JP6753203B2 - Steel plate and manufacturing method of the steel plate - Google Patents

Description

The present invention relates to a steel sheet and a method for manufacturing the steel sheet. In particular, the present invention relates to a steel sheet having a tensile strength of 590 MPa or more and a method for manufacturing the steel sheet.

In recent years, in the automobile field, there is an increasing need to use high-strength thin steel sheets for vehicle bodies and parts for the purpose of improving fuel efficiency and collision safety. Since resistance welding typified by spot welding is mainly used in processes such as vehicle body assembly and component mounting using high-strength steel sheets, steel sheets are required to have excellent weldability. Here, the resistance weldability required for a thin steel sheet for automobiles means that there is no welding defect and a predetermined tensile strength can be obtained in the joint after welding.

As shown in JIS Z 3136 and JIS Z 3137, the tensile strength of the resistance welded joint includes the tensile shear strength (TSS) measured by applying a tensile load in the shear direction and the tensile load in the peeling direction. There is a cross tensile strength (CTS) to be measured. It is known that TSS increases as the strength of the steel sheet increases. On the other hand, CTS increases as the tensile strength of the steel sheet increases up to a certain level, but when the tensile strength of the steel sheet exceeds a certain level, it hardly changes as the tensile strength of the steel sheet increases, or vice versa. It is known that it tends to decrease. It is considered that the reason why CTS does not increase with the tensile strength of the steel sheet is that it breaks in the nugget (fracture in the nugget) under low stress during the tensile test. Here, as shown in FIG. 1, the nugget refers to a portion that has been once melted by resistance welding and solidified.

Non-Patent Document 1 shows that the ease of peeling fracture in a nugget in a cross tensile test is related to the carbon equivalent Ceq obtained from the components of a steel sheet, and that Ceq is suppressed below a specific range in order to prevent fracture. Has been done. Therefore, generally, based on this finding, a method of adjusting the contained elements is used so as not to increase Ceq as much as possible.

Patent Documents 1 and 2 disclose high-yield ratio high-strength alloyed hot-dip galvanized steel sheets that improve spot weldability by reducing C to less than 0.1%.
Patent Document 3 discloses a high-strength cold-rolled steel sheet having excellent formability and weldability that secures spot weldability by setting Ceq to 0.25 or less.
Patent Document 4 discloses a high-strength cold-rolled steel sheet that improves spot weldability by reducing C to less than 0.1%.
Patent Document 5 discloses a high-strength steel plate having good weldability and stretch flangeability in which the ductility ratio of TSS and CTS is 0.5 or more by setting C to less than 0.1%.
Patent Document 6 discloses a high-strength cold-rolled steel sheet having both elongation, stretch flangeability and weldability by setting C to 0.25% or less.
Patent Document 7 discloses a technique for improving joint strength by suppressing fracture in the nugget by keeping the carbide and dendrite spacing in the nugget within an appropriate range.
Patent Document 8 describes a method of providing a spot welded joint having excellent weld strength by optimizing the carbon content and other components of the base material and keeping the average oxygen concentration of the surface layer of the raw steel sheet within an appropriate range. Proposed.
Patent Document 9 discloses a technique for obtaining a high-strength resistance welded joint by suppressing brittle fracture in the nugget by optimizing the metallographic structure, inclusion distribution and hardness in the nugget. However, the characteristics of steel materials suitable for producing high-strength resistance welded joints have not been clarified.

Japanese Unexamined Patent Publication No. 2005-105361 Japanese Unexamined Patent Publication No. 2005-105367 Japanese Unexamined Patent Publication No. 2006-219738 Japanese Unexamined Patent Publication No. 2006-283156 Japanese Unexamined Patent Publication No. 2009-263686 Japanese Unexamined Patent Publication No. 2010-285636 Japanese Patent No. 5043236 Japanese Unexamined Patent Publication No. 2012-102370 Japanese Unexamined Patent Publication No. 2014-180698

"Spot weldability of high-strength steel sheets for automobiles", Nippon Steel Technical Report No. 385, 2006, pp. 36-41

However, conventional techniques have left room for further study of fractures within the nugget in cross joints. In particular, when a steel plate having a high Ceq containing a high content of C, Mn, etc. is resistance welded, there is a problem that the cross joint may break in the nugget and the strength of the joint may decrease. If the joint strength is lowered even with a low probability, the reliability of the welded joint and the reliability of the member including the welded joint are impaired.
The present invention has been made in view of the above, and an object of the present invention is to provide a steel plate capable of obtaining a cross joint capable of stably obtaining high CTS / TSS for enhancing spot weldability.

That is, the gist of the present invention is as follows.
(1) The composition of the steel sheet is mass%.
C: 0.04 to 0.30%,
Si: 3.0% or less,
Al: 0.1% or less,
Mn: 0.8 to 7.0%,
Ni: 0.01-1.5%,
P: 0.03% or less,
S: 0.005% or less,
Mg: 0.0001 to 0.0015%,
O: Contains 0.004% or less,
A total of one or more of Ti, Zr, Hf, V, Nb, Ta, Sc, and Y: 0.01 to 0.2%.
Total of one or more types of Cr, Mo, W, Cu: 0 to 1.0%,
Total of 1 or 2 types of Ca and REM: 0 to 0.005%,
B: 0 to 0.005%,
Containing
The balance consists of Fe and impurities,
Further, the steel sheet is characterized in that the average distribution density of oxide-based inclusions containing Mg having a particle diameter of 3 μm or more is 10 pieces / mm ² or less.
(2) The steel sheet according to (1), wherein the Ceq calculated by the following formula is 0.22 or more.
Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (mass%)
(3) The steel sheet according to (1) or (2), wherein the average crystal grain size is 10 μm or less.
(4) The composition is characterized by containing one or more of Cr, Mo, W, and Cu in a total amount of 0.003% or more and 1.0% or less in mass% (1) to (1). The steel plate according to any one of 3).
(5) Any of (1) to (4), wherein the composition contains 1 or 2 types of Ca and REM in a total amount of 0.0002% or more and 0.005% or less in mass%. The steel plate described in 1.
(6) The steel sheet according to any one of (1) to (5), wherein the composition contains 0.001% or more and 0.005% or less of B in mass%.
(7) The method for manufacturing a steel sheet according to any one of (1) to (6), wherein a refractory member containing no Mg is used as a refractory member for holding the molten metal, and the temperature is 1570 ° C to 1630 ° C. A method for producing a steel sheet, characterized in that an alloy containing Mg is charged, and the time from the start of charging Mg into molten steel to the start of casting is 10 s to 300 s.
(8) (1) to be any method of manufacturing a steel sheet according to one of (6), using a refractory material containing Mg in tundish, the temperature of molten steel of 1570 ℃ ~1630 ℃ tundish Note A method for producing a steel sheet, which comprises holding for a time of 10 s to 300 s from the start of hot water pouring to the start of hot water pouring into a mold.

