JP5182855B2 - Resistance welded steel sheet - Google Patents

Description

The present invention relates to resistance welding of a high-tensile steel plate of 980 MPa class or higher, which has good resistance weldability and is used as a joining means for members composed of high-strength steel plates used for the purpose of reducing the weight of automobiles. .

2. Description of the Related Art Conventionally, as steel plates for automobiles, high tension has been advanced for the purpose of reducing the weight of automobiles. When resistance welding such as spot welding is performed as an adverse effect of increasing the tension of automobile parts, there is a problem that the toughness of the melt-solidified portion is lost due to rapid heating and rapid cooling, and the bonding strength is low or not stable. Therefore, by suppressing the additive element, particularly C (% by weight), joint strength is ensured by preventing toughness deterioration of the melt-solidified portion (for example, Patent Document 1).
JP 2005-21934 A

  However, suppressing the additive element, particularly the amount of C, suppresses the increase in the tensile strength of the steel sheet, and as a high-strength steel sheet for automobiles, there is a problem that the tensile strength remains at about 980 MPa class. In addition, there is an increase in strength by hot pressing. In this case, it contains about 0.2% (weight) of C, and it is quenched by heating and cooling by a mold, resulting in a material strength of about 1500 MPa. Due to the above-mentioned problems and the like, strength reliability is secured by restricting the mating material to be joined. Thus, suppressing C (weight%) makes it difficult to increase the tension of the material, and if C (weight%) is increased to some extent, the plate assembly is restricted. .

  An object of the present invention is to provide a resistance-welded steel sheet that can ensure the joint strength of a joint portion while being a high-tensile steel sheet having a material strength exceeding the 980 MPa class.

  As a result of investigations by the present inventors, even when the amount of C is added to 0.38 (wt%), the bonding strength is almost reduced by making the melt-solidified portion and the heat-affected zone into tempered martensite or tempered bainite structure. I found it not. Moreover, it was confirmed that by setting the hardness of the melt-solidified part in the range of Hv + 50 to +150 from the hardness of the base material, both the strength and toughness of the melt-solidified part can be ensured and good joint strength can be obtained. Moreover, it confirmed that welding strength was securable by suppressing addition of Mn as a material and adding Cr.

  In order to achieve the above-described object of the present invention, the present inventors have studied and found that it is effective to satisfy the following conditions.

That is,
(A) By weight, C: 0.15 to 0.38%, Si: 0.5 to 2.5%, Mn: 0.10 to 0.70%, Cr: 0.5 to 3.0% Ni: 0.1-3.0%, Cu: 0.01-3.0%, Al: 0.001-0.1%, the balance is made of iron and inevitable impurities, and the strength is 980 MPa. In the resistance-welded steel plate formed by spot welding of the base steel plate, the melt-solidified part and the heat-affected zone of the resistance-welded joint are tempered martensite or tempered bainite. A resistance-welded steel sheet.

(B) The resistance-welded steel sheet according to the above-mentioned resistance-welded steel sheet, wherein the hardness of the melt-solidified part of the resistance-welded joint is in the range of Hv: +50 to +150, based on the hardness of the base material.

In the present invention, the base steel plate used for resistance spot welding has a high tensile strength of 980 MPa or higher, but does not decrease the strength of the resistance welded portion , and exhibits an excellent effect that has never been achieved.

The reason for the limitation of the structure and hardness of the present invention and the composition of the material will be described below.
The melt-solidified zone and heat-affected zone of resistance welded joints are the parts that determine the joint strength, but the toughness and strength can be improved by making the structure of this part tempered martensite or tempered bainite as the main phase. Thus, even a steel plate containing a large amount of C can obtain good joint strength.

Furthermore, the hardness of the melted and solidified portion of the joint is Hv: +50 to +150 from the hardness of the base material, so that both strength and toughness can be achieved. Part strength can be obtained.
Further, by adjusting the composition of the material, it is possible to obtain a good weld strength while having a high strength. The reason for limiting the composition will be described below.

  C: C is the most effective component for increasing the strength. However, in order to satisfy the strength of the present invention, it cannot be satisfied if it is less than 0.15%. On the other hand, if it exceeds 0.38%, the toughness is deteriorated. It was determined to be 0.15 to 0.38 wt%.

Mn: Mn is the most important element. It is effective for reducing the austenitizing temperature and making the austenite finer, and is effective for improving the hardenability and tempering softening resistance, but less than 0.10 %. On the other hand, if the desired effect cannot be obtained, and if it exceeds 0.70 %, the toughness of the resistance welded joint may be deteriorated, so it was determined to be 0.10 to 0.70 %.

