JP2007308743A - High-tensile strength steel plate for resistance welding and method for joining the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-tensile strength steel plate having excellent resistance weldability in which the joining strength of a weld zone by resistance welding can be secured in spite of the high-tensile strength steel plate having its strength in a class of ≥1,180 MPa, and to provide a method for joining the high-tensile strength steel plates by resistance welding. <P>SOLUTION: The high-tensile strength steel plate has a composition containing 0.15 to 0.25% C and 0.1 to 1.0% Si, and in which the content of Mn is controlled to the range of 0.10 to 1.0%, 0.5 to 3.0% Cr and 0.01 to 2.0% Mo are added, and further, the ratio of Mn/(Mn+Cr) is controlled to <0.50, so as to precipitate metal carbide. More preferably, P and S as impurity components are suppressed to, by mass, ≤0.02% and ≤0.01%, respectively, and further, 0.1 to 3.0% Ni, 0.01 to 3.0% Cu, 0.001 to 0.1% Al and at least one element selected from 0.01 to 1.5% W, 0.001 to 1.0% V, 0.001 to 1.0% Ti, 0.001 to 1.0% Nb, 0.01 to 1.0% Ta and 0.001 to 1.0% B are added. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-tensile steel plate of 1180 MPa class or higher that is suitably used for reducing the weight of a vehicle body, for example, as a steel plate for automobiles, and in particular, high tensile strength for resistance welding that is excellent in resistance weldability as its joining means. The present invention relates to a joining method by resistance welding between a steel plate and the high-tensile steel plate.

In steel sheets for automobiles, high tension has been increasing for the purpose of reducing the weight of the car body. However, as a negative effect of increasing the tension in such steel sheets for automobiles, when resistance welding such as spot welding is performed, In some cases, the toughness of the melted portion is lost due to rapid heating or rapid cooling, resulting in a decrease in bonding strength or a failure to obtain the desired strength stably.
Therefore, in conventional high-strength steel sheets for automobiles, resistance weldability is ensured particularly by providing an upper limit for the C content (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 03-180445

  However, among various additive elements, in particular, suppressing the C content also suppresses the increase in the tensile strength of the steel sheet. Therefore, as a high-tensile steel sheet for automobiles, the actual situation is that it remains at about 980 MPa class. However, there is a problem that it is difficult to obtain a higher tension than this.

  The present invention has been made in view of such a situation in conventional high-strength steel sheets for automobiles. The object of the present invention is to join welds by resistance welding while having a strength of 1180 MPa class or more. An object of the present invention is to provide a high-tensile steel plate that can ensure strength and has excellent resistance weldability, and a joining method by resistance welding of such a high-tensile steel plate.

  As a result of repeating earnest studies to solve the above problems, the inventors of the present invention increased the addition amount of C, added Cr and Mo, while suppressing the addition amount of Mn, and further added the metal carbide into the steel sheet substrate. The inventors have found that the object can be achieved by precipitation, and have completed the present invention.

  This invention is based on the said knowledge, Comprising: The high strength steel plate for resistance welding of this invention has the tensile strength of 1180 Mpa or more, C: 0.15-0.40 mass%, Si: 0.1 1.0% by mass, Mn: 0.10 to 1.0% by mass, Cr: 0.5 to 3.0% by mass, Mo: 0.01 to 2.0% by mass, balance Fe and inevitable impurities The ratio of the content of Mn and Cr [Mn / (Mn + Cr)] is less than 0.50, and metal carbide is precipitated, and if necessary, impurity components P and S is suppressed to 0.02% by mass or less and 0.01% by mass or less, respectively, Ni: 0.1 to 3.0%, Cu: 0.01 to 3.0%, Al: 0.001 to 0 0.1%, W: 0.01 to 1.5%, V: 0.001 to 1.0%, Ti: 0.001 At least one element selected from the group consisting of 1.0%, Nb: 0.001 to 1.0%, Ta: 0.01 to 1.0%, and B: 0.001 to 1.0%. It is characterized by containing.

  In the joining method of the present invention, the high-tensile steel sheet for resistance welding of the present invention is resistance-welded, preferably cooled thereafter, reheated to a temperature range of Ac3 point to Ac3 point + 100 ° C., and further cooled. Then, it is tempered in a temperature range of 150 ° C. to Ac1 point.

  The high-strength automotive member of the present invention is composed of the above-described high-strength steel sheet for resistance welding of the present invention, and is characterized by being joined by the joining method of the present invention.

