JP6950826B2 - High-strength steel sheet, hot-rolled steel sheet manufacturing method, cold-rolled full-hard steel sheet manufacturing method, and high-strength steel sheet manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a high-strength steel sheet and a hot-rolled steel sheet, a method for manufacturing a cold-rolled full-hard steel sheet, and a method for manufacturing a high-strength steel sheet.

In recent years, CO ₂ emission regulations have become stricter due to heightened environmental problems, and in the automobile field, weight reduction of the vehicle body to improve fuel efficiency has become an issue. For this reason, thinning is being promoted by applying high-strength steel sheets to automobile parts, and steel sheets with a tensile strength (TS) of 780 MPa or more are being applied.

High-strength steel plates used for structural members and reinforcing members of automobiles are required to have excellent workability. In particular, in the processing of parts having a complicated shape, it is required that not only individual characteristics such as elongation and hole expandability are excellent, but also all of them are excellent.

Furthermore, press-formed parts are often combined by resistance welding (spot welding) and may be welded to hot-dip galvanized steel sheets. In this case, during spot welding, the zinc on the surface of the steel plate melts and residual stress is generated in the vicinity of the welded portion, so that there is a concern that liquid metal brittleness may occur and the steel plate may crack. This is a problem that can occur even if the high-strength steel sheet is a non-galvanized steel sheet, because if the other steel sheet to be welded is a galvanized steel sheet, the zinc melts. Further, after spot welding, stress is applied to the welded portion in order to maintain the rigidity of the entire automobile body, so there is a concern that delayed fracture may occur due to hydrogen entering from the usage environment.

Therefore, in order to apply a high-strength steel plate to the above applications, the high-strength steel plate has workability, is less likely to crack the resistance welded portion due to liquid metal brittleness or delayed fracture, and has excellent crack resistance to the resistance welded portion. It is necessary to be there.

So far, several means for improving the crackability of the resistance welded portion of the high-strength steel sheet have been reported. For example, Patent Document 1 discloses a technique for improving surface crack resistance during resistance welding by controlling the contents of Si, Al, and Mn.

Japanese Unexamined Patent Publication No. 2002-294398

However, in the steel sheet disclosed in Patent Document 1, it is difficult to simultaneously achieve a high tensile strength (TS) of 780 MPa or more, and it cannot be said that sufficient strength and hole expandability are ensured. As described above, it is difficult to realize a high-strength steel sheet having both excellent workability and crack resistance in the resistance welded portion, and even if other steel sheets are included, a steel sheet having these characteristics has been developed. The reality is that it has not been done.

The present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a high-strength steel plate having excellent workability and crack resistance in a resistance welded portion.
Here, the high-strength steel sheet in the present invention means a steel sheet having a tensile strength (TS) of 780 MPa or more.
In the present specification, excellent workability means having both excellent elongation and excellent hole expanding property. For example, excellent elongation and excellent hole-expanding property can mean those having an elongation of 14% or more and a hole-expanding rate of 35% or more, respectively, in the tests described later.
In the present specification, the crack resistance of the resistance welded portion means that the occurrence of cracks during resistance welding and the occurrence of cracks in the resistance welded portion due to hydrogen in the usage environment are both suppressed. For example, in the test described later, when the steel plate and the electrode are welded at a predetermined angle (for example, 6 degrees), the resistance welded portion does not crack, and hydrogen is added to the welded body by cathode electrolytic charge. It may mean that no breakage is observed after a predetermined time.

As a result of diligent studies by the present inventors, in order to improve both workability (elongation and hole widening) and resistance weld crackability in high-strength steel sheets, ferrite is used for the microstructure of the steel sheets. The body integration ratios of martensite, pearlite, and bainite should be set to a specific ratio, and these crystal grains should be refined to control the properties to be close to spherical (circular in the case of cross section). We found that it is effective to control the dispersion state of martensite and the properties of martensite, which have a large effect. The details are as follows.

(1) During punching in the hole expansion test, voids are generated in soft ferrite and hard martensite, and when the number of voids increases, the hole expansion property deteriorates. In order to suppress the formation of voids, it is effective to reduce the difference in hardness between ferrite and martensite, and the behavior of void formation changes depending on the shape of ferrite and martensite. It is effective to make the shape close to. By carrying out a predetermined heat treatment with a predetermined component composition, it becomes possible to contain auto-tempered martensite having a shape close to a spherical shape (circular in the case of a cross section), and the hole expanding property is dramatically improved.

(2) Cracks due to liquid metal brittleness during resistance welding are due to the melting of zinc on the surface of the steel plate during welding and the generation of residual stress near the weld, which is the heat-affected zone (HAZ) near the nugget. Occurs in. Even in an appropriate current range where spatter does not occur, if the tensile strength is as high as 780 MPa class, cracks (internal cracks) may occur on the surface where the steel plates overlap, and in particular, the electrode for welding is a steel plate. When welded at a predetermined angle, the internal stress increases and cracks are likely to occur. The crack in the resistance welded portion in the present specification includes this internal crack. When this internal crack occurs, there is a concern that the fatigue strength of the welded portion will decrease, so it is necessary to avoid this internal crack when used for automobiles and the like.
When observing the internal cracks, the grain boundaries are broken at the place where the martensite becomes a single phase after welding of the heat-affected zone (HAZ). It is effective, and for that purpose, it is important to make the crystal grains of the base material finer. Further, by increasing the grain boundary strength, the brittleness of the liquid metal is improved. By carrying out a predetermined heat treatment with a predetermined component composition, an optimum microstructure can be obtained and the crack resistance of the resistance welded portion is improved.

(3) When the manufactured automobile is actually repeatedly driven after being finished as an automobile body, hydrogen is electrochemically generated on the steel plate due to rain or the like, and a part of the automobile invades the steel plate. If no stress is generated in the steel sheet, delayed fracture due to this hydrogen does not occur, but stress may be generated by resistance spot welding. Even if hydrogen invades, the carbides serve as hydrogen trap sites, so that delayed fracture can be suppressed. For that purpose, it is useful to generate carbides by auto-tempering martensite.

The present invention is based on the above findings, and its gist structure is as follows.

[1] By mass%,
C: 0.05% or more and 0.18% or less,
Si: 0.8% or less,
Mn: 1.5% or more and 3.0% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.10% or less,
N: 0.010% or less,
Mo: 0.05% or more and 0.50% or less,
Ti: 0.01% or more and 0.10% or less, and
B: Contains 0.0002% or more and 0.0100% or less,
The balance has a component composition consisting of Fe and unavoidable impurities,
Where Mo, N, Ti and B are equations (1):
[Mo] + 2 x ([Ti] -3.4 x [N]) + 45 x [B] ≧ 0.20
(Here, [Mo], [Ti], [N] and [B] are the contents (mass%) of Mo, Ti, N and B, respectively, and [N] is 0% by mass. May be good.)
With volume fraction,
Ferrite is 30% or more and 70% or less,
Martensite is 20% or more and 70% or less,
Pearlite is 10% or less (including 0%) and bainite is 20% or less (including 0%).
here,
The ferrite has an average crystal grain size of 6 μm or less and an average aspect ratio of 2.0 or less.
The martensite has an average crystal grain size of 5 μm or less and an average aspect ratio of 2.0 or less.
The bainite has an average crystal grain size of 5 μm or less and has an average crystal grain size of 5 μm or less.
The volume of martensite grains having an average free path of 8.0 μm or less and having 10 or more carbides having a particle size of 0.1 μm or more in the martensite grains with respect to the total martensite is 8.0 μm or less. It has a microstructure with a fraction of 50% or more,
High-strength steel plate.

