JP6465040B2 - Manufacturing method of molded member - Google Patents

Description

  The present invention relates to a method for producing a molded member having excellent delayed fracture resistance. The present invention is suitable for manufacturing automobile frame members, reinforcing members, and the like.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of conservation of the global environment. For this reason, the movement to reduce the weight of the vehicle body itself has become active due to the thinning of the vehicle body material. On the other hand, from the viewpoint of occupant protection in the event of a vehicle collision, it is also required to improve the safety of the automobile body by increasing the strength. In order to satisfy the weight reduction and strengthening of the automobile body at the same time, it is effective to increase the strength and thickness of the component material. Recently, a high strength thin steel sheet (hereinafter referred to as a tensile strength (TS) of 1180 MPa or more) The thin steel plate is also simply referred to as a steel plate) has begun to be used for automobile frame members, reinforcing members and the like.

  However, as described in Non-Patent Document 1, a high-strength steel sheet having a TS of 1180 MPa or more is caused by hydrogen entering a member formed from the steel sheet during manufacture or use of an automobile, compared to a low-strength steel sheet. This increases the possibility of delayed fracture. For this reason, application of high-strength steel sheets having a TS of 1180 MPa or more is limited.

  Further, an automobile member is usually used after being processed into a desired shape by press molding. Delayed fracture often occurs in the part where the residual stress on the surface layer of the processed part is high tensile stress after being processed by the above press forming, and the delay in the part where such high tensile stress remains. Control of destruction is important.

  Conventionally, there are techniques described in Patent Documents 1 to 5 as a method for producing a high-strength steel sheet for automobile members having excellent delayed fracture resistance.

  In Patent Documents 1 to 3, delayed fracture resistance is improved by adding elements such as Ca, Mg, Mo, and V. Patent Documents 4 and 5 attempt to improve the delayed fracture resistance by limiting the steel structure.

  Patent Document 6 discloses a technique for improving delayed fracture resistance by performing shot peening treatment on high-strength mechanical structural steel.

Japanese Patent Laid-Open No. 2003-166035 JP 2004-359974 A Japanese Patent No. 3406904 Japanese Patent No. 3424619 JP 2005-220440 A JP 7-292434 A

Yoshino, Taji, Takagi, Hasegawa: Iron and Steel, Vol. 99, (2013), p302.

  However, all of Patent Documents 1 to 3 require the addition of special elements such as Ca, Mg, Mo, and V in order to improve delayed fracture resistance. For this reason, there exists a subject that the steel plate manufacturing cost increases. Further, Patent Documents 4 and 5 need to limit the steel structure and have not led to improvement of general-purpose delayed fracture resistance. Further, as disclosed in Patent Documents 1 to 5, there is a demand for a technique for improving delayed fracture resistance based on other points of view, instead of devising the composition and structure of a steel plate as a raw material.

  The technique disclosed in Patent Document 6 is a technique for improving delayed fracture resistance by suppressing the intrusion of hydrogen by subjecting a steel material to shot peening treatment. When inter-forming is added, even if shot peening is applied to the material steel plate, it does not lead to an improvement in delayed fracture resistance of the member.

  The present invention has been made to solve the above problems. It is an object of the present invention to provide a method for producing a molded member having excellent delayed fracture resistance without severely restricting the components and structure of a high-strength steel sheet that is a raw material for cold forming.

  In order to solve the above problems, the present inventors have made extensive studies. As a result, it has been found that the cold-formed member is subjected to shot peening treatment to reduce the residual stress of the tensile force on the surface of the formed member, thereby significantly suppressing the delayed fracture resistance of the cold-formed member. Based on this finding, the inventors have further studied and have completed the present invention. The gist of the present invention is as follows.

  [1] A step of cold forming using a steel sheet having a tensile strength of 1180 MPa or more, and a step of subjecting at least a portion having a surface layer residual stress of 500 MPa or more to shot peening treatment in a formed member after cold forming, The manufacturing method of the shaping | molding member containing.

  [2] In the shot peening process, the projection speed of the projection material is 50 m / s or more, the particle size of the projection material is 0.1 mm or more and 2.0 mm or less, and the hardness of the projection material is 400 or more in terms of Hv. The manufacturing method of the shaping | molding member of description.

  [3] The method for producing a molded member according to [1] or [2], wherein a coverage in the shot peened region is 30% or more.

  [4] The method for manufacturing a molded member according to any one of [1] to [3], wherein the surface layer has a residual stress of 200 MPa or less after the shot peening treatment.

  In the present invention, the residual stress is expressed as “+” for the tensile residual stress and “−” for the compressive residual stress, and continuously expressed as the residual stress.

  According to the present invention, with respect to a molded member manufactured by forming a steel plate having a TS of 1180 MPa or more into a desired shape by cold forming, the addition of a special component or the steel structure is performed by subjecting the surface of the molded member to shot peening treatment. It is possible to provide a high-strength member excellent in delayed fracture resistance without limiting the above.

