JPH07268561A - High strength stainless steel excellent in hot workability and free from welding softening - Google Patents

Abstract

PURPOSE:To impart excellent hot workability to a stainless steel and to prevent the generation of edge cracking caused by delta ferrite at the time of hot rolling by adding B to a high strength stainless steel. CONSTITUTION:This stainless steel has a compsn. contg., by weight, <=0.10% C, <=4.5% Si, <=5.0% Mn, <=0.04% P, <=0.020% S, 10.0 to 17.0% Cr, 3.0 to 10.0% Ni, <=0.10% N and 0.0010 to 0.0100% B, and the balance Fe with inevitable impurities, and in which the value of Nieq defined by the formula I is regulated to 13.0 to 17.5 and the value of deltaC expressed by the formula II lies from -20 to 20. If required, one or more kinds among Cu, Mo, Co or/and Ti, Nb and V are added thereto. This steel is subjected to inverse transforming treatment in the temp. range of 575 to 750 deg.C to form its structure into a composite phase one in which a fine austenitic phase is dispersed into a martensitic phase at a room temp., and it 15 used at >=1000N/mm<2> tensile strength.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural material such as a vehicle structure or a building, and by adding an appropriate amount of B, it is possible to prevent edge cutting during hot rolling. High strength stainless steel with excellent welding softening.

[0002]

2. Description of the Related Art Structures such as vehicle structures and buildings have been promoted to be lightweight due to high strength and thinness. The properties required to achieve this purpose include high yield strength, tensile strength, ductility for bending the shape as a structure member, correction of thermal strain during welding, and welding during welding. There is little decrease in the strength of the welded part due to the welding heat input. To meet these demands, for example, Japanese Patent Laid-Open No. 63-21
Japanese Patent No. 0234 discloses a method for producing high-strength stainless steel having excellent workability and no weld softening, C: 0.10% or less, N: 0.10% or less, Si: 4.5% or less, Mn:
5.0% or less, Cu: 1.0% or less, Ni: 3.0 to 1
0.0%, Cr: 10 to 17.0%, composition consisting of balance Fe and impurities, and Nieq component limited by limited formula, heat treatment temperature and time limited, excellent workability, no welding softening A method of making high strength stainless steel is disclosed.

Japanese Patent Application Laid-Open No. 63-2102 proposed here.
The heat treatment condition in JP-A-34-34 is that a heat treatment is performed within a temperature range of 575 to 750 ° C. for less than 60 minutes to cause reverse transformation from a martensite phase to an austenite phase, and at room temperature, a martensite phase and a fine grain are formed. By forming a multi-phase structure of austenite phase (reverse transformation treatment), it is possible to obtain high-strength stainless steel having excellent workability and no weld softening.

[0004]

However, the conventional materials used for the above heat treatment have the following problems. It has excellent mechanical properties such as proof stress and tensile strength, as well as workability, and can be given high weld softening resistance, but continuous casting during the production of heat treated material steel containing a large amount of ferrite-forming elements such as Si. When the slab is hot-rolled, cracks may occur from the edge of the hot-rolled material starting from δ ferrite, which may reduce the yield.

From the above, a crack generated during hot rolling is removed after hot rolling, a method of heating for a long time before hot rolling in order to reduce the amount of δ ferrite, or a reduction in hot workability. It was necessary to take measures such as reducing the amount of S, which influences to 10ppm or less, at the time of dissolution, and the cost for this was great. The present invention solves such cracks during hot rolling and provides a high-strength stainless steel excellent in hot workability and free from welding softening.

[0006] An object of the present invention is to solve the problem of edge cutting that occurs from δ ferrite during hot rolling, and it is an object of the present invention to provide a high-strength stainless steel which is excellent in hot workability and does not cause welding softening.

[0007]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the inventors have made a detailed study and as a result, have found that if an appropriate amount of B is added, it has excellent hot workability. As a result of intensive studies based on this knowledge, the present invention has been completed.

