Press forming method for high-tensile steel sheet and press-formed product made of high-tensile steel sheet

Abstract

To form a press-formed article having a tensile strength of the same order as super high tensile strength steel plate by using a high tensile strength steel plate having tensile strength lower than the tensile strength of super high tensile strength steel plate.SOLUTION: A press forming method for high tensile strength steel plate performs cold press forming of high tensile strength steel plate having a tensile strength lower than that of super high tensile strength steel plate, performs carburizing treatment or carbonitriding treatment of a press-formed article to add at least twice tensile strength before such kinds of treatment or a tensile strength of the same degree as the tensile strength of the super high tensile strength steel plate or more than the tensile strength. A press-formed article made of super high tensile strength steel plate is the press-formed article formed by the press forming method of the high tensile strength steel plate. Even when material is high tensile strength steel plate, the press-formed article has at least twice a tensile strength obtained before carburizing treatment or carbonitriding treatment.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a press forming method for high-tensile steel (High Tensile Strength Steel: HTSS), and a press-formed product made of a high-tensile steel sheet formed by the method.

Background Art

[0002] High-tensile steel is a material obtained by adding chromium (Cr), nickel (Ni), molybdenum (Mo), etc. to low-carbon steel, and there are several types depending on the type of added element. High-tensile steel has a higher tensile strength than ordinary steel, and since the strength can be ensured even when the plate thickness is reduced, weight reduction can be achieved. Since the carbon content is low, hardening due to the influence of heat during welding is less. Since it is a non-normalizing steel, it has the advantage of being inexpensive.

[0003] In recent years, aiming at improving the global environment, reduction of carbon dioxide (CO2) has been required internationally, and reduction of CO2 emissions has also been required for the automotive industry. As measures for reducing CO2 in the automotive industry, the development of hybrid vehicles (HA vehicles), plug-in hybrid vehicles (PHEV vehicles), and electric vehicles (EV vehicles) has been promoted. Along with this, weight reduction has been required for automotive parts from the aspects of improving the driving range, fuel efficiency, and operability. High-tensile steel is a material for automotive parts that meets these requirements.

[0004] While high-tensile steel has the above-mentioned advantages, on the other hand, it has the drawback that it is difficult to press-process because of its high tensile strength and hardness.

[0005] There are various high-tensile steel plates with different tensile strengths. For high-tensile steel plates used in automotive parts, there are cold-rolled steel plates (SPCC: Steel Plate Cold Commercial) and hot-rolled steel plates (SPHC: Steel Plate Hot Commercial), and those with a tensile strength of 270 - 780 MPa. High-tensile steel plates with particularly high tensile strength are called ultra-high-tensile steel plates. Although there is no specific defined standard for ultra-high-tensile steel plates, generally, it is said that the tensile strength is 980 MPa or more.

[0006] High-tensile steel plates with relatively low tensile strength, such as SPCC780 and below SPHC780, can be formed (processed) by general cold pressing. However, for ultra-high-tensile steel plates, it is difficult to perform press forming such as bending, punching, and drawing by general cold press forming. In the case of bending, the press-formed product is prone to chipping or cracking (breaking easily), and due to elastic recovery during forming unloading, the formed product may spring back, resulting in poor dimensional accuracy. In the case of punching, burrs are likely to occur. In either case, the mold is prone to wear and breakage, and the mold life is shortened. Therefore, ultra-high-tensile steel plates with particularly high tensile strength and surface hardness are often hot press formed (hot stamping formed).

[0007] In hot press forming, since the ultra-high-tensile steel plate is heated to about 900 °C and then press formed, and rapidly cooled and quenched after forming, there are problems such as many working steps, time-consuming work, slow production speed (low productivity), and high cost. Also, since the formed product after quenching has high hardness, it is difficult to perform processing such as trimming and piercing after press forming, and laser processing may be required. In this case, there are also difficulties such as the increase in initial cost associated with the introduction of laser processing equipment and the running cost associated with its use.

[0008] In order to solve the above problems, a method of directly water-cooling a steel sheet in a mold has been developed (Non-Patent Document 1). According to this method, it is said that the productivity can be increased three times compared to quenching at room temperature. However, this method is still hot press forming. To the best knowledge of the applicant of this application, a method for improving productivity in cold press forming is not known.

