JPH07109026B2 - High-strength stainless steel with excellent stress corrosion resistance and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to stress corrosion resistance used as a corrosion resistant structural member,
The present invention relates to a high strength stainless steel having excellent toughness and a method for manufacturing the same.

(Prior Art) Conventionally, SUS403 and SUS410 are structural members that require strength.
And martensitic stainless steel such as SUS420JI was used. These martensitic stainless steels
It had a tensile strength of 70 to 90 kg / mm ² and was excellent in strength but inferior in stress corrosion resistance and low in toughness.

On the other hand, ferritic stainless steel such as SUS410L
Excellent in stress corrosion resistance, but has a tensile strength of 42 kg
It was as low as / mm ² and was inferior in strength, and the crystal grains were likely to be coarsened during the heat treatment, and the toughness was not always satisfactory.

(Problems to be solved) As described above, although the martensitic stainless steel is excellent in strength, it is low in stress corrosion resistance and toughness, and the ferritic stainless steel is low in stress corrosion resistance. Is excellent, but its strength and toughness are low.In steels used for corrosion-resistant structural members that require strength, excellent stress corrosion resistance, 15kg ・ m / cm ²
There was no stainless steel having the above-mentioned toughness and high strength of 70 to 90 kg / mm ² .

(Means for Solving Problems) The present invention has been made in view of the drawbacks of the conventional steel, and the present invention reduces the C content of 12Cr stainless steel to 0.03% or less and the Si content of 0.20 to 0.50% and Mn content 0.30
~ 0.80%, S content 0.005% or less, N content 0.0080
By controlling to ~ 0.030%, we succeeded in obtaining stainless steel excellent in strength, toughness and stress corrosion resistance.

Further, the present invention can improve strength and toughness without lowering stress corrosion resistance when the steel is controlled-rolled at a finish rolling temperature of 700 to 850 ° C.

That is, the present invention is C0.03% or less, Si0.20 ~ 0.50%, Mn
0.30 to 0.80%, S0.005% or less, Cr11.00 to 13.00%, Ni0.
20% or less, Cu 0.15% or less, Mo 0.05 to 0.50%, N 0.0080 to
It contains 0.030%, the balance is Fe and impurity elements, and the tensile strength is 70 kgf / mm ² or more. The second invention steel is V0.05-0.25%, Nb0.05- It contains one or two of 0.25% and further improves the toughness. The third and fourth inventions are the steels of the first and second inventions from 1050 to
After heating to 1150 ° C, rolling at a rolling temperature of 700 to 1100 ° C, and controlling so that the finishing rolling temperature is in the temperature range of 700 to 850 ° C, and after finishing rolling, the cooling rate up to 600 ° C is 50 ° C. The strength and toughness are further improved by setting the ratio to be not less than / minute.

(Effects of the Invention) The present invention, by having the above-described structure, has a tensile strength of 70 to 90 kg / mm ² required as a corrosion-resistant structural member requiring strength, and an impact value of 15 kg · m / cm ² or more. And succeeded in obtaining excellent stress corrosion resistance, the present invention has high strength,
A stainless steel having high toughness and excellent stress corrosion resistance, and a method for producing the same.

The reasons for limiting the components of the steel of the present invention will be described below.

C is an element effective for obtaining strength, but on the other hand, it is an element that reduces toughness and stress corrosion resistance, and the upper limit was 0.03%.

Si is an element added as a deoxidizer, and the lower limit was made 0.20%. However, it is an element that reduces quenching hardness and deteriorates toughness due to coarsening of grain size, and the upper limit is 0.50%.
And

Mn is an element that has a deoxidizing effect on steel and improves hot workability, and the lower limit was made 0.30%. However, if Mn is contained more than necessary, it becomes brittle, so the upper limit was made 0.80%.

Cr is an element that imparts the basic corrosion resistance of the steel of the present invention,
The lower limit was set to 11.00%. However, Cr is a ferrite-forming element, and if its content increases too much, the strength decreases, so the upper limit was made 13.00%.

Ni is an element that reduces stress corrosion resistance, and the lower the content, the more preferable. Therefore, the upper limit was made 0.20%. Like Ni, Cu is an element that lowers the stress corrosion resistance, and the lower the content, the better. Therefore, the upper limit was made 0.15%.

