JP6524810B2 - Steel plate excellent in spot weld resistance and its manufacturing method - Google Patents

Description

  The present invention relates to a steel plate excellent in fracture resistance of a spot welded portion at the time of collision deformation and a method of manufacturing the same.

  In recent years, fuel efficiency improvement of a car is required for global environment protection, and the needs of a high-strength steel plate are increasing from a viewpoint of weight reduction of a car body, and a crew member's safety ensuring. The steel plate to be provided for automotive members is required to have good press formability and high strength, and the spot welds joining the members do not break at the time of collision deformation, and the occupant has sufficient shock absorbing ability. Important in terms of ensuring the safety of the

  A steel plate utilizing the TRIP effect of retained austenite is known as a steel plate having high strength and high elongation used for a shock absorbing portion. For example, Patent Document 1 discloses a steel plate excellent in both strength and ductility. However, it is not clear how to improve the spot-welded part fracture characteristics with high strength.

  Patent document 2 and patent document 3 devise welding conditions at the time of spot welding, and after passing a certain time after the completion of the spot welding, the material structure around the formed nugget is conducted again. A method is disclosed that improves joint test strength by reforming. However, it is not directly related to the fracture avoidance of the spot weld part at the time of impact deformation, and the knowledge about the steel plate is not described.

  Patent Document 4 discloses a method of improving joint strength by controlling the curvature of a nugget formed at the time of spot welding. However, it is not directly related to the spot weld part breakage avoidance at the time of collision deformation, and the knowledge about the steel plate is not described.

  Patent Document 5 discloses a method of 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, it is supposed that the joint strength represented by the cross tensile strength is improved. However, the method of Patent Document 5 is also not directly related to the spot weld portion breakage avoidance at the time of 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 is further annealed in a reducing atmosphere and then plated. Although it is stated that the inclusion of Si improves the joint strength, no mention is made of the prevention of breakage of the spot weld at the time of collision deformation.

Patent Document 7 discloses a method of subjecting a steel sheet to decarburizing annealing, and then heating the steel sheet to _one or more Ac points. According to this method, the surface layer of the steel sheet 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 welded portion at the time of collision deformation. Not.

Japanese Patent Application Laid-Open No. 11-279691 JP, 2010-115706, A JP, 2010-172946, A JP, 2014-180686, A JP, 2014-180698, A JP, 2014-185395, A Unexamined-Japanese-Patent No. 05-195149

  In order to make the best use of high strength, in view of the fact that the steel plate is required to have high strength and fracture resistance of the spot welded portion at the time of collision deformation from the viewpoint of securing the safety of the occupant to the steel plate. It is an object of the present invention to improve the fracture resistance of a spot welded portion which is concerned at the time of impact deformation, and to provide a steel plate which solves the problem and a method of manufacturing the same.

  The present inventors diligently studied a method for improving the spot weld 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. Further investigations have shown that if the surface layer of the high strength material is softened, the occurrence of fracture can be avoided. However, it was found that when the surface layer of the high strength material was softened too much, breakage occurred again.

  That is, it was found that spot weld resistance fracture characteristics are dramatically improved by appropriately controlling the hardness of the surface of the steel sheet and the center of the steel sheet.

  The present invention has been made based on the above findings, and the summary thereof is as follows.

  (1) A steel plate excellent in spot-welded part fracture characteristics characterized in that the ratio (Hs / Hm) of the hardness Hs of the surface layer of the steel sheet to the hardness Hm of the central part of the steel sheet is 0.4 or more and 0.8 or less.

(2) In the steel plate excellent in the spot-welded part breaking characteristics described in the above (1),
Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: not more than 0.10%, 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 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%, the balance being iron and unavoidable impurities,
The steel plate surface layer having a depth of 5 μm to 200 μm from the steel plate surface is composed of a decarburized ferrite layer containing 1 vol% or more of tempered martensite and the balance mainly consisting of ferrite having an average crystal grain size of 20 μm or less
The steel sheet core has a structure containing 3.0% by volume or more of tempered martensite and 5.0% by volume or more of retained austenite,
A steel plate excellent in spot weld resistance fracture characteristics, characterized in that it has mechanical properties having a tensile strength of 980 MPa or more in a tensile test in the direction perpendicular to rolling.

  (3) The chemical composition is, further, in mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more 0 . 30% or less of one type or two or more types are characterized in that the steel plate excellent in the spot-welded part fracture characteristics described in the above (2), characterized in that

  (4) The chemical composition further contains, by mass%, one or two or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The steel plate excellent in the spot-welded part fracture | rupture characteristic as described in said (2) or (3) characterized by the above-mentioned.

  (5) The chemical composition further contains, in mass%, Cu: 0.001% or more and 2.00% or less, and Ni: 0.001% or more and 2.00% or less. The steel plate excellent in the spot-welded part fracture | rupture characteristic in any one of said (2)-(4) characterized by the above-mentioned.

  (6) The spot resistance according to any one of the above (2) to (5), wherein the chemical composition further contains, by mass%, B: 0.0001% or more and 0.020% or less. Steel plate with excellent weld fracture characteristics.

  (7) A spot-welded part fracture characteristic characterized in that a hot-dip galvanized layer is formed on the surface of the steel plate excellent in spot-welded part fracture characteristic according to any one of the above (1) to (6) Excellent steel plate.

