WO2023075391A1 - Hot-rolled ferritic stainless steel sheet having excellent formability and method for manufacturing same - Google Patents

Abstract

The present invention pertains to a hot-rolled ferritic stainless steel sheet having excellent formability, which has an R-bar value of at least 1.08 and satisfies TS/YS≤1.5 representing the ratio of tensile strength (TS) to yield strength (YS) after a steel, containing 0.001-0.1% of C, 10.0-14.0% of Cr, 0.001-0.5% of Ti, 0.001-0.5% of Nb, 0.001-1.5% of Ni, 0.001-1.5% of Mn, 0.001-1.0% of Cu, 0.001-2.0% of Si, 0.001-0.1% of N, and 0.1% or less of Al, with the remainder comprising Fe and inevitable impurities, is hot-rolled and then pre-rolled at a reduction ratio of 30% or more. In addition, the present invention pertains to a method for manufacturing a hot-rolled ferritic stainless steel sheet having excellent formability, the method comprising the steps of: producing a slab by continuous casting a steel containing 0.001-0.1% of C, 10.0-14.0% of Cr, 0.001-0.5% of Ti, 0.001-0.5% of Nb, 0.001-1.5% of Ni, 0.001-1.5% of Mn, 0.001-1.0% of Cu, 0.001-2.0% of Si, 0.001-0.1% of N, and 0.1% or less of Al, with the remainder comprising Fe and inevitable impurities; reheating the produced slab; hot-rolling the reheated slab; pre-rolling the hot-rolled slab at a reduction ratio of 30% or more; and hot-annealing the pre-rolled slab.

Description

Ferritic stainless steel hot-rolled steel sheet with excellent formability and manufacturing method thereof

The present invention relates to a ferritic stainless steel hot-rolled steel sheet with excellent formability and a manufacturing method.

Ferritic stainless steel cold-rolled products have excellent high-temperature properties such as thermal expansion coefficient and thermal fatigue, are resistant to stress corrosion cracking, and have excellent high-temperature strength. Based on these characteristics, ferritic stainless steel is applied to automobile exhaust systems, household appliances, structures, home appliances, and elevators. It is important to secure formability in order to expand the application field of ferritic stainless steel. The deformation mechanism of ferritic stainless steel is by plastic induced transformation, and unlike austenitic stainless steel with high workability, deformation occurs by generation and movement of dislocations. It is characterized in that the formability of ferritic stainless steel is changed by controlling the impurities that hinder the movement of the ferritic stainless steel. In the past, many researchers have tried to improve the formability of ferritic stainless steel, but there is a problem that is vulnerable to elongation molding. In addition, since hot-rolled steel has a low strain rate and does not generate dislocations smoothly, it is difficult to secure formability as much as cold-rolled cold-rolled annealed steel even if it has the same thickness by itself, so hot-rolled steel is applied to products requiring high formability. there is a limit to

In order to solve this problem, the present invention controls the band structure formed in the hot-rolled material by rolling it at a predetermined reduction ratio before annealing the hot-rolled steel sheet and further activates the generation of dislocations to refine the crystal grains after the hot-rolled annealing and at the same time form An object of the present invention is to manufacture a stainless steel hot-rolled steel sheet that exhibits formability similar to that of cold-rolled steel even though it is a hot-rolled steel by increasing its properties.

Ferritic stainless steel hot-rolled steel sheet with excellent formability according to an embodiment of the present invention, in weight%, C: 0.001 ~ 0.1%, Cr: 10.0 ~ 14.0%, Ti: 0.001 ~ 0.5%, Nb: 0.001 ~ 0.5% , Ni: 0.001~1.5%, Mn: 0.001~1.5%, Cu: 0.001~1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, remaining Fe and unavoidable impurities After hot rolling, the R-bar value after pre-rolling with a reduction ratio of 30% or more is 1.08 or more, and satisfies TS / YS ≤ 1.5, which represents the ratio of yield strength (YS) and tensile strength (TS).

