JP2003049247A - Welded steel pipe having excellent hydroforming property and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded steel pipe which has excellent hydroforming properties, and to provide its production method. SOLUTION: A welded steel pipe having a composition containing, by mass, 0.03 to 0.2% C, <=2.0% Si, >1.0 to 1.5% Mn, <=0.01% S, <=0.1% Ti, <=0.1% Nb and <=1.0% Cr, and in which the contents of Al and P are controlled to the optimum ranges is used as a steel pipe stock, and is heated and soaked. The steel pipe is thereafter subjected to reduction rolling at a cumulative reduction ratio of >=35% and at a rolling finishing temperature of 500 to 900 deg.C. Thus, the welded steel pipe having a tensile strength of >=590 MPa, n×r of >=0.22 and excellent hydroforming properties is obtained. In the reduction rolling, the cumulative reduction ratio in the temperature range of the Ar3 transformation point or below is preferably controlled to >=20%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded steel pipe suitable for use as a structural member or an underbody member of an automobile, and more particularly to improving workability in hydroforming (hydroforming property).

[0002]

2. Description of the Related Art Hollow members having various cross-sectional shapes have been used as structural members for automobiles. Conventionally, as a method of manufacturing such hollow members, parts formed by press working of steel sheets are joined together. The method of joining and manufacturing by spot welding has been adopted. However, recently, hollow members for structural members of automobiles have been required to have a higher shock absorbing capacity at the time of collision, and therefore, materials having higher strength have been used. For this reason, it has become increasingly difficult to manufacture a member having no molding defect and excellent in shape and dimensional accuracy of a molded product by the conventional press molding method.

Hydroforming has recently attracted attention as a new molding method for solving such problems. Hydroforming is a molding method in which a high-pressure liquid is injected into a steel pipe to form a member having a desired shape, and the cross-sectional dimension of the steel pipe is changed by pipe expansion processing, etc.
This is an excellent molding method that can integrally mold members with complicated shapes and has the function of increasing strength and rigidity.

By the way, as a steel pipe used for this hydroforming, an electric resistance welded steel pipe made of a low-medium carbon steel sheet of C: 0.10 to 0.20% by mass, which can easily obtain strength and is inexpensive, is used. There were many.

[0005]

However, when hydroforming is performed on such an electric resistance welded steel pipe made of a low carbon steel plate, the workability of the electric resistance welded steel pipe is not good.
There was a problem that it was not possible to perform sufficient pipe expansion.
In order to solve such problems, it is conceivable to use an ultra-low carbon steel sheet having a significantly reduced carbon content as a raw material in order to improve the workability of the electric resistance welded steel pipe. However, in the case of ultra-low carbon ERW steel pipe, although the hydroforming property is good, the welding heat at the time of manufacturing the steel pipe coarsens and softens the crystal grains in the vicinity of the seam, so the deformation during pipe expansion is near the seam. There is a problem that the high ductility of the material cannot be fully exerted by concentrating locally. Therefore, there is a strong demand for a welded steel pipe having material properties and seam portion quality that can sufficiently withstand hydroforming.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to propose a welded steel pipe excellent in hydroforming and capable of withstanding severe hydroforming, and its production. To do.
Specific target properties of the welded steel pipe of the present invention are as follows: tensile strength TS is 590 MPa or more, preferably less than 780 MPa, product of n value and r value, n × r is 0.22 or more, preferably n value is 0.15 or more, or The r value is 1.5 or more.

[0007]

Means for Solving the Problems In order to achieve the above-mentioned objects, the present inventors diligently studied the composition of a welded steel pipe and the manufacturing method. As a result, a welded steel pipe having a C content in the range of 0.03 to 0.2 mass% has a cumulative diameter reduction ratio of 35% or more,
It was found that the product of n value and r value (n × r) and the hydroforming property are improved by performing reduction rolling at a rolling end temperature of 500 to 900 ° C.

