JP2004232022A - Dual phase type high tensile strength steel sheet having excellent elongation and stretch flanging property, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dual phase type high tensile strength steel sheet having excellent elongation and stretch flanging properties, and exhibiting the value of TS (tensile strength)×äEl (elongation)×λ}<SP>1/2</SP>satisfying ≥30,000, and to provide a production method therefor. <P>SOLUTION: The steel sheet has a composition comprising, by mass, 0.03 to 0.20% C, ≤2% Si, 0.5 to 3% Mn, ≤0.10% P, ≤0.01% S, 0.01 to 0.1% Sol.Al, ≤0.005% N, ≤1% Cr and ≤1% Mo, and the balance iron with inevitable impurities, and a structure in which the average crystal grain size of martensite is ≤2 μm, the average crystal grain size of ferrite is ≤2 μm, and the volume ratio of martensite is 20 to <60%. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【表２】

【００４３】
比較例Ａは、マルテンサイト体積率が低く、またフェライトの平均結晶粒径が大きいため、伸びフランジ性が悪い。
【００４４】
比較例Ｄ、Ｊは、マルテンサイトまたは両相の平均結晶粒径が大きいため、伸びフランジ性が優れない。比較例Ｇはマルテンサイト体積率が高すぎるため、伸びが劣る。
【００４５】
比較例Ｌは、Ｃ濃度が高く、強度が高過ぎ、伸びが劣る。また、スポット溶接性が優れない。比較例ＭはＭｎ濃度が低く、フェライトの平均結晶粒径が大きく、組織が粗いため、伸び特性は優れるが、伸びフランジ性が悪い。
【００４６】
（実施例２）
表１の実施例鋼Ｅについて、実施例１と同様に板厚１．２ｍｍの鋼版とした。この鋼板を表３に示す温度条件で熱処理を実施した。Ｔ１は前処理温度、ｎは前処理の回数、Ｔ２は焼鈍温度、Ｔｑは急冷開始温度、Ｔｔは焼戻し温度を示す。実施例１と同様に前処理での保持時間は１００秒とし、焼鈍温度Ｔ２での保持時間は５分とした。供試材の機械的性質を表４に示す。
【００４７】
【表３】