Mo is an element effective in improving general corrosion resistance, and the lower limit was made 0.05%. However, Mo is an expensive element, and since ferrite is formed to reduce strength, the upper limit was made 0.50%.

N is an element that imparts strength by solid solution strengthening and refines the crystal grains, and the lower limit was made 0.0080%. However, if N is added in a large amount, it not only lowers stress corrosion resistance but also reduces workability, and its upper limit is 0.030%.
And

It is an element that forms V and Nb carbides and refines the crystal grains to improve the toughness. But,
V and Nb are ferrite-forming elements, and the strength is lowered if they are contained in a large amount, so the upper limits of V and Nb are set to 0.25% and 0.20%, respectively.

S is an element that adversely affects the corrosion resistance of steel, and the upper limit of its content must be strictly controlled and was set to 0.005%.

Further, in the controlled rolling, the heating temperature was set to 1050 to 1150 ° C, because the temperature of less than 1050 ° C is ferrite and austenite region, the austenitization and diffusion cannot achieve the homogenization of the components, and the temperature of ferrite and austenite above 1150 ° C is not obtained. This is because ferrite grows in the region, the ferrite grains of the rolled material become coarse, and the strength and toughness decrease.

Further, the rolling temperature is set to 700 to 1100 ° C. because rolling is performed in the ferrite and austenite regions, and rolling is difficult at temperatures lower than this.

Also, the finish rolling temperature was set to 700 to 850 ° C because if it is less than 700 ° C, the deformation resistance is large and rolling becomes difficult.
This is because if the temperature exceeds ℃, recrystallization occurs, the effect of refining crystal grains by controlled rolling is small, and the improvement in toughness is small.

Furthermore, the reason why the cooling rate up to 600 ° C. is 50 ° C./min or more is that recrystallization occurs at a rate slower than 50 ° C./min and improvement in toughness cannot be expected.

Next, the characteristics of the steel of the present invention will be clarified by examples compared with the conventional steel and the comparative steel.

Table 1 shows the chemical composition of these test steels.

In Table 1, AK steels of the present invention, L to R steels are comparative steels, S to V steels are conventional steels, S steels are SUS403T steels, SUS410,
U steel is SUS420JI and V steel is SUS410L.

Then, a test steel having the chemical composition shown in Table 1 was melted in a high frequency melting furnace to manufacture a 30 kg ingot. Table 2 shows the steel ingot
Forged to 65 s and then rolled to 12 x 20 mm at 900 ° C
After heating for 30 minutes, air cooling, a test piece was prepared by cutting, strength, toughness, and stress corrosion resistance were measured, and the results are shown.

For strength, measure tensile strength using JIS No. 4 test piece, measure impact value using JIS No. 3 test piece for toughness, and for stress corrosion resistance diameter 5 mm, parallel part 30 mm
Using the test piece of No. 1, it was immersed in a 3% NaCl aqueous solution at 50 ° C by the uniaxial tension method, and a rupture test was performed.
Those that exceeded the time were rated as ◯, and those that were less than 2000 hours were rated as x.

As is known from Table 2, the S steel, which is a conventional steel, has an excellent tensile strength of 78 kg / mm ² , but has an impact value of
The toughness of 14.5 kg · m / cm ² is not always satisfactory, and the stress corrosion resistance is low at less than 2000 hours until rupture, and the conventional steels T and U are Like S steel, it has excellent strength, but its impact value is low at 6.7 and 9.8 kg · m / cm ² ,
Moreover, the stress corrosion resistance is also low, and the above S,
The martensitic stainless steels of T and U steels were excellent in strength, but were low in toughness and stress corrosion resistance.

In addition, the conventional steel V steel has excellent stress corrosion resistance since the time to fracture exceeds 2000 hours, but the tensile strength is 43 kg / mm ² and the impact value is 13.8 kg · m / cm ² . The strength and toughness were low.

Furthermore, the comparative steels L and M are excellent in strength and toughness but inferior in stress corrosion resistance, and the N steel is excellent in toughness and stress corrosion resistance but in strength. Is low, the P steel has high strength, but the toughness and stress corrosion resistance are inferior, and the Q and R steels have excellent toughness and stress corrosion resistance, The strength is slightly low.

