WO2003106723A1

WO2003106723A1 - High strength cold rolled steel plate and method for production thereof

Info

Publication number: WO2003106723A1
Application number: PCT/JP2003/007062
Authority: WO
Inventors: 長谷川　浩平; 松田　広志; 北野　総人; 河村　健二; 鶴丸　英幸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2002-06-14
Filing date: 2003-06-04
Publication date: 2003-12-24
Also published as: US20040238082A1; EP1514951A1; EP2017363A3; EP1514951B1; DE60335106D1; EP2017363A2; EP1514951A4

Abstract

A high strength cold rolled steel plate which has a chemical composition in mass %: C: 0.04 to 0.10 %, Si: 0.5 to 1.5 %, Mn: 1.8 to 3 %, P: 0.02 % or less, S: 0.01 % or less, Sol.Al: 0.01 to 0.1 %, N: 0.005 % or less, and the balance: Fe and inevitable impurities, and has a metal structure consisting substantially of a ferrite phase and a martensite phase. The high strength cold rolled steel plate exhibits good ductility of an elongation of 18 % or more and excellent stretch-flanging property of a bore expanding percentage of 60 % or more, in combination with a tensile strength of 780 MPa or more, and thus can be suitably used for a structural member of an automobile.

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet having a tensile strength of 780 MPa or more, which is suitable for mechanical structural members, particularly structural members of automobiles.

For structural members of automobiles, the use of high-strength cold-rolled steel sheets with a tensile strength of 780MPa or more is being studied from the viewpoint of light weight for improving fuel efficiency and safety for protecting occupants. However, such high-strength cold-rolled steel sheets are inferior in ductility and stretch flangeability as compared with soft cold-rolled steel sheets, so that press forming becomes difficult. Here, the stretch flangeability is a property that indicates the likelihood of cracking of the blank end face when a steel sheet is press-formed, and is evaluated by the hole expansion rate determined by the hole expansion test specified in the Japan Iron and Steel Federation Standard JFST1001-1996. Be valued.

Conventionally, a method for improving the stretch flangeability of a high-strength cold-rolled steel sheet as described below has been disclosed.

JP-B-7-59726, JP-A-2001-226741, JP-A-10-60593 and JP-A-9-263838 disclose that the steel composition and production conditions are optimized to control the metallographic structure. Then, the elongation Lalange nature is the best! : Fig. 1 Koichi Koshin cold steel sheet and its 31 methods are disclosed. More specifically, for example, in Japanese Patent Application Laid-Open No. 9-263838, the steel sheet after cold rolling is gradually cooled from the soaking temperature during annealing so that the second phase is uniformly dispersed in the ferrite phase, and then the cooling rate is increased. By adjusting the overaging temperature, the bainite phase is evenly dispersed in the ferrite phase to improve the strength and improve the stretch flangeability.

Japanese Patent Application Laid-Open No. 2001-355044 discloses that while increasing the strength of a ferrite phase, There is disclosed a high-strength cold-rolled steel sheet in which 2 to 20 retained austenite phases are formed in the light phase to achieve both high-strength elongation and elongation flangeability.

Japanese Patent Application Laid-Open No. 11-350038 discloses a method of producing a composite structure type high-strength cold-rolled steel sheet having excellent ductility and elongation flangeability and a tensile strength of about 980 MPa by combining steel components and production conditions. Have been.

Japanese Patent Application Laid-Open No. 9-41040 discloses that a steel sheet after cold rolling is annealed in the α + γ two-phase region, and is maintained in a temperature range from 650 ° C. to a stop temperature of pearlite transformation for 10 seconds or more. Cooling, then cooling to maintain the temperature range from the stop temperature of the perlite transformation to 450 ° C for 5 seconds or less, to produce a high-strength cold-rolled steel sheet with excellent stretch flangeability. It has been disclosed.

In addition, although there is no mention of stretch flangeability, the following technology relating to a high-strength cold-rolled steel sheet for the purpose of improving formability and the like is also disclosed.

