WO2009096596A1

WO2009096596A1 - High-strength steel sheet and process for production thereof

Info

Publication number: WO2009096596A1
Application number: PCT/JP2009/051915
Authority: WO
Inventors: Hiroshi Matsuda; Reiko Mizuno; Yoshimasa Funakawa; Yasushi Tanaka; Tatsuya Nakagaito; Saiji Matsuoka
Original assignee: Jfe Steel Corporation
Priority date: 2008-01-31
Filing date: 2009-01-29
Publication date: 2009-08-06
Also published as: EP2246456A1; KR20100101697A; US20110030854A1; EP2246456B1; CA2713181C; EP2246456A4; CN101932746B; CA2713181A1; JP5365217B2; MX2010008227A; JP2009203550A; KR101225321B1; CN101932746A; EP2246456B9

Abstract

Provided is a high-strength steel sheet having both high strength and excellent formability which exhibits a tensile strength of 900 MPa or above. The steel sheet has both a composition which contains by mass C: 0.1 to 0.3%, Si: 2.0% or less, Mn: 0.5 to 3.0%, P: 0.1% or less, S: 0.07% or less, Al: 1.0% or less and N: 0.008% or less with the balance being Fe and unavoidable impurities, and a structure which comprises, in terms of area fraction, ferrite: 5 to 80%, autotempered martensite: 15% or more, bainite: 10% or less, retained austenite: 5% or less, and as-quenched martensite: 40% or less and in which the average hardness of the autotempered martensite is 700HV or below and the average number of precipitated iron carbide particles of 5nm to 0.5μm in the autotempered martensite is 5×104 or above per mm2.

Description

Description High-strength steel sheet and manufacturing method thereof Technical Field

The present invention relates to a high strength steel plate having a tensile strength of 900 MPa or more and excellent in formability used in industrial fields such as automobiles and electricity. The high-strength steel sheet of the present invention includes a steel sheet surface that has been subjected to hot dip galvanization or alloyed hot dip galvanization. Background art

In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. For this reason, efforts are being made to reduce the thickness of the body by increasing the strength of the body material and to reduce the weight of the body itself. However, increasing the strength of the steel sheet causes a decrease in forming processability, and therefore, development of a material having both the strength of the steel and the high processability is desired. In response to these demands, various types of steel sheets with various microstructures have been developed, such as ferrite-martensite duplex steel (DP steel) and TRIP steel using transformation-induced plasticity of residual austenite.

For example, for DjP steel, Patent Document 1 specifies the component composition, hot rolling and annealing conditions, and has a high tensile strength: 588 to 882 MPa. Tensile steel sheet and method for producing the same, Patent Document 2 discloses a method for producing a high-tensile cold-rolled steel sheet having excellent bendability by prescribing conditions for hot rolling, cold rolling and annealing of steel having a predetermined composition. Proposed. Patent Document 3 describes a steel plate excellent in collision safety and formability by specifying a martensite fraction, its particle size, and mechanical characteristics, and a manufacturing method thereof. High strength steel sheet, high strength hot-dip galvanized steel sheet, high-strength galvannealed steel sheet and high-strength alloyed hot-dip galvanized steel sheet In the manufacturing method, Patent Document 5, a high-strength steel sheet with excellent stretch freezing properties and shape freezing properties and impact resistance properties is defined by defining the component composition, the ferrite particle size, the texture and the martensite fraction, High-strength hot-dip galvanized steel sheet Hot-dip galvanized steel sheet and its manufacturing method, Patent Document 6 proposes a high-strength steel sheet with excellent mechanical properties and its manufacturing method by specifying the composition of ingredients and the manufacturing method of martensite amount Has been. Furthermore, Patent Documents 7 and 8 include high-strength hot-dip galvanized steel sheets and high-strength hot-dip zinc, which are excellent in stretch flangeability and bendability by specifying the composition of the components and the production conditions in the hot-dip galvanizing line. A method and equipment for producing steel plates have been proposed.

As a steel sheet having a structure that contains other than martensite in the hard second phase, Patent Document 9 defines the component, particle size, hardness ratio, etc., with the hard second phase as martensite and / or vinyl. Steel sheets with excellent fatigue properties due to this, Patent Document 10 discloses a steel sheet with excellent stretch flangeability by defining the composition and hardness ratio thereof mainly with the bait or parlite as the second base. Proposed. Patent Document 11 describes a high strength, high ductility hot-dip galvanized steel plate with excellent hole expansibility consisting of bainite and martensite as a hard second phase and a manufacturing method thereof, and Patent Document 1 2 describes a hard second phase. Composite structure steel plate with excellent fatigue properties by containing both vanite and martensite in the phase, and by defining the mean free path of the entire hard phase by specifying the fraction, grain size and hardness of each constituent phase Patent Documents 13 and 13 define high-tensile steel sheets that are excellent in ductility and hole expandability by specifying the composition of components and the amount of retained austenite, and Patent Documents 14 and 14 describe the contents of beanite and retained austenite and / or martensite. A high-strength cold-rolled steel sheet with excellent workability has been proposed by prescribing the component composition and the fraction of each phase, etc., in a steel sheet containing steel. Patent Document 15 describes the distribution of hard second-phase grains in ferrite and the existence ratio of grains composed of tempered martensite and bainite, thereby providing high strength with excellent workability. A steel plate and a manufacturing method thereof have been proposed. Furthermore, as the main structure of the vanite, Patent Document 16 describes the ultrahigh tensile cold-rolled steel with excellent delayed fracture resistance with a tensile strength of l lSOMPa or more by specifying the component composition and manufacturing process. The manufacturing method of the plate, Patent Document 17 describes the manufacturing method of the ultra-high-tensile cold-rolled steel plate that has excellent bendability with a tensile strength of 980 MPa or more by specifying the component composition and the manufacturing method, Patent Document 18 proposes an ultra-high strength thin steel plate with a tensile strength of 9S0MPa or more and its manufacturing method that prevents hydrogen embrittlement by limiting the number of iron-based carbides in tempered martensite to a certain number. Has been.

However, the above-described invention has the following problems. Patent Documents 1-7, 9- No. 10 1 to 14 are inventions for steel sheets with a tensile strength of less than 900 MPa, and workability cannot often be ensured if the strength is further increased. Further, Patent Document 1 stipulates that annealing is performed in a single-phase region and the subsequent cooling is performed at 6 to 20 ° C / second to 400 ° C. In addition, it is necessary to raise the temperature before plating in order to cool to 400 ° C or less, so it is necessary to raise the temperature before plating. Cannot be produced with a continuous hot dip galvanized line. Furthermore, in Patent Documents 7 and 8, since it is necessary to generate tempered martensite during the heat treatment in the hot dip galvanizing line, equipment for reheating after cooling to the Ms point or lower is required. In Patent Document 11, the phase composition of the hard second phase is defined as the bainite and martensite, and the fraction is specified, but in the specified range, the characteristic variation is large, and in order to suppress the variation Therefore, precise control of operating conditions is required. In Patent Document 15 as well, in order to cool to the Ms point or lower in order to generate martensite before the transformation, it is necessary to provide reheating equipment. Since precise control is indispensable, the cost of equipment and operation increases. In Patent Documents 16 and 17, it is necessary to keep the structure in the temperature range where the bainite is formed after annealing in order to make the structure mainly composed of bainite, and it is difficult to ensure ductility. In this case, it is necessary to re-ripe more than the bath temperature. Patent Document 18 shows only an improvement in hydrogen embrittlement of a steel sheet, and hardly any consideration is given to workability except for a few studies of bending workability.