According to the present invention, a steel sheet having excellent spot weldability, that is, CTS / TSS can be obtained.

FIG. 1 is a cross-sectional view of a two-disc spot welded joint, with a central portion showing a nugget. FIG. 2 is an electron micrograph observing the origin of brittle fracture in the nugget. FIG. 3 is a preferable energization history when spot welding is performed.

In order to improve CTS / TSS, it is necessary to improve CTS itself. Therefore, the present inventor first observes the fracture surface of the nugget of the resistance welded joint having a low CTS in detail using a steel plate having a tensile strength of 590 MPa or more, and as a result, when brittle fracture occurs in the nugget. It was found that CTS decreased. Next, the cause of brittle fracture in the nugget was investigated. As a result, it was found that inclusions were present in the cross section as shown by the arrows in FIG. 2, and that these inclusions were the starting points of brittle fracture. Then, it was clarified that this inclusion is an oxide-based inclusion containing Si, Mn, Al, Mg and the like. In addition, the larger the inclusions and the larger the amount, the more likely it was that brittle fracture would occur. Furthermore, it was found that the CTS may be low even when the fracture surface of the nugget after the cross tensile test is a ductile fracture surface. In this case, a large number of oxide-based inclusions were observed in the ductile fracture surface. Therefore, it was predicted that the fracture of the nugget was caused by inclusions in both brittle fracture and ductile fracture.

Therefore, the inventor investigated the origin of oxide-based inclusions dispersed in the nugget. As a result, it was found that the oxides in the nugget have two different origins. That is, the first is the case where the portion melted by resistance welding comes into contact with the outside air through the gap between the plates to form an oxide, and the oxide is taken into the nugget, and the second is the joint. This was a case where the inclusions originally contained in the constituent steel sheet remained without being decomposed even in a molten state and were taken into the nugget as they were.

Next, the inventor performed resistance welding of steel sheets having various compositions, and investigated the size and amount of oxides formed when the molten portion came into contact with the outside air. As a result, although the reason is not clear, in the case of a steel sheet containing one or more of Ti, Zr, Hf, V, Nb, Ta, Sc, and Y, in the portion where the molten portion comes into contact with the outside air. It was found that oxides are less likely to be formed.

Furthermore, the inventor performs resistance welding of steel sheets containing oxide-based inclusions of various types and sizes, and the types and sizes of oxide-based inclusions in the material steel sheet and the types and sizes of the oxide-based inclusions remaining in the nugget after welding. The relationship between the type and size of oxide-based inclusions was investigated. As a result, the oxide-based inclusions containing no Mg tend to dissolve in solid solution during welding, while the oxide-based inclusions containing Mg contained in the steel sheet have an oxide size. It was found that even if it is small, it does not dissolve in solid solution during welding and tends to remain as it is in the nugget after welding. That is, it has been found that in order to reduce the distribution of coarse oxides in the nugget after welding, it is necessary to optimize the distribution of the oxide-based inclusions containing Mg in the material steel sheet.

Based on the above findings, the inventor has produced welded joints of steel sheets containing various steel sheet components and oxide-based inclusions containing Mg of various sizes, and having various metal structures and particle sizes. A high-strength thin steel sheet capable of obtaining high joint strength, that is, a high-strength thin steel sheet having excellent resistance weldability was completed by conducting a tensile test on a cross joint.
The details of each specific matter will be specifically described. All% for elements are mass%.
<Sheet steel composition >

"C": 0.04 to 0.30%
C is used for controlling the structure of the material steel sheet and strengthening the steel sheet. The C content is 0.04% or more in order to keep the tensile strength of the steel sheet above a certain level. On the other hand, in order to secure good tensile strength of the cross joint by controlling the distribution of other components and inclusions in the steel sheet, the C content is set to 0.30% or less. A more preferable range of the C content is 0.04% or more and 0.26% or less.

"Si": 3.0% or less Si is not an essential element and is used for deoxidizing molten steel, for example, and may be contained as an impurity. In order to secure CTS and fatigue strength, the Si content should be 3.0% or less. The Si-based oxide present in the steel sheet dissolves during resistance welding, and the solid-dissolved Si undergoes an oxidation reaction with the outside air in the nugget during resistance welding to form an oxide containing Si. This is because even if other elements are adjusted, a large amount of oxide formation cannot be suppressed in the outer peripheral portion of the nugget, and as a result, CTS and fatigue strength tend to decrease. The lower limit is not particularly limited, but if the Si content is reduced more than necessary, the manufacturing cost increases. Therefore, the preferable range of the Si content is 0.003% or more. Further, in order to control the metal structure and the crystal grain size and improve the strength, the more preferable range of the Si content is 0.5% or more, and the more preferable range of the Si content is 1.0% or more. When these are arranged, the preferable range is 0.03% or more and 3.0% or less, 0.5% or more and 3.0% or less, or 1.0% or more and 3.0% or less.

