Method for improving tensile strength of welded products and welded products

Abstract

To provide a method for improving tensile strength of a welded product to almost same as that of a welded product of an advanced high strength steel plate, even when the welded product is a welded product obtained by welding pressed components made of a steel plate or of a high-tensile strength steel plate having low tensile strength, and to provide a welded product having tensile strength equal to that of a welded product of an advanced high strength steel plate, and to provide a welded product having tensile strength equal to that of a welded product made of advanced high tensile strength steel plate.SOLUTION: In a method for improving tensile strength of a welded product, cold press molding is performed on steel material or a high-tensile steel plate having low tensile strength, two or more press-molded products are welded to obtain a welded product, carburization processing or carburization / nitriding processing is performed on the welded product and a welded portion thereof, and consequently the tensile strength of the welded product is improved up to two times or more the tensile strength of the welded product on which carburization processing or carburization / nitriding processing is not performed, preferably up to tensile strength equal to that of the advanced tensile steel plate or more. A welded product is produced by a method in which a welded product made of two or more press molded products made of steel material or high tensile steel plates which have low tensile strength and are obtained by cold press-molding and welded portions thereof receive carburization processing or carburization / nitriding processing, consequently the welded product and the welded portion thereof have tensile strength more than twice the tensile strength of the welded product on which carburization processing or carburization / nitriding processing is not performed, preferably tensile strength more than twice the tensile strength of the advanced tensile strength steel plate.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for improving the tensile strength of a welded product and to a welded product having high tensile strength. [Background technology]

[0002] High tensile strength steel (HTSS) is harder than regular steel, has excellent mechanical strength, especially tensile strength, and can be made lighter because it maintains its strength even when the plate thickness is thin. It is also less susceptible to scratches. For these reasons, it is used in a variety of fields, including automotive parts.

[0003] High-tensile steel is a material made by adding chromium (Cr), nickel (Ni), molybdenum (Mo), etc. to low-carbon steel, and there are several types depending on the type of added elements. Because it has a low carbon content, it does not harden much due to the effects of heat when welding. It also has the advantage of being inexpensive because it is a non-tempered steel.

[0004] In recent years, there has been an international call for the reduction of carbon dioxide (CO2) emissions in an effort to improve the global environment, and the automobile industry is also being asked to reduce its CO2 emissions. As part of the automobile industry's efforts to reduce CO2 emissions, the development of hybrid vehicles (HVs), plug-in hybrid vehicles (PHEVs), and electric vehicles (EVs) is underway. This has led to a demand for lighter automobile parts in order to improve driving distance, fuel efficiency, and operability. High-tensile steel is an automobile part material that meets these demands.

[0005] There are various types of high-tensile steel plates with different tensile strengths. High-tensile steel plates for automotive parts include cold-rolled steel plates (SPCC: Steel Plate Cold Commercial) and hot-rolled steel plates (SPHC: Steel Plate Hot Commercial). High-tensile steel plates with especially high tensile strength are called ultra-high-tensile steel plates. There is no specific definition of ultra-high-tensile steel plates, but they are generally said to have a tensile strength of 980 MPa or more. In general, the tensile strength of regular steel is said to be 270 MPa or more, while the tensile strength of high-tensile steel is said to be around 340 MPa to 790 MPa.

[0006] High-tensile steel plates below SPCC780 and SPHC780, which have relatively low tensile strength, can be formed (processed) using ordinary cold presses, but ultra-high tensile steel plates have high tensile strength and are hard, making it difficult to perform press forming such as bending, drilling, and drawing using ordinary cold press forming. When bending, the press-formed product is prone to chipping and cracking (easily broken), and the formed product may return to its original shape (spring back) due to elastic recovery when the forming load is removed, which can lead to poor dimensional accuracy. When drilling holes, burrs are likely to appear. In either case, the mold is prone to wear and breakage, shortening the life of the mold. For this reason, ultra-high tensile steel plates, which have particularly high tensile strength and surface hardness, are often hot press formed (hot stamping).

[0007] In hot press forming, ultra-high tensile steel plates are heated to around 900°C before press forming, and then rapidly cooled and quenched after forming, which means that there are many work steps, which is time-consuming, slow production speed (low productivity), and high costs. In addition, since the formed products become very hard after quenching, processing such as trimming and piercing after press forming is difficult, and laser processing may be required. In this case, there are also drawbacks such as high initial costs associated with the introduction of laser processing equipment and high running costs associated with its use.

[0008] In order to solve the above problems, a method of directly water-cooling the steel sheet in the mold has been developed (Non-Patent Document 1). This method is said to be able to increase productivity three times as much as quenching at room temperature. However, this method is still a hot press forming method. As far as the applicant knows, there is no known method of improving productivity in cold press forming.