In general, in order to increase the strength of a steel sheet, it is necessary to increase the ratio of the hard second phase to the entire structure. However, if the ratio of the hard second phase is increased, the workability of the steel sheet will be hard second. It is strongly influenced by the workability of the phase. This is because, when the proportion of the hard second phase is small, the ferrite itself, which is the parent phase, is deformed, so that the minimum additivity is ensured even when the additivity of the hard second phase is not sufficient. However, when the ratio of the hard second phase is large, the deformability of the hard second phase itself directly affects the formability of the steel sheet, not the deformation of the ferrite, and the workability is not sufficient. This is because the moldability deteriorates significantly.

For this reason, for example, in the case of cold-rolled steel sheets, martensite is produced by water quenching by adjusting the fraction of ferrite and hard second phase in a continuous annealing facility with a water quenching function. After the formation, the workability of the hard phase has been improved by maintaining the temperature rise and tempering the martensite.

However, in the case of equipment that cannot be tempered by heating or holding at a high temperature after such martensite is generated, the strength can be secured, but the workability of hard second phase such as martensite is possible. It was difficult to ensure.

For the purpose of securing stretch flangeability by utilizing hard phases other than martensite, ferritic is used as the parent phase, and the hard second phase containing carbide containing carbide is used as a hard phase. Workability has been secured and stretch flangeability has been secured, but in this case sufficient ductility could not be secured.

In addition, when utilizing the bainette, the problem was that the characteristics changed greatly due to variations in temperature and holding time in the bain formation region. In addition, when the second phase is martensite or residual austenite (including the vein containing residual austenite), in order to ensure stretch flangeability at the same time as ductility, for example, the second phase structure Consideration has been made such as making a mixed organization of martensite and bainto.

However, in order to make the second phase a mixed structure of various phases and to control the fraction with high precision, precise control of heat treatment conditions is necessary, which may cause problems in manufacturing stability. There were many.

Patent Document 1: Japanese Patent No. 1853389

Patent Document 2: Japanese Patent No. 3610883

Patent Document 3: Japanese Patent Laid-Open No. 11-61327

Patent Document 4: Japanese Patent Laid-Open No. 2003-213369

Patent Document 5: Japanese Unexamined Patent Publication No. 2003-213370

Patent Document 6: Special Table 2003-505604

Patent Document 7: JP-A-6-93340

Patent Document 8: JP-A-6-108152

Patent Document 9: Japanese Patent Laid-Open No. 7-11383

Patent Document 10: Japanese Patent Laid-Open No. 10-60593

Patent Document 1 1: JP-A-2005-281854

Patent Document 1 2: Japanese Patent No. 3231204 Patent Document 1 3 Japanese Patent Laid-Open No. 2001-207234

Patent Document 1 4 Japanese Patent Laid-Open No. 7-207413

Patent Document 1 5 JP 2005-264328 A

Patent Document 1 6 Japanese Patent No. 2616350

Patent Document 1 7 Japanese Patent No. 2621744

Patent Document 1 8 Japanese Patent No. 2826058 Disclosure of Invention

The present invention advantageously solves the above-mentioned problems, minimizes the generation of the strength that tends to vary in properties such as strength and formability, and can achieve both high strength and excellent formability. The purpose is to supply high-strength steel sheets of 900MPa or more together with their advantageous manufacturing method.

As for moldability, TS X T. is indicative of the EL and stretch flangeability, and shall be evaluated by a value, in the present invention, TS X T. El≥14500MPa ·%, tut ≥1 ^5% of target characteristics And

In order to solve the above-mentioned problems, the inventors studied the process of martensite formation, particularly the effect of steel sheet cooling conditions on the martensite.

As a result, if the heat treatment conditions after cold rolling are optimally controlled, the martensite after transformation is tempered at the same time as the martensite transformation, and the auto-tempered martensite generated by this treatment is brought to a predetermined ratio. By controlling and appropriately controlling the distribution of iron-based carbide in the autotempered martensite, the excellent formability and tensile strength targeted by the present invention: High strength steel with high strength of 900 MPa or more The knowledge that a board is obtained was acquired.

The present invention has been completed based on the above findings and has been further studied. The gist of the present invention is as follows.

1. Mass. /. so,

C: 0.1% or more 0.3% or less,

Si: 2.0% or less,

Mn: 0.5% or more 3.0% or less,

P: 0.1% down, S: 0.07% or less,

Al: 1.0% or less and

N: 0.008% or less

The balance consists of Fe and inevitable impurities, and the area ratio of steel structure is 5% to 80% ferrite, 15% auto tempered martensite, and 10% bainite residue. Austenite is 5% or less, as-quenched martensite is 40% or less, the average hardness of the auto-tempered martensite is HV≤700, and 5 nm or more and 0.5 i Di or less in the auto-tempered martensite A high-strength steel sheet characterized in that the average number of precipitations of iron-based carbide is 5 x 10 ⁴ or more per 1 mm ² and the tensile strength is 900 MPa or more.

2. The steel sheet is further mass%,

Cr: 0.05% or more and 5.0% or less, ·

V: 0.005% to 1.0% and

Mo: 0.005% to 0.5%

2. The high-strength steel sheet according to 1 above, which contains one or more elements selected from among the above.

3. The steel sheet is further mass%,

Ti: 0.01% or more and 0.1% or less,

Nb: 0.01% or more and 0.1% or less,

B: 0.0003% or more and 0.0050% or less,

Ni: 0.05% or more and 2.0% or less

Cu: 0.05% or more and 2.0% or less

3. The high-strength steel sheet according to 1 or 2 above, which contains one or more elements selected from the above.

4. The steel sheet is further mass%,

Ca: 0.001% to 0.005% and

REM: 0.001% or more and 0.005% or less

4. The high-strength steel sheet according to any one of 1 to 3, wherein the steel sheet contains one or two elements selected from among the above.

5 · Among iron tempered martensite, iron system of 0.1 zm or more and 0.5μηι or less The ratio of the auto-tempered martensite in which the number of precipitated carbides is 5 × 10 ² or less per 1 mm ² is 3% or more in terms of the area ratio with respect to the whole auto-tempered martensite. 5. A high-strength steel sheet according to any one of 4 to 4.

6. The high-strength steel plate according to any one of 1 to 5 above, wherein a surface of the steel plate is provided with a molten zinc adhesion layer.