FIG. 1 is a schematic view showing a sample preparation method for a delayed fracture test in the example. FIG. 2 is a drawing showing the relationship between the residual stress on the surface layer and the time until the failure in the delayed fracture test in the examples.

  Hereinafter, embodiments of the present invention will be described.

  Cold forming is performed using a steel plate having a TS of 1180 MPa or more, and then the surface of the formed member is subjected to shot peening treatment. The shot peening treatment may be performed on the entire surface of the molded member, or a portion where the residual stress of the surface layer is 500 MPa or more may be selectively performed. The reason why the shot peening treatment is not necessarily required for the portion where the residual stress of the surface layer is less than 500 MPa is that delayed fracture is less likely to occur even if the portion where the residual stress of tension is less than 500 MPa is not subjected to the shot peening treatment. On the other hand, as will be described later, when the shot peening treatment is performed, the residual stress of the surface layer is preferably set to 200 MPa or less.

  The residual stress of the surface layer can be measured by X-ray, and is measured at an arbitrary position and an arbitrary direction of the molded member to determine a position where the residual stress of the tensile force exceeds 500 MPa. At that time, a cold press forming simulation is performed in advance by CAE (Computer Aided Engineering), and a part where the tensile residual stress becomes high is predicted from the analysis result, thereby efficiently requiring shot peening processing. Can be determined. Here, the surface layer refers to a region from the surface of the molded member to a depth at which X-rays penetrate into the surface of the molded member in X-ray diffraction, and is usually 100 μm or less from the surface.

  The steel sheet used as the material for cold forming has a TS of 1180 MPa or more. As a result, the strength and thinning of the component material are realized. TS is obtained by a tensile test using a steel plate as a material. Moreover, as for the thickness of the steel plate which is a raw material, 0.6-5.0 mm is preferable.

  As a device for performing the shot peening process, any device such as a centrifugal type or a pneumatic type may be used.

  The projection speed, material, hardness, diameter, and the like of the projection material in the shot peening process are not limited and may be performed according to a conventional method.

  For example, the projection speed is preferably 50 m / s (s representing time means seconds) or more. If the projection speed is less than 50 m / s, there is a possibility that the reduction of the residual stress on the surface layer is insufficient.

  The projection material preferably has a Vickers hardness Hv of 400 or more. When the hardness of the projection material is less than 400 in terms of Vickers hardness, the hardness of the projection material is insufficient with respect to the hardness of the molded member, and the residual stress may be insufficiently reduced. The Vickers hardness of the projection material is determined by a micro Vickers hardness test.

  The particle size of the projection material is preferably 0.1 to 2.0 mm. If the particle size of the projection material is less than 0.1 mm, the kinetic energy may be too small to obtain a sufficient effect, and if it exceeds 2.0 mm, the surface roughness may be serious or the shape of the molded member may deteriorate. There is.

  The time for the shot peening treatment is not particularly limited, but in order to obtain the effect of the present invention, the treatment is preferably performed until the coverage is 30% or more, and more preferably 70% or more. The coverage is the ratio of the area of the portion where the projection material collides after the treatment and the surface is deformed to the area before the treatment of the region where the shot peening treatment is performed.

  Delayed fracture is likely to occur under tensile stress, and the improvement in delayed fracture resistance by shot peening treatment to the tensile residual stress part is due to the reduction in tensile residual stress. It is preferable to do. Furthermore, the effect becomes remarkable because the residual stress becomes a compressive stress.

  About the steel plate used as a raw material, the tensile strength TS should just be manufactured so that it may become 1180 Mpa or more. The tensile strength TS is measured by the method described in the examples described later. The steel plate may be either a hot-rolled steel plate or a cold-rolled steel plate, and the surface may be plated with Zn, Al, or the like. Although the example of a composition and steel structure of a steel plate are described below, the steel plate used as a raw material is not limited to the following content. In the following description, the mass% of the component composition is simply described as%.

  In order to set TS to 1180 MPa or more, C is preferably 0.10% or more. On the other hand, if C exceeds 0.5%, the toughness decreases. For this reason, C is preferably 0.1% or more and 0.5% or less. The upper limit side of the content is more preferably 0.3% or less.

Preferred ranges of other elements are as follows.
Si: 3.0% or less, Mn: 0.5 to 10%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 1.5%, N: 0.02% Hereinafter, O: 0.01% or less.

Furthermore, it is possible to contain other elements as described below as required.
Ti: 0.3% or less, Nb: 0.2% or less, V: 0.5% or less, Mo: 0.5% or less, Cr: 1% or less, B: 0.005% or less, Cu: 0.00%. One or more selected from 5% or less, Ni: 0.5% or less, and Sb: 0.03% or less.

  Usually, the balance other than the above is Fe and inevitable impurities. Inevitable impurities include, for example, Sn, Zn, Co and the like. Moreover, the effect is not lost even if it contains Mg, Ca, Zr, and REM within the range of a normal steel composition.