Prior Art Documents

Non-Patent Documents

[0009]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] The problem to be solved by the present invention is to increase the tensile strength using a high-tensile steel sheet having a lower tensile strength than an ultra-high-tensile steel sheet, preferably to improve it by two times or more or to the same level as or higher than the tensile strength of the ultra-high-tensile steel sheet. Here, the same level as the tensile strength of the ultra-high-tensile steel sheet generally refers to a tensile strength of about 980 MPa, which is generally referred to as an ultra-high-tensile steel sheet (the same applies hereinafter).

Means for Solving the Problems

[0011] The press forming method of the high-tensile steel sheet of the present invention cold press forms a high-tensile steel sheet having a lower tensile strength than an ultra-high-tensile steel sheet, and carburizes or carbonitrides the formed press-formed product, so that at least the tensile strength of the press-formed product is improved to two times or more, preferably to the same level as or higher than the tensile strength of the ultra-high-tensile steel sheet, compared to the tensile strength before the carburizing treatment or the carbonitriding treatment. Here, the carburizing treatment or the carbonitriding treatment refers to various general-purpose carburizing treatments or carbonitriding treatments (the same applies hereinafter).

[0012] The press-formed product made of high-tensile steel sheet of the present invention is a press-formed product obtained by forming a high-tensile steel sheet by the press-forming method of the present invention, which is carburized or carbonitrided and has a tensile strength of 2 times or more of the tensile strength before these treatments, preferably the same or more as the tensile strength of an ultra-high-tensile steel sheet.

Advantages of the Invention

[0013] Since the press-forming method of the high-tensile steel sheet of the present invention uses a high-tensile steel sheet having a lower tensile strength than an ultra-high-tensile steel sheet, it can be easily formed even by cold press-forming, the forming operation is easier than hot press-forming, and the productivity is improved, so that the cost can be reduced. There are few chips and cracks due to press-forming, and high-precision forming can be performed. Since post-processing such as trimming can be performed by press-forming, there is no need to introduce a laser processing machine, and there is no increase in initial cost and running cost. The wear and damage of the mold are also small. Since processes such as quenching and tempering are not required, the productivity is improved and the cost is reduced.

[0014] The press-formed product made of high-tensile steel sheet of the present invention, even if it is a high-tensile steel sheet, has at least a tensile strength of 2 times or more of the tensile strength before carburizing treatment or carbonitriding treatment, or the same or more as the tensile strength of an ultra-high-tensile steel sheet. Therefore, there are no chips or cracks, the dimensional accuracy is high, and trimming and other post-processing are easy. Even if the plate thickness is thin, the tensile strength can be maintained, so that the weight is reduced.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0016] (Embodiment of the press-forming method of high-tensile steel sheet) The press forming method of the high-tensile steel sheet of the present invention is to cold press form a high-tensile steel sheet having a tensile strength lower than that of an ultra-high-tensile steel sheet, and subject the press-formed product to carburizing treatment (carburizing quenching) or carbonitriding treatment (carbonitriding quenching) to make the tensile strength 2 times or more of the tensile strength before the treatment or equal to or higher than the tensile strength of the ultra-high-tensile steel sheet.

[0017] [Carburizing treatment] The carburizing treatment is a general-purpose carburizing treatment, which is a method of diffusing and infiltrating carbon on the surface of the press-formed product, and then performing quenching and tempering to improve the tensile strength of the press-formed product. In this case, the surface hardness of the press-formed product is also improved by quenching.

[0018] [Carbonitriding treatment] The carbonitriding treatment is a general-purpose carbonitriding treatment, which is a method of simultaneously diffusing and infiltrating carbon and nitrogen on the surface of the press-formed product, and then performing quenching and tempering to improve the tensile strength of the press-formed product. Also in this case, the surface hardness of the press-formed product is improved by quenching.

[0019] In either case of carburizing treatment or carbonitriding treatment, there are various carburizing treatments such as solid carburizing, liquid carburizing, drip carburizing, gas carburizing, vacuum carburizing, plasma carburizing, etc. However, the carburizing treatment and carbonitriding treatment in the present invention may be any of these carburizing treatments and carbonitriding treatments, and if possible, carburizing treatments and carbonitriding treatments under other conditions may also be used.

