JPH10183307A - High purity, ferritic and thin stainless steel sheet excellent in ridging resistance and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a high purity, ferritic and thin stainless steel sheet excellent in ridging-resistance. SOLUTION: A continuously cast slab of a high purity ferritic stainless steel in which the components of the steel are regulated so as to contain, by weight, <=0.015% C, <=0.020% N, <=1% Mn, 10.5 to 12% Cr and 8×(C+N) to 0.4% Ti and furthermore so as to satisfy either <=0.2% Si or <=0.03% Al, and the balance Fe with inevitable impurities is heated at <=1,150 deg.C, is subjected to hot rolling, is subjected to annealing of executing holding in the temp. range of 800 to 900 deg.C for >=2hr according to necessary and is thereafter subjected to cold rolling and annealing.

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 11% Cr high-purity ferritic stainless steel sheet having excellent ridging resistance.

[0002]

2. Description of the Related Art Ferritic stainless steel has excellent corrosion resistance and workability, and is inexpensive compared to austenitic stainless steel because it does not add expensive Ni. Therefore, it is widely used for home appliances, building materials, automobile exhaust system members, etc. It is used. In particular, C and N are extremely low, and T
A high-purity ferritic stainless steel in which the amounts of solid solution C and N in the steel are reduced by fixing these elements by i or the like is particularly excellent in press formability and deep drawability.

Generally, when a ferritic stainless steel sheet is press-formed, striped irregularities called ridging are produced along the rolling direction. Ridging not only impairs the aesthetics of the molded product, but also causes a polishing load to remove it, which is a problem when press-forming ferritic stainless steel.

There have been many reports on the ridging mechanism of ferritic stainless steels. However, a layered coarse ferrite band structure extending in the rolling direction existing in the solidification structure at the time of casting, hot rolling and hot rolling annealing structure ( Colony structure) remains after the subsequent cold rolling and annealing, and each structure shows different deformation behavior when press-formed due to the difference in plastic anisotropy, so that striped irregularities are parallel to the rolling direction. The idea that it occurs is mainstream. That is, in order to improve the ridging resistance, it is necessary to finely cut the coarse ferrite band structure in the hot-rolled sheet and the hot-rolled annealed sheet,
Many countermeasures have been developed so far.

For example, Japanese Patent Application Laid-Open No. 6-81036 discloses that the components are adjusted so that γmax (the maximum value of the austenite phase fraction generated during hot rolling) represented by the equation (1) is between 35 and 80. The resulting ferritic stainless steel slab is heated to 1150-1250 ° C. and hot-rolled, the finish rolling exit speed is set to 7.0 m / s or more, and 860 ° C.
A method in which finish rolling is completed as described above, and the film is wound at 650 ° C. or higher is disclosed. With this technology, ridging of 17% Cr ferritic stainless steel such as SUS430, which is a general-purpose steel, is greatly improved. However, it has no effect on 11% Cr-low C, N-Ti added steel (high-purity ferritic stainless steel) as handled in the present invention, and the ridging resistance is not improved.

Γmax = 420C + 470N + 23Ni + 7Mn-11.5Cr-11.5Si-23V-49Ti-50Nb-50Zr + 189 (1)

[0007]

In the case of 11% Cr high-purity ferritic stainless steel, increasing the γmax alone does not produce an austenite phase during hot rolling that is effective in dividing the ferrite band. That is, 1
Γm for 1% Cr high-purity ferritic stainless steel
With ax, the austenite phase fraction during hot rolling cannot be predicted. For example, C: 0.004%, N: 0.008%,
Mn: 0.4%, Cr: 11.0%, Ti: 0.16
%, Si: 0.5%, Al: 0.04%, the balance being 11% Cr high-purity ferritic stainless steel composed of Fe and unavoidable impurities, despite the fact that γmax is 57%, The austenite phase formed during hot rolling is only about 5% at most.

Accordingly, an object of the present invention is to increase the amount of austenite phase formed during hot rolling of 11% Cr high-purity ferritic stainless steel without adding a large amount of expensive alloying elements, and to cut a ferrite band. And to develop a method for reducing ridging.

[0009]

Means for Solving the Problems The present inventors have proposed that 11% C
r The method of generating austenite phase during hot rolling of a high-purity ferritic stainless steel to divide the ferrite band and improve ridging resistance was studied in detail in the laboratory. As a result, by lowering the amount of Si and Al in the steel composition and lowering the slab heating temperature to 1150 ° C. or less, even the 11% Cr high-purity ferritic stainless steel has sufficient austenite phase during hot rolling. It has been found that recrystallization proceeds at the interface between the austenite phase and the ferrite phase during hot rolling, so that a coarse ferrite band can be cut off and ridging resistance can be improved.

