JPH07216451A - Production of stainless steel material having high welding softening resistance, high strength, and high ductility - Google Patents

Abstract

PURPOSE:To obtain a high strength stainless steel material having 0.2% proof stress as high as >=900N/mm<2>, required of a structural member, and also having high ductility and welding softening resistance. CONSTITUTION:A strip or plate of stainless steel is used. This stainless steel has a composition which contains, by mass, <=0.10% C, <=4.5% Si, <=5.0% Mn, 10.0-17.0% Cr, 3.0-10.0% Ni, and <=0.10% N and further contains, selectively, <=4.0%, in total, of one or more elements among Cu, Mo, and Co and/or <=1.0%, in total, of one or more elements among Ti, Nb, V, and B and in which the value of gamma/r defined by gammar-210+650C+10Si+12(Ni+Mn+Cu+Mo+Co)+7Cr+150N is regulated to 3-35. Further, this stainless steel is composed of a dual phase structure consisting of martensitic phase and austenitic phase. After <=3% strain is applied to this steel strip or steel plate, heat treatment is done at 400-600 deg.C for <=30min.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high strength and high ductility stainless steel material having high welding softening resistance, which is used for structural materials such as vehicle structures and buildings.

[0002]

2. Description of the Related Art Structural materials are required to have high strength and, at the same time, are required to have ductility to some extent because they are often processed into a shape as a structural member by bending or the like.
In addition, since a welding assembly method is frequently used in terms of construction speed in construction, it is also required to have high resistance to softening against heat input during welding.

To meet these demands, for example, Japanese Patent Laid-Open No. 55-
In the publication No. 21566, C: 0.02% or less, N: 0.
02% or less, Si: 0.5% or less, Mn: 1.0 to 3.
5%, Cu: 1.0% or less, Ni: less than 0.1%, C
Disclosed is a martensitic stainless steel for structural use, which has r: 10 to 13.5%, is composed of balance Fe and impurities, and has a Cr equivalent defined by a limited formula and is excellent in weldability and workability. Since this steel is a marnsite type, it does not soften due to welding, but since it is tempered, it has a yield strength of about 40 kg / mm ² and a tensile strength of 60 kg / m 2.
It is about m ² .

Further, Japanese Patent Laid-Open No. 4-6215 discloses that
C: more than 0.03% to 0.10% or less, Si: 2.00%
Below, Mn: 4.0% or less, Cr: 16 to 30%, N
i: 6 to 20%, Nb: 0.05 to 0.50%, N:
Steel consisting of 0.1 to 0.3% and balance Fe and impurities is heated to 1100 to 1300 ° C., rolled at 1050 ° C. or higher so that the total reduction amount is 50% or higher, and then 8
Finish rolling is performed at 00 ° C to 1050 ° C so that the total reduction amount is 10% or more, and further 800 ° C to 500 ° C after rolling.
The method for producing a high-strength austenitic stainless steel with less welding softening is disclosed, characterized in that the average cooling rate is 500 ° C./min or more. Since this steel is austenitic stainless steel, it has excellent ductility and 0.2%
High yield strength and high tensile strength of 73.5 kg each
/ Mm ² and 98.1 kg / mm ² are obtained.

Further, Japanese Patent Laid-Open No. 4-56724 discloses that
C: 0.02% or less, Si: 1.0% or less, Mn: 1.
0% or less, Cu: 1.0% or less, Ni: 3 to 6%, C
r: 15 to 19%, Mo: 2% or less, Nb: 0.4% or less, balance Fe and impurities, and Cr
+ Mo: steel having a content of 15 to 19% is arranged so that ferrite and martensite are layered by hot rolling,
Disclosed is a method for producing high-strength stainless steel, which comprises performing quenching treatment in a temperature range of 800 to 1000 ° C. and then tempering treatment in a temperature range of 600 ° C. or higher and Ac1 point or lower.
The steel obtained by this manufacturing method has a 0.2% proof stress of 62 to 86.
kg / mm ^2, the tensile strength has a 74~95kg / mm ^2.