Method for manufacturing a ferritic stainless steel hot-rolled steel sheet excellent in formability according to another embodiment of the present invention, in weight%, C: 0.001 ~ 0.1%, Cr: 10.0 ~ 14.0%, Ti: 0.001 ~ 0.5%, Nb: 0.001~0.5%, Ni: 0.001~1.5%, Mn: 0.001~1.5%, Cu: 0.001~1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, remaining Fe and unavoidable impurities Manufacturing a slab by continuously casting a steel comprising a; reheating the manufactured slab; hot rolling the reheated slab; R-bar value is 1.08 or more, pre-rolling at a reduction ratio of 30% or more to satisfy TS / YS ≤ 1.5 representing the ratio of yield strength (YS) and tensile strength (TS); and hot rolling annealing.

According to the present invention, when the hot-rolled steel sheet is rolled at a reduction ratio of 30% or more before annealing, the band structure formed in the hot-rolled material is controlled and the generation of dislocations is additionally activated to refine the crystal grains after hot-rolling annealing and at the same time increase formability to heat Despite being a soft steel, it is possible to manufacture a stainless steel hot-rolled steel sheet exhibiting formability similar to that of a cold-rolled steel.

1 shows the microstructure of Example 1 (ferritic stainless steel 35% pre-rolling annealed material).

Figure 2 shows the microstructure of Example 2 (ferritic stainless steel 50% pre-rolling annealed material).

Figure 3 shows the microstructure of Comparative Example 1 (ferritic stainless steel hot-rolled annealed material (2.0t)).

Figure 4 shows the microstructure of Comparative Example 2 (a conventional cold-rolled annealed material of ferritic stainless steel).

Ferritic stainless steel hot-rolled steel sheet with excellent formability according to an embodiment of the present invention, in weight%, C: 0.001 ~ 0.1%, Cr: 10.0 ~ 14.0%, Ti: 0.001 ~ 0.5%, Nb: 0.001 ~ 0.5% , Ni: 0.001~1.5%, Mn: 0.001~1.5%, Cu: 0.001~1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, remaining Fe and unavoidable impurities After hot rolling, the R-bar value after pre-rolling with a reduction ratio of 30% or more is 1.08 or more, and satisfies TS / YS ≤ 1.5, which represents the ratio of yield strength (YS) and tensile strength (TS).

Various methods have been studied so far regarding methods for improving the formability of ferritic stainless steels. In general, there is a method of managing the hot-rolled reheating temperature at a low level or rolling at a high reduction ratio at the end of rough rolling to activate the introduction of dislocations to facilitate recrystallization. Since it is difficult to form, it causes various surface defects including sticking defects, and in severe cases, there is also a problem of fracture due to cracks, so there are clear limitations in practical application.

In order to overcome these disadvantages, in the present invention, the hot-rolled steel sheet is rolled at a predetermined reduction ratio before annealing to control the band structure formed in the hot-rolled material and additionally activates the generation of dislocations to refine the crystal grains after hot-rolling annealing and at the same time formability It is intended to provide a method for producing ferritic stainless steel exhibiting formability similar to that of cold-rolled steel by increasing the

[Ferritic stainless hot-rolled steel sheet]

Ferritic stainless steel hot-rolled steel sheet with excellent formability according to an embodiment of the present invention, in weight%, C: 0.001 ~ 0.1%, Cr: 10.0 ~ 14.0%, Ti: 0.001 ~ 0.5%, Nb: 0.001 ~ 0.5% , Ni: 0.001~1.5%, Mn: 0.001~1.5%, Cu: 0.001~1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, remaining Fe and unavoidable impurities After hot rolling, the R-bar value after pre-rolling with a reduction of 30% or more is 1.08 or more, and satisfies TS / YS ≤ 1.5, which represents the ratio of yield strength (YS) and tensile strength (TS), Preferably, the pre-rolling reduction ratio may be 35% or more.

In addition, according to another embodiment of the present invention, the ferritic stainless steel hot-rolled steel sheet having excellent formability is ferrite grain size when the crystal grain size is measured by EBSD (ELECTRON BACKSCATTERING DIFFRACTION) analysis method in the range of 15 ° to 180 ° between grain orientation differences may be 60 μm or less.

