〜0.0100%」
Nは、粗大な窒化物を形成し、延性および伸びフランジ性を劣化させることから、添加量を抑える必要がある。Nの含有量が0.0100%を超えると、この傾向が顕著となることから、N含有量の範囲を0.0100%以下とした。また、Nは、溶接時のブローホール発生の原因になることから少ない方が良い。Nの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Nの含有量を0.0001%未満にすると、製造コストの大幅な増加を招くことから、0.0001%を下限値とする。
“N: 0.0001 to 0.0100%”
N forms coarse nitrides and degrades ductility and stretch flangeability, so it is necessary to suppress the addition amount. When the N content exceeds 0.0100%, this tendency becomes remarkable, so the N content range is set to 0.0100% or less. Further, N is better because it causes blowholes during welding. The lower limit of the content of N is not particularly defined, and the effect of the present invention is exhibited. However, if the content of N is less than 0.0001%, a significant increase in manufacturing cost is caused, so 0.0001% Is the lower limit.
「O:0.0001〜0.0100%」
Oは、酸化物を形成し、延性および伸びフランジ性を劣化させることから、含有量を抑える必要がある。Oの含有量が0.0100%を超えると、伸びフランジ性の劣化が顕著となることから、O含有量の上限を0.0100%以下とした。Oの含有量は0.0080%以下であることが好ましく0.0060%以下であることがさらに好ましい。Oの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Oの含有量を0.0001%未満とすることは製造コストの大幅な増加を伴うため、0.0001%を下限とした。
“O: 0.0001 to 0.0100%”
O forms an oxide and deteriorates ductility and stretch flangeability, so the content needs to be suppressed. When the O content exceeds 0.0100%, the stretch flangeability deteriorates significantly, so the upper limit of the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0060% or less. Although the lower limit of the content of O is not particularly defined, the effects of the present invention are exhibited. However, if the content of O is less than 0.0001%, a significant increase in manufacturing cost is caused, so 0.0001% Was the lower limit.
本発明の高強度冷延鋼板または高強度亜鉛めっき鋼板の母材鋼板は、以上の元素を含有し、残部が鉄及び不可避的不純物よりなる組成を基本とするが、鋼板は更に、必要に応じて、以下に示す元素を含んでいてもよい。 The base steel plate of the high-strength cold-rolled steel plate or high-strength galvanized steel plate of the present invention contains the above elements, and the balance is basically composed of iron and unavoidable impurities. In addition, the following elements may be included.
「Ti:0.005〜0.150%」
Tiは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Tiの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Tiの含有量は0.150%以下であることが好ましい。成形性の観点から、Tiの含有量は0.120%以下であることがより好ましく、0.100%以下であることがさらに好ましい。Tiの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Tiによる強度上昇効果を十分に得るにはTiの含有量は0.005%以上であることが好ましい。母材鋼板の高強度化には、Tiの含有量は0.010%以上であることがより好ましく、0.015%以上であることがさらに好ましい。
“Ti: 0.005 to 0.150%”
Ti is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the Ti content exceeds 0.150%, precipitation of carbonitrides increases and the formability deteriorates, so the Ti content is preferably 0.150% or less. From the viewpoint of moldability, the Ti content is more preferably 0.120% or less, and further preferably 0.100% or less. The lower limit of the Ti content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the strength increasing effect by Ti, the Ti content is preferably 0.005% or more. In order to increase the strength of the base steel sheet, the Ti content is more preferably 0.010% or more, and further preferably 0.015% or more.
「Nb:0.005〜0.150%」
Nbは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Nbの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Nbの含有量は0.150%以下であることが好ましい。成形性の観点から、Nbの含有量は0.120%以下であることがより好ましく、0.100%以下であることがさらに好ましい。Nbの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Nbによる強度上昇効果を十分に得るにはNbの含有量は0.005%以上であることが好ましい。母材鋼板の高強度化には、Nbの含有量は0.010%以上であることがより好ましく、0.015%以上であることがさらに好ましい。
“Nb: 0.005 to 0.150%”
Nb is an element that contributes to an increase in the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization. However, if the Nb content exceeds 0.150%, carbonitride precipitation increases and the formability deteriorates, so the Nb content is preferably 0.150% or less. From the viewpoint of moldability, the Nb content is more preferably 0.120% or less, and further preferably 0.100% or less. The lower limit of the Nb content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Nb, the Nb content is preferably 0.005% or more. In order to increase the strength of the base steel sheet, the Nb content is more preferably 0.010% or more, and further preferably 0.015% or more.
「V:0.005〜0.150%」
Vは、析出物強化、フェライト結晶粒の成長抑制による細粒強化および再結晶の抑制を通じた転位強化にて、母材鋼板の強度上昇に寄与する元素である。しかし、Vの含有量が0.150%を超えると、炭窒化物の析出が多くなり成形性が劣化するため、Vの含有量は0.150%以下であることが好ましい。Vの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Vによる強度上昇効果を十分に得るにはVの含有量は0.005%以上であることが好ましい。
"V: 0.005-0.150%"
V is an element that contributes to increasing the strength of the base steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and dislocation strengthening by suppressing recrystallization. However, if the V content exceeds 0.150%, carbonitride precipitation increases and the formability deteriorates, so the V content is preferably 0.150% or less. The lower limit of the content of V is not particularly limited, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by V, the content of V is preferably 0.005% or more.
「B:0.0001〜0.0100%」
Bは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Bの含有量が0.0100%を超えると、熱間での加工性が損なわれ、生産性が低下する。このことから、Bの含有量は0.0100%以下であることが好ましい。生産性の観点から、Bの含有量は0.0050%以下であることがより好ましく、0.0030%以下であることがさらに好ましい。Bの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Bによる高強度化の効果を十分に得るには、Bの含有量を0.0001%以上とすることが好ましい。高強度化には、Bの含有量が0.0003%以上であることがより好ましく、0.0005%以上であることがより好ましい。
“B: 0.0001 to 0.0100%”
B is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. When the content of B exceeds 0.0100%, hot workability is impaired and productivity is lowered. Therefore, the B content is preferably 0.0100% or less. From the viewpoint of productivity, the B content is more preferably 0.0050% or less, and further preferably 0.0030% or less. The lower limit of the content of B is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by B, the content of B should be 0.0001% or more. preferable. For increasing the strength, the B content is more preferably 0.0003% or more, and more preferably 0.0005% or more.
「Cr:0.01〜2.00%」
Crは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Crの含有量が2.00%を超えると、熱間での加工性が損なわれ、生産性が低下することから、Crの含有量は2.00%以下であることが好ましい。Crの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Crによる高強度化の効果を十分に得るには、Crの含有量は0.01%以上であることが好ましい。
"Cr: 0.01-2.00%"
Cr is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Cr content exceeds 2.00%, hot workability is impaired and productivity is lowered. Therefore, the Cr content is preferably 2.00% or less. Although the lower limit of the Cr content is not particularly defined, the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cr, the Cr content may be 0.01% or more. preferable.
「Ni:0.01〜2.00%」
Niは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Niの含有量が2.00%を超えると、溶接性が損なわれることから、Niの含有量は2.00%以下であることが好ましい。Niの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Niによる高強度化の効果を十分に得るには、Niの含有量は0.01%以上であることが好ましい。
「Cu:0.01〜2.00%」
Cuは微細な粒子として鋼中に存在することで強度を高める元素であり、Cおよび/またはMnの一部に替えて添加することができる。Cuの含有量が2.00%を超えると、溶接性が損なわれることから、Cuの含有量は2.00%以下であることが好ましい。Cuの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Cuによる高強度化の効果を十分に得るには、Cuの含有量は0.01%以上であることが好ましい。
"Ni: 0.01-2.00%"
Ni is an element that suppresses phase transformation at high temperature and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Ni content exceeds 2.00%, weldability is impaired, so the Ni content is preferably 2.00% or less. The lower limit of the Ni content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Ni, the Ni content should be 0.01% or more. preferable.
