JP5365217B2 - High strength steel plate and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength steel sheet having excellent formability and high strength steel of 900 MPa or more, and a method for producing the same, 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.

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 vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. However, the development of a material having both high strength and high workability is desired since the increase in strength of the steel sheet causes a decrease in forming workability. In response to such demands, various composite steel sheets such as ferrite-martensite duplex steel (DP steel) and TRIP steel utilizing transformation-induced plasticity of retained austenite have been developed so far.
For example, with regard to DP steel, Patent Document 1 discloses a low-yield-ratio high-tensile steel sheet having a tensile strength of 588 to 882 MPa, which has excellent surface properties and bending workability, by specifying the composition of components, hot rolling and annealing conditions. And its manufacturing method, Patent Document 2, proposes a method for manufacturing a high-tensile cold-rolled steel sheet having excellent bendability by prescribing hot rolling, cold rolling, and annealing conditions for a steel having a predetermined composition. ing. Patent Document 3 discloses a steel sheet excellent in collision safety and formability by specifying a martensite fraction, its particle size and mechanical properties, and a method for producing the same, and Patent Document 4 includes a component composition and martens. High-strength steel sheet, high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet with excellent stretch flangeability and impact resistance characteristics by specifying the site fraction and the particle size thereof, and its production method, Patent Document 5 Is a high strength steel sheet, high strength hot dip galvanized steel sheet and high strength steel with excellent stretch flangeability, shape freezing properties and impact resistance characteristics by defining the composition of the composition, ferrite grain size, its texture and martensite fraction. The alloyed hot-dip galvanized steel sheet and its manufacturing method, Patent Document 6, specify the component composition, the amount of martensite and the manufacturing method, thereby providing excellent mechanical properties. High-strength steel sheet and a manufacturing method thereof are proposed. Furthermore, Patent Documents 7 and 8 describe the production of high-strength hot-dip galvanized steel sheets and high-strength hot-dip galvanized steel sheets that are excellent in stretch flangeability and bendability by prescribing the component composition and manufacturing conditions in the hot-dip galvanizing line. Methods and equipment have been proposed.

  As a steel sheet having a structure including other than martensite in the hard second phase, Patent Document 9 discloses that the hard second phase is martensite and / or bainite, and the components, particle size, hardness ratio, and the like are defined. A steel sheet excellent in fatigue characteristics, Patent Document 10, proposes a steel sheet excellent in stretch flangeability by defining the component composition and the hardness ratio thereof mainly including bainite or pearlite as the second phase. Patent Document 11 discloses a high-strength, high-ductility hot-dip galvanized steel sheet excellent in hole expansibility composed of bainite and martensite as a hard second phase and a manufacturing method thereof, and Patent Document 12 describes bainite and martensite as a hard second phase. A composite structure steel sheet containing both sites and having excellent fatigue properties by defining the fraction of each constituent phase, the particle size and hardness, and the mean free path of the entire hard phase, By specifying the amount of retained austenite, a high-tensile steel plate excellent in ductility and hole expansibility, Patent Document 14 specifies the composition of components and the fraction of each phase in a steel plate containing bainite and retained austenite and / or martensite. Thus, a high-strength cold-rolled steel sheet having excellent workability has been proposed. Patent Document 15 discloses a high-strength steel sheet excellent in workability and its manufacture by defining the distribution state of hard second phase grains in ferrite and the existence ratio of grains tempered martensite and bainite therein. A method has been proposed. Furthermore, as a structure mainly composed of bainite, Patent Document 16 defines a component composition and a manufacturing process, thereby providing an ultra-high-tensile cold-rolled steel sheet excellent in delayed fracture resistance with a tensile strength of 1180 MPa or more, and a manufacturing method thereof, Patent Document 17 discloses an ultra-high-tensile cold-rolled steel sheet excellent in bendability with a tensile strength of 980 MPa or more by defining the component composition and the manufacturing method, and Patent Document 18 includes tempered martensite. An ultra-high strength thin steel sheet having a tensile strength of 980 MPa or more and a manufacturing method thereof that prevent hydrogen embrittlement by limiting the number of iron-based carbides to a certain number have been proposed.

  However, the above-described invention has the following problems. Patent Documents 1 to 7, 9 to 10, and 12 to 14 are inventions for steel sheets having a tensile strength of less than 900 MPa, and workability cannot be ensured in many cases as the strength is further increased. Further, Patent Document 1 stipulates that annealing is performed in a single phase region, and thereafter cooling is performed at 6 to 20 ° C./second to 400 ° C. In the case of a hot dip galvanized steel sheet, the plating adhesion is taken into consideration. In addition, it is necessary to raise the temperature before plating because the cooling to 400 ° C is below the plating bath temperature, and it is manufactured in a continuous hot dip galvanizing line that does not have a heating device before the plating bath. Can not. Further, 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 necessary. In Patent Document 11, the fraction of the hard second phase is defined as bainite and martensite, but the characteristics vary widely within the specified range, and in order to suppress the variations, Precision control is required. Also in Patent Document 15, in order to cool to the Ms point or less in order to generate martensite before the bainite transformation, reheating equipment is necessary, and in order to obtain stable characteristics, precise control of operating conditions is required. Since it is essential, the cost of facilities and operations increases. In Patent Documents 16 and 17, in order to obtain a structure mainly composed of bainite, it is necessary to maintain in the bainite generation temperature range after annealing, and it is difficult to ensure ductility. Need to be reheated. Patent Document 18 merely shows an improvement in hydrogen embrittlement of a steel sheet, and the workability is hardly taken into consideration except for a slight study 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, when the ratio of the hard second phase is increased, the workability of the steel sheet is hard second. It is strongly influenced by the workability of the phase. This is because when the ratio of the hard second phase is small, the ferrite itself, which is the parent phase, is deformed, and the minimum workability is ensured even when the workability of the hard second phase is not sufficient. 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. This is because the deterioration of the property becomes remarkable.