In the following, unless otherwise specified, units are % by weight. In addition, when a part "includes" a certain component, it means that it may further include other components without excluding other components unless otherwise stated.

(Ingredient range)

C: 0.001 to 0.1%

If the amount of carbon (C) is large, the strength is improved, but the workability is lowered. In order to obtain sufficient strength, 0.001% or more of carbon (C) must be contained, but if it exceeds 0.1%, there is a problem in that workability is reduced.

Cr: 10.0 to 14.0%

The amount of chromium (Cr) is preferably 10.0% or more and 14.0% or less. Chromium (Cr) is the most important element added to secure corrosion resistance of stainless steel, and in the present invention, 10.0% or more of chromium (Cr) is added to increase corrosion resistance. When the amount of chromium (Cr) exceeds 14.0%, it causes hot rolling sticking defects, so it is preferably 14.0% or less.

Ti: 0.001 to 0.5%

The amount of titanium (Ti) is preferably 0.001% or more and 0.5% or less. If the amount of titanium (Ti) is less than 0.001%, the amount of crystallization of TiN is reduced, and the equiaxed crystallization rate of the slab is lowered, and the amount of dissolved C and N elements increases, resulting in a decrease in elongation. There is a problem of deterioration of workability.

Nb: 0.001 to 0.5%

The amount of niobium (Nb) is preferably 0.001% or more and 0.5% or less. Niobium (Nb) preferentially combines with carbon (C) and nitrogen (N) to form a precipitate that suppresses the deterioration of corrosion resistance. The content is limited to 0.001% or more and 0.5% or less.

Ni: 0.001 to 1.5%

The amount of nickel (Ni) is preferably 0.001% or more and 1.5% or less. Nickel (Ni) is an element that improves corrosion resistance, but when a large amount is added, it is preferably 1.5% or less because there is a risk of not only hardening but also stress corrosion cracking.

Mn: 0.001 to 1.5%

The amount of manganese (Mn) is preferably 0.001% or more and 1.5% or less. Manganese (Mn) has the effect of increasing the strength of the steel, but when it is excessively contained, the generation of Mn-based fume rapidly decreases weldability, and the ductility of the steel decreases due to the formation of excessive MnS precipitates.

Cu: 0.001~1.0%

The amount of copper (Cu) is preferably 1.0% or less. Copper (Cu) has an effect of improving corrosion resistance, but when it exceeds 1.0%, there is a problem in that workability is lowered.

Si: 0.001 to 2.0%

Silicon (Si) is an element added for deoxidation and ferrite stabilization of molten steel during steelmaking, and it is preferably 0.001% or more, but if its content is excessive, it causes hardening of the material and lowers the ductility of the steel. Limit it to 2.0% or less.

N: 0.001 to 0.1%

The amount of nitrogen (N) is preferably 0.001% or more and 0.1% or less. Nitrogen (N), like carbon (C), has the effect of increasing the strength of the material. If the amount of nitrogen (N) is less than 0.001%, TiN crystallization is lowered and the equiaxed crystallization rate of the slab is lowered, and if it exceeds 0.1%, impurities in the material increase and the elongation rate decreases.

Al: 0.1% or less

The amount of aluminum (Al) is preferably 0.1% or less. When the amount of aluminum (Al) exceeds 0.1%, there is a problem in that the elongation rate decreases due to an increase in impurities in the material.

[Method of manufacturing ferritic stainless hot-rolled steel sheet]

In the manufacturing method of a ferritic stainless steel hot-rolled steel sheet excellent in formability according to an embodiment of the present invention, in weight%, C: 0.001 to 0.1%, Cr: 10.0 to 14.0%, Ti: 0.001 to 0.5%, Nb: 0.001 ~0.5%, Ni: 0.001~1.5%, Mn: 0.001~1.5%, Cu: 0.001~1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, remaining Fe and unavoidable impurities Manufacturing a slab by continuously casting a steel comprising; reheating the manufactured slab; hot rolling the reheated slab; R-bar value is 1.08 or more, pre-rolling at a reduction ratio of 30% or more to satisfy TS / YS ≤ 1.5 representing the ratio of yield strength (YS) and tensile strength (TS); and

It includes the step of hot rolling annealing.

