JP2017002384A - Steel plate superior in spot weld zone fracture resistance characteristics and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel plate for maximally utilizing high strength of material by improving resistance to break at a spot weld zone feared to occur in collision deformation.SOLUTION: A steel plate superior in spot weld zone fracture resistance characteristics is provided in which a ratio (Hs/Hm) of a hardness Hs of outer layer of the steel plate to a hardness Hm of center portion of the steel plate is 0.4-0.8.SELECTED DRAWING: None

Description

  The present invention relates to a steel plate excellent in fracture resistance of a spot welded part during impact deformation and a method for producing the same.

  In recent years, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment, and the need for high-strength steel sheets is increasing from the viewpoint of reducing the weight of the vehicle body and ensuring the safety of passengers. Steel plates to be used for automobile members are required to have good press formability and high strength, and the spot welded part that joins the members does not break during collision deformation, and has sufficient shock absorbing ability. This is important from the viewpoint of ensuring safety.

  As a steel plate having good elongation and high strength used for the impact absorbing portion, a steel plate using the TRIP effect of retained austenite is known. For example, Patent Document 1 discloses a steel sheet having both excellent strength and ductility. However, it is not clear how to improve the spot-welded fracture resistance after increasing the strength.

  In Patent Document 2 and Patent Document 3, the welding conditions at the time of spot welding are devised, and after a certain period of time has elapsed after spot welding is energized, the energization is performed again, and the material structure around the formed nugget is determined. A technique for improving joint test strength by modification is disclosed. However, it is not directly related to the avoidance of breakage of the spot weld at the time of collision deformation, and no knowledge about the steel sheet is described.

  Patent Document 4 discloses a method for improving joint strength by controlling the curvature of a nugget formed during spot welding. However, it is not directly related to the avoidance of breakage of the spot weld at the time of collision deformation, and no knowledge about the steel sheet is described.

  Patent Document 5 discloses a method for improving the joint strength by improving the steel plate side. According to this, when the inclusion density and particle diameter in the nugget formed at the time of welding and the microstructure of the nugget are appropriately controlled, the joint strength represented by the cross tensile strength is improved. However, the method of Patent Document 5 is not directly related to avoidance of breakage of the spot weld during collision deformation.

  Patent Document 6 discloses a manufacturing method in which a steel sheet for hot stamping contains 0.7% or more of Si, and after annealing in a reducing atmosphere, plating is performed. Although inclusion of Si is said to improve joint strength, there is no description about avoidance of breakage of spot welds during collision deformation.

Patent Document 7 discloses a method of performing decarburization annealing on a steel plate and then heating the steel plate to Ac ₁ point or higher. According to this method, the steel sheet surface layer becomes a soft layer having a C content of 0.1% or less, and the bendability is improved. However, Patent Document 7 describes the avoidance of breakage of the spot weld during collision deformation. Not.

Japanese Patent Application Laid-Open No. 11-296991 JP 2010-115706 A JP 2010-172946 A JP 2014-180686 A JP 2014-180698 A JP 2014-185395 A JP 05-195149 A

  In order to make the most of the high strength, the steel plate, in view of ensuring the safety of the occupant, in addition to high strength, the fracture resistance of the spot welded portion during impact deformation is required. An object is to improve the fracture resistance of a spot welded portion which is feared at the time of impact deformation, and an object is to provide a steel plate which solves the problem and a method for manufacturing the same.

  The present inventors diligently studied a method for improving the spot welded portion fracture characteristics at the time of impact deformation. It was found that when the high-strength material and the low-strength material were joined, breakage occurred around the nugget on the high-strength material side, not on the low-strength material side. As a result of further investigation, it was found that the occurrence of fracture can be avoided by softening the surface layer of the high-strength material. However, it has been found that if the surface layer of the high-strength material is excessively softened, the fracture occurs again.

  That is, it has been found that the spot welded portion fracture resistance is drastically improved if the hardness of the steel plate surface layer and the steel plate center is appropriately controlled.

  This invention was made | formed based on the said knowledge, and the summary is as follows.

  (1) A steel sheet excellent in spot welded portion rupture resistance, wherein the ratio (Hs / Hm) of the hardness Hs of the steel sheet surface layer to the hardness Hm of the steel sheet center is 0.4 or more and 0.8 or less.

(2) In the steel plate excellent in the spot-welded fracture resistance described in (1) above,
Chemical composition is mass%, C: 0.03-0.70%, Si: 0.25-3.00%, Mn: 1.00-5.00%, P: 0.10% or less, S : 0.010% or less, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, comprising the balance iron and inevitable impurities,
The steel sheet surface layer having a depth of 5 μm or more and 200 μm or less from the steel sheet surface contains 1% by volume or more of tempered martensite, and the balance consists of a decarburized ferrite layer mainly composed of ferrite having an average crystal grain size of 20 μm or less,
The steel plate center portion is composed of a structure containing 3.0% by volume or more of tempered martensite and 5.0% by volume or more of retained austenite,
A steel sheet excellent in spot weld resistance fracture characteristics, characterized by having a mechanical property of a tensile strength of 980 MPa or more in a tensile test in the direction perpendicular to rolling.

  (3) The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0. The steel plate excellent in the spot-welded fracture resistance according to (2) above, containing one or more of 30% or less.

  (4) The chemical composition further contains, in mass%, one or two of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The steel plate excellent in the spot-welded fracture resistance described in (2) or (3) above.

  (5) The chemical composition further contains one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The steel plate excellent in the spot-welded fracture resistance according to any one of (2) to (4) above.

  (6) The spot-resistant spot according to any one of (2) to (5), wherein the chemical composition further contains B: 0.0001% to 0.020% by mass%. Steel sheet with excellent weld fracture characteristics.

  (7) Spot weld resistant fracture characteristics, characterized in that a hot-dip galvanized layer is formed on the surface of the steel sheet having excellent spot weld resistant fracture characteristics according to any one of (1) to (6). Excellent steel plate.

(8) The chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: 0.10% Hereinafter, S: 0.010% lower, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or lower at an average cooling rate of 2 to 200 ° C./second after the annealing step;
A steel sheet excellent in spot-welding fracture resistance, characterized by comprising a tempering step for tempering the cooled steel sheet after the cooling process, wherein the tempering process is performed in a temperature range of 100 to 600 ° C. for 1 second to 48 hours. Production method.

  (9) The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0. The method for producing a steel sheet excellent in spot-welded fracture resistance as described in (8) above, containing one or more of 30% or less.

  (10) The chemical composition further includes one or two of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00% in mass%. The method for producing a steel sheet excellent in spot-welded fracture resistance as described in (8) or (9) above.

  (11) The chemical composition further contains one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The manufacturing method of the steel plate excellent in the spot-welding part fracture | rupture characteristic in any one of said (8)-(10) characterized by the above-mentioned.

  (12) The spot-resistant spot according to any one of (8) to (11), wherein the chemical composition further contains B: 0.0001% to 0.020% by mass%. A method for producing a steel sheet having excellent weld fracture characteristics.