  As a steel structure, in order to set TS to 1180 MPa or more, it is preferable that martensite (including tempered martensite) and bainite contributing to high strength be 50% or more in total. As phases other than martensite and bainite, there are ferrite, pearlite, retained austenite, and the like, and these may be appropriately included depending on the required properties of the steel sheet. In particular, ferrite and retained austenite are effective phases for improving the ductility of the steel sheet. When ductility is required in press forming, it is preferable to contain these phases in an amount of 3 to 50%. The steel structure is obtained as an area ratio by observing the cross-sectional structure of the steel sheet using a scanning electron microscope (SEM).

  The molded member produced according to the present invention is preferably used for an automobile skeleton member, a reinforcing member and the like. Parts such as center pillars, front pillars, side sills, roof rails, cross members, front side members, rear side members, bumpers, door impact beams, etc. that have a significant contribution to improving the crashworthiness of automobiles and that require a high level of strength. Application to is effective.

  Hereinafter, the present invention will be specifically described based on examples. The technical scope of the present invention is not limited to the following examples.

A steel slab having the composition shown in Table 1 (unit: mass%, balance is Fe and inevitable impurities) is manufactured by continuous casting, reheated to 1250 ° C, finish rolling temperature 850 ° C, winding temperature 600 ° C, Hot rolling was performed to a plate thickness of 2.8 mm. Further, after pickling, cold rolled to give a cold-rolled sheet having a thickness of 1.4 mm, then heated to 800-900 ° C. and held in that temperature range for 300-900 seconds,
Steel types A and B were gradually cooled to a temperature range of 600 to 750 ° C. at an average cooling rate of 10 ° C./s, then cooled to room temperature, and then tempered at 150 to 250 ° C. for 10 minutes.
Steel types C and G were cooled to 350 to 450 ° C. at an average cooling rate of 20 ° C./s, held in that temperature range for 10 minutes, and then cooled to room temperature.
Steel types D to F were cooled to 150 to 300 ° C. at an average cooling rate of 20 ° C./s, then reheated to 350 to 450 ° C., held in that temperature range for 10 minutes, and then cooled to room temperature.
Each steel plate obtained was subjected to temper rolling at an elongation of 0.2%. Using the cold-rolled steel sheet thus manufactured, the tensile strength and delayed fracture characteristics were investigated.

Details of each test method are as follows.
(Tensile test)
Using a JIS No. 5 test piece sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet, a tensile test based on JISZ2241 was performed to determine TS (tensile strength). The results are shown in Table 2.

(Delayed fracture test)
The sample preparation method for the delayed fracture test performed in this example is schematically shown in FIG. A 100 mm × 30 mm test piece taken from the cold-rolled steel sheet with the longitudinal direction perpendicular to the rolling direction was used. The test piece was sampled by machining so that the end face had a three-end finish (the peak portion is indicated by ▽). The obtained test piece was subjected to U-bending with a bending radius of 5 mm, and then the member was tightened with a bolt from the spring-backed state, and the residual stress on the surface layer was changed depending on the tightening amount. Such stress load due to bending and bolt tightening simulates cold forming and actual introduction of residual stress in the formed member in the production of the formed member. Then, the shot peening process was performed from the outer side of the bending part of the sample fastened with the bolt (see FIG. 1). As a comparison, a sample not subjected to shot peening was also evaluated. The shot peening treatment was performed on the entire surface of the sample, and under some conditions, a region where the stress was 500 MPa or more was selectively performed near the bending vertex.

  Thereafter, the surface stress (corresponding to the residual stress) in the bending deformation direction at the bending apex portion was measured by the X-ray diffraction method.

  The obtained sample was immersed in 0.1 N hydrochloric acid, and the time until cracking was measured. The determination of destruction was made visually, and the case where the specimen was immersed for 96 hours and no cracks occurred was regarded as no occurrence of destruction. The test was performed with the number of samples N = 3, and the shortest crack occurrence time was taken as the fracture time.

  Table 2 shows the time to break in each test in the presence or absence of shot peening treatment, shot peening conditions, surface residual stress, presence or absence of breakage, and breakage conditions. Residual stress is indicated by positive tensile stress and negative compressive stress.

  FIG. 2 shows the relationship between the residual stress on the surface layer and the time to failure in the delayed fracture test in this example. The arrows in the figure indicate that they were not broken even after being immersed for 96 hours. When the shot peening treatment is not performed, a high tensile stress remains on the surface layer, and fracture occurs in the delayed fracture test. On the other hand, in the example of the present invention, by performing the shot peening treatment, the tensile residual stress of the surface layer is reduced and no breakage occurs.

Claims (2)

Cold forming using a steel plate having a tensile strength of 1180 MPa or more;
A step of performing shot peening treatment at least on a portion where the residual stress of the surface layer is 500 MPa or more in the molded member after cold forming, and in the shot peening treatment, the projection speed of the projection material is 60 m / s or more. The particle diameter of the material is 0.1 mm or more and 2.0 mm or less, the hardness of the projection material is 500 or more in terms of Hv, and after the shot peening treatment, the residual stress of the surface layer at the location is −650 MPa or more and −410 MPa or less. Manufacturing method of molded member. The method for manufacturing a molded member according to claim 2 , wherein a coverage in the shot peened region is 30% or more.

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