Japanese Patent Publication No. 58-55219 and Japanese Patent No. 2545316 disclose a method of producing a high-strength cold-rolled steel sheet by strictly defining the chemical composition range and annealing under specific continuous annealing conditions.

Japanese Patent Publication No. Hei 7-68583 describes the mechanical properties by specifying the contents of Si and Mn in steel, reheating conditions before hot rolling, soaking conditions and atmosphere for continuous annealing after cold rolling, etc. A method for producing a two-phase structure type high-strength cold-rolled steel sheet having excellent spot weldability and chemical conversion treatment properties is disclosed.

Japanese Patent Application Laid-Open No. 8-30212 discloses that the structure after hot rolling is uniformly refined so that there is no band gap, and the structure after continuous annealing is a structure in which a ferrite phase and a martensite phase are uniformly and finely distributed. A method for producing a high-strength cold-rolled steel sheet having high ductility and good bendability is disclosed.

^ Hei 5-57332 discloses that after a steel containing Si and a relatively large amount of Mn is heated to an austenitic single phase region above the Ac3 transformation point, the low-temperature transformation phase such as ferrite phase and martensite phase is cooled during the cooling process. A method for producing a high-strength cold-rolled steel sheet having a low yield ratio and having a yield ratio of 0.65 or less by forming a composite structure with the same and having excellent surface properties and bendability is disclosed.

Japanese Patent Publication No. Hei 1-35051 and Japanese Patent Publication No. A method for producing a high-strength cold-rolled steel sheet having excellent ductility by controlling a heat temperature, a water quenching start temperature, and an overaging treatment temperature is disclosed.

^ Hei 7-74412 ゃ Tokuhei 3-68927 discloses that after cold rolling, annealing at a high temperature range reduces the concentration of C and reduces the austenite phase to 5% or less. Thus, there is disclosed a method for producing a high-strength cold-rolled steel sheet having excellent bendability, drawability, and anti-crack resistance. However, the above-described related art has the following problems.

In the technology of Japanese Patent Publication No. Hei 7-59726, overaging treatment at a high temperature of 350 ° C or more is indispensable. (Inventive steels 9, 10, and 13 in Table 1 contain 0.17% or more in order to provide characteristics with a tensile strength of 980MPa or more). Therefore, if spot welding is performed during vehicle assembly, the toughness of the spot welds will deteriorate, and the joint strength will decrease. In addition, high overage treatment increases energy costs in production and reduces productivity. Further, when the tensile strength is 980 MPa or more, the hole expansion ratio is as low as 56 (inventive steel 9 in Table 1), and the stretch flangeability is insufficient.

According to the technology disclosed in Japanese Patent Application Laid-Open No. 2001-226741, an austempering heat treatment is indispensable after homogenization in a continuous annealing step in order to generate a bainite phase, but this heat treatment does not provide stable steel sheet properties. There is a problem.

In the technique disclosed in Japanese Patent Application Laid-Open No. 2001-355044, the formation of a bainite phase is indispensable because the retained austenite phase remains, and the strength is reduced. The tensile strength shown in the examples is as low as 600 to 800 MPa, and a tensile strength of 780 MPa or more cannot be obtained stably. In order to increase the strength, TC-, Si, and Mn—large amounts of additional force]] — requires deterioration of weldability and weldability.

In the technique disclosed in Japanese Patent Application Laid-Open No. 11-350038, since the C content is as high as 0.10 to 15, the stretch flangeability and the toughness of the spot weld are inferior.

In the technology disclosed in JP-A-9-41040 and JP-A-9-263838, since the metal structure is a ferrite phase and a pearlite phase or a ferrite phase and a payinite phase, They have low tensile strength of 400-70 OMPa.

In JP-A-10-60593, JP-B-58-55219, JP-B-7-68583 and JP-A-2545316, a tensile strength of 400 to 700 MPa cannot be obtained.