"Al": 0.1% or less Al is not an essential element and is used for deoxidizing molten steel, for example, and may be contained as an impurity. To ensure CTS, the Al content should be 0.1% or less. This is because the CTS may decrease due to an increase in the oxide containing Al. The lower limit is not particularly limited, but if the Al content is reduced more than necessary, the production cost increases. Therefore, the preferable range of the Al content is 0.003% or more and 0.1% or less.

"Mn": 0.8 to 7.0%
Mn is mainly used for controlling the metal structure and crystal grain size of the raw steel sheet. In order to increase the tensile strength of the steel sheet to 590 MPa or more, the Mn content is set to 0.8% or more. Further, in order to further improve the strength, the preferable range of the Mn content is 1.0% or more, and the more preferable range is 3.0% or more. On the other hand, in order to secure CTS and fatigue strength, the Mn content is set to 7.0% or less. This is because Mn reacts with the outside air in the nugget during resistance welding, and a large amount of oxide is formed on the outer peripheral portion of the nugget, and as a result, CTS and fatigue strength may decrease. A more preferable range of the Mn content is 5.0% or less. The preferred ranges for organizing these are 1.0% or more and 8.0% or less, 3.0% or more and 8.0% or less, 0.8% or more and 5.0% or less, 1.0% or more and 5.0%. Below, or 3.0% or more and 5.0% or less.

"Ni": 0.01-1.5%
Ni is an important element in the present invention and has an effect of suppressing brittle fracture of the nugget. Due to this effect, improvement of CTS / TSS can be expected even when fine inclusions of several μm or less are present in the nugget. In order to exert the effect, the Ni content is 0.01% or more. In order to raise the CTS / TSS to a higher level, the preferable range of the Ni content is 0.02% or more, and the more preferable range is 0.2% or more. On the other hand, in order to suppress the decrease in CTS, the Ni content is set to 1.5% or less, and the preferable range of the Ni content is 1.0% or less. The preferred ranges for organizing these are 0.02% or more and 1.5% or less, 0.2% or more and 1.5% or less, 0.01% or more and 1.0% or less, 0.02% or more and 1.0%. Or less, or 0.2% or more and 1.0% or less.

"P": 0.03% or less P is not an essential element and may be contained as an impurity in steel. In order to secure CTS / TSS, the P content should be 0.03% or less. This is because the tendency of brittle fracture of the nugget is strengthened and the fracture in the nugget may not be suppressed. A more preferable range of P content is 0.02% or less. The lower limit is not particularly limited, but the production cost increases when the content is reduced, so the preferable range of the P content is 0.003% or more. When these are arranged, the preferable range is 0.02% or less, 0.003% or more and 0.03% or less, or 0.003% or more and 0.02% or less.

"S": 0.005% or less S is not an essential element and may be contained as an impurity in steel. To ensure CTS / TSS, the S content should be 0.005% or less. This is because the presence of S in the nugget in a solid solution state or as a sulfide may enhance the brittle fracture tendency of the nugget and may not suppress the fracture in the nugget. The preferred range of S content is 0.003% or less. The lower limit is not particularly limited, but if the content is reduced more than necessary, the production cost increases, so the preferable range of the S content is 0.0001% or more. When these are arranged, the preferable range is 0.003% or less, 0.0001% or more and 0.005% or less, or 0.0001% or more and 0.003% or less.

"Mg": 0.0001 to 0.0015%
Mg contributes to the miniaturization of oxide-based inclusions in the steel sheet and is an important element in the present invention. In order to make the oxide-based inclusions finer, the Mg content is set to 0.0001% or more. On the other hand, in order to secure CTS and fatigue strength, the range of Mg content is set to 0.0015% or less. This is because a large amount of coarse oxide-based inclusions containing Mg may be formed in the steel sheet, and as a result, a large amount of oxide may remain in the nugget after welding and the CTS may decrease. The preferred range of Mg content is 0.0010% or less.

"O": 0.004% or less O is not an essential element and may be contained as an impurity. To ensure CTS, the O content should be 0.004% or less. This is because O is mainly present in the material steel sheet as oxide-based inclusions, and coarse oxide-based inclusions in the material steel sheet may remain in the nugget after welding and cause a decrease in CTS. Further, if the O content exceeds 0.004%, the fatigue strength may also decrease. The preferred range of O content is 0.003% or less. The smaller the O content is, the more preferable it is. However, if the O content is reduced more than necessary, the production cost increases. Therefore, the preferable range of the O content is 0.0003% or more. When these are arranged, the preferable range is 0.003% or less, 0.0003% or more and 0.004% or less, or 0.0003% or more and 0.003% or less.

"One or more of Ti, Zr, Hf, V, Nb, Ta, Sc, Y": 0.01-0.2% in total
Ti, Zr, Hf, V, Nb, Ta, Sc, and Y are important elements in the present invention and are used for controlling the particle size of the raw steel sheet and controlling the oxide-based inclusions in the nugget after welding. is there. In order to secure CTS / TSS, the content of these elements should be 0.01% or more. When these elements are contained, the crystal grain size in the nugget after welding is also refined, or the generation of oxide-based inclusions during welding is suppressed, so that brittle fracture in the nugget is suppressed, and CTS is improved, which in turn is improved. This is because it has the effect of improving CTS / TSS. On the other hand, if it exceeds 0.2%, the density of oxide-based inclusions in the nugget after welding may increase and CTS / TSS may decrease, so the content of these elements should be 0.2% or less. To.

"One or more types of Cr, Mo, W, Cu": 0 to 1.0% or less in total One or two or more types of Cr, Mo, W, Cu are for adjusting the strength of the material steel sheet, etc. , May be included. However, in order to secure CTS, the total content of these elements should be 1.0% or less. This is because the amount of inclusions in the nugget may increase and the CTS may decrease. The content of Cr, Mo, W or Cu or any combination thereof is, for example, 0.003% or more. Further, Cr, Mo and W are elements forming carbonitrides and have an effect on the grain refinement of HAZ and in the nugget after welding, and therefore, Cr, Mo or W or any combination thereof is contained. The preferred range of the total amount is 0.1% or more.