7. The high-strength steel plate according to any one of 1 to 5 above, wherein a surface of the copper plate is provided with an alloyed molten zinc adhesion layer.

8. The steel slab having the composition described in any one of 1 to 4 above is hot-rolled and then cold-rolled into a cold-rolled steel sheet, and then the cold-rolled steel sheet is at least 700 ° C and 950 ° C. After annealing for 15 seconds or more and 600 seconds or less in the following first temperature range, the cooling conditions in the second temperature range from the first temperature range to 420 ° C are changed from the first temperature range to 550 ° C. The average cooling rate is 3 ° C / second or more, the time required for cooling from 550 ° C to 420 ° C is 600 seconds or less, and the third temperature range from 250 ° C to 420 ° C is 50 ° C / second. A method for producing a high-strength steel sheet, which is cooled at the following speed to cause martensite transformation in the third temperature range, and at the same time, tempering the martensite after transformation. .

9. When the steel sheet is cooled at a cooling rate of 50 ° C / sec or less in the third temperature range of 250 ° C or more and 420 ° C or less, the temperature range of at least (Ms point one 50) ° C or less is 1.0. Cooling at a rate of not less than ° C / sec and not more than 50 ° C / sec, causing martensite transformation in the third temperature range, and at the same time performing auto-tempering treatment to temper the martensite after transformation 9. The method for producing a high-strength steel plate as described in 8 above.

1 0. In the steel slab, the martensite transformation start point Ms is approximated by M represented by the following equation (1), which is 300 ° C or higher, and is described in the above 8 or 9 Manufacturing method of high strength steel sheet.

Record

M (° C) = 540-361X {[C%] / (l- [a%] / 100)}-6X [Si%]-40X [Μη%] + 30Χ [Al%]

-20 X [Cr%]-35 X [V%]-10 X [Mo%] one 17 X [Ni%] one 10 X [Cu%] · · · (1)

However, the [X%] is mass ^_0/0 of component element X of the steel strip, Non%] is polygonal Blow wells area ratio (%). According to the present invention, an appropriate amount of auto-tempered martensite is contained in the steel sheet, and the distribution state of carbides in the auto-tempered martensite is appropriately controlled, thereby achieving high strength and excellent performance. Tensile strength with both workability and excellent ductility: A high-strength steel sheet with a strength of 900 MPa or more can be obtained. Therefore, it greatly contributes to reducing the weight of automobile bodies.

In addition, since the method for producing a high-strength steel sheet according to the present invention does not require reheating of the steel sheet after quenching, no special production equipment is required, and hot-dip galvanizing or galvannealing is performed. Since it can be easily applied to processes, it contributes to cost reduction in process savings. Brief description of the drawings ''

Fig. 1 is a schematic diagram showing a quenching and tempering process for obtaining a normal tempered martensite.

FIG. 2 is a schematic diagram showing a phototempering process for obtaining a phototempered martensite according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be specifically described.

First, the reason why the weaving of the steel sheet is limited as described above in the present invention will be described.

Ferrite area ratio: 5% to 80%

Workability and tensile strength: To achieve both 900MPa and higher, the ratio of ferrite to the hard phase described below is important, and the ferrite area ratio must be 5% or more and 80% or less. . If the area ratio of the ferrite is less than 5%, ductility cannot be secured. On the other hand, if the ferrite area ratio exceeds 80%, the area ratio of the hard phase cannot be secured and the strength becomes insufficient. Preferred Fuwerai DOO area ratio is in the range of more than 10% ⁶ 5% or less.

Auto Tempered Martensite Area Ratio: 15% or more

In the present invention, the phototempered martensite is not a so-called tempered martensite obtained by quenching and tempering as in the prior art, but is obtained by simultaneously proceeding martensite transformation and tempering by autotempering. Means an organization. The structure is not a uniform tempered structure formed by heating and tempering after completion of the martensite transformation by quenching, as in normal quenching / tempering treatment, but in the region below the Ms point. This is a structure in which martensite with different tempering conditions is mixed by controlling the cooling process of the steel and proceeding through the martensite transformation and tempering step by step. -This auto temper martensite is a hard phase for increasing strength. If the area ratio of auto-tempered martensite is less than 15%, strength cannot be secured and work hardening of the ferrite cannot be accelerated. Therefore, the area ratio of auto-tempered martensite must be 15% or more. Preferably it is 30% or more.

In the present invention, the steel sheet structure is preferably composed of ferrite and autotempered martensite in the above-described range. In forming these structures, other phases such as vanite, residual austenite, and as-quenched martensite may be formed, but if these are within the allowable ranges described below, Even if a phase is formed, there is no problem. These allowable ranges are described below.

Veneer area ratio: 10% or less (including 0%)

Venite is a hard phase that contributes to high strength. However, it is desirable that the steel structure does not contain as much as possible because the characteristics may change greatly depending on the temperature range of formation and increase the material variation. Is acceptable up to 10%. Preferably it is 5% or less.

Residual austenite area ratio: 5% or less (including 0%)

Residual austenite is transformed during processing to become hard martensite, which reduces elongation flangeability. For this reason, it is desirable to have as little as possible in the steel structure, but up to 5% is acceptable. Preferably it is 3% or less.

Quenched martensite area ratio: 40% or less (including 0%)

As-quenched martensite is extremely inferior in workability, so it is desirable that it is as small as possible in the steel structure, but up to 40% is acceptable. Preferably, it is 30% or less. Quenched martensite can be distinguished from autotempered martensite by the fact that carbides are not observed by observation with a scanning electron microscope (SEM) or transmission electron microscope (TEM). Average hardness of auto-tempered manoleite sites: HV≤700

If the average hardness of the autotempered martensite is 700 <HV, the stretch flangeability deteriorates significantly, so HV 700 is set. Preferably HV≤630.

Iron-based carbides in hot tempered martensite:

Size: 5 ηπι or more, 0.5 πι or less, Average number of precipitations: 5 X 10 ⁴ or more per 1 mm ² Autotempered martensite is a manotenite site that has been heat-treated (autotempered) by the method of the present invention. However, even when the average hardness of auto-tempered martensite is HV≤700, if auto-tempering is not appropriate, the workability is reduced. The degree of auto-tempering can be confirmed by the production status (distribution state) of iron-based carbides in auto-tempered martensite. Among these iron-based carbides, those with a size of 5 nm or more and 0.5 μππι or less and the average number of precipitates are 5 × 10 ⁴ or more per 1 mm ² , the desired autotempering treatment is applied It can be judged. The reason why iron carbide is less than 5 nm is not considered because it does not affect the workability of autotempered martensite. On the other hand, iron-based carbides with a size exceeding 0.5 μπι may not reduce the strength of auto-tempered martensite, but have a minor effect on workability and are not subject to judgment. When the number of iron-based carbides is less than 5 × 10 ⁴ per nun ² , the effect of improving workability, particularly stretch flangeability, cannot be obtained, so it is judged that the phototempering treatment is inappropriate. The preferred number of iron-based carbides is in the range of 1 × 10 ⁵ or more and 1 × 10 ⁶ or less per 1 mm ² , more preferably 4 × 10 ⁵ or more and 1 × 10 ⁶ or less. The iron-based carbide mentioned here is mainly Fe ₃ C, but may include other _ε carbides.