"Cu: 0.01-2.00%"
Cu is an element that increases the strength by being present in the steel as fine particles, and can be added instead of a part of C and / or Mn. If the Cu content exceeds 2.00%, weldability is impaired, so the Cu content is preferably 2.00% or less. The lower limit of the Cu content is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Cu, the Cu content should be 0.01% or more. preferable.
「Mo:0.01〜1.00%」
Moは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Moの含有量が1.00%を超えると、熱間での加工性が損なわれ、生産性が低下する。このことから、Moの含有量は1.00%以下であることが好ましい。Moの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Moによる高強度化の効果を十分に得るには、Moの含有量は0.01%以上であることが好ましい。
"Mo: 0.01-1.00%"
Mo is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the Mo content exceeds 1.00%, hot workability is impaired and productivity is lowered. For this reason, the Mo content is preferably 1.00% or less. The lower limit of the content of Mo is not particularly defined, and the effect of the present invention is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by Mo, the content of Mo is 0.01% or more. preferable.
「W:0.01〜1.00%」
Wは高温での相変態を抑制し、高強度化に有効な元素であり、Cおよび/またはMnの一部に代えて添加してもよい。Wの含有量が1.00%を超えると、熱間での加工性が損なわれ、生産性が低下することから、Wの含有量は1.00%以下であることが好ましい。Wの含有量の下限は、特に定めることなく本発明の効果は発揮されるが、Wによる高強度化の効果を十分に得るには、Wの含有量は0.01%以上であることが好ましい。
"W: 0.01-1.00%"
W is an element that suppresses phase transformation at high temperatures and is effective for increasing the strength, and may be added instead of a part of C and / or Mn. If the W content exceeds 1.00%, hot workability is impaired and productivity is lowered. Therefore, the W content is preferably 1.00% or less. The lower limit of the W content is not particularly defined, and the effects of the present invention are exhibited. However, in order to sufficiently obtain the effect of increasing the strength by W, the W content may be 0.01% or more. preferable.
「Ca、Ce、Mg、Zr,Hf、REMの1種または2種以上を合計で0.0001〜0.5000%」
Ca、Ce、Mg、Zr,Hf、REMは、成形性の改善に有効な元素であり、1種又は2種以上を添加することができる。しかし、Ca、Ce、Mg、Zr、Hf、REMの1種または2種以上の含有量の合計が0.5000%を超えると、却って延性を損なう恐れがある。このため、各元素の含有量の合計は0.5000%以下であることが好ましい。Ca、Ce、Mg、Zr,Hf、REMの1種または2種以上の含有量の下限は、特に定めることなく本発明の効果は発揮されるが、母材鋼板の成形性を改善する効果を十分に得るには、各元素の含有量の合計が0.0001%以上であることが好ましい。成形性の観点から、Ca、Ce、Mg、Zr,Hf、REMの1種または2種以上の含有量の合計が0.0005%以上であることがより好ましく、0.0010%以上であることがさらに好ましい。
なお、REMとは、Rare Earth Metalの略であり、ランタノイド系列に属する元素をさす。本発明において、REMやCeはミッシュメタルにて添加されることが多く、LaやCeの他にランタノイド系列の元素を複合で含有する場合がある。不可避不純物として、これらLaやCe以外のランタノイド系列の元素を含んだとしても本発明の効果は発揮される。また、金属LaやCeを添加したとしても本発明の効果は発揮される。
“Totally 0.0001 to 0.5000% of one or more of Ca, Ce, Mg, Zr, Hf, and REM”
Ca, Ce, Mg, Zr, Hf, and REM are effective elements for improving formability, and one or more of them can be added. However, if the total content of one or more of Ca, Ce, Mg, Zr, Hf, and REM exceeds 0.5000%, the ductility may be impaired. For this reason, the total content of each element is preferably 0.5000% or less. The lower limit of the content of one or more of Ca, Ce, Mg, Zr, Hf, and REM is not particularly defined and the effect of the present invention is exhibited, but the effect of improving the formability of the base steel sheet In order to obtain it sufficiently, the total content of the elements is preferably 0.0001% or more. From the viewpoint of formability, the total content of one or more of Ca, Ce, Mg, Zr, Hf, and REM is more preferably 0.0005% or more, and 0.0010% or more. Is more preferable.
Note that REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanoid series. In the present invention, REM and Ce are often added by misch metal and may contain a lanthanoid series element in combination with La and Ce. Even if these lanthanoid series elements other than La and Ce are included as inevitable impurities, the effect of the present invention is exhibited. Even if the metal La or Ce is added, the effect of the present invention is exhibited.
(鋼板組織)
次に、鋼板組織を規定した理由について説明する。
(Steel sheet structure)
Next, the reason for defining the steel sheet structure will be described.
「ベイナイト:50%以上」
ベイナイトは、強度と曲げ性のバランスに優れた組織であり、900MPa以上の引張最大強度を確保するためには、主相とするベイナイトの体積分率を50%以上とする必要がある。体積分率が50%未満では、900MPa以上の強度を確保するのが困難であるとともに、曲げ性が劣化することから好ましくない。更には、体積分率を60%以上とすることが、より好ましい。
“Bainnight: 50% or more”
Bainite is a structure excellent in the balance between strength and bendability, and in order to ensure the maximum tensile strength of 900 MPa or more, the volume fraction of bainite as a main phase needs to be 50% or more. If the volume fraction is less than 50%, it is difficult to secure a strength of 900 MPa or more, and the bendability deteriorates, which is not preferable. Furthermore, it is more preferable that the volume fraction is 60% or more.
「残留オーステナイト:3〜30%」
鋼板の組織は、板厚の1/4を中心とした1/8厚〜3/8厚の範囲において、体積分率で3〜30%の残留オーステナイトを含有する。残留オーステナイトは、延性を大きく向上させることで、曲げ加工時に発生するネッキング抑制に効果がある。一方で、残留オーステナイトは、破壊の起点となって曲げ性を劣化させる。このため、亜鉛めっき鋼板では、母材鋼板の組織に含まれる残留オーステナイトを体積分率で3〜30%に制限する。
"Residual austenite: 3-30%"
The structure of the steel sheet contains 3-30% residual austenite in volume fraction in the range of 1/8 thickness to 3/8 thickness centered on 1/4 of the plate thickness. Residual austenite is effective in suppressing necking that occurs during bending by greatly improving ductility. On the other hand, retained austenite becomes a starting point of fracture and deteriorates bendability. For this reason, in the galvanized steel sheet, the retained austenite contained in the structure of the base steel sheet is limited to 3 to 30% by volume fraction.
「フレッシュマルテンサイト:15%以下」
鋼板の組織は、上述の残留オーステナイトの他に、板厚の1/4を中心とした1/8厚〜3/8厚の範囲において、フレッシュマルテンサイトを15%以下に制限することが必要である。本発明の鋼板(亜鉛めっき鋼板の場合は母材鋼板)の組織がこのような組織を有するものである場合、900MPa以上の強度と優れた曲げ性を有する高強度鋼板となる。
フレッシュマルテンサイトは、引張強度を大きく向上させるが、一方で破壊の起点となって曲げ性を大きく劣化させるため、母材鋼板の組織に体積分率で15%以下に制限することが好ましい。曲げ性を高めるにはフレッシュマルテンサイトの体積分率を10%以下とすることがより好ましく、5%以下とすることが更に好ましい。 フレッシュマルテンサイトとは、鉄基炭化物を含まないマルテンサイトであり、非常に硬くて脆い。この結果、曲げ加工を行った場合、割れの起点となり曲げ性を大幅に劣化させてしまう。このことから、体積率は出来るだけ小さくすることが望ましい。
"Fresh martensite: 15% or less"
In addition to the above-mentioned retained austenite, the structure of the steel sheet needs to limit fresh martensite to 15% or less in the range of 1/8 thickness to 3/8 thickness centered on 1/4 of the thickness. is there. When the structure of the steel sheet of the present invention (base steel sheet in the case of a galvanized steel sheet) has such a structure, it becomes a high-strength steel sheet having a strength of 900 MPa or more and excellent bendability.