(8) 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% Below, below S: 0.010%, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or less at an average cooling rate of 2 to 200 ° C./sec after the annealing step;
After the cooling step, the steel plate excellent in spot-welded part fracture characteristics is characterized by comprising a tempering step of tempering to hold the cooled steel plate in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less. Production method.

  (9) The chemical composition further includes, in mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more 0 30% or less of 1 type or 2 types or more being contained, The manufacturing method of the steel plate excellent in the spot-welding part breaking characteristic as described in said (8) characterized by the above-mentioned.

  (10) The chemical composition further contains one or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00% by mass%. The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic as described in said (8) or (9) characterized by the above-mentioned.

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

  (12) The spot resistance according to any one of the above (8) to (11), wherein the chemical composition further contains, by mass%, B: 0.0001% to 0.020%. A method of manufacturing a steel plate 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% Below, below S: 0.010%, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold 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;
A plating step of subjecting the cooled steel plate to hot dip galvanization after the first cooling step;
A second cooling step of cooling the plated steel plate to a temperature range of 200 ° C. or less at an average cooling rate of 5 ° C./sec or more after the plating step;
After the second cooling step, there is provided a tempering step of tempering the plated steel sheet which has been cooled and held in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less. Excellent steel plate manufacturing method.

  (14) The chemical composition further includes, in mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more 0 30% or less of 1 type or 2 types or more containing, The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic as described in said (13) characterized by the above-mentioned.

  (15) The chemical composition further contains, by mass%, one or two or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic as described in said (13) or (14) characterized by the above-mentioned.

  (16) The chemical composition further contains, in mass%, Cu: 0.001% or more and 2.00% or less, and Ni: 0.001% or more and 2.00% or less. The manufacturing method of the steel plate excellent in the spot-welding part fracture | rupture characteristic in any one of said (13)-(15) characterized by the above-mentioned.

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

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

It is a figure which shows typically the collision test which confirms the fracture | rupture aspect of a spot welding part in a hat-shaped shaping | molding member.

  The steel plate (hereinafter sometimes referred to as "the steel plate of the present invention") excellent in the fracture resistance of the spot welded portion according to the present invention has a ratio (Hs / Hm) of hardness Hs of surface layer of steel plate to hardness Hm of center portion of steel plate .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 manufacturing method of the steel plate excellent in the spot-welded part fracture characteristic of the present invention (hereinafter sometimes referred to as "the manufacturing method of the present invention") is as follows.
Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: not more than 0.10%, S : Below 0.010%, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or less at an average cooling rate of 2 to 200 ° C./sec after the annealing step;
After the above-mentioned cooling process, it is characterized by including a tempering-and-tempering process which holds a cooled steel plate in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less.

In the production method of the present invention, the chemical composition is, in 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% below, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold 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;
A plating step of subjecting the cooled steel plate to hot dip galvanization after the first cooling step;
A second cooling step of cooling the plated steel plate to a temperature range of 200 ° C. or less at an average cooling rate of 5 ° C./sec or more after the plating step;
After the said 2nd cooling process, it is characterized by providing the tempering process which gives a tempering hold | maintained to the cooled plated steel plate in the temperature range of 100-600 degreeC 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 investigation of the method of solving the above-mentioned problems by the present inventors will be described.

  Usually, the fracture resistance at the time of collision of a spot welded part is often evaluated by a breaking load in a joint test such as shear tensile strength (TSS) or cross tensile strength (CTS). Although these joint test strengths increase with high strength of steel plate, shear tensile strength increases with increase of steel plate strength up to 590MPa grade steel plate, and when steel plate strength exceeds 590MPa, the increase rate is low Become.

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

  In the past, the fracture at the time of collision of a high strength steel sheet spot at collision was regarded as a phenomenon related to the cross tensile strength which decreases as the strength of the steel sheet increased. (See, for example, Patent Document 5).

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

  Therefore, as a result of performing a treatment to soften the surface layer of a 1470 MPa grade steel plate and performing a collision experiment again, the ratio (Hs / Hm) of the hardness Hs of the surface layer of the steel plate to the hardness Hm of the central portion of the steel plate is within the required range. It has been found that breakage of the high strength material side can be avoided, and excellent fracture resistance is developed.

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

  The upper limit of the ratio (Hs / Hm) of the hardness Hs of the surface of the steel sheet to the hardness Hm of the central part of the steel sheet is 0.8. This is because when the ratio (Hs / Hm) exceeds 0.8, the surface layer softening effect does not appear. The lower limit of the hardness ratio (Hs / Hm) is 0.4. This is because the fracture started to occur again when the hardness ratio (Hs / Hm) is less than 0.4.

  Although it is not clear why it is possible to avoid breakage of the spot welded portion on the high strength material side by softening the surface layer of the steel sheet, the following two mechanisms are assumed.

  One is a mechanism considered that the fracture is due to a local bending deformation. That is, by bending the surface layer of the steel sheet, the bendability is improved and it is considered that the breakage 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 restraint effect of the steel sheet surface disappears It can be considered as a critical value.

  The other one is the improvement effect of the fracture limit in the region of high stress triaxiality. The base material around the spot welds is considered to be in a state of higher stress triaxiality than uniaxial tension, because the base material is constrained by the welds under complicated stress relationships. In the region where the stress triaxiality is high, the softening of the surface layer of the steel sheet may dramatically improve the fracture limit.