(13) Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.000%, Mn: 1.00 to 5.00%, P: 0.10% Hereinafter, S: 0.010% lower, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A first cooling step of cooling the annealed steel sheet to a temperature range of 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the first cooling step, a plating step of performing hot dip galvanizing on the cooled steel sheet,
A second cooling step for cooling the plated steel sheet to a temperature range of 200 ° C. or lower at an average cooling rate of 5 ° C./second or higher after the plating step;
After the second cooling step, the galvanized steel sheet is provided with a tempering step for tempering for 1 second to 48 hours in a temperature range of 100 to 600 ° C. Excellent steel plate manufacturing method.

  (14) The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0. The method for producing a steel sheet excellent in spot-welded fracture resistance as described in (13) above, containing one or more of 30% or less.

  (15) The chemical composition further includes one or two of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00% in mass%. The manufacturing method of the steel plate excellent in the spot-welding part fracture resistance according to the above (13) or (14).

  (16) The chemical composition further includes one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The method for producing a steel sheet excellent in spot-welded fracture resistance according to any one of the above (13) to (15).

  (17) The spot resistance according to any one of (13) to (16) above, wherein the chemical composition further contains B: 0.0001% to 0.020% by mass%. A method for producing a steel sheet having excellent weld fracture characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, the steel plate excellent in the fracture resistance of the spot weld part at the time of a collision deformation can be manufactured and provided.

In a hat-shaped molded member, it is a figure which shows typically the collision test which confirms the fracture | rupture aspect of a spot weld part.

  The steel sheet excellent in the spot-welded fracture resistance of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”) has a ratio (Hs / Hm) of the hardness Hs of the steel sheet surface layer to the hardness Hm of the steel sheet center part. .4 or more and 0.8 or less.

  The steel sheet of the present invention is characterized in that a hot dip galvanized layer is formed on the surface.

The method for producing a steel sheet excellent in the spot-welded fracture resistance of the present invention (hereinafter sometimes referred to as “the present invention production method”) is:
Chemical composition is mass%, C: 0.03-0.70%, Si: 0.25-3.00%, Mn: 1.00-5.00%, P: 0.10% or less, S : 0.010% lower, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or lower at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the cooling step, the chilled steel sheet is provided with a tempering step for tempering the steel sheet for 1 second to 48 hours in a temperature range of 100 to 600 ° C.

Further, the production method of the present invention has a chemical composition of mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.000%, Mn: 1.00 to 5.00%, P: 0.10% or less, S: 0.010% lower, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A first cooling step of cooling the annealed steel sheet to a temperature range of 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the first cooling step, a plating step of performing hot dip galvanizing on the cooled steel sheet,
A second cooling step for cooling the plated steel sheet to a temperature range of 200 ° C. or lower at an average cooling rate of 5 ° C./second or higher after the plating step;
After the second cooling step, the cooled plated steel sheet is provided with a tempering step in which tempering is performed in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less.

  Hereinafter, the steel sheet of the present invention and the manufacturing method of the present invention will be described. First, the inventors will discuss the method for solving the above-mentioned problems.

  Usually, the fracture resistance at the time of a collision of a spot weld is often evaluated by a fracture load in a joint test such as shear tensile strength (TSS) or cross tensile strength (CTS). These joint test strengths increase with increasing strength of the steel sheet, but the shear tensile strength increases with increasing steel sheet strength up to the 590 MPa class steel sheet. When the steel sheet strength exceeds 590 MPa, the rate of increase is low. Become.

  The cross tensile strength increases with increasing strength of the steel sheet up to a 590 MPa grade steel sheet, but decreases when the steel sheet strength exceeds 590 MPa.

  Conventionally, fractures at the time of collision of spot welds in high-strength steel sheets are regarded as a phenomenon related to the cross tensile strength that decreases as the strength of the steel sheet increases, and based on that assumption, efforts to improve the cross tensile strength Has been made (for example, see Patent Document 5).

  However, according to the results of the collision experiments conducted by the present inventors, in a spot welded portion of a steel plate and a mild steel plate of 1470 MPa class (tensile strength), originally, the strength is small and the deformation is small, and the risk of breakage is small. A fracture occurred in the base material around the welded portion (nugget) of the supposed 1470 MPa class steel plate.

  Then, the process which softens the surface layer of a 1470 MPa class steel plate is performed, and as a result of performing a collision experiment again, the ratio (Hs / Hm) of the hardness Hs of the steel plate surface layer and the hardness Hm of the steel plate center part is within a required range. In this case, it was found that the fracture on the high strength material side can be avoided and that excellent fracture resistance is exhibited.

  The hardness Hs of the steel sheet surface layer is the average hardness of the steel sheet surface layer from the steel sheet surface to a depth of 5 μm or more and 100 μm or less, and the hardness Hm of the steel sheet center part is the average hardness of the sheet thickness center part.

  The upper limit of the ratio (Hs / Hm) of the hardness Hs of the steel sheet surface layer to the hardness Hm of the steel sheet center is 0.8. This is because when the ratio (Hs / Hm) exceeds 0.8, the surface softening effect was not exhibited. The lower limit of the hardness ratio (Hs / Hm) is 0.4. This is because fracture began to occur again when the hardness ratio (Hs / Hm) was less than 0.4.

  The reason why the breakage of the spot welded portion on the high strength material side can be avoided by softening the steel sheet surface layer is not clear, but the following two mechanisms are assumed.

  One is a mechanism that considers the fracture to be due to local bending deformation. That is, it is a mechanism that the bendability is improved by softening the surface layer of the steel sheet and that fracture can be avoided. The upper limit of the hardness ratio (Hs / Hm) is a critical value that can ensure sufficient bendability, and the lower limit of the hardness ratio (Hs / Hm) is that the steel sheet surface layer becomes too soft and the restraining effect of the steel sheet surface layer disappears. It can be regarded as a critical value.

  The other is the effect of improving the fracture limit in a region where the stress triaxiality is high. Since the base material around the spot welded portion is constrained by the welded portion under a complicated stress relationship, it is considered that the stress has a higher degree of triaxiality than uniaxial tension. In a region where the stress triaxiality is high, the softening of the steel sheet surface layer may dramatically improve the fracture limit.

  Also in this case, the range of the hardness ratio (Hs / Hm) (0.4 or more and 0.8 or less) can be considered as a region where the effect of dramatically improving the fracture limit is exhibited.

  In the steel sheet of the present invention, the most important point is to control the ratio (Hs / Hm) of the hardness Hs of the steel sheet surface layer and the hardness Hm of the steel sheet center to an appropriate range (0.4 to 0.8). The object is to avoid the breakage of the spot weld at the time of impact deformation.

  Since the ratio of the hardness Hs of the steel sheet surface layer to the hardness Hm of the steel sheet center part (Hs / Hm) is not an index that directly depends on the chemical composition and mechanical properties of the steel plate, the chemical composition and mechanical properties of the steel sheet of the present invention are: Although there is no particular limitation, according to the test results of the present inventors, the effect of the ratio (Hs / Hm) is greatly expressed in a steel sheet having a tensile strength of 980 MPa or higher. Furthermore, the effect of the ratio (Hs / Hm) is more greatly expressed in a steel sheet having a tensile strength of 1300 MPa or higher.