The present invention has been completed by further studies based on the above findings. That is, the first aspect of the present invention is C: 0.03 to 0.2%, Si: 2.0% or less, Mn: more than 1.0% and 1.5% or less, P: 0.1% or less, and S:
0.01% or less, Cr: 1.0% or less, Al: 0.1% or less, Nb: 0.
A welded steel pipe having a composition of 1% or less, Ti: 0.1% or less, N: 0.01% or less, and the balance Fe and unavoidable impurities, and a tensile strength TS of 590 MPa or more, preferably 780 M or more.
It is a welded steel pipe excellent in hydroforming, characterized in that it is less than Pa, the product of n value and r value, and n × r is 0.22 or more, and in the first aspect of the present invention, the n value is 0.15 or more. Alternatively, the r value is preferably 1.5 or more, and in the first aspect of the present invention, in addition to the above composition, the following A
Group or Next Group B Group A: Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: 0.01% or less, one or more of group B: Ca: 0.02% or less, REM : 0.02% or less, preferably 1 group or 2 groups of 1 or 2 groups.

The second aspect of the present invention, in mass%, is C: 0.
03-0.2%, Si: 2.0% or less, Mn: 1.0% to 1.5% or less, P: 0.1% or less, S: 0.01% or less, Cr: 1.0% or less, Al: 0.1% or less, Nb: 0.1% or less, A welded steel pipe having a composition containing Ti: 0.1% or less and N: 0.01% or less is used as a material steel pipe, and after the material steel pipe is subjected to heat treatment or soaking, the cumulative reduction ratio is 35% or more, and the rolling end temperature is : 500
Welded steel pipe having a tensile strength TS of 590 MPa or more, preferably less than 780 MPa, a product of n value and r value, and n × r of 0.22 or more, which is characterized by being subjected to draw rolling at up to 900 ° C. In the second aspect of the present invention, it is preferable that the reduction rolling is a reduction rolling having a cumulative diameter reduction ratio of 20% or more in a temperature range not higher than the Ar ₃ transformation point.

In the second aspect of the present invention, in addition to the above composition, the following group A or B group A group: Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: 0.01% or less. % Or less, 1 type or 2 types or more, B group: Ca: 0.02% or less, REM: 0.02% or less, 1 type or 2 types of 1 or 2 groups are preferably contained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION First, reasons for limiting the composition of the welded steel pipe of the present invention will be explained. In the following, mass% relating to the composition will be simply expressed as%. C: 0.03 to 0.2% C is an element that contributes to an increase in the strength of steel, but if it is contained in excess of 0.2%, the formability decreases. On the other hand, 0.03
If the content is less than%, it becomes difficult to secure a desired tensile strength, and the crystal grains tend to become coarse during welding, which causes strength reduction and uneven deformation. Therefore, C is limited to the range of 0.03 to 0.2%. From the viewpoint of improving moldability, it is preferably 0.05 to 0.1%.

Si: 2.0% or less Si is an element that increases the strength of steel. From the viewpoint of increasing the strength, it is preferable to contain 0.01% or more, but 2.0% or less.
If it is contained in excess, exceeding the above range, not only the surface properties are significantly deteriorated, but also the ductility is lowered and the hydroforming property is lowered. Therefore, in the present invention, Si is limited to 2.0% or less. The content is preferably 1.0% or less.

Mn: more than 1.0% and less than 1.5% Mn is an element that improves the strength without lowering the surface properties and weldability, and is contained in an amount of more than 1.0% in order to secure the desired strength. On the other hand, when the content exceeds 1.5%, the limit expansion ratio LBR during hydroforming decreases, and the hydroforming property decreases. Therefore, Mn
Was limited to more than 1.0% and less than 1.5%.

P: 0.1% or less P is an element useful for increasing strength. From the viewpoint of increasing strength, P is preferably contained in 0.01% or more, but if it is contained in excess of 0.1%, the weldability is significantly deteriorated. To do. For this reason,
In the present invention, P is limited to 0.1% or less. It should be noted that if the strengthening with P is not required so much or the weldability becomes a problem, it is desirable to set it to 0.05% or less.