In order to confirm the formation of carbides, it is effective to observe the mirror-polished sample with SEM (scanning electron microscope) or ΤΕΜ (transmission electron microscope). Carbide identification can be performed by, for example, SEM-EDS (energy dispersive X-ray analysis), EPMA (electron beam microanalyzer), FE-AES (field emission-Auger electron spectroscopy) of cross-section polished samples.

Further, in the steel sheet of the present invention, in the above-described autotempered martensite, the amount of the autotempered martensite further limiting the size and number of iron-based carbides precipitated in the autotempered martensite is appropriately determined. As below can do.

0. Ιμπι or more 0. Less than 5 × 10 ² iron carbide precipitates per lnrni ² Phototempered martensite: 3% or more in area ratio with respect to the entire autotempered martensite

Ductility is further improved by increasing the proportion of iron tempered martensite with an iron carbide content of not less than 0 and not more than 0.5 μπι at 5 × 10 ² per 1 mm ² . In order to obtain such an effect, the ratio of auto-tempered martensite in which the number of precipitates of iron carbide of 0.1 lm or more and 0.5 / im or less is 5 × 10 ² or less per 1 mm ² is determined by auto-tempered martensite. It is preferable to set the area ratio to 3% or more with respect to the whole. Incidentally, deposition numbers of 0.5 ^ m following iron-based carbide or 0.5 is lmm ² per 5X10 ² or less auto-tempered - Domarutensai DOO is, to significantly degrade the workability to be present in multiple quantities in the steel sheet, It is preferable that the ratio of such auto-tempered martensite is 40% or less in terms of the area ratio with respect to the entire auto-tempered martensite. More preferably, it is 30% or less.

Also, the ratio of auto-tempered martensite where the number of precipitates of iron-based carbides from 0. to 0.5 111 or less is 5X10 ² or less per lmm ² is 3% or more in terms of the area ratio with respect to the entire auto-tempered martensite. In this case, the iron-based carbides contained in the autotempered martensite increase in fine iron-based carbides, so that the average number of iron carbide precipitates in the entire auto-tempered martensite increases. Therefore, it is preferable that the average number of iron carbide precipitates of 5 nm or more and 0.5 m or less in the auto-tempered martensite be in the range of 1 × 10 ⁵ or more and 5 × 10 ⁶ or less per 1 mm ² . More preferably, it is in the range of 4 × 10 ⁵ or more and 5 × 10 ⁶ or less. As described above, the details of the reason why ductility is further improved are not clear, but are thought to be as follows. The ratio of auto-tempered martensite where the number of precipitates of relatively large iron-based carbides of 0.1 / im or more and 0. or less is 5 X10 ² or less per lmm ² is 3 in terms of the area ratio to the entire auto-tempered martensite. When more than% is present, the autotempered martensite structure is a structure in which a portion containing a relatively large amount of iron-based carbide and a portion containing a relatively large amount of iron-based carbide are mixed. The portion with relatively small iron-based carbides contains hard iron-tempered martensite because it contains a lot of fine iron-based carbides. On the other hand, a lot of relatively large iron-based carbides The part that contains it is soft auto-tempered martensite. By making this hard auto-tempered martensite surrounded by soft auto-tempered martensite, it is possible to suppress the deterioration of stretch flangeability caused by the difference in hardness in the auto-tempered martensite, and it is soft. By dispersing hard martensite in the hot tempered martensite, it is considered that the work hardening ability is increased and the ductility is improved.

Next, the reason why the component composition is set in the above range in the steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass%.

C: 0.1% or more 0.3% or less

C is an indispensable element for increasing the strength of steel sheets. If the C content is less than 0.1%, it is difficult to ensure both the strength of the steel sheet and workability such as ductility and stretch flangeability. On the other hand, if the amount of c exceeds 0.3%, the weld zone and the heat-affected zone are hardened, and the weldability deteriorates. Therefore, in the present invention, the C content is in the range of 0.1% to 0.3%. Preferably, it is in the range of 0.12% or more and 0.223% or less.

Si: 2.0% or less

Si is an effective element for strengthening the solid solution of ferrite, and it is preferable to contain 0.1% or more in order to ensure ductility and hardness of the ferrite. Occurrence causes deterioration of surface properties, plating adhesion and adhesion. Therefore, the Si content is 2.0% or less. Preferably, it is 1.6% or less.

Mn: 0.5% or more 3.0% or less

Mn is an effective element for strengthening steel. It is an element that stabilizes austenide and is an element necessary to secure the area ratio of the hard phase. For this purpose, Mn should be added in an amount of 0.5% or more. On the other hand, if Mn exceeds 3.0% and is added excessively, it causes deterioration of forgeability. Therefore, the Mn content should be 0.5% or more and 3.0% or less. The range is preferably 1.5% or more and 2.5% or less.

P: 0.1% or less

P causes embrittlement due to grain boundary segregation and degrades impact resistance, but is acceptable up to 0.1%. Also, when alloying hot dip galvanizing, a p content exceeding 0.1% significantly delays the alloying rate. Therefore, the P content is 0.1% or less. Preferably it is 0.05% or less. S: 0.07% or less

S is an inclusion such as MnS, and it is preferable to reduce it as much as possible because it causes deterioration of impact resistance and cracks along the metal flow of the weld. Permissible. A preferable amount of S is 0.04% or less.

A1: 1.0% or less

A1 is a ferritic element and is an effective element for controlling the amount of ferrite generated during manufacturing. However, excessive A1 content degrades slab quality during steelmaking. Therefore, the amount of A1 is 1.0% or less. Preferably, it is 0.5% or less. It should be noted that if the content of A1 is too small, deoxidation may become difficult, so the amount of A1 is preferably 0.01% or more.

N: 0.008% or less

N is an element that causes the most deterioration in the aging resistance of steel. The smaller the amount, the better. If it exceeds 0.008%, the deterioration of aging resistance becomes significant. Therefore, the N content is 0.008% or less. Preferably it is 0.006% or less.

In addition to the basic components described above, the steel plate of the present invention can appropriately contain the components described below as necessary.

Cr: 0.05% or more and 5.0% or less, V: 0.005% or more and 1.0% or less Opium Mo: One or more selected from 0.005% or more and 0.5% or less

Cr, V, and Mo have an action of suppressing the formation of pearlite during cooling from the annealing temperature, and can be added as necessary. The effect is obtained with Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% or more. On the other hand, if Cr is added in excess of 5.0%, V: 1.0%, Mo: 0.5%, the area ratio of the hard phase becomes excessive, resulting in an increase in strength more than necessary. Therefore, when these elements are contained, it is preferable that Cr: 0.005% to 5.0%, V: 0.005% to 1.0%, Mo: 0.005% to 0.5%.