  The reason for this is that, as described above, the bendability or the fracture resistance under high stress triaxiality is greatly deteriorated in a steel plate of 980 MPa class or higher, and further 1300 MPa class or higher, whereas the ratio (Hs It is considered that the effect of the ratio (Hs / Hm) is remarkably exhibited by appropriate control of / Hm).

  As described above, the chemical composition of the steel sheet of the present invention need not be particularly limited, but the reason for limiting the preferable chemical composition in which the effect of the above ratio (Hs / Hm) is surely greatly expressed will be described. Hereinafter,% relating to the chemical composition means mass%.

(A) Chemical composition C: 0.03% or more and 0.70% or less C is an element effective for obtaining high tensile strength. If it is less than 0.03%, the required tensile strength cannot be obtained, so C is made 0.03% or more. Preferably it is 0.05% or more. On the other hand, if it exceeds 0.70%, the weldability of the steel sheet is lowered, so C is made 0.70% or less. Preferably it is 0.45% or less.

Si: 0.25% to 3.00% Si is an essential element for suppressing the precipitation of cementite, promoting the retention of austenite, and increasing the elongation. Si is an element that strengthens ferrite, homogenizes the structure, and contributes to high strength.

  If it is less than 0.25%, the effect of addition cannot be sufficiently obtained, so Si is made 0.25% or more. Preferably it is 0.40%. In view of facilitating growth of the decarburized ferrite layer and generation of austenite, 0.60% or more is more preferable. On the other hand, if it exceeds 3.00%, defects occur in the plating step, so Si is made 3.00% or less. Preferably it is 2.50% or less.

Mn: 1.00% or more and 5.00% or less Mn is an essential element for dispersing martensite in the ferrite of the decarburized layer. Mn is an element that contributes to the improvement of strength and elongation by generating MA while suppressing precipitation of cementite.

  If it is less than 1.00%, the effect of addition cannot be sufficiently obtained, so Mn is made 1.00% or more. Preferably it is 1.90% or more. On the other hand, if it exceeds 5.00%, the weldability of the steel sheet is lowered, so Mn is made 5.00% or less. Preferably it is 4.20% or less, More preferably, it is 3.50% or less.

P: 0.10% or less P is an impurity element and an element that impairs weldability. If it exceeds 0.10%, the weldability is remarkably deteriorated, so P is made 0.10% or less. Preferably it is 0.02% or less. The lower limit includes 0%, but if P is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

S: 0.010% or less S is an impurity element, which forms MnS in steel and inhibits the hole expanding property. If it exceeds 0.010%, the hole expanding property is remarkably deteriorated, so S is made 0.010% or less. Preferably it is 0.005% or less, More preferably, it is 0.002% or less.

  The lower limit includes 0%, but if S is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

sol. Al: 0.001% or more and 1.50% or less Al is a deoxidizing element and is an element that makes the steel material sound. If it is less than 0.001%, the effect of addition is not sufficiently exhibited. Al is made 0.001% or more. Preferably it is 0.200% or more.

  On the other hand, if it exceeds 1.50%, inclusions increase and moldability deteriorates. Al is 1.50% or less. Further, Al is effective in suppressing the precipitation of cementite and increasing the amount of retained austenite, similarly to Si. Al is preferably 1.00% or less.

N: 0.020% or less N is an impurity element, which forms a nitride during continuous casting and causes cracks in the slab. If it exceeds 0.020%, slab cracks occur frequently, so N is made 0.020% or less. Preferably it is 0.010% or less.

  The lower limit includes 0%, but if N is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

  In addition to the above elements, the chemical composition of the steel sheet of the present invention includes Ti: 0 to 0.30%, Nb: 0 to 0.30%, and V: 0 to 0.30% in order to improve the characteristics of the steel sheet of the present invention. Cr: 0 to 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0 One or more of 0.010%, REM: 0 to 0.10%, and Bi: 0 to 0.050% may be included.

Ti: 0 to 0.30%
Nb: 0 to 0.30%
V: 0 to 0.30%
Ti, Nb, and V are elements that form precipitates that act as nuclei of crystal grains, refine the crystal grains, and contribute to improvement in strength and toughness.

  However, if each of Ti, Nb, and V exceeds 0.30%, the effect of addition is saturated and the manufacturing cost increases. Therefore, any element of Ti, Nb, and V is 0.30% or less. And Preferably, any element is 0.20% or less.

  The lower limit of any element of Ti, Nb, and V is 0%, but if any element is less than 0.001%, the effect of addition is not sufficiently exhibited, so Ti, Nb, and V Any element is preferably 0.001% or more. More preferably, any element is 0.007% or more.

  Ti and Nb promote the C concentration to austenite due to the formation of ferrite in the steel partially or completely austenitized by heat treatment, and contribute to the formation of MA. Or both are preferably 0.010% or more, and more preferably 0.030% or more.

Cr: 0 to 2.00%
Mo: 0 to 2.00%
Cr and Mo, like Mn, are elements that stabilize austenite and promote transformation strengthening and contribute to increasing the strength of the steel sheet.

  However, if each of Cr and Mo exceeds 2.00%, the effect of addition is saturated and the manufacturing cost increases, so both elements of Cr and Mo are made 2.00% or less. Preferably, Cr is 1.00% or less, and Mo is 0.50% or less.

  The lower limit of both elements Cr and Mo is 0%, but if any element is less than 0.001%, the effect of addition is not sufficiently exhibited, so both elements Cr and Mo are 0.001%. The above is preferable. More preferably, Cr is 0.100% or more, and Mo is 0.050% or more.

Cu: 0 to 2.00%
Ni: 0 to 2.00%
Cu and Ni are elements that exhibit a corrosion-inhibiting effect, and are elements that concentrate on the surface of the steel sheet to suppress the penetration of hydrogen into the steel sheet and suppress the delayed fracture of the steel sheet.

  However, if both elements of Cu and Ni exceed 2.00%, the effect of addition is saturated and the manufacturing cost increases, so both elements of Cu and Ni are made 2.00% or less. Preferably, any element is 0.08% or less.

  The lower limit of both elements Cu and Ni is 0%. However, if both elements are less than 0.001%, the effect of addition cannot be sufficiently obtained, so both elements Cu and Ni are 0.001%. % Or more is preferable. More preferably, any element is 0.010% or more.

B: 0 to 0.020%
B is an element that suppresses nucleation from the grain boundary, enhances the hardenability of the steel sheet, and contributes to increasing the strength of the steel sheet. Moreover, B is an element which contributes to the improvement of the elongation of a steel plate by effectively generating MA.

  However, if it exceeds 0.020%, the effect of addition is saturated and the manufacturing cost increases, so B is made 0.020% or less. Preferably it is 0.010% or less. The lower limit is 0%, but if it is less than 0.0001%, the effect of addition cannot be sufficiently obtained, so B is preferably 0.0001% or more. More preferably, it is 0.0010% or more.

