JP2018162477A - High-strength steel plate and method for producing the same, resistance spot welded joint, and automobile member - Google Patents

Abstract

A high-strength steel sheet having high strength and elongation and excellent in cross tensile force when spot welded is provided.
A Ti-based precipitate having a predetermined component composition, the retained austenite is 20 to 50% by volume, the average crystal grain size of the retained austenite is 3 μm or less, and the grain size is less than 0.1 μm. The steel structure has a total number of Nb-based precipitates and V-based precipitates of 15 or more per 100 μm ² .
[Selection figure] None

Description

The present invention relates to a high-strength steel sheet suitable as a member (hereinafter also referred to as an automobile member) used in the automobile industry, and a method for producing the same.
The present invention also relates to a resistance spot welded joint using the above-described high-strength steel sheet as a base steel sheet, and an automotive member having the resistance spot welded joint.

In recent years, CO ₂ emission regulations have become stricter due to increasing environmental problems, and in the automobile field, it has become a challenge to reduce the weight of the vehicle body in order to improve fuel efficiency.
For this reason, the use of high-strength steel sheets has been promoted to reduce the thickness of automobile parts. At present, the application of steel sheets with a tensile strength of 900 MPa or more is being studied.

  Moreover, it is requested | required that the steel plate used for the member for motor vehicles, especially the structural member and reinforcement member of a motor vehicle should be excellent in a moldability. For this reason, it is necessary to improve elongation as well as strength for application to automobile members.

An example of a steel sheet having both strength and elongation is a TRIP steel sheet containing a predetermined amount of retained austenite in the steel structure.
Here, in order to generate a predetermined amount of retained austenite in the steel structure, it is preferable to lower the martensitic transformation point (Ms point) to room temperature or lower, for example, adding a predetermined amount of Mn. It is possible to lower the Ms point.

By the way, in assembling an automobile, press-molded parts are often joined and combined by resistance spot welding in terms of cost and efficiency. Therefore, it is necessary to improve the strength at the welded portion as the strength of the steel plate increases.
Generally, the tensile strength (tensile shear strength) of a resistance spot welded joint increases proportionally with an increase in the tensile strength of a steel plate for welding (base material steel plate). However, when the tensile strength of the base steel plate exceeds 900 MPa class, the fracture form of the resistance spot welded joint tends to be interface fracture, and the cross tensile force of the resistance spot welded joint decreases in reverse to the tensile strength of the base steel plate. Tend to.

For such a problem, for example, Patent Document 1 discloses:
“A method of spot welding high strength steel sheets in which a nugget is formed on the joint surface between two or more thin steel sheets,
Among the two or more thin steel plates, at least one tensile strength is 750 to 1850 MPa, and each plate thickness is 0.8 to 3.6 mm, and the carbon equivalent Ceq. The high-strength steel plates that are in the range of 0.22 to 0.55 mass% are overlapped,
After carrying out welding energization with a predetermined pressure EF1, set to a predetermined PEF1, and after energization with a predetermined post-energization current PC1 and a predetermined post-energization time Pt1,
Next, a method of spot welding a high-strength steel sheet characterized by performing electrode holding for a predetermined electrode holding time Ht "
Is disclosed.

JP-A-2015-93282 International Publication 2016/0676623 International Publication 2016/0676624 International Publication 2016/0676625 International Publication 2016/0676626

  Here, the technique of patent document 1 controls welding conditions, such as applied pressure and an energizing current, after adjusting a carbon equivalent. However, in the technique of Patent Document 1, since the carbon equivalent is limited to a certain range, the material of the base steel plate is limited, and it is difficult to sufficiently secure retained austenite that contributes to the improvement of elongation. There is. In addition, when the welding conditions are controlled as in the technique of Patent Document 1, the welding time and thus the assembly process of the automobile may be prolonged, which causes a problem that productivity is lowered.

In addition, Patent Documents 2 to 5 disclose high-strength steel sheets that have both high strength and elongation, but the steel sheets of Patent Documents 2 to 5 all have joint strength when welded, particularly No consideration is given to the cross tension force when joined by resistance spot welding.
For this reason, the present condition is that development of a steel plate having high strength and elongation and excellent weldability is required.

The present invention has been developed in view of the above-described present situation, and provides a high-strength steel sheet having high strength and elongation, and excellent in cross tensile force when spot-welded, together with its manufacturing method. With the goal.
Another object of the present invention is to provide a resistance spot welded joint using the high-strength steel plate as a base material, and an automobile member provided with the resistance spot welded joint.
Here, the “cross tensile force” is a cross tensile force (maximum tensile load until the test piece breaks) measured in a cross tensile test based on JIS Z 3137 (1999).
The “high strength steel plate” also includes a high strength steel plate (high strength plated steel plate) having a plating layer on the surface.

  The inventors have intensively studied to develop a high-strength steel sheet having high strength and elongation and excellent in cross tensile force when spot-welded, and obtained the following knowledge.

(1) In order to realize high strength and elongation at the same time, it is effective to make the structure containing a predetermined amount of retained austenite after setting the Mn content to 3.2% or more.

(2) However, when the Mn content is 3.2% or more, when resistance spot welding is performed, segregation of Mn, P, and S becomes remarkable in the solidification cell at the end of the nugget. Since segregated portions of Mn, P, and S are easily brittle fracture, the cross tensile force is greatly reduced.
That is, in resistance spot welding, a part of the steel sheet is melted and liquefied as the temperature of the base steel sheet rises, and then solidifies to form a nugget. In a steel sheet having a relatively small Mn content, the phase transformation behavior in the nugget during solidification is transformed in the order of liquid phase, δ phase, γ phase, and α ′ phase.
However, in a steel sheet in which the Mn content is 3.2% or more, the phase transformation behavior after a part of the steel sheet is melted and liquefied is different from the above, specifically, depending on the distribution state of Mn concentration, In a part of the region such as the end of the nugget, a portion that transforms directly from the liquid phase to the γ phase occurs without transformation from the liquid phase to the δ phase. Here, the solid solubility limit of the γ phase with respect to Mn, P and S is lower than that of the δ phase. Therefore, when transforming directly from the liquid phase to the γ phase, Mn at the end of the nugget and P And S are easily segregated.
As a result, the cross tension force in the welded joint is greatly reduced.