[2] The composition of the components is mass%, and further
V: 0.06% or less, and
Nb: The high-strength steel sheet of [1] containing at least one selected from the group consisting of 0.05% or less.

[3] The composition of the components is mass%, and further
Cr: 0.80% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sb: 0.02% or less, and
The high-strength steel plate according to [1] or [2], which contains one or more selected from the group consisting of 0.0050% or less: one or more selected from Ca and REM.

[4] For a steel material having any of the composition of [1] to [3], the reduction rate of the final pass of finish rolling: 12% or more, the reduction rate of the pass immediately before the final pass: 11% or more, Finish rolling end temperature: Hot rolling under the conditions of 840 ° C or higher and 950 ° C or lower,
After the hot rolling, the primary cooling is performed at a primary average cooling rate of 70 ° C./s or higher to a primary cooling stop temperature of 700 ° C. or lower.
After the primary cooling, the secondary cooling is performed at a secondary average cooling rate of 5 ° C./s or more and 50 ° C./s or less to a winding start temperature of 610 ° C. or lower.
Then wind up,
Manufacturing method of hot-rolled steel sheet.

[5] A method for producing a cold-rolled full-hard steel sheet, in which the hot-rolled steel sheet obtained by the manufacturing method of [4] is pickled and cold-rolled.

[6] The cold-rolled full-hard steel sheet obtained by the manufacturing method of [5] is heated at an average heating rate of 0.5 ° C./min or more and 5.0 ° C./min or less to a soaking temperature of 750 ° C. or more and 900 ° C. or less. A method for producing a high-strength steel plate, which comprises holding at the above temperature for 1 hour or more and then cooling to room temperature at an average cooling rate of 1.0 ° C./h or more and 100 ° C./h or less.

According to the present invention, a high-strength steel sheet having high tensile strength, excellent workability (excellent elongation and hole expandability), and excellent resistance weld cracking resistance is provided together with a method for producing the same.
Further, according to the present invention, there is provided a method for manufacturing a hot-rolled steel sheet and a method for manufacturing a cold-rolled full-hard steel sheet, which can bring about the above-mentioned high-strength steel sheet.
By using the high-strength steel plate of the present invention as a member of a structural part of an automobile, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Further, the manufacturing method of the present invention makes it possible to stably provide such a high-strength steel plate. Therefore, the industrial utility value of the present invention is great.

Hereinafter, the present invention will be specifically described.
[Component composition of high-strength steel sheet]
The composition of the high-strength steel plate according to the embodiment of the present invention will be described. The unit in the component composition is "mass%", but unless otherwise specified, it is simply expressed as "%".

C: 0.05% or more and 0.18% or less
C is an element effective for increasing the strength of the steel sheet, and is an important element that contributes to the formation of the second phase (including pearlite, bainite, and martensite) other than ferrite, which is the first phase. If the C content is less than 0.05%, it is difficult to secure the required volume fractions of pearlite, bainite, and martensite, and it is difficult to obtain the desired strength. Therefore, the content is set to 0.05% or more. It is preferably 0.075% or more. If the C content is excessive, the hardness after resistance welding becomes hard, the toughness at the time of resistance welding decreases, and the crackability of the resistance welded portion decreases. In addition, high-hardness martensite with insufficient carbide formation becomes excessive, and good hole-spreading property and delayed fracture characteristics cannot be obtained. Further, the volume fraction of martensite increases, it becomes difficult to secure the volume fraction of ferrite, and the elongation decreases. Therefore, the content should be 0.18% or less. It is preferably 0.16% or less.

Si: 0.8% or less
Si is an element that contributes to high strength by solid solution strengthening of ferrite, but if the content is excessive, the toughness during resistance welding decreases and the crack resistance of the resistance welded portion deteriorates. In addition, martensite in which a predetermined amount of carbide is produced cannot be obtained, and good hole expandability and delayed fracture characteristics cannot be obtained. Therefore, the content is set to 0.8% or less. It is preferably 0.50% or less. More preferably, it is 0.30% or less. The lower limit is not particularly limited, but 0.005% or more is preferable because extremely low Si conversion increases the cost.

Mn: 1.5% or more and 3.0% or less
Mn is an element that contributes to high strength by promoting solid solution strengthening and the formation of the second phase. It is also an element that stabilizes austenite and is necessary for controlling the volume fraction of the second phase. In order to obtain these effects, the content should be 1.5% or more. It is preferably 2.1% or more. More preferably, it is 2.2% or more. On the other hand, if the content is excessive, an Mn band is generated, so that the hole expanding property and the resistance welded portion cracking property are deteriorated. Therefore, the content is set to 3.0% or less. It is preferably 2.6% or less.

P: 0.05% or less
P is an element that contributes to higher strength by strengthening the solid solution, but if the content is excessive, segregation to the grain boundaries becomes significant and the grain boundaries become embrittlement, resulting in a decrease in resistance weld cracking resistance. do. Therefore, the content is set to 0.05% or less. It is preferably 0.04% or less. The lower limit is not particularly limited, but an extremely low P ratio increases the steelmaking cost, so 0.0005% or more is preferable.

S: 0.005% or less
As the content of S increases, the production of sulfides such as MnS increases, and it becomes the starting point of voids during punching in the hole expanding test, and is an element that reduces the hole expanding property. Therefore, the content is set to 0.005% or less. Preferably, it is 0.0045% or less. The lower limit is not particularly limited, but an extremely low S ratio increases the steelmaking cost, so 0.0002% or more is preferable.

Al: 0.01% or more and 0.10% or less
Al is an element necessary for deoxidation, and in order to obtain this effect, its content should be 0.01% or more. On the other hand, if it exceeds 0.10%, the effect will be saturated, so the content should be 0.10% or less. It is preferably 0.06% or less.

N: 0.010% or less
N is an element that forms a coarse nitride and deteriorates the hole-expanding property. This tendency becomes remarkable when N exceeds 0.010%. Therefore, the content should be 0.010% or less. It is preferably 0.008% or less. The lower limit is not particularly limited, but an extremely low N conversion increases the cost, so 0.0005% or more is preferable.