Ca: 0 to 0.010%
Ca is an element that spheroidizes inclusions such as MnS and contributes to the improvement of the deformability of steel. However, if it exceeds 0.010%, the effect of addition is saturated and the manufacturing cost increases, so Ca is made 0.010% or less. Preferably it is 0.005% or less.

  The lower limit is 0%, but if it is less than 0.0005%, the effect of addition cannot be sufficiently obtained, so Ca is preferably 0.0005% or more. More preferably, it is 0.0010% or more.

REM: 0 to 0.10%
REM, like Ca, is an element that spheroidizes inclusions such as MnS and contributes to the improvement of the deformability of steel. However, if it exceeds 0.10%, the effect of addition is saturated and the manufacturing cost increases, so REM is made 0.10% or less. Preferably it is 0.05% or less.

  The lower limit is 0%, but if it is less than 0.0005%, sufficient effects cannot be obtained, so REM is preferably 0.0005% or more. More preferably, it is 0.0010% or more.

  Note that REM includes lanthanoid elements (15 elements from La to Ln), Sc (scandium), and Y (yttrium). Especially, it is preferable to contain 1 or more types of elements of La, Ce, and Y. More preferably, it is La and / or Ce.

Bi: 0 to 0.050%
Bi is an element effective for improving punching workability and machinability. However, if it exceeds 0.0050%, the effect of addition is saturated and the manufacturing cost increases, so Bi is made 0.050% or less. Preferably it is 0.020% or less.

  The lower limit is 0%, but if it is less than 0.005%, the effect of addition cannot be obtained sufficiently, so Bi is preferably 0.005% or more. More preferably, it is 0.010% or more.

The balance: iron and inevitable impurities The balance of the chemical composition of the steel sheet of the present invention consists of iron and inevitable impurities. Inevitable impurities are elements that are inevitably mixed from steel raw materials (ores, scraps, etc.) or due to various factors in the manufacturing process. Inevitable impurities are allowed in an amount that does not impair the properties of the steel sheet of the present invention.

  Next, the structure of the steel sheet that can ensure the ratio (Hs / Hm) will be described. The structure of the steel sheet of the present invention is basically a structure in which the steel sheet surface layer is a soft decarburized ferrite layer made of ferrite in which tempered martensite is dispersed, and the steel sheet center part includes a structure containing tempered martensite and residual 4 austenite. It is.

(B) Structure of steel plate Steel plate surface layer: Decarburized ferrite layer The surface layer of the steel plate of the present invention is a decarburized ferrite layer. In the steel sheet of the present invention, the ratio of the hardness Hs to the hardness Hm at the center of the steel sheet (Hs / Hm) is 0.4 or more and 0.8 or less by making the steel sheet surface layer a decarburized ferrite layer, and spot welding at the time of impact deformation Avoid breaking parts.

  It is known that when the steel sheet surface layer is decarburized, the steel sheet surface layer is softened, but when the surface layer of the steel sheet of the present invention is decarburized by a normal method, the ferrite particle diameter of the decarburized layer becomes larger than 20 μm, The total area of ferrite grain boundaries is reduced. Since deformation concentrates on the ferrite grain boundary, it is understood that the ratio of the surface layer hardness Hs to the central portion hardness Hm (Hs / Hm) cannot be controlled within an appropriate range for a steel sheet having a tensile strength of 980 MPa or more. It was.

  As a result of examining the method for solving the above problems, the present inventors have reduced the average particle diameter of ferrite in the decarburized ferrite layer, dispersed martensite in the ferrite of the decarburized ferrite layer, It has been found that tempering the dispersed martensite is effective in controlling the ratio (Hs / Hm) of the hardness Hs of the surface layer to the hardness Hm of the central portion within an appropriate range. The aspect of the decarburized ferrite layer obtained based on this knowledge is as follows.

Average crystal grain size of ferrite in the decarburized ferrite layer: 20 μm or less The decarburized ferrite layer is mainly composed of ferrite, and the average crystal grain size of this ferrite is 20 μm or less. When the average particle diameter of the ferrite in the decarburized ferrite layer exceeds 20 μm, the total area of the ferrite grain boundary decreases. Since deformation concentrates in a narrow region, if the average grain size of ferrite in the decarburized ferrite layer exceeds 20 μm, it becomes difficult to control the surface layer hardness within an appropriate range. The diameter is 20 μm or less.

  The average particle diameter of ferrite is preferably small, and the lower limit is not particularly limited, but it is difficult to make the average particle diameter of ferrite 0.5 μm or less with the current technical level.

Structure of decarburized ferrite layer Tempered martensite: 1 vol% or more Remaining structure: mainly ferrite The decarburized ferrite layer contains 1 vol% or more of tempered martensite in total. If the tempered martensite is less than 1% by volume, non-uniform deformation occurs in the steel sheet, and it is difficult to control the hardness within an appropriate range.

  The upper limit of the amount of tempered martensite in the decarburized ferrite layer is equal to the amount of tempered martensite contained in the base material of the steel sheet (that is, the portion excluding the decarburized ferrite layer).

  Since the decarburized ferrite layer is formed by decarburizing the steel sheet surface layer, the amount of tempered martensite in the decarburized ferrite layer does not exceed the amount of tempered martensite in the base material. . When the amount of tempered martensite in the decarburized ferrite layer exceeds the amount of tempered martensite in the base material, decarburization does not occur in the decarburized ferrite layer.

  By making martensite contained in the decarburized ferrite layer not tempered martensite (ferrite that has not been tempered) but tempered martensite, the hardness can be controlled within an appropriate range. The balance of the structure of the decarburized ferrite layer is mainly ferrite.

  The remainder of the structure of the decarburized ferrite layer may include, for example, a structure such as bainite, retained austenite, and pearlite within a range that does not affect the control of hardness (for example, 5% by volume or less).

Decarburized ferrite layer region: depth of 5 μm or more and 200 μm or less from the steel plate surface The decarburized ferrite layer is a layer in a region from the steel plate surface to a depth of 5 μm or more and 200 μm or less. That is, the thickness of the decarburized ferrite layer is 5 μm or more and 200 μm or less. If the thickness of the decarburized ferrite layer is less than 5 μm, the effect of softening the surface layer is not sufficiently exhibited, so the thickness of the decarburized ferrite layer is set to 5 μm or more.

  On the other hand, if the thickness of the decarburized ferrite layer exceeds 200 μm, it becomes difficult to ensure the tensile strength of the entire steel sheet, so the thickness of the decarburized ferrite layer is set to 200 μm or less.

Structure of steel plate center Tempered martensite: 3.0% by volume or more Residual austenite: 5.0% by volume or more In order to obtain a steel sheet having good workability, a structure containing MA in the structure of the base material of the steel sheet. It is effective to make the structure tempered at a relatively low temperature where residual austenite remains. Thereby, local elongation can be improved, maintaining favorable total elongation by MA.

