JP2005281816A - High strength cold rolled steel sheet having satisfactory formability and excellent projection weldability, and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheethaving satisfactory formability and excellent projection weldability by making the mechanical properties of a steel sheet having a tensile strength (TS) of ≥780 MPa, an elongation (El) of ≥20% and a yield ratio (YR) of ≤65% compatible with projection weldability. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.075 to 0.105% C, 0.50 to 0.65% Si and 2.45 to 2.65% Mn, and in which the content of P is restricted to ≤0.020%, S to ≤0.005% and Al to ≤0.05%, and the balance substantially Fe with inevitable impurities. The steel sheet has a tensile strength (TS) of ≥780 MPa, an elongation (El) of ≥20% and a yield ratio (YR) of ≤65%, and thus has high strength and satisfactory formability, and also has excellent projection weldability. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a high-strength cold-rolled steel sheet and a method for producing the same, and in particular, the shape of the front side member of an automobile is complex, and the formability is good and the projection weldability is good for use in producing a member to be projection welded. The present invention relates to an excellent high-strength cold-rolled steel sheet and a method for producing the same.

  Steel materials used for automobile body components and exterior materials are required to have high strength in order to reduce the weight of the vehicle body. For this reason, various high-strength steel plates are provided. Among them, members that have complicated shapes such as the front side members of automobiles and weld nuts to join other members have a high tensile strength (TS) of over 780 MPa and an elongation (El) of 20%. Above (preferably 22% or more), the yield ratio (YR) is required to be 65% or less, and the projection is required to be good.

  In order to satisfy the high strength and good formability among the requirements for such steel materials for automobiles, for example, Patent Document 1 discloses the amount of components such as C, Mn, Nb, Ti, and Cr, B. A composite structure having a second phase structure is obtained by optimizing the temperature, and further, a hot dip galvanizing is performed while retaining in a temperature range of about 500 ° C. after recrystallization annealing, and the subsequent cooling rate is regulated. Means have been disclosed that prevent the two phases from being hardened more than necessary and obtain a low yield ratio high strength high strength hot dip galvanized steel sheet.

Japanese Patent Laid-Open No. 4-236741

  However, the means disclosed in Patent Document 1 can provide a galvanized steel sheet having high strength and good formability, but lacks consideration for projection welding. Projection welding, especially butt projection welding, is a process similar to upset welding and requires that no oxides remain on the joint surface. Therefore, special considerations differing from the mechanical properties of steel sheets are made. There must be.

  In the present invention, the mechanical properties of a steel sheet that have a tensile strength (TS) of 780 MPa or more, an elongation (El) of 20% or more, and a yield ratio (YR) of 65% or less, which could not be achieved by such conventional techniques. An object of the present invention is to propose a high-strength cold-rolled steel sheet having excellent formability and projection weldability, and a method for producing the same.

  The high-strength cold-rolled steel sheet according to the present invention contains, by mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, S: 0.005% or less. , Al: Limited to 0.05% or less, substantially composed of Fe except for the inevitable impurities of the balance, tensile strength (TS) is 780MPa or more, elongation (El) is 20% or more, yield ratio (YR) is 65% It has the following high strength, good formability, and excellent projection weldability. This high-strength cold-rolled steel sheet can further improve its mechanical properties without impairing the projection weldability by further containing B: 0.00015 to 0.0020%.

  The high-strength cold-rolled steel sheet having excellent projection weldability contains, by mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, and S: It is limited to 0.005% or less, Al: 0.05% or less, and the steel material consisting essentially of Fe, excluding the remaining inevitable impurities, is hot-rolled and then cold-rolled, and the resulting cold-rolled sheet is heated to 775-830 ° C. After heating, it can be produced by applying a heat treatment for rapid cooling to 300 ° C. or lower at a rate of 15 ° C./s or higher. In this case, the steel material can further contain B: 0.00015 to 0.0020%.

  According to the present invention, it is possible to supply a high-strength cold-rolled steel sheet having good formability and excellent projection weldability. As a result, it becomes possible to easily and reliably form a member having a complicated shape, such as a front side member of an automobile, and join by projection welding, thereby contributing to weight reduction of the automobile.

  The high-strength cold-rolled steel sheet according to the present invention contains, by mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, S: 0.005% or less. Al: It is limited to 0.05% or less, and has a composition substantially composed of Fe except for the remaining inevitable impurities. Hereinafter, the reasons for limiting these components will be described.

  C is required to give a predetermined strength to the steel sheet and to make a composite structure type high-strength steel sheet by heat treatment adopted when manufacturing the steel according to the present invention, to achieve high strength and good formability. Element. However, when the content is too small, sufficient strength, for example, 780 MPa or more cannot be imparted to the steel sheet. On the other hand, when the content is too high, the strength is too high and the workability is impaired. It causes slab cracking. Therefore, the C content is 0.075 to 0.105% (mass ratio, the same applies hereinafter), preferably 0.080 to 0.100%.