[0006]

However, in recent years, structures such as vehicle structures and buildings have been promoted to be lighter by making them thinner. Along with this, there is an increasing demand for structural materials that exhibit even higher yield strength and tensile strength than before, and that also have good ductility and weld softening resistance. With conventional technology,
It is difficult to obtain a material that has a high level of strength, ductility, and weld softening resistance characteristics that can meet such new needs. That is, if the structure is made into martensite, it is possible to increase the strength and the welded part is not softened, but its ductility is reduced. Further, if tempering is performed to secure ductility, the strength is sacrificed. on the other hand,
If the structure is made austenite or ferrite, high ductility can be achieved, but on the other hand, it is difficult to satisfy high strength and weld softening resistance.

The present invention has been devised in order to solve such a problem, and has a well-balanced martensite phase capable of achieving high strength and free from welding softening and an austenite phase having ductility. By subjecting the adjusted duplex stainless steel to low temperature heat treatment in a strained state, 0.2% proof stress is 900 N / mm ² or more, tensile strength is 1000 N / mm ² or more, and elongation is high. It is an object of the present invention to provide a stainless steel material having a high ductility of 15% or more and having a high weld softening resistance.

[0008]

[Means for Solving the Problems]
C: 0.10% or less, Si: 4.5% or less, Mn:
5.0% or less, Cr: 10.0% to 17.0%, Ni:
3.0 to 10.0%, N: 0.10% or less, and if necessary, one or more kinds of Cu, Mo, Co in total 4.0% or less, and / or Ti, Nb, V, 1 of B
1.0% or less in total of at least one kind, consisting of balance Fe and unavoidable impurities, and γr = −210 + 650C + 10Si + 12 (Ni + M
n + Cu + Mo + Co) + 7Cr + 150N and a value of γ r defined in the range of 3 to 35, a stainless steel strip or steel plate having a multiphase structure of a martensite phase and an austenite phase is strained with 3% or less and then 400 It is achieved by a method for producing a high-strength and high-ductility stainless steel material, which is characterized by performing a heat treatment for 30 minutes or less in a temperature range of to 600 ° C.

[0009]

The reason for limiting the composition of the material steel to which the method of the present invention is applied is as follows.

C: C is an austenite forming element,
Although it is effective in forming an austenite phase at high temperature and strengthening the austenite phase and martensite phase after heat treatment, if it is too large, the elongation rate decreases and the corrosion resistance of the welded portion deteriorates. The upper limit is 10%.

N: N, like C, is an austenite forming element, is effective in forming an austenite phase at high temperatures, and is effective in increasing the strength and strengthening of the austenite phase after heat treatment. Since it lowers 0.
The upper limit is 10%.

Si: Si is effective in strengthening the martensite phase and austenite phase after heat treatment, but if too much, it promotes solidification cracking during solidification or welding, so 4.5
% Is the upper limit.

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

Cr: Cr is a basic component that imparts corrosion resistance. If it is less than 10%, it is not effective. On the other hand, if it exceeds 17%, austenite single phase or austenite-based metallographic structure is obtained at high temperature. Since a large amount of forming element is required, and as a result, a desired structure cannot be obtained when brought to room temperature, the upper limit is 17.0%.

Ni: Ni is an austenite forming element, and is an element necessary for austenitizing the metal structure at high temperature and adjusting the amount of austenite at room temperature. The required Ni content differs depending on the contents of other elements. At least about 3% is required for austenitizing at high temperature and adjusting the Ms point, 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 that is mixed from raw materials and auxiliary raw materials during smelting, but if it is contained in a large amount, it makes the steel brittle, so it is desirable to limit it to 0.040% or less.

S: S is also an unavoidable impurity mixed from raw materials and auxiliary raw materials during smelting, but if it is contained in a large amount, it makes the steel brittle, so it is desirable to limit it to 0.030% or less.

