JPH04341521A - Production of hot rolled strip of ferrite single phase stainless steel excellent in ridging resistance - Google Patents

Abstract

PURPOSE:To improve the ridging resistance of a ferrite single phase steel. CONSTITUTION:At the time of subjecting a cast slab of a ferritic stainless steel having a composition consisting of <=0.03% C, <=2% Si, <=2% Mn, 18-30% Cr, <=0.03% N, 0.4-1.0% Nb, <=1.0% Al, and the balance Fe with inevitable impurities to hot rolling, the cast slab is held or reheated at or to a temp. between 1150 and 1300 deg.C in a heating furnace, subjected to roughing having a reduction pattern where reduction of <=30% per pass is done twice or more at an interval of deg.1 sec at 950-1100 deg.C, held at a temp. in the region between 900 and < 1000 deg.C for <=60sec, and then subjected to finish rolling including reduction of <=30% per pass.

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 hot-rolled ferritic single-phase stainless steel strip having excellent ridging resistance.

0002

2. Description of the Related Art Ferritic stainless steel, represented by SUS430, is widely used in products such as automobile parts, kitchen equipment, and household electrical equipment. This is because ferritic stainless steel has relatively excellent corrosion resistance, and
This is because it is superior in price to austenitic stainless steel because it does not contain i. However, when ferritic stainless steel is press-molded, a striped pattern of irregularities along the rolling direction called ridging occurs, which impairs the aesthetic appearance of the molded product and reduces its commercial value. Therefore, reducing or eliminating ridging has become a major issue in the production of ferritic stainless steel, and much research has been conducted on methods to prevent it.

[0003] In order to prevent ridging, it is important to obtain a uniform recrystallized structure in a hot rolled sheet.
In the publication No. 4016, low-temperature hot rolling is performed, and then 800~
A method is disclosed in which the ridging resistance is improved by performing box-shaped annealing at 830° C., followed by cold rolling and final annealing. In addition, Japanese Patent Publication No. 57-61096 discloses a method of hot rolling with a rolling reduction of 20% or more using a rolling mill with different diameters, followed by hot-rolled sheet annealing, cold rolling, and finish annealing, and furthermore, Japanese Patent Publication No. 1-111816 In the publication, hot rolling is performed at 850°C or higher, immediately cooled at 10°C/sec or higher, rolled up at 550°C or lower to form a two-phase structure of ferrite and martensite, and then cold rolled at a cumulative reduction rate of 50% or higher. A method has been proposed to improve the ridging property by applying All of these are intended primarily for ferritic stainless steels that have ferrite and austenite phases during hot rolling, and are not intended only for so-called ferritic single-phase steels.

0004

[Problems to be Solved by the Invention] In SUS430, the austenite phase precipitated during hot rolling can break up the band-like structure that causes ridging, so it is possible to reduce ridging by controlling the amount of austenite. On the other hand, although ferritic single phase steel has a high Cr concentration and is superior to SUS430 in terms of corrosion resistance, it generally has poorer ridging resistance than SUS430, and also has no austenite phase precipitation during hot rolling, making ridging easier. cannot be reduced.

[0005] Therefore, the present invention aims to improve the ridging resistance of ferritic single-phase steel.
The purpose of the present invention is to provide a method for producing a ferritic single-phase stainless steel hot-rolled steel strip that has excellent ridging resistance that could not be obtained using the production technology of No. 0.

[0006]

[Means for Solving the Problems] That is, the present invention provides C
: 0.03% or less, Si: 2% or less, Mn: 2% or less,
Ferritic stainless steel consisting of Cr: 18% or more and 30% or less, N: 0.03% or less, Nb: 0.4% or more and 1.0% or less, Al: 1.0% or less, balance Fe and inevitable impurities. When hot rolling a cast piece, after holding or reheating the cast piece at a temperature of 1150°C to 1300°C in a heating furnace, a reduction of 30% or more per pass within 1 second in a temperature range of 950°C to 1100°C. After rough rolling with a rolling pattern applied twice or more at intervals of 9
A ferritic single-phase stainless steel hot-rolled steel with excellent ridging resistance, characterized by being heat-retained in a temperature range of 00°C or more and less than 1000°C for 60 seconds or more, and then subjected to finish rolling including a reduction of 30% or more per pass. A method for manufacturing a belt is provided.