  According to the present invention, by mass ratio, the C content is 0.15 to 0.40%, the Si content is 0.1 to 1.0%, and the Mn content is 0.10 to 1.0. % Within the range of 0.5% to 3.0% Cr and 0.01% to 2.0% Mo, and the Mn / (Mn + Cr) ratio is less than 0.50. Since metal carbide is precipitated in the base, the substantial amount of C acting on the weld during resistance welding is reduced, so that a high tensile strength of 1180 MPa or more can be obtained as a high-strength steel sheet, and at the same time, the C content Even if it increases, the intensity | strength of a resistance weld part can be stabilized and resistance weldability can be ensured.

  Below, the high-strength steel sheet for resistance welding of the present invention and the joining method thereof will be described in more detail together with the action of each alloy component and the reasons for limiting the numerical values thereof. In the present specification, “%” means mass percentage unless otherwise specified.

In the present invention, by controlling the addition amount of Mn, it is found that even if C is added to 0.25%, the bonding strength is hardly lowered, but there is a concern that the hardenability is lowered by the reduction of Mn. Therefore, in addition to Mn as an additive element, Cr is added as an element that contributes to ensuring the strength of resistance welds, and by adding Mo, the toughness of resistance welds is remarkably improved. It is possible to achieve both the strength of the steel sheet and the strengthening of the steel sheet.
However, if the C content in the steel exceeds 0.25%, the toughness of the melted part decreases and the strength of the resistance welded part decreases, so that metal carbides, particularly alloy carbides, in the steel sheet before welding are precipitated. Therefore, when resistance welding is performed, the C content that contributes substantially can be suppressed, and even when the C content is added to 0.40%, the bonding strength is hardly lowered.

  Below, the reason for limitation of each component element in this invention is demonstrated.

C: 0.15-0.40%
C is the most effective component for increasing the strength of the steel sheet. In the present invention, C needs to be added within a range of 0.15% or more and 0.40% or less in order to obtain a desired strength, preferably About 0.2 to 0.3%, more preferably about 0.2 to 0.25% is added.
That is, when the C content is less than 0.15%, the desired strength cannot be ensured. On the other hand, when the C content exceeds 0.40%, the toughness of the resistance welded portion is deteriorated. , 0.15 to 0.40% of range.

Si: 0.10 to 1.0%
Si is an element effective for deoxidation and strength increase. To obtain such an effect, it is necessary to add 0.1% or more.
On the other hand, if the Si content exceeds 1.0%, toughness deterioration may occur, so the Si content is specified in the range of 0.10 to 1.0% in the present invention.

Mn: 0.10 to 1.0%
Mn is the most important component in the present invention, and is an element effective in reducing the austenitizing temperature and making the austenite finer and improving the hardenability and temper softening resistance. , Not less than 0.10 and not more than 1.0%, preferably 0.1 to 0.5%, more preferably about 0.1 to 0.3%.
That is, if the Mn content is less than 0.1%, the desired effect cannot be obtained. On the other hand, if the Mn content exceeds 1.0%, the toughness of the resistance welded portion may be deteriorated. It is specified in the range of -1.0%.

Cr: 0.5 to 3.0%
Cr is an element effective for improving hardenability, and is an element that has a strong effect of delaying softening due to tempering by dissolving in cementite. Therefore, in order to obtain such an effect, it is necessary to contain at least 0.5%, preferably 1% or more. However, if the Cr content exceeds 3.0% and is added excessively, In addition to saturation of the effect, the toughness is lowered, so in the present invention, it is specified within the range of 0.5 to 3.0%.

Mo: 0.01 to 2.0%
Mo is also an important element in the present invention, and is an effective component for hydrogen substitution as well as forming alloy carbides to make fine crystal grains.
However, when the addition amount of Mo is less than 0.01%, alloy carbide cannot be formed, and thus such an effect cannot be obtained. On the other hand, Mo is an expensive alloy element. It is specified in the range of 0.01 to 2.0%.

Mn / (Mn + Cr) <0.50
Mn and Cr are added within the above ranges, respectively, but within this range, it is necessary to make the Mn content smaller than the Cr content. That is, when this ratio is 0.50 or more, the toughness of the resistance welded portion is deteriorated, which is not preferable.

Ni: 0.1 to 3.0%
Ni is an element that lowers the austenitizing temperature and is effective in refining austenite and is effective in improving corrosion resistance. However, if its content is less than 0.1%, the desired effect cannot be obtained. Even if added over 3.0%, the effect tends to be saturated.
Therefore, when adding Ni, since it is also an especially expensive element, the content is specified in the range of 0.1 to 3.0% in the present invention.

Cu: 0.01 to 3.0%
Cu is effective for strengthening the structure and is an element that contributes to suppression of hydrogen embrittlement by fine precipitation, so 0.01% or more is added as necessary. However, excessive addition causes deterioration of workability. Therefore, when Cu is added, the content is specified in the range of 0.01 to 3.0% in the present invention.