1. % By weight, C: 0.10% or less,
Si: 4.5% or less, Mn: 5.0% or less, P: 0.
04% or less, S: 0.020% or less, Cr: 10.0
.About.17.0%, Ni: 3.0-10.0%, N: 0.
10% or less, B: 0.0010 to 0.0100%, the balance consisting of Fe and unavoidable impurities, and defined by Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) Nieq value is 13.0 to 17.5 δc = -87-182 (C + N) + 16Si-3.0Ni + 5Cr Hot workability with δc value in the range of -20 to 20 High strength stainless steel with excellent welding softening.

2. % By weight, C: 0.10% or less,
Si: 4.5% or less, Mn: 5.0% or less, P: 0.
04% or less, S: 0.020% or less, Cr: 10.0
.About.17.0%, Ni: 3.0-10.0%, N: 0.
10% or less, B: 0.0010 to 0.0100%, and the total of one or more of Cu, Mo, and Co is 4.
Nieq value of 0% or less, balance of Fe and inevitable impurities, and Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) + Cu + Mo + 0.2Co Is defined as 13.0 to 17.5 δc = -87-182 (C + N) + 16Si-3.0 (Ni + Cu + Co) + 5Cr and the value of δc is in the range of -20 to 20 High-strength stainless steel with excellent workability and no welding softening.

3. % By weight, C: 0.10% or less,
Si: 4.5% or less, Mn: 5.0% or less P: 0.0
4% or less, S: 0.020% or less, Cr: 10.0%
.About.17.0%, Ni: 3.0-10.0%, N: 0.
10% or less, B: 0.0010 to 0.0100%, and the total of one or more of Ti, Nb, and V is 1.
0% or less, the balance consisting of Fe and unavoidable impurities, and the value of Nieq defined by Nieq = Ni + Mn + 0.5Cr + 0.3Si is 13.0 to 17.5 δc = -87 + 16Si. High-strength stainless steel with excellent hot workability and no weld softening, with a δc value defined by -3.0 (Ni + Cu + Co) + 5Cr in the range of -20 to 20.

4. % By weight, C: 0.10% or less,
Si: 4.5% or less, Mn: 5.0% or less P: 0.0
4% or less, S: 0.020% or less, Cr: 10.0%
.About.17.0%, Ni: 3.0-10.0%, N: 0.
10% or less, B: 0.0010 to 0.0100%, and the total of one or more of Cu, Mo, and Co is 4.
0% or less, 1.0% or less of the total of one or more of Ti, Nb, and V, the balance consisting of Fe and inevitable impurities, and Nieq = Ni + Mn + 0.5Cr + 0.3Si + The Nieq value defined by Cu + Mo + 0.2Co is 13.0 to 17.5 δc = -87 + 16Si-3.0 (Ni + Cu + Co) + 5Cr and the δc value is -20 to 20. High strength stainless steel with excellent hot workability and no welding softening in the range of.

[0012]

The chemical composition of the present invention as described above will be described as follows.

C: C is an austenite forming element,
It is effective for forming austenite phase at high temperature when performing reverse transformation treatment, and is effective for strengthening austenite phase and martensite phase after reverse transformation treatment. 0.10% is made the upper limit because it deteriorates the corrosion resistance.

N: N, like C, is an austenite forming element, is effective in forming an austenite phase at high temperatures, and is effective in increasing and strengthening the strength of the austenite phase after reverse transformation treatment, but if too much, Since the elongation rate is lowered, 0.10% is made the upper limit.

Si: Si is effective in strengthening the martensite phase and austenite phase after reverse transformation treatment, but if too much, it promotes solidification cracking during solidification or welding, and significantly impairs hot workability. Therefore, the upper limit is 4.5%.

Mn: Mn is an austenite forming element and is an element necessary for adjusting the Ms point, but if it is too much, it will be a harmful effect during steelmaking, so 5.0% is made the upper limit.