Mo: 0.05% or more and 0.50% or less
Mo is an element that contributes to high strength by promoting the formation of the second phase. It is also an element that stabilizes austenite during annealing and is necessary for controlling the volume fraction of the second phase. Further, it is an important element in the present invention from the viewpoint that hardenability can be ensured even if the cooling rate during annealing is low. In order to obtain these effects, the content should be 0.05% or more. It is preferably 0.08% or more. On the other hand, if the content is excessive, the second phase is excessively generated, and the elongation and hole expansion property are deteriorated. Therefore, the content is set to 0.50% or less. It is preferably 0.43% or less.

Ti: 0.01% or more and 0.10% or less
Ti is an element that contributes to high strength by forming fine carbonitrides. It also contributes to suppressing the reaction of B, which is an essential element in the present invention, with N. Furthermore, it is an important element in the present invention because it can control the grain growth of fine carbonitrides during annealing and generate ferrite and martensite that are close to spheres. In order to obtain these effects, the content should be 0.01% or more. It is preferably 0.03% or more. More preferably, it is 0.04% or more. On the other hand, if a large amount of Ti is added, the elongation is significantly reduced, so the content thereof should be 0.10% or less. It is preferably 0.09% or less.

B: 0.0002% or more and 0.0100% or less
B is an element that improves hardenability and promotes the formation of the second phase, thereby contributing to higher strength, ensuring hardenability, and not lowering the martensitic transformation start point. Further, since the grain boundary strength is improved by segregating at the grain boundaries, it is effective for delayed fracture resistance. In order to obtain these effects, the content should be 0.0002% or more. It is preferably 0.0005% or more. On the other hand, if the content is excessive, the toughness is deteriorated and the resistance welding crack resistance is lowered. Therefore, the content should be 0.0100% or less. It is preferably 0.0050% or less.

In the present invention, the contents of Mo, Ti, N and B need to satisfy the formula (1).
[Mo] + 2 x ([Ti] -3.4 x [N]) + 45 x [B] ≧ 0.20 ・ ・ ・ (1)

Here, [Mo], [Ti], [N] and [B] are the contents (mass%) of Mo, Ti, N and B, respectively, and [N] is 0% by mass. good.

The above formula (1) is an index for securing a predetermined structure and ensuring strength, workability and crack resistance of the resistance welded portion. If the value on the left side is less than 0.20, the predetermined structure is obtained. It may be difficult to obtain strength, workability, and crackability of the resistance welded portion. Therefore, the value on the left side is 0.20 or more. It is preferably 0.21 or more. The upper limit is 1.15, which is preferably 0.85 or less because it is difficult to secure ductility as the quenching element increases.

The high-strength steel plate of the present invention can further contain one or more of the following components.

V: 0.06% or less
V is an element that contributes to the increase in strength by forming fine carbonitrides. When V is contained, the content is preferably 0.01% or more in order to obtain this effect. More preferably, it is 0.015% or more. On the other hand, if it is contained in a large amount, the toughness at the time of resistance welding is lowered, which may adversely affect the crackability of the resistance welded portion. Therefore, the content should be 0.06% or less. It is preferably 0.055% or less.

Nb: 0.05% or less
Like V, Nb is an element that contributes to higher strength by forming fine carbonitrides. When Nb is contained, the content is preferably 0.005% or more in order to obtain this effect. More preferably, it is 0.01% or more. On the other hand, if it is contained in a large amount, the toughness at the time of resistance welding is lowered and the crackability of the resistance welded portion is deteriorated. Therefore, the content is set to 0.05% or less. It is preferably 0.035% or less.

Cr: 0.80% or less
Cr is an element that contributes to high strength by promoting the formation of the second phase. In addition, Cr is an element that stabilizes austenite during annealing, and is effective in controlling the volume fraction of the second phase. When Cr is contained, the content thereof is preferably 0.01% or more in order to obtain these effects. More preferably, it is 0.05% or more. If the content is excessive, the second phase is excessively formed, which may adversely affect the elongation and bending workability, and the surface oxide is excessively formed, which deteriorates the galvanizing property and the chemical conversion treatment property. Therefore, the content is set to 0.80% or less. It is preferably 0.65% or less.

Cu: 0.50% or less
Cu is an element that contributes to high strength by strengthening solid solution, and also contributes to high strength by promoting the formation of the second phase. When Cu is contained, the content thereof is preferably 0.05% or more in order to obtain these effects. More preferably, it is 0.12% or more. On the other hand, if the content exceeds 0.50%, the effect is saturated and surface defects due to Cu are likely to occur. Therefore, the content is set to 0.50% or less. It is preferably 0.33% or less.

Ni: 0.50% or less
Like Cu, Ni is an element that contributes to high strength by strengthening solid solution, and also contributes to high strength by promoting the formation of the second phase. When Ni is contained, the content is preferably 0.05% or more in order to obtain these effects. More preferably, it is 0.08% or more. Ni is particularly effective when Cu is contained because it has the effect of suppressing surface defects caused by Cu when it is contained at the same time as Cu. On the other hand, if it is contained in a large amount, the toughness at the time of resistance welding is lowered, which may adversely affect the crackability of the resistance welded portion. Therefore, the content is set to 0.50% or less. It is preferably 0.35% or less.

Sb: 0.02% or less
Sb is an element that has the effect of suppressing the decarburized layer generated on the surface layer of the steel sheet and contributes to the improvement of the crack resistance of the resistance welded portion by making the hardness distribution of the surface layer uniform. When Sb is contained, the content thereof is preferably 0.002% or more in order to obtain these effects. More preferably, it is 0.005% or more. On the other hand, when the content exceeds 0.02%, the rolling load increases and the productivity decreases. Therefore, the amount of Sb should be 0.02% or less. It is preferably 0.015% or less.

One or more selected from Ca and REM: 0.0050% or less
Ca and REM are elements that spheroidize the shape of sulfides to reduce the adverse effect on hole-spreading properties. When one or more selected from Ca and REM are contained, the content is preferably 0.0005% or more in order to obtain these effects. More preferably, it is 0.0008% or more. On the other hand, when the content exceeds 0.0050%, the effect is saturated. Therefore, the content is set to 0.0050% or less. Preferably, it is 0.0032% or less.

The rest other than the above is Fe and unavoidable impurities. Examples of unavoidable impurities include Zn, Co, Sn, Zr, etc., and the permissible range of their contents is Zn: 0.01% or less, Co: 0.10% or less, Sn: 0.10% or less, Zr: 0.10. % Or less.

[Microstructure of high-strength steel plate]
Next, the microstructure of the high-strength steel plate according to the primary embodiment of the present invention will be described in detail. The microstructure is
With volume fraction,
Ferrite is 30% or more and 70% or less,
Martensite is 20% or more and 70% or less,
Pearlite is 10% or less (including 0%) and bainite is 20% or less (including 0%).
The ferrite has an average crystal grain size of 6 μm or less and an average aspect ratio of 2.0 or less.
The martensite has an average crystal grain size of 5 μm or less and an average aspect ratio of 2.0 or less.
The bainite has an average crystal grain size of 5 μm or less and has an average crystal grain size of 5 μm or less.
The volume of martensite grains having an average free path of 8.0 μm or less and having 10 or more carbides having a particle size of 0.1 μm or more in the martensite grains with respect to the total martensite is 8.0 μm or less. The fraction is 50% or more.

