JPH06158233A - Production of ferritic stainless steel thin slab excellent in toughness and ferritic stainless steel strip by the same thin slab - Google Patents

Abstract

PURPOSE:To improve the cold rollability of ferritic stainless steel after being produced into a thin slab. CONSTITUTION:A thin slab contg., by weight, <=0.030% C, <=1.0% Si, <=1.0% Mn, 10 to 30% Cr and <=0.03% N, and the balance Fe with inevitable impurities and furthermore satisfying the gamma potential (gammap')=420C%+470N%+23Ni%+9Cu%+7Mn%-11.5Cr%-11.5Si%-12Mo%-23V%-47Nb% -49Ti%-52Al%+179<=0, and in which sheet thickness is regulated to <=5mm and the average grain size is regulated to <=150m is obtd. Furthermore, the slab is directly subjected to cold rolling to obtain a steel strip.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel thin cast piece and a method for producing a ferritic stainless steel strip by the cast piece.

[0002]

2. Description of the Related Art As a conventional technique, for example, Japanese Patent Application Laid-Open No. 62-54017 discloses a method for producing a thin cast piece of ferritic stainless steel by rapid solidification. In this technology, after the cast slab is cooled at a cooling rate of air cooling or higher, it is heat-treated at a temperature of 700 to 1000 ° C. or subjected to a young hot working to finely disperse the slab structure. It is intended to improve the cold rolling property.

[0003]

DISCLOSURE OF THE INVENTION The present invention is to obtain a slab or steel strip which has improved heat-treatment or hot working after the production of a thin cast slab and has improved cold rolling property.

[0004]

In order to achieve the above object, the present invention specifies the chemical composition and slab structure of ferritic stainless steel. That is, the gist of the present invention is C: 0.030% or less by weight%, Si: 1.
0% or less, Mn: 1.0% or less, Cr: 10 to 30% and N: 0.030% or less, and if necessary, Ni:
0.3-5.0%, and if necessary, Mo: 0.1-5.
0%, Cu: 0.2 to 1.0%, Ti: 0.05 to 1.
0%, Al: 0.05 to 1.0%, Nb: 0.1 to 1.
0%, V: 0.1-1.0%, one or more, or if necessary, B: 0.0003-0.0030%, balance Fe and unavoidable impurities, and gamma potential (hereinafter gamma _P 'hereinafter) is _{γ P' = 420C% + 470N} % + 23Ni% + 9Cu% + 7Mn% -11.5Cr% -11.5Si% -12Mo% -23V% -47Nb% -49Ti% -52Al% + 179 ≦ When continuously casting a ferritic stainless steel satisfying 0 to a thin cast piece, the cast piece has a plate thickness of 5 mm or less and an average crystal grain size of the cast piece of 150 μm or less.

The present invention will be described in detail below along with its operation.

[0006]

First, the reason why the chemical composition of steel is limited as described above in the present invention will be explained. C has a strong effect on the toughness of the steel and adversely affects the cold rolling property, so the content is 0.0
30% or less. Since Si and Mn are effective as deoxidizing agents for steel, they are contained in 1.0% or less each. If it exceeds 1.0%, the mechanical properties deteriorate.

Cr is an essential component for improving the corrosion resistance of steel, but if it is less than 10%, the effect is weak, and if it exceeds 30%, the toughness is remarkably deteriorated and the manufacturability becomes extremely difficult, so that the appropriate range is 10. ~ 35%. Since N deteriorates the toughness of steel similarly to C, the upper limit of the content is set to 0.030%. Furthermore, in order to further improve the toughness and corrosion resistance characteristics, the following components are contained in addition to the above chemical components. Ni
Is effective in improving the toughness of high-Cr materials, but if its content is less than 0.3%, its characteristics disappear, and if it exceeds 5.0%, gamma (γ) is conversely generated in the high temperature range and toughness is increased. Is deteriorated, so the range is made 0.3 to 5.0%.

To improve the corrosion resistance, Mo, Cu, Ti, A
It is effective to add 1, Nb and V, and one kind or two or more kinds are selected and contained. Mo has a remarkable effect in improving the corrosion resistance, but if it is less than 0.1%, its characteristics cannot be obtained, and if it exceeds 5.0%, the workability deteriorates and the cost increases, so The range is 5.0%. Cu is contained in the range of 0.2 to 1.0% in order to improve the corrosion resistance. If it exceeds the upper limit, γ is generated in the high temperature region to deteriorate the toughness.