"One or more of Ca and REM": 0 to 0.005% in total For deoxidation of material steel sheet and control of inclusions in nuggets, Ca or REM (Rare Earth Metal) Alternatively, a combination thereof may be contained. However, in order to secure CTS, the total content of these elements should be 0.005% or less. This is because the amount of coarse inclusions in the nugget may increase and the CTS may decrease. The lower limit is not particularly limited, but the inclusion of Ca or REM or a combination thereof in the material steel sheet refines the inclusions formed in the nugget and further increases CTS, so that the total content of these elements is increased. The preferable range of is 0.0002% or more.

REM refers to a total of 15 elements of lanthanoids, and the content of REM means the total content of these 15 elements. Lanthanoids are industrially added, for example as mischmetal.

"B": 0 to 0.005% or less B may be contained in order to control the structure of the material steel sheet. In order to secure CTS, the content of B is set to 0.005% or less. This is because coarse inclusions containing B may be formed and the CTS may decrease. The preferred range of B content is 0.002% or less. The B content is, for example, 0.001% or more. When these are arranged, the preferable range is 0.002% or less, 0.001% or more and 0.005% or less, or 0.001% or more and 0.002% or less.

In the steel component of the present embodiment, the balance other than the above-mentioned elements is Fe and impurities. The types of other elements contained as impurities are not particularly limited . Various elements may be appropriately contained in an amount that is mixed in a normal manufacturing process as long as the effects of the present invention are not impaired. Examples of the impurity element include N, Sb, Sn, Co, As, Pb and Bi. N, Sb, Sn, Co, As, Pb, and Bi are each allowed to be mixed in an amount of 0.003% or less, preferably less than 0.001% in each .

Further, the surface of the material steel sheet to be used may be plated. The type of plating and the thickness of plating are not particularly limited, and the effects shown in the present invention can be obtained.

"Average distribution density of oxide-based inclusions having a particle size of 3 μm or more containing Mg": 10 pieces / mm ² or less The oxide-based inclusions containing Mg contained in the material steel sheet remain after resistance welding. Affects CTS. Even if inclusions containing Mg less than 3 μm remain in the nugget after resistance welding, the probability of becoming a starting point of fracture is low. If the number of oxide-based inclusions of 3 μm or more exceeds 10 pieces / mm ² , the CTS may decrease, so the amount was limited to 10 pieces / mm ² or less. A more desirable upper limit is 5 pieces / mm ² or less. The lower limit is not particularly limited, and the lower the lower limit, the more desirable. The average distribution density of oxide-based inclusions containing Mg is 1/8 from the front or back surface of this cross section by mirror-polishing the surface (TD surface) that includes the axis perpendicular to the surface of the steel sheet and the axis in the rolling direction. In the range of ~ 3/8, a total area of 2 mm ² or more was observed using an FE-SEM equipped with an energy dispersive X-ray detector (EDS), and the number in the observation field was divided by the observation area. Let it be a value. The accelerating voltage shall be measured at 10 to 20 kV, and the observation magnification may be any magnification capable of measuring inclusions of 1 μm or more, and is preferably 500 times or more. Here, it is assumed that the particle size of the oxide adopts a diameter equivalent to a circle. The oxide-based inclusions containing Mg are inclusions containing 2% or more of Mg and 20% or more of O in mass% of each particle, and the acid sulfide containing S is also included. Including. Further, the oxide-based inclusions containing Mg may be an oxide containing Ti, Al, Ca or REM other than Mg in the oxide and an acid sulfide in addition to the Mg oxide. In addition, for inclusions that are present in combination with Mg-containing oxide-based inclusions and other oxides, nitrides, or sulfides, the size of Mg-containing oxide-based inclusions shall be measured. ..

"CTS (kN) / TSS (kN) (ratio of cross tensile strength (CTS) to shear tensile strength (TSS)": 4 × ^10-7 × [TS] when the tensile strength of the steel plate is TS (MPa) ] ² −1.3 × 10 ^-3 × TS] +1.23 or more CTS / TSS is also called ductility ratio, and is one index for evaluating the tendency of peeling fracture in the nugget when the cross joint is pulled. If the CTS / TSS is less than 4 × ^10-7 × [TS] ² −1.3 × 10 ^-3 × [TS] +1.23, the tendency of peeling fracture is strong and it becomes difficult to use it as a practical joint. 4, 4 × 10 ^-7 × [TS] ² −1.3 × 10 ^-3 × [TS] +1.23 or more is desirable. The evaluation of CTS and TSS was performed on a test material that was spot-welded using steel plates of the same steel type and the same plate thickness under the condition that the cross-sectional nugget diameter was 4.8√t to 5.2√t. Evaluation shall be made in accordance with JIS Z 3136 and JIS Z 3137. Regarding the electrode diameter, shape, pressing force, and current conditions when performing spot welding, a nugget with a cross-sectional diameter of 4.8√t to 5.2√t can be obtained, and welding defects that affect the joint strength are present. It may be appropriately selected so that a small number of nuggets can be obtained, but it is desirable to perform the current conditions based on the energization history shown in FIG.
That is, a molten nugget is formed by a welding current Wc, then de-energized for tC: 15 to 60 ms, and then at a current amount of 0.7 to 0.9 times that of Wc for tP: 50 to 300 ms. After that, energization is performed, and finally, TH: 15 to 40 ms is maintained while the pressing force is maintained.

"Carbon equivalent Ceq": 0.22% or more If the Ceq of the steel sheet is less than 0.22%, the CTS according to the technique of the present invention may break not in the nugget but in the base metal in the cross joint tensile test. The increase effect decreases. Therefore, in the present invention, Ceq is preferably 0.22% or more. Here, as the carbon equivalent Ceq, Ceq = C + Si / 30 + Mn / 20 + 2P + 4S described in Non-Patent Document 1 is used. A more preferable lower limit is 0.30% or more, and a more preferable lower limit is 0.35% or more.