[0007]

[Operation] The present invention will be explained in more detail below.

First, the experiments that formed the basis of the present invention will be explained. The present inventors aimed to improve the ridging resistance by obtaining a uniform recrystallized structure in ferritic single-phase steel.
1%, Si: 0.25%, Mn: 0.24% or less, N:
0.015%, Cr: 19.6%, Nb: 0.4-1.
0%, Al: 0.10%, various compositions of ferritic stainless steel ingots were melted and made into 100mm x 160mm x
A 70 mm block was cut out and used as a test material. After heating this test piece to 1200°C, apply a reduction of 35% per pass to 0.2 to 3.0 in a temperature range of 900°C to 1150°C.
Rough rolling with a rolling pattern applied twice at intervals of less than 1 second, and immediately after the rough rolling, holding at 920°C for 0 to 600 seconds, followed by finish rolling including a reduction of 10 to 50% or more per pass, It was made into a hot-rolled plate with a thickness of 4 mm. This hot-rolled sheet was annealed at 950°C for 2 minutes, then made into a cold-rolled sheet with a thickness of 0.8 mm, and finished annealed at 920°C for 30 seconds. After applying a 20% tensile strain to this cold-rolled annealed plate using a JIS No. 5 tensile test piece, the ridging waviness height was measured using a surface roughness meter and evaluated based on the following criteria. Rigging grade Steel plate waviness height 1
Good 10~20μm2
20-30
μm3 30
~40μm4
40-70μm5 poor
>70μm

Regarding the above evaluation criteria, if the ridging grade is 2.0 or less, there will be no practical problem. Also

[Equation 1] is the average value of L (rolling direction), C (perpendicular direction to the rolling direction), and D (direction at 45 degrees to the rolling direction) after applying 15% tensile strain to a JIS No. 13 B test piece.

It was calculated as [Equation 2]. These experimental results are shown in Figure 2, Figure 3, Figure 4,
Shown in FIGS. 5 and 6.

FIG. 2 shows a test piece containing 0.6% Nb.
1 in the temperature range of 900°C to 1150°C after heating to 200°C
After rough rolling with a rolling pattern that applies a rolling reduction of 10% to 50% twice within 1 second per pass, it is rolled at 920°C.
The graph shows the relationship between rough rolling temperature, reduction ratio, and ridging grade under hot rolling conditions including holding for 0 seconds and 40% reduction in finish rolling, where ○ indicates ridging grade of 2.0 or less, and ● indicates Rigging grade is 2.0
It represents something that exceeds. As can be seen from the figure, in order to obtain a ridging grade of 2.0 or less, which does not cause any practical problems, a minimum reduction of 30% or more per pass in a temperature range of 950° C. or higher and 1100° C. or lower is required.

[0011] Figure 3 shows the test piece heated to 1200°C.
Nb content under hot rolling conditions including rough rolling with a rolling pattern in which 35% reduction is applied twice within 1 second at 0°C, followed by holding at 920°C for 90 seconds, and further including 40% reduction in finish rolling. The relationship between the amount and the ridging grade is shown, and it can be seen that the ridging resistance is improved by adding 0.4% or more of Nb. This means that Nb is N
This is thought to be due to the fine precipitation of b(C,N), which becomes the nucleus of recrystallized grains and promotes recrystallization.