(3) To solve the problem (2) above,
(A) suppressing the contents of P and S at a higher level,
(B) refining retained austenite grains in the steel structure;
(C) By adding Nb, Ti and / or V and optimizing the cooling pattern after hot rolling in the manufacturing process of the steel sheet, fine Ti-based precipitates, Nb-based precipitates and / or V Producing system precipitates,
This is very important.
That is, as described above, the decrease in the cross tensile force in the welded joint is caused by embrittlement due to segregation of Mn, P, and S, but in particular, the effect of P segregation is large.
Here, any of the Ti-based precipitates, Nb-based precipitates, and V-based precipitates described above may react with P during the temperature increase in resistance spot welding to form a compound with P. Since the P compound thus produced is finely dispersed when the liquid phase is solidified during welding, segregation of P at the nugget edge is suppressed. Further, by making the retained austenite grains finer, Mn, P and S are easily dispersed during resistance spot welding, and segregation of these elements is suppressed. The inventors believe that these synergistic effects make it possible to sufficiently increase the cross tensile force while maintaining high strength and elongation.
The present invention was completed after further studies based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.07 to 0.25%
Si: 0.01 to 2.00%
Mn: 3.20-8.40%
P: 0.0001 to 0.0085%,
S: 0.0001-0.0041%,
Al: 0.01 to 2.00% and N: 0.010% or less,
Contains one or more selected from Ti: 0.005-0.100%, Nb: 0.005-0.100%, V: 0.005-0.100%,
The balance has a component composition consisting of Fe and inevitable impurities,
The residual austenite is 20 to 50% by volume, and the average crystal grain size of the residual austenite is 3 μm or less,
Particle size: having a steel structure in which the number of Ti-based precipitates, Nb-based precipitates and V-based precipitates less than 0.1 μm is 15 or more per 100 μm ² in total.
A high-strength steel sheet characterized by that.

2. The component composition is selected from B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less and REM: 0.0050% or less The high-strength steel sheet according to 1 above, which contains one or more kinds.

3. 3. The high-strength steel plate according to 1 or 2 above, wherein a plating layer is provided on the surface.

4). 4. The high-strength steel plate according to 3 above, wherein the plating layer is a hot dip galvanizing layer or an alloyed galvanizing layer.

5. A method for producing a high-strength steel sheet having the steel structure according to 1 above,
The steel slab having the component composition according to 1 or 2 is hot rolled at a finish rolling outlet temperature: 800 ° C. or higher and 1000 ° C. or lower to form a hot rolled plate, and the hot rolled plate is used as a first cooling, Cool to an average cooling rate of 75 ° C / s or higher to a temperature of 720 ° C or lower to 600 ° C or higher, and then as a second cooling, to a cooling rate of 5 ° C / s to 50 ° C / s or lower to 550 ° C or lower Cool, and then wind up the hot-rolled sheet at an average winding temperature of 550 ° C. or lower,
Pickling the hot-rolled sheet to remove scale,
Holding the hot-rolled sheet in a temperature range of not less than Ac ₁ transformation point + 20 ° C. and not less than Ac ₁ transformation point + 120 ° C. for not less than 600 s and not more than 21600 s;
The hot-rolled sheet is cold-rolled to form a cold-rolled sheet,
The cold-rolled sheet is held for 20 s or more and 900 s or less in a temperature range of Ac ₁ transformation point + 10 ° C. or more and Ac ₁ transformation point + 100 ° C. or less, and then cooled.
The manufacturing method of the high strength steel plate characterized by the above-mentioned.

6). The high-strength steel sheet according to 5 above, wherein the cold-rolled sheet is cooled and then subjected to galvanizing treatment or galvanizing treatment and further subjected to alloying treatment at 400 to 650 ° C or lower. Production method.

7). A plurality of base material steel plates are joined under the nugget, and at least one of the base material steel plates is a high strength steel plate according to any one of the above 1-4, is a resistance spot welded joint,
In the joint surface of the base steel plate, the area ratio of the P-enriched portion in the region from the end portion of the nugget to 50 μm toward the inside is 5% or less.
A resistance spot welded joint.
Here, the P concentration part is a part where the concentration of P becomes 0.3 mass% or more when the distribution state of P in the region is subjected to surface analysis.

8). 8. The resistance spot welding according to 7, wherein an area ratio of an S-concentrated portion in a region from the end portion of the nugget to 50 μm toward the inside is 5% or less on the joint surface of the base steel plate. Fittings.
Here, the S concentration part is a part where the concentration of S becomes 0.3% by mass or more when the distribution state of S in the region is surface-analyzed.

9. An automobile member comprising the resistance spot welded joint according to 7 or 8 above.

According to the present invention, a high-strength steel plate having high strength and elongation and excellent in cross tensile force when spot-welded can be obtained.
In addition, by applying the high-strength steel sheet of the present invention to a member for an automobile, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, which is advantageous in terms of productivity, and industrial utility value is extremely large.

It is a figure which shows the example of the resistance spot welded joint in which the two base material steel plates piled up were joined under the nugget interposition. It is a figure which shows the example of the resistance spot welded joint in which the three base material steel plates piled up were joined under the nugget interposition.

  Hereinafter, the present invention will be specifically described. First, the component composition of the high-strength steel sheet according to one embodiment of the present invention will be described. In addition, although the unit in a component composition is all "mass%", unless otherwise indicated below, it shows only with "%".

C: 0.07 to 0.25%
C is an element that contributes to increasing the strength of the steel sheet, and is an element necessary for obtaining a tensile strength of 980 MPa or more. C is an element that contributes to securing retained austenite and forming Ti-based precipitates, Nb-based precipitates, and V-based precipitates. For this reason, C content shall be 0.07% or more.
However, when C is excessively contained, the toughness of the nugget is reduced when resistance spot welding is performed. Therefore, the C content is 0.25% or less. Preferably it is 0.21% or less.

Si: 0.01-2.00%
Since Si strengthens the solid solution of ferrite, it contributes to increasing the strength of the steel sheet. In order to obtain such effects, the Si content is set to 0.01% or more.
However, when Si is contained excessively, the surface characteristics (chemical conversion properties and plating properties) required for automobile steel plates deteriorate. For this reason, Si content shall be 2.00% or less. Preferably it is 1.80% or less, More preferably, it is 1.60% or less.