For this reason, for example, in the case of a cold-rolled steel sheet, after the martensite is generated by water quenching by adjusting the fraction of ferrite and hard second phase in a continuous annealing facility having a water quenching function, the temperature is raised and maintained. Thus, the workability of the hard second phase has been improved by tempering the martensite.
However, in the case of equipment that cannot be tempered by heating or holding at a high temperature after generating such martensite, it is possible to ensure the strength, but workability of hard second phase such as martensite. It was difficult to ensure.

For the purpose of securing stretch flangeability by utilizing a hard phase other than martensite, the workability of the hard second phase is ensured by using ferrite as the parent phase and bainite or pearlite containing carbide in the hard second phase. In this case, sufficient ductility cannot be secured.
Further, when bainite is used, there is a problem that the characteristics change greatly due to variations in temperature and holding time in the bainite formation region. Further, even when the second phase is martensite or retained austenite (including bainite containing retained austenite), in order to ensure stretch flangeability as well as ductility, for example, the second phase structure is composed of martensite and bainite. Studies such as a mixed organization have been conducted.
However, in order to make the second phase a mixed structure of various phases and to control the fraction and the like with high accuracy, it is necessary to precisely control the heat treatment conditions, which may cause problems in manufacturing stability and the like. There were many.

Japanese Patent No. 1853389 Japanese Patent No. 3610883 Japanese Patent Laid-Open No. 11-61327 Japanese Patent Laid-Open No. 2003-213369 JP 2003-213370 A Special Table 2003-505604 JP-A-6-93340 JP-A-6-108152 Japanese Patent Laid-Open No. 7-11383 Japanese Patent Laid-Open No. 10-60593 JP 2005-281854 JP Japanese Patent No. 3321204 JP 2001-207234 A JP-A-7-207413 JP 2005-264328 A Japanese Patent No. 2616350 Japanese Patent No. 2621744 Japanese Patent No. 2826058

The present invention advantageously solves the above-mentioned problems, minimizes the generation of bainite whose properties such as strength and formability are likely to vary, and has a tensile strength of 900 MPa or more that can achieve both high strength and excellent formability. It is an object of the present invention to supply a high-strength steel sheet together with its advantageous production method.
The formability is evaluated by TS × T.EL and the λ value which is an index of stretch flangeability. In the present invention, TS × T.El ≧ 14500 MPa ·% and λ ≧ 15% are the target characteristics. To do.

In order to solve the above problems, the inventors have studied the martensite formation process, particularly the influence of the cooling condition of the steel sheet on the martensite.
As a result, if the heat treatment conditions after cold rolling are optimally controlled, martensite after transformation is tempered simultaneously with martensite transformation, and the autotempered martensite generated by this treatment is controlled to a predetermined ratio. In addition, by appropriately controlling the distribution of iron-based carbides in autotempered martensite, a high-strength steel sheet having excellent formability and tensile strength targeted by the present invention: high strength of 900 MPa or more is obtained. I got the knowledge that

The present invention has been completed through further studies based on the above findings, and the gist of the present invention is as follows.
1. % By mass
C: 0.1% or more and 0.3% or less,
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, the balance is made of Fe and inevitable impurities, and the steel structure has an area ratio of 5% to 80% ferrite and 15% or more autotempered martensite. In addition, the bainite is 10% or less, the retained austenite is 5% or less, the as-quenched martensite is 40% or less, the average hardness of the autotempered martensite is HV ≦ 700, and the autotempered martensite A high-strength steel sheet characterized in that the average number of precipitation of iron-based carbides of 5 nm to 0.5 μm is 5 × 10 ⁴ or more per mm ² and the tensile strength is 900 MPa or more.

2. The steel sheet is further in mass%,
Cr: 0.05% to 5.0%,
V: 0.005% to 1.0% and
Mo: The high-strength steel sheet according to 1 above, which contains one or more elements selected from 0.005% to 0.5%.

3. The steel sheet is further in 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% to 2.0% and
Cu: The high-strength steel sheet according to 1 or 2 above, which contains one or more elements selected from 0.05% to 2.0%.

4). The steel sheet is further in mass%,
Ca: 0.001% to 0.005% and
REM: The high-strength steel sheet according to any one of 1 to 3 above, which contains one or two elements selected from 0.001% to 0.005%.

5. Of the autotempered martensite, the proportion of autotempered martensite in which the number of precipitated iron carbides of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² is the entire autotempered martensite. The high-strength steel sheet according to any one of 1 to 4 above, wherein the area ratio is 3% or more with respect to the steel sheet.

6). 6. The high-strength steel plate according to any one of 1 to 5, wherein a hot-dip galvanized layer is provided on the surface of the steel plate.

7). 6. The high-strength steel plate according to any one of 1 to 5, wherein an galvannealed layer is provided on the surface of the steel plate.

8). It is a manufacturing method of the high strength steel plate of any one of said 1 thru | or 7, Comprising: The steel slab which becomes a component composition of any one of said 1 thru | or 4 is cold-rolled after hot rolling. The cold rolled steel sheet is then subjected to annealing for 15 seconds to 600 seconds in a first temperature range of 700 ° C. or more and 950 ° C. or less, and then the second temperature from the first temperature range to 420 ° C. The cooling conditions in the temperature range are: the average cooling rate from the first temperature range to 550 ° C is 3 ° C / second or more, the time required for cooling from 550 ° C to 420 ° C is 600 seconds or less, 250 ° C to 420 ° C The third temperature range is cooled at a rate of 50 ° C./second or less, and martensite transformation is caused in the third temperature range, and at the same time, an autotempering process is performed to temper the martensite after transformation. Manufacturing method of high strength steel sheet.