[0020] In the present invention, the depth of diffusion and infiltration of carbon by carburizing treatment (carburizing depth), the depth of diffusion and infiltration of carbon and nitrogen by carbonitriding treatment (carbonitriding depth), tensile strength, etc. are set according to the use of the press-formed product and the specifications required for the press-formed product. The surface hardness can be set in the same way.

[0021] For parts of automobiles, machine tools, and various industrial machines, especially for automobile parts, parts made of high-tensile steel sheets are required in terms of weight reduction, toughness, etc. The press forming method of the high-tensile steel sheet of the present invention can make the tensile strength equal to or higher than that of the ultra-high-tensile steel sheet even when the plate thickness is reduced, so it is particularly suitable for press forming of automobile parts.

[0022] (Embodiments of the carbonitriding treatment and press-formed products made of high-tensile steel sheets in the present invention) Automobile parts were formed by cold press forming using a high-tensile steel sheet of SPHC270 with a thickness of 2 mm and a high-tensile steel sheet of SPHC590 with a thickness of 2 mm, respectively. The formed press-formed product 1 was subjected to carbonitriding treatment by the carbonitriding method in the present invention using a general-purpose carbonitriding apparatus, and carbon and nitrogen were penetrated in the thickness direction from both the front and back surfaces of the press-formed product 1 (Fig. 1).

[0023] [Carbonitriding method in the present invention] An example of the carbonitriding method in the present invention is shown in Fig. 2. In the embodiment of Fig. 2, purging was performed at 880 °C for 30 minutes, carburizing was performed for 130 minutes, and then quenching was performed to 840 °C in 15 minutes, oil quenching (OQ: OIL Quench) was performed at 60 °C, and then tempering was performed at 180 °C for 90 minutes, and then air cooling (AC) was performed. The quenching to 840 °C in 15 minutes was to prevent the occurrence of distortion of the press-formed product. Although diffusion was not performed in this embodiment (Fig. 2), in the carbonitriding treatment in the present invention, diffusion can be performed as necessary, and the temperature, time, etc. can be arbitrarily set according to the purpose of the carbonitriding treatment. The press-formed products of SPHC270 and SPHC590 obtained by the carbonitriding treatment had the following characteristics.

[0024] [Carburizing depth, surface hardness] The carburizing depth and surface hardness were as shown in Table 1. For the surface hardness, three places of the press-formed product subjected to carbonitriding treatment were measured. The surface hardness at the three places had little difference and was substantially uniform.

[0025]

Table 1

[0026] [Hardness distribution] The hardness distribution after carbonitriding treatment (hardness distribution in the depth direction from the surface of the high-tensile steel sheet), and the core hardness was as shown in Table 2.

[0027]

Table 2

[0028] The tensile strength after carbonitriding treatment was as shown in Table 3.

Table 3

[0029] From Table 3, for both the press-formed products of the high-tensile steel sheet SPHC270 and the high-tensile steel sheet SPHC590, the tensile strength after carbonitriding treatment became 2 to 3 times or more that before the carbonitriding treatment. In particular, for the high-tensile steel sheet SPHC590, it was 1227 to 1320 MPa, reaching the same level as or higher than the tensile strength of ultra-high-tensile steel sheets.

[0030] [Carburizing treatment] The inventors have not conducted experiments on carburizing treatment. Basically, carburizing treatment is the same as carbonitriding treatment, and the difference lies in the presence or absence of nitrogen penetration. Therefore, effects such as the tensile strength, surface hardness, and hardness distribution after treatment are approximately the same as those in the case of carbonitriding.

Industrial applicability

[0031] The above embodiments are merely examples of the present invention. The present invention is not limited to the above embodiments, and the environment, conditions of carburizing treatment or carbonitriding treatment, the devices used for those treatments, etc. can be changed according to the thickness of the high-tensile steel sheet to be carburized, the specifications required for the press-formed product, and other conditions.

[0032] The press forming method of the high-tensile steel sheet of the present invention can be used not only for automobile parts but also for forming metal parts such as machine tools and various industrial equipment.