FIG. 4 shows a test piece of ferritic single phase steel containing 0.5% Nb heated to 1200°C and then roughly rolled for 3.
The relationship between the pass interval time and the ridging grade of a cold-rolled annealed plate when 5% reduction is applied twice is shown (however, after rough rolling, the temperature was held at 920°C for 90 seconds, and then 40% reduction was applied in finish rolling). include). As is clear from the figure, if the pass interval time is set to 1 second or less, the ridging resistance is significantly improved.

FIG. 5 shows a test piece of ferritic single phase steel containing 0.5% Nb heated to 1200°C and then subjected to rough rolling with a rolling pattern in which 35% reduction is applied twice within 1 second at 1050°C. It shows the relationship between holding time and ridging after rolling (including 40% reduction in finish rolling). When the holding time is 60 seconds or more, the ridging grade shows a good value.

Furthermore, FIG. 6 shows that a test piece of ferritic single phase steel containing 0.4% Nb was heated to 1050°C after being heated to 1200°C.
This shows the relationship between rolling reduction and ridging in finish rolling when holding at 920°C for 90 seconds after rough rolling with a rolling pattern in which 35% reduction is applied twice within 1 second at ℃. By applying a reduction of 30% or more, it is possible to obtain a ridging grade that does not cause any practical problems. This can be confirmed by applying strong reduction rolling with a short pass interval time in rough rolling and then holding it at an isothermal temperature.
It is thought that b(C,N) is finely precipitated and that strain is accumulated by performing strong reduction rolling in the subsequent finish rolling, leading to the promotion of recrystallization.

Next, the reasons for limiting the hot rolling conditions in the present invention will be explained. Slab heating temperature 1100℃ or higher13
The reason for setting the temperature below 00°C is that rolling resistance is large when the temperature is below 1100°C, and the load on the rolls becomes large. This is because it becomes difficult to apply a rolling reduction pattern of more than 100° C., and when the temperature exceeds 1300° C., coarsening of ferrite grains occurs.

1 in the temperature range of 950°C to 1100°C
The reason why a reduction of 30% or more was applied twice or more at intervals of 1 second or more per pass is because the Nb (C, N) precipitation peak is in the temperature range of 950°C to 1100°C, and it is also necessary to improve the ridging resistance. This is because the greater the rolling reduction rate in one pass and the shorter the time between passes, as is clear from FIG. 4, are more effective in improving the properties.

Furthermore, holding the heat for 60 seconds or more in the temperature range of 900°C or more and less than 1000°C is because, as is clear from FIG. This is because no improvement can be expected.

[0018] Also, in finish rolling, 30
The reason for including the reduction of % or more is to improve the ridging resistance by accumulating strain and promoting recrystallization during annealing of the hot rolled sheet (see FIG. 6).

By performing rolling under the above-mentioned hot rolling conditions, Nb(C,N) is finely precipitated during hot rolling, and furthermore, strain is accumulated, and during annealing of the mother plate after hot rolling, Recrystallization is promoted, leading to improved ridging resistance.

Further, the reasons for limiting the components in the present invention will be explained. First, the reason why Cr is set to 18% or more and 30% or less is because if it is less than 18%, a two-phase structure of ferrite and austenite phase occurs during hot rolling, and sufficient corrosion resistance cannot be obtained. If it exceeds 30%, it will slow down, so the upper limit was set at 30%.

[0021] The Nb content is set to 0.4% or more and 1.0% or less in order to finely precipitate Nb as Nb(C,N), promote recrystallization, and improve the ridging resistance.
．． It is sufficient if it is 4% or more, and the effect is 1 from Figure 3.
．． If it exceeds 0%, it will reach saturation, so set the upper limit to 1.0%.
And so. The reason why Al is 1.0% or less is that Al is 1.0% or less.
This is because if added in excess of %, the toughness will decrease.

[0022] C and N are elements that have a large effect on the tensile properties of steel sheets, and if added in large amounts they will have an adverse effect on formability, so the upper limit for both is set at 0.03%.