Cr: Cr is a basic component which imparts corrosion resistance. When it is less than 10%, it has no effect. On the other hand, when it exceeds 17%, austenite single phase or a metal structure mainly composed of austenite is obtained at high temperature. Since a large amount of forming element is required, and as a result, a desired structure cannot be obtained at room temperature, the upper limit is 17.0%.

Ni: Ni is an austenite-forming element, and is an element necessary for adjusting the amount of austenite at room temperature cooling after the austenitization of the metal structure at high temperature and the reverse transformation treatment. The required Ni content differs depending on the contents of other elements. Austenitization and Ms at high temperature
At least about 3% is required for the point adjustment, but even if the amount of other components is reduced, if Ni exceeds about 10%, the desired structure cannot be obtained.

P: P is an unavoidable impurity mixed from raw materials and auxiliary raw materials during smelting, but if a large amount is contained, the steel becomes brittle, so 0.04% is made the upper limit.

S: S is also an unavoidable impurity mixed from raw materials and auxiliary raw materials during smelting, but if contained in a large amount, it embrittles steel and impairs hot workability, so 0.020% is added. The upper limit.

B: Most important in the present invention,
As an example of the results of the investigation of the effect of B on the hot workability, Table 1 shows the composition of steel and the amount of edge cutting in hot rolling. δc of about 11 B-free steel and B of 0.000
4, 0.0010, 0.0048, 0.0098, 0.
It is a steel added with 0176%. B addition amount 0.0004%
In the hot rolling, edge cutting occurred from the end of the hot rolled material, and when 0.0010% or more was added, edge cutting was not observed. Therefore, the lower limit is 0.0010%. This B
Although the reason for the effect of improving hot workability is not always clear at this point in time, the influence of B on the amount of δ ferrite and the size of δ ferrite is shown in FIGS. 1 and 2. As can be seen from the above, the amount and size of δ-ferrite significantly change even with the addition of 0.0010%, and
At 100% or more, the change is saturated. On the other hand, a large amount of B
Since the addition causes adverse effects such as the occurrence of surface defects,
The upper limit of B is 0.0100%.

[0022]

[Table 1]

Cu: Cu is an element which is originally effective in improving the corrosion resistance, but is effective in lowering the Ms point in the present invention. If it exceeds about 4.0%, the hot workability is significantly impaired, so the upper limit is 4%.

Mo: Mo is also effective in improving the corrosion resistance, increasing the strength of austenite, and lowering the Ms point, but since it is expensive, it raises the price of steel materials, so 4.
Limited to 0%.

Co: Co is an effective component that does not excessively lower the Ms point, despite its large austenitizing effect in the high temperature range. It is a very effective element for adjusting the composition of a system having a large Cr content, but if it is too much, it will increase the cost of steel, so it is limited to 4.0%.

All of the above three elements of Cu, Mo and Co are effective for adjusting the martensite forming ability in relation to other components while improving the corrosion resistance. Equivalent in this sense.

Ti: Ti is a carbonitride forming element, and is an element effective for suppressing the generation of a Cr-deficient layer due to the precipitation of Cr carbide during welding and suppressing the growth of austenite phase crystal grains. The upper limit is 1.0% because it may cause flaws or scum formation during welding.

Nb: Nb is an element effective in suppressing the generation of a Cr-depleted layer due to the precipitation of Cr carbide during welding and in suppressing the growth of austenite phase crystal grains, but if it is too much, it causes solidification cracking during casting or welding. The upper limit is 1.0% because it not only accelerates but also impairs the ductility of the material.

V: V is an element effective for suppressing the generation of a Cr-depleted layer due to the precipitation of Cr carbide during welding and suppressing the growth of austenite phase crystal grains, but if too much V impairs the ductility of the material, it is 1.0 % Is the upper limit.