9. When cooling the steel sheet at a cooling rate of 50 ° C./second or less in the third temperature range of 250 ° C. or more and 420 ° C. or less, at least a temperature range of (Ms point−50) ° C. or less is 1.0 ° C./second or more and 50 ° C./second. 9. The production of the high-strength steel sheet according to 8 above, wherein cooling is performed at the following speed to cause martensite transformation in the third temperature range, and at the same time, performing autotempering to temper the martensite after transformation. Method.

10. In the steel slab, the martensitic transformation start point Ms is approximated by M represented by the following formula (1), and the M is 300 ° C. or higher. Production method.
M (° C.) = 540−361 × {[C%] / (1− [α%] / 100)} − 6 × [Si%] − 40 × [Mn%] + 30 × [Al%] −
20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%] − 10 × [Cu%] (1)
However, [X%] is the mass% of the component element X of the steel slab, and [α%] is the area ratio (%) of polygonal ferrite.

According to the present invention, by containing an appropriate amount of autotempered martensite in the steel sheet and appropriately controlling the distribution state of carbides in the autotempered martensite, high strength and excellent workability are achieved. And a high strength steel plate with a tensile strength of 900 MPa or more with excellent ductility. Therefore, it greatly contributes particularly to the weight reduction of the automobile body.
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, it does not require special production equipment, and is also suitable for hot dip galvanizing or alloying hot dip galvanizing processes. Since it can be easily applied, it contributes to process saving and cost reduction.

It is the schematic diagram which showed the hardening and tempering process of obtaining a normal tempered martensite. It is the schematic diagram which showed the autotempering process of obtaining autotempered martensite according to this invention.

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

Ferrite area ratio: 5% or more and 80% or less In order to achieve both workability and tensile strength: 900 MPa or more, the ratio of ferrite to the hard phase described below is important, and the ferrite area ratio is 5% or more and 80 % Or less is required. If the ferrite area ratio is less than 5%, ductility cannot be ensured. On the other hand, if the ferrite area ratio exceeds 80%, the area ratio of the hard phase cannot be secured and the strength is insufficient. A preferable ferrite area ratio is in the range of 10% to 65%.

Area ratio of autotempered martensite: 15% or more In the present invention, autotempered martensite is not so-called tempered martensite obtained by quenching / tempering treatment as in the prior art, but martensite transformation by autotempering treatment. It means a structure obtained by simultaneously proceeding tempering. The structure is not a uniformly tempered structure formed by heating and tempering after completion of martensitic transformation by quenching, as in normal quenching / tempering treatment, but a cooling process in the region below the Ms point. Is a structure in which martensite transformation and its tempering are advanced step by step to mix martensite with different tempering conditions.
This auto temper martensite is a hard phase for increasing the strength. If the area ratio of autotempered martensite is less than 15%, strength cannot be secured and work hardening of ferrite cannot be accelerated, so the area ratio of autotempered martensite needs to 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 bainite, retained austenite, and as-quenched martensite may be formed. However, these phases are formed within the allowable range described below. There is no problem. Hereinafter, these allowable ranges will be described.

Area ratio of bainite: 10% or less (including 0%)
Although bainite is a hard phase that contributes to high strength, its characteristics may change greatly depending on the temperature range of its formation, which may increase the material variation. Is acceptable. Preferably it is 5% or less.

Residual austenite area ratio: 5% or less (including 0%)
Residual austenite is transformed during processing into hard martensite, which reduces stretch flangeability. For this reason, it is desirable that there is 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%)
Since as-quenched martensite is extremely inferior in workability, 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. Note that as-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 autotempered martensite: HV ≦ 700
When the average hardness of the autotempered martensite is 700 <HV, the stretch flangeability deteriorates remarkably, so HV ≦ 700. Preferably, HV ≦ 630.

Iron-based carbides in autotempered martensite:
Size: 5 nm or more and 0.5 μm or less, Average number of precipitates: 5 × 10 ⁴ or more per 1 mm ² Autotempered martensite is martensite that has been heat-treated (autotempered) by the method of the present invention. Even when the average hardness of domartensite is HV ≦ 700, if the autotempering treatment is inappropriate, the workability deteriorates. The degree of autotempering can be confirmed by the production status (distribution state) of iron carbide in autotempered martensite. Among these iron-based carbides, those having a size of 5 nm or more and 0.5 μm or less have an average number of precipitations of 5 × 10 ⁴ or more per 1 mm ^2, so that it can be determined that a desired autotempering treatment has been performed. . The reason why the size of the iron-based carbide is less than 5 nm is not considered because it does not affect the workability of autotempered martensite. On the other hand, iron carbides having a size exceeding 0.5 μm may reduce the strength of autotempered martensite, but the effect on the workability is negligible, so it is not subject to judgment. If the number of iron-based carbides is less than 5 × 10 ⁴ per 1 mm ² , the effect of improving workability, particularly stretch flangeability, cannot be obtained, so it is judged that autotempering 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 referred to here is mainly Fe ₃ C, but may include other ε carbides and the like.
In order to confirm the formation of carbides, it is effective to observe a mirror-polished sample by SEM (scanning electron microscope) or TEM (transmission electron microscope). The carbide can be identified by, for example, SEM-EDS (energy dispersive X-ray analysis), EPMA (electron beam microanalyzer), FE-AES (field emission-Auger electron spectroscopy) of a cross-section polished sample.