Japanese Patent Publication No. 1-35051, Japanese Patent Publication No. 1-35052, Japanese Patent Publication No. 3-68927, Japanese Patent Publication No. 8-30212, Japanese Patent Publication No. 5-57332, and Japanese Patent Publication No. 7-74412 With technology, stable and excellent stretch flangeability cannot be obtained. DISCLOSURE OF THE INVENTION An object of the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet having an elongation of 18% or more, a hole expansion ratio of 60 or more, and a tensile strength of 780 MPa or more. The purpose is mass: C: 0.04-0.10%, Si: 0.5-1.5%, Mn: 1.8-3%, P: 0.02% or less, S: 0.01% or less, Sol.Al: 0.01-0.1, N: 0.005% or less, the balance can be achieved by a high-strength cold-rolled steel sheet composed of iron and unavoidable impurities and having a substantially metallic structure substantially composed of a ferrite phase and a martensite phase.

This high-strength cold-rolled steel sheet is manufactured by hot rolling and then cold rolling a steel slab having the above-described components to produce a steel sheet; and subjecting the cold-rolled steel sheet to 750 ° C to 870 ° C for at least lOsec. Manufacture of high-strength cold-rolled steel sheet comprising: a heating step, a step of cooling the heated steel sheet to 550 to 750, and a step of cooling the cooled steel sheet to 300 ° C or less at a cooling rate exceeding 100 Vsec. It can be realized by the method.

Perform ¾l ^ form

For a high-strength steel sheet having a tensile strength of 780 MPa or more, it is necessary that the metal structure be substantially a two-phase structure of a ferrite phase and a martensitic phase. there must, Shin Pi flangeability, spot weldability, the chemical convertibility to further decreases ₀ Therefore, the present inventor and others have proposed an excellent stretch flange having a tensile strength of 780 MPa or more even when the C content is low, and an excellent ductility and a hole expansion rate of 60% or more with an elongation of 18 or more. Investigations were conducted on steel sheets with good properties.C: 0.04-0.10%, Si: 0.5-1.5%, Mn: 1.8-3%, P: 0.02 or less, S: 0.01% or less, Sol.Al : 0.01% to 0.1%, Ν: 0.005% or less, the balance being iron and unavoidable impurities, and a metal structure that can be realized by a steel sheet substantially consisting of a ferrite phase and a martensite phase.

Hereinafter, the details of the present invention will be described.

1) Ingredients

C: C has a significant effect on tensile strength and is an important element for strengthening the martensite phase, which is a quenched structure. If the C content is less than 0.04%, a tensile strength of 780 MPa or more cannot be obtained, and if it exceeds 0.10%, the stretch flangeability and spot weldability are significantly reduced. Therefore, the C content is 0.04 to 0.10%. In order to obtain a tensile strength of 780MPa or more and less than 980MPa without impairing the stretch flangeability and spot weldability, the amount of C should be 0.04 or more and less than 0.070%; ^, 980 ^> 3 or more and 11801 To obtain a tensile strength of less than ^ 3, the C content is more preferably set to 0.070 to 0.10%.

Si: Si is effective in increasing the ductility of a dual phase steel sheet of a ferrite phase and a martensite phase. If the Si content is less than 0.5, the effect is not sufficient, and if it exceeds 1.5, a large amount of Si oxide is formed on the steel sheet surface in the hot rolling process, and surface defects occur. Therefore, the amount of Si is assumed to be 0.5 to 1.5. From the viewpoint of chemical conversion property, the amount of Si is desirably 1.0% or less.

Mn: Mn is an important element for suppressing the formation of ferrite phase in the cooling step of continuous annealing. "rT amount; i." 8% Leiman Morozou More than 3% —exceeds—exceeds—Slab cracking occurs during unstructured fabrication. Therefore, the amount of Mn is set to 1.8 to 3%. In order to produce a steel sheet stably in the continuous annealing process, the Mn content is desirably 2.0 to 2.5%.

P: If the P content exceeds 0.02%, the spot weldability deteriorates significantly, so the P content should be 0.02% or less. S: If the S content exceeds 0.01, the spot weldability deteriorates significantly, so the S content should be 0.01 or less.

Sol.Al: A1 is added to deoxidize and precipitate N as A1N. If the amount of Sol.Al is less than SO.01, deoxidation and precipitation of A1N will not be sufficiently performed, and if it exceeds 0.1, the effect will be saturated and the cost will increase. Therefore, the amount of 301.1 should be 0.01-0.1%.