Measurement methods such as volume fraction, average crystal grain size, aspect ratio, mean free path, etc .:
The volume fractions of ferrite, martensite, bainite and pearlite are 3000 times higher using SEM (scanning electron microscope) after polishing the sheet thickness section parallel to the rolling direction of the steel plate and corroding it with 3 vol.% Nital. Observe 10 visual fields (1 visual field is 50 μm × 40 μm) at a position 1/4 from the surface in the plate thickness direction at magnification, and measure the area fraction by the point counting method (based on ASTM E562-83 (1988)). It is measured and the area fraction is used as the volume fraction. The volume fraction obtained in this way is the volume fraction with respect to the entire hot press member. In the present invention, granular carbides are observed in the microstructure, but when calculating the area fraction of each phase, if carbides are present in each phase, the area including the areas of these carbides is also included. It is a fraction. The same applies to the following.

For the average crystal grain size of ferrite, bainite and martensite, by using Image-Pro of Media Cybernetics, a photograph in which the crystal grains of ferrite, bainite and martensite were previously identified from the steel plate structure photograph was taken. The area of each phase was made calculable, the circle-equivalent diameter was calculated for the crystal grains of ferrite, bainite, and martensite, and the circle-equivalent diameter values were averaged for each of ferrite, bainite, and martensite. It is a thing.

Regarding the aspect ratios of ferrite and martensite, the major axis length and minor axis length were calculated for each of the 25 crystal grains having the largest average crystal grain size from the above photographs, and the major axis length was calculated for each crystal grain. Is divided by the minor axis length and the values are averaged.

Among the martensites, the volume fraction of the martensite grains in which 10 or more carbides having a particle size of 0.1 μm or more are present in the martensite grains is obtained as follows. After polishing the sheet thickness section parallel to the rolling direction of the steel sheet, it is corroded with 3 vol.% Nital, and at a magnification of 20000 times using TEM (transmission electron microscope), about the position of 1/4 from the surface in the sheet thickness direction. , 10 fields (1 field is 0.5 μm × 0.5 μm) were observed, and 25 martensite grains were selected in descending order of average crystal grain size, and the point counting method (ASTM E562-83 (ASTM E562-83)) was used for each. According to 1988)), the area division is calculated and used as the body integration ratio. In addition, for the selected 25 martensite grains, the number of carbides having a particle size of 0.1 μm or more existing in the grains is counted. At that time, the particle size of the carbide shall be the diameter equivalent to a circle. Obtain the surface integral of martensite grains in which 10 or more carbides having a particle size of 0.1 μm or more are present in the grains, and use this as the volume fraction. _{The ratio of the total volume fraction (X c} ) of martensite grains containing 10 or more carbides with a particle size of 0.1 μm or more in the martensite grains to the total volume fraction (X) of 25 martensite grains. Is determined, and the volume fraction (Xc / X × 100) of the martensite grains having 10 or more carbides having a particle size of 0.1 μm or more in the grains is used.

L_Ｍ：マルテンサイトの平均自由行程
ｄ_Ｍ：マルテンサイトの平均結晶粒径
π：円周率
f：マルテンサイトの面積率（＝体積分率）The mean free path of martensite is calculated by the following formula 1.

L _M: mean free path of martensite d _M: average grain martensite diameter [pi: pi
f: Martensite area ratio (= volume fraction)

Ferrite:
When the volume fraction of ferrite exceeds 70%, it is difficult to achieve a tensile strength of 780 MPa. Therefore, the volume fraction is set to 70% or less. It is preferably 65% or less. On the other hand, when the volume fraction is less than 30%, the second phase is excessively generated, and voids are likely to be generated at the time of punching in the hole expanding test, so that the hole expanding property is deteriorated. Furthermore, if the volume fraction is less than 30%, the elongation decreases. Therefore, the volume fraction is set to 30% or more. It is preferably 35%, more preferably 40% or more.

When the average crystal grain size of ferrite exceeds 6 μm, the crystal grains become coarser during resistance welding, resulting in deterioration of toughness, cracking of the resistance welded portion, and further deterioration of the delayed fracture characteristics of the resistance welded portion. Therefore, the average crystal grain size of ferrite should be 6 μm or less. On the other hand, the lower limit is not particularly limited, but industrially, it can be 0.5 μm or more. 5.5 μm or less is preferable, and 2 μm or more is preferable.

When the average aspect ratio of ferrite exceeds 2.0, voids generated during punching in the hole expansion test are easily connected during hole expansion, so that the hole expansion property deteriorates. Therefore, the average aspect ratio is set to 2.0 or less. On the other hand, the lower limit is 1.0 and may be 1.0. 1.8 or less is preferable, and 1.3 or more is preferable. More preferably, it is 1.6 or less.

Martensite:
The volume fraction of martensite shall be 20% or more to ensure the desired strength. It is preferably 23% or more. On the other hand, when the volume fraction exceeds 70%, the void formation at the time of punching in the hole expanding test is excessively increased, so that the hole expanding property is deteriorated. Therefore, the volume fraction is set to 70% or less. It is preferably 65% or less, and more preferably 60% or less.

When the average crystal grain size of martensite exceeds 5 μm, the crystal grains become coarser during resistance welding, which deteriorates toughness and tends to cause cracks in the resistance weld, and voids formed at the interface between martensite and ferrite are generated. It is easy to connect and the hole expandability deteriorates. Therefore, the average crystal grain size of martensite should be 5 μm or less. On the other hand, the lower limit is not particularly limited, but industrially, it can be 0.2 μm or more. It is preferably 4.5 μm or less, and preferably 2.5 μm or more.

When the average aspect ratio of martensite exceeds 2.0, the voids generated during punching in the drilling test are easily connected at the time of drilling, so that the drilling property deteriorates. Therefore, the average aspect ratio is set to 2.0 or less. On the other hand, the lower limit is 1.0 and may be 1.0. 1.8 or less is preferable, and 1.3 or more is preferable.

In addition, among martensite, martensite grains having 10 or more carbides having a particle size of 0.1 μm or more in the grains have a volume fraction of 50% or more in all martensite. If the volume fraction is less than 50%, a large number of high-hardness martensites will be produced, and void formation will be excessively increased during punching, resulting in deterioration of hole expandability. Furthermore, since the hydrogen trap sites in the resistance welded zone and the heat-affected zone (HAZ) are insufficient, it is difficult to obtain sufficient delayed fracture resistance. Therefore, the volume fraction is set to 50% or more. Preferably, it is 55% or more. On the other hand, the upper limit can be 95%, preferably 90% or less.