The hot rolling reheating temperature is in the range of 1100 ° C to 1280 ° C. If the slab is reheated to a temperature of 1280℃ or higher, there is a high possibility of a coarse band structure remaining, which may reduce formability. there is

Hereinafter, the present invention will be described in more detail through examples.

The following examples are presented to sufficiently convey the spirit of the present invention to those skilled in the art to which the present invention belongs, and the present invention is not limited to only the examples presented here, and may be embodied in other forms. there is.

(Example)

A slab was prepared by continuously casting steel having the composition shown in Table 1 below, and after reheating the slab, hot rolling was performed, with a hot-rolled thickness of 2.0 to 5.0 t, a thickness after pre-rolling of 2.0 t, and a reduction ratio of 0 to 50%. For ferritic stainless steel hot-rolled steel sheets that were pre-rolled and then hot-rolled and annealed, grain size, R value (R ₀ , R ₄₅ , R ₉₀ , R-bar), yield strength, tensile strength, TS/YS, and Elongation was measured.

division (weight%)	C	Si	Mn	Cr	Ni	Cu	Ti	Nb	N	Al
Example 1	0.007	0.446	0.26	11.12	0.075	0.013	0.19	0.002	0.0077	0.032
Example 2	0.006	0.447	0.26	11.30	0.073	0.010	0.21	0.003	0.0052	0.022
Comparative Example 1	0.006	0.472	0.37	11.15	0.067	0.009	0.18	0.002	0.0057	0.023
Comparative Example 2	0.008	0.466	0.36	11.17	0.072	0.008	0.22	0.002	0.0079	0.052

It shows the microstructure of the conventional cold-rolled annealed material (2.0t) including the hot-rolled annealed material (2.0t) according to the load rate. Comparative Example 1 is a conventional 2.0t hot-rolled steel and has an average grain size of about 120 μm, which is very coarse. Example 1 shows the microstructure of a material hot-rolled after pre-rolling a 3.1t thick hot-rolled steel sheet at a reduction rate of about 35%, and as shown in Table 2, the grain size is about 60㎛, Compared to the microstructure of Comparative Example 1 in which the conventional hot-rolled 2.0t steel sheet was annealed, it was found that the crystal grains were finer to a level of 50%. It can be assumed that the crystal grains are refined due to the additional generation of dislocations introduced when rolling the hot-rolled material at a reduction ratio of 30% or more. Example 2 shows the microstructure of a material obtained by pre-rolling a 4.0t thick hot-rolled steel sheet at a reduction rate of about 35% and then hot-rolling and annealing, and the grain size is fine to about 50 μm, increasing the pre-rolling reduction rate It can be seen that the crystal grains become finer as time goes on. Comparative Example 2 is a conventional cold-rolled annealed material obtained by cold-rolling and cold-rolling a 5.0t hot-rolled material after hot-rolling/pickling and then cold-rolling at a reduction ratio of about 60%. It shows a grain size similar to the rolling Example 2.

division	hot rolled thickness	Thickness after pre-rolling	Pre-rolling reduction rate	Grain size
Example 1	3.1t	2.0t	35%	59.5㎛
Example 2	4.0t	2.0t	50%	50.7㎛
Comparative Example 1	2.0t	2.0t	0%	121.5㎛
Comparative Example 2	5.0t	2.0t	Cold rolled rolling reduction 60%	54.9㎛

Table 3 shows the formability (R value) of hot-rolled annealed materials according to the pre-rolling reduction ratio and the formability of conventional cold-rolled annealed materials. The R-bar value, which represents the perpendicular anisotropy of the material, has the same properties in the plane direction of the plate, but the properties in the thickness direction are different from those in the plane direction. Since it has , the average value of R values measured by stretching in the rolling direction and 0 degree, 45 degree, and 90 degree directions is used (R-bar=(R ₀ + 2R ₄₅ + R ₉₀ )/4).