"Average grain size of steel plate": 10 μm or less The crystal grain size of the steel plate may affect the grain size of the nugget after welding, which changes the fracture behavior in the nugget during the tensile test. It affects CTS. When the crystal grain size of the steel sheet exceeds 10 μm, the effect of increasing CTS due to the change of oxide-based inclusions in the nugget becomes small, so the preferable range is set to 10 μm or less. 7 μm or less is a more preferable range.
The crystal grain size of the steel sheet is measured as follows. The surface (TD surface) including the axis perpendicular to the surface of the steel sheet and the axis in the rolling direction in the surface is mirror-polished, and further polished by electrolytic polishing or colloidal silica polishing. A region within a thickness range of 1/8 to 3/8 from the front surface or the back surface of this cross section is subjected to crystal orientation analysis by the SEM / EBSD method. The step size of the analysis may be sufficiently smaller than the crystal grain size, but it is preferably 0.8 μm or less. In addition, the measurement measures an area of 1 mm ² or more. By the crystal orientation analysis of EBSD, the boundary where there is a crystal orientation difference of 15 ° or more between adjacent measurement points is defined as the grain boundary, the region surrounded by the grain boundary is regarded as one crystal grain, and the equivalent circle diameter of the crystal grain is defined. Is the particle size. Then, the average value of the grain sizes of the crystal grains in the measurement region is taken as the average grain size. Here, the crystal grain is a region surrounded by a grain boundary having a crystal orientation difference of 15 ° or more with an adjacent region.
The spot weldability changes depending on the characteristics of the metal of the welded portion, and the characteristics of the spot weldability change depending on the metal of the welded portion generated through the welding process. Since the metal characteristics of the welded portion are the controlling factor for spot weldability, there is virtually no influence of the structure of the steel plate, and the metal structure of the steel plate is not particularly limited as long as other required characteristics are satisfied. , Ferrite, bainite, martensite, austenite, pearlite, or a mixed structure of two or more kinds may be used. For example, since the tensile strength is 590 MPa or more, a structure of 100% ferrite having precipitation strengthening as the main strengthening mechanism may be used, and a structure of bainite and martensite having 100% of bainite and martensite as the main strengthening mechanism may be used. A DP structure in which ferrite and martensite are mixed may be used.

"Tensile strength of steel sheet": 590 MPa or more is more preferable If the tensile strength TS of the steel sheet is less than 590 MPa, it may break not in the nugget but in the base metal in the cross joint tensile test. The increasing effect is less than that of a steel sheet of 590 MPa or more. Therefore, in the present invention, it is more preferable to apply it to a steel sheet having a tensile strength TS of 590 MPa or more.
<Manufacturing method of steel sheet>

The steel sheet according to this embodiment has the above-mentioned chemical composition and structure regardless of the manufacturing method, and the effect can be obtained. However, according to the manufacturing method shown below, it is preferable because the steel sheet according to the present embodiment can be stably obtained.
Specifically, the method for producing a steel sheet according to the present embodiment preferably includes the following steps.
<Process of adding Mg to molten steel and discharging hot water>

Since oxide-based inclusions containing Mg are formed in molten steel, their size and distribution are controlled in the process until solidification starts. When the alloy containing Mg is charged, the alloy containing Mg is charged when the temperature of the molten steel is 1570 ° C. to 1630 ° C., and the time from the start of charging Mg into the molten steel to the start of casting is set to 10 s to 300 s. Is desirable. This is because when the temperature of the molten steel is less than 1570 ° C., the number of oxides containing Mg of 3 μm or more in the slab increases, and when the temperature exceeds 1630 ° C., the coarsening of the oxide containing Mg progresses rapidly. Because. Further, if the time until the start of casting is less than 10 s, the number of oxides containing Mg of 3 μm or more in the slab increases, and if it exceeds 300 s, the oxides containing Mg aggregate and coalesce and become coarse. Therefore, the weldability tends to deteriorate. In the case of a continuous casting facility, it is desirable to add an alloy containing Mg in the tundish when the molten steel is poured into the tundish. At this time, since the pouring into the tundish and the pouring into the mold are continuously performed in the tundish, the holding time can be managed by the time from the start of adding Mg to the molten steel to the start of pouring into the mold. It's simple.

It is also possible to mix a small amount of Mg using a refractory material. In this case, use a refractory material containing Mg in the tundish, pour molten steel at a temperature of 1570 to 1630 ° C into the tundish, and hold it for 10 seconds or more from the start of pouring to the start of pouring into the mold. Is desirable. This is because if the pouring temperature is less than 1570 ° C. and the holding time in the tundish is less than 10 s, the mixing of Mg into the molten steel does not proceed sufficiently and contains Mg of 3 μm or more in the slab. This is because the number of oxides increases, and when it exceeds 300 s, the oxides containing Mg that have been aggregated and coarsened tend to increase. The agglomerated coarsened Mg-containing oxide tends to improve or deteriorate in weldability by floating and being removed in the tundish or being incorporated into the slab.
<Hot rolling process>

The manufacturing method of the hot rolling process such as the method of controlling the crystal grain size of the steel sheet is not particularly limited, and the slab heating conditions, the hot rolling conditions, and the cooling conditions may be appropriately selected according to a conventional method.
In order to reduce the grain size, it is preferable that the time from the end of rough rolling to the start of finish rolling (multiple rolling mills are arranged in series and continuously roll) is 60 seconds or less, and the finish is finished. the temperature to complete the rolling is preferably set to _a r3 _~930 ℃. The time from the end of finish rolling to cooling is preferably 3 seconds or less, and it is preferable to cool at a minimum cooling rate of 8 ° C./sec or more until it becomes 700 ° C. or less. However, if the total cooling rate is less than 8 ° C./sec within 3 seconds in the cooling, the cooling rate is temporarily less than 8 ° C./sec because it does not affect the fineness of the crystal grains. Does not prevent that. When the final product is a hot-rolled plate, it is desirable that the winding temperature is 650 ° C or lower in order to refine the metal structure. On the other hand, when there is a cold rolling / annealing step after the hot rolling process, the winding temperature is 300 ° C. or higher because the hot rolling plate may become hard and the cold ductility deteriorates if the winding temperature is low. Is preferable.
<Cold rolling process and plating process>