The present invention is based on this finding and has the following constitution. That is, (1) mass%
And C: 0.015% or less, N: 0.020% or less, M
n: 1% or less, Cr: 10.5 to 12%, Ti: 8 ×
(C + N) -0.4%, and further Si: 0.2%
Or Al: 0.03% or less, and if necessary, B: 0.005% or less, with the balance being Fe and unavoidable impurities. 1150 continuous cast slab
℃ or less, hot rolling is performed, and if necessary, 800
(2) a method for producing a high-purity ferritic stainless steel sheet excellent in ridging resistance, characterized in that the steel sheet is annealed for 2 hours or more in a temperature range of from 900C to 900C and then cold rolled and annealed. By mass%, C: 0.015% or less,
N: 0.020% or less, Mn: 1% or less, Cr: 10.
5-12%, Ti: 8 × (C + N) -0.4%, Si: 0.2% or less, Al: 0.03% or less,
Further, it is a high-purity ferritic stainless steel excellent in ridging resistance, which further contains B: 0.005% or less as necessary, and the balance consists of Fe and unavoidable impurities. By setting the Si content to 0.1% or less in this steel, the ridging resistance can be further improved.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS By reducing one or both of the amount of Si and the amount of Al in a high-purity ferritic stainless steel, the .gamma.max value does not change much, but the austenite phase forming ability in the steel increases, and the slab further increases. By setting the heating temperature to 1150 ° C. or lower, an austenite phase can be generated during hot rolling.

FIG. 1 shows C: 0.004%, N: 0.008.
%, Mn: 0.4%, Cr: 11.0%, Ti: 0.1
6% further (a) 0.5% of Si, 0.04 of Al
%, (B) Si: 0.06%, Al: 0.04%,
(C) 8% steel with 0.5% Si and 0.01% Al
It is the result of having measured the austenite phase ratio in the steel in the equilibrium state at the time of keeping for 30 minutes in each temperature range of 50 ° C-1200 ° C. (B) steel with reduced Si content as in (b), (c)
The austenitic phase is formed at 1150 ° C. or lower in steel having a reduced Al content as described above, and reaches its maximum at 1000 ° C. 100
The austenite phase ratio at 0 ° C. is only 5% for (a), 80% for (b) and 45% for (c),
It can be seen that the austenite phase ratio is increased by decreasing the amount or Al amount. The effect of Si reduction is greater than the effect of Al reduction.

When an austenite phase is formed in the ferrite phase, the interface becomes a nucleation site for recrystallization, and a coarse ferrite band can be separated by accelerating recrystallization during hot rolling, particularly at a later stage of rough rolling. Ridging resistance can be increased.

FIG. 2 shows C: 0.004% and N: 0.008.
%, Mn: 0.4%, Cr: 11.0%, Ti: 0.1
A high-purity ferritic stainless steel vacuum-melted IC slab with a Si content of 6% and Al: 0.04% was used for 114
The results of measuring the ridging height when heated to 0 ° C., hot-rolled in a laboratory hot rolling mill, further cold-rolled, and finally annealed are shown. The effect of reducing ridging is recognized in a region where the amount of Si is 0.2% or less, and ridging is further suppressed in a region where the amount of Si is 0.1% or less.

When higher ridging resistance is required, it is effective to add an annealing step of maintaining the hot-rolled steel strip in a temperature range of 800 ° C. to 900 ° C. for 2 hours or more. By performing this annealing, the structure after hot rolling is completely recrystallized, and the recrystallized structure is made finer, so that the ferrite band is more finely divided.

Next, the limited range of the present invention will be described. The reasons for limiting the component content of the steel subject to the present invention are as follows. C: It is necessary to be 0.015% or less. 0.015%
When the content exceeds the range, the corrosion resistance of the weld heat affected zone is likely to be deteriorated due to the precipitation of Cr carbide at the grain boundary. In addition, since it is an interstitial solid solution element, it strengthens the steel, and if it exceeds 0.015%, the workability deteriorates. For these reasons, the upper limit is made 0.015%.

N: It is necessary to be 0.020% or less.
Since it is an interstitial solid-solution element, it strengthens the steel. If the content exceeds 0.020%, the workability deteriorates. Therefore, the upper limit is set to 0.020%.