Although fresh martensite greatly improves the tensile strength, on the other hand, it becomes a starting point of fracture and greatly deteriorates the bendability. Therefore, it is preferable to limit the structure of the base steel sheet to 15% or less in terms of volume fraction. In order to improve bendability, the volume fraction of fresh martensite is more preferably 10% or less, and further preferably 5% or less. Fresh martensite is martensite that does not contain iron-based carbides, and is very hard and brittle. As a result, when bending is performed, it becomes a starting point of cracking and the bendability is greatly deteriorated. For this reason, it is desirable to make the volume ratio as small as possible.
上記鋼板組織に加え、焼き戻しマルテンサイト、フェライト、パーライト、セメンタイトの1種以上を含んでもよい。以下に説明する範囲であれば、本発明の目的を達成することができる。 In addition to the steel sheet structure, one or more of tempered martensite, ferrite, pearlite, and cementite may be included. If it is the range demonstrated below, the objective of this invention can be achieved.
「焼戻しマルテンサイト:47%以下」
焼戻しマルテンサイトは、引張強度を大きく向上させる組織であり、母材鋼板の組織に体積分率で47%以下含まれていてもよい。焼き戻しマルテンサイトとは、マルテンサイトを200〜500℃で保持することで、θ、ε、η等の鉄基炭化物を析出させたマルテンサイトであり、フレッシュマルテンサイトに比べ、割れの発生の原因となり難い。このことから、焼き戻しマルテンサイトを含有しても良い。しかしながら、体積分率が47%超となると、過度に降伏応力が高くなりすぎてしまい形状凍結性が劣化するため、体積分率は47%以下とする必要がある。
“Tempered martensite: 47% or less”
Tempered martensite is a structure that greatly improves the tensile strength, and may be contained in the structure of the base steel sheet in a volume fraction of 47% or less. Tempered martensite is martensite in which iron-based carbides such as θ, ε, and η are precipitated by holding martensite at 200 to 500 ° C., and causes cracking compared to fresh martensite. It ’s hard to be. Therefore, tempered martensite may be contained. However, if the volume fraction exceeds 47%, the yield stress becomes excessively high and the shape freezing property deteriorates, so the volume fraction needs to be 47% or less.
「フェライト:30%以下」
フェライトは、延性の向上に有効な組織であり、母材鋼板の組織に体積分率で30%以下含まれても良い。また、フェライトは軟質な組織であるため、体積分率が30%を超えると十分な強度が得られない場合がある。このことから、フェライト体積分率は、30%以下とする必要がある。
"Ferrite: 30% or less"
Ferrite is an effective structure for improving ductility, and may be contained in the structure of the base steel sheet in a volume fraction of 30% or less. Moreover, since ferrite is a soft structure, if the volume fraction exceeds 30%, sufficient strength may not be obtained. For this reason, the ferrite volume fraction needs to be 30% or less.
「パーライト:5%以下」
パーライトが多くなると、延性が劣化する。このことから、母材鋼板の組織に含まれるパーライトの体積分率は、5%以下であることが好ましく、3%以下であることがより好ましい。
"Perlite: 5% or less"
When the amount of pearlite increases, the ductility deteriorates. Therefore, the volume fraction of pearlite contained in the base steel sheet structure is preferably 5% or less, more preferably 3% or less.
「その他」
その他の組織として、粗大なセメンタイトなど上記以外の組織が含まれていてもよい。しかし、母材鋼板の組織中に粗大なセメンタイトが多くなると、曲げ性が劣化する。このことから、母材鋼板の組織に含まれる粗大なセメンタイトの体積分率は、10%以下であることが好ましく、5%以下であることがより好ましい。粗大なセメンタイトとは、公称粒径で2μm以上のセメンタイトを意味する。セメンタイトは、鉄に比べてもろく、鉄とセメンタイトの界面強度も小さいことから、曲げ成形中に割れやボイド形成の起点となり、曲げ性を劣化させる。このことから、粗大なセメンタイトの体積率は小さくする必要がある。一方、ベイナイト組織や焼き戻しマルテンサイト中に含まれる微細な鉄基炭化物は、曲げ性を劣化させないことから含有しても良い。
"Other"
Other structures such as coarse cementite may be included as other structures. However, when coarse cementite increases in the structure of the base steel sheet, the bendability deteriorates. From this, the volume fraction of coarse cementite contained in the base steel sheet structure is preferably 10% or less, and more preferably 5% or less. Coarse cementite means cementite having a nominal particle size of 2 μm or more. Cementite is fragile compared to iron, and the interfacial strength between iron and cementite is also small. Therefore, it becomes a starting point of crack formation and void formation during bending, and deteriorates bendability. For this reason, it is necessary to reduce the volume ratio of coarse cementite. On the other hand, fine iron-based carbides contained in the bainite structure or tempered martensite may be contained because the bendability is not deteriorated.
以上のような各組織の体積分率は、例えば、以下に示す方法により測定できる。
残留オーステナイトの体積分率は、母材鋼板の板面に平行かつ1/4厚の面を観察面としてX線回折を行い、面積分率を算出し、それを持って体積分率と見なすことができる。
また、フェライト、パーライト、ベイナイト、セメンタイト、焼戻しマルテンサイトおよびフレッシュマルテンサイトの体積分率は、母材鋼板の圧延方向に平行な板厚断面を観察面として試料を採取し、観察面を研磨、ナイタールエッチングし、板厚の1/4を中心とした1/8厚〜3/8厚の範囲を電界放射型走査型電子顕微鏡(FE−SEM:Field Emission Scanning Electron Microscope)で観察して面積分率を測定し、それを持って体積分率と見なすことができる。
なお、各組織の体積分率の測定位置を、表面から1/4厚、1/8厚〜3/8厚としたのは、鋼板表層は脱炭が原因で鋼板組織が1/8厚〜3/8厚と異なっており、板厚中心もMn偏析が原因でマルテンサイトを多く含む組織となり、他の位置と鋼板組織が大きく異なるためである。
The volume fraction of each tissue as described above can be measured by the following method, for example.
The volume fraction of retained austenite is X-ray diffracted using a plane parallel to the plate surface of the base steel sheet and a thickness of 1/4 as the observation surface, and the area fraction is calculated and taken as the volume fraction. Can do.
The volume fractions of ferrite, pearlite, bainite, cementite, tempered martensite and fresh martensite were collected by taking a sample with the thickness cross section parallel to the rolling direction of the base steel sheet as the observation surface, and polishing the observation surface. Etching and observing the range of 1/8 to 3/8 thickness centered on 1/4 of the plate thickness with a field emission scanning electron microscope (FE-SEM). You can measure the rate and take it as a volume fraction.
In addition, the measurement position of the volume fraction of each structure was set to 1/4 thickness, 1/8 thickness to 3/8 thickness from the surface. The steel sheet surface layer was 1/8 thickness from the thickness due to decarburization. This is because the thickness center is a structure containing a lot of martensite due to Mn segregation, and the steel sheet structure is greatly different from other positions.
(鋼板の硬度比)
次に、鋼板表層の硬度と前記母材鋼板の1/4厚の硬度の比を規定した理由について説明する。
(Hardness ratio of steel sheet)
Next, the reason for defining the ratio between the hardness of the steel sheet surface layer and the hardness of the quarter thickness of the base steel sheet will be described.
本発明者らは、前記のような化学成分と組織を有する鋼板に、脱炭処理を施して鋼板表層を軟化することにより、優れた曲げ性を得ることができることを見出した。すなわち、鋼板の表層の硬度Hvsと母材鋼板の1/4厚の硬度Hvbの比「Hvs/Hvb」を、0.35〜0.90とすることで、優れた曲げ性が得られる。
この硬度比を0.35以上としたのは、硬度比が0.35未満とすると鋼板が軟化しすぎてしまい900MPa以上の引張最大強度を確保することが難しくなるためである。一方、0.90超では、本発明の効果である曲げ性向上の効果が得られないためである。
The present inventors have found that excellent bendability can be obtained by subjecting a steel sheet having the above chemical components and structure to decarburization to soften the steel sheet surface layer. That is, excellent bendability can be obtained by setting the ratio “Hvs / Hvb” of the hardness Hvb of the surface layer of the steel sheet and the hardness Hvb of the quarter thickness of the base steel sheet to 0.35 to 0.90.