On the other hand, the tensile strengths of the steels A to K of the present invention are
75 ~ 89kg / mm ² , impact value is 16.9 ~ 25.0kg ・ m / cm ² , strength,
The toughness is excellent, and the time until fracture is 2000 hours or more, which is also excellent in stress corrosion resistance, and the steel of the present invention is excellent in both strength, toughness, and stress corrosion resistance. .

Table 3 shows the results obtained by subjecting the test steel to controlled rolling under the conditions shown in Table 3 and measuring the strength, toughness and stress corrosion resistance.

The strength, toughness, and stress corrosion resistance were measured by the same test methods as shown in Table 2 above.

As known from Table 3, heat at 1050-1150 ° C., 70
Tensile strength of the steels A and D, which are the steels of the present invention that have been finish-rolled at 0 to 850 ° C and controlled to be rolled at a cooling rate of up to 600 ° C at 50 ° C / min or more, are 81 to 86kg / mm. ² and the impact value was 18.5 to 24.8 kg · m / cm ² , the strength and toughness were improved by performing the controlled rolling.

For stress corrosion resistance,
It was more than 2000 hours, and the stress corrosion resistance did not decrease due to the controlled rolling.

As described above, the steel of the present invention improves the stress corrosion resistance by lowering the C, S, Ni, Cu contents and limiting the upper limit of the N amount, and lowers the C content and Si.
The toughness is improved by limiting the upper limit of the amount, and the strength is also improved by further containing N.

Further, the steel of the present invention is further subjected to controlled rolling,
The steel of the present invention is a high-strength stainless steel excellent in stress corrosion resistance and toughness and a method for producing the same, which can further improve strength and toughness, and has high practicality as a corrosion structural member.

Claims (4)

[Claims] 1. A weight ratio of C 0.03% or less, Si 0.20 to 0.50
%, Mn 0.30 to 0.80%, S0.005% or less, Cr11.00 to 13.00
%, Ni 0.20% or less, Cu 0.15% or less, Mo 0.05 to 0.50%, N
High strength stainless steel with excellent stress corrosion resistance, containing 0.0080% to 0.030%, the balance Fe and impurity elements, and having a tensile strength of 70 kgf / mm ² or more. 2. C0.03% or less by weight ratio, Si 0.20 to 0.50
%, Mn 0.30 to 0.80%, S0.005% or less, Cr11.00 to 13.00
%, Ni 0.20% or less, Cu 0.15% or less, Mo 0.05 to 0.50%, N
Contains 0.0080% to 0.030%, V0.05 to 0.25%,
Nb 0.05 to 0.20%, containing 1 or 2 kinds, and the balance
Consisting of Fe and impurity elements, tensile strength 70kgf / mm ²
High strength stainless steel excellent in stress corrosion resistance characterized by the above. 3. C0.03% or less by weight ratio, Si 0.20 to 0.50
%, Mn 0.30 to 0.80%, S0.005% or less, Cr11.00 to 13.00
%, Ni 0.20% or less, Cu 0.15% or less, Mo 0.05 to 0.50%, N
Steel containing 0.0080% to 0.030% and the balance Fe and impurity elements is heated to 1050 to 1150 ° C, then rolled at a rolling temperature of 700 to 1100 ° C, and the finish rolling temperature is 700
The stress resistance is characterized by controlling the temperature range to 850 ℃, and after finishing rolling, the cooling rate up to 600 ℃ is 50 ℃ / min or more, and the tensile strength is 70 kgf / mm ² or more. A method for producing high-strength stainless steel having excellent corrosiveness. 4. C0.03% or less by weight ratio, Si 0.20 to 0.50
%, Mn 0.30 to 0.80%, S0.005% or less, Cr11.00 to 13.00
%, Ni 0.20% or less, Cu 0.15% or less, Mo 0.05 to 0.50%, N
Contains 0.0080% to 0.030%, V0.05 to 0.25%,
Nb 0.05 to 0.20%, containing 1 or 2 kinds, and the balance
Steel consisting of Fe and impurity elements is heated to 1050 to 1150 ° C, then rolled at a rolling temperature of 700 to 1100 ° C, and the finish rolling temperature is controlled to be in the temperature range of 700 to 850 ° C, and further finish rolling is performed. After that, cooling rate up to 600 ℃ 50 ℃ /
And a tensile strength of 70 kgf / mm ² or more, and a method for producing high-strength stainless steel with excellent stress corrosion resistance.