S: 0.01% or less S exists as non-metallic inclusions in steel, but the steel pipe may break (burst) at the time of hydroforming starting from these non-metallic inclusions, and the hydroforming property deteriorates. To do. For this reason, it is preferable to reduce S as much as possible, but if it is reduced to 0.01% or less, the influence on the decrease in hydroforming property is reduced.
Was limited to 0.01% or less. From the viewpoint of further improving the hydroforming property, it is preferably 0.005% or less, and more preferably 0.001% or less.

Al: 0.1% or less Al is a useful element that acts as a deoxidizing agent and suppresses coarsening of crystal grains, and it is desirable to contain 0.01% or more. However, if the content exceeds 0.1%, the amount of oxide inclusions increases and the cleanliness decreases. Therefore, Al
Was limited to 0.1% or less. From the viewpoint of reducing the number of crack initiation points during hydroforming, it is preferably 0.05% or less.

N: 0.01% or less N is an element that combines with Al to refine the crystal grains, and for this purpose, it is desirable to contain 0.001% or more.
If the content exceeds 01%, the ductility deteriorates. Therefore, N is limited to 0.01% or less. Cr: 1.0% or less Cr is an element that increases strength and improves corrosion resistance. Since such an effect becomes remarkable when the content is 0.01% or more, it is preferable to contain 0.01% or more. On the other hand, 1.0%
If it is contained in excess, the ductility and weldability deteriorate. Therefore, in the present invention, Cr is limited to 1.0% or less.

Nb: 0.1% or less, Nb is an element that effectively contributes to the refinement of crystal grains and the increase of strength when contained in a small amount. Such an effect is noticeable when the content is 0.01% or more. However, if the content exceeds 0.1%, the hot deformation resistance of the steel increases, the manufacturability is impaired, and the ductility decreases. Therefore, Nb is 0.1%
Limited to:

Ti: 0.1% or less, Ti, like Nb, is an element that effectively contributes to the refinement of crystal grains and the increase in strength. Such an effect is noticeable when the content is 0.01% or more. However, if the content exceeds 0.1%, the hot deformation resistance of the steel increases, the manufacturability is impaired, and the ductility decreases. Therefore, Ti is 0.1%
Limited to:

In the present invention, in addition to the above component composition,
Further, Group A: Cu: 1.0% or less, Ni: 1.0% or less, Mo:
1.0% or less, B: 0.01% or less, 1 or 2 or more, Group B: Ca: 0.02% or less, REM: 0.02% or less, 1 or 2 types, 1 or 2 groups Can be included. Group A: Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: 0.01% or less One or more of Cu, Ni, Mo, and B all have impaired ductility. However, it is a useful element capable of improving strength, and can be selected and contained if necessary. Such effects are
The content of Mo is 0.1% or more, and the content of B is 0.001% or more. On the other hand, Cu, Ni, and Mo are 1.0%, and B is 1.0%.
If the content exceeds 0.01%, the effect is saturated, the effect commensurate with the content cannot be expected, it is economically disadvantageous, and the hot workability and cold workability of steel are deteriorated. Therefore, C
u: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less,
B: It is preferably limited to 0.01% or less.

Group B: Ca: 0.02% or less, REM: 0.02% or less One or two types of Ca and REM each have a non-metallic inclusion in a spherical form,
It is an element effective in improving the hydroforming property, and can be selected and contained if necessary. Such effects are
REM content of 0.0020% or more is recognized. On the other hand, 0.
If the content exceeds 02%, the amount of inclusions becomes too large and the cleanliness decreases. Therefore, both Ca and REM are preferably 0.02% or less. When both Ca and REM are used together, the total amount is preferably 0.03% or less.

The balance other than the above components is Fe and inevitable impurities. The welded steel pipe of the present invention having the above-described composition has a high tensile strength TS of 590 MPa or more, preferably less than 780 MPa, and further has a product of n value and r value, n × r
A steel pipe with a high bulge formability of 0.22 or more. On the other hand, if n × r is less than 0.22, the bulge formability is insufficient. In addition, it is preferable that the n value is 0.15 or more from the viewpoint that the deformation proceeds uniformly. Further, from the viewpoint of suppressing local thinning, the r value is preferably 0.15 or more.