Al: 0.001 to 0.1%
Since Al is an element effective for deoxidation, 0.001% or more is added as necessary. On the other hand, excessive addition generates inclusions and causes deterioration of workability. Therefore, in the present invention, even if Al is added, the content is specified in the range of 0.001 to 0.1%.

P: 0.02% or less P is an element to be removed as much as possible in order to reduce the grain boundary strength. Therefore, the upper limit is preferably set to 0.02%.

S: 0.01% or less S, like P, is an element to be removed as much as possible in order to reduce the grain boundary strength, and the upper limit is desirably set to 0.01%.

W: 0.01 to 1.5%
V: 0.001 to 1.0%
Ti: 0.001 to 1.0%
Nb: 0.001 to 1.0%
Ta: 0.01 to 1.0%
B: 0.001 to 1.0%
These elements all have the same effect as Mo, and are components that contribute to the refinement of crystal grains by forming carbides. Therefore, these elements can be used alone or in combination of two or more. It can add within the said range.
In addition, when adding 2 or more types of elements, when the total amount of them becomes excessive, it becomes inclusions and tends to cause toughness deterioration. Therefore, it is desirable to keep the total amount of these elements within 5% by mass.

  The high-strength steel sheet for resistance welding of the present invention has the above-described chemical components, but by applying a quenching and tempering treatment, the base structure after tempering has an average prior austenite grain size of 3 to 10 μm, preferably 3 to 3 μm. A tempered martensite structure, a bainite structure, or a tempered bainite structure that is 5 μm or less.

  That is, if the average prior austenite grain size of the base structure is less than 3 μm, toughness deterioration may occur. On the other hand, when it exceeds 10 μm, the strength may be lowered.

  In addition, about the magnitude | size of the said metal carbide | carbonized_material or an alloy carbide | carbonized_material, and its formation density, it can control by controlling tempering temperature and time.

FIG. 1 is a schematic diagram showing a stress-strain diagram by a tensile test using a plate-like test piece (for example, a No. 5 test piece or a No. 13 test piece defined in JIS Z2201). The difference between the tensile strength (TS) and the breaking stress is defined as the degree of stress reduction (SD).
In the above stress-strain diagram, those exhibiting uniform elongation and having a value of a stress reduction (SD) of 180 MPa or more until rupture had good toughness.

The high-strength steel sheet for resistance welding according to the present invention can be galvanized, whereby the steel sheet can be made into a galvanized steel sheet having excellent rust prevention properties.
For such zinc plating, for example, methods such as electroplating, hot dipping, and thermal spraying can be applied, and the plating method is not particularly limited.

  The high-strength steel sheet for resistance welding of the present invention is excellent in resistance weldability, and can be assembled into various machine parts, for example, automotive parts by resistance spot welding or resistance seam welding, etc. It can contribute to weight reduction.

  And about the resistance welding part of the said high strength steel plate for resistance welding, the resistance welded fusion | melting part can be again heated to the temperature range of Ac3 point-Ac3 point +100 degreeC, and it can be made to cool, thereby, resistance The joint strength of the welded portion can be made more stable.

  Furthermore, after reheating and cooling the molten part by resistance welding, it can be tempered in a temperature range of 150 ° C. to Ac1 point, thereby making the molten part more preferable in terms of toughness, The joint strength can be made more stable.

  EXAMPLES Hereinafter, although this invention is further explained in full detail based on an Example, this invention is not limited to these Examples.

  Using each steel sheet having a thickness of 2.0 mm having chemical components and mechanical properties shown in Table 1, after holding in a furnace at 570 to 650 ° C. for 1 hour, cooling with water and conforming to JIS Z 3136 and JIS Z 3137 The tensile shear test piece and the cruciform tensile test piece were prepared by cutting, and the weld joint surface was degreased, and then spot welding was performed under the conditions of a pressure of 4950 to 6450 kN, an energization time of 19 to 23 cyc, and a current value of 10 to 14 kA. went.

  At this time, the mechanical properties of the steel sheet were subjected to a tensile test using a No. 5 test piece specified in JIS Z 2201, and the microstructure of the base material, the melted part and the heat-affected part was observed after polishing the cross section, Etching was performed, and observation at 1000 to 5000 times with SEM was performed together with observation at 100 to 1000 times with an optical microscope. The prior austenite particle size was measured according to JIS G 0551. The precipitation form of the metal carbide was controlled by the tempering temperature, and all the tempering times were 1 hour.

  The obtained spot welded test pieces were each subjected to a tensile shear test and a cross tensile test in accordance with the above-mentioned standards. The cross-sectional test was based on JIS Z 3139 and measured the nugget diameter.