【表４】

【００４８】
比較例鋼Ｅ３は前処理温度がＡｅ_３点以上であり、フェライト粒が針状に析出しており、このため伸びフランジ性が低下した。比較例Ｅ５、Ｅ７は焼鈍温度がＡｅ_３点以上であり、粗大なマルテンサイトが存在したため、伸びフランジ性が劣化した。比較例Ｅ９〜Ｅ１１は、本発明の特徴である繰返し焼入れ処理を行っておらず、組織が微細化していないため、伸びフランジ性が劣る。
【００４９】
【発明の効果】
本発明によれば、フェライト・マルテンサイトの二相型高張力鋼板の伸びおよび伸びフランジ性を大幅に向上させることができ、従来高強度鋼板の適用が困難であった難成形の部材にも適用を拡大することができ、自動車の軽量化、安全性向上などに対し、極めて有益である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel sheet suitable for a strength member of an automobile part requiring excellent elongation and stretch flangeability, and a method for producing the same.
[0002]
[Prior art]
Recently, the use of high-strength steel sheets for automobile parts has been studied to improve the safety of automobiles and reduce the weight of vehicle bodies. Since steel sheets for automobile structural members are press-formed, characteristics such as elongation and stretch flangeability are required. However, the method of improving the elongation and the method of improving the stretch flange formability have a conflicting relationship, and it has been difficult to improve both at the same time.
[0003]
A conventional method for improving stretch flangeability will be described with reference to the following literature. Patent Literatures 1 and 2 disclose a high-strength steel sheet having a tensile strength of 780 MPa or more in consideration of stretch flangeability and a method for manufacturing the same. These Patent Literatures 1 and 2 disclose that a structure containing fine bainite having an average crystal grain size of 5.0 μm or less in a fraction of 80% or more has excellent workability. However, the metal structures of the steels of Patent Documents 1 and 2 have bainite as a main phase and do not target two phases of ferrite and martensite.
[0004]
Patent Document 3 discloses a method for manufacturing a high-strength steel sheet having excellent local elongation, that is, excellent stretch flangeability. According to the method of Patent Document 3, when the overaging treatment is performed at a temperature in the range of 350 to 600 ° C., the hardness ratio between the ferrite phase and the low-temperature transformation generation phase is reduced, so that the local elongation is improved. However, since the method of Patent Document 3 requires an overaging treatment at a high temperature of 350 ° C. or more (400 ° C. or more in the example), a reduction in tensile strength of a material inevitably generated by the overaging treatment is compensated. Therefore, a large amount of C as a strengthening element must be added.
[0005]
In most cases, automotive steel sheets are assembled by spot welding after forming by press forming, etc. However, if the amount of C in the steel is large, the toughness of the spot welds will deteriorate and the joining strength will decrease, reducing the C content. Was required. Furthermore, in addition to the above-mentioned problems in the material properties of the products, there are also problems from the viewpoint of manufacturability. That is, the higher the overaging temperature, the higher the energy cost in the production and the lower the productivity, so that a lowering of the overaging temperature has been required.
[0006]
Next, a description will be given of conventional findings regarding the improvement of characteristics due to the refinement of the structure.
Patent Document 4 discloses that when one or more of the elements of Nb, V, and Ti are added, this has the effect of making crystal grains fine, and improves elongation and stretch flangeability. .
[0007]
However, in the steel of Patent Literature 4, addition of elements such as Nb, Ti, and V increases alloy cost and deteriorates manufacturability such as hot rolling, and also generates carbides such as NbC and VC to reduce elongation. Deteriorate. For this reason, it is desirable not to add these elements as much as possible.
[0008]
[Patent Document 1]
JP 2001-226741 A (page 3, paragraph 0008, page 5, paragraph 0024)
[0009]
[Patent Document 2]
JP 2001-220647 A (Paragraph 0008 on page 3 and Paragraph 0025 on page 5)
[0010]
[Patent Document 3]
Japanese Patent Publication No. 7-59726 (upper right column on page 4)
[0011]
[Patent Document 4]
JP-A-11-350038 (paragraph 0013 on page 3)
[0012]
[Problems to be solved by the invention]
The present invention improves the above-mentioned drawbacks and provides a two-phase high-strength steel sheet having excellent elongation and stretch flangeability, having a TS × (El × λ) ^1/2 value of 30,000 or more, and a method for producing the same. The purpose is to do.
[0013]
[Means for Solving the Problems]
The present inventors tried to produce steel based on the data of the examples of Patent Documents 1 and 2, and obtained a high-tensile steel of TS × El19000 or more and TS × λ74000 or more described in the document. I couldn't do that. Originally, the metal structure of the steels of Patent Documents 1 and 2 has bainite as a main phase and does not target the two-phase structure of ferrite and martensite. It has been found that a tensile steel sheet is not provided.
[0014]
Further, in the method of Patent Document 3, since overaging treatment at a high temperature of 350 ° C. or more (400 ° C. or more in the example) is indispensable, a reduction in tensile strength of a material inevitably generated by this overaging treatment is compensated. Therefore, a large amount of C as a strengthening element must be added.
[0015]
The present inventors have conducted intensive studies to solve such problems, and as a result, have come to invent a steel sheet excellent in both elongation and stretch flangeability and a method for producing the same, and have an average crystal of martensite and ferrite. It has been found that stretch flangeability can be improved without deteriorating various properties such as elongation and weldability by making both particles extremely fine and uniform and further setting each volume ratio to an appropriate value. This effect is considered to be due to the fact that by realizing the structure, deterioration of the end face of the material in blanking, which is a problem in stretch flange processing, could be suppressed. As a method of manufacturing the steel sheet, after hot rolling or further cold rolling, a steel sheet appropriately formed into a two-phase structure of ferrite and martensite is further quenched after maintaining a soaking temperature in a two-phase temperature region of ferrite and austenite. By doing so, it has been found that the above-mentioned target metal structure can be achieved.