N: Since N deteriorates the formability of the steel sheet, it is desirable that N be as small as possible. However, if it is reduced more than necessary, the cost of refining increases. Therefore, the N content is set to 0.005% or less which does not substantially impair the formability.

Cr: 0.01 to 1.0, Mo: 0.01 to 0.5, B: 0.0001 to 0.0020, i: 0.001 to 0.05%, Nb: 0.001 to 0.05, V: 0.001 to 0.05¾, Zr: When at least one element selected from 0.001 to 0.05% is contained, the structure can be easily adjusted during continuous annealing, and carbides and nitrides are formed in the steel during the hot and cold rolling process. This has the effect of suppressing the coarsening of the crystal grains and improving the stretch flangeability. If the content of each element is less than the lower limit, such effects are not sufficient, and if the content exceeds the upper limit, ductility is inferior.

2) Metal fibers

The metal structure consists essentially of two phases, a ferrite phase and a martensite phase. In addition to the above two phases, the bainite phase and the austenite phase each containing iron as a main constituent element do not impair the effects of the present invention as long as the volume fraction is less than 2%. Cementite, which is a compound containing iron, may be contained in the ferrite, in the martensite phase, or at the interface between the ferrite phase and the martensite phase. It should be noted that the compounds such as A1N and MnS have almost no effect of the present invention as long as the component elements and the impurity elements are within the scope of the present invention.

If the volume ratio of the martensite phase is 30 to 45, the tensile strength is in the range of 780 MPa to less than 980 MPa, and if the volume ratio is 45 to 60, the tensile strength is in the range of 980 to 1180 MPa. can get.

Note that tempering treatment of the martensite phase can be performed as appropriate within a range in which the desired strength is achieved. 3) Manufacturing method

First, a slab composed of the above components is produced by a continuous production method or an ingot-sufficient lump method, and is reheated or subjected to direct rolling. The final rolling temperature (finish temperature) in hot rolling is desirably 870 ° C or higher at the Ar3 transformation point or higher in order to improve the ductility and the stretch flangeability by miniaturizing the thread. The hot-rolled steel sheet is wound after cooling, but the winding temperature is desirably 620 ° C or less to improve ductility and stretch flangeability.

Next, cold rolling is performed to obtain a desired thickness. At this time, the cold rolling reduction is desirably 55 or more in order to improve ductility and stretch flangeability by refining the structure.

Finally, the steel sheet after cold rolling is annealed in a continuous annealing furnace under the following conditions.

i) Caloric fever: 750-870. L Osec or more at C

If the heating temperature is lower than 750 ° C, a sufficient amount of austenite phase is not generated, so that high strength cannot be achieved. . If the caloric heat time is less than 10 sec, the austenite phase is not sufficiently formed, and the strength cannot be increased.

ii) —Next cooling (slow cooling): Cooling end temperature 550 ~ 750 ° C

If the cooling end temperature is lower than 550 ° C, the volume ratio of the ferrite phase becomes too high, resulting in insufficient strength. If it exceeds 750 ° C, the subsequent rapid cooling deteriorates ductility and deteriorates the flatness of the steel sheet. The cooling rate at this time is adjusted so that the volume ratio of the austenite phase can be adjusted to 30 to 45% or 45 to 60% in the range of 550 to 750 ° C depending on the components, that is, the martensite phase It is desirable that the temperature be 20 ° C / sec or less so that the volume ratio of the mixture can be adjusted to 30 to 45% or 45 to 60%.

iii) secondary cooling (quenching): Beyond cooling rate 100 ° C / sec, 100 ° / sec under situ baked cooling end temperature: 300 ° C or less first cooling speed one time; NOTE ^ ⁷ becomes, can not be ensured high strength. It is desirable to rapidly cool at a cooling rate of 500 ° C / sec or more in order to stably achieve high strength. If the cooling end temperature exceeds 300 ° C, a bainite phase is formed or an austenite phase remains, and the stretch flangeability deteriorates. In order to stably obtain excellent elongation flangeability, it is desirable that the cooling end be 100 ° C or less.