The reason why the hardness ratio is set to 0.35 or more is that if the hardness ratio is less than 0.35, the steel sheet is too soft and it is difficult to ensure the maximum tensile strength of 900 MPa or more. On the other hand, if it exceeds 0.90, the effect of improving the bendability which is the effect of the present invention cannot be obtained.
なお、ここで用いられる「硬度」は、鋼板表層と鋼板の圧延方向に平行な板厚断面における板厚の1/4の位置とで、ビッカース硬度試験機を用いて押込み荷重10g重でそれぞれ10点づつ硬度を測定し、その平均値をそれぞれの硬度とした。 The “hardness” used here is a steel plate surface layer and a 1/4 position of the plate thickness in a plate thickness cross section parallel to the rolling direction of the steel plate, and 10% indentation load of 10 g using a Vickers hardness tester. The hardness was measured point by point, and the average value was defined as each hardness.
本発明者らは、硬度と曲げ性の関係を調査するにあたって、予備試験として、曲げ性と鋼板特性の関係を調査したところ、1/8厚〜3/8厚の範囲であれば、平均硬度は位置に依らないこと、及び、板厚中心(1/2厚)では、Mnの中心偏析が原因で鋼板組織が異なり、1/8厚〜3/8厚の位置とは、平均硬度も異なることを見出した。このことから、鋼板母材の硬度を代表可能な1/4の位置での硬さを母材の硬さ(Hvb)とした。 When investigating the relationship between hardness and bendability, the present inventors investigated the relationship between bendability and steel sheet characteristics as a preliminary test. Does not depend on the position, and at the thickness center (1/2 thickness), the steel sheet structure is different due to the center segregation of Mn, and the average hardness is also different from the position of 1/8 thickness to 3/8 thickness. I found out. From this, the hardness at the 1/4 position where the hardness of the steel plate base metal can be represented is taken as the base material hardness (Hvb).
一方、脱炭条件と鋼板表層の硬さの関係を調査したところ、脱炭が進めば進むほど、鋼板表層の硬度は低下するとともに軟化した領域が板厚方向に広がっていくこと、鋼板表面からある深さ位置での硬度を測定することで、軟化層の厚みや軟化度合いを代表できることを見出した。このことから、鋼板表面から20μm位置の硬度を測定し、めっき鋼板であれば、めっき層/地鉄界面から20μm位置の硬度を測定し、鋼板表層の硬度(Hvs)とした。 On the other hand, when investigating the relationship between the decarburization conditions and the hardness of the steel sheet surface layer, as the decarburization progresses, the hardness of the steel sheet surface layer decreases and the softened area expands in the sheet thickness direction. It was found that the thickness and the degree of softening of the softened layer can be represented by measuring the hardness at a certain depth position. From this, the hardness at the position of 20 μm from the surface of the steel sheet was measured, and in the case of a plated steel sheet, the hardness at the position of 20 μm was measured from the plating layer / base metal interface to obtain the hardness (Hvs) of the steel sheet surface layer.
ここで測定位置を表面から20μmとしたのは、次の理由による。
軟化位置では、鋼板硬度がHv100〜400であり、圧痕サイズが8〜13μm程度となり、測定位置が鋼板表面に近すぎる場合は、正確な硬度測定が難しかった。一方、測定位置が鋼板表面から離れすぎると、軟化層が含まれないことから、曲げ性と鋼板表層の硬度の間の関係を正確に求めることが出来なかった。このことから、測定値を20μm位置とした。
なお、鋼板表層の硬度測定にあたっては、研磨時の鋼板表面のダレを防止するため、鋼板に当て板を行い樹脂埋め込みした後、研磨、硬度測定を行うとよい。
Here, the measurement position was set to 20 μm from the surface for the following reason.
In the softening position, the steel sheet hardness was Hv 100 to 400, the indentation size was about 8 to 13 μm, and when the measurement position was too close to the steel sheet surface, accurate hardness measurement was difficult. On the other hand, if the measurement position is too far from the steel sheet surface, the softening layer is not included, and thus the relationship between the bendability and the hardness of the steel sheet surface layer could not be obtained accurately. From this, the measured value was set at a position of 20 μm.
In measuring the hardness of the steel sheet surface layer, in order to prevent sagging of the surface of the steel sheet during polishing, it is preferable to perform polishing and hardness measurement after applying a plate to the steel sheet and embedding resin.
(鋼板の形態)
本発明の高強度鋼板は、鋼板表層の硬さが上記範囲を満たす限り、冷延鋼板、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板並びに電気亜鉛めっき鋼板のいずれであってもよい。
亜鉛めっき層としては、特に限定されず、例えば、溶融亜鉛めっき層としては、Feを7質量%未満含有し、残部がZn、Alおよび不可避的不純物からなるものなどが、合金化溶融亜鉛めっき層としては、Feを7〜15質量%含有し、残部がZn、Alおよび不可避的不純物からなるものなどが使用できる。
また、亜鉛めっき層は、Pb、Sb、Si、Sn、Mg、Mn、Ni、Cr、Co、Ca、Cu、Li、Ti、Be、Bi、Sr、I、Cs、REMの1種または2種以上を含有、あるいは混入するものであってもよい。合金化亜鉛めっき層が、上記の元素の1種または2種以上を含有、あるいは混入するものであっても、本発明の効果は損なわれず、その含有量によっては耐食性や加工性が改善される等好ましい場合もある。
(Plate form)
The high-strength steel sheet of the present invention may be any of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, an alloyed hot-dip galvanized steel sheet, and an electrogalvanized steel sheet as long as the hardness of the steel sheet surface layer satisfies the above range.
The galvanized layer is not particularly limited. For example, as the galvanized layer, an alloyed galvanized layer containing less than 7% by mass of Fe and the balance consisting of Zn, Al and unavoidable impurities is used. As for, what contains 7-15 mass% of Fe, and the remainder consists of Zn, Al, and an unavoidable impurity etc. can be used.
Further, the galvanized layer may be one or two of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, I, Cs, and REM. The above may be contained or mixed. Even if the alloyed galvanized layer contains or mixes one or more of the above-mentioned elements, the effects of the present invention are not impaired, and the corrosion resistance and workability are improved depending on the content. In some cases, it is preferable.
さらに、本発明の高強度鋼板は、冷延鋼板の表面あるいは亜鉛めっき鋼板のめっき層の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜層を有するものであってもよい。
リン酸化物および/またはリンを含む複合酸化物からなる皮膜は、鋼板を加工する際に潤滑剤として機能させることができ、鋼板の表面や合金化亜鉛めっき層を保護することができる。
Furthermore, the high-strength steel sheet of the present invention may have a coating layer made of a complex oxide containing phosphorus oxide and / or phosphorus on the surface of a cold-rolled steel sheet or the surface of a galvanized steel sheet. .
A film made of a phosphorus oxide and / or a composite oxide containing phosphorus can function as a lubricant when processing the steel sheet, and can protect the surface of the steel sheet and the alloyed galvanized layer.
(鋼板の製造方法)
次に、本発明の高強度鋼板を製造する方法について詳細に説明する。
鋼板を製造するには、まず、上述した化学成分(組成)を有するスラブを鋳造する。熱間圧延に供するスラブは、連続鋳造スラブや薄スラブキャスターなどで製造したものを用いることができる。されに、鋳造後に直ちに熱間圧延を行う連続鋳造−直接圧延(CC−DR)のようなプロセスを用いてもよい。
(Steel plate manufacturing method)
Next, the method for producing the high-strength steel plate of the present invention will be described in detail.
To manufacture a steel plate, first, a slab having the above-described chemical component (composition) is cast. As the slab used for hot rolling, a slab produced by a continuous casting slab, a thin slab caster or the like can be used. In addition, a process such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting may be used.