[0016]
The gist of the present invention is as follows.
[0017]
(1) In mass%, C: 0.03 to 0.20%, Si: 2% or less, Mn: 0.5 to 3%, P: 0.10% or less, S: 0.01% or less, Sol . Al: 0.01 to 0.1%, N: 0.005% or less, Cr: 1% or less, Mo: 1% or less, the balance being iron and unavoidable impurities, the average crystal grain of martensite A two-phase high-strength steel sheet excellent in elongation and stretch flangeability, characterized in that the diameter is 2 μm or less, the average crystal grain size of ferrite is 2 μm or less, and the volume fraction of martensite is 20% or more and less than 60%. .
[0018]
(2) above the steel chemical composition was melted, after which hot rolling was maintained at a temperature of Ae ₃ point or less than ₁ point Ae 10 seconds or more, from more than _one point Ar to 300 ° C., cooling rate 100 ° C. / heat treatment of quenching in seconds at least once, further kept at Ae ₁ point or more Ae ₃ point or less of the temperature range 10 seconds or more, from more than _one point Ar to 300 ° C., cooling rate 100 ° C. / sec or higher Quenching at a rate of 2 μm or less for the average crystal grain size of martensite, 2 μm or less for the average crystal grain size of ferrite, and a structure in which the volume fraction of martensite is 20% or more and less than 60%. A method for manufacturing a two-phase high strength steel sheet with excellent stretch flangeability.
[0019]
(3) The above ingredients were melted a steel composition, which was hot-rolled, after further cold rolling, and held at a temperature of at least ₁ point Ae Ae ₃ point or less 10 seconds or more, 300 from _one or more point Ar C., at least one heat treatment of quenching at a cooling rate of 100 ° C./sec or more, and further holding for 10 seconds or more in a temperature range of ₁ Ae or more and ₃ Ae or less, and from ₁ Ar or more to 300 ° C. Rapid cooling at a cooling rate of 100 ° C./sec or more, the average crystal grain size of martensite is 2 μm or less, the average crystal grain size of ferrite is 2 μm or less, and the volume fraction of martensite is 20% or more and less than 60%. A method for producing a two-phase high-strength steel sheet having excellent stretchability and stretch flangeability.
[0020]
Incidentally, in the steel having the composition of Patent Document 4, simply adding an element such as Nb, Ti, V, or the like, the present invention has found that a method of improving elongation and stretch flangeability is "ferrite grain size of 2 μm or less, martensite. A microstructure having a particle size of 2 μm or less cannot be obtained. In addition, the addition of Nb, Ti, and V increases alloy cost and deteriorates manufacturability such as hot rolling, and also generates carbides such as NbC and VC and deteriorates elongation. It is desirable. Further, Patent Literature 4 does not show a quantitative critical condition relating to tissue control found in the present invention.
[0021]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention and the reasons for each limitation will be described.
[0022]
First, the reasons for limiting the composition of steel in the present invention are as follows.
[0023]
1) C: 0.03 to 0.20%
C is an important element for strengthening martensite in a quenched structure. If the C content is less than 0.03%, the effect of increasing the strength becomes insufficient. On the other hand, if the C content exceeds 0.20%, good weldability cannot be obtained, so the upper limit was made 0.20%.
[0024]
2) Mn: 0.5-3%
Mn is an important element for suppressing the formation of ferrite. If it is less than 0.5%, the effect is not sufficient, and if it exceeds 3%, slab cracking occurs in the continuous casting step, so Mn is set to 0.5 to 3%.
[0025]
3) Si: 2% or less Si is a solid solution strengthening element, and is an effective element for obtaining a steel sheet with high strength and high elongation. If it exceeds 2%, a large amount of Si oxide is formed on the surface of the steel sheet, and the chemical conversion property is deteriorated.
[0026]
4) P: 0.1% or less, S: 0.01% or less P and S are impurities in the steel of the present invention, and are preferably lower in consideration of the workability of the steel sheet. S was set to 1% or less, and S was set to 0.01% or less.
[0027]
5) Sol. Al: 0.01 to 0.1%
Al is used as a deoxidizing material. However, if it is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and if it is 0.1% or more, the effect of adding Al is saturated and uneconomical. For the above reasons, Sol. Al is defined in the range of 0.01 to 0.1%.
[0028]
6) N: 0.005% or less N is an impurity contained in the crude steel and degrades the formability of the raw steel sheet. Therefore, it is desirable to remove and reduce as much as possible in the steel making process. However, if N is reduced unnecessarily, the refining cost increases. Therefore, the content is set to 0.005% or less, which is substantially harmless.
[0029]
7) Cr: 1.0% or less Cr is added as necessary to increase the strength of the steel sheet. If the content is 1% or less, the effects of the present invention are not impaired, and the corrosion resistance is sometimes improved.
[0030]
8) Mo: 1.0% or less Mo is a precipitation strengthening element, but if it is too much, it causes a decrease in ductility and the price is expensive, so the upper limit is made 1.0%.
[0031]
9) Other additive components Nb, Ti, V, Zr, B, and other carbon-nitride-forming elements may be contained in trace amounts for the purpose of adjusting the strength and the like so long as the properties and productivity of the steel are not deteriorated.