Mn: 3.20-8.40%
Mn is an element that lowers the martensite transformation start temperature, and is an important element for securing retained austenite in the steel structure. In order to obtain such effects, the Mn content is set to 3.20% or more. Preferably it is 3.50% or more, more preferably 3.80% or more.
However, when Mn is contained excessively, when resistance spot welding is performed, Mn segregation at the end of the nugget is promoted, resulting in a decrease in joint strength, particularly a cross tensile force. In addition, the plating property also decreases. Therefore, the Mn content is 8.40% or less. Preferably it is 7.50%, More preferably, it is 7.00% or less.

P: 0.0001-0.0085%
P contributes to increasing the strength of the steel sheet by solid solution strengthening. Therefore, the P content is 0.0001% or more.
Further, as described above, when a large amount of Mn is contained, Mn tends to segregate at the nugget end during resistance spot welding. As a result, the freezing point at the end of the nugget is lowered, and instead of transformation from the liquid phase to the δ phase and then the γ phase, there is a portion that transforms directly from the liquid phase to the γ phase. To do. As a result, the joint strength, particularly the cross tension force, is reduced. From the viewpoint of suppressing such P segregation, the P content is set to 0.0085% or less. Preferably it is 0.0078% or less, More preferably, it is 0.0071% or less.

S: 0.0001-0.0041%
S, like P, contributes to increasing the strength of the steel sheet. For this reason, S content shall be 0.0001% or more.
However, similarly to P, S also causes a decrease in joint strength of a spot welded joint, particularly a cross tensile force, due to segregation. Therefore, the S content is 0.0041% or less. Preferably it is 0.0035% or less, More preferably, it is 0.0029% or less.

Al: 0.01-2.00%
Al is an element necessary for deoxidation. In order to obtain such an effect, the Al content is set to 0.01% or more.
However, when Al is contained excessively, the plating property is lowered. For this reason, Al content shall be 2.00% or less. Preferably it is 1.50% or less.

N: 0.010% or less N forms coarse nitrides and lowers joint strength due to void formation due to such coarse nitrides. Here, when the N content exceeds 0.010%, this tendency becomes remarkable. For this reason, N content shall be 0.010% or less. Preferably it is 0.0050% or less.
However, from the viewpoint of securing predetermined amounts of Ti-based precipitates such as nitrides, Nb-based precipitates, and V-based precipitates, the N content is preferably set to 0.0003% or more.

Moreover, in the high-strength steel sheet of the present invention, in addition to the above components, one or more selected from Ti: 0.005 to 0.100%, Nb: 0.005 to 0.100%, and V: 0.005 to 0.100% are contained. It is necessary.
Ti: 0.005-0.100%
Ti contributes to refinement of the steel structure by forming fine precipitates with C and N. In addition, Ti contributes to the suppression of segregation of P by forming the P compound by reacting the fine precipitates described above with P during resistance spot welding. In order to obtain such an effect, the Ti content is set to 0.005% or more. Preferably it is 0.008% or more. However, if Ti is excessively contained, the plating property is lowered. For this reason, Ti content shall be 0.100% or less. Preferably it is 0.080% or less.

Nb: 0.005-0.100%
Nb, like Ti, contributes to the refinement of the steel structure and the suppression of segregation of P during resistance spot welding by forming fine precipitates with C and N. In order to obtain such an effect, the Nb content is set to 0.005% or more. Preferably it is 0.008% or more. However, when Nb is contained excessively, the plating property is lowered as in the case of Ti. For this reason, Nb content shall be 0.100% or less. Preferably it is 0.080% or less.

V: 0.005-0.100%
V, as well as Ti and Nb, contributes effectively to refinement of the steel structure and suppression of P segregation during resistance spot welding by forming fine precipitates with C and N. In order to obtain such an effect, the V content is set to 0.005% or more. Preferably it is 0.008% or more. However, when V is excessively contained, the plating property is lowered as in the case of Ti. For this reason, the V content is 0.100% or less. Preferably it is 0.080% or less.

  In addition to the above components, B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less, and REM: 0.0050% or less One or more selected from among them can be contained.

B: 0.010% or less B is an element that improves the hardenability, increases the strength of HAZ (welding heat affected zone), and contributes to the improvement of joint strength, and can be contained as necessary. From the viewpoint of obtaining such an effect, the B content is preferably 0.0003% or more. However, when the B content exceeds 0.010%, the effect is saturated. Therefore, when B is contained, its content is set to 0.010% or less.

Cu: 0.50% or less
Cu is an element contributing to high strength by solid solution strengthening, and can be contained as required. From the viewpoint of obtaining such an effect, the Cu content is preferably 0.05% or more. However, if the Cu content exceeds 0.50%, the effect is saturated and surface defects due to Cu are likely to occur. Therefore, when Cu is contained, its content is 0.50% or less.

Ni: 0.50% or less
Ni, like Cu, is an element that contributes to high strength by solid solution strengthening, and can be contained as necessary. From the viewpoint of obtaining such an effect, the Ni content is preferably 0.05% or more. Further, when Ni is contained simultaneously with Cu, there is an effect of suppressing surface defects caused by Cu. For this reason, it is preferable to contain Ni simultaneously with Cu. However, when the Ni content exceeds 0.50%, the effect is saturated. Therefore, when Ni is contained, its content is 0.50% or less.

Cr: 0.50% or less
Cr is an element that improves the hardenability, increases the strength of the HAZ (welding heat affected zone), and thus contributes to the improvement of joint strength, and can be contained as necessary. In order to obtain such an effect, the Cr content is preferably 0.05% or more. However, when the Cr content exceeds 0.50%, martensite is excessively generated. Therefore, when Cr is contained, its content is 0.50% or less.

Mo: 0.50% or less
Mo, like Cr, is an element that contributes to increasing the strength of the HAZ (welding heat affected zone) and thus improving the joint strength, and can be contained if necessary. In order to obtain such an effect, the Mo content is preferably 0.01% or more. However, when the Mo content exceeds 0.50%, the effect is saturated. Therefore, when Mo is contained, its content is 0.50% or less.

Ca: 0.0050% or less
Ca is an element that spheroidizes the shape of the sulfide, improves the adverse effects of segregation of S on the spot welded portion, particularly the nugget end, and contributes to the improvement of joint strength, and may be included as necessary. it can. In order to obtain such an effect, the Ca content is preferably 0.0005% or more. However, when the Ca content exceeds 0.0050%, the Ca sulfide may deteriorate the bendability. Therefore, when Ca is contained, the content is made 0.0050% or less.