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

  In the steel sheet of the present invention, the most important point is that the ratio (Hs / Hm) of the hardness Hs of the surface layer of the steel sheet to the hardness Hm of the center part of the steel sheet is controlled within an appropriate range (0.4 to 0.8). It is in the ability to avoid the breakage of the spot weld at the time of impact deformation.

  The ratio (Hs / Hm) of the hardness Hs of the surface of the steel sheet to the hardness Hm of the central part of the steel sheet is not an index that directly depends on the chemical composition or mechanical properties of the steel sheet. Although it is not necessary to limit in particular, according to the test results of the present inventors, the effect of the above-mentioned ratio (Hs / Hm) is largely expressed in a steel plate with a tensile strength of 980 MPa or more. Furthermore, in the steel plate having a tensile strength of 1300 MPa or more, the effect of the above ratio (Hs / Hm) is more largely expressed.

  The reason is that, as described above, the above-mentioned ratio (Hs) is significantly deteriorated in the steel sheet having high flexural strength or high stress triaxiality and with a steel sheet of 980 MPa grade or more and further 1300 MPa grade or more. It is considered that the appropriate control of / Hm) causes the effect of the above ratio (Hs / Hm) to be significantly expressed.

  As described above, the chemical composition of the steel sheet of the present invention does not need to be particularly limited, but the reason for limiting the preferred chemical composition in which the effect of the above ratio (Hs / Hm) reliably appears largely 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 to obtain high tensile strength. If it is less than 0.03%, the required tensile strength can not be obtained, so C is made 0.03% or more. Preferably, it is 0.05% or more. On the other hand, when the content exceeds 0.70%, the weldability of the steel sheet is reduced, so C is made 0.70% or less. Preferably it is 0.45% or less.

Si: 0.25% or more and 3.00% or less Si is an essential element for suppressing the precipitation of cementite, promoting the retention of austenite, and enhancing the elongation. Further, Si is an element that strengthens ferrite, makes the structure uniform, and contributes to high strength.

  If it is less than 0.25%, the addition effect is not sufficiently obtained, so Si is made 0.25% or more. Preferably it is 0.40%. 0.60% or more is more preferable at the point which the growth of a decarburized ferrite layer and formation of austenite become easy. On the other hand, if it exceeds 3.00%, a defect occurs 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 element essential for dispersing martensite in ferrite of the decarburized layer. Moreover, Mn is an element which produces M-A, suppressing precipitation of cementite, and contributes to the improvement of intensity | strength and elongation.

  If it is less than 1.00%, the addition effect is not 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 reduced, 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 is an element that inhibits weldability. If the content exceeds 0.10%, the weldability is significantly reduced, so P is made 0.10% or less. Preferably it is 0.02% or less. Although the lower limit includes 0%, when P is reduced to less than 0.0001%, the manufacturing cost is significantly increased, so the practical lower limit is 0.0001% on the practical steel sheet.

S: 0.010% or less S is an impurity element and is an element that forms MnS in steel and inhibits hole expandability. If it exceeds 0.010%, the hole expansibility is significantly reduced, so S is made 0.010% or less. Preferably it is 0.005% or less, more preferably 0.002% or less.

  Although the lower limit includes 0%, when S is reduced to less than 0.0001%, the manufacturing cost is significantly increased, so the practical lower limit is 0.0001% on the 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 steel materials sound. If it is less than 0.001%, the effect of addition is not sufficiently expressed, so sol. Al is 0.001% or more. Preferably it is 0.200% or more.

  On the other hand, if it exceeds 1.50%, inclusions increase and the formability deteriorates, so sol. Al is 1.50% or less. Also, 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, and forms nitrides during continuous casting to cause cracks in the slab, and therefore, the smaller amount is a preferable element. If it exceeds 0.020%, cracks in the slab frequently occur, so N is made 0.020% or less. Preferably it is 0.010% or less.

  Although the lower limit includes 0%, when N is reduced to less than 0.0001%, the manufacturing cost is significantly increased, so the practical lower limit is 0.0001% on a practical steel sheet.

  The chemical composition of the steel plate of the present invention is Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.30% in order to improve the characteristics of the steel plate of the present invention other than the above elements. 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, make the crystal grains finer, and contribute to the improvement of strength and toughness.

  However, if each of Ti, Nb, and V exceeds 0.30%, the addition effect is saturated and the manufacturing cost is increased. Therefore, 0.30% or less of any of Ti, Nb, and V is also included. I assume. Preferably, each element content is 0.20% or less.

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

  Since Ti and Nb promote C enrichment to austenite by the formation of ferrite and contribute to the formation of M-A in a steel whose structure is partially or completely austenitized by heat treatment, one of Ti and Nb Or 0.010% or more is preferable, and both are more preferable.

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

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

  The lower limit of both elements of Cr and Mo is 0%, but if any element is less than 0.001%, the addition effect is not sufficiently expressed, so that both elements of 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 which exert a corrosion suppressing effect, and are elements which are concentrated on the surface of the steel plate to suppress the penetration of hydrogen into the steel plate and to suppress the delayed fracture of the steel plate.

  However, when any element of Cu and Ni exceeds 2.00%, the addition effect is saturated and the manufacturing cost is increased, so that both elements of Cu and Ni are 2.00% or less. Preferably, each element is at most 0.08%.