When the mean free path of martensite exceeds 8.0 μm, the void formation during punching in the drilling test is excessively increased, and the drilling property is deteriorated. Further, a concentration distribution of Mn or the like occurs in the heat-affected zone (HAZ) after resistance welding, and internal cracking during resistance welding and delayed fracture characteristics after resistance welding are reduced. Therefore, the mean free path of martensite should be 8.0 μm or less. The mean free path can be, for example, 3.0 μm or more.

Parlite:
Since pearlite contributes to high strength, it may be generated in the microstructure, but since it contains a lot of hard cementite, if the volume fraction exceeds 10%, void formation after punching during the drilling test is excessive. As the number increases, the hole-spreading property deteriorates. Therefore, the volume fraction is set to 10% or less. It is preferably 5% or less, and may be 0%.

Bainite:
Bainite may be formed in the microstructure because it contributes to high strength, but since it contains a high dislocation density, void formation after punching in the drilling test increases excessively when the volume fraction exceeds 20%. Therefore, the hole expanding property deteriorates. Therefore, the volume fraction is set to 20% or less. It is preferably 15% or less, and may be 0%.

If the average crystal grain size of bainite exceeds 5 μm, the crystal grains become coarser during resistance welding, resulting in deterioration of toughness, internal cracking, and delayed fracture after resistance welding. Therefore, the average crystal grain size of bainite should be 5 μm or less. On the other hand, the lower limit is not particularly limited, but industrially, it can be 0.5 μm or more. 4 μm or less is preferable, and 2 μm or more is preferable.

Remaining organization:
The microstructure of the high-strength steel plate of the present invention is basically composed of ferrite, martensite, pearlite and bainite (however, the volume fraction of pearlite and bainite may be 0%). In addition, retained austenite and unrecrystallized ferrite may be contained. Even in this case, the object of the present invention can be achieved if ferrite, martensite, pearlite and bainite satisfy the above conditions. The residual structure other than ferrite, martensite, pearlite and bainite is preferably 5% or less, more preferably 3% or less, and may be 0%.

The high-strength steel sheet of the present invention may include a plating layer. The plating layer is not particularly limited, and may be, for example, a hot-dip plating layer, an electroplating layer, or an alloyed plating layer. The plating metal is not particularly limited, and may be aluminum plating or the like in addition to zinc plating. The high-strength steel sheet of the present invention includes a plated steel sheet.

The thickness of the high-strength steel plate is not particularly limited and can be 0.4 mm or more and 3.0 mm or less. The plate thickness is preferably 0.5 mm or more, more preferably 0.55 mm or more, more preferably 2.8 mm or less, and more preferably 2.6 mm or less. When the high-strength steel sheet includes a plating layer, the plate thickness refers to the plate thickness of the base steel plate excluding the plating layer.

[Manufacturing method of high-strength steel plate]
Next, a method for manufacturing a high-strength steel plate according to an embodiment of the present invention will be described. This manufacturing method
Conditions for a steel material having the above composition, a rolling reduction of 12% or more in the final pass of finish rolling, a rolling reduction of 11% or more in the pass immediately before the final pass, and a finishing rolling temperature of 840 ° C or more and 950 ° C or less. Hot rolled with
After the hot rolling, the primary cooling is performed at a primary average cooling rate of 70 ° C./s or higher to a primary cooling stop temperature of 700 ° C. or lower.
After the primary cooling, the secondary cooling is performed at a secondary average cooling rate of 5 ° C./s or more and 50 ° C./s or less to a winding start temperature of 610 ° C. or lower.
Then, it is wound to obtain a hot-rolled steel sheet,
After pickling the hot-rolled steel sheet, it is continuously cold-rolled to obtain a cold-rolled full-hard steel sheet.
Next, the cold-rolled steel sheet is heated at an average heating rate of 0.5 ° C./min or more and 5.0 ° C./min or less to a soaking temperature of 750 ° C. or more and 900 ° C. or less, held at the above temperature for 1 hour or more, and then 1.0 ° C./min. It is a method including cooling to room temperature at an average cooling rate of h or more and 100 ° C./h or less.

<Manufacturing method of hot-rolled steel sheet>
In the present invention, the hot rolling start temperature of the steel slab is preferably 1100 ° C. or higher and 1300 ° C. or lower, and after casting the steel slab, hot rolling is performed at a temperature of 1100 ° C. or higher and 1300 ° C. or lower without reheating. It is preferable to start hot rolling or after reheating to 1100 ° C. or higher and 1300 ° C. or lower. That is, in the present invention, in addition to the conventional method of producing a steel slab, which is once cooled to room temperature and then reheated, a method of charging the rolled pieces into a heating furnace without cooling, or keeping the rolled pieces. Energy-saving processes such as a method of rolling immediately after heating, or a method of direct rolling or direct rolling of rolling as it is after casting can be applied without any problem.

-Rolling rate of the final pass of finish rolling: 12% or more In the present invention, setting the rolling rate of the final pass of finish rolling to 12% or more means that many shear zones are introduced in the austenite grains and after hot rolling. Martensite in hot-rolled steel sheets, cold-rolled full-hard steel sheets, and high-strength steel sheets after annealing by increasing the number of ferrite transformation nucleation sites and making the structure of hot-rolled steel sheets finer, and eliminating the Mn band. It is necessary because it can reduce the average free-rolling process of. It is also necessary to obtain a predetermined aspect ratio for ferrite and martensite. Preferably, the reduction rate of the final pass is 13% or more. More preferably, the reduction rate of the final pass is 16% or more. On the other hand, the upper limit is not particularly limited, but is 30% in order to prevent the plate thickness from fluctuating in the width direction of the plate due to the increase in the hot spread load and the crack resistance of the resistance welded portion from deteriorating. The following is preferable.

-Reduction rate of the pass immediately before the final pass: 11% or more In the present invention, if the reduction rate of the path immediately before the final pass is 11% or more, the strain accumulation effect is further enhanced and a shear band is formed in the austenite grains. It is necessary from the viewpoint that the Mn band can be eliminated and the mean free path of martensite can be reduced by introducing a large number of them, further increasing the nucleation sites of ferrite transformation, and making the structure of the hot-rolled steel plate finer. .. It is also necessary to obtain a predetermined aspect ratio for ferrite and martensite. Preferably, the reduction rate of the pass immediately before the final pass is 12% or more. On the other hand, the upper limit is not particularly limited, but is 16% in order to prevent the plate thickness from fluctuating in the width direction of the plate due to the increase in the hot spread load and the crack resistance of the resistance welded portion from deteriorating. The following is preferable.