REM: 0.0050% or less
REM, like Ca, is an element that spheroidizes the shape of the sulfide, improves the adverse effects due to segregation of S at the spot welds, especially the nugget ends, and contributes to the improvement of joint strength. Can be contained. In order to obtain such an effect, the REM content is preferably 0.0005% or more. However, when the REM content exceeds 0.0050%, the effect is saturated. Therefore, when it contains REM, the content shall be 0.0050% or less.

  Components other than the above are Fe and inevitable impurities. Here, the inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.10% or less, Zn: 0.01% or less , Co: 0.10% or less.

Next, the steel structure of the high-strength steel plate according to one embodiment of the present invention will be described.
Volume ratio of retained austenite: 20-50%
Residual austenite is a structure that undergoes work-induced martensite transformation and contributes to an improvement in the balance between strength and elongation. In order to obtain such an effect, the volume ratio of retained austenite is set to 20% or more. Preferably it is 26% or more.
On the other hand, if the volume fraction of retained austenite exceeds 50%, the steel sheet is excessively hardened due to the processing-induced martensitic transformation, and therefore voids are easily generated during punching in a hole expansion test or the like, and the hole expandability is deteriorated. Therefore, the volume ratio of retained austenite is 50% or less.

Here, the volume ratio of retained austenite is obtained by polishing the steel sheet to a ¼ surface in the plate thickness direction (a surface corresponding to ¼ of the plate thickness in the depth direction from the steel plate surface). It is obtained by measuring the diffracted X-ray intensity.
Specifically, using the Kα ray of Mo as the radiation source at an acceleration voltage of 50 keV, the X-ray diffraction method (apparatus: RINT2200 made by Rigaku), the [200] plane, [211] plane of iron ferrite, [ The integrated intensity of X-ray diffraction lines on the [220] plane and the [200] plane, [220] plane, and [311] plane of austenite are measured, and the X-ray diffraction handbook (2000) ) Rigaku Electric Corporation, p. 26, The volume ratio of retained austenite is obtained from the calculation formula described in 62-64.

  Examples of the remaining structure other than retained austenite include ferrite, bainite, martensite, pearlite, and non-recrystallized ferrite. In particular, when high ductility is required, it is preferable to contain a large amount of ferrite. When particularly high strength is required, it is preferable to include a large amount of bainite and martensite. Moreover, even if the above phases are mixed, a desired effect can be obtained.

Average crystal grain size of retained austenite: 3 μm or less The average crystal grain size of residual austenite is 3 μm or less. When the average crystal grain size of retained austenite exceeds 3 μm, Mn, P and S are segregated without being dispersed during resistance spot welding, and the joint strength, particularly the cross tensile force, is reduced. Therefore, the average crystal grain size of retained austenite is 3 μm or less. Preferably, it is 2.7 μm or less.

Here, the average crystal grain size of retained austenite is obtained as follows.
That is, a cross section of the plate thickness parallel to the rolling direction of the steel plate is polished, and observed with 10 fields of view at a magnification of 5000 by FE-SEM (field emission scanning electron microscope). Next, in the obtained structural photograph, the residual austenite grains were identified by observing the same field of view with an EBSD (electron beam backscatter diffraction analyzer), and each image was obtained from the structural photograph using Media-Cybernetics Image-Pro. The equivalent circle diameter of the retained austenite grains is calculated, and these values are averaged to obtain the average crystal grain size of the retained austenite. The particle size of the retained austenite serving as the measurement limit (lower limit) is 0.1 μm.

Particle size: Number of Ti-based precipitates, Nb-based precipitates and V-based precipitates of less than 0.1 μm: 15 or more per 100 μm ^{2 in} total In order to ensure a desired cross tensile force in a resistance spot welded joint, Particle size: It is necessary that 15 or more fine Ti-based precipitates, Nb-based precipitates, and V-based precipitates (hereinafter also referred to as Ti-based precipitates) having a particle size of less than 0.1 μm are present per 100 μm ² .
That is, Ti-based precipitates having a particle size of less than 0.1 μm partially react with P when the temperature rises in resistance spot welding to form a compound with P. Since the P compound thus produced is finely dispersed when the liquid phase is solidified during welding, segregation of P at the nugget edge is suppressed. In addition, Ti-based precipitates having a grain size of less than 0.1 μm contribute to refinement of crystal grains such as retained austenite. These synergistic effects improve the cross tensile force of the resistance spot welded joint.
However, when the total number of Ti-based precipitates having a particle size of less than 0.1 μm is less than 15 per 100 μm ² , the above effect cannot be obtained sufficiently.
Accordingly, a total of 15 Ti-based precipitates having a particle size of less than 0.1 μm are present per 100 μm ² . Preferably it is 20 or more, more preferably 25 or more.
Ti-based precipitates and the like are Ti, Nb and V carbides, nitrides and carbonitrides, and Ti, Nb and V composite carbides, composite nitrides, and composite carbonitrides.

The number of Ti-based precipitates having a particle size of less than 0.1 μm is determined as follows.
That is, the thickness cross section parallel to the rolling direction of the steel sheet was observed with a TEM (Transmission Electron Microscope) at 10 times each at a magnification of 10000 times and 20000 times, and a Ti-based precipitate having an equivalent circle diameter of less than 0.1 μm, Nb The total number of system precipitates and V system precipitates is counted, and the number of Ti-based precipitates having a particle size of less than 0.1 μm per 100 μm ² is determined. The particle size of the Ti-based precipitate, the Nb-based precipitate, and the V-based precipitate that is the measurement limit (lower limit) is 2 nm.

In addition, you may provide the plating layer on the steel plate surface. The type of plating is not particularly limited, and examples thereof include zinc plating such as hot dip galvanizing, alloying galvanizing, and electrogalvanizing, and aluminum plating such as hot dip aluminum plating.
Also, the composition of the plating layer is not particularly limited and may be a general one. However, in the case of hot dip galvanization or alloyed galvanization, the plating adhesion amount per side should be ²⁰ to 120 g / m ^2. Is preferred. This is because, when the plating adhesion amount is less than 20 g / m ^2, it is difficult to ensure the corrosion resistance, while when it exceeds 120 g / m ² , the plating peel resistance may be deteriorated.

  The thickness of the high-strength steel plate is not particularly limited, but is about 0.4 to 3.0 mm. However, the plate thickness here does not include the thickness of the plating layer.