  Further, in the steel sheet of the present invention, in the above autotempered martensite, the amount of autotempered martensite further limiting the size and number of iron-based carbides precipitated in the autotempered martensite is appropriately set as follows. It can be like this.

Auto-tempered martensite in which the number of precipitates of iron-based carbides of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² : 3% or more in terms of area ratio relative to the entire auto-tempered martensite Auto-tempered martens The ductility is further improved by increasing the ratio of the number of precipitates of iron-based carbides of 0.1 μm or more and 0.5 μm or less of 5 × 10 ² or less 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 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² is determined as the whole auto-tempered martensite. Preferably, the area ratio is 3% or more. In addition, if auto-tempered martensite with a number of iron-based carbides of 0.1 μm or more and 0.5 μm or less being 5 × 10 ² or less per 1 mm ² exists in a large amount in the steel sheet, the workability will be significantly deteriorated. The proportion of such autotempered martensite is preferably 40% or less in terms of the area ratio relative to the entire autotempered martensite. More preferably, it is 30% or less.

The ratio of autotempered martensite in which the number of precipitates of iron carbide of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² is 3% or more in terms of the area ratio with respect to the entire autotempered martensite. In this case, since the iron-based carbide contained in the autotempered martensite contains a large amount of fine iron-based carbide, the average number of iron carbide precipitates in the entire autotempered martensite increases. Therefore, it is preferable that the average number of iron-based carbides of 5 nm to 0.5 μm in autotempered martensite be in the range of 1 × 10 ⁵ to 5 × 10 ⁶ per mm ² . More preferably, it is the range of 4 × 10 ⁵ or more and 5 × 10 ⁶ or less.

Although the details of the reason why the ductility is further improved as described above are not clear, it is considered as follows. The ratio of auto-tempered martensite in which the number of precipitates of relatively large iron-based carbides of 0.1 to 0.5 μm is 5 × 10 ² or less per 1 mm ² is 3% or more in terms of the area ratio with respect to the entire auto-tempered martensite. When 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 a relatively small amount of iron-based carbide is hard autotempered martensite because it contains a lot of fine iron-based carbide. On the other hand, a portion containing a relatively large amount of iron-based carbide is soft autotempered martensite. By making this hard auto-tempered martensite surrounded by soft auto-tempered martensite, the deterioration of stretch flangeability caused by the hardness difference in the auto-tempered martensite can be suppressed, and soft It is considered that by dispersing hard martensite in such auto-tempered martensite, work hardening ability is increased and 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 compositions shall mean the mass%.

C: 0.1% or more and 0.3% or less C is an element indispensable for increasing the strength of a steel sheet. If the C content is less than 0.1%, it is possible to ensure both the strength of the steel sheet and workability such as ductility and stretch flangeability. Have difficulty. On the other hand, if the amount of C exceeds 0.3%, the welded part and the heat-affected part are markedly 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 0.12% or more and 0.23% or less of range.

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 to ensure ductility and ferrite hardness. However, excessive addition of Si causes surface properties due to the occurrence of red scale and the like. And deterioration of plating adhesion and adhesion. Therefore, the Si content is 2.0% or less. Preferably, it is 1.6% or less.

Mn: 0.5% to 3.0%
Mn is an element effective for strengthening steel. Moreover, it is an element which stabilizes austenite, and is an element necessary for ensuring the area ratio of a hard phase. For this purpose, Mn needs to be added in an amount of 0.5% or more. On the other hand, if Mn is added in excess of 3.0%, castability is deteriorated. Therefore, the Mn content is 0.5% or more and 3.0% or less. Preferably it is 1.5 to 2.5% of range.

P: 0.1% or less P causes embrittlement due to grain boundary segregation and deteriorates impact resistance, but is acceptable up to 0.1%. In addition, when alloying hot dip galvanizing is performed, an amount of P exceeding 0.1% significantly delays the alloying speed. 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 impact resistance deterioration and cracks along the metal flow of the weld, but from the viewpoint of manufacturing cost Up to 0.07% is allowed. A preferable amount of S is 0.04% or less.

Al: 1.0% or less
Al is a ferrite-forming element and is an effective element for controlling the amount of ferrite produced during production. However, excessive inclusion of Al deteriorates slab quality during steelmaking. Therefore, the Al content is 1.0% or less. Preferably, it is 0.5% or less. In addition, when there is too little content of Al, since deoxidation may become difficult, Al amount is preferable 0.01% or more.

N: 0.008% or less N is an element that most deteriorates 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.

  Moreover, in the steel plate of this invention, the component described below other than the above-mentioned basic component can be appropriately contained as necessary.

One or more selected from Cr: 0.05% to 5.0%, V: 0.005% to 1.0% and Mo: 0.005% to 0.5%
Cr, V, and Mo have an effect of suppressing the formation of pearlite during cooling from the annealing temperature, and can be added as necessary. The effect is obtained when Cr: 0.05% or more, V: 0.005% or more, Mo: 0.005% or more. On the other hand, excessive addition over Cr: 5.0%, V: 1.0%, Mo: 0.5% leads to an unnecessarily high strength due to an excessive area ratio of the hard phase. 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%.

  Moreover, about Ti, Nb, B, Ni, and Cu, 1 type selected from these or 2 types or more can be contained, The reason for limitation of the containing range is 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, and the effect can be obtained at 0.01% or more. On the other hand, when it exceeds 0.1%, workability and shape freezing property are lowered. Accordingly, the Ti and Nb contents are preferably in the range of 0.01% to 0.1%, respectively.