スラブの熱間圧延において、スラブ加熱温度は、Ar3変態点以上の仕上げ圧延温度を確保するため、また、スラブ加熱温度の低下は、過度の圧延荷重の増加を招き、圧延が困難となったり、圧延後の母材鋼板の形状不良を招いたりする懸念があるため、1050℃以上にする必要がある。スラブ加熱温度の上限は特に定めることなく、本発明の効果は発揮されるが、加熱温度を過度に高温にすることは、経済上好ましくないことから、スラブ加熱温度の上限は1350℃以下とすることが望ましい。 In the hot rolling of the slab, the slab heating temperature is to secure a finish rolling temperature equal to or higher than the Ar3 transformation point, and the decrease in the slab heating temperature leads to an excessive increase in rolling load, making rolling difficult. Since there is a concern of causing a shape defect of the base steel plate after rolling, it is necessary to set the temperature to 1050 ° C. or higher. The upper limit of the slab heating temperature is not particularly defined, and the effect of the present invention is exhibited. However, since it is not economically preferable to make the heating temperature excessively high, the upper limit of the slab heating temperature is 1350 ° C. or less. It is desirable.
熱間圧延は、Ar3変態点温度以上の仕上げ圧延温度で完了する必要がある。仕上げ圧延温度がAr3変態点を下回ると、フェライト及びオーステナイトの二相域圧延となり、熱延板組織が不均質な混粒組織となり、冷間圧延工程及び連続焼鈍工程を経たとしても不均質な組織は解消されず、延性や曲げ性に劣る鋼板となる。
一方、仕上げ圧延温度の上限は特に定めることなく、本発明の効果は発揮されるが、仕上げ圧延温度を過度に高温とした場合、その温度を確保するためにスラブ加熱温度を過度に高温にしなければならない。このことから、仕上げ圧延温度の上限温度は、1100℃以下とすることが望ましい。
Hot rolling needs to be completed at a finish rolling temperature not lower than the Ar3 transformation point temperature. If the finish rolling temperature is lower than the Ar3 transformation point, it becomes a two-phase rolling of ferrite and austenite, and the hot-rolled sheet structure becomes a heterogeneous mixed grain structure, even if it undergoes a cold rolling process and a continuous annealing process. Is not eliminated, and the steel sheet is inferior in ductility and bendability.
On the other hand, the upper limit of the finish rolling temperature is not particularly defined, and the effect of the present invention is exhibited. However, when the finish rolling temperature is excessively high, the slab heating temperature must be excessively high in order to secure the temperature. I must. For this reason, the upper limit temperature of the finish rolling temperature is desirably 1100 ° C. or lower.
なお、Ar3変態点は、各元素の含有量(質量%)を用いた次の式により計算する。
Ar3=901−325×C+33×Si−92×(Mn+Ni/2+Cr/2
+Cu/2+Mo/2)+52×Al
The Ar3 transformation point is calculated by the following formula using the content (mass%) of each element.
Ar3 = 901-325 * C + 33 * Si-92 * (Mn + Ni / 2 + Cr / 2)
+ Cu / 2 + Mo / 2) + 52 × Al
熱間圧延の巻き取り温度は、熱延鋼板の表面に形成される酸化物の厚さが過度に増大して、酸洗性が劣化することを防止するため、750℃以下とする。酸洗性をより一層高めるために、巻き取り温度は720℃以下であることが好ましく、700℃以下であることがさらに好ましい。
一方、巻き取り温度が400℃未満となると熱延鋼板の強度が過度に高まり、冷間圧延が困難となるため、巻き取り温度は400℃以上とすることが望ましい。冷間圧延の負荷を軽減するため、巻き取り温度は420℃以上とすることが好ましい。ただし、400℃未満で巻き取ったとしても、その後、箱型炉にて焼鈍を行い、熱延板の軟化処理を行うことで、冷間圧延が可能となることから、400℃未満で巻き取っても構わない。
The hot rolling coiling temperature is set to 750 ° C. or lower in order to prevent the thickness of the oxide formed on the surface of the hot-rolled steel sheet from increasing excessively and deteriorating the pickling property. In order to further improve the pickling property, the winding temperature is preferably 720 ° C. or lower, and more preferably 700 ° C. or lower.
On the other hand, when the coiling temperature is less than 400 ° C., the strength of the hot-rolled steel sheet is excessively increased and cold rolling becomes difficult, and therefore the coiling temperature is preferably 400 ° C. or higher. In order to reduce the cold rolling load, the winding temperature is preferably 420 ° C. or higher. However, even if it winds at less than 400 degreeC, since it can be cold-rolled by annealing in a box furnace and performing a softening process of a hot-rolled sheet after that, it winds at less than 400 degreeC. It doesn't matter.
次に、このようにして製造した熱延鋼板に、酸洗を行うことが好ましい。酸洗は、熱延鋼板の表面の酸化物を除去するものであることから、母材鋼板のめっき性向上のために重要である。また、酸洗は、一回でも良いし、複数回に分けて行っても良い。 Next, it is preferable to perform pickling on the hot-rolled steel sheet thus manufactured. Pickling removes oxides on the surface of the hot-rolled steel sheet, and is therefore important for improving the plateability of the base steel sheet. Moreover, pickling may be performed once or may be performed in a plurality of times.
酸洗後の熱延鋼板は、板厚の調整や形状矯正を目的として冷間圧延される。冷間圧延は、板厚精度が高く優れた形状を有する母材鋼板を得るために、圧下率を30〜80%の範囲とすることが好ましい。圧下率が30%未満であると、形状を平坦に保つことが困難であり、最終製品の延性が劣悪となる恐れがある。冷間圧延における圧下率は35%以上であることが好ましく、40%以上であることがより好ましい。一方、圧下率が80%を超える圧下率では、冷延荷重が大きくなりすぎて冷延が困難となる。このことから、圧下率は80%以下であることが好ましい。ただし、冷延率80%を超えて冷間圧延を行ったとしても本発明の効果である優れた曲げ性を得ることはできる。
なお、冷間圧延工程において、圧延パスの回数、各圧延パス毎の圧下率については特に規定することなく本発明の効果は発揮される。
The hot-rolled steel sheet after pickling is cold-rolled for the purpose of plate thickness adjustment and shape correction. In cold rolling, in order to obtain a base steel plate having a high thickness accuracy and an excellent shape, it is preferable that the rolling reduction is in the range of 30 to 80%. If the rolling reduction is less than 30%, it is difficult to keep the shape flat, and the ductility of the final product may be deteriorated. The rolling reduction in cold rolling is preferably 35% or more, and more preferably 40% or more. On the other hand, when the rolling reduction ratio exceeds 80%, the cold rolling load becomes too large and cold rolling becomes difficult. Therefore, the rolling reduction is preferably 80% or less. However, even if cold rolling is performed at a cold rolling rate exceeding 80%, excellent bendability that is the effect of the present invention can be obtained.
In the cold rolling process, the effect of the present invention is exhibited without particularly defining the number of rolling passes and the rolling reduction for each rolling pass.
次に、得られた冷延鋼板を、焼鈍ラインを通板させて、(Ac3変態点−20)℃〜(Ac3変態点+100)℃の温度域で焼鈍を行う。曲げ性に優れた鋼板を得るためには、焼鈍中に表層の脱炭処理を行い、鋼板表層を軟化させる必要がある。脱炭処理とは、焼鈍の際の炉内雰囲気を下記範囲にすることで、鋼板表層に含まれるCを大気中へと拡散さ、鋼板表層のC濃度を低下させ、硬質組織の分率を低下させる処理である。 Next, the obtained cold-rolled steel sheet is passed through an annealing line and annealed in a temperature range of (Ac3 transformation point−20) ° C. to (Ac3 transformation point + 100) ° C. In order to obtain a steel sheet with excellent bendability, it is necessary to decarburize the surface layer during annealing to soften the steel sheet surface layer. The decarburization treatment means that the atmosphere in the furnace during annealing is in the following range, C contained in the steel sheet surface layer is diffused into the atmosphere, the C concentration of the steel sheet surface layer is reduced, and the fraction of the hard structure is reduced. It is a process to reduce.