Next, the manufacturing method of the high strength steel plate which concerns on one Embodiment of this invention is demonstrated.
That is, a steel slab having the above composition is hot-rolled at a finish rolling exit temperature: 800 ° C. or higher and 1000 ° C. or lower to form a hot-rolled plate, and the hot-rolled plate is subjected to 75 ° C. / s at an average cooling rate of s or higher and cooled to a temperature of 720 ° C. or lower and 600 ° C. or higher, and then, as a second cooling, cooled at a cooling rate of 5 ° C./s or higher and 50 ° C./s or lower to 550 ° C. or lower, Thereafter, the hot rolled sheet is wound at an average winding temperature of 550 ° C. or less,
Pickling the hot-rolled sheet to remove scale,
Holding the hot-rolled sheet in a temperature range of not less than Ac ₁ transformation point + 20 ° C. and not less than Ac ₁ transformation point + 120 ° C. for not less than 600 s and not more than 21600 s;
The hot-rolled sheet is cold-rolled to form a cold-rolled sheet,
The cold-rolled sheet is held in the temperature range of Ac ₁ transformation point + 10 ° C. or higher and Ac ₁ transformation point + 100 ° C. or lower and over 900 s to 21600 s or less, and then cooled
Is.
Hereinafter, the manufacturing conditions will be described.

Heating temperature of steel slab: 1100 ° C or higher and 1300 ° C or lower Ti-based precipitates, etc. existing in the steel slab heating stage will exist as coarse precipitates in the finally obtained steel plate, Since it does not contribute to the improvement of the joint strength, it is preferable to re-dissolve Ti-based precipitates and the like precipitated during casting when the steel slab is heated.
Here, when the heating temperature of the steel slab is less than 1100 ° C., it is difficult to sufficiently dissolve the above precipitates, particularly carbides, and the risk of occurrence of trouble during hot rolling due to an increase in rolling load is increased. May cause problems. Therefore, the heating temperature of the steel slab is preferably 1100 ° C. or higher.
Also, from the viewpoint of scaling off defects such as bubbles and segregation on the surface of the slab, reducing cracks and irregularities on the steel sheet surface, and achieving a smooth steel sheet surface, the heating temperature of the steel slab should be 1100 ° C or higher. preferable. More preferably, it is 1150 ° C or higher.
On the other hand, when the heating temperature of the steel slab exceeds 1300 ° C., there is a risk of increasing the scale loss as the amount of oxidation increases. For this reason, the heating temperature of the steel slab is preferably 1300 ° C. or lower. More preferably, it is 1250 ° C. or lower.

  In order to prevent macro segregation, the steel slab is preferably manufactured by a continuous casting method, but can also be manufactured by an ingot-making method or a thin slab casting method. Moreover, after manufacturing a steel slab, the conventional method of once cooling to room temperature and heating again after that can be used. Furthermore, after steel slabs are manufactured, energy-saving processes such as direct feed rolling and direct rolling, which do not cool to room temperature, are charged in a heating furnace as they are, or are rolled immediately after being kept warm, are also problematic. Applicable without any problem. Furthermore, steel slabs are made into sheet bars by rough rolling under normal conditions, but if the heating temperature is lowered, a bar heater or the like is used before finish rolling from the viewpoint of preventing troubles during hot rolling. It is preferable to heat the sheet bar.

Finishing rolling exit temperature of hot rolling: 800 ° C. or higher and 1000 ° C. or lower The heated steel slab is hot rolled by rough rolling and finish rolling to become a hot rolled sheet. At this time, when the finish rolling exit temperature exceeds 1000 ° C., the amount of oxide (scale) generated increases rapidly, the interface between the base iron and the oxide becomes rough, the surface of the steel plate after pickling and cold rolling. The quality tends to deteriorate. In addition, if some of the hot-rolled scale remains after pickling, it adversely affects the ductility of the steel sheet and the joint strength of the welded joint. Furthermore, the desired amount of retained austenite cannot be obtained. For this reason, the finish rolling exit temperature of hot rolling is set to 1000 ° C. or less. Preferably it is 960 degrees C or less.
On the other hand, when the finish rolling outlet temperature is less than 800 ° C., the rolling load increases, the rolling load increases, and the rolling reduction in a state where the austenite is not recrystallized becomes high. As a result, an abnormal texture develops, the in-plane anisotropy in the final product becomes remarkable, and not only the material uniformity is impaired, but also the ductility of the steel sheet itself is lowered. Also, a desired amount of retained austenite cannot be obtained. For this reason, the finish rolling exit temperature of hot rolling is set to 800 ° C. or higher. Preferably it is 820 degreeC or more.

First cooling after hot rolling: cooling to a temperature of 720 ° C or lower and 600 ° C or higher with an average cooling rate of 75 ° C / s or higher Fine Ti in the steel structure of the final steel sheet (hereinafter also referred to as final structure) In order to secure a predetermined amount of system precipitates, etc., by appropriately adjusting the cooling conditions after hot rolling, the precipitates such as Ti, Nb and V carbonitrides of hot-rolled sheet obtained after hot rolling It is important to control the precipitation state. In addition, there is an effect that the final structure is refined by controlling the distribution state of the structure of the hot-rolled sheet and the precipitation state of the precipitate.
Here, when the average cooling rate in the first cooling after hot rolling is less than 75 ° C./s, the formation of Ti-based precipitates such as carbonitrides is accelerated, and these precipitates become coarse, This makes it difficult to refine the final structure. As a result, the joint strength of the resistance spot welded joint, particularly the cross tensile force, is reduced. For this reason, the average cooling rate in the 1st cooling after hot rolling shall be 75 degrees C / s or more. Preferably, it is 85 ° C./s or more. The upper limit is not particularly limited, but is about 300 ° C./s.
Moreover, when the cooling stop temperature of the first cooling exceeds 720 ° C., pearlite is excessively generated in the hot-rolled sheet, the steel structure of the hot-rolled sheet becomes inhomogeneous, and it becomes difficult to refine the final structure, The joint strength of the resistance spot welded joint, particularly the cross tension force, is reduced. Therefore, the cooling stop temperature of the first cooling is set to 720 ° C. or lower.
Furthermore, as will be described later, in order to secure a predetermined amount of fine Ti-based precipitates and the like in the final steel structure, it is necessary to perform the second cooling under appropriate conditions following the first cooling. For this reason, the cooling stop temperature of the first cooling is set to 600 ° C. or higher.