[0023] The reason why Si is set to 2% or less is that Si is SiO2.
This is because inclusions are formed in the system and the moldability is deteriorated. Furthermore, since adding a large amount of Mn causes embrittlement of the steel, the upper limit was set at 2%.

[0024]

EXAMPLES The present invention will be specifically explained below based on examples. (Example 1) High Cr stainless steel ingots of the three steel types A, B, and C shown in Table 1 were melted and hot rolled to a thickness of 4 mm under the hot rolling conditions shown in Table 2 and Figure 1. Rolled plate at 950℃ x 2
After annealing for minutes, it was made into a cold-rolled plate with a thickness of 0.7 mm, and was heated at 920°C
After finishing annealing and pickling for 30 seconds, the material was investigated. The results are shown in Table 2. Among the steels A and B of the present invention having different amounts of Nb added, No. 2 satisfies the hot rolling conditions of the present invention. 1
, 2, 4, 5, and 8, which does not satisfy even one of the hot rolling conditions of the present invention. 3, 6, 7, 9, 10 are ridging grades,

[Equation 3] It can be seen that both are inferior. Furthermore, Nb-free steel C
In No. 1, which satisfies the hot rolling conditions of the present invention. 11. No. not satisfied. 12 both No. Rigging grade compared to 1, 2, 4, 5, 8,

[Equation 4] It can be seen that both are inferior. Therefore, the present invention can obtain maximum effects by satisfying all of the heating temperature conditions, rough rolling conditions, holding conditions, and finish rolling conditions recited in the claims.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

Effects of the Invention As is clear from the above examples, the method for producing a ferritic single-phase stainless steel hot-rolled steel strip with excellent ridging resistance according to the present invention can be applied to C, N, Cr, Nb, Si, M
n, the content of Al is limited, and especially the content of Cr is 18% or more.
% or less, Nb is added in a component system that does not form a two-phase structure of ferrite phase and austenite phase during hot rolling, and further, a rolling reduction of 30% or more per pass for 1 second in a temperature range of 950°C or more and less than 1100°C. After rough rolling with a rolling pattern applied twice or more at intervals of 900
Retain heat for 60 seconds or more in a temperature range of ℃ or higher and lower than 1000℃,
Thereafter, by performing hot rolling under hot rolling conditions in which finish rolling includes a reduction of 30% or more per pass, it was possible to obtain a ferritic single phase steel having excellent ridging resistance and high workability.

[0028]

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing a hot rolling pass schedule in Example 1.

FIG. 2 is a diagram showing the relationship between rough rolling temperature, rolling reduction, and ridging grade.

FIG. 3 is a diagram showing the relationship between the Nb content of ferritic single phase steel (19% Cr) and the ridging grade of a cold-rolled finish annealed sheet.

FIG. 4 is a diagram showing the relationship between pass interval time and ridging grade in rough rolling.

FIG. 5 is a diagram showing the relationship between the holding time after rough rolling and the ridging grade.

FIG. 6 is a diagram showing the relationship between rolling reduction and ridging grade in finish rolling.

Claims (1)

[Claims] [Claim 1] C: 0.03% or less, Si: 2% or less, Mn: 2% or less, Cr: 18% or more and 30% or less, N:
When hot rolling a ferritic stainless steel cast piece consisting of 0.03% or less, Nb: 0.4% or more and 1.0% or less, Al: 1.0% or less, and the balance Fe and unavoidable impurities, a heating furnace is used. After holding or reheating at a temperature of 1150℃ or higher and 1300℃ or lower, the temperature is 950℃ or higher and 110℃ or higher.
Pressure reduction of 30% or more per pass in the temperature range below 0℃
After performing rough rolling with a pattern of reduction applied twice or more at intervals of less than 2 seconds, heat is maintained in a temperature range of 900°C or more and less than 1000°C for 60 seconds or more, followed by finish rolling that includes a reduction of 30% or more per pass. A method for producing a ferritic single-phase stainless steel hot-rolled steel strip with excellent ridging resistance.