B: 0.0003% or more and 0.0050% or less B has an action of suppressing the formation / growth of ferrite from the austenite grain boundary, and can be contained as necessary. The effect can be obtained at 0.0003% or more, while if it exceeds 0.0050%, the workability decreases. Therefore, when B is contained, the content is preferably in the range of 0.0003% to 0.0050%. In addition, in containing B, it is preferable to suppress the production | generation of BN in order to acquire the said effect, For this reason, it is preferable to make it contain together with Ti.

Ni: 0.05% to 2.0% and Cu: 0.05% to 2.0%
Ni and Cu promote internal oxidation and improve plating adhesion when hot dip galvanizing is performed. The effect is obtained at 0.05% or more. On the other hand, the content exceeding 2.0% lowers the workability of the steel sheet. Ni and Cu are effective elements for strengthening steel. Therefore, the contents of Ni and Cu are preferably set in the range of 0.05% or more and 2.0% or less, respectively.

Ca: 0.001% or more and 0.005% or less and REM: One or two selected from 0.001% or more and 0.005% or less
Ca and REM are effective elements for spheroidizing the shape of sulfide and improving the adverse effect of sulfide on stretch flangeability. The effect can be obtained at 0.001% or more. On the other hand, a content exceeding 0.005% causes an increase in inclusions and the like, and causes surface and internal defects. 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, components other than those described 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, variations in manufacturing conditions. This is preferable for suppressing variation in characteristics due to.

  In the present invention, a hot dip galvanized layer or an alloyed hot dip galvanized layer may be provided on the surface of the steel sheet.

Next, the reason for limiting the preferred manufacturing method and conditions of the steel sheet of the present invention will be described.
First, after manufacturing the steel slab adjusted to said suitable component composition, it hot-rolls, and then cold-rolls to make a cold-rolled steel sheet. In the present invention, these treatments are not particularly limited, and may be performed according to ordinary methods.
Here, suitable manufacturing conditions are as follows. After heating the steel slab to 1100 ° C or higher and 1300 ° C or lower, finish hot rolling at a temperature of 870 ° C or higher and 950 ° C or lower, that is, the hot rolling finish temperature is set to 870 ° C or higher and 950 ° C or lower. Is wound at a temperature of 350 ° C. or higher and 720 ° C. or lower. Next, after pickling the hot-rolled steel sheet, cold rolling is performed at a rolling reduction of 40% or more and 90% or less to obtain a cold-rolled steel sheet.
In addition, although the case where it manufactures through each process of normal steelmaking, casting, and hot rolling is assumed for a hot-rolled steel plate, for example, a part or all of a hot rolling process is abbreviate | omitted by thin casting etc. May be.

  The obtained cold-rolled steel sheet is annealed for 15 seconds or more and 600 seconds or less in a first temperature range of 700 ° C. or more and 950 ° C. or less, specifically in an austenite single phase region or a two-phase region of an austenite phase and a ferrite phase. Apply. If the annealing temperature is less than 700 ° C, or if the annealing time is less than 15 seconds, the carbides in the steel sheet will not dissolve sufficiently or the recrystallization of ferrite will not be completed, and the target ductility and stretch flangeability will not be achieved. It may not be obtained. On the other hand, when the annealing temperature exceeds 950 ° C., the growth of austenite grains is remarkable, which causes coarsening of constituent phases caused by subsequent cooling, and may deteriorate ductility and stretch flangeability. In addition, annealing exceeding 600 seconds causes an increase in cost due to a great energy consumption. For this reason, the annealing temperature and annealing time are in the range of 700 ° C. or more and 950 ° C. or less and 15 seconds or more and 600 seconds or less, respectively. A preferable annealing temperature and annealing time are 760 ° C. or more and 920 ° C. or less and 30 seconds or more and 400 seconds or less, respectively.

The cold-rolled steel sheet after annealing is cooled at a rate of 3 ° C / second or more from the first temperature range to 550 ° C in the second temperature range from the first temperature range to 420 ° C, and from 550 ° C to 420 ° C. Cool down the cooling time to 600 seconds or less. Thereafter, the third temperature range of 250 ° C. or more and 420 ° C. or less is cooled at a rate of 50 ° C./second or less.

  The cooling condition in the second temperature range from the first temperature range to 420 ° C. is important in order to suppress precipitation of phases other than the intended ferrite and autotempered martensite phases. 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, 700 ° C. to 550 ° C., which is the lower limit temperature of the first temperature range, is less than 3 ° C./second, pearlite or the like is deposited, and the target structure cannot be obtained. In some cases, a cooling rate of 3 ° C./second 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 a temperature range in which the bainite transformation proceeds by holding for a long time. When the time required for cooling from 520 ° C. to 420 ° C. exceeds 600 seconds, the bainite transformation proceeds and the target structure may not be obtained. For this reason, the time required for cooling from 550 ° C. to 420 ° C. is 600 seconds or less. A more preferable time is 400 seconds or less.

  After the treatment in the second temperature range, it is led to the third temperature range. In this third temperature range, martensite transformation is generated, and at the same time, the tempered martensite is tempered, and the tempered martensite is optimally controlled inside the carbide. Is the greatest feature of the present invention.

  Normal martensite can be 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 tempered martensite with good workability, it is common practice to reheat the tempered steel sheet for tempering. FIG. 1 schematically shows the above steps. In such normal quenching / tempering treatment, the martensite transformation is completed by quenching, and then the temperature is raised and the tempering treatment is performed to obtain a uniform tempered structure.