  Therefore, the structure of the central part of the steel sheet of the present invention needs to contain 3.0% by volume or more of tempered martensite in volume ratio. Preferably it is 5.0 volume% or more. The tempered martensite is preferably 20.0% by volume or more in terms of achieving higher strength.

  Moreover, the structure of the central part of the steel sheet of the present invention needs to contain 5.0% by volume or more of retained austenite. Preferably it is 8.0 volume% or more.

  Part or all of the tempered martensite and retained austenite of the steel sheet of the present invention needs to form MA.

  In addition, let the area ratio of the structure | tissue measured in plate | board thickness 1/4 position be a volume ratio of a structure | tissue.

  The balance of the structure is preferably ferrite and / or bainite. In order to promote the formation of MA, it is preferable that no cementite of 5 μm or more is contained in the ferrite grains and martensite grains. Elongation is improved by tempering M-A at a relatively low temperature such that residual austenite remains. In order to improve bendability, all martensite is preferably tempered martensite.

Treatment of Steel Plate Surface A hot dip galvanized layer may be formed on the surface of the steel plate of the present invention. The hot dip galvanized layer may be formed by a normal hot dip galvanizing method. By forming the hot dip galvanized layer, the corrosion resistance of the steel sheet surface is improved. The adhesion amount of hot dip galvanizing is preferably ²⁰ to 120 g / m ² per side.

  Next, the manufacturing method of the present invention will be described.

  In order to generate MA in the metal structure, it is necessary to have a C concentration gradient in austenite. Austenite having a high C concentration remains as retained austenite, and austenite having a low C concentration is transformed into martensite.

  As a result, an MA structure is obtained. Since the MA structure contains retained austenite and martensite is hard, strain is concentrated in a relatively soft matrix. As a result, high strength and good elongation can be obtained in the MA structure.

  However, excessively hard martensite inhibits elongation, so it is appropriately tempered so that residual austenite remains. By this tempering, it is possible to produce a steel sheet having excellent elongation and having tempered martensite formed in the decarburized layer of the steel sheet surface layer and having an appropriate surface hardness.

  In order to form the decarburized ferrite layer, it is necessary to heat the steel sheet at an appropriate average heating rate and to anneal in an appropriate atmosphere.

The manufacturing method of the present invention (Method A) is a heating step of heating a steel sheet having a chemical composition of the steel sheet of the present invention with an average heating rate in a temperature range of 100 to 720 ° C. being 1 to 50 ° C./second,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or lower at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the cooling step, the cooled steel sheet is provided with a tempering step in which tempering is performed in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less.

Further, the production method of the present invention (Method B)
A heating step of heating the steel plate having the chemical composition of the steel plate of the present invention at an average heating rate of 1 to 50 ° C./second in a temperature range of 100 to 720 ° C.
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A first cooling step of cooling the annealed steel sheet to a temperature range of 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the first cooling step, a plating step of performing hot dip galvanizing on the cooled steel sheet,
A second cooling step of cooling the plated steel sheet to 200 ° C. or less at an average cooling rate of 5 ° C./second or more after the plating step;
After the second cooling step, the cooled plated steel sheet is provided with a tempering step in which tempering is performed in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less.

  Below, the process conditions of this invention manufacturing method are demonstrated.

Heating process (Method A and Method B)
Heating temperature range: 100-720 ° C
Average heating rate in the temperature range: 1 to 50 ° C./second A steel plate having a chemical composition of the steel sheet of the present invention is heated at an average heating rate in the temperature range of 100 to 720 ° C. of 1 to 50 ° C./second.

  The steel plate may be either a hot rolled steel plate or a cold rolled steel plate. The hot-rolled steel sheet is preferably a hot-rolled steel sheet that is finished at a finishing temperature of 800 ° C. or higher and 1100 ° C. or lower and wound at a winding temperature of 350 ° C. or higher and 750 ° C. or lower. It is not limited to a cold-rolled steel sheet and a cold-rolled steel sheet manufactured under specific conditions.

  When the lower limit of the heating temperature range is less than 100 ° C, the heating temperature range is widened and it becomes difficult to adjust the average heating rate, so the lower limit of the heating temperature range is set to 100 ° C. The upper limit of the heating temperature range is 720 ° C. because the lower limit of the annealing temperature in the next annealing step is 720 ° C.

  When the average heating rate in the temperature range of 100 to 720 ° C. is less than 1 ° C./second, cementite does not dissolve during heating, the tensile strength of the finally obtained steel sheet decreases, and ferrite in the decarburized layer Therefore, it is difficult to disperse martensite in the glass, so that the average heating rate is 1 ° C./second or more. Preferably, it is 10 ° C./second or more.

  On the other hand, if the average heating rate exceeds 50 ° C./second, coarse ferrite is generated during heating, the tensile strength is reduced, and it becomes difficult to disperse martensite in the ferrite of the decarburized layer. The heating rate is 50 ° C./second or less. Preferably it is 40 degrees C / sec or less.

Annealing process (Method A and Method B)
Atmospheric components: 2 to 20% by volume of hydrogen, remaining nitrogen and impurities Atmospheric dew point: -30 ° C to 20 ° C
Holding time: 10 to 600 seconds After the heating step, the material steel plate is annealed under the above conditions.

  If the hydrogen concentration in the atmosphere is less than 2%, the oxide film on the surface of the steel sheet cannot be reduced, and the chemical conversion property and plating wettability of the steel sheet deteriorate, so the hydrogen concentration in the atmosphere is set to 2% by volume or more. Preferably it is 5 volume% or more. On the other hand, if the hydrogen concentration in the atmosphere exceeds 20% by volume, the dew point cannot be kept below 20 ° C., so the hydrogen concentration in the atmosphere is set to 20% by volume or less. Preferably it is 15 volume% or less.

  If the dew point of the atmosphere is −30 ° C. or lower, the thickness of the decarburized ferrite layer cannot be grown to 5 μm or more, so the dew point of the atmosphere is set to be higher than −30 ° C. Preferably it is −20 ° C. or higher. On the other hand, if the dew point of the atmosphere exceeds 20 ° C., dew condensation occurs in the facility and the operation of the facility is hindered. Therefore, the dew point of the atmosphere is set to 20 ° C. or less. Preferably it is 10 degrees C or less.

When the annealing temperature (holding temperature) is less than 720 ° C., the required amount of austenite cannot be secured, and after cooling, the required amount of martensite is not generated, so the annealing temperature (holding temperature) is set to 720 ° C. or higher. In order to obtain a uniform structure that contributes to improvement in bendability, it is preferable to maintain the annealing temperature in the temperature range of the austenite single phase region (Ac ₃ points or more).

When the annealing temperature is maintained in the temperature range of the austenite single phase region, heating from 720 ° C. to Ac ₃ point over 30 seconds or more is preferable for forming a desired decarburized ferrite layer on the steel sheet surface. On the other hand, if the annealing temperature exceeds 950 ° C., it becomes difficult to disperse martensite in the ferrite of the decarburized layer, so the annealing temperature is set to 950 ° C. or less. Preferably it is 920 degrees C or less.