The welded steel pipe of the present invention has a limit expansion ratio LB
R is preferably 40% or more. Here, the limit expansion ratio LBR is defined by the following equation LBR (%) = (d _max −d _O ) / d _O × 100. Here, d _max is the maximum outer diameter (mm) at the time of burst (breaking), and d ₀ is the outer diameter before the test (m
m). d _max is a value obtained by dividing the perimeter of the burst portion by the circular constant π. The limit expansion ratio LBR in the present invention is a value obtained by carrying out a free bulge test in which axial compression is applied.

The free bulge test can be carried out, for example, by expanding the halves of the molds 2a and 2b shown in FIG. 1 by using the hydroforming apparatus having the configuration shown in FIG. FIG. 1 is a sectional view of a mold used.
Each of the upper mold 2a and the lower mold 2b has a steel pipe holding portion 3 composed of a semi-cylindrical surface substantially equal to the outer diameter d _O of the test steel pipe on both end sides in the length direction, while in the length direction. In the center,
It has a deforming portion 6 including a semi-cylindrical deforming portion 4 having a diameter d _C and a tapered deforming portion 5 having an inclination angle θ = 45 °. The length l _c of the deformed portion 6 is twice the outer diameter d _O of the steel pipe.
Further, the diameter d _C of the semi-cylindrical deformation portion 4 may be about twice the outer diameter d _O of the steel pipe.

Using the upper mold 2a and the lower mold 2b, the test steel pipe 1 is sandwiched so that the steel pipe 1 fits into the steel pipe holding portion 3 as shown in FIG. In this state, a liquid such as water is supplied from both ends of the test steel pipe 1 to the inner surface side of the test steel pipe 1 through the shaft pressing cylinder 7a to apply the hydraulic pressure P,
Freely bulge the circular cross section until it bursts. Then, the maximum outer diameter dmax at the time of bursting (breaking) is measured.

Numeral 8 is a die holder, and numeral 9 is an outer ring, both of which are for holding a steel pipe in a die. In hydroforming, there are cases where both ends of the tube are fixed and cases where a compressive force is applied from both ends of the tube (referred to as axial compression). Generally, it is possible to obtain a higher limit expansion ratio LBR by axial compression, and in the present invention, a compressive force is appropriately applied from both ends of the pipe so that a high limit expansion ratio LBR can be obtained. The compression force can be applied by applying a compression force F in the axial direction to the shaft pressing cylinders 7a and 7b in FIG.

Next, a method for manufacturing the welded steel pipe of the present invention will be described. In the present invention, the welded steel pipe having the above-mentioned composition is used as the raw material steel pipe, but the manufacturing method of the raw material steel pipe is not particularly limited. Cold or warm (or hot) roll forming or bending of steel strip into an open tube, both edges of the open tube are heated above the melting point using induction heating and squeeze roll abutting welded The electrical resistance welding method, or the solid pressure welding method in which both edges of the open pipe are heated to a solid pressure welding temperature range below the melting point by using induction heating and squeeze roll is used for abutting pressure welding, or forging welding method is used. Can also be preferably used. The strip steel used in the production of the raw material steel pipe, after smelting the steel of the above composition, slab by the continuous casting method or the ingot-segmentation method,
The slab is a hot rolled steel sheet by hot rolling, or a cold rolled steel sheet by cold rolling-annealing, a hot rolled steel sheet,
Alternatively, a cold rolled steel plate can be preferably used.

In the second aspect of the present invention, the material steel pipe as described above is first subjected to heat treatment or soaking. The condition of the heat treatment applied to the material steel pipe is not particularly limited, but 700
It is preferable to set the temperature to ˜1100 ° C. in order to satisfy the drawing rolling conditions described later. If the raw material steel pipe is manufactured warm or hot and it retains a sufficient temperature during the drawing rolling, it is sufficient to carry out a soaking treatment to make the pipe temperature distribution uniform. is there. It goes without saying that heat treatment is performed when the temperature of the raw steel pipe is low.