The ratio of the reduction rate of the pass immediately before the final pass to the reduction rate of the final pass is preferably 1.1 or less. If the reduction rate of the pass immediately before the final pass is excessively higher than the reduction rate of the final pass, the predetermined aspect ratio may not be obtained for ferrite and martensite. The number of passes other than the final pass and the pass immediately before the final pass, and the reduction rate are not particularly limited. For example, the number of passes may be two, the last pass and the pass immediately before the last pass, and the total number of passes may be 4 or more including these two, for example, 5 or more and 20 or less. It can be done, preferably 16 or less.

-Finish rolling end temperature Hot rolling improves the crack resistance of the resistance welded part after annealing by making the structure uniform in the steel sheet and reducing the anisotropy of the material. In order to secure the ratio and improve the hole expansion property, it is necessary to end in the austenite single phase region. Therefore, the finish rolling end temperature is set to 840 ° C or higher. It is preferably 870 ° C. or higher. On the other hand, when the finish rolling end temperature exceeds 950 ° C., the hot-rolled structure becomes coarse and the crystal grains after annealing also become coarse. Therefore, the finish rolling end temperature is set to 950 ° C or lower. It is preferably 930 ° C or lower.

・ Cooling conditions after finish rolling After cooling from the finish rolling end temperature to the first cooling stop temperature of 700 ° C or less at the primary average cooling rate of 70 ° C / s or more as the primary cooling, 5 as the secondary cooling. Cool to 610 ° C or lower at the secondary average cooling rate of ° C / s or higher and 50 ° C / s or lower.

After the completion of hot rolling, austenite undergoes ferrite transformation during the cooling process, but the ferrite becomes coarse at high temperatures. Therefore, by performing rapid cooling after completion of hot rolling, the structure is homogenized as much as possible, and at the same time, precipitates. Suppress generation. Therefore, as the primary cooling, the temperature is cooled to the primary cooling stop temperature of 700 ° C. or lower at the primary average cooling rate of 70 ° C./s or higher.
If the primary average cooling rate is less than 70 ° C / s, ferrite and martensite will be coarsened, resulting in inhomogeneous steel sheet structure of hot-rolled steel sheets, resulting in hole expandability, resistance weld cracking resistance, and delayed fracture characteristics. descend. Therefore, the primary average cooling temperature is set to 70 ° C./s or higher. It is preferably 75 ° C./s or higher. The upper limit of the primary average cooling temperature is not particularly limited, but can be, for example, 150 ° C./s or less.
When the first cooling stop temperature in the primary cooling exceeds 700 ° C, the structure of the hot-rolled steel sheet becomes coarse, the final microstructure becomes coarse, and the hole expandability, resistance weld cracking resistance, and delayed fracture characteristics deteriorate. do. Therefore, the first cooling stop temperature is set to 700 ° C. or lower. It is preferably 680 ° C or lower. The first cooling stop temperature is not particularly limited as long as it is higher than the winding start temperature of 610 ° C. or lower, and can be, for example, 630 ° C. or higher.

Subsequent secondary cooling is cooling from the above cooling stop temperature to the winding start temperature of 610 ° C or lower at a secondary average cooling rate of 5 ° C / s or more and 50 ° C / s or less. When the secondary average cooling rate is less than 5 ° C / s, ferrite or pearlite is excessively generated in the microstructure of the hot-rolled steel sheet, and the hole expansion property and resistance weld cracking resistance of the thin steel sheet after annealing are reduced. do. The secondary average cooling rate is 5 ° C./s or higher, preferably 8 ° C./s or higher. When the secondary average cooling rate exceeds 50 ° C./s, martensite in which a predetermined amount of carbide is formed cannot be obtained, and good perforation property and good delayed fracture fracture characteristics cannot be obtained. The secondary average cooling rate can be 45 ° C./s or less, preferably 40 ° C./s or less.
On the other hand, when cooled to a temperature higher than 610 ° C, the structure of the hot-rolled steel sheet becomes coarse, the structure after annealing becomes coarse, and the hole expansion property, resistance weld cracking resistance, and delayed fracture characteristics of the thin steel sheet deteriorate. .. At the above average cooling rate, the temperature is cooled to a winding start temperature of 610 ° C. or lower, and then winding is started. The lower limit of the winding start temperature is not particularly limited, but is 300 from the viewpoint of avoiding that the temperature at the time of winding becomes too low, excessively hard martensite is generated, and the cold rolling load increases. ℃ or higher is preferable.

・ Winding start temperature: 610 ℃ or less When the winding start temperature is over 610 ℃, coarse pearlite is generated in the structure of the hot-rolled steel sheet, and coarse martensite is generated in the final steel sheet structure, and the hole expandability And the crackability of the resistance welded part deteriorates. The winding start temperature is preferably 600 ° C. or lower. The winding start temperature is preferably 300 ° C. or higher.

After the above winding, the steel sheet is cooled by air cooling or the like, and is used for manufacturing the following cold-rolled full-hard steel sheet. When a hot-rolled steel sheet is a transaction target as an intermediate product, it is usually a transaction target in a cooled state after winding.

The thickness of the hot-rolled steel sheet is not particularly limited, and is preferably 0.8 mm or more and 5.0 mm or less, and more preferably 1.1 mm or more and 4.5 mm or less.

<Manufacturing method of cold-rolled full-hard steel sheet>
The cold-rolled full-hard steel sheet can be manufactured by cold-rolling the hot-rolled steel sheet obtained by the above-mentioned manufacturing method. In the present specification, the cold-rolled full-hard steel sheet means a cold-rolled steel sheet that is used for various purposes in a state where no annealing treatment is performed after cold rolling, that is, in a full-hard state. Various uses include the production of high-strength steel sheets, and high-strength steel sheets may be continuously produced after the production of cold-rolled full-hard steel sheets.

The cold rolling conditions can be appropriately set from the viewpoint of, for example, a desired thickness. In the present invention, cold rolling is performed at a rolling reduction of 30% or more in order to promote recrystallization of ferrite and prevent unrecrystallized ferrite from remaining excessively and deteriorating ductility and perforation property. Is preferable. Normally, the rolling reduction rate for cold rolling is 30% or less.

Pickling is performed before the cold rolling for the purpose of removing scale on the surface of the hot-rolled steel sheet. Pickling conditions can be set as appropriate.

The thickness of the cold-rolled full-hard steel sheet is not particularly limited, and is preferably 0.4 mm or more and 3.0 mm or less, and more preferably 0.5 mm or more and 2.8 mm or less.

<Manufacturing method of high-strength steel plate>
A high-strength steel sheet can be manufactured by heating and cooling (annealing) a cold-rolled full-hard steel sheet.

This annealing step is a step for advancing the recrystallization of ferrite and forming fine ferrite, martensite and bainite in the microstructure for high strength, and is 0.5 ° C./min or more and 5.0 ° C./min or less. It is heated to a soaking temperature of 750 ° C. or higher and 900 ° C. or lower at an average heating rate, held at the above temperature for 1 hour or longer, and then cooled to room temperature at an average cooling rate of 1.0 ° C./h or higher and 100 ° C./h or lower.