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

Mo: Mo is also effective in improving the corrosion resistance, increasing the strength of austenite, and lowering the Ms point, but it is an expensive material, and if too much, the price of steel material increases, so 4.0%. Restricted to.

Co: Co has a large austenitizing effect in a high temperature range and lowers the Ms point (the Ms point is not excessively lowered in spite of the large austenitizing effect). C
It is a very effective element for adjusting the composition of a system with a large r content, but if it is too much, it will increase the price of steel, so it is 4.0%.
Restricted to.

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 in common. Equivalent in this sense.

Ti: Ti is a carbide 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 for suppressing the growth of austenite phase crystal grains. The upper limit is 1.0% because it may cause scum formation during welding.

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

B: B is effective in suppressing crystal grain growth of the austenite phase and improving hot workability, but if it is too much, it impairs the ductility of the steel, so the upper limit is 1.0%.

The above four elements of Ti, Nb, V and B are
These are carbide and nitride forming elements, both of which suppress the growth of austenite crystal grains and their effects are remarkable. In that sense, they are equivalent.

The chemical composition of the steel in the present invention is within the above composition range, and the value of γr defined by the following equation is 3 to.
It must be within the range that satisfies 35. γr = -210 + 650C + 10Si + 12 (Ni + M
The value of (n + Cu + Mo + Co) + 7Cr + 150Nγr is a value that serves as an index of the amount of austenite, and substantially corresponds to the volume% of the austenite phase at room temperature. γ
When the value of r is less than 3, high ductility cannot be obtained because the austenite phase is small. Further, when the value of γr exceeds 35, a large amount of austenite phase does not result in high strength even if the heat treatment specified in the present invention is applied, and the welded portion may soften when subjected to the heat history of welding. . Therefore, the γr value is limited to 3 to 35.

The hot-rolled or annealed steel material having the above composition has a multiphase structure of a martensite phase and an austenite phase at room temperature, and the austenite phase has a volume ratio of about 3 to 35%. The inventors of the present invention apply a strain of 3% or less to a steel strip or steel sheet having a multi-phase structure of a martensite phase and an austenite phase in the above composition range, and then 30 minutes or less in a temperature range of 400 to 600 ° C. It was found that the above-described object of the present invention can be achieved by applying the heat treatment of.

The reason for limiting the heat treatment conditions in the method of the present invention is as follows. The hot rolled or annealed material having a multi-phase structure of martensite phase and austenite phase is about 110
Although it has a tensile strength of about 0 N / mm ² , its 0.2% proof stress is only about 600 to 800 N / mm ² , and it cannot be said that it has a sufficiently high yield strength as a structural material. It is the most important point of the present invention to subject the above-mentioned steel to heat treatment. If the heat treatment temperature is lower than 400 ° C, the yield strength will not increase so much, and if it exceeds 600 ° C, the reverse transformation from the martensite phase to the austenite phase will occur and the yield strength will decrease.
C is the lower limit temperature and 600 C is the upper limit temperature. Further, the heat treatment time is 30 minutes or less in consideration of productivity because the effect can be obtained even in a short time. The increase in 0.2% proof stress and elongation due to this heat treatment is considered to be a phenomenon due to strain aging and rearrangement of dislocations. For heat treatment equipment,
It is possible to use batch furnaces and continuous annealing pickling lines.