After quenching, hold at the cooling end temperature for 5 to 20 minutes or temper at 150 to 390 ° C for 5 to 20 minutes. May be performed. By this tempering treatment, the martensite phase formed by rapid cooling is tempered, and the ductility and the stretch flangeability are improved. The tempering temperature is 150. If the temperature is less than C or the tempering time is less than 5 minutes, such effects cannot be sufficiently obtained. On the other hand, if the tempering temperature exceeds 390 ° C or the tempering time exceeds 20 minutes, the strength is remarkably reduced, and a tensile strength of 780 MPa or more cannot be obtained.

Further, it is preferable to subject the obtained steel sheet to temper rolling at a rolling reduction of 0.1 to 0.7 to completely eliminate the yield elongation.

The steel sheet of the present invention can be subjected to electroplating or hot-dip galvanization, or can be coated with a solid lubricant. Example 1

Steel No. 1 having the composition shown in Table 1: After slab of L0 was manufactured, re-heated at 1250 ° C, hot-rolled at a finishing temperature of about 860 ° C, and gradually cooled at about 20 C / sec. The rolled steel sheet with a sheet thickness of 2.8 mm was manufactured by simulating a one-hour winding at 600 and 600 mm. Next, cold rolling was performed to produce a cold-rolled steel sheet having a thickness of 1.2 pieces, and a heat treatment simulating continuous annealing was performed. In continuous annealing, the temperature rises at a heating rate of about 20 ° C / sec, and calories are heated at 830 ° C for 300 seconds, about 10 ° C / 36. The temperature was gradually cooled to 700 by using, and the temperature was rapidly cooled in jet water at a temperature of 7 ° C and 20 ° C. The rapid cooling rate was about 2000 ° C / sec. Finally, tempering treatment was performed at 300 ° C for 15 minutes, and after cooling, temper rolling of 0.3% was performed to produce steel sheets Nos. 1 to 10. Then, the tensile property values and the hole expansion ratio (λ) of the steel sheets No. l to 10 were measured.

The tensile properties values, and along connexion taken in the rolling direction and the direction perpendicular to JIS5 test piece No. (JISZ2201), JISZ ²²⁴ 1 rows test according to Rere, yield strength (YP), tensile strength (TS), Shin t (Ei) was determined:

The hole expansion ratio was determined by conducting a test in accordance with the evaluation method for stretch flangeability of the Iron and Steel Federation Standard (JFST1001-1996).

The target values of the present invention are TS≥780 MPa, Ε1≥18%, and λ≥60%.

Table 2 shows the results.

In the steel sheet Nos. 2, 3, 4, 9, and 10, which are examples of the invention, TS≥780MPa, El≥18%, λ≥60%, indicating high strength and excellent ductility and stretch flangeability. On the other hand, steel sheet No. 1, which is a comparative example, has a low TS due to a low C content, steel sheet No. 5 has a high C content, and a low Mn content has a significantly low L, and steel sheet No. 6 has a low Si content. Therefore, L is low, and steel plate No. 7 has low Mn content and TS; L is low, and steel plate No. 8 has high Mn content and low El.

o

table 1

o

Steel ί "f ^^^^" (mass%) Remarks

No. C Si Mn; P S Sol. Al N B Cr Mo Ti Nb V Zr

1 0.032 1.1 2.3 0.012 0.004 0.030 0.003 <0.0001 go 0.001 <0.001 go 0.001 go 0.001 go 0.001 <0.001 Comparative example

2 0.054 1.0 2.3 0 | .015 0.002 0.030 0.003 <0.0001 less 0.001 <0.001 less 0.001 less 0.001 <0.001 less 0.001 Invention example

3 0.065 1.4 2.1 0.010 0.003 0.030 0.003 <0.0001 <0.001 <0.001 go 0.001 go 0.001 go 0.001 go 0.001 Invention example

4 0.081 0.8 • 2.0 0.006 0.001 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 0.001 Invention example

5 0.112 0.9 1.3 0'.008 0.007 0.030 0.003 0.0001 0.001 <0.001 0.001 <0.001 <0.001 0.001 Comparative example