  If the holding time is less than 10 seconds, it is difficult to grow the thickness of the decarburized ferrite layer to 5 μm or more, so the holding time is set to 10 seconds or more. Preferably it is 20 seconds or more. On the other hand, if the holding time exceeds 600 seconds, the annealing effect is saturated and not only the productivity decreases, but also the decarburized layer grows excessively and the tensile strength decreases, so the holding time is 600 seconds or less. To do. Preferably it is 540 seconds or less.

Cooling step (Method A)
Cooling temperature range: 400 ° C. or lower Average cooling rate: 2 to 200 ° C./second After the annealing step, the steel sheet is cooled to a temperature range of 400 ° C. or lower at an average cooling rate of 2 to 200 ° C./second.

  If the upper limit of the cooling temperature range exceeds 400 ° C, the cooling effect cannot be sufficiently obtained, so the upper limit of the cooling temperature range is set to 400 ° C. When the average cooling rate is less than 2 ° C./second, cementite precipitates and martensite is not generated in the decarburized layer. Therefore, the average cooling rate is 2 ° C./second or more. Preferably, it is 5 ° C./second or more.

  On the other hand, if the average cooling rate exceeds 200 ° C./second, the concentration gradient of C in the austenite becomes insufficient and the MA structure does not occur, so the average cooling rate is set to 200 ° C./second or less. Preferably it is 100 degrees C / sec or less.

Tempering annealing process (Method A)
Temperature range: 100-600 ° C
Holding time: 1 second or more and 48 hours or less After the cooling step (Method A), the cooled steel sheet is tempered for 1 second or more and 48 hours or less in a temperature range of 100 to 600 ° C.

  Tempering is performed to properly temper the martensite of the MA structure. By this tempering, martensite is softened, so that the elongation of the steel sheet is improved. In addition, tempering concentrates C in the untransformed retained austenite and hardens the retained austenite, thereby improving the uniform elongation UEl of the steel sheet.

  If the lower limit of the temperature range is less than 100 ° C., or if the holding time is less than 1 second, the martensite of the steel sheet and the decarburized ferrite layer becomes hard, and the hardness of the steel sheet surface layer cannot be controlled to an appropriate range. The lower limit of the temperature range is 100 ° C., and the holding time is 1 second or longer. The holding time is preferably 20 seconds or longer.

  On the other hand, when the upper limit of the temperature range exceeds 600 ° C. or the holding time exceeds 48 hours, the retained austenite of the steel sheet and the decarburized ferrite layer is decomposed, the martensite becomes too soft, and the elongation is increased. Therefore, the upper limit of the temperature range is 600 ° C., and the holding time is 48 hours or less. The holding time is preferably 40 hours or less.

  However, isothermal holding at the highest temperature is preferable in terms of suppressing variations in the characteristics of the steel sheet. The maximum temperature reached is preferably 250 to 500 ° C. In addition, it is preferable that all martensite of MA structure is tempered.

  After the tempering step, the flatness of the steel plate may be corrected with a leveler. Further, the steel plate may be coated with a film having oiling or lubricating action.

First cooling step (Method B)
Cooling temperature range: 450-600 ° C
Average cooling rate: 2 to 200 ° C./second In the case of subjecting a steel plate to hot dip galvanization in the subsequent plating step, the steel plate is subjected to 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./second after the annealing step. Cool to temperature range.

  If the upper limit of the cooling temperature range exceeds 600 ° C., the cooling effect cannot be sufficiently obtained, so the upper limit of the cooling temperature range is set to 600 ° C. On the other hand, if the lower limit of the cooling temperature range is less than 450 ° C., the cooling temperature range becomes wider and it becomes difficult to control the average cooling rate, so the lower limit of the cooling temperature range is set to 450 ° C. The average cooling rate of 2 to 200 ° C./second is as described above.

Plating process (Method B)
After a 1st cooling process, after performing isothermal holding | maintenance and cooling to a steel plate as needed, hot dip galvanization is given to a steel plate. Note that, if necessary, the hot dip galvanizing may be alloyed to alloy the plating. The bath temperature and bath composition of hot dip galvanizing may be a normal bath temperature and a normal bath composition, and are not particularly limited. The plating adhesion amount is not particularly limited, and may be within a normal range. For example, the adhesion amount per one side of the steel sheet is in the range of ^{20 to} 120 g / m2.

  The alloying treatment is preferably performed under conditions such that the Fe concentration in the plating layer is 7% by mass or more. The alloying treatment is performed, for example, by heating at 490 to 560 ° C. for 5 to 60 seconds. When the alloying treatment is not performed, the Fe concentration of the hot dip galvanizing may be less than 7% by mass. The weldability of the hot dip galvanized steel sheet is lower than that of the alloyed hot dip galvanized steel sheet, but the corrosion resistance of the hot dip galvanized steel sheet is good.

Second cooling step (Method B)
Cooling temperature range: 200 ° C. or less Average cooling rate: 5 ° C./second or more After the plating step, the hot dip galvanized steel sheet is cooled from the plating temperature (bath temperature) to 200 ° C. or less at an average cooling rate of 5 ° C./second or more. . This cooling produces stable austenite, and most of the stable austenite remains as austenite even after tempering.

  In the second cooling step, hard martensite is generated simultaneously with stable austenite, but the hard martensite becomes ductile martensite by subsequent tempering.

  If the upper limit of the cooling temperature range exceeds 200 ° C, stable austenite is difficult to be generated, so the upper limit of the cooling temperature range is set to 200 ° C. The upper limit of the cooling temperature region is preferably 100 ° C.

  If the average cooling rate (the value obtained by dividing the difference between the cooling start temperature and the cooling end temperature by the cooling time) is less than 5 ° C / second, stable austenite is difficult to be generated, so the average cooling rate is 5 ° C / second or more. To do. Preferably, it is 10 ° C./second or more. The upper limit of the cooling rate is not particularly defined, but is preferably 500 ° C./second or less from the viewpoint of economy.

Tempering annealing process (Method B)
Temperature range: 100-600 ° C
Holding time: 1 second or more and 48 hours or less After the second cooling step (Method B), the cooled plated steel sheet is tempered for 1 second or more and 48 hours or less in a temperature range of 100 to 600 ° C. The process conditions in the tempering process (Method B) are the same as the process conditions in the tempering process (Method A).

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on these one example conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Example 1
Formed into a hat type (flange width 20 mm, 30 mm height, member width 50 mm, member length 500 mm), a 1.4 mm thick tensile strength 1470 MPa grade steel sheet subjected to surface softening treatment under various conditions shown in Table 1 A 1.4 mm thick (90 mm wide, 500 mm long) mild steel plate (0.2% proof stress 220 MPa, tensile strength 310 MPa, total elongation 41%), weld distance 40 mm, nugget diameter 3.5 A hat-shaped molded member was produced by spot welding aiming at √t.