The material steel pipe that has been subjected to the heat treatment or soaking is then subjected to squeeze rolling. The reduction rolling shall have a cumulative reduction ratio of 35% or more. If the cumulative reduction ratio is less than 35%, n
Value and r value cannot be expected to be improved, and the workability and hydroforming property deteriorate. Therefore, in the present invention, the cumulative reduction ratio is 35
Limited to at least%. The upper limit of the cumulative diameter reduction ratio is preferably 95% from the viewpoint of suppressing the increase of the uneven thickness ratio and productivity. Incidentally, it is more preferably 35 to 90%. Further, from the viewpoint of further improving the r value, it is important to perform reduction rolling at a high reduction ratio in the ferrite region to develop a rolling texture. Therefore, it is preferable that the cumulative diameter reduction rate in the temperature range below the Ar ₃ transformation point is 20% or more.

The rolling end temperature in the drawing rolling is 500 to 90.
Set to 0 ° C. Drawing rolling temperature is less than 500 ℃ or 900 ℃
If it exceeds, the n-value and the r-value cannot be expected to be improved and the workability is deteriorated, or the limit pipe expansion ratio LBR in the free bulge test cannot be expected to be increased and the hydroforming property is deteriorated. Therefore, in the present invention, the rolling end temperature of the reduction rolling is set to 500 to 90.
Limited to 0 ° C.

For the reduction rolling, it is preferable to use a rolling mill train called a reducer in which a plurality of hole rolling mills are arranged in tandem. In the present invention, n × r is improved at a tensile strength of TS590 MPa or more and the hydroforming property of a high-strength steel pipe is remarkably improved only by subjecting the raw material steel pipe having the above-mentioned composition range to the above-mentioned reduction rolling. It is possible.

[0032]

Example A steel sheet having the composition shown in Table 1 (hot rolled steel sheet or annealed cold rolled steel sheet) is cold or warm rolled into a tubular shape to form an open tube, and then the edge end is heated by induction heating. Then, butt joint and welded steel pipe (outer diameter: 146 mm φ,
Thickness: 2.6 mm) Using these welded steel pipes as material steel pipes, the material steel pipes were subjected to drawing rolling under the conditions shown in Table 2 to obtain product steel pipes.

Tensile test pieces (JIS 12 A test pieces) were sampled from the obtained product steel pipe in the longitudinal direction, and the tensile properties (YS, TS, El), n value and r value of the product steel pipe were obtained. .
The n value is the ratio of the change in true strain to the change in true stress between 5% and 10%, that is, n = (lnσ _{10 %} −lnσ _5% ) / (lne _10%
-Lne _5% ). Note that σ is true stress and e is true strain.

The r value is defined by the ratio of the true strain of the sheet width to the true strain of the sheet thickness in the tensile test. r = ln (W _i / W _f ) / ln (T _i / T _f ), where W _{i is the} initial strip width, W _{f is the} final strip width T _{i is the} initial strip thickness, and T _{f is the} final strip width. Plate thickness However, since the plate thickness measurement involves a considerable error, the value of r is determined by the following equation assuming that the volume of the test piece is usually constant.

R = ln (W _i / W _f ) / ln (L _f W _f / L _i W _i ), where W _i : initial plate width, W _f : final plate width L _i : initial length L _f : Final length In the present invention, the r value is the longitudinal direction when a strain gauge having a gauge length of 2 mm is attached to a tensile test piece and a tensile strain of 6 to 7% is performed. The true strain and the true strain in the width direction were measured and calculated by the above formula.

Further, the product steel pipe was cut into a length of 500 mm to obtain a test body for hydroforming. This test piece is shown in Figure 2.
As shown in (1), the test piece was set in a hydroforming apparatus, and water was supplied from both ends of the test piece to cause free bulge deformation of a circular cross section to cause a burst. The maximum outer diameter d _max of the test body at the time of bursting was measured, and the limiting tube expansion ratio LBR was calculated from the following formula LBR (%) = (d _max −d _O ) / d _O × 100. Here, d _O is the outer diameter of the test body (outer diameter of the product pipe). Incidentally, the die size used was, 127 mm is l _c in FIG. 1, d _C is 127 mm, r _d is 5 mm, l _o is 550 mm, theta is the 45 °.