6 0.062 2.1 0'.014 0.006 0.030 0.003 <0.0001 <0.001 <0.001 go 0.001 <0.001 <0.001 <0.001 Comparative example 0.068 0.9 1.5 Ot.012 0.003 0.030 0.003 go 0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Comparative example

8 0.045 1.2 3.6 0, .010 0.002 0.030 0.003 Less 0.0001 <0.001 <0.001 <0.001 <0.001 Less 0.001 <0.001 Comparative example

9 0.058 0.9 1.9 Ο, .ΟΙΟ 0.001 0.030 0.003 0.0010 0.020 <0.001 less 0.001 <0.001 less 0.001 <0.001 Invention example

10 0.045 0.8 2.0 0.010 0.003 0.030 0.003 go 0.0001 <0.001 <0.001 <0.001 go 0.001 Invention example

o o

o

cm

Example 2

Using a slab of steel No. 2 shown in Table 1, cold rolling was performed under the same conditions as in Example 1, and continuous annealing and tempering were performed under the conditions shown in Table 3. Finally, in the same manner as in Example 1, a temper rolling of 0.3 was performed to produce steel sheets Nos. A to H. Then, the same tests as in Example 1 were performed on steel sheets No. A to H.

Table ⁴ shows the results.

The steel sheets Nos. E, G, and H, which are examples of the invention, all have TS ≥ 780 MPa, 1 ≥ 18%, and λ ≥ 60, indicating that they have high strength and are excellent in ductility and stretch flangeability. On the other hand, the steel sheet No. ある as a comparative example has a low TS and L due to a high heating temperature. This is probably because the metal structure mainly composed of the martensite phase was coarse. Steel plate No. C has low TS and λ due to short heating time. This is presumably because the austenite phase was not sufficiently generated during heating, and a martensite phase with a sufficient volume fraction was not obtained after quenching. Steel sheet No. D has low TS and I due to low annealing temperature. This is probably because the ferrite phase was formed during slow cooling and the volume fraction of the martensite phase after quenching was reduced. Steel sheet No. F has a low quenching rate and a high quenching end temperature, so TS and λ are low.

Heating Slow cooling End of slow cooling End of rapid cooling Tempering Tempering Steel sheet Steel-> Temperature Time Speed Speed Time Remarks

No. o. (° C) (sec) (° C / sec) c) (° C / sec) (° C) CO) (sec)

A 2 _: 830 150 5.0 680 2000 40 Invention example

B 2 890 200 5.7 720 2000 40 Comparative example

C 2 830 5 4.7 690 2000 40 Comparative example

D 2 ¹ 830 120 10.0 530 2000 40 Comparative example

E 2 '830 300 6.0 650 300 200 Invention example

F 2 i 840 160 3.8 f 25 30 400 Comparative example

G 2, 850 60 5.7 680 2000 40 200 15 Invention example

H 2, 830 150 5.0 680 2000 40 300 15 Invention example

Steel sheet Martensite Tensile property value Hole expansion ratio Remarks

No. Volume ratio () YP (MPa) TS (MPa) El () λ (%)

A 39 492 820 23.2 83 Invention example

B 29 450 750 25.3 30 Comparative example

C 25 438 730 • 45 Comparative example

D 24 432 720 • 26.4 50 Comparative example

E 44 510 850 99 Invention example

F 20 390 650 29.2 55 Comparative example

G 39 516 860 22.1 85 Invention example

H 42 504 840

92 Invention example o

cr

Example 3

After manufacturing slurries of steel Nos. 1 to 9 having the components shown in Table 5, hot rolling, cold rolling, continuous annealing, and temper rolling were performed under the same conditions as in Example 1 to obtain a steel sheet N. l-9 were prepared. Then, in the same manner as in Example 1, the yield strength (YP), the tensile strength (TS), the elongation (El), and the hole expansion ratio (λ) were measured.

Table 6 shows the results.