In addition, Ti, Nb, B, Ni and Cu can contain one or more selected from these, and the reasons for limiting the range of inclusion are as follows.

Ti: 0.01% to 0.1% and Nb: 0.01% to 0.1%

Ti and Nb are effective for precipitation strengthening of steel. On the other hand, if it exceeds 0.1%, the workability and the shape freezing property decrease. Accordingly, the content of Ti and Nb is preferably in the range of 0.01% to 0.1%. B: 0.003% or more 0.005% or less

B has an action of suppressing the formation and growth of ferrite from the austenite grain boundary, and can be contained as necessary. The effect is obtained with 0.000 ^{to 3%} or more, while the workability and exceeds 0.0050% decreases. Therefore, when B is contained, the content is preferably in the range of 0.0003% or more and 0.0050% or less. In addition, when B is included, it is preferable to suppress the formation of BN in order to obtain the above-mentioned effect. For this reason, it is preferable to include B in combination with Ti.

Ni: 0.05% or more 2. 0% or less Opium Cu: 0.05% or more 2. 0% or less

Ni and Cu promote internal oxidation and improve plating adhesion when hot-dip zinc plating is applied. The effect is obtained at 0.05% or more respectively. On the other hand, if the content exceeds 2.0%, the workability of the steel sheet is lowered. Ni and Cu are effective elements for strengthening steel. Therefore, the contents of Ni and Cu are preferably in the range of 0.05% or more and 2.0% or less, respectively.

Ca: 0.001% or more and 0.005% or less. REM: 0.001% or more and 0.005% or less.

Ca and REM are effective elements for spheroidizing the shape of the sulfide and improving the adverse effect of the sulfide on the stretch flangeability. The effect is obtained at 0.001% or more for each. On the other hand, a content exceeding 0.005% leads to an increase in inclusions, etc., and causes internal defects on the surface. Therefore, when Ca and REM are contained, the content is preferably in the range of 0.001% to 0.005%.

In the steel sheet of the present invention, the components other than the above are Fe and inevitable impurities. However, as long as the effects of the present invention are not impaired, the inclusion of components other than those described above is not rejected.

Further, as will be described later, it is preferable for stable production that the composition of the steel sheet of the present invention satisfies M≥300 ° C, which is a relational expression with the area ratio of polygonal ferrite, that is, manufacturing. This is preferable for suppressing variation in characteristics due to variation in conditions.

In the present invention, the surface of the steel sheet may be provided with a molten zinc plated layer or an alloyed molten zinc plated layer. Next, the reason for limiting the preferred manufacturing method and conditions of the steel sheet of the present invention will be described. First, a steel slab adjusted to the above-mentioned preferred component composition is manufactured, hot-rolled, and then cold-rolled to obtain a cold-rolled steel sheet. In the present invention, these treatments are not particularly limited, and may be performed according to ordinary methods.

Here, preferable manufacturing conditions are as follows. After the billet was heated to 1 ³ 00 ° C or less than 1100 ° C, 870 ° C or higher 950T:. The following finishing hot rolling at a temperature, i.e. hot pressure a rolling finish temperature 870 ° C or higher 950 ° C The obtained hot-rolled steel sheet is rolled up at a temperature of 350 ° C or higher and 720 ° C or lower. Next, after pickling the hot-rolled steel sheet, it is cold-rolled at a rolling reduction of 40% or more and 90% or less to obtain a cold-rolled steel sheet.

It is assumed that hot-rolled steel sheets are manufactured through normal steelmaking, forging, and hot rolling processes. For example, some or all of the hot rolling process is performed by thin forging. It may be omitted and manufactured.

The obtained cold-rolled steel sheet is used for 15 seconds in the first temperature range of 700 ° C or more and 950 ° C or less, specifically in the austenite single-phase region or in the two-phase region of the austenite phase and ferrite phase. Annealing is performed for 600 seconds or less. If the annealing temperature is less than 700 ° C, or if the annealing time is less than 15 seconds, the carbide in the steel sheet will not dissolve sufficiently, or the recrystallization of the fly will not be completed, and the target ductility will be reduced. Stretch flangeability may not be obtained. On the other hand, when the annealing temperature exceeds 950 ° C, the austenite grains grow significantly, which causes coarsening of the constituent phases caused by subsequent cooling, which may deteriorate ductility and stretch flangeability. In addition, annealing for over 600 seconds results in an increase in costs associated with enormous energy consumption. Therefore, the annealing temperature and annealing time should be in the range of 700 ° C to 950 ° C and 15 seconds to 600 seconds, respectively. Preferable annealing temperature and annealing time are 760 ° C to 920 ° C and 30 seconds to 400 seconds, respectively.

Whether the cold-rolled steel sheet after annealing is in the second temperature range from the first temperature range to 420 ° C.

Cool down to 550 ° C at a rate of 3 ° C / s or more, and cool down from 5δ0 to 420 ° C using 600s or less. Thereafter, a third temperature range below 2 ⁵ 0 ° C or 420 ° C is cooled at a rate 50 ° C / sec.

Cooling conditions in the second temperature range from the first temperature range to 420 ° C are important in order to suppress the precipitation of phases other than the intended ferritic tempered martensite phase. The Of these, the temperature range from the first temperature range to 550 ° C is a temperature range where pearlite transformation is likely to occur. When the average cooling rate from the first temperature range, that is, from 700 ° C to 550 ° C, which is the lower limit temperature of the first temperature range, is less than 3 ° C / sec, perlite, etc. is deposited, and the target and Therefore, a cooling rate of 3 t / sec or more is necessary. Preferably, it is 5 ° C / second or more. On the other hand, although the upper limit of the cooling rate is not particularly defined, a special cooling facility is required to obtain a cooling rate of 200 ° C / second or higher, and therefore 200 ° C / second or lower is preferable.

The temperature range from 550 ° C to 420 ° C is the temperature range where the bainitic transformation proceeds by holding for a long time. If the time required for cooling from 520 ° C to 420 ° C exceeds 600 seconds, the targeted transformation may progress and the target structure may not be obtained. For this reason, the time required for cooling from 550 ° C to 420 ° C should be 600 seconds or less. A more preferable time is 400 seconds or less.

After the treatment in this second temperature range, it leads to the third temperature range. In this third temperature range, the martensite transformation is generated, and at the same time, the tempering process is performed to temper the martensite after transformation, and the precipitation state of carbide in the interior is optimally controlled. Obtaining a martensite is the greatest feature of the present invention. Normal martensite is obtained by quenching with water cooling after annealing. This martensite is a hard phase and contributes to increasing the strength of the steel sheet but is inferior in workability. Therefore, in order to make this martensite a tempered martensite with good workability, it is common practice to reheat the tempered steel sheet for tempering. Figure 1 schematically shows the above process. In such a normal quenching / tempering process, after the martensite transformation is completed by quenching, the structure is tempered uniformly by raising the temperature and tempering.