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

B: 0 to 0.020%
B is an element that suppresses nucleation from grain boundaries, enhances the hardenability of the steel plate, and contributes to the strengthening of the steel plate. Moreover, B is an element which produces M-A effectively and contributes to the improvement of the elongation of a steel plate.

  However, if it exceeds 0.020%, the effect of addition saturates 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 less than 0.0001%, sufficient addition effect can not be obtained, so B is preferably at least 0.0001%. More preferably, it is 0.0010% or more.

Ca: 0 to 0.010%
Ca is an element which spheroids inclusions such as MnS and contributes to the improvement of the deformability of steel. However, if it exceeds 0.010%, the addition effect saturates 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 less than 0.0005%, sufficient addition effect can not be 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 spheroids inclusions such as MnS and contributes to the improvement of the deformability of steel. However, if it exceeds 0.10%, the addition effect saturates and the manufacturing cost increases, so the REM is made 0.10% or less. Preferably it is 0.05% or less.

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

  REM is a meaning including lanthanoid elements (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among them, it is preferable to contain one or more 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 addition effect is saturated and the manufacturing cost is increased, so Bi is made 0.050% or less. Preferably it is 0.020% or less.

  The lower limit is 0%, but if less than 0.005%, the addition effect can not be sufficiently obtained, and therefore, Bi is preferably 0.005% or more. More preferably, it is 0.010% or more.

Remainder: Iron and unavoidable impurities The remainder of the chemical composition of the steel sheet of the present invention consists of iron and unavoidable impurities. Unavoidable impurities are elements which are inevitably mixed from steel raw materials (ores, scraps, etc.) or due to various factors of the production process. Unavoidable impurities are acceptable in an amount that does not impair the characteristics of the steel sheet of the present invention.

  Next, the structure of the steel plate capable of securing the above ratio (Hs / Hm) will be described. In the steel sheet according to the present invention, the steel sheet surface layer is basically a soft decarburized ferrite layer consisting of ferrite in which tempered martensite is dispersed, and the core of the steel sheet comprises tempered martensite and residual 4 austenite. It is.

(B) Structure of Steel Plate Surface Layer of Steel Plate: 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 surface layer of the steel sheet is a decarburized ferrite layer, so that the ratio (Hs / Hm) of hardness Hs to hardness Hm at the center of the steel sheet is 0.4 or more and 0.8 or less. Avoid breakage of parts.

  Decarburization of the steel sheet surface layer is known to soften the steel sheet surface layer, but when the surface layer of the steel sheet of the present invention is decarburized by the usual method, the ferrite particle size of the decarburized layer becomes larger than 20 μm, The total area of ferrite grain boundaries is reduced. Since deformation is concentrated at ferrite grain boundaries, it has been found that the ratio (Hs / Hm) of surface layer hardness Hs to center portion hardness Hm can not be controlled to an appropriate range in steel plates having a tensile strength of 980 MPa or more. The

  As a result of examining the method for solving the above problems, the present inventors have made the average particle diameter of ferrite of the decarburized ferrite layer smaller, disperse the martensite in the ferrite of the decarburized ferrite layer, and 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 grain size of ferrite of decarburized ferrite layer: 20 μm or less The decarburized ferrite layer mainly consists of ferrite, and the average grain size of this ferrite is 20 μm or less. When the average grain size of ferrite in the decarburized ferrite layer exceeds 20 μm, the total area of ferrite grain boundaries decreases. Since deformation is concentrated 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 to an appropriate range, so the average grain size of ferrite in the decarburized ferrite layer The diameter is 20 μm or less.

  The smaller the average grain size of ferrite is preferable, and the lower limit is not particularly limited. However, it is difficult in the present technical level to set the average grain size of ferrite to 0.5 μm or less.

Structure of Decarburized Ferrite Layer Tempered Martensite: 1% by Volume or More Remaining Structure: Mainly Ferrite The decarburized ferrite layer contains 1% by volume or more of tempered martensite in total. When the tempered martensite is less than 1% by volume, the steel sheet is deformed nonuniformly, and it is difficult to control the hardness to 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 surface of the steel sheet, the amount of tempered martensite in the decarburized ferrite layer does not exceed the amount of tempered martensite in the base material. . If the amount of tempered martensite in the decarburized ferrite layer exceeds the amount of tempered martensite in the base material, no decarburization occurs in the decarburized ferrite layer.

  By setting the martensite contained in the decarburized ferrite layer to be tempered martensite instead of fresh martensite (ferment which is not tempered), the hardness can be controlled within an appropriate range. The balance of the structure of the decarburized ferrite layer is mainly ferrite.

  The remaining part of the structure of the decarburized ferrite layer may contain, for example, structures of bainite, retained austenite, and pearlite within the range not affecting the control of hardness (for example, 5% by volume or less).

Region of Decarburized Ferrite Layer: Depth from 5 μm to 200 μm from Steel Plate Surface The decarburized ferrite layer is a layer from the surface of the steel plate to a depth of 5 μm to 200 μm. 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 surface layer softening effect is not sufficiently exhibited, so the thickness of the decarburized ferrite layer is set to 5 μm or more.

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

Structure of center of steel sheet Tempered martensite: 3.0% by volume or more Retained austenite: 5.0% by volume or more In order to obtain a steel sheet having good formability, a structure including 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 at which retained austenite remains. Thereby, local elongation can be improved while maintaining good total elongation by M-A.