Second cooling after hot rolling: Cooling to 550 ° C. or less at an average cooling rate of 5 ° C./s or more and 50 ° C./s or less After the completion of the first cooling, second cooling is performed. Here, when the average cooling rate in the second cooling is less than 5 ° C./s, Ti-based precipitates and the like are coarsened, and it is difficult to refine the final structure. For this reason, the average cooling rate in the second cooling after hot rolling is set to 5 ° C./s or more.
On the other hand, if the average cooling rate in the second cooling exceeds 50 ° C./s, precipitation of Ti and the like will be insufficient even after the winding time and will remain in solid solution, so that it is difficult to refine the final structure. Become. For this reason, the average cooling rate in the second cooling after hot rolling is set to 50 ° C./s or less. Preferably it is 40 degrees C / s or less.
Moreover, when the cooling stop temperature of the second cooling exceeds 550 ° C., the Ti-based precipitates and the like are coarsened. For this reason, the cooling end temperature of the second cooling is set to 550 ° C. or lower. In addition, although it does not specifically limit about a minimum, 400 degreeC or more is preferable.

Average coiling temperature after hot rolling: 550 ° C or less When the average coiling temperature after hot rolling exceeds 550 ° C, the ferrite grain size of the hot-rolled sheet structure increases and it is difficult to ensure the desired strength. Become. Further, Ti-based precipitates and the like are coarsened, making it difficult to refine the final structure, and the desired residual austenite grain size cannot be obtained. Therefore, the average winding temperature after hot rolling is set to 550 ° C. or lower.
The lower limit of the average coiling temperature after hot rolling is not particularly limited, but if it is less than 300 ° C., the hot rolled sheet strength is excessively increased, and the rolling load in cold rolling is increased, Since the plate shape is defective, productivity is lowered. Therefore, the average winding temperature after hot rolling is preferably 300 ° C. or higher.

  Note that rough rolling sheets may be joined to each other during hot rolling to continuously perform finish rolling. Moreover, you may wind up a rough rolling board once. Moreover, in order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient at the time of lubrication rolling into the range of 0.10 or more and 0.25 or less.

  The hot-rolled sheet thus manufactured is pickled. Since pickling can remove oxides (scale) on the surface of the steel sheet, it is important for ensuring good chemical conversion properties and plating quality of the high-strength steel sheet as the final product. Moreover, pickling may be performed once and may be performed in multiple times.

Hot-rolled sheet annealing (first heat treatment) conditions: Ac ₁ transformation point + 20 ° C or higher, Ac ₁ transformation point + 120 ° C or lower, maintained for 600s or more and 21600s or less The annealing temperature (holding temperature) for hot-rolled sheet annealing is Ac ₁ transformation In both cases where the point + 20 ° C or below the Ac ₁ transformation point + 120 ° C and the annealing time (holding time) is less than 600 s, the concentration of Mn in the austenite does not proceed and the final annealing (cooling) It becomes difficult to secure a sufficient amount of retained austenite after sheet annealing), and ductility decreases. On the other hand, if the holding time exceeds 21600 s, the concentration of Mn in the austenite is saturated, and not only the effect on ductility in the steel sheet obtained after the final annealing is reduced, but also the cost is increased.
For this reason, in hot-rolled sheet annealing, it is held for a period of 600 s to 21600 s in a temperature range of Ac ₁ transformation point + 20 ° C. to Ac ₁ transformation point + 120 ° C.
Preferably, it is held for a period of 1000 s or more and 10000 s or less in a temperature range of Ac ₁ transformation point + 35 ° C. or more and Ac ₁ transformation point + 100 ° C. or less.

  The above heat treatment method may be any annealing method such as continuous annealing or batch annealing. In addition, after the above heat treatment, it is cooled to room temperature, but the cooling method and cooling rate are not particularly specified, and any cooling such as furnace cooling in batch annealing, gas jet cooling in air cooling and continuous annealing, mist cooling and water cooling, etc. I do not care. The pickling may be performed according to a conventional method.

  Next, the hot-rolled sheet subjected to the hot-rolled sheet annealing is cold-rolled to obtain a cold-rolled sheet. The cold rolling conditions are not particularly limited, and may be according to a conventional method. By setting the reduction ratio to 15% or more, austenite is easily generated finely during cold-rolled sheet annealing (second heat treatment), and fine retained austenite is easily obtained. Further, the upper limit value of the cold rolling reduction is preferably about 80% from the viewpoint of the load load of the cold rolling.

Cold-rolled sheet annealing (second heat treatment) conditions: Ac ₁ transformation point + 10 ° C or higher, Ac ₁ transformation point + 100 ° C or lower, maintained for 20s or more and 900s or less The annealing temperature (holding temperature) for cold-rolled sheet annealing is Ac ₁ If the transformation point + 10 ° C. or less than Ac ₁ transformation point + 100 ° C. greater than the annealing time (holding time) in the case of less than 20s are all not proceed enriched of Mn into the austenite, a sufficient amount of It becomes difficult to secure retained austenite, and ductility is lowered. On the other hand, if the holding time exceeds 900 s, crystal grain growth is promoted, so that the crystal grains become coarse and the desired average austenite crystal grain size cannot be obtained.
Therefore, in cold-rolled sheet annealing, the temperature is maintained for 20 s or more and 900 s or less in a temperature range of Ac ₁ transformation point + 10 ° C. or more and Ac ₁ transformation point + 100 ° C. or less.
Preferably, the temperature is maintained for 60 seconds or more and 600 seconds or less in a temperature range of Ac ₁ transformation point + 25 ° C. or more and Ac ₁ transformation point + 90 ° C. or less.

  In addition, the cold-rolled sheet after the cold-rolled sheet annealing obtained as described above is subjected to plating treatment such as hot-dip galvanizing treatment, alloyed zinc plating treatment, electrogalvanizing treatment, hot-dip aluminum plating treatment on the surface. In addition, a high-strength steel sheet having a galvanized layer such as a hot dip galvanized layer, an alloyed galvanized layer, and an electrogalvanized layer and an aluminum plated layer such as a hot dip aluminum plated layer can be obtained.