本発明では、焼鈍の際の炉内雰囲気をlog(水分圧/水素分圧)が−3.0〜0.0の範囲として、脱炭を行う。雰囲気ガスの水分圧と水素分圧の比の対数を−3.0〜0.0とすることで、焼鈍を行うことによる冷延鋼板表層からの脱炭を適度に促進できる。
水分圧と水素分圧の比の対数が−3.0未満であると、焼鈍を行うことによる冷延鋼板表層からの脱炭が不十分となる。脱炭を促進するために、水分圧と水素分圧の比の対数は、−2.5以上であることが好ましい。一方、水分圧と水素分圧の比の対数が0.0超であると、焼鈍を行うことによる冷延鋼板表層からの脱炭が過度に促進されて、鋼板の強度が不十分となる恐れがある。鋼板の強度を確保するために、水分圧と水素分圧の比の対数は、−0.3以下であることが好ましい。
また、焼鈍を行う際の雰囲気は、窒素と水蒸気と水素とを含み、窒素を主体とするものであることが好ましく、窒素と水蒸気と水素の他に、酸素が含まれていてもよい。
In the present invention, decarburization is performed by setting the atmosphere in the furnace at the time of annealing to a log (moisture pressure / hydrogen partial pressure) in the range of −3.0 to 0.0. By setting the logarithm of the ratio between the moisture pressure of the atmospheric gas and the hydrogen partial pressure to −3.0 to 0.0, decarburization from the surface layer of the cold-rolled steel sheet by performing annealing can be appropriately promoted.
When the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is less than −3.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is insufficient. In order to promote decarburization, the logarithm of the ratio between the water pressure and the hydrogen partial pressure is preferably −2.5 or more. On the other hand, if the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is more than 0.0, decarburization from the surface layer of the cold-rolled steel sheet by annealing is excessively promoted, and the strength of the steel sheet may be insufficient. There is. In order to ensure the strength of the steel sheet, the logarithm of the ratio between the moisture pressure and the hydrogen partial pressure is preferably −0.3 or less.
Moreover, the atmosphere at the time of annealing contains nitrogen, water vapor | steam, and hydrogen, and it is preferable that it is mainly nitrogen, and oxygen may be contained in addition to nitrogen, water vapor | steam, and hydrogen.
また、焼鈍の際の温度域を(Ac3変態点−20)℃〜(Ac3変態点+100)℃とするのは、焼鈍時のオーステナイト体積率を高め、鋼板組織をベイナイトを主相とする組織とするためである。
焼鈍温度が(Ac3変態点−20)℃未満では、焼鈍時に形成したオーステナイトの体積率が小さく、ベイナイトの体積分率を50%以上とすることが出来ず、強度と曲げ性のバランスに劣る。このことから、焼鈍温度の下限を(Ac3変態点−20)℃とした。一方で、焼鈍温度が過度に高温になりすぎると、経済的に好ましくないばかりでなく、ロールや製造設備の劣化が顕著となるので、焼鈍温度は、(Ac3変態点+100)℃以下とすることが望ましい。ただし、経済性を除く効果である優れた曲げ性を得ることができる。
Moreover, the temperature range in the case of annealing shall be (Ac3 transformation point-20) degreeC-(Ac3 transformation point +100) degreeC to raise the austenite volume rate at the time of annealing, and to make a steel plate structure into a bainite main phase. This is because.
When the annealing temperature is less than (Ac3 transformation point−20) ° C., the volume fraction of austenite formed during annealing is small, the volume fraction of bainite cannot be made 50% or more, and the balance between strength and bendability is poor. From this, the lower limit of the annealing temperature was set to (Ac3 transformation point−20) ° C. On the other hand, if the annealing temperature is excessively high, not only is it not economically preferable, but also the deterioration of the roll and the production equipment becomes remarkable, so the annealing temperature is set to (Ac3 transformation point +100) ° C. or lower. It is desirable. However, excellent bendability, which is an effect excluding economic efficiency, can be obtained.
なお、Ac3変態点は次の式により計算する。
Ac3=910−203×(C)0.5−15.2×Ni+44.7×Si+104×V
+31.5×Mo−30×Mn−11×Cr−20×Cu+700×P
+400×Al+400×Ti
The Ac3 transformation point is calculated by the following formula.
Ac3 = 910−203 × (C) 0.5 −15.2 × Ni + 44.7 × Si + 104 × V
+ 31.5 × Mo-30 × Mn-11 × Cr-20 × Cu + 700 × P
+ 400 × Al + 400 × Ti
本発明において、上記の焼鈍温度および雰囲気での滞留時間は、20秒〜600秒とする。上記の滞留時間が20秒未満であると、硬質組織分率が少なくなりすぎてしまい900MPa以上の高強度を確保することが難しい。即ち、オーステナイトは、炭化物が溶解することで形成するものの、溶解にはある程度の時間を要する。20秒未満の焼鈍では、炭化物が溶解する時間が不足し、十分な量のオーステナイトを確保することが出来ない。その結果、900MPa以上の強度を確保することが難しい。そのことから、焼鈍温度時間の下限を20秒とした。一方で、600秒超の滞在は、その効果が飽和するばかりでなく、生産性の劣化を招くことから好ましくない。このことから、焼鈍温度の上限は、600秒とした。 In the present invention, the residence time in the annealing temperature and atmosphere is set to 20 seconds to 600 seconds. If the residence time is less than 20 seconds, the hard tissue fraction becomes too small and it is difficult to ensure a high strength of 900 MPa or more. That is, austenite is formed by dissolution of carbides, but it takes some time for dissolution. When annealing is performed for less than 20 seconds, a sufficient amount of austenite cannot be secured due to insufficient time for the carbide to dissolve. As a result, it is difficult to ensure a strength of 900 MPa or more. Therefore, the lower limit of the annealing temperature time was set to 20 seconds. On the other hand, staying longer than 600 seconds is not preferable because not only the effect is saturated but also productivity is deteriorated. For this reason, the upper limit of the annealing temperature was set to 600 seconds.
焼鈍後の冷却は、700℃〜500℃の温度範囲での平均冷却速度が0.5℃/秒以上とするのが望ましい。この温度範囲での平均冷却速度が0.5℃/秒未満であると、この温度範囲での滞在時間が長時間となってフェライトやパーライトが多量に生成される。このため、900MPa以上の強度を確保することが難しくなる。一方、500℃/秒を上回る冷却速度では、過度の設備投資を必要とするばかりでなく、板内の温度バラツキの増大等を招く懸念がある。 As for cooling after annealing, it is desirable that an average cooling rate in a temperature range of 700 ° C. to 500 ° C. is 0.5 ° C./second or more. When the average cooling rate in this temperature range is less than 0.5 ° C./second, the residence time in this temperature range is long and a large amount of ferrite and pearlite is generated. For this reason, it becomes difficult to ensure the strength of 900 MPa or more. On the other hand, at a cooling rate exceeding 500 ° C./second, not only an excessive facility investment is required, but there is a concern that the temperature variation in the plate increases.
続く冷却過程で、300〜500℃の温度範囲で10〜1000秒間の保持を行う。保持の温度範囲を、300〜500℃とするのは、この温度域でベイナイト変態が最も早く進行するためである。
保持温度が300℃未満では、ベイナイト変態の進行が遅く、オーステナイト中へのCの濃化が十分起こらないため、引き続く冷却過程でオーステナイトがマルテンサイトへと変態してしまう。この結果、残留オーステナイト体積率を3%以上とすることが出来ないばかりか、フレッシュマルテンサイト体積率が15%超となってしまう。この結果、曲げ性に劣る。
一方保持温度が500℃超では、ベイナイト変態の進行が遅く、その体積率を50%超とするのが難しいとともに、パーライト組織が形成し、強度を大幅に低下させることから好ましくない。このことから、冷却停止温度と保持温度は、300〜500℃とする。
In the subsequent cooling process, holding is performed for 10 to 1000 seconds in a temperature range of 300 to 500 ° C. The reason why the holding temperature range is set to 300 to 500 ° C. is that the bainite transformation proceeds most rapidly in this temperature range.