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

  Further, the structure of the central portion of the steel sheet of the present invention needs to contain retained austenite at 5.0% by volume or more. 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 M-A.

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

  The balance of the tissue is preferably ferrite and / or bainite. It is preferable that the ferrite grains and the martensite grains do not contain cementite of 5 μm or more in order to promote the formation of M-A. Elongation is improved by tempering MA at a relatively low temperature such that retained austenite remains. In order to improve bendability, all martensite is preferably tempered martensite.

Treatment of Steel Sheet Surface A hot-dip galvanized layer may be formed on the surface of the steel sheet of the present invention. The hot dip galvanized layer may be formed by a conventional 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 galvanization is preferably ²⁰ to 120 g / m ² per one side.

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

  In order to generate M-A in the metal structure, it is necessary to have a concentration gradient of C in austenite. Austenite with high C concentration remains as retained austenite, and austenite with low C concentration transforms to martensite.

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

  However, since excessively hard martensite inhibits elongation, it is suitably tempered so that retained austenite remains. By this tempering, it is possible to produce a steel sheet which has excellent elongation and which forms tempered martensite in the decarburized layer on the surface of the steel sheet and which has an appropriate surface hardness.

  In order to form a decarburized ferrite layer, the steel sheet needs to be heated under an appropriate average heating rate and annealed under 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 present invention steel sheet, with an average heating rate in the temperature range of 100 to 720 ° C.
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or less at an average cooling rate of 2 to 200 ° C./sec after the annealing step;
It is characterized by providing a tempering process which performs a tempering which holds the cooled steel plate in the temperature range of 100-600 ° C for 1 second or more and 48 hours or less after the above-mentioned cooling process.

Further, the production method of the present invention (Method B) is
A heating step of heating a steel plate having a chemical composition of the present invention steel plate, with an average heating rate in a temperature range of 100 to 720 ° C. as 1 to 50 ° C./sec,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold 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;
A plating step of subjecting the cooled steel plate to hot dip galvanization after the first cooling step;
A second cooling step of cooling the plated steel plate to 200 ° C. or less at an average cooling rate of 5 ° C./sec or more after the plating step;
After the said 2nd cooling process, it is characterized by providing the tempering process which gives a tempering hold | maintained to the cooled plated steel plate in the temperature range of 100-600 degreeC for 1 second or more and 48 hours or less.

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

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

  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 finished with finish hot rolling at a finish temperature of 800 ° C. or more and 1100 ° C. or less and wound at a take-up temperature of 350 ° C. or more and 750 ° C. or less. It is not limited to a cold rolled steel plate and a cold rolled steel plate manufactured under specific conditions.

  If the lower limit of the heating temperature range is less than 100 ° C., the heating temperature range widens, and adjustment of the average heating rate becomes difficult, so the lower limit of the heating temperature range is 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.

  If the average heating rate in the temperature range of 100 to 720 ° C. is less than 1 ° C./sec, cementite does not dissolve during heating, and the tensile strength of the finally obtained steel plate decreases, and the ferrite of the decarburized layer The average heating rate is 1 ° C./sec or more because it is difficult to disperse martensite in the steel. Preferably it is 10 degrees C / sec or more.

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

Annealing process (Method A and Method B)
Atmospheric component: 2 to 20% by volume of hydrogen and the balance nitrogen and impurities Atmospheric dew point: -30 ° C to 20 ° C or less Annealing temperature: 720 to 950 ° C
Holding time: 10 to 600 seconds After the heating step, the steel sheet 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 can not be reduced, and the chemical conversion and the plating wettability of the steel sheet deteriorate, so the hydrogen concentration in the atmosphere is 2% by volume or more. Preferably, it is 5% by volume or more. On the other hand, when the hydrogen concentration in the atmosphere exceeds 20% by volume, the dew point can not be maintained at 20 ° C. or lower, so the hydrogen concentration in the atmosphere is made 20% by volume or less. Preferably it is 15 volume% or less.

  When the dew point of the atmosphere is -30 ° C or less, the thickness of the decarburized ferrite layer can not be increased to 5 μm or more, so the dew point of the atmosphere is -30 ° C or more. Preferably it is -20 degreeC or more. 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, so 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 can not be secured, and after cooling, the required amount of martensite is not generated, so the annealing temperature (holding temperature) is made 720 ° C. or higher. In order to obtain a uniform structure which contributes to the improvement of bendability, it is preferable to keep the annealing temperature in the temperature range of the austenite single phase range (Ac ₃ points or more).

When maintaining the annealing temperature in the temperature range of the austenite single phase region, it is preferable to heat from 720 ° C. to the Ac ₃ point over 30 seconds to form a desired decarburized ferrite layer on the steel sheet surface. On the other hand, when the annealing temperature exceeds 950 ° C., it becomes difficult to disperse martensite in the ferrite of the decarburized layer, so the annealing temperature is made 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 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 saturates and not only the productivity decreases, but the decarburized layer grows excessively and the tensile strength decreases, so the holding time is set to 600 seconds or less Do. Preferably, it is 540 seconds or less.

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

  If the upper limit of the cooling temperature range exceeds 400 ° C., the cooling effect is not sufficiently obtained, so the upper limit of the cooling temperature range is set to 400 ° C. If the average cooling rate is less than 2 ° C./sec, cementite will precipitate and martensite will not form in the decarburized layer, so the average cooling rate should be 2 ° C./sec or more. Preferably it is 5 degrees C / sec or more.