-Average heating rate: 0.5 ° C / min or more and 5.0 ° C / min or less By controlling the heating temperature after annealing, the crystal grains after annealing can be made finer. When heated rapidly, recrystallization is less likely to proceed, crystal grains with a large aspect ratio are likely to be formed, and the hole-spreading property is lowered. In addition, martensite in which a predetermined amount of carbide is produced cannot be obtained, and good delayed fracture fracture characteristics cannot be obtained. Therefore, the average heating rate should be 5.0 ° C / min or less. It is preferably 4.5 ° C./min or less.
Further, if the heating rate is too low, the ferrite and martensite grains become coarse, and the mean free path of the predetermined martensite cannot be obtained, the hole expanding property deteriorates, and the resistance welded portion cracking property and the delayed fracture property are deteriorated. descend. Therefore, the average heating rate is set to 0.5 ° C./min or more. It is preferably 1.0 ° C./min or higher.

・ Soaking temperature (holding temperature): 750 ° C or more and 900 ° C or less The soaking temperature shall be a two-phase range of ferrite and austenite or a temperature range of austenite single-phase range. If the temperature is lower than 750 ° C, the ferrite fraction increases, making it difficult to secure the strength. In addition, martensite in which a predetermined amount of carbide is produced cannot be obtained, and good perforation property and good delayed fracture fracture characteristics cannot be obtained. Therefore, the soaking temperature is set to 750 ° C or higher. If the soaking temperature is too high, the crystal grain growth of austenite becomes remarkable, and the crystal grains become coarse, so that the crack resistance of the resistance welded portion is lowered. Therefore, the soaking temperature is set to 900 ° C or lower. It is preferably 880 ° C. or lower.

-Heat soaking time: 1 hour or more At the above soaking temperature, a soaking time of 1 hour or more is required for the progress of recrystallization, spheroidization of crystal grains, and austenite transformation of some or all structures. .. Further, when the soaking time is less than 1 hour, the ferrite fraction increases, which makes it difficult to secure the strength. In addition, martensite in which a predetermined amount of carbide is produced cannot be obtained, and good perforation property and good delayed fracture fracture characteristics cannot be obtained. Preferably, it is 2 hours or more. On the other hand, the upper limit is not particularly limited, but if the soaking time is excessively long, microsegregation of Mn is promoted, and the hole expanding property and the resistance welded portion cracking property may deteriorate. Therefore, 100 hours or less is preferable. More preferably, it is 72 hours or less.

・ Cooling conditions during annealing: Cool to room temperature at an average cooling rate of 1.0 ° C / h or more and 100 ° C / h or less After the above soaking, from the soaking temperature to room temperature, 1.0 ° C / h or more and 100 ° C / h or less It is necessary to cool at the average cooling rate. If the average cooling rate is less than 1.0 ° C./h, ferrite transformation proceeds during cooling and the volume fraction of the second phase decreases, making it difficult to secure the strength. Therefore, the average cooling rate should be 1.0 ° C / h or higher. It is preferably 1.3 ° C./h or higher. On the other hand, when the average cooling rate exceeds 100 ° C./h, martensite is excessively generated, the TS becomes high and the El becomes low, and the desired aspect ratio of ferrite and martensite cannot be obtained, so that the hole expandability deteriorates. .. In addition, martensite in which a predetermined amount of carbide is produced cannot be obtained, and good delayed fracture fracture characteristics cannot be obtained. Therefore, the average cooling rate is set to 100 ° C./h or less. It is preferably 73 ° C./h or less.

After annealing, the steel sheet may be plated to provide a plating layer. The plating layer may be either a hot-dip plating layer or an electroplating layer, or may be an alloyed plating layer. The plating metal is not particularly limited, and may be aluminum plating or the like in addition to zinc plating. However, in the high-strength steel sheet of the present invention, hole widening property and resistance weld cracking property can be ensured by controlling the above-mentioned composition and microstructure without plating treatment.

Hot-dip galvanizing is preferable for the plating treatment. Hot dip galvanizing can be performed under normal conditions. The temperature of the steel sheet immersed in the plating bath is preferably (hot-dip galvanizing bath temperature −40) ° C. or higher (hot-dip galvanizing bath temperature +50) ° C. or lower. When the temperature of the steel sheet immersed in the plating bath is lower than (hot-dip galvanizing bath temperature -40) ° C, a part of the molten zinc solidifies when the steel sheet is immersed in the plating bath, which deteriorates the plating appearance. There is. Therefore, the lower limit is preferably set to (hot-dip galvanizing bath temperature −40) ° C. Further, if the temperature of the steel sheet immersed in the plating bath exceeds (hot-dip galvanizing bath temperature + 50) ° C., the temperature of the plating bath rises, which causes a problem in mass productivity. Therefore, it is preferable to set the upper limit to (hot-dip galvanizing bath temperature + 50) ° C.

After plating, zinc plating can be alloyed in a temperature range of 450 ° C. or higher and 600 ° C. or lower. By alloying in a temperature range of 450 ° C. or higher and 600 ° C. or lower, the Fe concentration during plating can be controlled to 7% or higher and 15% or lower, and the adhesion of plating and the corrosion resistance after painting are improved. Below 450 ° C, alloying does not proceed sufficiently, leading to a decrease in sacrificial anticorrosion action and slidability, and at a temperature higher than 600 ° C, the progress of alloying becomes remarkable and powdering resistance deteriorates. ..

For hot-dip galvanizing, it is preferable to use a zinc plating bath containing 0.10% by mass or more and 0.20% by mass or less of Al. After plating, wiping can be performed to adjust the basis weight of the plating.

Temper rolling may be carried out after annealing. In this case, the preferable range of the elongation rate is 0.05% or more and 2.0% or less.

Hereinafter, examples of the present invention will be described.
The present invention is not limited by the following examples, and can be carried out with appropriate modifications within a range that can be adapted to the gist of the present invention, all of which are the technical scope of the present invention. include.

After melting and casting steel with the composition shown in Table 1 to produce a slab with a thickness of 220 mm, the hot rolling heating temperature is 1250 ° C and the finishing rolling end temperature (FDT) is finished under the conditions shown in Table 2. Hot rolling was performed with a total number of rolling passes of 7 to obtain a hot-rolled steel sheet.
Next, after cooling to the first cooling stop temperature at the first average cooling rate (cooling speed 1) shown in Table 2, it is cooled to the winding start temperature (CT) at the second average cooling temperature (cooling speed 2). , Winding was performed to obtain a hot-rolled steel sheet (HR).

The obtained hot-rolled steel sheet was pickled and then cold-rolled to the thickness shown in Table 2 to produce a cold-rolled full-hard steel sheet (FH).