On the other hand, the autotempering process is a process that cools the third temperature range at a constant speed as shown in Fig. 2, and is extremely productive without quenching and tempering by reheating. It is a high method. A steel plate containing auto-tempered martensite obtained by this auto-tempering treatment has strength and workability equivalent to or higher than the steel plate tempered by quenching / reheating shown in Fig. 1. In addition, the auto-tempering process continuously and gradually advances the martensite transformation and its tempering by performing continuous cooling (including stepwise cooling and holding) in the third temperature range. It is possible to obtain a structure in which martensites with different tempering conditions are mixed. Although martensite with different tempering conditions has different properties such as strength and workability, it is possible to obtain the desired characteristics for the entire steel sheet by optimally controlling the amount of martensite with different tempering conditions by autotempering. It is. In addition, autotempering is not accompanied by rapid cooling to a low temperature range that completes all martensitic transformations, so that the residual stress in the steel sheet is small and it is advantageous to obtain a steel sheet with an excellent plate shape. It is.

In the present invention, the third temperature range is 250 ° C. or more and 420 or less. In the temperature range above 420 ° C, the bainitic transformation is likely to occur as described above. On the other hand, in the temperature range below 250, autotempering takes a long time, so continuous annealing line or continuous molten zinc plating line. In this process, the progress of the autotemper is insufficient. In this third temperature range, martensite transformation occurs, and at the same time, the tempered martensite is tempered to make it a hot tempered martensite. Must be less than ° C / sec. If the cooling rate exceeds 50 ° C / sec, the autotempering process may be insufficient and the workability of martensite may not be ensured. On the other hand, when the cooling rate is less than 0.1 ° C / second, the strength cannot be secured due to the progress of the bainitic transformation or excessive progress of the phototemper. Therefore, the cooling rate is preferably 0.1 ° C / second or more.

Further, in the method for producing a steel sheet of the present invention, the following constitution can be added as necessary.

When cooling a steel sheet at a cooling rate of 50 ° C / sec or less in the third temperature range of 250 ° C or more and 420 ° C or less, at least the temperature range of (Ms point 50) ° C or less is 1.0 ° C. It is preferable to cool at a cooling rate in the range of not less than 50 / C and not more than 50 ° C / second. This is because the precipitation of carbides in autotempered martensite is more appropriately controlled in the third temperature range, and the number of precipitations of iron-based carbides between Ο. Ϊ́ μ πι and 0.5 μιη is 1 This is because the ratio of the auto tempered martensite, which is 5 x 10 ² or less per mm ² , is 3% or more in terms of the area ratio with respect to the entire auto tempered martensite. When the cooling rate exceeds 50 ° C / sec, the progress of the phototemper is insufficient and the desired autotempered martensite cannot be obtained, and the workability of the martensite may not be ensured. On the other hand, when the cooling rate is less than 1.0 T / sec, the precipitation of iron-based carbides of 0.1 / xm or more and 0.5 / im or less Can not be the number out to 3% or more in area ratio to the total O over preparative temper Domaru beet preparative percentage of O over preparative temper Domaru beet preparative is l mm ² per 5 X 10 ² or less, the desired ductility and strength obtained Therefore, the cooling rate should be 1.0 ° C / second or more. Here, the Ms point can be obtained by measurement of thermal expansion during cooling or measurement of electrical resistance, as is usually done. Alternatively, M obtained by an approximate expression (1) of the Ms point described later may be used.

Furthermore, in the method for producing a steel sheet of the present invention, when the M shown by the following formula (1) is 300 ° C. or higher, the auto-tempering treatment can be performed stably.

M (° C) = 540- 361 X {[C%] / (1-[α%] / 100)}-6 X [Si%]-40 X [Mn%] + 30 X [Al%]

-20 X [Cr%]-35 X [V%]-10 X [Mo%]-17 X [Ni%] One 10 X [Cu%] · · · (1)

However, [X%] is the mass of alloy element X. / ₀ , [«%] is the area ratio (%) of polygonal ferrite.

The される expressed by the above equation (1) is an approximate expression of the Ms point at which the martensitic transformation that is empirically determined starts. This M is greatly related to the precipitation behavior of iron carbide from the martensite. it is conceivable that. Therefore, M can be used as an indicator that can stably obtain a wheat tempered martensite containing 5 × 10 ⁴ or more iron-based carbides of 5 nm or more and 0.5 · π μπι or less per lmm ² . Even if M is less than 300 ° C, autotempered martensite can be obtained, but the temperature at which martensite transformation and autotemper progress is low, so these processes tend to slow down and the desired auto than in the case of M≥ ³ 00 ° C in order to obtain tempering Domaru beet DOO, slows the cooling rate or prolonged cryostat is required, there is a possibility to significantly deteriorate the production efficiency, M 300 It is preferable to set it to ° C or higher.

The area ratio of polygonal ferrite is measured by, for example, image processing analysis of SEM photographs of 1000 to ³ 000 times. Polygonal ferrite is observed in the steel sheet after annealing and cooling under the above conditions. In order to increase the M to 300 ° C or more, after manufacturing a cold-rolled steel sheet having a desired component composition, the area ratio of polygonal ferrite is determined, and the alloy element content determined from the component composition of the steel sheet is combined with (1 Find the value of M from the formula. If M is less than 300 ° C, the polygonal ferrite In order to reduce the area ratio, for example, the annealing temperature in the first temperature range is set to a higher temperature, the average cooling rate from the first temperature range to 550 ° C is increased, and the desired heat treatment conditions are adjusted as appropriate. M may be obtained, and the content of the component composition in the formula (1) may be adjusted.

In addition, the steel sheet of the present invention can be subjected to hot dip zinc alloying and galvannealing. The hot dip galvanizing and alloyed hot dip galvanizing treatments are preferably carried out in a continuous hot dip galvanizing line, satisfying the annealing and cooling conditions under the conditions described above. Here, the hot dip galvanizing treatment and the alloying treatment are preferably performed in a temperature range of 420 ° C. or higher and 550 ° C. or lower. In this case, including the hot dip galvanizing treatment or further alloying treatment time, the temperature is 550 ° C. The time required for cooling from C to 420 ° C, that is, the holding time in the temperature range of 420 ° C to 550 ° C should be 600 seconds or less.

The method of galvanizing and galvanizing hot dip galvanizing is as follows. First, let the steel plate enter the squeeze bath and adjust the amount of adhesion by gas wiping. The amount of dissolved A1 in the plating bath is in the range of 0.12% to 0.22% in the case of hot dip zinc, and in the range of 0.08% to 0.18% in the case of alloyed hot dip galvanizing. And In the case of hot dip galvanizing, the temperature of the plating bath may be in the range of 450 ° C or higher and 500 ° C or lower. The temperature during alloying is preferably in the range of 450 ° C to 550 ° C. If the alloying temperature exceeds 550 ° C, carbides may precipitate excessively from the untransformed austenite or, in some cases, may become perlite, and the desired strength and ductility may not be obtained. In addition, powdering properties are also degraded. On the other hand, when the temperature during alloying is less than 450 ° C, alloying does not proceed.