  As shown in FIG. 1, both ends 100 mm of the hat-shaped molded member 1 are restrained by a fixing jig 2 from above and below with a 1470 MPa class material positioned upward, and a spherical mass of 100 kg with a tip radius of 50 mm is placed at the center. The falling weight 3 was collided at 3 m / sec. After the collision, the hat-shaped molded member 1 was collected, and the mode (presence / absence) of breakage of the spot welded portion was examined.

  The surface hardness Hs and the center hardness Hm of the 1470 MPa class steel were measured by performing a Vickers hardness test with a pressing load of 500 gf on the cross section of the steel sheet. The hardness Hs of the surface layer portion was evaluated by measuring 10 points of Vickers hardness with a pressing load of 500 gf on the surface softened layer and calculating the average value. Further, the hardness Hm of the central portion was evaluated by calculating an average value of 10 Vickers hardness measurements with a pressing load of 500 gf in a region of +100 μm and −100 μm from the central portion.

  Steel a was not softened, but at the spot weld near the part where the falling weight collided, the weld was broken at the base material of the 1470 MPa class steel. Steels b and c were subjected to softening treatment, but the welds were broken at substantially the same places as steel a that had not been softened.

  However, for steels d, e, f, g, and h having a hardness ratio (Hs / Hm) in the range of 0.4 to 0.8, no spot weld fracture occurred. As described above, this is considered to be because either or both of the bendability and the fracture resistance under high stress triaxiality are improved by the softening of the surface layer.

  On the other hand, for steels i and j having a hardness ratio (Hs / Hm) of less than 0.4, the spot welded portion was broken. Although the details are unknown, it is considered that when the surface layer portion is softened while the hardness of the center portion is high, the stress distribution becomes non-uniform between the surface layer portion and the center portion, resulting in breakage.

(Example 2)
Steel having the chemical composition shown in Table 2 was melted to produce a slab having a thickness of 40 mm. This slab was hot-rolled at the rolling completion temperature shown in Table 3 so as to have the thickness shown in Table 3. After hot rolling, water spray cooling at about 30 ° C./second was performed, and hot rolled steel sheets were produced at the coiling temperatures shown in Table 3.

  Winding is simulated by water spray cooling to the coiling temperature, then charging into the furnace, holding at the coiling temperature for 60 minutes, and then cooling to the furnace below 100 ° C at a cooling rate of 20 ° C / hour. I did. The obtained hot-rolled steel sheet was scale-removed by pickling and cold-rolled to the thickness shown in Table 4.

  The obtained steel plate was heat-treated under the conditions shown in Table 3. That is, first, the test material was heated to the annealing temperature at the indicated average heating rate, held at the temperature for the indicated time for annealing, and then cooled to 400 ° C. or less at the indicated average cooling rate. Then, after confirming that the cooling stop temperature was reached, it was heated at a rate of 20 ° C./second to the heat treatment temperature (maximum temperature reached) shown in Table 3 and held for the time indicated as the heat treatment time.

  By this final heat treatment, all martensite generated before this is tempered. The fact that martensite was tempered was confirmed by confirming the presence of carbides in martensite by SEM observation after nital corrosion.

  The following measurements were performed on the obtained test materials. These measurement results are summarized in Table 4.

  The decarburized layer thickness is an average value of values calculated by performing image analysis of an electron microscope observation image of a cross section orthogonal to the rolling direction and a cross section orthogonal to the sheet width direction (direction orthogonal to the rolling direction). The area ratio of martensite and tempered martensite was measured every 1 μm from the surface layer, the position where the value became constant was defined as the decarburized layer end, and the distance from the surface to the decarburized layer end was defined as the decarburized layer thickness.

  The structure of the decarburized layer, the particle diameter of the decarburized layer ferrite, and the area ratio of the tempered martensite of the decarburized layer were calculated by performing image analysis of an electron microscope observation image of the decarburized layer.

  The area ratio of ferrite in the surface layer part, the retained austenite area ratio in the center part, and the tempered martensite area ratio were determined. These area ratios are average values of values calculated by performing an image analysis of an electron microscope observation image of a cross section orthogonal to the rolling direction and a cross section orthogonal to the sheet width direction (direction orthogonal to the rolling direction). .

  Tensile test: JIS No. 5 tensile test piece was sampled from various heat-treated materials so that the direction perpendicular to the rolling direction was the tensile direction, the tensile test was performed, yield strength (YS), tensile strength (TS), and The total elongation (El) was measured. The measurement results are also shown in Table 4.

  Hardness measurement: Hm was measured by performing a Vickers hardness test with a pressing load of 500 gf on a steel sheet cross section. The hardness Hs of the surface layer portion was evaluated by measuring 10 points of Vickers hardness measurement with a pressing load of 500 gf in the surface decarburized layer and calculating the average value. In addition, the hardness Hm of the central portion was evaluated by measuring 10 points of Vickers hardness measurement with a pressing load of 500 gf in an area of +100 μm and −100 μm from the central portion, and calculating an average value thereof. The evaluation results are also shown in Table 4.

  Spot welded portion fracture evaluation: Evaluated in the same manner as in Example 1. Various heat treatment materials are formed into a hat shape (flange width 20 mm, 30 mm height, member width 50 mm, member length 500 mm), and a mild steel plate (0.2 mm in width and 90 mm in length) having a thickness of 1.4 mm % Hat yield strength of 220 MPa, tensile strength of 310 MPa, total elongation of 41%), a distance between welds of 40 mm, and a nugget diameter of 3.5√t.

  With respect to this hat-shaped molded member, both ends 100 mm were constrained from above and below with the heat treatment material positioned above, and a falling weight of 100 kg having a spherical shape with a tip end radius of 50 mm was collided at 3 m / sec. (See FIG. 1). After the collision, the hat-shaped molded member was collected, and the mode (presence / absence) of breakage of the spot welded portion was examined. A spot welded part with no fracture was evaluated as good and a spot welded part was evaluated as defective. The evaluation results are also shown in Table 4.

  The steel sheet shown in Table 4 as an example according to the present invention had a hardness ratio Hs / Hm between 0.4 and 0.8, and showed good characteristics without breakage of the spot weld. No. Nos. 5 and 31 have a hardness ratio Hs / Hm in a predetermined range and show excellent spot-welded fracture resistance, but it has been found that the strength is low from the characteristics of the chemical components and production conditions.

(Example 3)
Steel having the chemical composition shown in Table 2 was melted to produce a slab having a thickness of 40 mm. This slab was hot-rolled to the indicated thickness at the rolling completion temperature shown in Table 5. After hot rolling, water spray cooling of about 30 ° C./second was performed, and a hot-rolled steel sheet was manufactured at the indicated winding temperature. Winding is simulated by water spray cooling to the coiling temperature, then charging into the furnace, holding at the coiling temperature for 60 minutes, and then furnace cooling to below 100 ° C at a cooling rate of 20 ° C / hour. I did.

  The obtained hot-rolled steel sheet was scale-removed by pickling and cold-rolled to the thickness of the steel sheet shown in Table 6. The obtained steel sheet was subjected to hot dip galvanizing and alloying treatment under the heat treatment conditions in the galvannealing treatment shown in Table 5. That is, first, the test material was heated to the annealing temperature at the indicated average heating rate, held at that temperature for the indicated time, and then annealed, and then cooled to 500 ° C. at the indicated cooling rate.