It can be seen that the R-bar value of Comparative Example 1 (normal 2.0t hot rolled annealed material) is very poor at about 1.01 level, but the R-bar value gradually increases as the pre-rolling reduction gradually increases. In addition, it can be seen that the R-bar value of Example 2 (50% pre-rolled hot-rolled annealed material) is about 1.21, which is similar to the R-bar value of Comparative Example 2 (normal cold-rolled annealed material). .

division	hot rolled thickness	Thickness after pre-rolling	Pre-rolling reduction rate	R 0	R45	R90	R-bar
Example 1	3.1t	2.0t	35%	0.77	1.02	1.51	1.08
Example 2	4.0t	2.0t	50%	1.16	0.99	1.7	1.21
Comparative Example 1	2.0t	2.0t	0%	0.54	0.77	1.01	0.77
Comparative Example 2	5.0t	2.0t	Cold rolling reduction 60%	1.22	0.85	2.14	1.27

Table 4 shows the tensile properties of each material, and the tensile strength value of Example 2 (2.0t material hot-rolled after about 50% pre-rolling) is the tensile strength value of Comparative Example 2 (normal cold-rolled material) There is no significant difference in contrast, but the yield strength value appears as high as about 10% or more in Example 2. Compared to normal cold-rolled products that undergo a total of two or more annealing processes of hot-rolling and cold-rolling annealing, pre-rolling materials have higher yield strength than conventional cold-rolled annealing materials because they additionally introduce dislocation and undergo only one hot-rolling annealing process. It is considered to represent These characteristics can be applied to products that require high-strength materials, but it is considered to be helpful in extending the lifespan of existing products.

division	hot rolled thickness	Thickness after pre-rolling	Pre-rolling reduction rate	YS (MPa)	TS (MPa)	TS/YS		EL(%)
Example 1	3.1t	2.0t	35%	306	415	1.36		33
Example 2	4.0t	2.0t	50%	293	405	1.38		36
Comparative Example 1	2.0t	2.0t	0%	270	419	1.55		34
Comparative Example 2	5.0t	2.0t	Cold rolling reduction 60%	261	397	1.52		35

Although the ferritic stainless steel hot-rolled steel sheet according to an embodiment of the present invention is a hot-rolled material, it exhibits formability similar to that of a cold-rolled material, and thus industrial applicability is recognized.

Claims (3)

1. In % by weight, C: 0.001 to 0.1%, Cr: 10.0 to 14.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5%, Ni: 0.001 to 1.5%, Mn: 0.001 to 1.5%, Cu: 0.001 to 0.001% 1.0%, Si 0.001~2.0%, N: 0.001~0.1%, Al: 0.1% or less, after hot rolling the steel containing the remaining Fe and unavoidable impurities, R- after pre-rolling at a reduction ratio of 30% or more A ferritic stainless steel hot-rolled steel sheet with excellent formability that has a bar value of 1.08 or more and satisfies TS/YS≤1.5 representing the ratio of yield strength (YS) and tensile strength (TS).
2. The method of claim 1,

   A ferritic stainless steel hot-rolled steel sheet with excellent formability, with a ferrite grain size of 60㎛ or less when the grain size is measured by the EBSD (ELECTRON BACKSCATTERING DIFFRACTION) analysis method in the range of 15° to 180° for the orientation difference between grains.
3. In % by weight, C: 0.001 to 0.1%, Cr: 10.0 to 14.0%, Ti: 0.001 to 0.5%, Nb: 0.001 to 0.5%, Ni: 0.001 to 1.5%, Mn: 0.001 to 1.5%, Cu: 0.001 to 0.001% 1.0%, Si 0.001 ~ 2.0%, N: 0.001 ~ 0.1%, Al: 0.1% or less, the remaining Fe, and manufacturing a slab by continuously casting a steel containing unavoidable impurities;

   reheating the manufactured slab;

   hot rolling the reheated slab;

   R-bar value is 1.08 or more, pre-rolling at a reduction ratio of 30% or more to satisfy TS / YS ≤ 1.5 representing the ratio of yield strength (YS) and tensile strength (TS); and

   Method for producing a ferritic stainless steel hot-rolled steel sheet with excellent formability comprising the step of hot-rolling annealing.

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