The above three elements of Ti, Nb, and V are carbide and nitride forming elements, and both suppress the grain growth of austenite, and their effects are remarkable. In that sense, they are equivalent.

Even within the composition range of the steel of the present invention, it is defined as Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) + Cu + Mo + 0.2Co which is an index of the amount of austenite. Steel having a nickel equivalent value of less than 13 has a too high Ms point, and is subjected to heat treatment as shown in the method for producing high-strength stainless steel excellent in workability and free from welding softening proposed in JP-A-63-210234. Also fails to achieve the desired high elongation. Further, in the case of steel having this value larger than 17.5, the welded part is softened when subjected to heat history such as welding, and the desired high strength member cannot be obtained. The formula for the nickel equivalent value is
In consideration of the contribution of each component element to the transformation, a coefficient is determined based on the austenite forming ability of Ni and converted into the amount of Ni.

The value of δc defined by δc = -87-182 (C + N) + 16Si-3.0 (Ni + Cu + Co) + 5Cr, which is an index of the amount of δ ferrite in the cast slab, is -20. In the following cases, since there are many austenite phases at the time of hot rolling, the edge cutting at the time of hot rolling is not recognized even if B is not added. Therefore, it is not necessary to add B and the lower limit was made -20. Further, when the value of δc is 20 or more, a large amount of δ ferrite, which is a starting point of edge cutting, is precipitated and the improvement effect by the addition of B does not occur, so the upper limit was made 20.

[0033]

EXAMPLES The present invention will be specifically described below with reference to Examples. The compositions of the present invention and comparative examples are shown in Table 2. Sample No.
1-No. 10 is steel having the composition targeted by the present invention. No. 11-No. Steel No. 14 is a comparative example steel, and No. 14 1
Steel No. 1 has Nieq and δc within the range, but B is not added. No. 12, No. 13, No. The composition of each of the No. 14 steels is within the specified composition range. 12 and N
o. The No. 13 steels have Nieq of 18.3 and 12.4, which are outside the scope of the present invention. In addition, No. 14 steel has δc of 22.0
That is outside the scope of the present invention.

[0034]

[Table 2]

The method of sample preparation is shown in FIG. As a sample steel, a 30 kg steel ingot was melted in a vacuum high frequency furnace, cut into a plate thickness of 33 mm × width 100 mm × length 150 mm from a columnar crystal part of the steel ingot, and hot rolling was performed. Moreover, after cutting out a 15 mm square × 110 mm length as a hot tensile test piece, 1220
A test was performed by applying a heat treatment of ° C x 2 hours. For hot rolling, after soaking at an extraction temperature of 1220 ° C for 2 hours, a thickness of 6 mm
Finished in 5 passes. After that, 1050 ℃ x 6 in the furnace
Annealing for min and pickling were performed, and after reverse transformation treatment at 700 ° C. × soaking for 0 min, pickling was performed. A tensile test was performed using this material. Further, the amount and the diameter of δ ferrite were measured by an optical microscope using a hot rolled plate.

Table 3 shows the results of hot rolling, the amount of δ ferrite in the hot rolled sheet, the grain size, the reduction ratio in the hot tensile test at 1000 ° C., and the mechanical properties after the reverse transformation treatment in the examples. According to Table 3, the finishing temperatures in hot rolling are all around 940 ° C. Steel No. 1 of the present invention during hot rolling
No. No edge cutting of 10 was observed, the amount of δ ferrite after hot rolling was as small as 1 to 8%, and the δ ferrite grain size was as fine as 6 to 15 μm. Further, the drawing ratio of the present invention steel in the hot tensile test at 1000 ° C. is 60% or more, and the present invention steel has high strength in the mechanical properties after the reverse transformation treatment as well as being excellent in hot workability. It has high ductility. On the other hand, comparative steel No. 11 does not contain B, so δ
Large amount of ferrite and large diameter. Therefore, edge cutting also occurs, and the drawing ratio by the hot tensile test at 1000 ° C is 50% or less. In addition, comparative steel No. No. 14 contained B, but δc outside the range of the present invention was as large as 21.7, so comparative steel
No. The amount of δ ferrite is larger than 11 and the diameter is also large.
Therefore, deafening of the ears is also recognized, and the drawing ratio is as low as 38%.
On the other hand, comparative steel No. 12, No. The No. 13 steel has a Nieq content outside the range of the present invention, so the amount of δ-ferrite is small and no cutting edge is observed. No. 12 steel has low yield strength and tensile strength, No. 13 steel has low elongation.