The results obtained are shown in Table 3.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

Each of the examples of the present invention has a tensile strength of 590 MPa or more, a high n value and a high r value, and n × r of 0.22 or more is excellent in workability and has excellent hydroforming property. . On the other hand, in the comparative example outside the scope of the present invention, n ×
It is a product tube that has low r and poor workability, low LBR and poor hydroforming, and is not suitable for hydroformed members.

[0042]

As described in detail above, according to the present invention,
It becomes possible to manufacture welded steel pipes for structural members, which are excellent in workability, and particularly excellent in hydroforming properties, at low cost without lowering productivity, and to achieve a marked industrial effect.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a mold used for a free bulge test.

FIG. 2 is a cross-sectional view showing an example of the configuration of a hydroforming apparatus used for a free bulge test.

[Explanation of symbols]

1 test body (test steel pipe) 2 mold 2a Upper mold 2b Lower mold 3 Steel pipe holder 4 Semi-cylindrical deformation part 5 Tapered deformation part 6 Deformation part 7a, 7b Axial push cylinder 8 Mold holder 9 Outer ring

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI Theme Coat (reference) C22C 38/54 C22C 38/54 (72) Inventor Masanori Nishimori 1-1, Kawasaki-cho, Handa-shi, Aichi Prefecture Made by Kawasaki Iron Co., Ltd. Chita Works (72) Inventor Yuji Hashimoto 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Pref. Iron Co., Ltd. Chita Works (72) Inventor Mitsuo Kimura 1-1 Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Co., Ltd. Chita Works (72) Inventor Akira Ito 1-1, Kawasaki-cho Handa City, Aichi Pref. Iron Co., Ltd. Chita Works (72) Inventor Nori Okabe 1-1, Kawasaki-cho, Handa City, Aichi Prefecture Kawasaki Steel Co., Ltd. Chita Works (72) Akiya Nagahama Takuya 1-1, Kawasaki-cho, Handa-shi, Aichi Kawasaki Steel Co., Ltd. Chita Works F-Term CB02 CC01 CC02 CC03 CC04

Claims (6)

[Claims] 1. In mass%, C: 0.03 to 0.2%, Si: 2.0% or less, Mn: more than 1.0% and 1.5% or less, P: 0.1% or less, S: 0.01% or less, Cr: 1.0% or less, Al A welded steel pipe containing 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, N: 0.01% or less, and a balance of Fe and inevitable impurities, and a tensile strength TS of 590 MPa or more, n
A welded steel pipe with excellent hydroforming properties, characterized in that the product of the value and r value, n × r, is 0.22 or more. 2. The n value is 0.15 or more or the r value is
It is 1.5 or more, The welded steel pipe of Claim 1 characterized by the above-mentioned. 3. The welded steel pipe according to claim 1, further comprising, in addition to the composition, one or two of the following groups A and B. One or more of group A: Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: 0.01% or less, Group B: Ca: 0.02% or less, REM: 0.02% or less One or two of 4. In mass%, C: 0.03 to 0.2%, Si: 2.0% or less, Mn: more than 1.0% and 1.5% or less, P: 0.1% or less, S: 0.01% or less, Cr: 1.0% or less, Al : 0.1% or less, Nb: 0.1% or less, Ti: 0.1% or less, N: 0.01% or less is used as a material steel pipe, and the material steel pipe is subjected to heat treatment or soaking treatment, and then cumulative shrinkage. The tensile strength TS is 590 MPa or more, the product of n value and r value, and n × r is 0.22 or more, which is characterized by performing reduction rolling at a porosity of 35% or more and a rolling end temperature of 500 to 900 ° C. A method for manufacturing a welded steel pipe having excellent hydroforming properties. 5. The method for producing a welded steel pipe according to claim 4, wherein the reduction rolling is performed by reduction rolling having a cumulative diameter reduction ratio of 20% or more in a temperature range equal to or lower than the Ar ₃ transformation point. 6. The method for producing a welded steel pipe according to claim 1, further comprising, in addition to the composition, one or two of the following groups A and B. One or more of group A: Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, B: 0.01% or less, Group B: Ca: 0.02% or less, REM: 0.02% or less One or two of