  On the other hand, the autotempering process is a process for cooling the third temperature range at a constant speed as shown in FIG. 2, and is a highly productive method that does not involve tempering by quenching and reheating. It is. The steel plate containing autotempered martensite obtained by this autotempering treatment has the same or higher strength and workability as the steel plate tempered by quenching and reheating shown in FIG. In addition, the autotempering process enables continuous and stepwise martensitic transformation and tempering by performing continuous cooling (including stepwise cooling and holding) in the third temperature range. It is possible to obtain an organization in which different martensites are mixed. Martensite with different tempered conditions has different properties such as strength and workability, but by optimally controlling the amount of martensite with different tempered conditions by autotempering, it is possible to obtain the desired characteristics as a whole steel sheet. is there. Furthermore, since autotempering does not involve rapid cooling to a low temperature range that completes all martensitic transformations, the residual stress in the steel sheet is small, and it is also advantageous in that a steel sheet having an excellent plate shape can be obtained. is there.

  In the present invention, the third temperature range is 250 ° C. or higher and 420 ° C. or lower. In the temperature range above 420 ° C, bainite transformation is likely to occur as described above. On the other hand, in the temperature range below 250 ° C, autotempering takes a long time. Temper progress is insufficient. In this third temperature range, martensite transformation is caused, and at the same time, the tempered martensite is converted to autotempered martensite, and the cooling rate of the steel plate in the third temperature range is 50 ° C / second or less. It is necessary to. When the cooling rate exceeds 50 ° C./second, the progress of the autotemper 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 bainite transformation may proceed or the autotemper may proceed excessively, so that the strength cannot be ensured. It is preferable to set it to at least ° C / second.

  Moreover, in the manufacturing method of the steel plate of this invention, the following structures can be added suitably as needed.

When the steel sheet is cooled at a cooling rate of 50 ° C / second 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 -50) ° C or less is 1.0 ° C / second or more and 50 ° C / second or less. It is preferable to cool at a cooling rate in the range. This is because the precipitation of carbides in auto-tempered martensite is more appropriately controlled in the third temperature range, and the number of precipitation of iron-based carbides of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² per 1 mm ^2. This is because the ratio of the autotempered martensite as described below is 3% or more in terms of the area ratio with respect to the entire autotempered martensite. When the cooling rate exceeds 50 ° C./second, the progress of autotemper is insufficient, and desired autotempered martensite cannot be obtained, and the workability of martensite may not be ensured. On the other hand, when the cooling rate is less than 1.0 ° C / second, the ratio of autotempered martensite in which the number of precipitates of iron carbide of 0.1 µm or more and 0.5 µm or less is 5 × 10 ² or less per 1 mm ² Since the area ratio with respect to the entire domartenite cannot be 3% or more and desired ductility and strength cannot be obtained, the cooling rate is 1.0 ° C./second or more. Here, the Ms point can be obtained by measurement of thermal expansion during cooling or measurement of electric resistance, as is usually done. Moreover, you may use M calculated | required by the approximate expression (1) of Ms point mentioned later.

Furthermore, in the manufacturing method of the steel plate of this invention, when M shown by following (1) Formula is 300 degreeC or more, an auto temper process can be performed stably.
M (° C.) = 540−361 × {[C%] / (1− [α%] / 100)} − 6 × [Si%] − 40 × [Mn%] + 30 × [Al%] −
20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%] − 10 × [Cu%] (1)
However, [X%] is the mass% of the alloy element X, and [α%] is the area ratio (%) of polygonal ferrite.
M represented by the above equation (1) is an approximate expression of the Ms point at which martensitic transformation starts empirically, and this M is greatly related to the precipitation behavior of iron carbide from martensite. it is conceivable that. Therefore, M can be used as an index for stably obtaining autotempered martensite containing 5 × 10 ⁴ or more iron-based carbides of 5 nm to 0.5 μm per mm 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 be slow, and the desired autotempered martensite. Compared to the case of M ≧ 300 ° C. in order to obtain the site, it is necessary to slow down the cooling rate or hold the low temperature for a long time, and there is a possibility that the production efficiency is remarkably deteriorated. preferable.
The area ratio of polygonal ferrite is measured, for example, by image processing / analysis of 1000 to 3000 times SEM photographs. Polygonal ferrite is observed in the steel sheet after annealing and cooling under the above conditions. In order to set the M to 300 ° C. or higher, after manufacturing a cold-rolled steel sheet having a desired component composition, the area ratio of polygonal ferrite is obtained, and the content of the alloy element obtained from the component composition of the steel sheet is combined with the above formula (1). The value of M may be obtained from When M is less than 300 ° C, the average cooling rate from the first temperature range to 550 ° C, for example, is set so that the annealing temperature in the first temperature range is higher so that the area ratio of polygonal ferrite becomes smaller. The heat treatment conditions may be adjusted as appropriate so that the desired M can be obtained, and the content of the component composition in the formula (1) may be adjusted.

  Further, the steel sheet of the present invention can be subjected to hot dip galvanizing and alloying hot dip galvanizing. The hot dip galvanizing and alloying hot dip galvanizing treatment is preferably performed in a continuous hot dip galvanizing line while satisfying the annealing and cooling conditions under the above-described conditions. 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, from the hot dip galvanizing treatment or further including the alloying treatment time to 550 ° C. to 420 ° C. The time required for cooling, that is, the holding time in the temperature range from 420 ° C. to 550 ° C. may be 600 seconds or less.

