JP5014807B2 - Steel sheet for hot press - Google Patents

Description

  The present invention relates to a steel sheet for hot pressing that simultaneously improves the strength of a member by press forming and quenching, and more particularly, to a steel sheet for hot pressing used for manufacturing a frame part, a reinforcing part, a suspension part, and the like of an automobile body.

  In recent years, weight reduction of automobiles has been strongly desired from the viewpoint of the global environment. In a part using a steel plate such as a body, a high strength steel plate is adopted to reduce the thickness of the steel plate and reduce the weight. However, increasing the strength of the steel sheet causes a decrease in workability and press formability at the time of manufacturing the parts, and in particular, it is more difficult to ensure product accuracy due to springback and the like.

  In order to solve these problems, the development of steel plates with excellent workability and processing methods that improve product accuracy have been proposed so far. However, the strength of steel plates applied to automobiles has further increased, especially tensile strength ( In a high-strength steel sheet having a TS (Tensile Strength) exceeding 1180 MPa, there is a limit to the parts that can be applied from the viewpoints of the above workability and product accuracy.

  In recent years, the hot press method (press quench method) has come to be used as a practical technique as a method of simultaneously satisfying the high strength, workability, and product accuracy of steel sheets. For example, it is disclosed in Patent Document 1. In this method, the steel sheet is heated to an austenite region of about 800 ° C. or higher, then press-formed, and simultaneously quenched by cooling after forming to obtain a high-strength material. This hot press method improves the workability such as cracking during molding and makes it possible to manufacture parts with relatively good product accuracy. In addition, the residual stress introduced during press molding is reduced. Although the sensitivity of delayed fracture, which is a problem with high-strength steel sheets with a tensile strength exceeding 1180 MPa, also decreases, the following problems remain.

  As described above, in the hot press method (press quench method), the steel plate is heated to about 800 ° C. or higher, so that the surface of the steel plate is oxidized to produce iron oxide, so-called scale. When press molding is performed with such a scale on the surface of the steel sheet, the scale peels off due to molding and remains in the mold part, causing indentation scratches during press molding of the next sample. It may be necessary to clean the interior, which may hinder productivity. For this reason, in the hot press method, it is desired that the scale generated during heating has adhesion to the processing strain introduced during press molding. In addition, when coating is performed in the next process with the scale remaining, the adhesion to the coating film is low, resulting in a decrease in corrosion resistance. As a result, after press molding, the process before coating is represented by shot blasting. A scale removal process is required.

  For such a problem, it is possible to suppress the generation of scale and to ensure the paintability and corrosion resistance without setting the scale removal step by applying a surface treatment to the steel sheet by plating. However, the actual situation is that an increase in cost cannot be avoided by plating.

  In addition, spot welding is usually used for joining parts in the manufacture of automobiles, but steel sheets used for hot pressing have many additional components to ensure strength and hardenability after quenching. In addition, the spot welded portion is increased in hardness, and the reliability (breaking form) of the joint of the spot welded portion is lowered.

  In other words, in the steel plate used for the hot press method, there is still no steel plate that has not been plated, and has not been considered for the strength and scale adhesion of parts after hot pressing and quenching, and the quality of spot welds. It is.

Japanese Patent Laid-Open No. 10-96031 JP 2003-147499 A

  It is an object of the present invention to provide a hot press steel sheet that overcomes the above-described problems and satisfies the requirements of component strength after hot pressing and quenching, scale adhesion, and quality of spot welds. To do.

  The gist of the present invention is as follows.

(1) In mass%
C: 0.15-0.3%
Si: 0.05 to 0.5%,
Mn: 0.5 to 3.0%
P: 0.005-0.1%,
S: 0.02% or less Al: 0.01-3.0%,
N: 0.01% or less,
Cr: 0.05 to 0.5%,
B: 0.0002 to 0.01%
Steel, the balance of which is iron and inevitable impurities, satisfying the following formulas (A) and (B), and hot rolling, pickling, and cold rolling with a rolling rate of 20 to 60% A steel sheet for hot pressing, which is an unannealed material that has been subjected to heat treatment and has a tensile strength of 1180 MPa or more after press quenching.
0.5 ≦ (Cr / Si) ≦ 1.5 (A)
0.35 ≧ C + Si / 30 + Mn / 20 + Cr / 20 + 4 × B + 2 × P + 4 × S
(B)

(2) Further, in mass% Ti: 0.002 to 0.5%,
The steel sheet for hot pressing as described in (1) above, wherein the balance is steel composed of iron and inevitable impurities.