Explanation of symbols

[0033] 1 Press-formed product 2 Core part

Claims

1. A high-tensile steel sheet having a lower tensile strength than an extra-high-tensile steel sheet is cold-press formed, and the press-formed product is carburized or carbonitrided so as not to penetrate to the core part in the thickness direction, and a tensile strength of 2 times or more the tensile strength before the carburizing treatment or carbonitriding treatment is added to the press-formed product. A method for press-forming a high-tensile steel sheet, characterized by the above.

2. A high-tensile steel sheet having a lower tensile strength than an extra-high-tensile steel sheet is cold-press formed, and the press-formed product is carburized or carbonitrided so as not to penetrate to the core part in the thickness direction, and a tensile strength equal to or higher than that of the extra-high-tensile steel sheet is added to the press-formed product. A method for press-forming a high-tensile steel sheet, characterized by the above.

3. A high-tensile steel sheet having a lower tensile strength than an extra-high-tensile steel sheet is cold-press formed, and the press-formed product is carburized or carbonitrided so as not to penetrate to the core part in the thickness direction, and a tensile strength of 2 times or more the tensile strength before the carburizing treatment or carbonitriding treatment is added to the press-formed product, and a surface hardness equal to or higher than the surface hardness before the carburizing treatment or carbonitriding treatment is added. A method for press-forming a high-tensile steel sheet, characterized by the above.

4. A high-tensile steel sheet having a lower tensile strength than an extra-high-tensile steel sheet is cold-press formed, and the press-formed product is carburized or carbonitrided so as not to penetrate to the core part in the thickness direction, and a tensile strength equal to or higher than that of the extra-high-tensile steel sheet is added to the press-formed product, and a surface hardness equal to or higher than the surface hardness before the carburizing treatment or carbonitriding treatment is added. A method for press-forming a high-tensile steel sheet, characterized by the above.

5. A press-formed product made of a high-tensile steel sheet formed by the method for press-forming a high-tensile steel sheet according to Claim 1, wherein the press-formed product after the carburizing treatment or carbonitriding treatment has a tensile strength of 2 times or more the tensile strength before the carburizing treatment or carbonitriding treatment. A press-formed product made of a high-tensile steel sheet, characterized by the above.

6. A press-formed product made of a high-tensile steel sheet formed by the method for press-forming a high-tensile steel sheet according to Claim 2, wherein the press-formed product after the carburizing treatment or carbonitriding treatment has a tensile strength equal to or higher than that of the extra-high-tensile steel sheet. A press-formed product made of a high-tensile steel sheet, characterized by the above.

7. A press-formed product made of a high-tensile steel sheet formed by the method for press-forming a high-tensile steel sheet according to Claim 3, The press-formed product after carburizing treatment or carbonitriding treatment has a tensile strength that is 2 times or more the tensile strength before carburizing treatment or carbonitriding treatment, and has a surface hardness equal to or higher than the surface hardness before carburizing treatment or carbonitriding treatment. A press-formed product made of high-tensile steel sheet, characterized by the above.

8. A press-formed product made of high-tensile steel sheet formed by the press-forming method of the high-tensile steel sheet described in Claim 4, The press-formed product after carburizing treatment or carbonitriding treatment has a tensile strength equal to or higher than the tensile strength of the ultra-high-tensile steel sheet, and has a surface hardness equal to or higher than the surface hardness before carburizing treatment or carbonitriding treatment. A press-formed product made of high-tensile steel sheet, characterized by the above.

9. A press-formed product made of high-tensile steel sheet according to any one of Claims 5 to 8, The core part of the press-formed product after carburizing treatment or carbonitriding treatment has not been subjected to those treatments, and the core part hardness is the same as or almost the same as the core part hardness before carburizing treatment or carbonitriding treatment. A press-formed product made of high-tensile steel sheet, characterized by the above.

10. A press-formed product made of high-tensile steel sheet according to any one of Claims 5 to 8, The core part of the press-formed product after carburizing treatment or carbonitriding treatment has not been subjected to those treatments, and the hardness distribution from the surface or back surface side to the core part side decreases as it goes to the core part side, and is the same as or almost the same as the core part hardness before carburizing treatment or carbonitriding treatment. A press-formed product made of high-tensile steel sheet, characterized by the above.

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