Ti combines with C and N to prevent the precipitation of Cr carbonitrides at the grain boundaries and to improve the intergranular corrosion resistance, but if the content is less than 0.05%, that characteristic cannot be obtained. If it exceeds 1.0%, the workability deteriorates, so the content is made 0.05 to 1.0%. Al has similar characteristics to Ti, so 0.05
Included in the range of ~ 1.0%.

Since Nb and V also have the same characteristics as Ti, they are contained in the respective ranges of 0.1 to 1.0%.
Further, B is effective for improving the resistance to intergranular cracking in hot and cold working, but if it is less than 0.0003%, that characteristic cannot be obtained, and if it exceeds 0.0030%, the characteristic becomes poor. Since it is saturated, its content range is 0.0003 to 0.0030.
%.

In the present invention, the above chemical components are further regulated so that the value of γ _P ′ represented by the following equation is 0% or less. That is, when γ _P ′ = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% -11.5 Cr% -11.5 Si% -12 Mo% -23 V% -47 Nb% -49 Ti% -52 Al% + 179 ≦ 0, the slab structure is martensitic. Since the formation of sites can be prevented, the slab toughness can be greatly improved.

Next, the relationship between the thickness of the slab and the average crystal grain of the slab, which is another feature of the present invention, will be described. In order to secure the slab toughness necessary for directly cold-rolling a thin slab, the present inventors reduced C and N in steel as much as possible and focused on the crystal grain size of the slab structure. The experiment was done. Chemical composition: C: 0.017%, Si: 0.42%, M
n: 0.31%, Cr: 17.20%, N: 0.018
%, Ni: 0.28%, V: 0.045%, Ti: 0.
05%, the balance Fe and unavoidable impurities, and γ
A thin cast slab having a plate thickness of 5 and 6 mm was produced from the molten steel regulated to _P '=-2.9% by a twin drum casting method. At this time,
In order to change the crystal grain size of the slab, the molten steel temperature of the tundish is changed in the temperature range of 1555 to 1595 ° C., and cooling to the slab immediately after casting is changed by a combination of gas cooling and steam cooling 680 It was wound up at ℃.

Using the thin cast piece produced under the above conditions, the cold rolling property was evaluated by the Charpy impact test. A Charpy impact value of at least ² Kgfm / cm ^{2 is} necessary to prevent plate breakage during cold rolling, and the processing temperature (hereinafter referred to as vT ₂ (° C)) when this value can be secured is I asked. Considering manufacturing in winter, vT ₂ needs to be 0 ° C or lower. The relationship between the average crystal grain size of the slab and vT ₂ is shown in FIG. Here, the average crystal grain size of the cast slab is a value obtained by converting the sizes of the columnar crystal structure and the equiaxed crystal structure of the thin cast slab into a size equivalent to a circle.

As is clear from the figure, vT ₂ tends to decrease to a low temperature region and toughness tends to improve as the crystal grain size of the slab becomes smaller. In order to keep vT ₂ at 0 ° C. or lower, the thickness of the slab should be 5 mm or less, and the average crystal grain size of the slab should be 150 μm or less.

In the present invention, such a slab is annealed and descaled, cold-rolled at 50 to 90%, and then annealed normally, for example, at a temperature range of 850 to 1100 ° C.
Annealing for 1 to 3 minutes is performed, and then temper rolling is performed to obtain a steel strip having a desired thickness.

As described above, even if the cast slab is directly cold-rolled, no plate breakage or surface cracking occurs during rolling, and a steel strip having excellent surface properties can be manufactured.

As described above, according to the present invention, the chemical composition is such that C + N is 0.04% or less and γ _P ′ is 0% or less, and the average crystal grain size of the slab is 150 μm or less. As a result, the slab toughness is improved, and direct cold rolling without annealing or hot rolling treatment of the slab becomes possible.

[0018]

EXAMPLE A ferritic stainless steel having the chemical composition shown in Table 1 was melted and the temperature of the molten steel was 1555 by the twin drum method.
After casting into a thin cast piece having a thickness of 3 to 5 mm while adjusting the temperature range to -1595 ° C, cooling was changed by a combination of gas cooling and steam cooling, and wound at 650 ° C. Then, the coil is descaled by a combination of shot blasting and sulfuric acid pickling, and cold rolling is performed at a rolling rate of 50% or more at 900 ° C.
After annealing for minutes, temper rolling was performed.