The plating treatment conditions may be in accordance with conventional methods, but when performing hot dip galvanizing treatment, the temperature of the steel plate immersed in the plating bath is (hot dip galvanizing bath temperature) -40 ° C to (hot dip galvanizing bath temperature) + 50 ° C. It is preferable to set it as the range.
Here, when the temperature of the steel plate immersed in the plating bath is less than (hot dip galvanizing bath temperature) −40 ° C., when the steel plate is immersed in the plating bath, a part of the molten zinc is solidified, and the plating appearance is reduced. May deteriorate. On the other hand, 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 may cause a problem in mass productivity.
The hot dip galvanizing bath temperature is not particularly limited, but is preferably about 440 ° C to 500 ° C. In addition, hot dip galvanizing is preferably performed using a hot dip galvanizing bath having an Al content of 0.10% by mass or more and 0.20% by mass or less.

  In addition, after the hot dip galvanizing treatment, an alloying treatment of hot dip galvanizing may be performed. By performing the alloying treatment, the Fe concentration in the plating layer can be set to about 7 to 15% by mass, and the adhesion of plating and the corrosion resistance after coating are further improved. However, when the alloying treatment temperature is less than 450 ° C., alloying does not proceed sufficiently, and there is a possibility that the sacrificial anticorrosive action and the sliding property are lowered. On the other hand, when the alloying treatment temperature exceeds 600 ° C., alloying may proceed excessively and the powdering resistance may be lowered. Therefore, the alloying temperature for hot dip galvanizing is preferably 450 ° C. or higher and 600 ° C. or lower.

  Further, the plating adhesion amount can be adjusted by wiping after the plating treatment.

  Furthermore, temper rolling may be applied to the cold-rolled sheet after the cold-rolled sheet annealing obtained as described above. A suitable elongation is in the range of 0.05 to 2.0%.

Next, a resistance spot welded joint according to an embodiment of the present invention will be described.
That is, the resistance spot welded joint according to an embodiment of the present invention is a resistance spot welded joint in which a plurality of base steel plates are joined together with nuggets as shown in FIGS. The above-described high-strength steel plate is used for at least one of the steel plates.
In the resistance spot welded joint, it is important that the area ratio of the P-concentrated portion in the region from the end of the nugget to 50 μm toward the inside is 5% or less on the joint surface of the base steel plate. .

The area ratio of the P-enriched part in the region from the end part of the nugget to 50 μm toward the inside: 5% or less As described above, the cross tension force of the resistance spot welded joint is Mn, P and S, especially P Largely affected by segregation. That is, in the P-concentrated portion where P is segregated, brittle cracks are likely to be generated. In particular, such a P-concentrated portion is the end of the nugget at the joint surface of the base steel plate as shown in FIG. If there is a large amount in the region up to 50 μm from the part to the inside, the interface breaks due to brittle fracture, and the joint strength, especially the cross tension force, is greatly reduced.
Therefore, the area ratio of the P-concentrated portion in the region from the end portion of the nugget to 50 μm toward the inside is set to 5% or less. Preferably it is 4% or less. The lower limit is not particularly limited, and may be 0%.

The area ratio of the S-concentrated portion in the region from the end portion of the nugget to 50 μm toward the inside: 5% or less The cross tension force of the resistance spot welded joint is also affected by the segregation of S. For this reason, it is preferable that the area ratio of the S enriched portion in the region from the end portion of the nugget to 50 μm toward the inside is 5% or less. More preferably, it is 4% or less. The lower limit is not particularly limited, and may be 0%.

Here, the above-described P concentration portion is a region where the concentration of P becomes 0.3% by mass or more when the distribution state of P in the region is subjected to surface analysis. The S concentration part is a part where the concentration of S becomes 0.3% by mass or more when the distribution state of S in the region is surface-analyzed.
In addition, the area ratio of the P-concentrated portion and the S-concentrated portion was determined by using an electron beam microprobe analyzer (EPMA) in a region from the end of the nugget to 50 μm toward the inside at a magnification of 1000 × and a pixel of 0.25. Surface analysis is performed as μm × 0.25 μm, and quantitative analysis of P and S is performed. And each site | part which becomes the density | concentration of P and S 0.3 mass% or more is identified, and it calculates | requires by dividing | segmenting the total of the area of those site | parts by the area in the area | region to 50 micrometers from the edge part of a nugget toward the inside. .

In addition, the above-described high-strength steel plate is applied to at least one of the base steel plates for the resistance spot welded joint in which the area ratio of the P-enriched portion and the S-enriched portion in the vicinity of the end portion of the nugget is reduced. You can get it. Preferably, the above-described high-strength steel plate is applied to all the base material steel plates.
Furthermore, the number of base steel plates may be three or more, and may be, for example, a resistance welding spot welded joint as shown in FIGS. 2 (a) and 2 (b).

  In addition, although resistance spot welding conditions should just follow a conventional method, from a viewpoint of reducing the area ratio in the P concentrating part and S concentrating part vicinity of the edge part of a nugget, applied pressure: 1000-5000N, energization time: It is preferable to set 5 to 50 cycles (50 Hz), hold time: 1 to 99 cycles (50 Hz), and welding current: 3000 to 10,000 A.

  Moreover, the member for motor vehicles which concerns on one Embodiment of this invention is equipped with said resistance spot welded joint, Specifically, the member for frame | skeleton etc. are mentioned.

Steel having the composition shown in Table 1 with the balance being Fe and inevitable impurities was melted and cast into a steel slab. The obtained steel slab was heated to 1240 ° C., hot-rolled under the conditions shown in Table 2, cooled, wound up, and a hot-rolled sheet having a thickness of 2.4 mm was obtained. Next, the hot-rolled sheet is pickled and annealed under the conditions shown in Table 2, cooled to room temperature, cold-rolled at a reduction ratio of 50%, and a cold-rolled sheet having a thickness of 1.2 mm. It was. Subsequently, the obtained cold-rolled sheet was subjected to cold-rolled sheet annealing under the conditions shown in Table 2 in a continuous annealing line (CAL) or a continuous hot-dip galvanizing line (CGL). In addition, some parts are further subjected to hot dip galvanizing treatment (including galvanizing treatment after hot dip galvanizing treatment) to obtain hot dip galvanized steel sheet (GI) or galvannealed steel sheet (GA). did.
Here, the plating treatment conditions are all hot dip galvanizing bath temperature: 460 ° C., hot dip galvanizing bath Al concentration: 0.14 mass% (when alloying treatment), 0.18 mass% (when not alloying treatment), Plating adhesion amount per side: 45 g / m ² (double-sided plating). The alloying treatment temperatures are as shown in Table 2.