When the holding temperature is less than 300 ° C., the progress of bainite transformation is slow, and C does not sufficiently concentrate in the austenite, so that austenite is transformed into martensite in the subsequent cooling process. As a result, the retained austenite volume fraction cannot be increased to 3% or more, and the fresh martensite volume fraction exceeds 15%. As a result, the bendability is inferior.
On the other hand, if the holding temperature exceeds 500 ° C., the progress of the bainite transformation is slow, and it is difficult to make the volume ratio more than 50%, and a pearlite structure is formed and the strength is greatly reduced. For this reason, the cooling stop temperature and the holding temperature are set to 300 to 500 ° C.
また、保持時間を10〜1000秒保持としたのは、十分な量のベイナイト変態を起こさせるためである。保持時間が、10秒未満では、体積率50%以上のベイナイト組織を得ることが出来ない。一方、1000秒以下としたのは、過度の保持は生産性を低下させることから好ましくない。 The holding time is set to 10 to 1000 seconds in order to cause a sufficient amount of bainite transformation. If the holding time is less than 10 seconds, a bainite structure having a volume ratio of 50% or more cannot be obtained. On the other hand, the reason for setting it to 1000 seconds or less is not preferable because excessive retention reduces productivity.
なお、本発明で言う保持とは、鋼板が冷却途中で上記の温度域で上記の時間の間滞在することを意味する。したがって、冷却を一旦停止して等温で保持する場合のみを意味するのではなく、この温度域での除加熱や除冷を含む。 In addition, the holding said by this invention means that a steel plate stays for the said time in said temperature range in the middle of cooling. Therefore, it does not mean only the case where the cooling is temporarily stopped and kept at an isothermal temperature, but includes heating and cooling in this temperature range.
溶融亜鉛めっき鋼板の製造にあたっては、脱炭処理と300〜500℃までの冷却と保持を行った後、(亜鉛めっき浴温度―40)℃〜(亜鉛めっき浴温度+50)℃に加熱、あるいは、冷却を行い溶融亜鉛めっき浴へ浸漬させ、めっきを行う。
めっき浴浸漬板温度は、溶融亜鉛めっき浴温度より40℃低い温度から溶融亜鉛めっき浴温度より50℃高い温度までの温度範囲とすることが望ましい。浴浸漬板温度が溶融亜鉛めっき浴温度−40)℃を下回ると、めっき浴浸漬進入時の抜熱が大きく、溶融亜鉛の一部が凝固してしまいめっき外観を劣化させる場合があることから、下限を(溶融亜鉛めっき浴温度−40)℃とする。
In the production of the hot dip galvanized steel sheet, after decarburization treatment and cooling and holding to 300 to 500 ° C., heating to (zinc plating bath temperature−40) ° C. to (zinc plating bath temperature + 50) ° C., or Cool and dip in a hot dip galvanizing bath for plating.
The plating bath immersion plate temperature is preferably in a temperature range from a temperature 40 ° C. lower than the hot dip galvanizing bath temperature to a temperature 50 ° C. higher than the hot dip galvanizing bath temperature. If the bath immersion plate temperature is lower than the hot dip galvanizing bath temperature −40) ° C., the heat removal at the time of immersion in the plating bath is large, and part of the molten zinc may solidify and deteriorate the plating appearance. The lower limit is (hot dip galvanizing bath temperature −40) ° C.
ただし、浸漬前の板温度が(溶融亜鉛めっき浴温度−40)℃を下回っても、めっき浴浸漬前に再加熱を行い、板温度を(溶融亜鉛めっき浴温度−40)℃以上としてめっき浴に浸漬させても良い。また、めっき浴浸漬温度が(溶融亜鉛めっき浴温度+50)℃を超えると、めっき浴温度上昇に伴う操業上の問題を誘発する。また、めっき浴は、純亜鉛に加え、Fe、Al、Mg、Mn、Si、Crなどを含有しても構わない。 However, even if the plate temperature before immersion is lower than (hot dip galvanizing bath temperature −40) ° C., reheating is performed before immersion in the plating bath, and the plate temperature is set to (hot dip galvanizing bath temperature −40) ° C. or higher. It may be immersed in. On the other hand, if the plating bath immersion temperature exceeds (hot dip galvanizing bath temperature + 50) ° C., operational problems accompanying the rise of the plating bath temperature are induced. Further, the plating bath may contain Fe, Al, Mg, Mn, Si, Cr, etc. in addition to pure zinc.
また、めっき層の合金化を行う場合には、460℃以上で行う。合金化処理温度が460℃未満であると合金化の進行が遅く、生産性が悪い。600℃を超えると、オーステナイト中に炭化物が析出してしまい、オーステナイトが分解するため、700MPa以上の強度と良好な曲げ性の確保が難しくなるので、これが上限である。 Moreover, when alloying a plating layer, it carries out at 460 degreeC or more. When the alloying treatment temperature is less than 460 ° C., the progress of alloying is slow and the productivity is poor. If it exceeds 600 ° C., carbide precipitates in the austenite and austenite is decomposed, so that it is difficult to ensure a strength of 700 MPa or more and good bendability, so this is the upper limit.
冷延鋼板の表面の亜鉛めっきは、上述の溶融亜鉛めっきで行ったものに限らず、電気めっきで行っても構わない。その場合には常法に従って行えばよい。 The galvanization of the surface of the cold-rolled steel sheet is not limited to that performed by hot dip galvanization described above, and may be performed by electroplating. In that case, it may be carried out according to a conventional method.
また、表面の潤滑などを目的に、本発明の冷延鋼板の表面や亜鉛めっき鋼板のめっき層の表面に、リン酸化物および/またはリンを含む複合酸化物からなる皮膜を付与しても構わない。 Further, for the purpose of surface lubrication, a film made of a complex oxide containing phosphorus oxide and / or phosphorus may be applied to the surface of the cold rolled steel sheet of the present invention or the surface of the plated layer of the galvanized steel sheet. Absent.
なお、前述の焼鈍後等にスキンパス圧延を行うこともできる。その際の圧下率は、0.1〜1.5%の範囲が好ましい。0.1%未満では効果が小さく、制御も困難であることから、これが下限となる。1.5%を超えると生産性が著しく低下するのでこれを上限とする。スキンパスは、インラインで行っても良いし、オフラインで行っても良い。また、一度に目的の圧下率のスキンパスを行っても良いし、数回に分けて行っても構わない。 In addition, skin pass rolling can also be performed after the above-mentioned annealing. The rolling reduction at that time is preferably in the range of 0.1 to 1.5%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit. Since productivity will fall remarkably when it exceeds 1.5%, this is made an upper limit. The skin pass may be performed inline or offline. Further, a skin pass having a desired reduction rate may be performed at once, or may be performed in several steps.
本発明を、実施例を用いてさらに詳しく説明する。
表1に示すA〜V、a〜dの化学成分(組成)を有するスラブを鋳造し、スラブ加熱温度1230℃に加熱した後、表2−1、2の条件で熱間圧延を行った。その後、熱延板を酸洗した後、表2−1〜3の冷延率で1.2mmまで冷間圧延を行い、冷延鋼板とした。その後、表2−1〜3の条件で焼鈍を行い脱炭処理を施した後、種々の温度に冷却を行い高強度冷延鋼板を製造した。なお、表2、3では、表1の鋼に試料番号を付けて鋼種としている。
溶融めっき鋼板及び合金化溶融亜鉛めっき鋼板の製造にあたっては、冷却-保持を施した鋼板を、亜鉛めっき浴に浸漬し、その後室温まで冷却した。めっき浴中のめっき浴中の有効Al濃度は、0.07〜0.17mass%の範囲とした。一部の鋼板については、亜鉛めっき浴に浸漬後、各条件にて合金化処理を行い、室温まで冷却した。その際の目付け量としては、両面とも約35g/m2とした。最後に、得られた鋼板について0.4%の圧下率でスキンパス圧延を行った。
The present invention will be described in more detail with reference to examples.