The plating adhesion amount is preferably 20 to 150 _g / m ² per side. If the amount of plating deposition is less than 20 g / _m ^2, the corrosion resistance is degraded. On the other hand, even if the plating adhesion amount exceeds 150 g / m ² , the corrosion resistance is saturated, which only increases costs. Further, the alloying degree is preferably 7 to 1δ% by mass in terms of Fe content in the plating layer. If the degree of alloying is less than 7% by mass, unevenness in alloying will occur and the appearance will deteriorate, or the so-called ζ phase will be generated and the slidability will deteriorate. On the other hand, if the degree of alloying exceeds 15% by mass, a large amount of hard and brittle Γ phase is formed and the plating adhesion deteriorates.

In the present invention, the holding temperature in the first temperature range, the second temperature range, etc. must be set. However, it does not have to be constant, and even if it fluctuates within the specified range, the gist of the present invention is not impaired. The same applies to the cooling rate. Moreover, as long as the thermal history is satisfied, the steel sheet may be annealed and treated with an automatic tempering treatment by any equipment. Further, the present invention also includes temper rolling the steel plate of the present invention for shape correction after autotempering. Example

Example 1

EXAMPLES Hereinafter, the present invention will be further described with reference to examples, but the following examples are not intended to limit the present invention. In addition, changing the configuration within the scope of the gist configuration of the present invention is included in the scope of the present invention.

Steel strips with the composition shown in Table 1 were heated to 1250 ° C, then hot-rolled steel sheet hot rolled at 880 ° C was scraped at 600 ° C, and then the hot-rolled steel plate was pickled, Cold-rolled at a rolling rate of 65% to obtain a cold rolled steel sheet with a thickness of 1.2 mm. The obtained cold-rolled steel sheet was heat-treated under the conditions shown in Table 2. None of the samples in the table was quenched. The holding times in Table 2 are the holding times at the holding temperatures in Table 2, and the annealing time in the first temperature range of 700 ° C to 950 ° C is 600 for all conditions in the table. It was less than a second. For hot-dip zinc plating, plating bath temperature: 463 ° C, basis weight (per side): 50g / m ²

The test was carried out under the conditions of (double-sided). In addition, galvannealed alloying was further alloyed under the condition that the Fe% (iron content) in the adhesion layer was 9% by mass. The obtained steel sheet was subjected to temper rolling at a rolling rate (elongation rate) of 0.3% regardless of the presence or absence of plating.

table 1

(mass%)

Note) Underline indicates outside the proper range.

Table 2

* 1) Underline indicates outside the proper range.

* 2) GR: No plating (cold rolled steel sheet), GI: Hot dip zinc, GA: Hot galvanized alloy

Various properties of the steel sheet thus obtained were evaluated by the following methods.

In order to adjust the structure of the steel plates, two samples were cut from each steel plate, one was polished as it was, and the other was polished after 200 t x 2 hours of heat treatment. The polished surface was a cross section in the plate thickness direction parallel to the rolling direction. By observing the polished surface with a scanning electron microscope (SEM) at 3000 times the steel structure, the area ratio of each phase was measured, and the phase structure of each crystal grain was identified. Observation was performed for 10 fields, and the area ratio was the average of 10 fields. For autotempered martensite, polygonal ferrite, and vein, the area ratio was determined using polished samples. As-quenched martensite (untempered martensite) and residual austenite were obtained using a sample that had been heat-treated at 200 ° C for 2 hours. The reason for preparing the sample that was heat-treated at 200 ° C for 2 hours was to distinguish martensite that was not tempered and residual austenite during SEM observation. In SEM observation, it is difficult to distinguish martensite that has not been tempered from residual austenite. When martensite is turned back, iron-based carbides are formed in the martensite. The presence of these iron-based carbides makes it possible to distinguish from residual austenite. The heat treatment at 200 ° CX for 2 hours can temper the martensite without affecting the area other than the martensite, that is, without changing the area ratio of each phase. This makes it possible to distinguish from residual austenite. As a result of comparing SEM observations of both the polished sample and the sample heat-treated at 200 ° C for 2 hours, it was confirmed that there was no change in the phases other than martensite.

Next, the size and number of iron carbides in the hot tempered martensite were measured by SEM observation. Needless to say, the sample was the same as that observed above, but the sample was not heat-treated at 200 ° C. for 2 hours. Depending on the precipitation state and size of the iron-based carbide, it was observed in the range of 10,000 to 30,000 times. The size of the iron-based carbide is evaluated by the average value of the major axis and minor axis of each precipitate, and the number of those whose size is 5 mn or more and 0.5 μ αι or less is counted. The number per 1 mm ² was obtained. Observation was performed in 5 to 20 fields, and the average value was calculated from the total number of all fields in each sample to obtain the number of iron-based carbides in each sample (number per 1 _mm ^{2 of} autotempered martensite).

Auto Tempered Martensite Hardness HV is loaded with ultra micro Vickers Heavy: Measured at 0.02N, and then confirmed the structure of the autotempered martensite from which iron carbide was deposited by confirming the indentation with SEM. did.

For strength, JIS No. 5 test piece was cut out from the direction parallel to the rolling direction of the steel sheet, and the tensile test was conducted in accordance with JIS Z2241. Tensile strength (TS), yield strength (YS) and total elongation (T.E1) were measured, and the product of tensile strength and total elongation (TSXT.E1) to evaluate the balance between strength and elongation was calculated. In the present invention, the case of TSXT.E1≥14500 (MPa-%) was determined to be good.

The stretch flangeability was evaluated in accordance with Japan Iron and Steel Federation Standard JFST1001. After cutting each steel plate into lOOmmXIOOmm, clearance: punching out a 10mm diameter hole with 12% of the plate thickness, then using a 75mm diameter die, wrinkle holding force: 88.2kN, 60 ° Insert a conical punch into the hole, measure the hole diameter at the crack initiation limit, obtain the critical hole expansion rate (%) from the equation (2), and determine the stretch flangeability from this critical hole expansion rate value. evaluated. In the present invention, λ≥15% is considered good.

Limit hole expansion rate λ (%) = {(D _f -D ₀ ) / D. } X100 · · '(2)

However, D _f is the hole diameter D when the crack occurs. Is the initial hole diameter.

The above evaluation results are shown in Table 3.

Underline indicates out of range.

As is apparent from the table, the steel sheet of the present invention, pull ^ Strength: This is 900MPa or more, or, TS X T. E1≥1 ⁴⁵ 00 - shows the (MPa%) Oyopi Shinpi flangeability Since the average value is ¹⁵ % or more, it can be confirmed that both high strength and good workability are achieved. Of the invention examples, those having M of 300 ° C or higher are particularly excellent in that the stretch flangeability, particularly when the strength is increased, does not deteriorate.