[0009] High-tensile steel and ultra-high-tensile steel have the drawback that the tensile strength (weld strength) of the heat-affected zone (HAZ) weakens due to the heat input during welding. In addition, they have poor ductility and are prone to weld cracks. If welding is done at high temperatures, there is a possibility of low-temperature cracking. Particular care must be taken to prevent weld cracks when welding with steel materials that have different characteristics. If the HAZ becomes hard, it becomes brittle, less stretchable, its toughness deteriorates, and it becomes more susceptible to brittle fracture. These drawbacks are common to all types of welding, including spot welding, arc welding, and laser welding. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2023-010222 A [Non-patent literature]

[0011] [Non-Patent Document 1] Unipres Corporation News Release dated May 20, 2013 (https: / / www.unipres.co.jp / asset / 3912 / view) Summary of the Invention [Problem to be solved by the invention]

[0012] The problem to be solved by the present invention is to provide a tensile strength improving method capable of increasing the tensile strength of a welded product obtained by welding press parts made of steel or high tensile steel plate having a weak tensile strength to the same level as a welded product made of ultra-high tensile steel plate, and to provide a welded product which is a press part made of steel or high tensile steel plate having a weak tensile strength but has a tensile strength as high as a welded product made of ultra-high tensile steel plate. The term "high tensile steel plate having a weak tensile strength" as used herein refers to a high tensile steel plate having a tensile strength of about 340 MPa to 500 MPa. In addition, the term "same level as the tensile strength of ultra-high tensile steel plate" refers to a tensile strength at least twice the tensile strength before the tensile strength is increased, preferably at the same level as or higher than the tensile strength of ultra-high tensile steel plate, which is 980 MPa (the same applies below). [Means for solving the problem]

[0013] The method for improving the tensile strength of a welded product of the present invention is a method in which two or more press parts made of cold press-formed steel plate or high tensile steel plate with weak tensile strength are welded together to form a welded product, and the welded product is carburized or carbonitrided to improve the tensile strength of the welded product to at least twice the tensile strength before the carburized or carbonitrided treatment, and preferably to the same level as or higher than the tensile strength of ultra-high tensile steel plate. The carburized or carbonitrided treatment is also performed on the welded parts of the welded product. The carburized or carbonitrided treatment referred to here refers to various general-purpose carburizing processes. Reason Or carbonitriding treatment (the same applies below).

[0014] The welded product of the present invention is a welded product obtained by welding two or more press parts made of cold press-formed steel plate or high tensile steel plate with weak tensile strength, and then carburizing or carbonitriding the welded product to have a tensile strength of at least twice the tensile strength before carburizing or carbonitriding, preferably equal to or greater than the tensile strength of ultra-high tensile steel plate. In this case, the welded parts of the welded product also have a tensile strength similar to that of the press-formed product.

[0015] The welding may be any of general-purpose spot welding, arc welding, laser welding, and the like. Effect of the Invention

[0016] The method for improving the tensile strength of a welded product according to the present invention has the following effects. (1) Since two or more press parts made of steel plate or high tensile steel plate with weak tensile strength are welded together and then carburized or carbonitrided, welding is easy and does not have the above-mentioned problems that arise when welding high tensile steel plate with strong tensile strength. (2) The carburizing or carbonitriding treatment increases the tensile strength to at least twice the tensile strength before the treatment, and preferably to the same level or higher than that of ultra-high tensile steel plate, so that the welded product has excellent tensile strength even if it is made of steel plate or high tensile steel plate with weak tensile strength. (3) Since two or more press parts made of steel plate or high-tensile steel plate with weak tensile strength are used, press forming is easy, there are few chips or cracks due to press forming, and press forming with high dimensional accuracy is possible. Since trimming and other processing can be performed by press forming before carburizing or carbonitriding, there is no need to introduce a laser processing machine, and there is no increase in initial costs and running costs. There is also little wear and damage to the mold. Since processes such as quenching and tempering are not required, productivity is improved and costs are reduced. (4) The carburizing or carbonitriding treatment is performed on the welded parts, including the welded parts, so the tensile strength of the entire welded part is uniform.