  The high-strength steel sheet for hot pressing according to the present invention can provide a steel sheet excellent in scale adhesion and spot welding quality while ensuring the strength of parts after hot pressing and quenching, and is manufactured by the steel sheet according to the present invention. It is considered that the use of such a member can greatly contribute to the improvement of reliability in automobile manufacturing, and consequently to the reduction in weight and safety of automobiles, and the industrial contribution is great.

  The present inventors have investigated various additive components, additive amounts, and the influence of additive components in order to satisfy the parts strength and scale adhesion after hot pressing and quenching, and spot welding characteristics.

  As a result of investigating the scale adhesion, it was found that the ratio of Cr and Si added in the steel (Cr / Si) has an effect, and if this ratio is within a specific range, the adhesion is good.

  In addition, hot rolling, cold rolling is performed on the steel plate after pickling, and then continuous annealing is not performed, that is, adhesion is good in an unannealed material, and the rolling rate is 20 to 60%. It was also found that the adhesion was further improved under the conditions. Next, when the spot weldability was investigated, when the hot-pressed and hardened steel plate was spot-welded, the strength of the welded part and the heat-affected zone changes with respect to the strength of the base metal due to the thermal history of the welded part. When the joint strength was evaluated, it was found that the fracture form of the spot welded part may exhibit a peeling rupture in which the welded part (nugget) does not remain or an external nugget rupture in which the nugget remains. And when C, Si, Mn, P, S, Al, N, Cr, B, Ti are contained in the steel, among these, C, Si, Mn, Cr, B, P, S are predetermined. It has been found that when the relational expression is satisfied, the fracture form of the spot welded portion tends to exhibit a nugget-out fracture.

  Based on the above knowledge, a steel sheet for hot pressing having desired characteristics can be obtained by optimizing the additive components of the steel sheet and setting the mutual additive amount ratio within a predetermined range.

  The present invention is described in detail below.

  First, the reason why each component of steel is limited to a predetermined range will be described below.

  C is an important element that affects the strength of parts after hot pressing and quenching, and if it is less than 0.15%, sufficient strength cannot be secured. Further, if it exceeds 0.3%, the fracture form at the time of spot welding is deteriorated, and at the same time, the cementite that becomes the starting point of crack generation at the time of punching in the manufacture of parts is increased to easily cause delayed fracture. 3%.

  Si is an element that affects hot pressing, hardenability, and scale adhesion, and is an element that suppresses cementite precipitation. However, if it is less than 0.05%, a sufficient effect cannot be obtained. On the other hand, if it exceeds 0.5%, the effect is saturated, which is costly and disadvantageous economically.

  Mn is an element that affects hot press and hardenability, and is effective in increasing the strength of the steel sheet. However, this effect cannot be obtained at less than 0.5%, so the lower limit was set to 0.5%. Further, if it exceeds 3.0%, co-segregation with P and S is promoted and the spot welded portion becomes brittle, so 3.0% is made the upper limit.

  P is an element that promotes intergranular fracture due to intergranular segregation, and a lower value is desirable. However, if it is attempted to make it less than 0.005%, it is not preferable because it causes an increase in the cost of steelmaking in the production process. On the other hand, if it exceeds 0.1%, intergranular fracture will be promoted and spot welds will become brittle, so the upper limit is made 0.1%.

  S is an element that generates non-metallic inclusions such as MnS and degrades punchability, or promotes hot embrittlement and embrittlement of spot welds. A lower value is desirable, but extremely low is an increase in manufacturing cost. Since it is not preferable, it is made 0.02% or less.

  Al is added in an amount of 0.01% or more for deoxidation. Increasing the addition amount increases inclusions such as alumina, which causes generation of coarse cracks on the punched surface, so the upper limit was made 3.0%. .

  Since N produces a coarse compound when the amount of addition increases, it causes the occurrence of coarse cracks on the punched surface, or forms BN by combining with B described later, thereby reducing the effect of B addition, so the addition is small. Is preferable. In particular, when the content exceeds 0.01%, the effect becomes significant, so the upper limit was made 0.01%. Moreover, although it is desirable that the content is less, it is not preferable in view of manufacturing cost to make it less than 0.0005%, so the lower limit value is preferably made 0.0005%.

  Cr is an element that affects the hardenability and scale performance of the steel sheet, and is necessary for ensuring the strength and scale adhesion after hot pressing and quenching. However, since these effects cannot be obtained if the content is less than 0.05%, the lower limit is set to 0.05%. Moreover, addition of a large amount not only saturates the hardenability but also deteriorates the scale adhesion and increases the delayed fracture susceptibility, so the upper limit is made 0.5%.