Prior to this heat treatment, the 0.2% proof stress is further increased by applying strain to the steel strip or steel sheet having the multiphase structure of the martensite phase and the austenite phase. If the amount of strain applied exceeds 3%, the elongation becomes 10% or less and the ductility decreases, so the upper limit is 3%. Also,
It was confirmed that the same effect can be obtained in both rolling and bending as to the method of applying strain. That is, although strain may be applied by performing light rolling, in the case of manufacturing using a continuous annealing line, a strain applying bending roll is installed in the pass line of the coil, and the radius of the bending roll is adjusted. The same effect can be obtained by applying an appropriate amount of strain to the surface of the steel strip or the steel sheet.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically illustrated below with reference to the accompanying drawings. FIG. 1 shows a sample preparation method. The sample steel is made by melting a 30 kg ingot in a vacuum high-frequency furnace and heating at a temperature of 12
After forging at 50 ° C. into a plate having a thickness of 35 mm and a width of 155 mm, it is cut into a plate having a thickness of 30 mm and a width of 150 mm,
After soaking at 1230 ° C. for 2 hours, it was hot-rolled to a thickness of 6 mm, and a part thereof was subjected to a test as a hot-rolled material. The other part was annealed at 1050 ° C. for 5 minutes, pickled, and then part was subjected to a test as a hot rolled annealed material, and the other part was cold rolled to a thickness of 3 mm and annealed at 1050 ° C. for 5 minutes. Then, it was subjected to a test as a cold rolled annealed material.

Table 1 shows the chemical composition of the sample. Sample No.
1 to 8 are steels in which the content of each element and the γr value are within the ranges specified in the present invention and which have the chemical composition targeted by the present invention. No. A, B and C are comparative steels, and the content of each element is within the specified range, but γr is A,
B is less than 3 and C is more than 35.

[0033]

[Table 1]

Table 2 shows the tensile property values obtained by the conventional method and the method of the present invention for the hot rolled material, the hot rolled annealed material and the cold rolled annealed material. In Table 2, what is referred to as a conventional method is a tensile property value in a hot-rolled material as hot-rolled or an annealed material as-annealed at 1050 ° C. for 5 minutes. Further, what is referred to as the method of the present invention is the same as the material obtained by the above-mentioned conventional method.
It is a tensile property value in a material that is subjected to a heat treatment at 500 ° C. for 5 minutes after applying a 5% strain. Tensile test is J
The test was performed using IS13 tensile test pieces.

[0035]

[Table 2]

According to Table 2, the 0.2% proof stress of the material obtained by the conventional method among the steels of the present invention is 600 to 800.
It is as low as N / mm ² . On the other hand, in the case of the material obtained by the method of the present invention, that is, the material obtained by the conventional method, which is further heat-treated at 500 ° C. for 5 minutes after applying a strain of 1.5%, 0.2 % Yield strength is 900 N / m
It can be seen that the value has increased to a high value of m ² or more, and the elongation has also improved. On the other hand, the comparative steels A and B have a low value of γr, which is an index of the amount of austenite, so that the material subjected to the heat treatment of the method of the present invention also has a low elongation. On the contrary, C of the comparative steel has a high elongation because the value of γr is high, but high yield strength cannot be obtained even if the heat treatment of the method of the present invention is performed.

The weld softening test was evaluated by performing MIG welding on a plate having a thickness of 6 mm under the conditions of a current of 200 A, a voltage of 25 V and a speed of 300 mm / min. FIG. 3 shows the hardness distribution in the central portion of the plate thickness in the plate cross section orthogonal to the weld bead. The example of the present invention is No. 1 in Table 1. No. 2 in Table 1 was used for the comparative example. Each of the hot-rolled materials of C steel was used as a raw material, and after applying a strain of 1.5% by the method of the present invention, heat-treated at 500 ° C. for 5 minutes was used as a sample. In the example of the present invention, the hardness of the welded portion is slightly lower than that of the base material portion, but the Vickers hardness is maintained at a high level of 330 or more in all the portions. On the other hand, in the comparative example, the hardness of the welded portion is significantly lower than that of the base material portion, and the hardness level is also significantly lower than that of the examples of the present invention. From this, it can be said that the steel of the present invention has a high resistance to softening of welding and is a material that can maintain high strength during welding.