[0017] The welded product of the present invention has the following effects. (1) Although it is a welded product obtained by welding pressed parts made of steel plate or high-tensile steel plate with weak tensile strength, after welding, carburizing or carbonitriding treatment is performed to improve the tensile strength to at least twice the tensile strength before the treatment, and preferably to the same level as or greater than the tensile strength of ultra-high tensile steel plate, it is a welded product with excellent tensile strength and no defects associated with welding, as in the case of welding pressed parts made of ultra-high tensile steel plate with high tensile strength. (2) The overall tensile strength of the welded product, including the welded points, is improved, resulting in a welded product with uniform tensile strength throughout. [Brief description of the drawings]

[0018] [Figure 1] FIG. 2 is an explanatory diagram of an example of a welded product of the present invention. [Diagram 2] FIG. 2 is an explanatory diagram of the carbonitriding treatment of a welded product according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] (Embodiment of the method for improving the tensile strength of a welded product) The method for improving the tensile strength of a welded product of the present invention involves cold press forming a press-formed product of a desired shape from a steel plate or a high-tensile steel plate with low tensile strength. Two or more press-formed products are welded together to form a welded product, which is then carburized (carburizing and quenching) or carbonitrided (carbonitriding and quenching) to improve the tensile strength of the welded product to at least twice the tensile strength before the carburizing or carbonitriding treatment, preferably to the same level as or higher than the tensile strength of an ultra-high tensile steel plate. In this case, the surface of the welded portion of the welded product is also carburized or carbonitrided.

[0020] The cold press is a general-purpose cold press, and the welding may be general-purpose spot welding, arc welding, laser welding, or any other type of welding.

[0021] [Carburizing treatment] The carburizing treatment is a general-purpose carburizing treatment in which carbon is diffused and penetrated into the surface of a welded product formed by welding two or more press-formed products, and then the welded product is quenched and tempered to improve the tensile strength of the welded product. In this case, the surface hardness of the welded product is also improved by quenching.

[0022] [Carbonitriding treatment] The carbonitriding process is a general-purpose carbonitriding process in which carbon and nitrogen are simultaneously diffused and infiltrated into the surface of a welded product formed by welding two or more press-formed products, and then the welded product is quenched and tempered to improve the tensile strength of the welded product. In this case, the surface hardness of the welded product is also improved by quenching.

[0023] In both the carburizing treatment and the carbonitriding treatment, there are various types of carburizing treatment such as solid carburizing, liquid carburizing, drop type carburizing, gas carburizing, vacuum carburizing and plasma carburizing, and the carburizing treatment and the carbonitriding treatment in the present invention may be any of these carburizing treatments or carbonitriding treatments, or may be carburizing treatment and carbonitriding treatment under other conditions if possible.

[0024] In the present invention, the depth of diffusion and penetration of carbon by carburizing (carburization depth), the depth of diffusion and penetration of carbon and nitrogen by carbonitriding (carbonitriding depth), tensile strength, etc. are set according to the application of the welded product and the specifications required for the welded product. The same applies to surface hardness.

[0025] Parts for automobiles, machine tools, and various industrial equipment, particularly automobile parts, are required to be made of high tensile steel plates from the viewpoints of weight reduction, toughness, etc. According to the method for improving tensile strength of the present invention, even if the plate thickness is reduced, the tensile strength can be made equal to or greater than that of high tensile steel plates or ultra-high tensile steel plates having high tensile strength, and is particularly suitable for welding of automobile parts.

[0026] (Embodiment of Welded Products) Automotive parts were formed by cold press forming two or more press-formed products 1 (Fig. 1) that were formed, and the butted end faces (welds) 3 were welded together to form a welded product 4. A general-purpose carbonitriding device was used to perform the carbonitriding treatment on the welded product 4, and carbon and nitrogen were permeated in the thickness direction from both the front and back of the welded product 4. In this case, the surfaces of the welded parts of the welded product were also carburized or carbonitrided to improve tensile strength. Hereinafter, the front and back sides subjected to carbonitriding treatment may be referred to as the carbonitrided portion or the carbonitrided layer (the carburized portion or the carburized layer in the case of carburizing treatment).

[0027] [Carbonitriding method according to the present invention] An example of the carbonitriding method of the present invention is shown in FIG. 2. In the embodiment of FIG. 2, the material is purged at 880°C for 30 minutes, carburized for 130 minutes, quenched to 840°C in 15 minutes, oil quenched at 60°C (OQ: OIL Quench), tempered at 180°C for 90 minutes, and then air cooled (AC). The reason for quenching to 840°C in 15 minutes is to prevent distortion of the press-formed product. Although diffusion is not performed in this embodiment, the carbonitriding treatment of the present invention 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 product of SPHC270 and the press-formed product of SPHC590 obtained by the carbonitriding treatment had the following characteristics.

[0028] [Carburization depth, surface hardness] The carburized depth and surface hardness are shown in Table 1. The surface hardness of the press-formed product that had been carbonitrided was Surface (carbonitrided or carburized area) The surface hardness was measured at three points. There was no significant difference in the surface hardness at the three points, and the surface hardness was approximately uniform.

[0029] [Table 1]

[0030] [Hardness distribution] The hardness distribution (hardness distribution in the depth direction from the surface of the high tensile steel plate) and core hardness after carbonitriding treatment were as shown in Table 2.