[0037]

[Table 3]

[0038]

As described above in detail, according to the present invention, by adding an appropriate amount of B, a high-strength stainless steel excellent in hot workability can be obtained, which brings the present invention to this field. The effect is great.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a relationship between a δ ferrite content and a B content after hot rolling.

FIG. 2 is an explanatory diagram of a relationship between a δ ferrite diameter after hot rolling and a B content.

FIG. 3 is a diagram showing a method for producing a sample in the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiro Fujimoto 4976 Nomura-Minamimachi, Shinnanyo-shi, Yamaguchi Nisshin Steel Co., Ltd.

Claims (4)

[Claims] 1. By weight%, C: 0.10% or less, Si: 4.5% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.020% or less, Cr : 10.0 to 17.0%, Ni: 3.0 to 10.0%, N: 0.10% or less, B: 0.0010 to 0.0100%, with the balance being Fe and unavoidable impurities And the value of Nieq defined by Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) is 13.0 to 17.5 δc = -87-182 (C + N) + High-strength stainless steel with excellent hot workability and δc value defined by 16Si-3.0Ni + 5Cr in the range of -20 to 20 without weld softening. 2. In% by weight, C: 0.10% or less, Si: 4.5% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.020% or less, Cr : 10.0 to 17.0%, Ni: 3.0 to 10.0%, N: 0.10% or less, B: 0.0010 to 0.0100%, and further Cu, Mo, Co The total content of one or more kinds is 4.0% or less, the balance is Fe and unavoidable impurities, and Nieq = Ni + Mn + 0.5Cr + 0.3Si + 20 (C + N) + Cu + Mo + 0.2. The value of Nieq defined by Co is 13.0 to 17.5 δc = -87-182 (C + N) + 16Si-3.0 (Ni + Cu + Co) +5 The value of δc defined by Cr is -20. High-strength stainless steel with excellent hot workability and no welding softening in the range of up to 20. 3. In% by weight, C: 0.10% or less, Si: 4.5% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.020% or less, Cr 10.0 to 17.0%, Ni: 3.0 to 10.0%, N: 0.10% or less, B: 0.0010 to 0.0100%, and further Ti, Nb, V, The total content of one or more of Ni is 1.0% or less, the balance is Fe and inevitable impurities, and the value of Nieq defined by Nieq = Ni + Mn + 0.5Cr + 0.3Si is 13.0 to 17.5 δc = -87 + 16Si-3.0 (Ni + Cu + Co) + 5Cr with δc value in the range of -20 to 20 High strength stainless steel with excellent hot workability and no weld softening . 4. In% by weight, C: 0.10% or less, Si: 4.5% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.020% or less, Cr : 10.0 to 17.0%, Ni: 3.0 to 10.0%, N: 0.10% or less, B: 0.0010 to 0.0100%, and further Cu, Mo, Co The total content of one or more elements is 4.0% or less, the total content of one or more elements of Ti, Nb, V is 1.0% or less, the balance is Fe and inevitable impurities, and Nieq = Ni + The value of Nieq defined by Mn + 0.5Cr + 0.3Si + Cu + Mo + 0.2Co is defined by 13.0 to 17.5 δc = -87 + 16Si-3.0 (Ni + Cu + Co) + 5Cr High strength stainless steel with excellent δc value in the range of -20 to 20 with excellent hot workability and no welding softening.