  Si is an element that increases the ferrite phase fraction and thereby improves the elongation (El) in a high-strength steel sheet having a composite structure. Further, in projection welding, it is an element that improves the projection weldability by making it easy to discharge from the welding surface by making the oxide generated on the welding surface into a silicate system. In order to obtain this effect, 0.50% or more of Si is contained in the present invention. However, Si concentrates on the surface of the steel sheet, thereby reducing paintability and plating properties. This effect is particularly likely to occur in so-called non-pickling materials. When the Si content exceeds 0.65%, the Si concentration on the surface of the steel sheet becomes very high due to the surface concentration of Si, and the corrosion resistance after coating is remarkably deteriorated. Therefore, in the present invention, the Si content is set to 0.50 to 0.65%.

  Mn is a so-called austenite formation promoting element. Like C, Mn gives a predetermined strength to the steel sheet, and a high-strength steel sheet having a composite structure type by heat treatment adopted when manufacturing the steel according to the present invention. It is an element necessary to achieve and improve moldability. However, when the content is less than 2.45%, sufficient strength, for example, a tensile strength of 780 MPa or more cannot be imparted to the steel sheet, while when the content is higher than 2.65%, an appropriate composite structure is obtained. It cannot be performed, and the strength is too high, which causes the workability to be damaged. Therefore, the Mn content is 2.45 to 2.65%.

  If P exceeds 0.020%, projection weldability is impaired, and segregation at the center of the slab causes a ferrite band to be formed, which deteriorates bending workability in a direction perpendicular to the rolling direction. Further, if S exceeds 0.005%, it impairs the projection weldability and also the workability. For these reasons, P and S are limited to 0.020% or less and 0.005% or less, respectively.

  Al is added to the steel as a deoxidizer during steelmaking. In addition, there is an effect of making the crystal grains finer. However, even if it exceeds 0.05%, the effect is only saturated, so it is limited to 0.05% or less.

  Other components are Fe except for inevitable impurities. Inevitable impurities include O, N and other trump elements. It is desirable to suppress these contents as low as possible.

  B is an element that promotes the formation of a hard phase in the composite structure by segregating in the austenite grain boundary in a solid solution state and allowing austenite to exist stably to a relatively low temperature. In the present invention, B can be contained in the range of 0.00015 to 0.00200 (1.5 to 20 ppm), whereby an appropriate composite structure can be obtained. B may be contained in an amount of 1 ppm or less as an inevitable impurity, but the present invention does not exclude such a case.

  The high-strength cold-rolled copper sheet according to the present invention has a high tensile strength (TS) of 780 MPa or higher, an elongation (El) of 20% or more (more preferably 22% or more), and a yield ratio (YR) of 65% or less. Have excellent moldability. The structure of the high-strength cold-rolled steel sheet according to the present invention that achieves both high strength and formability is a composite containing a martensite phase in the second phase (also referred to as a hard phase) other than the ferrite phase. It is an organization. By setting it as such a composite structure, it can have high intensity | strength and favorable moldability. The hard phase generally consists of a martensite phase and a bainite phase.

  By having the above composition and structure, the steel according to the present invention has a tensile strength (TS) of 780 to 860 MPa, a proof stress (yield point) (YP) of 460 to 550 MPa, and an elongation (El) of 20 to 24%. It has properties and is excellent in projection weldability.

  Steel having such characteristics contains, by mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, S: 0.005% or less, Al: 0.05 %, And a composition consisting essentially of Fe except the inevitable impurities of the remainder, or in addition to this, a slab having a composition containing B: 0.00015 to 0.0020%, which is heated and heated by a conventional method. To obtain a hot-rolled sheet having a thickness of, for example, about 2.6 mm, and pickling and cold-rolling the obtained hot-rolled sheet according to a conventional method to obtain a cold-rolled sheet having a final thickness of, for example, about 1.8 mm. Can be manufactured by performing the following heat treatment. The heat treatment is preferably performed by a continuous line.

Heating temperature: 775 ~ 830 ℃
The heating temperature of the cold-rolled sheet is performed in a ferrite-austenite two-phase region in order to increase the strength of the product structure as a composite structure having a ferrite phase and a hard phase and to provide formability. When the heating temperature is less than 775 ° C., an amount of austenite sufficient to generate a hard phase is not generated, resulting in insufficient product strength. On the other hand, when the heating temperature is higher than 830 ° C., the amount of ferrite phase becomes too small, resulting in insufficient product ductility.

Cooling condition: 15 ° C./s or more The cooling rate from the ferrite-austenite two-phase region is to transform austenite into a hard phase, thereby obtaining a composite structure to obtain high strength and good moldability. As a condition. When the cooling rate is less than 15 ° C./s, the austenite phase undergoes pearlite transformation or the upper bainite phase is formed, and a product having desired strength and ductility cannot be obtained. From the viewpoint of the material, it is not necessary to set an upper limit of the cooling rate, but if it exceeds 30 ° C./s, the steel sheet is not uniformly cooled, and the flatness of the steel sheet is impaired. Accordingly, the cooling condition should be rapid cooling of 15 ° C./s or more, preferably 15 to 30 ° C./s. In addition, although the said rapid cooling may be started immediately after the said heating, the area | region of about 10-20 degreeC after a heating can be gradually cooled, and it can also cool at the said speed | rate after an appropriate time.