[0038]

As described in the above embodiments, according to the method of the present invention, a 0.2% proof stress of 900 N / mm ² or more, excellent ductility and high weld strength and high strength without welding softening are obtained. A steel strip or steel plate of high-strength stainless steel can be manufactured industrially stably, and the effect of the present invention is extremely large particularly in the field of structural members and the like that require softening resistance to welding.

[Brief description of drawings]

FIG. 1 is a system diagram showing a method for producing a sample in the present invention.

FIG. 2 is a graph showing the relationship between the distance from the center of the weld bead and the Vickers hardness in the plate cross section regarding the weld softening resistance.

Claims (4)

[Claims] 1. In mass%, C: 0.10% or less, S
i: 4.5% or less, Mn: 5.0% or less, Cr: 10.
0% to 17.0%, Ni: 3.0 to 10.0%, N:
0.10% or less, balance Fe and unavoidable impurities, and γr = −210 + 650C + 10Si + 12 (Ni + M
n) + 7Cr + 150N The value of γ r defined in the range of 3 to 35 is applied to the steel strip or steel plate of the stainless steel having the multi-phase structure of the martensite phase and the austenite phase. High heat resistance of softening of 0.2%, which is characterized by heat treatment for 30 minutes or less in a temperature range of 600 to 600 ° C.
A method for producing a high strength and high ductility stainless steel material having a proof stress of 900 N / mm ² or more. 2. Mass%, C: 0.10% or less, S
i: 4.5% or less, Mn: 5.0% or less, Cr: 10.
0% to 17.0%, Ni: 3.0 to 10.0%, N:
0.10% or less, and further, one or more of Cu, Mo, and Co including 4.0% or less in total, the balance Fe and unavoidable impurities, and γr = −210 + 650C + 10Si + 12 (Ni + M
n + Cu + Mo + Co) + 7Cr + 150N and a value of γ r defined in the range of 3 to 35, a stainless steel strip or steel plate having a multiphase structure of a martensite phase and an austenite phase is strained with 3% or less and then 400 High heat resistance of softening of 0.2%, which is characterized by heat treatment for 30 minutes or less in a temperature range of 600 to 600 ° C.
A method for producing a high strength and high ductility stainless steel material having a proof stress of 900 N / mm ² or more. 3. In mass%, C: 0.10% or less, S
i: 4.5% or less, Mn: 5.0% or less, Cr: 10.
0% to 17.0%, Ni: 3.0 to 10.0%, N:
Containing 0.10% or less, and 1 of Ti, Nb, V, B
1.0% or less in total of at least one kind, consisting of balance Fe and unavoidable impurities, and γr = −210 + 650C + 10Si + 12 (Ni + M
n) + 7Cr + 150N The value of γ r defined in the range of 3 to 35 is applied to the steel strip or steel plate of the stainless steel having the multi-phase structure of the martensite phase and the austenite phase and then 400% High heat resistance of softening of 0.2%, which is characterized by heat treatment for 30 minutes or less in a temperature range of 600 to 600 ° C.
A method for producing a high strength and high ductility stainless steel material having a proof stress of 900 N / mm ² or more. 4. In mass%, C: 0.10% or less, S
i: 4.5% or less, Mn: 5.0% or less, Cr: 10.
0% to 17.0%, Ni: 3.0 to 10.0%, N:
0.1% or less, further 4.0% or less in total of one or more of Cu, Mo and Co, and 1.0% or less in total of one or more of Ti, Nb, V and B, and the balance Fe. And unavoidable impurities, and γr = −210 + 650C + 10Si + 12 (Ni + M
n + Cu + Mo + Co) + 7Cr + 150N and a value of γ r defined in the range of 3 to 35, a stainless steel strip or steel plate having a multiphase structure of a martensite phase and an austenite phase is strained with 3% or less and then 400 High heat resistance of softening of 0.2%, which is characterized by heat treatment for 30 minutes or less in a temperature range of 600 to 600 ° C.
A method for producing a high strength and high ductility stainless steel material having a proof stress of 900 N / mm ² or more.