In the steel sheets Nos. 1, 2, 3, 8, and 9, which are examples of the invention, TS≥780MPa, El≥18% ^ ≥60%, high strength, excellent ductility and excellent stretchability. I understand. On the other hand, steel sheet No. 4, which is a comparative example, has a high C content and therefore has low E1 and L, steel plate No. 5 has a high C content, and a low Mn content has a significantly low L, and steel plate No. 6 has a low Si content. L is low because I is low, and steel sheet No. has high Mn content!

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Table 5

Steel Chemical composition (mass) Remarks

No. C Si Mn P s Sol.Al N B Cr Mo Ti Nb V Zr

1 0.065 1.1 2.3 0.012 0.004 0.030 0.003 go 0.0001 <0.001 go 0.001 go 0.001 <0.001 <0.001 go 0.001 Invention example

2 0.073 1.0 2.3 0.015 0.002 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 <0.001 0.001 Invention example

3 0.095 1.4 2.1 0.010 0.003 0.030 0.003 <0.0001 <0.001 <0.001 <0.001 <0.001 less 0.001 less 0.001 Invention example

4 0.112 0.8 2.0 q.006 0.001 0.030 0.003 <0.0001 less 0.001 <0.001 less 0.001 less 0.001 <0.001 <0.001 Comparative example

5 0.134 0.9 1.3 0.008 0.007 0.030 0.003 <0.001 <0.001 0.001 0.001 0.001 <0.001 <0.001 Comparative example

6 0.081 2.1 0.014 0.006 0.030 0.003 <0.0001 <0.001 <0.001 less 0.001 <0.001 less 0.001 <0.001 Comparative example

7 0.078 1.2 3.6 0.010 0.002 0.030 0.003 g 0.0001 g 0.001 <0.001 <0.001 <o.ooi k 0.001 <0.001 Comparative example

8 0.083 0.9 1.9 0.010 0.001 0.030 0.003 0.0010 0.020 g 0.001 g 0.001 <0.001 ON g 0.001 g 0.001 Invention example

9 0.088 0.8 2.0 0.010 0.003 0.030 0.003 <0.0001 g 0.001 <0.001 <0.001 <0.001 Invention example

o

t

Steel plate Steel Mapletensite Tensile property value Hole expansion ratio Remarks

No. No. Volume ratio () YP (MPa) TS (MPa) El (%) λ (%)

1 1 50 696 870 21.8 61 Invention example

2 2 55 808 1010 70 Invention example

3 3 51 816 1020 18.6 65 Invention example

4 4 56 1000 1250 15.2 35 Comparative example

5 5 32 792+ 990 30 Comparative example

6 6 46 744 930 45 Comparative example

7 7 80 1024 1280 13.2 55 Comparative example

8 8 47 808 1010 73 Invention example

9 9 53 800 1000 71 Invention example o

Example 4

Using a slab of steel No. 3 shown in Table 5, cold rolling was performed under the same conditions as in Example 1, and continuous annealing and tempering were performed under the conditions shown in Table 7. Finally, in the same manner as in Example 1, 0.3% temper rolling was performed to produce steel sheets Nos. A to J. Then, the same test as in Example 1 was performed for the steel sheets No. A to J.

Table 8 shows the results.

In the steel sheets Nos. B, G, D, and J, which are examples of the invention, the slip deviation is TS≥780MPa, Ε1≥18% ^ λ ≥60%, which means that they have high strength and excellent ductility and stretch flangeability. Understand. On the other hand, the steel sheet No. で, which is a comparative example, has a low E1 due to a low heating temperature. Steel plate No. C has a low heating temperature, so I is low. This is probably because the metal structure mainly composed of the martensite phase became coarse. Steel plate No. D is low because the heating time is short. This is presumably because the austenite phase was not sufficiently formed during heating, and the martensite phase with sufficient volume fraction was not obtained after quenching. Steel sheet No. E has a low E1 due to the high annealing end temperature. Steel sheet No. F has low TS and L due to low annealing temperature. This is probably because the ferrite phase was formed during slow cooling and the volume fraction of the martensite phase after quenching was reduced. For steel sheet No. F, TS and λ are low because the rapid cooling rate is low and the rapid cooling end is high.