Cooling stop temperature: 300 ° C or more The cooling stop temperature is determined as a temperature at which a hard phase having an appropriate hardness is generated by the above cooling, and good moldability can be obtained even at high strength, and the relationship with the heating temperature is taken into consideration . Fig. 1 shows the heating temperature and cooling stop for cold-rolled sheets (C: 0.09%, Si: 0.60%, Mn: 2.5%, P: 0.012%, S: 0.0032%, Al: 0.038%, remaining Fe and inevitable impurities) It is a related figure which shows the influence which temperature has on the mechanical property of a perpendicular direction (C direction) with respect to the rolling direction of the steel strip of a product. As shown here, when the cooling stop temperature is higher than 300 ° C., the strength (TS) of the steel sheet is less than 780 MPa, or the ductility (El) is less than 20%. Furthermore, even when the cooling stop temperature is 300 ° C. or lower, when the heating temperature is lower than 775 ° C. or higher than 830 ° C., the strength of the steel sheet becomes insufficient or the ductility becomes insufficient. In FIG. 1, when both the strength and ductility are satisfied, it is separately confirmed that the yield ratio (YR) also satisfies 65% or less.

  The cooling stop temperature is not particularly limited as long as it is 300 ° C. or lower. When the heat treatment is performed in a continuous annealing line, the lower limit of the rapid cooling stop temperature is approximately 240 ° C. Further, the cooling after the rapid cooling is not particularly limited, and may be performed according to the specifications and capacity of the equipment. For example, a so-called accelerated cooling with a cooling rate of 2 to 4 ° C./s may be performed in a section up to 150 ° C., and then wound around a coil and allowed to cool naturally. Thereby, while preventing a hard phase from softening, cooling efficiency can be improved. In that case, it can hold | maintain for 20 to 60 s in the vicinity of the said cooling stop temperature, and can also carry out accelerated cooling and natural cooling after that.

  Cold-rolled sheets having a thickness of 1.6 mm having various compositions shown in Table 1 were manufactured, and subjected to heat treatment under the conditions shown in Table 2. Table 2 shows the mechanical properties, formability, and projection weldability of the obtained product. The mechanical properties of the obtained product are as follows. Test specimens taken from the direction perpendicular to the rolling direction (C direction) are processed into JIS No. 5 test specimens, and tensile strength (TS) and elongation at break (in accordance with JIS Z 2241) El) and yield strength (YS) were measured. The yield ratio (YR = (YS / TS) × lOO (%)) was calculated from the obtained tensile strength (TS) and yield strength (YS). Here, when the elongation (El) is 20% or more and the yield ratio (YR) is 65% or less, the workability is good (△), and when the elongation (El) is 22% or more, the workability is excellent (○). As evaluated. Further, the projection weldability of the obtained product was measured by peeling the hexagon nut after projecting the hexagon nut using a welding machine based on JIS C 9307, and measuring the peel strength. When the peel strength in the conventional material (steel plate No. 3) was 100, the case of having a peel strength of 125 or more was evaluated as ○, and the case of less than 125 was evaluated as ×.

  In addition, although the structure | tissue of the product was observed with the optical microscope, it confirmed that it was a composite structure which has a martensite phase while having a ferrite phase in this invention steel plate. Moreover, although the hole expansibility was investigated about this invention steel plate, it was confirmed that it is a hole expansibility ratio (lambda)> = 25% and there is no problem also in stretch flangeability. Furthermore, as a real part, it was molded into a side frame and a hexagon nut was tested for projection welding. However, there were no problems such as cracking during molding, and the projection weldability was much better than conventional materials. It was confirmed.

It is a relationship figure which shows the influence which the heating temperature and cold stop temperature of a cold-rolled sheet exert on the mechanical property of the steel strip C direction of a product.

Claims (4)

  In mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, S: 0.005% or less, Al: 0.05% or less, the balance Formability characterized by consisting essentially of Fe, excluding inevitable impurities, tensile strength (TS) of 780 MPa or more, elongation (El) of 20% or more, and yield ratio (YR) of 65% or less. A high-strength cold-rolled steel sheet with good projection weldability.   The high-strength cold-rolled steel sheet having good formability and excellent projection weldability according to claim 1, further comprising B: 0.00015 to 0.0020%.   In mass ratio, C: 0.075 to 0.105%, Si: 0.50 to 0.65%, Mn: 2.45 to 2.65%, P: 0.020% or less, S: 0.005% or less, Al: 0.05% or less, the balance A steel material consisting essentially of Fe, excluding inevitable impurities, is hot-rolled and then cold-rolled. The resulting cold-rolled sheet is heated to 775-830 ° C and then at a rate of 15 ° C / s or higher up to 300 ° C or lower. A method for producing a high-strength cold-rolled steel sheet having good formability and excellent projection weldability, characterized by performing a heat treatment for rapid cooling. The method for producing a high-strength cold-rolled steel sheet having good formability and excellent projection weldability according to claim 3, wherein the steel material further contains B: 0.00015 to 0.0020%.

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