The methods of hot dip galvanizing and alloying hot dip galvanizing are as follows. First, the steel sheet is infiltrated into the plating bath, and the amount of adhesion is adjusted by gas wiping or the like. The amount of dissolved Al in the plating bath is in the range of 0.12% to 0.22% in the case of hot dip galvanizing, and in the range of 0.08% to 0.18% in the case of alloyed hot dip galvanizing.
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, and when alloying hot dip galvanizing is further performed, the temperature during alloying Is preferably in the range of 450 ° C to 550 ° C. When the alloying temperature exceeds 550 ° C., the carbides may be excessively precipitated from untransformed austenite or, in some cases, pearlite may not be obtained, so that desired strength and ductility may not be obtained. Also, the powdering property is deteriorated. 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 is less than 20 g / m ² , the corrosion resistance will deteriorate. On the other hand, even if the plating adhesion amount exceeds 150 g / m ² , the corrosion resistance effect is saturated and only increases the cost. Moreover, it is preferable that an alloying degree shall be 7-15 mass% by Fe content in a 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 or the like does not necessarily 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. Further, as long as the thermal history is satisfied, the steel sheet may be annealed and auto-tempered by any equipment. Furthermore, it is also included in the scope of the present invention to perform temper rolling on the steel sheet of the present invention for shape correction after autotempering.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further, the following Example does not limit this 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.

After heating the steel slab with the composition shown in Table 1 to 1250 ° C, the hot-rolled steel sheet hot rolled at 880 ° C was rolled up at 600 ° C, then pickled and then 65% rolled. The steel sheet was cold-rolled at a rate of 1.2 mm to obtain a cold-rolled steel sheet. 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 time in Table 2 is the holding time at the holding temperature in Table 2, and the annealing time in the first temperature range of 700 ° C. or more and 950 ° C. or less is 600 seconds or less for any of the conditions in the table. there were.
The hot dip galvanization was performed under the conditions of a plating bath temperature: 463 ° C. and a basis weight (per one side): 50 g / m ² (double-side plating). Further, the alloying hot dip galvanizing was further alloyed under the condition that the Fe% (iron content) in the plating layer was 9% by mass. The obtained steel sheet was subjected to temper rolling with a rolling rate (elongation rate): 0.3% regardless of whether or not plating was present.

Various properties of the steel sheet thus obtained were evaluated by the following methods.
In order to investigate 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 heat treatment at 200 ° C. × 2 hours. 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 an average value of 10 fields. Autotempered martensite, polygonal ferrite, and bainite were obtained by polishing the area ratios of the samples as they were. As-quenched martensite (untempered martensite) and retained austenite were determined for the area ratio using a sample subjected to heat treatment at 200 ° C. for 2 hours. The reason why a sample subjected to heat treatment at 200 ° C. for 2 hours was prepared is to distinguish martensite that has not been tempered and residual austenite at the time of SEM observation. In SEM observation, it is difficult to distinguish martensite that has not been tempered from retained austenite. When martensite is tempered, iron-based carbides are formed in martensite, and the presence of the iron-based carbides makes it possible to distinguish from retained austenite. Heat treatment at 200 ° C for 2 hours can temper martensite without affecting other than martensite, that is, without changing the area ratio of each phase. This makes it possible to distinguish from austenite. In addition, as a result of SEM observation and comparison 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-based carbides in autotempered martensite were measured by SEM observation. Needless to say, the sample was the same as that observed in the above-described structure observation, but was not subjected to heat treatment at 200 ° C. for 2 hours. It was observed in the range of 10,000 to 30,000 times depending on the precipitation state and size of the iron-based carbide. 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 nm or more and 0.5 μm or less is counted, and the number per 1 mm ^{2 of} autotempered martensite. Asked. Observation was performed in 5 to 20 visual fields, and the average value was calculated from the total number of all visual fields in each sample to obtain the number of iron-based carbides in each sample (number per 1 mm ^{2 of} autotempered martensite).

  The hardness of autotempered martensite HV is measured with a load of 0.02N with ultra micro Vickers, and then the indentation of iron-based carbides is confirmed by checking the indentation with SEM. Was confirmed, and the average value of 10 or more measured values was used.

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

Stretch flangeability was evaluated in accordance with Japan Iron and Steel Federation Standard JFST1001. After cutting each steel plate to 100mm x 100mm, clearance: punching out a hole with a diameter of 10mm at 12% of the plate thickness, using a die with an inner diameter of 75mm, with a wrinkle holding force of 88.2kN, Measure the hole diameter at the crack initiation limit by pushing a 60 ° conical punch into the hole, obtain the critical hole expansion rate (%) from the formula (2), and evaluate the stretch flangeability from the value of this critical hole expansion rate did. In the present invention, λ ≧ 15% is considered good.
Limit hole expansion rate λ (%) = {(D _f −D ₀ ) / D ₀ } × 100 (2)
However, _{D f} is the hole diameter at crack initiation (mm), _{D 0} is the initial hole diameter (mm).

  The above evaluation results are shown in Table 3.

As is apparent from the table, the steel sheet of the present invention has a tensile strength of 900 MPa or more, and TS × T.El ≧ 14500 (MPa ·%) and the value of λ indicating stretch flangeability is 15% or more. Therefore, 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, in particular, the stretch flangeability does not deteriorate even when the strength is increased.
On the other hand, sample Nos. 6 and 7 have a martensite hardness of 700 <HV and the number of iron-based carbides in martensite is less than 5 × 10 ⁴ per mm ² or does not contain iron-based carbides. Therefore, the tensile strength: 900 MPa or more is satisfied, but the value of λ is less than 15% and the workability is inferior. 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./second. In samples No. 3 and 8, the martensite hardness satisfies HV ≦ 700, but the tensile strength: 900 MPa because the number of iron-based carbides in martensite is less than 5 × 10 ⁴ per mm ^2. Is satisfactory, but the value of λ is less than 15% and the workability is poor. This is because the cooling rate in the third temperature range of Samples Nos. 3 and 8 is 55 ° C./second and does not satisfy the condition of 50 ° C./second or less. In particular, the sample No. 8 has a relatively high C, so TS × T.EL was 14500 MPa ·% or less.
Based on the above, auto-tempered martensite with a sufficient martensite hardness of HV ≦ 700 and the number of iron-based carbides in martensite being 5 × 10 ⁴ or more per 1 mm ² is sufficiently applied. It can be confirmed that the steel sheet of the present invention includes both high strength and workability.