  On the other hand, if the average cooling rate exceeds 200 ° C./sec, the concentration gradient of C in austenite becomes insufficient and no M-A structure occurs, so the average cooling rate is set to 200 ° C./sec or less. Preferably it is 100 degrees C / s or less.

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

  Tempering is performed to properly temper martensite of the M-A structure. By this tempering, the martensite is softened, so the elongation of the steel sheet is improved. Further, tempering causes C to concentrate in untransformed retained austenite and hardens the retained austenite, so that the uniform elongation UEl of the steel sheet is improved.

  When the lower limit of the temperature range is less than 100 ° C. or the holding time is less than 1 second, the martensite of the steel plate and the decarburized ferrite layer becomes hard, and the hardness of the steel plate surface layer can not be controlled within an appropriate range. The lower limit of the temperature range is 100 ° C., and the holding time is 1 second or more. The holding time is preferably 20 seconds or more.

  On the other hand, if the upper limit of the temperature range exceeds 600 ° C. or the holding time exceeds 48 hours, the retained austenite of the steel plate and the decarburized ferrite layer decomposes or the martensite becomes too soft, resulting in elongation. The upper limit of the temperature range is 600 ° C., and the holding time is 48 hours or less. The retention time is preferably 40 hours or less.

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

  After the tempering process, the flatness of the steel plate may be corrected by a leveler. In addition, the steel sheet may be coated with an oil or lubricating film.

First cooling step (method B)
Cooling temperature range: 450 to 600 ° C
Average cooling rate: 2 to 200 ° C./s When hot-dip galvanizing is applied to the steel plate in the plating step, the steel plate is subjected to 450 to 600 ° C. at an average cooling rate of 2 to 200 ° C./s after the annealing step. Cool to temperature range.

  If the upper limit of the cooling temperature range exceeds 600 ° C., the cooling effect is not 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 widens and control of the average cooling rate becomes difficult, so the lower limit of the cooling temperature range is 450 ° C. The average cooling rate is 2 to 200 ° C./sec as described above.

Plating process (Method B)
After the first cooling step, the steel sheet is subjected to isothermal holding and cooling, if necessary, and then the steel sheet is subjected to hot dip galvanization. If necessary, hot-dip galvanizing may be subjected to an alloying treatment to alloy the plating. The bath temperature and bath composition of hot dip galvanization may be normal bath temperatures and normal bath compositions, and are not particularly limited. The plating adhesion amount is also not particularly limited, and may be within the normal range. For example, the adhesion amount per one side of the steel plate is in the range of ^{20 to} 120 g / m ² .

  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 process is performed, for example, by heating at 490 to 560 ° C. for 5 to 60 seconds. If alloying treatment is not performed, the Fe concentration in hot-dip galvanizing may be less than 7% by mass. The weldability of the hot-dip galvanized steel sheet is lower than the weldability 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./sec or more After the plating step, the galvanized steel sheet is cooled from the plating temperature (bath temperature) to 200 ° C. or less at an average cooling rate of 5 ° C./sec or more. . This cooling generates stable austenite, and most of the stable austenite remains as austenite even after tempering.

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

  When the upper limit of the cooling temperature range exceeds 200 ° C., it is difficult to form stable austenite, so the upper limit of the cooling temperature range is 200 ° C. The upper limit of the cooling temperature range 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./sec, stable austenite is difficult to form, so the average cooling rate is 5 ° C./sec or more. Do. Preferably it is 10 degrees C / sec or more. The upper limit of the cooling rate is not particularly limited, but is preferably 500 ° C./second or less from the viewpoint of economy.

Tempering blunting process (Method B)
Temperature range: 100 to 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 subjected to tempering to be held in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less. The process conditions in the tempering process (method B) are the same as the process conditions in the tempering process (method A).

  Next, although the Example of this invention is described, the conditions in an Example are one condition example employ | adopted in order to confirm the practicability and effect of this invention, and this invention is the one condition example. 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 scope of the present invention.

Example 1
Forming a 1470MPa class steel sheet with a thickness of 1.4mm that has been subjected to surface layer softening treatment under various conditions shown in Table 1 into a hat type (flange width 20mm, 30mm height, member width 50mm, member length 500mm) And a rectangular (width 90 mm, length 500 mm) soft steel plate (0.2% proof stress 220MPa, tensile strength 310MPa, total elongation 41%) and a distance between welds of 40mm, nugget diameter 3.5mm Spot welding was performed with 狙 い t aiming to produce a hat-shaped molded member.

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

  The hardness Hs of the surface layer of the 1470 MPa class steel and the hardness Hm of the central portion were measured by performing a Vickers hardness test with a pressing load of 500 gf on the cross section of the steel plate. The hardness Hs of the surface layer portion was evaluated by measuring the Vickers hardness at a pressing load of 500 gf at 10 points on the surface layer soft layer and calculating the average value thereof. The hardness Hm of the central portion was evaluated by measuring the Vickers hardness at a pressing load of 500 gf at 10 points from +100 μm and −100 μm from the central portion, and calculating the average value.