Table 7

j

Heating Slow cooling End of slow cooling End of rapid cooling End of rapid cooling

Mis Steel Time Degree / Dish F & =

ϋ & Freezing / Dish 1 hour

No. No. (° C) (sec) (° C / sec) (° C) (° C / sec) CO (° C) (sec)

A 3 740 300 3.0 650 2000 40 Comparative example

B '3 830 150 5.0 680 2000 40 Invention example

C '3 890 200 5.7 720 2000 40 Comparative example

D 13 830 5 4.F 690 2000 40 Comparative example

E3: 830 270 1.7 780 2000 40 Comparative example

F | 3 830 120 10.0 530 2000 40 Comparative example

G 3 830 300 6.0 650 300 200 Invention example

H 3 840 160 3.8 725 30 400 Comparative example

I, ³ 850 60 5.7 680 2000 40 200 15 Invention example

J 3 830 150 5.0 680 2000 40 300 15 Invention example

Claims

The scope of the claims

1.In mass, C: 0.04-0.10, Si: 0.5-1.5%, Mn: 1.8-3, P: 0.02 or less, S: 0.01 or less, Sol.Al: 0.01-0.1, N: 0.005% or less, balance Is a high-strength cold-rolled steel sheet consisting of iron and unavoidable impurities, and a metal structure substantially consisting of a ferrite phase and a martensite phase.

2. The high-strength cold-rolled steel sheet according to claim 1, having a mass of C: 0.04% or more and less than 0.070%, and a tensile strength of 780 MPa or more and less than 980 MPa.

3. The high-strength cold-rolled steel sheet according to claim 1, which has a mass of C: 0.070 to 0.10% and a tensile strength of 980 MPa or more and less than 1180 MPa.

4. Furthermore, in mass%, Cr: 0.01 ~ 1.0%, Mo: 0.01 ~ 0.5%, B: 0.0001 ~ 0.0020¾, Ti: 0.001 ~ 0.05% _N Nb: 0.001 ~ 0.05%, V: 0.001 ~ 0.05, Zr : The high-strength cold-rolled steel sheet according to claim 1, containing at least one element selected from the group consisting of 0.001 to 0.05.

5. Furthermore, in mass, Cr: 0.01 to 1.0, Mo: 0.01 to 0.5, B: 0.0001 to 0.0020, Ti: 0.001 to 0.05%, Nb: 0.001 to 0.05, V: 0.001 to 0.05%, Zr: 0.001 to 0.05 %. The high-strength cold-rolled steel sheet according to claim 2, containing at least one element selected from the group consisting of:

6. Furthermore, in mass%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.5%, B: 0.0001 to 0.0020, i: 0.001 to 0.05%, Nb: 0.001 to 0.05, V: 0.001 to 0.05%, Zr : The high-strength cold-rolled steel sheet according to claim 3, containing at least one element selected from the group consisting of 0.001 to 0.05.

7. The high-strength cold-rolled steel sheet according to claim 2, wherein the martensite phase has a volume fraction of 30 to 45%.

8. The high-strength cold-rolled steel sheet according to claim 5, wherein the volume fraction of the martensite phase is as large as 30 to 45.

9. The high-strength cold rolled sheet according to claim 3, wherein the volume fraction of the martensite phase is 45 to 60.

10. The high-strength cold-rolled steel sheet according to claim 6, wherein the volume fraction of the martensite phase is 45 to 60.

11. A step of producing a steel sheet by subjecting a steel slab having the composition according to any one of claims 1 to 6 to hot rolling and then cold rolling.

A step of heating the steel sheet after the cold rolling at 750 to 870 ° C for 10 sec ^ J ^; and a step of cooling the heated steel sheet to 550 to 750 ° C,

Cooling the steel sheet after cooling to 300 ° C or less at a cooling rate exceeding 100 ° C / sec;

Method for producing a high-strength cold-rolled steel sheet having:

12. The method for producing a high-strength cold-rolled steel sheet according to claim 11, wherein the steel sheet is cooled at a cooling rate of 20 ° C / sec or less within a temperature range of 550 to 750 ° C.