When the final product is a cold-rolled steel sheet or a plated steel sheet, hot-rolling may be performed under the above conditions, followed by pickling and cold-rolling. When heat treatment is performed on a continuous annealing line, for example, annealing is performed in the range of 750 to 900 ° C. with a residence time of 200 s or less, and then in the temperature range of up to 500 ° C., the cooling rate is at least 3 ° C./s or more. It may be cooled with. For fine graining of crystal grains, the cold rolling ratio is preferably 40% or more, the annealing temperature is preferably 850 ° C. or lower, and the cooling rate is low in the temperature range up to 500 ° C. after annealing. It is preferably 5 ° C./s or higher.

The above-mentioned plating may be performed on a continuous annealing / plating line, or may be performed on a dedicated plating facility separate from the annealing line. The composition of the plating is not particularly limited, and any of hot-dip galvanizing, alloying hot-dip galvanizing, and electroplating may be used.

It should be noted that all of the above embodiments merely show examples of embodiment in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner by these. That is, the present invention can be implemented in various aspects without departing from the technical idea or its main features.

According to the steel sheet according to the present invention as described above, a high-strength resistance welded joint can be stably obtained by the above configuration. Specifically, a welded joint in which the ratio of the cross tensile strength (CTS) to the shear tensile strength (TSS) satisfies the following equation is stably obtained when the tensile strength of the steel sheet is TS (MPa).

^{CTS / TSS> 4 × 10 -7} × [TS] 2 -1.3 × 10 -3 × [TS] +1.23

As a result, a welded joint using a high-strength steel plate having a high carbon equivalent can be realized, which contributes to weight reduction of automobile members. Furthermore, by reducing the variation in joint strength, it is expected to contribute to improving the safety of automobile bodies, and to improve the manufacturing cost and productivity of parts by simplifying post-weld heat treatment and reducing the number of resistance welding points. it can.

Hereinafter, the present invention will be described in more detail with reference to examples of the steel sheet according to the present invention, but the present invention is not limited to the following examples, and is within the scope of the above and the following. It is also possible to carry out with appropriate modifications, all of which are within the technical scope of the present invention.
<Example 1>

In producing steel pieces by melting steel having the composition shown in Table 1 below, an alloy containing Mg was put into the molten steel at 1600 ° C., and the molten steel was cast after 60 seconds. After cooling this steel piece to room temperature, it was heated to 1200 to 1250 ° C., rough-rolled, and subsequently finished-rolled. Finish rolling termination temperature of finish rolling is performed at A _r3 ~930 ℃, was cooled temperature range after completion of the finish rolling to 650 ° C. within 3 seconds so that the lowest cooling rate 10 ° C. / s or higher After that, a winding treatment was performed at 500 to 650 ° C.
For the cold-rolled steel sheet, the hot-rolled steel sheet is pickled, cold-rolled at a rolling rate of 40 to 70%, and subsequently annealed with a residence time of 100 s between 750 and 900 ° C. up to 500 ° C. The temperature range was cooled at a minimum cooling rate of 3 ° C./s or more, and further heat treatment was performed with a residence time of 20 to 400 s between 300 and 500 ° C.

CR: Cold-rolled steel sheet, HR: Hot-rolled steel sheet Total of Ti-Y: Total of Ti, Zr, Hf, V, Nb, Ta, Sc, Y

Both the hot-rolled steel sheet and the cold-rolled steel sheet were finally temper-rolled under the condition of an elongation rate of 0.3%.
The obtained steel sheet was evaluated as follows. First, a tensile test piece conforming to JIS Z2201 was sampled with the direction perpendicular to rolling as the longitudinal direction of the test piece, and a tensile test was performed according to JIS Z2241 to evaluate the mechanical properties.

As for the crystal grain size of the steel plate, observe the 1/4 thickness part of the cross-sectional sample, and evaluate the region surrounded by the grain boundaries with an inclination angle of 15 ° or more as one crystal grain using the EBSD method, and evaluate 50 pieces each. It was measured as the average nominal particle size of the above crystal grains.

The distribution density of oxide-based inclusions having a particle size of 3 μm or more containing Mg in the steel sheet was evaluated as follows. First, the cross-sectional sample is mirror-polished, and the size and composition of the 1/4-thick part are investigated by observing the image and elemental analysis of an area of 2 mm ² or more using an SEM equipped with a characteristic X-ray detector. did. Then, the average density was calculated by counting oxides containing Mg and having a diameter equivalent to a circle and having a size of 3 μm or more and dividing by the observed area.

Two types of welded joints, one for shear tension and the other for cross tension, were manufactured in accordance with JIS Z3136 and JIS Z3137. Servo gun type spot welding is used for cold-rolled steel sheets with a plate thickness of t = 1.2 mm and for hot-rolled steel sheets with a plate thickness of t = 2.0 mm, respectively, by superimposing combinations of the same steel type and plate thickness. Welding was performed by machine. In spot welding, the main welding is performed at a pressing force of 5000 N for a main energization time of 240 ms (in the case of a cold-rolled steel sheet) or 400 ms (in the case of a hot-rolled steel sheet), followed by a non-energization for 40 ms, and then a main current of 80 Energization was performed for 200 ms at a current value of%, and finally a hold of 20 ms was performed. The welding current value of the main energization was adjusted so that the cross-sectional nugget diameter was 5√t. That is, in the case of a cold-rolled steel sheet with t = 1.2 mm, the cross-sectional nugget diameter is 5.5 mm ± 0.2 mm, and in the case of a hot-rolled steel sheet with t = 2.0 mm, the cross-sectional nugget diameter is 7. The welding current value of the main energization was set so as to be 1 mm ± 0.2 mm. Finally, based on the JIS, the shear joint tensile strength (TSS) and the cross joint tensile strength (CTS) were measured using a tensile tester.
Table 2 shows the TS of the material, the grain size, the density of oxide-based inclusions of 3 μm or more, and the evaluation results of TSS and CTS.