  In order to confirm the effect of further ductility improvement by appropriate control of the distribution state of iron-based carbides in autotempered martensite, the cooling rate in the temperature range of 250 ° C or higher (Ms point -50) ° C in the third temperature range A sample was manufactured in the same manner as the sample shown in Table 2, except that the value was changed as shown in Table 4. In Table 4, sample Nos. 9, 11, 13, 14 and 26 have a temperature range of 250 ° C. or higher (Ms point −50 ° C.) with respect to the same sample as the same sample No. shown in Table 2. It has been clarified. Note that M (° C.) was used as the Ms point.

Various properties of the steel sheet thus obtained were evaluated in the same manner as in Example 1. Of the autotempered martensite, the amount of autotempered martensite in which the number of precipitates of iron carbide of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² was determined by the following method. .
As mentioned above, SEM observation of a sample that has not been heat-treated at 200 ° C. for 2 hours in the range of 10,000 to 30000 times, and the size of the iron-based carbide is the average value of the major axis and minor axis of each precipitate Evaluation was made to measure the area ratio of autotempered martensite having a size of 0.1 μm or more and 0.5 μm or less. Observation was performed in 5 to 20 fields of view.

  The results are shown in Table 5.

  From Table 5, Sample Nos. 11, 26, 27, 29, and 30 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 to 50 ° C./second are It can be confirmed that the distribution state of the iron-based carbide in the autotempered martensite is appropriately controlled and TS × T.EL ≧ 17000 MPa ·%, indicating that the ductility is improved.

Claims (10)

% By mass
C: 0.1% or more and 0.3% or less,
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, the balance is made of Fe and inevitable impurities, and the steel structure has an area ratio of 5% to 80% ferrite and 15% or more autotempered martensite. In addition, the bainite is 10% or less, the retained austenite is 5% or less, the as-quenched martensite is 40% or less, the average hardness of the autotempered martensite is HV ≦ 700, and the autotempered martensite A high-strength steel sheet characterized in that the average number of precipitation of iron-based carbides of 5 nm to 0.5 μm is 5 × 10 ⁴ or more per mm ² and the tensile strength is 900 MPa or more. The steel sheet is further in mass%,
Cr: 0.05% to 5.0%,
V: 0.005% to 1.0% and
Mo: One or more elements selected from 0.005% or more and 0.5% or less are contained, The high-strength steel sheet according to claim 1 characterized by things. The steel sheet is further in 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% to 2.0% and
Cu: One or more elements selected from 0.05% to 2.0% are contained, and the high-strength steel sheet according to claim 1 or 2. The steel sheet is further in mass%,
Ca: 0.001% to 0.005% and
The high-strength steel sheet according to any one of claims 1 to 3, comprising one or two elements selected from REM: 0.001% or more and 0.005% or less. Of the autotempered martensite, the proportion of autotempered martensite in which the number of precipitated iron carbides of 0.1 μm or more and 0.5 μm or less is 5 × 10 ² or less per 1 mm ² is the entire autotempered martensite. The high-strength steel sheet according to any one of claims 1 to 4, characterized in that the area ratio is 3% or more.   The high-strength steel sheet according to any one of claims 1 to 5, wherein a hot-dip galvanized layer is provided on a surface of the steel sheet.   The high-strength steel plate according to any one of claims 1 to 5, wherein an galvannealed layer is provided on the surface of the steel plate. It is a manufacturing method of the high strength steel plate of any one of Claims 1 thru | or 7, Comprising: The steel slab which becomes the component composition of any one of Claims 1 thru | or 4 is cold-rolled after hot rolling. Cold-rolled steel sheet by cold rolling, and then the cold-rolled steel sheet is subjected to annealing for 15 seconds to 600 seconds in a first temperature range of 700 ° C to 950 ° C, The cooling conditions in the second temperature range are as follows: the average cooling rate from the first temperature range to 550 ° C is 3 ° C / second or more, the time required for cooling from 550 ° C to 420 ° C is 600 seconds or less, and 250 ° C to 420 ° C. A third temperature range of ℃ or less is cooled at a rate of 50 ℃ / second or less, and martensite transformation is caused in the third temperature range, and at the same time, auto-tempering treatment is performed to temper the martensite after transformation. A method for producing a high-strength steel sheet.   When cooling the steel sheet at a cooling rate of 50 ° C./second or less in the third temperature range of 250 ° C. or more and 420 ° C. or less, at least a temperature range of (Ms point−50) ° C. or less is 1.0 ° C./second or more and 50 ° C./second. The high-strength steel sheet according to claim 8, wherein the high-strength steel sheet according to claim 8 is cooled at the following speed to cause martensitic transformation in the third temperature range, and at the same time, performs autotempering to temper the martensite after transformation. Production method. 10. The high-strength steel sheet according to claim 8, wherein in the steel slab, a martensite transformation start point Ms is approximated by M represented by the following formula (1), and the M is 300 ° C. or higher. Manufacturing method.
M (° C.) = 540−361 × {[C%] / (1− [α%] / 100)} − 6 × [Si%] − 40 × [Mn%] + 30 × [Al%] −
20 × [Cr%] − 35 × [V%] − 10 × [Mo%] − 17 × [Ni%] − 10 × [Cu%] (1)
However, [X%] is the mass% of the component element X of the steel slab, and [α%] is the area ratio (%) of polygonal ferrite.

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