The Ac ₁ transformation point (° C.) in Table 1 was determined using the following formula.
Ac ₁ transformation point (° C.) = 751−16 × (% C) + 11 × (% Si) −28 × (% Mn) −5.5 × (% Cu) −16 × (% Ni) + 13 × (% Cr) +3 .4x (% Mo)
Here, (% C), (% Si), (% Mn), (% Cu), (% Ni), (% Cr), (% Mo) are the contents of each element in steel (mass% ).

About each steel plate thus obtained, the volume ratio of retained austenite and the average crystal grain size, and the total of Ti-based precipitates, Nb-based precipitates and V-based precipitates having a particle size of less than 0.1 μm were obtained by the method described above. I asked for a number. These results are shown in Table 3.
In any steel sheet, the remaining structure other than retained austenite was ferrite, bainite, martensite, pearlite, and / or non-recrystallized ferrite.

  Further, the steel sheet obtained as described above was subjected to a tensile test and a cross tensile test in the following manner to evaluate the tensile strength and elongation, and the cross tensile force.

(1) Tensile test From the steel plate obtained as described above, a JIS No. 5 tensile test piece was sampled so that the direction perpendicular to the rolling direction was the longitudinal direction (tensile direction), and in accordance with JIS Z 2241 (1998). Tensile strength (TS) and elongation (El) were measured.
The target tensile strength (TS) is 980 MPa or more. The target growth (El) is 24% or more.
The evaluation results are shown in Table 3.

(2) Cross tension test Two 50 × 150mm test pieces were cut out from each steel plate obtained as described above, and these were superposed and resistance spot welding was performed, so that cross tension conforming to JIS Z 3137 was achieved. A test piece was prepared.
Here, in the resistance spot welding described above, a resistance spot welding machine of a servo motor pressurization type single-phase direct current (50 Hz) attached to a welding gun was used. The pair of electrode tips used for welding was an alumina-dispersed copper DR type electrode having a radius of curvature at the tip: R40 and a tip diameter: 6 mm. The welding pressure was 3500 N, the energization time was 15 cycles (50 Hz), the hold time was 1 cycle (50 Hz), and the welding current was adjusted so that the nugget diameter was 5√t (t is the thickness of the steel plate).
And using the obtained cross tension test piece, the cross tension test based on JIS Z 3137 (1999) was done, and the cross tension force was measured.
The target cross tensile force is 5.0 kN or more.
The evaluation results are shown in Table 3.

It can be seen that all of the inventive examples have high strength and elongation, and are excellent in cross tension.
On the other hand, in the comparative example, desired characteristics are not obtained for any one or more of tensile strength, elongation, and cross tension force.

Claims (9)

% By mass
C: 0.07 to 0.25%
Si: 0.01 to 2.00%
Mn: 3.20-8.40%
P: 0.0001 to 0.0085%,
S: 0.0001-0.0041%,
Al: 0.01 to 2.00% and N: 0.010% or less,
Contains one or more selected from Ti: 0.005-0.100%, Nb: 0.005-0.100%, V: 0.005-0.100%,
The balance has a component composition consisting of Fe and inevitable impurities,
The residual austenite is 20 to 50% by volume, and the average crystal grain size of the residual austenite is 3 μm or less,
Particle size: having a steel structure in which the number of Ti-based precipitates, Nb-based precipitates and V-based precipitates less than 0.1 μm is 15 or more per 100 μm ² in total.
A high-strength steel sheet characterized by that.   The component composition is selected from B: 0.010% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less, Mo: 0.50% or less, Ca: 0.0050% or less and REM: 0.0050% or less The high-strength steel sheet according to claim 1, comprising one or more kinds.   The high-strength steel sheet according to claim 1 or 2, wherein the surface is provided with a plating layer.   The high-strength steel sheet according to claim 3, wherein the plating layer is a hot-dip galvanizing layer or an alloyed galvanizing layer. A method for producing a high-strength steel sheet having the steel structure according to claim 1,
The steel slab having the component composition according to claim 1 or 2 is hot-rolled at a finish rolling exit temperature: 800 ° C or higher and 1000 ° C or lower to form a hot-rolled plate, and the hot-rolled plate is used as the first cooling. , With an average cooling rate of 75 ° C / s or higher, cooling to a temperature of 720 ° C or lower and 600 ° C or higher, and then as the second cooling, at a cooling rate of 5 ° C / s or higher and 50 ° C / s or lower, 550 ° C or lower And then winding the hot-rolled sheet at an average winding temperature of 550 ° C. or lower,
Pickling the hot-rolled sheet to remove scale,
Holding the hot-rolled sheet in a temperature range of not less than Ac ₁ transformation point + 20 ° C. and not less than Ac ₁ transformation point + 120 ° C. for not less than 600 s and not more than 21600 s;
The hot-rolled sheet is cold-rolled to form a cold-rolled sheet,
The cold-rolled sheet is held for 20 s or more and 900 s or less in a temperature range of Ac ₁ transformation point + 10 ° C. or more and Ac ₁ transformation point + 100 ° C. or less, and then cooled.
The manufacturing method of the high strength steel plate characterized by the above-mentioned.   The high-strength steel sheet according to claim 5, wherein the cold-rolled sheet is cooled and then subjected to galvanization or galvanization, and further subjected to alloying at 400 to 650 ° C or lower. Manufacturing method. A plurality of base material steel plates are joined under the nugget, and at least one of the base material steel plates is a high-strength steel plate according to any one of claims 1 to 4, which is a resistance spot welded joint. ,
In the joint surface of the base steel plate, the area ratio of the P-enriched portion in the region from the end portion of the nugget to 50 μm toward the inside is 5% or less.
A resistance spot welded joint.
Here, the P concentration part is a part where the concentration of P becomes 0.3 mass% or more when the distribution state of P in the region is subjected to surface analysis. 8. The resistance spot according to claim 7, wherein an area ratio of an S-concentrated portion in a region from the end portion of the nugget to 50 μm toward the inside is 5% or less on the joint surface of the base steel plate. Welded joints.
Here, the S concentration part is a part where the concentration of S becomes 0.3% by mass or more when the distribution state of S in the region is surface-analyzed.   An automotive member comprising the resistance spot welded joint according to claim 7 or 8.

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