Regarding resistance weld strength, when the base metal strength exceeds the 590 MPa class, TSS (tensile shear strength) increases, but CTS (cruciform tensile strength) tends not to increase.
Therefore, it is necessary to compare CTS as the contact strength of the structure. However, since TSS and CTS are loads, by dividing by the nugget area (π · (ND / 2) ² , ND = nugget diameter), TSS ′: tensile shear stress, CTS ′: cross tensile stress, and joint strength In order to compare these values, evaluation was made with a value CTSS obtained by adding the CTS components of TSS ′ and CTS ′. That is, TSS ′, CTS ′, and CTSS are calculated by the following equations.
Tensile shear stress: TSS ′ = TSS / π · (ND / 2) ²
Cross tensile stress: CTS ′ = CTS / π · (ND / 2) ²
CTSS = CTS ′ + TSS ′ ^{(sin θ)} , θ = 30 °

It is also known that resistance-welding forms a heat-affected zone that changes systematically under the influence of welding heat, which softens the base metal and lowers the joint strength. Therefore, the melted part by resistance welding and its periphery were cut in the plate thickness direction, embedded in resin, mirror-polished, measured for hardness according to JIS Z 2244, and confirmed for softening.
The results are also shown in Table 1 together with “O” for those without softening and “X” for those with softening confirmed.

  About the fracture form of the resistance weld, it can be said that it is the factor that has the most influence on the stability of the joint strength, and when taking the form of the plug fracture described in JIS Z 3136 is a stable strength, Table 1 shows the case of plug rupture as “◯” and the case of interface rupture as “x”.

  Each performance after spot welding of the steel sheet of the present invention includes delayed fracture resistance, joint strength, presence / absence of softening of the base material around the molten part that affects the joint strength, fracture forms that cause variations in joint strength, It has excellent performance, and the tensile strength of the base material is 1180 MPa class or higher. The high strength steel sheet for resistance welding according to the present invention enables both high strength and resistance weldability, and has an unprecedented performance. It was confirmed that

  In addition, when the resistance welding portion using the steel plate of Example 1 was reheated to 800 ° C. by high-frequency heating and cooled, a heat history corresponding to the tempering treatment was simply given. Since it had a martensite structure with an average particle diameter of 3 μm and each performance was equivalent or better, it was estimated that the film had high toughness.

The schematic diagram which shows the stress-strain diagram by the tension test using a plate-shaped test piece (For example, No. 5 test piece prescribed | regulated to JISZ2201, and No. 13 test piece).

Claims (10)

By mass ratio, C: 0.15 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.0%, Cr: 0.5 to 3.0%, Mo: Containing 0.01 to 2.0%, consisting of the balance Fe and inevitable impurities, and having a tensile strength of 1180 MPa or more, the contents of Mn and Cr satisfy the following formula (1), and metal carbide is precipitated. A high-strength steel sheet for resistance welding.
Mn / (Mn + Cr) <0.50 (1) By mass ratio, C: 0.15 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.10 to 1.0%, P ≦ 0.02, S ≦ 0.01, Cr : 0.5-3.0%, Ni: 0.1-3.0%, Cu: 0.01-3.0%, Al: 0.001-0.1%, W: 0.01- 1.5%, V: 0.001 to 1.0%, Ti: 0.001 to 1.0%, Nb: 0.001 to 1.0%, Ta: 0.01 to 1.0%, and B : It contains at least one selected from the group consisting of 0.001 to 1.0%, consists of the balance Fe and inevitable impurities, has a tensile strength of 1180 MPa or more, and the contents of Mn and Cr are represented by the following formula ( A high-tensile steel sheet for resistance welding characterized by satisfying 1).
Mn / (Mn + Cr) <0.50 (1)   The base structure is any one of a tempered martensite structure, a bainite structure, and a tempered bainite structure, and its average prior austenite grain size is 3 to 10 µm and tempered at 550 to 650 ° C. Or the high-tensile steel plate for resistance welding of 2.   The high-tensile steel sheet for resistance welding according to claim 3, wherein the average prior austenite grain size of the base structure is 3 to 5 µm.   Zinc plating is given, The high-tensile steel plate for resistance welding as described in any one of Claims 1-4 characterized by the above-mentioned.   The joining method characterized by carrying out resistance welding of the high strength steel plate for resistance welding of any one of Claims 1-5.   The welding method according to claim 6, wherein after the resistance welding, cooling is performed, and cooling is performed by reheating to a temperature range of Ac3 point to Ac3 point + 100 ° C.   The joining method according to claim 6 or 7, wherein after reheating and cooling, tempering is performed in a temperature range of 150 ° C to Ac1 point.   A high-strength automotive member comprising the high-strength steel plate for resistance welding according to any one of claims 1 to 5.   A high-strength automotive member, which is joined by the joining method according to any one of claims 6 to 8.

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