  B is an element effective for improving the hardenability. In order to make such an effect effective, 0.0002% or more must be added. However, the effect is saturated even if added excessively, so 0.01% was made the upper limit.

  Ti is a strong carbide-forming element, which generates precipitates and inclusions to ensure the strength of the hot-pressed and quenched members and prevents N from being bonded by fixing N and quenching B Fully demonstrate the effects of sex. In order to exert this effect, 0.002% or more must be added. However, if excessively added, coarse precipitates or crystallized substances are formed, which causes generation of coarse cracks on the punched surface, so 0.5% is made the upper limit.

Furthermore, in the present invention, it is important that the amount of each component added satisfies the following formulas (A) and (B).
0.5 ≦ (Cr / Si) ≦ 1.5 (A)
0.35 ≧ C + Si / 30 + Mn / 20 + Cr / 20 + 4 × B + 2 × P + 4 × S
(B)
When the value of the formula (A) is less than 0.5 or exceeds 1.5, the scale adhesion after hot pressing and quenching decreases. Preferably, the range of 0.9 to 1.2 provides better scale adhesion. In the steel sheet containing C, Si, Mn, P, S, Al, N, Cr, B, and Ti as components in steel in the laboratory, the inventors changed the amounts of Si and Cr. Using the steel plates having various component compositions, hot pressing and quenching were performed, and the behavior of the scale was investigated. Heating was performed at 930 ° C. × 3 min in the air, and then quenched with a flat plate mold. As a result, when the ratio of Cr and Si in the steel (Cr / Si) was 0.5 or more and 1.5 or less, the scale adhesion was good. When the state of this scale was investigated, it was found that FeCr ₂ O ₄ , (Fe, Mn, Cr) ₂ SiO ₄ was present at the interface between the scale and the steel plate.

On the other hand, when the addition ratio of Cr and Si in the steel (Cr / Si) was less than 0.5, Fe ₂ SiO ₄ was present at the interface between the scale and the steel plate. Further, when the addition ratio of Cr and Si in the steel (Cr / Si) exceeded 1.5, FeCr ₂ O ₄ was present at the interface between the scale and the steel plate. The reason why the scale adhesion improves is not clear, but since the oxide generated at the interface between the scale and the steel sheet differs depending on the ratio of Cr and Si in the steel (Cr / Si), FeCr ₂ O ₄ , ( The presence of the oxide of Fe, Mn, Cr) ₂ SiO _{4 at} the interface between the scale and the steel plate is presumed to improve the consistency between the scale and the steel plate.

  Next, when the value of the formula (B) exceeds 0.35, when the fracture form is evaluated by a peel test (JIS Z3144) of the spot welded part, the welded part (nugget) does not remain as the fractured form of the spot welded part. Since there are many cases of peeling and breaking, the upper limit of the formula (B) is set to 0.35. When the formula (B) is satisfied, that is, when the addition amount of C, Si, Mn, Cr, B, P, and S is in accordance with the present invention, the fracture mode is a nugget fracture or a partial nugget fracture. There was found. This is because C, Si, Mn, Cr, B increase the strength of the welded part and the heat affected zone, and P and S tend to segregate to the grain boundary due to heat input during welding. If it is added excessively, it is considered that the welded portion becomes brittle and tends to peel and break. In the case of peeling fracture, since the joint strength of the welded portion may be lower than the strength of the base material, the reliability of the performance of the component may be impaired. For this reason, in order to ensure the reliability of component performance, it is important to satisfy the above formula (B). When the formula (B) is satisfied, the fracture mode is a partial fracture in the nugget or an external nugget fracture, and a fracture mode in which the welded portion (nugget) remains in one of the steel plates. Furthermore, if the value of the formula (B) is 0.33 or less, the probability of fracture outside the nugget increases, which is more preferable from the viewpoint of welded part quality.

  In the method for producing a high-strength steel sheet according to the present invention, various conditions can be appropriately selected according to the use and required characteristics.