TS: Tensile strength of steel sheet (MPa)
d: Average grain size (μm)
ρ: Average distribution density of oxide-based inclusions containing Mg and having a particle size of 3 μm or more (pieces / mm ² )
TSS: Shear joint tensile strength (kN)
CTS: Cross joint tensile strength (kN)
TSS ^{formula: 4 × 10 -7 × [TS} ] 2 -1.3 × 10 -3 × [TS] +1.23

Comparative examples A, P, and R have low CTS / TSS because the total content of one or more of Ti, Zr, Hf, V, Nb, Ta, Sc, and Y is less than the specified amount. , Spot weldability was poor.
AE and AF, which are comparative examples, contain Mg of 3 μm or more because the total content of one or more of Ti, Zr, Hf, V, Nb, Ta, Sc, and Y exceeds the specified amount. The number of oxide-based inclusions was large, the CTS / TSS was low, and the spot weldability was poor.
In Comparative Example S, the content of C was lower than the specified amount, and in Comparative Example W, the content of Mn was lower than the specified amount, so that the TS was low and the strength was inferior.
Since the content of C in Comparative Example T and the content of Mn in Comparative Example X exceeded the specified amounts, the CTS / TSS was low and the spot weldability was poor.
In U, which is a comparative example, since the Si content exceeded the upper limit, the number of oxide-based inclusions containing Mg of 3 μm or more was large, the CTS / TSS was low, and the spot weldability was poor. In Comparative Example V, since the Al content exceeded the upper limit, the number of oxide-based inclusions containing Mg of 3 μm or more was large, the CTS / TSS was low, and the spot weldability was poor.
In Y and Z, which are comparative examples, the CTS / TSS was low and the spot weldability was poor because the Ni content was out of the specified range.
In AA, AB, and AD, which are comparative examples, CTS / TSS was low and spot weldability was poor because P, S, and O exceeded the specified amounts, respectively. In AC, which is a comparative example, since Mg exceeded the specified amount, the number of oxide-based inclusions containing Mg of 3 μm or more was large, CTS / TSS was low, and spot weldability was poor.
On the other hand, B to O and Q, which are examples of the present invention, have an alloy composition within the specified range of the present invention, and the distribution of oxide-based inclusions containing Mg of 3 μm or more is also appropriate, so that good spot weldability is obtained. was gotten.
<Example 2>

Using the B steel shown in Table 1 above, steel pieces were produced under various Mg addition methods and conditions. The addition of Mg to the molten steel was carried out by a method of charging an alloy containing Mg into the molten steel in a tundish, and the temperature of the molten steel at the time of charging the alloy and the time from the addition to the start of casting were changed. Further, two tests were conducted on the tundish wall in contact with the molten steel, in the case of using a refractory containing 3% or more of Mg and in the case of using a refractory containing no Mg. Table 3 shows the conditions.

After heating this steel piece to 1200 to 1250 ° C., hot rolling was performed. Finish rolling is performed at a finish temperature _A r3 _~930 ℃ the finish rolling, after the final finish rolling within 3 seconds performs a cooling temperature range up to 650 ° C. at a minimum cooling rate of 10 ° C. / s or higher, 600 to 650 ° C. The winding process was performed in. Next, the hot-rolled steel sheet was pickled, cold-rolled at a rolling rate of 50%, and subsequently annealed with a residence time of 100 s between 750 and 900 ° C., and the temperature range up to 500 ° C. was set to 3 ° C./. Cooling was performed at a cooling rate of s or more, and further heat treatment was performed with a residence time of 20 to 400 s between 300 and 500 ° C. Finally, temper rolling was performed under the condition of an elongation rate of 0.3%.

For the obtained steel sheet, the TS of the material, the density of oxide-based inclusions having a crystal grain size of 3 μm or more, and the TSS and CTS of the resistance welded joint were evaluated by the same method as in Example 1. The results are shown in Table 3. As shown in these examples, the temperature at which the Mg-containing alloy is charged into the molten steel or the time from the start of alloy injection to the start of hot water discharge changes, so that oxidation containing Mg having a particle size of 3 μm or more is changed. The distribution of physical inclusions has changed, and the CTS / TSS has changed.

T ₁ : Molten steel pouring temperature (° C) into the tundish
T ₂ : Mg alloy charging start temperature (° C)
t ₁ : Time from pouring molten steel into the tundish to the start of casting (s)
t ₂ : Time from the start of charging the Mg alloy to the start of casting (s)
TS: Tensile strength of steel sheet (MPa)
d: Average grain size (μm)
ρ: Average distribution density of oxide-based inclusions containing Mg and having a particle size of 3 μm or more (pieces / mm ² )
TSS: Shear joint tensile strength (kN)
CTS: Cross joint tensile strength (kN)
TSS ^{formula: 4 × 10 -7 × [TS} ] 2 -1.3 × 10 -3 × [TS] +1.23

B-1 to B-6 are examples in which a Mg alloy is added to the surface of the tundish without using a refractory containing Mg. From these examples, it is shown that the oxide-based inclusions containing Mg in the steel sheet can be controlled by optimizing the injection start temperature of the Mg alloy and the time from pouring to discharging.
In Comparative Example B-1, the charging start temperature of the Mg alloy is too low, and in Comparative Example B-3, the charging start temperature of the Mg alloy is too high. Therefore, oxidation containing Mg having a particle size of 3 μm or more The number of physical inclusions exceeded the regulation, and as a result, the CTS / TSS was low and the spot weldability was poor.
Since B-4, which is a comparative example, takes too short a time from the start of charging the Mg alloy to the casting time, B-6, which is a comparative example, takes too long a time from the start of charging the Mg alloy to the casting time. The number of oxide-based inclusions containing Mg having a particle size of 3 μm or more exceeded the specified value, and as a result, CTS / TSS was low and spot weldability was poor.
On the other hand, in B-2 and B-5, which are examples of the present invention, the molten steel pouring temperature into the tundish, the Mg alloy pouring start temperature, the time from the molten steel pouring into the tundish to the casting start, and the Mg alloy pouring Since the time from the start to the start of casting is appropriate, the number of oxide-based inclusions containing Mg having a particle size of 3 μm or more is sufficiently small, and as a result, CTS / TSS is high and good spot weldability can be obtained. It was.