[0032]
Next, the reasons for limiting the organization will be described.
[0033]
10) Martensite average crystal grain size is 2 μm or less;
Ferrite average crystal grain size is 2μm or less,
Martensite volume fraction of 20% or more and less than 60%,
The steel in the present invention has a two-phase structure of ferrite and martensite, and each has an average crystal grain size of 2 μm or less. This suppresses generation and connection of voids at the interface between ferrite and martensite and in the vicinity of inclusions during blanking and stretch flange processing, and significantly improves stretch and stretch flangeability.
[0034]
If the martensite volume ratio is less than 20%, the amount of ferrite is too large to achieve a fine structure. Moreover, sufficient strength cannot be obtained as a high-tensile steel sheet. On the other hand, when the martensite volume ratio is 60% or more, the structure becomes mainly martensite and is not a fine structure. In addition, elongation becomes insufficient. For this reason, in the present invention, the martensite volume ratio is in the range of 20% or more and less than 60%.
[0035]
Next, the reasons for limiting the manufacturing conditions will be described.
The slab is produced by continuous casting or ingot making. The slab is cooled and then reheated or hot rolled. The final rolling temperature in the hot rolling is desirably Ar ₃ or more in order to improve elongation and stretch flangeability. At a final rolling temperature lower than the Ar ₃ point, a two-phase structure is formed at the final rolling stage, so that the ferrite grains are significantly coarsened, and a steel sheet having good workability cannot be obtained even if cold rolling and annealing are performed. The hot rolled sheet is taken up after cooling. The winding temperature is desirably 620 ° C. or lower in order to improve elongation and stretch flangeability.
[0036]
Further, this may be cold-rolled to a desired thickness. The cold rolling reduction at this time is desirably 50% or more to improve elongation and stretch flangeability.
[0037]
Then, it is heated to a temperature range from Ae ₁ point to Ae ₃ point or less, preferably Ae ₃ point −50 ° C. to Ae ₃ point −10 ° C. or less, and after holding for 10 seconds or more, a cooling rate of 300 ° C. or less to 100 ° C. Rapid cooling at more than / sec. If the heating temperature is lower than _one Ae point, no austenite phase is obtained during high-temperature holding, so that martensite cannot be obtained after rapid cooling, and high strength cannot be achieved. At a temperature exceeding Ae ₃ points, austenite becomes a single phase and the structure becomes coarse, so that elongation and stretch flangeability deteriorate. If the holding time is less than 10 seconds, the possibility of the presence of undissolved carbides increases and the amount of austenite decreases, so that the holding time is set to 10 seconds or more.
[0038]
If the cooling rate to 300 ° C. is less than 100 ° C./sec, martensitic transformation does not occur, and the alloy content must be increased to obtain high tensile strength. After heating to a temperature range of Ae ₁ point or more and Ae ₃ points or less, preferably Ae ₃ points −50 ° C. or more and Ae ₃ points −10 ° C. or less and holding for 10 seconds or more, the cooling rate is 100 ° C. to 300 ° C. or less. One or more heat treatments, preferably two or more heat treatments, for quenching at a rate of at least per second.
[0039]
Furthermore, after heating to Ae ₁ point or more and Ae ₃ point or less and holding for 10 seconds or more, it is rapidly cooled from the rapid cooling start temperature of Ar ₁ point or more to 300 ° C. at a cooling rate of 100 ° C./second or more. If the cooling rate is less than 100 ° C./sec, martensitic transformation does not occur, and the alloy addition amount must be increased in order to obtain high tensile strength. If the holding temperature exceeds ₃ points of Ae, it becomes a single phase of austenite and the structure becomes coarse, so that the elongation and stretch flangeability deteriorate.
[0040]
When the cooling rate to the quenching start temperature of _one or more points of Ar is 50 ° C./second or less, the distribution state of the martensite phase can be made uniform. Therefore, it is preferable to perform slow cooling at 50 ° C./second or less. If the quenching start temperature is less than _one Ar point, martensite cannot be obtained, and high strength cannot be achieved. Further, the tempering treatment may be performed in a temperature range of 150 to 425 ° C. By the tempering treatment, the martensite phase is softened, and the stretch flangeability is further improved.
[0041]
(Example 1)
Steel having the composition shown in Table 1 was melted, and the cast slab was hot-rolled and cold-rolled to obtain a steel sheet having a thickness of 1.2 mm. Thereafter, each steel was kept at 780 to 850 ° C. for 100 seconds and then quenched in water as it was. This heat treatment (hereinafter referred to as pretreatment) was repeated 0 to 2 times depending on the sample to change the grain size of martensite and ferrite. The cooling rate was about 2000 ° C./sec. Then, the temperature was held at 780 to 850 ° C. for 5 minutes, and gradually cooled to a quenching start temperature (600 to 740 ° C.) at a cooling rate of 50 ° C./second or less, followed by water quenching. Thereafter, a tempering treatment was performed at 300 ° C. for 15 minutes. Table 1 shows the average crystal grain size of ferrite and martensite and the volume ratio of martensite. The average crystal grain size was determined by a line segment method. The tensile test of the manufactured steel sheet was performed in accordance with JIS Z2241, while the stretch flangeability was performed in accordance with Japan Iron and Steel Federation Standard JFST1001-1996. Table 2 shows the mechanical properties and the TS × (El × λ) ^1/2 value of the steel sheet.
[0042]
[Table 1]