On the other hand, Sample Nos. 6 and 7 have a martensite hardness of 700 and HV, and the number of iron-based carbides in the strong martensite is less than 5 × 10 ⁴ per mm ² or contains iron-based carbides. Therefore, tensile strength: 900MPa or more is satisfactory, but the value is less than 15% and the workability is poor. This is because the cooling rate in the third temperature range of Sample Nos. 6 and 7 is high and does not satisfy the condition of 50 ° C / sec. In samples No. 3 and 8, the martensite hardness satisfies HV≤700, but the number of iron carbides in the martensite is less than 5 X 10 ⁴ per mm ² , indicating the tensile strength. Satisfaction: 900MPa is satisfied, but the value of λ is less than 15% and the workability is poor. This is because the cooling rate of Sample Nos. 3 and 8 in the third temperature range is 55 / sec and does not satisfy the condition of 50 ° C / sec or less. Especially in the sample No. 8, TS X T. EL was 14500MPa ·% or less because of its relatively high C.

Based on the above, the auto-tempered martensite with sufficient martensite hardness HV ≤ 700 and the number of iron-based carbides in martensite being 5 x 10 ⁴ or more per 1 mm ² is sufficiently treated. It can be confirmed that the steel sheet of the present invention includes both high strength and workability.

Example 2 ''

In order to confirm the effect of further ductility improvement by appropriate control of the distribution state of iron-based carbides in the hot tempered martensite, temperatures of 250 ° C or higher (Ms point-50) ° C or lower in the third temperature range Samples were produced in the same manner as the samples shown in Table 2, except that the cooling rate of the area was changed as shown in Table 4. In Table 4, sample No. 9, 11, 13, 14 and 26, for the same sample and the same sample No. shown in Table 2, 250 ° C or higher (Ms point one ⁵ 0 ° C) The following temperature range is clarified. Note that M (° C) was used as the Ms point. Table 4

* 1) CR: No plating (cold rolled steel sheet), GI: Hot dip galvanizing, GA: Hot galvanizing galvanizing

Various properties of the steel sheet thus obtained were evaluated in the same manner as in Example 1. Among the autotempered Domaru beet bets, the amount of autotempered Domaru beet preparative precipitation number of 0.5 / xm following iron-based carbide or Ο.ΐμηι is 5X10 ²偭以under per lmm ² is was determined by the following method .

As described above, SEM observation of a sample that was not heat-treated at 200 ° CX for 2 hours was performed in the range of 10,000 to 3000 times, and the size of the iron-based carbides was determined as the average of the major axis and minor axis of each precipitate. The area ratio of autotempered martensite having a size of not less than 0.3 μπι and not more than 0.5; zm was measured. Observation was performed with 5 to ²⁰ fields of view.

The results are shown in Table 5.

From Table 5, Sample Nos. 11, 26, and 27 with a cooling rate in the temperature range of 250 ° C or higher (Ms point — 50) ° C or lower in the range of 1.0 ° C / second or more and 50 ° C / second or less. 29 and ³⁰ show that TS XT. EL ≥ 17000MPa ·% and the ductility is improved by properly controlling the distribution of iron carbide in autotempered martensite.

Table 5

Claims

The scope of the claims

1. In mass%,

C: 0.1% or more 0.3% or less,

Si: 2.0% or less,

Mn: 0.5% or more 3.0% or less,

P: 0.1% or less,

S: 0.07% or less,

A1: 1.0% or less

N: 0.008% or less

The balance consists of Fe and inevitable impurities, and the area ratio of the steel structure is 5% to 80% ferrite, and 15% or more tempered martensite. Residual austenite is 5% or less, as-quenched martensite is 40% or less, the average hardness of the autotempered martensite is HV≤700, and 5 nm or more in the autotempered martensite is 0. A high-strength steel sheet characterized in that the average number of precipitation of iron-based carbides of 5 μπι or less is 5 × 10 ⁴ or more per 1 mm ² and the tensile strength is 900 MPa or more.

2. The steel sheet is further in mass%,

Cr: 0.05% or more 5.0% or less,

V: 0.005% or more and 1.0% or less

Mo: 0.005% or more 0.5% or less

2. The high-strength steel sheet according to claim 1, comprising one or more elements selected from the group consisting of:

3. The steel sheet is further mass%,

Ti: 0.01% or more 0.1% or less,

Nb: 0.01% or more 0.1% or less

B: 0.0003% or more and 0.0050% or less,

Ni: 0.05% or more 2. 0% or less Cu: 0.05% or more 2. 0% or less

The high-strength steel sheet according to claim 1 or 2, which contains one or more elements selected from the group consisting of.

4. The steel sheet is further in mass%,

Ca: 0.001% or more 0.005% or less

REM: 0.001% or more 0.005% or less

4. The high-strength steel sheet according to claim 1, comprising one or two elements selected from the group consisting of:

5. The auto-tempered martensite in which the number of precipitates of iron-based carbides in the range of 0.3 μm to 0.5 / zm is 5 × 10 ² or less per 1 mm ² . The high-strength steel sheet according to any one of claims 1 to 4, wherein a ratio of is an area ratio of 3% or more with respect to the entire autotempered martensite.

6. The high-strength steel sheet according to any one of claims 1 to 5, wherein a hot-dip zinc plating layer is provided on the surface of the steel sheet.

7. The high strength steel sheet according to any one of claims 1 to 5, wherein a surface of the steel sheet is provided with an alloyed molten zinc adhesion layer.

8. The steel slab having the composition according to any one of claims 1 to 4 is hot-rolled and then cold-rolled into a cold-rolled steel plate, and then the cold-rolled steel plate is 700 ° C or higher and 950 ° After annealing for 15 seconds or more and 600 seconds or less in the first temperature range of C or lower, the cooling conditions in the second temperature range from the first temperature range to 420 ° C are set to 550 ° C from the first temperature range. The average cooling rate to C is 3 C / second or more, the time required for cooling from 550 ° C to 420 ° C is 600 seconds or less, and the third temperature range from 250 to 420 ° C is 50 ° C / second. A method for producing a high-strength steel sheet, which is cooled at the following speed to cause martensite transformation in the third temperature range, and at the same time, tempering the martensite after transformation. .

9. When the steel sheet is cooled at a cooling rate of 50 ° C / second or less in the third temperature range of 250 ^ to 420 ° C, the temperature range of at least (Ms point 50) ° C is 1.0 ° C. 9. Cooling at a rate of not less than 50 / sec and not more than 50 / sec, and causing a martensite transformation in the third temperature range, and simultaneously performing a tempering process of tempering the martensite after the transformation. Manufacturing method of high strength steel sheet.

10. The high strength according to claim 8 or 9, wherein in the steel slab, a martensitic transformation start point Ms is approximated by M represented by the following formula (1), and the value is 300¾ or more. A method of manufacturing a steel sheet.

Record

M (° C) = 540— 361X {[C%] / (1- [α%] / 100)}-6 X [Si%] -40 X [Μη%] + 30 X [Α1%]

-20 X [Cr%]-35 X [V%]-10 X [Mo%]-17 X [Ni%]-10 X [Cu%] · · ■ (1)

However, [X%] is the mass% of the component element X in the billet and is the area ratio (%) of polygonal ferrite.