  Although the steel a was not softened, in the spot welded portion in the vicinity of the portion where the falling weight collided, breakage of the welded portion occurred in the base material portion of the 1470 MPa grade steel. Although the steels b and c were softened, the welds were broken at almost the same position as the steel a which was not 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 fracture of the spot weld occurred. This is considered to be because, as described above, either or both of the bendability or the fracture resistance under high stress triaxiality is improved by the surface softening.

  On the other hand, for the steels i and j having a hardness ratio (Hs / Hm) of less than 0.4, breakage of the spot weld occurred. Although the details are unknown, if the surface layer portion is softened while the hardness of the central portion is high, the stress distribution becomes uneven between the surface layer portion and the central portion, and as a result, breakage is considered to have occurred.

(Example 2)
A 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 thickness shown in Table 3 at the rolling completion temperature shown in Table 3. After hot rolling, water spray cooling at about 30 ° C./sec was applied, and a hot rolled steel sheet was manufactured at a winding temperature shown in Table 3.

  Winding is simulated by water spray cooling to the winding temperature, then charging into the furnace, holding for 60 minutes at the winding temperature, and then furnace cooling to less than 100 ° C. at a cooling rate of 20 ° C./hour. I The obtained hot rolled steel sheet was descaled by pickling, and cold rolling was performed to a thickness shown in Table 4.

  The obtained steel plate was subjected to heat treatment 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 and annealed, and then cooled to 400 ° C. or less at the indicated average cooling rate. Then, after confirming that the cooling stop temperature was reached, the substrate was heated to the heat treatment temperature (maximum reached temperature) shown in Table 3 at a speed of 20 ° C./sec, and held for a time shown as the heat treatment time.

  By this last heat treatment, all martensite formed before this is subjected to tempering. That the martensite was tempered was confirmed by confirming the presence of carbides in the martensite in SEM observation after nital corrosion.

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

  The decarburized layer thickness is an average value of values calculated by performing image analysis of electron microscopic observation images of two cross sections orthogonal to the rolling direction and the plate 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, and the position where the value was constant was taken as the decarburized layer end, and the distance from the surface to the decarburized layer end was taken as the decarburized layer thickness.

  The structure of the decarburized layer, the decarburized layer ferrite particle size, and the tempered martensite area ratio of the decarburized layer were calculated by performing image analysis of an electron microscopic 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 image analysis of electron microscopic observation images of two cross sections orthogonal to the rolling direction and the plate width direction (direction orthogonal to the rolling direction). .

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

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

  Spot weld part fracture evaluation: It evaluated similarly to Example 1. FIG. Various heat-treated materials are formed into a hat shape (flange width 20 mm, height 30 mm, member width 50 mm, member length 500 mm), and a rectangular 1.4 mm thick rectangular (width 90 mm, length 500 mm) mild steel plate (0.2 A hat-shaped molded member was manufactured by spot welding with a% proof stress of 220 MPa, a tensile strength of 310 MPa, a total elongation of 41%), a distance between welds of 40 mm, and a nugget diameter of 3.5√t.

  In this hat-shaped molded member, both ends 100 mm were constrained from above and below with the heat-treated material positioned upward, and a falling weight with a spherical mass of 100 kg having a tip radius of 50 mm collided at 3 m / sec at the central portion ( See Figure 1). After the collision, the hat-shaped molded member was recovered, and the mode (presence or absence) of breakage of the spot welded portion was examined. The thing which does not have a fracture | rupture in a spot welding part was evaluated as favorable thing, and what was inferior. The evaluation results are shown in Table 4 together.

  The steel plates shown in Table 4 as an example according to the present invention exhibited good characteristics without breaking of the spot welded portion, with the hardness ratio Hs / Hm being between 0.4 and 0.8. No. 5 and 31 have hardness ratios Hs / Hm in a predetermined range and show excellent spot weld resistance fracture characteristics, but their strengths are low from the characteristics of their chemical components and manufacturing conditions.

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

  The obtained hot rolled steel sheet was descaled by pickling, and cold rolling was performed so as to obtain the thickness of the steel sheet shown in Table 6. The hot-dip galvanizing and the alloying treatment were performed on the obtained steel sheet according to the heat treatment conditions in the alloying galvanizing 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 annealing for the indicated time, and then cooled to 500 ° C. at the indicated cooling rate.

  Then, after holding in a temperature range of 500 to 460 ° C. under the indicated conditions, heat treatment simulating hot dip galvanizing is performed at 460 ° C., and thereafter, for some test materials, alloying heat treatment at 510 ° C. A heat treatment was applied to simulate and cooling was started at the indicated cooling rate.

  The test material thus cooled was partially removed, and immediately after cooling, tempering was performed to maintain the temperature under the tempering conditions shown in Table 5 (temperature is the highest achieved temperature). The rate of temperature rise to the tempering temperature was 20 ° C./sec. By this tempering, all martensite formed before tempering is tempered. That the martensite was tempered was confirmed by confirming the presence of carbides in the martensite in SEM observation after nital corrosion.

  The measurement on 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 plates shown in Table 6 had good hardness ratio Hs / Hm between 0.4 and 0.8 and no fracture of spot welds.

  As described above, according to the present invention, it is possible to manufacture and provide a steel plate excellent in the fracture resistance of the spot welded portion at the time of impact deformation. Since the steel plate of the present invention is a steel plate suitable as a steel plate for structural parts of an automobile that absorbs energy at the time of collision deformation, the present invention is highly applicable in the steel plate manufacturing industry and the automobile industry.