[Table 2]

[0043]
In Comparative Example A, the stretch flangeability is poor because the martensite volume ratio is low and the average crystal grain size of ferrite is large.
[0044]
Comparative Examples D and J have poor stretch flangeability because the average crystal grain size of martensite or both phases is large. Comparative Example G is inferior in elongation because the martensite volume ratio is too high.
[0045]
Comparative Example L has a high C concentration, too high strength, and poor elongation. Also, the spot weldability is not excellent. Comparative Example M has a low Mn concentration, a large average crystal grain size of ferrite, and a coarse structure, and thus has excellent elongation characteristics but poor stretch flangeability.
[0046]
(Example 2)
Example steel E of Table 1 was made into a steel plate having a plate thickness of 1.2 mm as in Example 1. This steel sheet was heat-treated under the temperature conditions shown in Table 3. T1 is the pretreatment temperature, n is the number of pretreatments, T2 is the annealing temperature, Tq is the quenching start temperature, and Tt is the tempering temperature. As in Example 1, the holding time in the pretreatment was 100 seconds, and the holding time at the annealing temperature T2 was 5 minutes. Table 4 shows the mechanical properties of the test materials.
[0047]
[Table 3]

[Table 4]

[0048]
The comparative example steel E3 had a pretreatment temperature of _three or more Ae points, and ferrite grains were precipitated in the form of needles, so that the stretch flangeability was reduced. In Comparative Examples E5 and E7, the annealing temperature was Ae ₃ or more, and the presence of coarse martensite resulted in poor stretch flangeability. In Comparative Examples E9 to E11, the repeated quenching treatment, which is a feature of the present invention, was not performed, and the structure was not refined.
[0049]
【The invention's effect】
According to the present invention, the elongation and stretch flangeability of a ferritic / martensitic dual-phase high-strength steel sheet can be significantly improved, and the present invention can also be applied to difficult-to-form members in which conventional high-strength steel sheets have been difficult to apply. This is extremely useful for reducing the weight of vehicles and improving safety.

Claims (3)

In mass%, C: 0.03 to 0.20%, Si: 2% or less, Mn: 0.5 to 3%, P: 0.10% or less, S: 0.01% or less, Sol. Al: 0.01 to 0.1%, N: 0.005% or less, Cr: 1% or less, Mo: 1% or less, the balance being iron and unavoidable impurities, the average crystal grain of martensite A two-phase high-strength steel sheet excellent in elongation and stretch flangeability, characterized in that the diameter is 2 μm or less, the average crystal grain size of ferrite is 2 μm or less, and the volume fraction of martensite is 20% or more and less than 60%. . A steel having the composition described in claim 1 is melted, and after hot rolling, the steel is held at a temperature of at least _one point of Ae and no more than _three points of Ae for at least 10 seconds, and a cooling rate of at least _one point of Ar to 300 ° C. subjected at least once to heat treatment of quenching at 100 ° C. / sec or more, and further kept at Ae ₁ point or more Ae ₃ point or less of the temperature range 10 seconds or more, from more than _one point Ar to 300 ° C., cooling rate 100 ° C. / sec Elongation characterized by having a structure in which the average crystal grain size of martensite is 2 μm or less, the average crystal grain size of ferrite is 2 μm or less, and the volume fraction of martensite is 20% or more and less than 60%. And a method for producing a two-phase high strength steel sheet with excellent stretch flangeability. Smelted claims 1 component composition according steel which was hot rolled, after further cold rolling, and held for 10 seconds or more at a temperature of Ae ₃ point or less than ₁ point Ae, from more than _one point Ar up to 300 ° C., subjected at least once to heat treatment of quenching at a cooling rate 100 ° C. / sec or more, and further kept at Ae ₁ point or more Ae ₃ point or less of the temperature range 10 seconds or more, from more than _one point Ar to 300 ° C. Quenching at a cooling rate of 100 ° C./sec or more to obtain a structure in which the average crystal grain size of martensite is 2 μm or less, the average crystal grain size of ferrite is 2 μm or less, and the volume fraction of martensite is 20% or more and less than 60%. A method for producing a two-phase high-strength steel sheet having excellent stretchability and stretch flangeability.