  For example, a high-strength steel plate can be manufactured according to the following method. First, steel adjusted in the range of the above-mentioned component composition is melted in a converter and made into a slab by a continuous casting method. After this slab is kept in a high temperature state or cooled to room temperature, it is inserted into a heating furnace, finish-rolled in a temperature range of 1000 to 1250 ° C., and wound at a temperature of 700 ° C. or less to obtain a hot-rolled steel sheet. Next, pickling and cold rolling are performed to form a cold-rolled steel sheet, which becomes a hot-press steel sheet. Scale adhesiveness improves by making it the steel plate for hot press, without performing normal continuous annealing after cold rolling, and leaving unannealed. The reason for this is not clear, but due to the introduction of strain due to rolling, when the scale is generated in the heating process in the hot press method, the rate of oxidation may change due to the change in the diffusion rate of Fe in the steel sheet. It is estimated that it has an effect on improving adhesion. For this reason, when continuous annealing is performed, the strain introduced in the cold rolling process is removed by annealing, so that the effect of improving adhesion is lost. Moreover, it was confirmed that the scale adhesion is further improved by setting the rolling rate in the cold rolling step to 20 to 60%. This effect is not recognized when the rolling rate is less than 20%, and when it exceeds 60%, the effect is saturated, and the strength of the steel sheet increases due to work hardening, and cutting becomes difficult. To do.

  Next, examples will be described.

  Steels having the component compositions shown in Table 1 were melted in a converter and slabs were obtained by continuous casting according to a conventional method. These slabs are heated in a heating furnace at a temperature of 1140 ° C. to 1250 ° C., hot-rolled at a finishing temperature of 810 ° C. to 880 ° C., wound up at 600 ° C. to 660 ° C., and hot rolled steel sheet (sheet thickness) : 2.0 mm). And about some things, the rolling rate of the cold rolling was changed after pickling, and it was set as the cold-rolled steel plate, and also the continuous annealing was given to some ones after cold rolling. Using the above steel plate, heating was performed in a heating furnace under the condition of 930 ° C. × 3 min, and the hat-shaped product was hot-pressed to produce a part having a member tensile strength of 1470 MPa class. (Hat shape: width 100 mm, length: 300 mm, height: 60 mm) In the evaluation of the component strength, the tensile strength after hot pressing was 1470 MPa or more. Evaluation of scale adhesion was taken out from the mold after cooling to near normal temperature, and the peeled state on the scale was evaluated visually, and the one without scale peeling was regarded as acceptable. Furthermore, an evaluation sample was cut out from this press product, and the tensile strength and spot welding quality after hot pressing and quenching were evaluated. The tensile test was performed according to JIS Z 2201. The fracture form evaluation as spot welding quality was performed by a peel test according to JIS Z 3144. For the welding, a stationary spot welder is used, the applied pressure is 4.4 kN, the energization time is 0.3 s, the holding time is 0.08 s, and the current value is 4√t ( t: plate thickness mm). The evaluation of spot welding quality was classified into A: peeling fracture, B: fracture in nugget, C: fracture outside nugget, and B and C in which the nugget remained were regarded as acceptable. The results are shown in Table 2.

  From the results shown in Table 2, the tensile strength after quenching was about 1470 MPa or more in the range of the steel of the present invention, and it was confirmed that the desired component tensile strength was obtained. On the other hand, when it was outside the scope of the present invention, the tensile strength was low. Moreover, in the range of this invention steel, scale adhesiveness also becomes favorable, but when it remove | deviated from the range of this invention, scale adhesiveness was inferior. Furthermore, in the range of the steel of the present invention, the fracture mode at the time of spot welding is satisfied, but when it is outside the range of the present invention, the tendency of peeling fracture is exhibited and the steel of the present invention satisfies the desired characteristics. Recognize.

Claims (2)

In mass%
C: 0.15-0.3%
Si: 0.05 to 0.5%,
Mn: 0.5 to 3.0%
P: 0.005-0.1%,
S: 0.02% or less Al: 0.01-3.0%,
N: 0.01% or less,
Cr: 0.05 to 0.5%,
B: 0.0002 to 0.01%
Steel, the balance of which is iron and inevitable impurities, satisfying the following formulas (A) and (B), and hot rolling, pickling, and cold rolling with a rolling rate of 20 to 60% A steel sheet for hot pressing, which is an unannealed material that has been subjected to heat treatment and has a tensile strength of 1180 MPa or more after press quenching.
0.5 ≦ (Cr / Si) ≦ 1.5 (A)
0.35 ≧ C + Si / 30 + Mn / 20 + Cr / 20 + 4 × B + 2 × P + 4 × S
(B) Furthermore, in mass% Ti: 0.002 to 0.5%,
The steel sheet for hot pressing according to claim 1, wherein the steel is made of iron and inevitable impurities.