After casting a slab having chemical components (compositions) A to V and a to d shown in Table 1 and heating to a slab heating temperature of 1230 ° C., hot rolling was performed under the conditions of Tables 2-1 and 2. Then, after pickling the hot-rolled sheet, it was cold-rolled to 1.2 mm at the cold-rolling rate shown in Tables 2-1 to 3 to obtain a cold-rolled steel sheet. Then, after annealing on the conditions of Tables 2-1 to 3 and performing a decarburization treatment, the steel was cooled to various temperatures to produce high-strength cold-rolled steel sheets. In Tables 2 and 3, sample numbers are given to the steels in Table 1 to indicate steel types.
In the production of the hot dip galvanized steel sheet and the alloyed hot dip galvanized steel sheet, the cooled and retained steel sheet was immersed in a galvanizing bath and then cooled to room temperature. The effective Al concentration in the plating bath in the plating bath was in the range of 0.07 to 0.17 mass%. Some steel sheets were immersed in a galvanizing bath, then alloyed under various conditions, and cooled to room temperature. The weight per unit area was about 35 g / m 2 on both sides. Finally, skin pass rolling was performed on the obtained steel sheet at a rolling reduction of 0.4%.
一部の高強度冷延鋼板は、脱脂後、電気亜鉛めっきした。めっき条件は、50℃、8重量%の硫酸溶液中で電流密度を15A/dm2として、約12秒間電解酸洗した後、下記の3種のめっき浴(Znめっき、Zn−NiめっきおよびZn−Coめっき)で、付着量が30g/m2および60g/m2となるように、めっきを施した。めっき浴温度:50±2℃、電流密度:60A/dm2、めっき液の流速:1m/秒とした。 Some high-strength cold-rolled steel sheets were electrogalvanized after degreasing. The plating conditions were as follows: the current density was 15 A / dm 2 in an sulfuric acid solution at 50 ° C. and 8 wt%, and after the electrolytic pickling for about 12 seconds, the following three plating baths (Zn plating, Zn—Ni plating and Zn -Co plating), plating was performed so that the adhesion amounts were 30 g / m 2 and 60 g / m 2 . Plating bath temperature: 50 ± 2 ° C., current density: 60 A / dm 2 , plating solution flow rate: 1 m / sec.
曲げ性の評価は、JIS Z 2248に基づき、得られた鋼板を圧延方向に垂直な方向に鋼板を切り出し、端面を機械研削し、35mm×100mmの試験片を作製し、先端のRが0.5〜6mmの90°のダイとパンチを用いて90度V曲げ試験を行うことにより実施した。曲げ試験後のサンプル表面をルーペで観察し、割れやネッキングがない、最小曲げ半径を限界曲げ半径と定義した。限界曲げ半径が1mm以下の鋼板を曲げ性に優れる鋼板と定義した。
また、得られた鋼板から試料を作成して、先述の方法で鋼板組織と硬度を測定した。また、引張り試験をJIS Z 2241に準拠して実施した。
The evaluation of bendability was based on JIS Z 2248. The obtained steel plate was cut out in a direction perpendicular to the rolling direction, the end face was mechanically ground to produce a 35 mm × 100 mm test piece, and the tip R was 0.00. The 90-degree V bending test was performed using a 5-6 mm 90 ° die and punch. The surface of the sample after the bending test was observed with a magnifying glass, and the minimum bending radius without cracking or necking was defined as the critical bending radius. A steel plate having a limit bending radius of 1 mm or less was defined as a steel plate having excellent bendability.
Further, a sample was prepared from the obtained steel plate, and the steel plate structure and hardness were measured by the method described above. Moreover, the tension test was implemented based on JISZ2241.
結果を表3−1〜3に示す。なお、表3で、鋼板の欄は、鋼板の形態を示し、CR:冷延鋼板、GI:溶融亜鉛めっき鋼板、GA:溶融亜鉛めっき鋼板、EG:電気亜鉛めっき鋼板をそれぞれ示す。
本発明の条件を満たすものは、900MPa以上の引張最大強度と良好な曲げ性を両立している。強度(TS)−全伸び(El)のバランス(TS×El)も18000(MPa・%)以上と良好であった。
The results are shown in Tables 3-1 to 3-1. In Table 3, the column of the steel sheet indicates the form of the steel sheet, CR: cold-rolled steel sheet, GI: hot-dip galvanized steel sheet, GA: hot-dip galvanized steel sheet, and EG: electrogalvanized steel sheet.
Those satisfying the conditions of the present invention have both a maximum tensile strength of 900 MPa or more and good bendability. The balance of strength (TS) -total elongation (El) (TS × El) was also as good as 18000 (MPa ·%) or more.
本発明は、自動車用の構造用部材、補強用部材、足廻り用部材に好適な、引張最大強度900MPa以上を有する曲げ性に優れた高強度鋼板を安価に提供するものであり、自動車の軽量化に大きく貢献することが期待でき、産業上の効果は極めて高い。 The present invention provides a high-strength steel sheet excellent in bendability having a maximum tensile strength of 900 MPa or more suitable for structural members, reinforcing members, and suspension members for automobiles at low cost. It can be expected to contribute greatly to industrialization, and the industrial effect is extremely high.
Claims (16)
C:0.075〜0.300%、
Si:0.30〜2.50%、
Mn:1.30〜3.50%、
P:0.001〜0.050%、
S:0.0001〜0.0100%、
Al:0.001〜1.500%、
N:0.0001〜0.0100%、
O:0.0001〜0.0100%
を含有し、残部が鉄および不可避的不純物からなる引張最大強度900MPa以上の高強度鋼板であり、その組織が、体積分率で、主相としてベイナイトを50%以上含有し、3〜30%の残留オーステナイトを含有するものであり、鋼板表層の硬度(Hvs)と鋼板の1/4厚の硬度(Hvb)の比(Hvs/Hvb)が0.35〜0.90であることを特徴とする曲げ性に優れた高強度冷延鋼板。 % By mass
C: 0.075 to 0.300%,
Si: 0.30 to 2.50%,
Mn: 1.30 to 3.50%,
P: 0.001 to 0.050%,
S: 0.0001 to 0.0100%,
Al: 0.001-1.500%,
N: 0.0001 to 0.0100%,
O: 0.0001 to 0.0100%
Is a high-strength steel plate having a tensile maximum strength of 900 MPa or more consisting of iron and inevitable impurities, and the structure contains 50% or more of bainite as a main phase in a volume fraction of 3 to 30%. Residual austenite is contained, and the ratio (Hvs / Hvb) of the hardness (Hvs) of the steel sheet surface layer to the 1/4 thickness (Hvb) of the steel sheet is 0.35 to 0.90. High-strength cold-rolled steel sheet with excellent bendability.
Ti:0.005〜0.150%、
Nb:0.005〜0.150%、
V:0.005〜0.150%
の1種または2種以上を含有することを特徴とする請求項1または2に記載の曲げ性に優れた高強度冷延鋼板。 The steel sheet is further in mass%,
Ti: 0.005 to 0.150%,
Nb: 0.005 to 0.150%,
V: 0.005-0.150%
The high-strength cold-rolled steel sheet with excellent bendability according to claim 1 or 2, characterized by containing one or more of the following.
B:0.0001〜0.0100%、
Cr:0.01〜2.00%、
Ni:0.01〜2.00%、
Cu:0.01〜2.00%、
Mo:0.01〜1.00%、
W:0.01〜1.00%
の1種または2種以上を含有することを特徴とする請求項1〜3のいずれか1項に記載の曲げ性に優れた高強度冷延鋼板。 The steel sheet is further in mass%,
B: 0.0001 to 0.0100%,
Cr: 0.01 to 2.00%
Ni: 0.01 to 2.00%,
Cu: 0.01-2.00%,
Mo: 0.01 to 1.00%,
W: 0.01-1.00%
The high-strength cold-rolled steel sheet excellent in bendability according to any one of claims 1 to 3, characterized by containing one or more of the following.
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