  Cr: Cr is an element effective for improving the hardenability and is an element having a strong effect of delaying softening due to tempering by dissolving in cementite. Therefore, it is necessary to contain at least 0.5% or more. Preferably, 1% or more is contained, but if added excessively, the effect is saturated and the toughness is lowered, so the upper limit was set to 3.0%.

  Si: Si is an element effective for deoxidation and strength increase. Therefore, the content including the one added as a deoxidizer and remaining in the steel is set to 0.2% or more. However, since excessive addition may cause toughness deterioration, the upper limit was set to 2.5 wt%.

  Ni: Ni is an element that lowers the austenitizing temperature and is effective for refining austenite and is effective for improving corrosion resistance. If it is less than 0.1%, the desired effect cannot be obtained, while if it exceeds 3.0%, the effect is saturated. Since it is an especially expensive element, it was determined as 0.1 to 3.0%.

  Cu: Cu is effective for strengthening and contributes to improvement of hydrogen embrittlement by fine precipitation. However, excessive addition causes deterioration of workability, so it was determined to be 0.01 to 3.0%.

  Al: Al is an element effective for deoxidation. However, excessive addition causes deterioration of workability by making inclusions, so it was determined to be 0.001 to 0.1%.

P: P is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably made 0.01%.

S: S is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is preferably made 0.01%.

  Table 1 lists the components and mechanical properties of Examples and Comparative Examples. A tensile shear test piece and a cross tensile test piece in accordance with JIS Z 3136 and JIS Z 3137 were prepared from each steel sheet shown in Table 1 by cutting, and the welded joint surface was degreased, and then the applied pressure was 4950 to 6450 kN and the energization time was 19 Spot welding was performed under conditions of ˜23 cyc and a current value of 10 to 14 kA. Test piece numbers 28 to 30 were tempered by a high-frequency heat treatment apparatus in which a coil was arranged on the outer periphery of the nugget after the spot welding.

The mechanical properties of the steel sheet were subjected to a tensile test using JIS Z 2201 No. 5 test piece. For observation of the structure of the base material, the melted part, and the heat-affected zone, the cross-section was polished and then etched with a nital solution, and the optical microscope was 100 to 1000 times and SEM observation 1000 to 5000 times.
The obtained spot-welded specimens were subjected to a tensile shear test and a cross tensile test in accordance with JIS Z 3136 and JIS Z 3137, respectively. The cross-sectional test was based on JIS Z 3139 and measured the nugget diameter.

With regard to the strength of the joint portion subjected to resistance welding, when the base material strength exceeds the 590 MPa class, TSS increases, but CTS tends not to increase. Therefore, as a comparison of effects, a comparison was made by examining the relationship of the following formula (3). As a judgment, the joint portion was evaluated by comparing the ratio of TSS and material strength and the product of CTS and material strength. (In Table 1; 590 MPa class or more: ○, 590 MPa class or less: x)
TSS / (Material strength) + CTS x (Material strength) ... Formula (3)

  It is known that resistance welding systematically changes around the melted part due to the influence of heat. As a result, the base material strength is softened to reduce the joint strength, so the presence or absence of softening (in Table 1; no softening: ○, with softening: x) is cut in the cross-sectional direction at the melt-solidified part and its periphery. After resin filling, it was mirror-polished and confirmed by measuring the hardness according to JIS Z 2244.

  The fracture form can be said to be the factor most attributable to the stability of the joint strength, and it can be said that the case of taking the form of the plug fracture described in JIS Z 3136 is a stable strength. (In Table 1, plug rupture: ○, interface rupture: x)

Claims (3)

C: 0.15-0.38%, Si: 0.2-2.5%, Mn: 0.10-0.70%, Cr: 0.5-3.0%, Ni: A base material containing 0.1 to 3.0%, Cu: 0.01 to 3.0%, Al: 0.001 to 0.1%, the balance being iron and inevitable impurities, and having a strength exceeding 980 MPa In a resistance-welded steel sheet formed by spot welding of steel sheets, the melt-solidified part and the heat-affected zone of the resistance-welded joint are tempered martensite or tempered bainite as the main phase. Resistance-welded steel sheet.   2. The resistance welded steel sheet according to claim 1, wherein the hardness of the melt-solidified part of the resistance welded joint is in a range of + Hv50 to +150 from the hardness of the base material. The resistance welded steel sheet according to claim 1 or 2 , wherein the base material is galvanized.