Table 2 shows the toughness of the slab obtained in the above process and the occurrence of plate breakage during cold rolling.

[0020]

[Table 1]

[0021]

[Table 2]

In Nos. 1 to 7 of the present invention, the crystal grain size of the slab is 150 μm or less, and vT ₂ is the target value of 0 ° C. or less. In all cases, no sheet breakage occurred in cold rolling, No surface cracking of the steel strip as the final product occurred. In contrast, Comparative Example No. 8 has high C and N and high γ _P ′, and martensite is formed in the slab structure, resulting in poor toughness. No. 9 has a good chemical composition, but the slab has a large grain size and is inferior in toughness. Further, No. 10 has a high γ _P ′ and a large slab crystal grain, resulting in poor toughness. All the coils of Nos. 8 to 10 caused plate breakage during cold rolling, and it was difficult to manufacture a thin plate.

[0023]

As described above, the present invention can significantly improve the toughness of the ferritic stainless steel slab obtained by the thin wall casting method, and the post-treatment of annealing or hot rolling can be omitted to directly cool the slab. It enables hot rolling, and its industrial effects such as reduction of manufacturing cost and shortening of delivery time are enormous.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an average crystal grain size of a thin cast piece and vT ₂ .

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical indication C21D 8/02 D 7412-4K 9/46 R C22C 38/48 (72) Inventor Shinichi Teraoka Chiba Prefecture 20-1 Shintomi, Futtsu-shi Nippon Steel Corp. Technology Development Division

Claims (8)

[Claims] 1. By weight%, C: 0.030% or less, S
i: 1.0% or less, Mn: 1.0% or less, Cr: 10
30% and N: 0.030% or less, the balance Fe and unavoidable impurities, and the gamma potential (γ _P ′) is γ _P ′ = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% -11. 5Cr% -11.5Si% -12Mo% -23V% -47Nb% -49Ti% -52Al% + 179 ≦ 0, and the slab plate thickness is 5 mm or less and the average grain size of the slab is 150 μm or less. Ferrite stainless steel thin-walled slab with excellent toughness, which is characterized by being present. 2. A ferritic stainless steel thin cast piece having excellent toughness according to claim 1, further containing Ni: 0.3 to 5.0% by weight. 3. Mo: 0.1 to 5.0% by weight,
Cu: 0.2-1.0%, Ti: 0.05-1.0%,
Al: 0.05 to 1.0%, Nb: 0.1 to 1.0%,
And V: 0.1 to 1.0% of one type or two or more types, and the ferritic stainless steel thin cast piece having excellent toughness according to claim 1 or 2. 4. The ferritic stainless steel thin cast piece with excellent toughness according to claim 1, further comprising B: 0.0003 to 0.0030% by weight. 5. C: 0.030% or less by weight%, S
i: 1.0% or less, Mn: 1.0% or less, Cr: 10
30%, N: 0.030% or less, and the gamma potential (γ _p ′) is further γ _p ′ = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% −11.5 Cr% −11.5 Si% -12 Mo% A ferritic stainless steel satisfying −23 V% −47 Nb% −49 Ti% −52 Al% + 179 ≦ 0 is continuously cast into a thin cast piece having a plate thickness of 5 mm or less and an average crystal grain size of 150 μm or less, and then descaled. A method for producing a ferritic stainless steel strip, which comprises cold rolling and then annealing. 6. The ferritic stainless steel strip according to claim 5, wherein the ferritic stainless steel containing Ni: 0.3 to 5.0% by weight is continuously cast to produce a thin slab, and then cold rolled. Production method. 7. Mo: 0.1 to 5.0% by weight,
Cu: 0.2-1.0%, Ti: 0.05-1.0%,
A thin slab is produced by continuously casting a ferritic stainless steel containing one or more of Al: 0.05 to 1.0% and V: 0.1 to 1.0%, and then cold rolled. Claim 5
Or the method for producing a ferritic stainless steel strip according to item 6. 8. The ferrite system according to claim 5, 6 or 7, further comprising continuously casting ferritic stainless steel containing B: 0.0003 to 0.0030% by weight to produce a thin-walled slab and then cold rolling. Manufacturing method of stainless steel strip.