B-7 to B-9 are examples in which a refractory material containing Mg is used on the surface of the tundish and an alloy containing Mg is not added. When a refractory material containing Mg is used, even if an alloy containing Mg is not added, the Mg in the steel sheet can be optimized by optimizing the temperature of molten steel pouring into the tundish and the time from pouring to hot water. It is shown that the oxide-based inclusions containing the above can be controlled.
In Comparative Example B-7, the molten steel pouring temperature into the tundish is too low, and in Comparative Example B-9, the time from the start of charging the Mg alloy to the casting time is too short, so the particle size is 3 μm. The number of oxide-based inclusions containing Mg as described above exceeded the specified number, and as a result, CTS / TSS was low and spot weldability was poor.
On the other hand, B-8, which is an example of the present invention, contains Mg having a particle size of 3 μm or more because the temperature of molten steel pouring into the tundish and the time from pouring the molten steel into the tundish to the start of casting are both appropriate. The number of oxide-based inclusions was sufficiently small, and as a result, CTS / TSS was high and good spot weldability was obtained.

B-10 to B-12 are examples in which a refractory material containing Mg is used on the surface of the tundish and an alloy containing Mg is further added. In this case as well, it is shown that the oxide-based inclusions containing Mg in the steel sheet can be controlled by controlling the pouring temperature of the molten steel into the tundish, the starting temperature of alloy injection, and the time until the hot water is discharged.
In Comparative Example B-10, the molten steel pouring temperature and the Mg alloy injection start temperature into the tundish were too low, and in Comparative Example B-12, the Mg alloy injection start temperature was too high, so that the particles were particles. The number of oxide-based inclusions containing Mg having a diameter of 3 μm or more exceeded the specified value, and as a result, CTS / TSS was low and spot weldability was poor.
On the other hand, in B-11, which is an example of the present invention, the molten steel pouring temperature into the tundish, the Mg alloy pouring start temperature, the time from the molten steel pouring into the tundish to the casting start, and the casting start from the Mg alloy pouring start. Since the time to time was appropriate, the number of oxide-based inclusions containing Mg having a particle size of 3 μm or more was sufficiently small, and as a result, CTS / TSS was high and good spot weldability was obtained.

According to the present invention, a steel plate having excellent spot weldability, that is, CTS / TSS can be obtained by the above configuration, so that a highly reliable and high-strength resistance welded joint can be stably obtained. Even in high-strength steel plates with high carbon equivalents used as materials for frames, members, chassis, etc. used for automobile frames and panels, resistance welded joints with high strength and extremely low frequency of abnormal fracture can be obtained. As a result, it becomes possible to realize a welded joint using a steel plate having a high carbon equivalent, which has been limited in use in the conventional steel plate, and contributes to weight reduction of automobile members. Furthermore, by reducing the frequency of occurrence of low joint strength due to brittle fracture in the nugget, it contributes to improving the safety of the automobile body, and through simplification of post-weld heat treatment and reduction of the number of resistance welding spots, the member It can be expected to contribute to the improvement of manufacturing cost and productivity, and its industrial effect is extremely high.

Claims (8)

The composition of the steel sheet is mass%,
C: 0.04 to 0.30%,
Si: 3.0% or less,
Al: 0.1% or less,
Mn: 0.8 to 7.0%,
Ni: 0.01-1.5%,
P: 0.03% or less,
S: 0.005% or less,
Mg: 0.0001 to 0.0015%,
O: Contains 0.004% or less,
A total of one or more of Ti, Zr, Hf, V, Nb, Ta, Sc, and Y: 0.01 to 0.2%.
Total of one or more types of Cr, Mo, W, Cu: 0 to 1.0%,
Total of 1 or 2 types of Ca and REM: 0 to 0.005%,
B: 0 to 0.005%,
Containing
The balance consists of Fe and impurities,
Further, the steel sheet is characterized in that the average distribution density of oxide-based inclusions containing Mg having a particle diameter of 3 μm or more is 10 pieces / mm ² or less. The steel sheet according to claim 1, wherein the Ceq calculated by the following formula is 0.22 or more.
Ceq = C + Si / 30 + Mn / 20 + 2P + 4S (mass%) The steel sheet according to claim 1 or 2, wherein the average crystal grain size is 10 μm or less. The composition is mass%
The steel sheet according to any one of claims 1 to 3, wherein one or more of Cr, Mo, W, and Cu are contained in a total amount of 0.003% or more and 1.0% or less. .. The composition is mass%
The steel sheet according to any one of claims 1 to 4, wherein one or two of Ca and REM are contained in a total amount of 0.0002% or more and 0.005% or less. The composition is mass%
The steel sheet according to any one of claims 1 to 5, wherein B is contained in an amount of 0.001% or more and 0.005% or less. The method for producing a steel sheet according to any one of claims 1 to 6, wherein a refractory member containing no Mg is used as a refractory member for holding the molten metal, and Mg is added at a temperature of 1570 ° C to 1630 ° C. A method for producing a steel sheet, characterized in that the alloy to be contained is charged, and the time from the start of charging Mg into molten steel to the start of casting is 10 s to 300 s. A method of manufacturing a steel sheet according to any one of claims 1 to 6, using a refractory material containing Mg in tundish, pouring temperature of molten steel of 1570 ℃ ~1630 ℃ tundish , A method for producing a steel sheet, which comprises holding for a time of 10 s to 300 s from the start of pouring to the start of pouring into a mold.