The obtained cold-rolled steel sheet was annealed in a box-type annealing furnace (BAF) under the conditions shown in Table 2 to obtain a cold-rolled steel sheet (CR). A part of the product was hot-dip galvanized and then alloyed to obtain an alloyed hot-dip galvanized steel sheet (GA). Here, the plating treatment is as follows: zinc plating bath temperature: 460 ° C., zinc plating bath Al concentration: 0.14% by mass (when alloying treatment is performed), 0.18% by mass (when alloying treatment is not performed), plating adhesion per one side. The amount was 45 g / m ² (double-sided plating). For some steel sheets, non-alloy hot-dip galvanized steel sheets (GI) were used without galvanizing.

The microstructure (volume fraction, average crystal grain size, aspect ratio, volume fraction of specific martensite grains, mean free path) of the obtained steel sheet was measured by the above-mentioned method. The results are shown in Table 3.

The characteristics were evaluated as follows. The results are shown in Table 3.

Tensile strength (TS) and elongation (El):
From the obtained steel sheet, JIS No. 5 tensile test pieces were collected so that the direction perpendicular to rolling was the longitudinal direction (tensile direction) of JIS No. 5 tensile test pieces, and the tensile strength (JIS Z 2241: 2011) was applied. TS) and elongation (El) were measured. A steel sheet having an El (%) of 14% or more was used as a steel sheet having good elongation.

Drilling property:
From the obtained steel plate, in accordance with JIS Z 2256: 2010, punch a hole of 10 mmφ with a clearance of 12.5%, set it on the testing machine so that the burr is on the die side, and then use a 60 ° conical punch. The hole expansion ratio (λ) was measured by molding. A steel sheet having a λ (%) of 35% or more was used as a steel sheet having good perforation property.

Resistance to weld cracking:
One test piece cut to 50 × 150 mm was used with the direction perpendicular to the rolling direction of the obtained steel sheet as the length, and the other was a 590 MPa class hot-dip galvanized steel sheet with a plate thickness of 1.2 mm. Resistance welding (spot welding) was carried out. Resistance spot welding was carried out using a single-phase direct current (50 Hz) resistance welder with a servomotor pressurization type, with the plate assembly of two steel plates tilted at 6 °. The welding conditions were a pressing force of 3.5 kN and a hold time of 0.1 seconds. The welding current and welding time were adjusted so that the nugget diameter was 4.2 mm. After welding, the test piece was cut in half, and the cross section was observed with an optical microscope. Those with no cracks of 0.1 mm or more were considered to have good crack resistance (◯).

For delayed fracture after resistance spot welding, resistance welding (spot welding) was performed using two tensile shear test pieces obtained in accordance with JIS Z 3136: 1999. A single-phase direct current (50 Hz) resistance welder with a servomotor pressurization type was used to perform resistance spot welding on a plate set in which two steel plates were stacked. The welding conditions were a pressing force of 3.5 kN and a hold time of 0.1 seconds. The welding current and welding time were adjusted so that the nugget diameter was 4.2 mm.
The load when the steel sheet was peeled off was measured by a tensile shear test in accordance with JIS Z 3136: 1999 on the obtained welded body. The peel strength at this time was defined as FS, and a tensile shear test piece was prepared by the same method as described above, and a load of 0.8 × FS was applied. Then, it was immersed in a 3.0% NaCl + 0.3% NH ₄ SCN solution at room temperature, and hydrogen was added by cathodic electrolytic charge. The current density was 1.5 mA / cm ² , and the counter electrode was platinum. Those that did not break even after 100 hours were considered to have good delayed fracture resistance (◯).

It can be seen that all of the invention examples are steel sheets having a high strength of TS of 780 MPa or more and having both workability (elongation and hole widening property) and resistance weld cracking resistance.

Claims (4)

By mass%
C: 0.05% or more and 0.18% or less,
Si: 0.8% or less,
Mn: 1.5% or more and 3.0% or less,
P: 0.05% or less,
S: 0.005% or less,
Al: 0.01% or more and 0.10% or less,
N: 0.010% or less,
Mo: 0.05% or more and 0.50% or less,
Ti: 0.01% or more and 0.10% or less, and
B: Contains 0.0002% or more and 0.0100% or less,
The balance has a component composition consisting of Fe and unavoidable impurities,
Where Mo, N, Ti and B are equations (1):
1.15 ≧ [Mo] ＋ 2 × ([Ti] −3.4 × [N]) ＋45 × [B] ≧ 0.20
(Here, [Mo], [Ti], [N] and [B] are the contents (mass%) of Mo, Ti, N and B, respectively, and [N] is 0% by mass. May be good.)
With volume fraction,
Ferrite is 30% or more and 70% or less,
Martensite is 20% or more and 70% or less,
Pearlite is 10% or less (including 0%) ,
Bainite is 20% or less (including 0%) and
The residual structure other than the ferrite, the martensite, the pearlite and the bainite is 5% or less.
here,
The ferrite has an average crystal grain size of 6 μm or less and an average aspect ratio of 2.0 or less.
The martensite has an average crystal grain size of 5 μm or less and an average aspect ratio of 2.0 or less.
The bainite has an average crystal grain size of 5 μm or less and has an average crystal grain size of 5 μm or less.
The volume of martensite grains having an average free path of 8.0 μm or less and having 10 or more carbides having a particle size of 0.1 μm or more in the martensite grains with respect to the total martensite is 8.0 μm or less. It has a microstructure with a fraction of 50% or more,
High-strength steel plate. The component composition is mass%, and further
V: 0.06% or less, and
Nb: The high-strength steel sheet according to claim 1, which contains at least one selected from the group consisting of 0.05% or less. The component composition is mass%, and further
Cr: 0.80% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Sb: 0.02% or less, and
The high-strength steel sheet according to claim 1 or 2, which contains at least one selected from the group consisting of 0.0050% or less: one or more selected from Ca and REM. Finishing a steel material having the component composition according to any one of claims 1 to 3 with a reduction rate of 12% or more in the final pass of finish rolling, a reduction rate of 11% or more in the pass immediately before the final pass. Rolling end temperature: Hot rolling under the conditions of 840 ° C or higher and 950 ° C or lower,
After the hot rolling, the primary cooling is performed at a primary average cooling rate of 70 ° C./s or higher to a primary cooling stop temperature of 700 ° C. or lower.
After the primary cooling, the secondary cooling is performed at a secondary average cooling rate of 5 ° C./s or more and 50 ° C./s or less to a winding start temperature of 610 ° C. or lower.
Then, to obtain a hot-rolled steel sheet Ri the winding,
The obtained hot-rolled steel sheet is pickled and cold-rolled to obtain a cold-rolled full-hard steel sheet .
The obtained cold-rolled full-hard steel sheet is heated at an average heating rate of 0.5 ° C./min or more and 5.0 ° C./min or less to a soaking temperature of 750 ° C. or more and 900 ° C. or less, and held at the above temperature for 1 hour or more. The method for producing a high-strength steel plate according to any one of claims 1 to 3, wherein the high-strength steel plate is cooled to room temperature at an average cooling rate of 1.0 ° C./h or more and 100 ° C./h or less.