  Then, after maintaining at a temperature range of 500 to 460 ° C. under display conditions, a heat treatment simulating hot dip galvanizing is performed at 460 ° C., and then some test materials are alloyed at 510 ° C. Heat treatment was performed to simulate the above, and cooling was started at the indicated cooling rate.

  The test material thus cooled was partially removed, and immediately after cooling, tempering was performed while maintaining the temperature under the tempering conditions shown in Table 5 (temperature is the highest temperature reached). The rate of temperature increase to the tempering temperature was 20 ° C./second. By this tempering, all martensite generated before tempering is tempered. The fact that martensite was tempered was confirmed by confirming the presence of carbides in martensite in SEM observation after nital corrosion.

  The measurement for the obtained test material is the same as in Example 2. These measurement results are summarized in Table 6.

  As an example according to the present invention, the steel sheet shown in Table 6 had a hardness ratio Hs / Hm between 0.4 and 0.8, and exhibited good characteristics without breakage of spot welds.

  As described above, according to the present invention, it is possible to manufacture and provide a steel plate having excellent rupture resistance of the spot welded portion during impact deformation. Since the steel sheet of the present invention is an optimal steel sheet for structural parts of automobiles that absorbs energy at the time of impact deformation, the present invention has high applicability in the steel sheet manufacturing industry and the automobile industry.

1 Hat-shaped forming member 2 Fixing jig 3 Falling weight

Claims (17)

  A steel plate having excellent spot welded portion fracture resistance, wherein the ratio (Hs / Hm) of the hardness Hs of the steel plate surface layer to the hardness Hm of the central portion of the steel plate is 0.4 or more and 0.8 or less. In the steel plate excellent in spot welded portion fracture characteristics according to claim 1,
Chemical composition is mass%, C: 0.03-0.70%, Si: 0.25-3.00%, Mn: 1.00-5.00%, P: 0.10% or less, S : 0.010% or less, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, comprising the balance iron and inevitable impurities,
The steel sheet surface layer having a depth of 5 μm or more and 200 μm or less from the steel sheet surface contains tempered martensite at 1% by volume or more, and the remaining structure is a decarburized ferrite layer mainly composed of ferrite having an average crystal grain size of 20 μm or less,
The steel plate center portion is composed of a structure containing 3.0% by volume or more of tempered martensite and 5.0% by volume or more of retained austenite,
A steel sheet excellent in spot weld resistance fracture characteristics, characterized by having a mechanical property of a tensile strength of 980 MPa or more in a tensile test in the direction perpendicular to rolling.   The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0.30%. The steel plate excellent in spot-welded fracture resistance according to claim 2, comprising one or more of the following.   The chemical composition further includes one or two of Cr: 0.001% or more and 2.00% or less and Mo: 0.001% or more and 2.00% or less in mass%. The steel plate excellent in the spot-welding part fracture resistance according to claim 2 or 3.   The chemical composition further includes one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The steel plate excellent in the spot-welded fracture resistance according to any one of claims 2 to 4.   The said chemical composition contains B: 0.0001% or more and 0.020% or less further by the mass%, The spot-welding part fracture resistance of any one of Claims 2-5 characterized by the above-mentioned. Excellent steel plate.   The steel plate excellent in the spot-welding part rupture characteristic characterized by the hot-dip galvanized layer being formed in the surface of the steel plate excellent in the spot-welding part rupture characteristic of any one of Claims 1-6 . Chemical composition is mass%, C: 0.03-0.70%, Si: 0.25-3.00%, Mn: 1.00-5.00%, P: 0.10% or less, S : 0.010% or less, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or lower at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the cooling step, the steel plate having excellent spot-welded fracture resistance, comprising a tempering step of tempering the cooled steel plate for 1 second to 48 hours in a temperature range of 100 to 600 ° C. Manufacturing method.   The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0.30%. The manufacturing method of the steel plate excellent in the spot-welding part fracture resistance of Claim 8 characterized by containing the following 1 type (s) or 2 or more types.   The chemical composition further includes one or two of Cr: 0.001% or more and 2.00% or less and Mo: 0.001% or more and 2.00% or less in mass%. The manufacturing method of the steel plate excellent in the spot welding part fracture | rupture characteristic of Claim 8 or 9.   The chemical composition further includes one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The manufacturing method of the steel plate excellent in the spot-welding part fracture | rupture characteristic of any one of Claims 8-10.   The said chemical composition contains further B: 0.0001% or more and 0.020% or less by the mass%, The spot-welding part fracture resistance of any one of Claims 8-11 characterized by the above-mentioned. Steel sheet manufacturing method with excellent performance. Chemical composition is mass%, C: 0.03-0.70%, Si: 0.25-3.00%, Mn: 1.00-5.00%, P: 0.10% or less, S : 0.010% lower, sol. Al: 0.001 to 1.50%, N: 0.020% or less, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30%, Cr: 0 to 0 2.00%, Mo: 0 to 2.00%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, B: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.10% and Bi: 0 to 0.050%, and the steel sheet composed of the remaining iron and impurities, with an average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / second Heating process for heating,
After the heating step, the heated steel sheet is composed of 2 to 20% by volume of hydrogen, the balance nitrogen and impurities, and in a temperature range of 720 to 950 ° C. in an atmosphere having a dew point of more than −30 ° C. and not more than 20 ° C. An annealing process for applying annealing for 10 to 600 seconds,
A first cooling step of cooling the annealed steel sheet to a temperature range of 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./second after the annealing step;
After the first cooling step, a plating step of performing hot dip galvanizing on the cooled steel sheet,
A second cooling step of cooling the plated steel sheet to 200 ° C. or less at an average cooling rate of 5 ° C./second or more after the plating step;
After the second cooling step, the galvanized steel sheet is provided with a tempering step for tempering for 1 second to 48 hours in a temperature range of 100 to 600 ° C. Excellent steel plate manufacturing method.   The chemical composition further includes, in mass%, Ti: 0.001% to 0.30%, Nb: 0.001% to 0.30%, and V: 0.001% to 0.30%. The manufacturing method of the steel plate excellent in the spot-welding part fracture characteristic of Claim 13 characterized by containing the following 1 type (s) or 2 or more types.   The chemical composition further includes one or two of Cr: 0.001% or more and 2.00% or less and Mo: 0.001% or more and 2.00% or less in mass%. The manufacturing method of the steel plate excellent in the spot welding part fracture | rupture characteristic of Claim 13 or 14.   The chemical composition further includes one or two of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00% in mass%. The manufacturing method of the steel plate excellent in the spot-welding-part fracture resistance of any one of Claims 13-15.   The chemical composition further contains, in mass%, B: 0.0001% or more and 0.020% or less, the spot welded portion fracture resistance according to any one of claims 13 to 16. Steel sheet manufacturing method with excellent performance.

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