1 Hat-shaped molded member 2 Fixing jig 3 Drop weight

Claims (16)

Ri der ratio (Hs / Hm) of 0.4 to 0.8 with Vickers hardness Hm of pressing load 500gf Vickers hardness Hs and the steel plate center of the steel plate surface layer of the pressing load 500gf,
Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: not more than 0.10%, 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 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%, the balance being iron and unavoidable impurities,
A steel plate surface layer with a depth of 5 μm to 200 μm from the steel plate surface contains 1 volume% or more of tempered martensite, and the remaining structure is a decarburizing consisting of ferrite having an average crystal grain size of 20 μm or less and other structures of 5 volume% or less Consists of a ferrite layer,
The steel sheet core has a structure containing 3.0% by volume or more of tempered martensite and 5.0% by volume or more of retained austenite,
A steel plate excellent in spot weld resistance fracture characteristics, characterized in that it has mechanical properties having a tensile strength of 980 MPa or more in a tensile test in the direction perpendicular to rolling . Further, the chemical composition is, by mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more and 0.30% or less steel sheet excellent in resistance spot weld rupture properties according to claim 1, characterized in that it contains more or less of one or. The chemical composition is characterized in that it further contains, by mass%, one or two or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The steel plate excellent in the spot-welded part breaking characteristic according to claim 1 or 2 . The chemical composition is characterized in that it further contains, by mass%, one or more of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00%. The steel plate excellent in the spot-welded part breaking characteristic according to any one of claims 1 to 3 according to any one of claims 1 to 3 . The spot weld resistance according to any one of claims 1 to 4 , wherein the chemical composition further contains, by mass%, B: 0.0001% or more and 0.020% or less. Excellent steel plate. A hot-dip galvanized layer is formed on the surface of the steel plate excellent in spot-welded part fracture characteristics according to any one of claims 1 to 5 , A steel plate excellent in spot-welded part fracture characteristics . It is a manufacturing method which manufactures the steel plate excellent in the spot-welding part breaking characteristic according to any one of claims 1 to 5,
Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: not more than 0.10%, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold for 10 to 600 seconds,
A cooling step of cooling the annealed steel sheet to a temperature range of 400 ° C. or less at an average cooling rate of 2 to 200 ° C./sec after the annealing step;
A steel plate excellent in spot-welded part fracture characteristics characterized by comprising a tempering step of tempering the cooled steel sheet in the temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less after the cooling step. Manufacturing method. Further, the chemical composition is, by mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more and 0.30% or less The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic of Claim 7 characterized by containing following 1 type or 2 types or more. The chemical composition is characterized in that it further contains, by mass%, one or two or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic of Claim 7 or 8 made into Claim 10 or 10 . The chemical composition is characterized in that it further contains, by mass%, one or more of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00%. claims 7-9 any one steel sheet excellent manufacturing method of the resistance spot welds rupture properties according to the to. The spot weld resistance according to any one of claims 7 to 10 , wherein the chemical composition further contains, by mass%, B: 0.0001% or more and 0.020% or less. Steel sheet manufacturing method. It is a manufacturing method which manufactures the steel plate excellent in the spot-welding part breaking characteristic according to claim 6,
Chemical composition is mass%, C: 0.03 to 0.70%, Si: 0.25 to 3.00%, Mn: 1.00 to 5.00%, P: not more than 0.10%, S : Below 0.010%, 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 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: A steel plate containing 0 to 0.10% and Bi: 0 to 0.050%, the balance being iron and impurities, the average heating rate in the temperature range of 100 to 720 ° C being 1 to 50 ° C / sec. Heating process to heat,
After the above heating step, the heated steel plate is made of 2 to 20% by volume of hydrogen, the balance is nitrogen and impurities, and the dew point is a temperature range of 720 to 950 ° C. in an atmosphere of more than −30 ° C. and 20 ° C. or less Annealing process which carries out annealing to hold 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;
A plating step of subjecting the cooled steel plate to hot dip galvanization after the first cooling step;
A second cooling step of cooling the plated steel plate to 200 ° C. or less at an average cooling rate of 5 ° C./sec or more after the plating step;
After the second cooling step, there is provided a tempering step of tempering the plated steel sheet which has been cooled and held in a temperature range of 100 to 600 ° C. for 1 second or more and 48 hours or less. Excellent steel plate manufacturing method. Further, the chemical composition is, by mass%, Ti: 0.001% or more and 0.30% or less, Nb: 0.001% or more and 0.30% or less, and V: 0.001% or more and 0.30% or less The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic of Claim 12 characterized by containing following 1 type or 2 types or more. The chemical composition is characterized in that it further contains, by mass%, one or two or more of Cr: 0.001% to 2.00% and Mo: 0.001% to 2.00%. The manufacturing method of the steel plate excellent in the spot-welded part fracture | rupture characteristic of Claim 12 or 13 made into it. The chemical composition is characterized in that it further contains, by mass%, one or more of Cu: 0.001% to 2.00% and Ni: 0.001% to 2.00%. method for manufacturing a steel sheet excellent in resistance spot weld rupture properties according to any one of claims 12 to 14,. The spot weld resistance according to any one of claims 12 to 15 , wherein the chemical composition further contains, by mass%, B: 0.0001% or more and 0.020% or less. Steel sheet manufacturing method.

Images