[0031] [Table 2] The core hardness is the hardness after carbonitriding, but in the carbonitriding treatment of the present invention, carbon and nitrogen do not penetrate to the core 2 (FIG. 1), so it is the same (almost the same) as before the carbonitriding treatment. Hereinafter, the core portion 2 into which carbon and nitrogen are not permeated may be referred to as a non-carbonitrided portion or a non-carbonitrided layer (a non-carburized portion or a non-carburized layer in the case of carburizing treatment).

[0032] The tensile strength after carbonitriding is shown in Table 3. [Table 3]

[0033] As can be seen from Table 3, the tensile strength of both the press-formed products of the high-tensile steel plate SPHC270 and the press-formed products of the high-tensile steel plate SPHC590 after carbonitriding was 2 to 3 times that before carbonitriding. In particular, the tensile strength of the high-tensile steel plate SPHC590 was 1227 to 1320 MPa, which was equal to or greater than that of ultra-high-tensile steel plate.

[0034] [Carburizing treatment] Although the inventors have not conducted any experiments on carburizing treatment, carburizing treatment is basically the same as carbonitriding treatment, with the difference being the presence or absence of nitrogen penetration, and therefore the tensile strength, surface hardness, hardness distribution, etc. after treatment have approximately the same effects as those of carbonitriding treatment.

[0035] [Welded steel plate products] Although the inventors have not conducted any experiments on carburizing or carbonitriding treatments on welded products made of steel plate, the process is basically the same as when welded products made of high-tensile steel plate are carburized or carbonitrided, so that the tensile strength, surface hardness, hardness distribution, carburized depth, etc. after treatment are approximately the same as those of welded products made of high-tensile steel plate. Since steel plate has a lower tensile strength and a softer surface hardness than high-tensile steel plate, the tensile strength, surface hardness, hardness distribution, carburized depth, etc. after treatment are greater than those of welded products made of high-tensile steel plate. [Industrial Applicability]

[0036] The above embodiment is merely one example of the present invention, and the present invention is not limited to the above embodiment, and the environment, conditions, and devices used for the carburizing or carbonitriding treatment can be changed according to the plate thickness of the welded product to be carburized or carbonitrided, the specifications required for the welded product, and other conditions.

[0037] The method for improving the tensile strength of a welded product of the present invention and the welded product with improved tensile strength can be used not only for automobile parts, but also for metal parts of machine tools, various industrial equipment, etc. [Explanation of symbols]

[0038] 1 Press-molded products 2 core 3. Welding 4 Welded parts

Claims

1. A press-molded product is a welded product in which two or more press-molded products are welded together, The two or more press-formed products are each obtained by cold press forming a high-tensile steel sheet, The welded product has a carburized portion or a carbonitrided portion that is carburized or carbonitrided on both the front side and the back side, the carburized depth of the carburized or carbonitrided portion on the front and back sides is several tenths of a millimeter from each of the front and back sides, a core portion between the carburized portions or carbonitrided portions on both sides is a non-carburized portion or a non-carbonitrided portion, The tensile strength of the carburized or carbonitrided welded product is at least twice the tensile strength of the high tensile steel plate before the carburized or carbonitrided treatment, or is equal to or greater than the tensile strength of an ultra-high tensile steel plate. A welded product characterized by:

2. A press-molded product is a welded product in which two or more press-molded products are welded together, Each of the two or more press-formed products is formed by cold press forming a high-tensile steel plate having a thickness of several mm, The welded product has a carburized portion or a carbonitrided portion that is carburized or carbonitrided on both the front side and the back side, the carburized depth of the carburized or carbonitrided portion on the front and back sides is several tenths of a millimeter from each of the front and back sides, a core portion between the carburized portions or carbonitrided portions on both sides is a non-carburized portion or a non-carbonitrided portion, The tensile strength of the carburized or carbonitrided welded product is at least twice the tensile strength of the high tensile steel plate before the carburized or carbonitrided treatment, or is equal to or greater than the tensile strength of an ultra-high tensile steel plate. A welded product characterized by:

3. In the welded product according to claim 1 or 2, The carburized depth of the carburized or carbonitrided portion on the front and back sides is 0.1 mm to 0.7 mm from each of the front and back sides. A welded product characterized by:

4. In the welded product according to claim 1 or 2, The core hardness after carburizing or carbonitriding is the same or almost the same as the core hardness before carburizing or carbonitriding. A welded product characterized by:

5. The welded product according to claim 1 or 2, The surface hardness after carburizing or carbonitriding is equal to or greater than the surface hardness before carburizing or carbonitriding. A welded product characterized by:

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