Mn must be 1% or less. It is an element that is effective in deoxidizing steel and also enhances the austenite phase forming ability of steel.
Since the amount of nS generated increases and the corrosion resistance deteriorates, the upper limit is set to 1%.

Cr: 10.5% or more and 12% or less. It is a basic element of stainless steel, and must contain at least 10.5% or more in order to obtain necessary corrosion resistance. However, if the content exceeds 12%, toughness and workability deteriorate, and the alloy cost further increases. Therefore, the upper limit is set to 12%.

It is necessary to be at least 8 times Ti: C + N and 0.4% or less. Ti is easily bonded to C and N, and the workability can be enhanced by the action of substantially reducing the amounts of C and N dissolved in the matrix. But this effect is 8
It does not appear if the content is less than × (C + N). Also, 0.4
%, The hot workability of the steel is reduced, which causes flaws during hot rolling. Therefore, the upper limit is set to 0.4%.

Si, Al: It is necessary to reduce the content of one or both of Si and Al so that the content of Si is 0.2% or less and the content of Al is 0.03% or less. Each element is an effective element as a steel deoxidizer,
By reducing the content of these elements, the austenite phase forming ability during hot rolling of 11% Cr high-purity ferritic stainless steel can be increased. This effect appears when Si: 0.2% or less, preferably 0.1% or less, and Al: 0.03% or less. The contents of Si and Al can be selected depending on the heating temperature of the slab as described later.

B: For applications that require higher secondary workability, it is effective to add 0.005% or less. B has the effect of repairing defects in the steel generated when processing the product and improving the secondary workability. However, if the B content exceeds 0.005%, the hot workability of the steel is significantly reduced, and cracks and flaws occur during hot rolling, so the upper limit was made 0.005%.

When the content of only one of the Si content and the Al content is reduced, the slab heating temperature during hot rolling needs to be 1150 ° C. or less. During the slab heating, it corresponds to a thermodynamic equilibrium state, and the austenite phase ratio as shown in FIG. 1 with respect to the slab heating temperature. By generating an austenitic phase in the steel at the stage of slab heating before hot rolling, during hot rolling (temperature reduction,
(Deformation), thereby facilitating the increase of the austenite phase, thereby contributing to the splitting of the ferrite band.

When the slab heating temperature exceeds 1150 ° C.,
During the slab heating (equilibrium state), almost no austenite phase is formed, making it difficult to form the austenite phase during hot rolling. As a result, the re-forming during hot rolling starting from the interface between the austenite phase and the ferrite phase is started. The effect of dividing the ferrite band by the crystal cannot be obtained.

When the slab heating temperature is 1150 ° C. or lower, the austenite phase ratio during slab heating increases as the temperature decreases, and the amount becomes maximum at 1000 ° C. As a result, the amount of austenite phase generated during hot rolling increases as the slab heating temperature decreases, the effect of dividing the ferrite band increases, and the ridging resistance further improves. However, if the slab heating temperature is lower than 1050 ° C., a flaw may be generated on the surface of the steel sheet during hot rolling. Therefore, the lower limit of the slab heating temperature is preferably 1050 ° C.

Furthermore, since the temperature at which the amount of austenite is maximized is 1000 ° C., setting the rough rolling end temperature during hot rolling to 1000 ° C. or more promotes recrystallization during hot rolling, It is effective to divide the band.

When both the amount of Si and the amount of Al are reduced below the specified amounts, even if the slab heating temperature exceeds 1150 ° C., a sufficient austenite phase is formed during hot rolling. There is no need to limit to:

For applications requiring higher ridging resistance, the annealing performed on the hot-rolled steel strip is performed at 800 ° C. to 900 ° C.
It is necessary to keep the temperature in the range of 2 hours or more. If the annealing temperature is lower than 800 ° C., the structure after annealing cannot be completely recrystallized, and if the temperature exceeds 900 ° C., the structure after annealing becomes coarse due to grain growth. If the annealing time is less than 2 hours, the structure after annealing cannot be completely recrystallized.

[0029]

EXAMPLE Seven types of high purity ferritic stainless steels A to E having the chemical components shown in Table 1 were melted and continuously cast into 250 mm thick slabs. The slab was heated at a slab heating temperature shown in Table 2, hot-rolled to a thickness of 3.2 mm, and then, if necessary, annealed with a hot-rolled sheet, pickled, and then 0.9%.
It was cold-rolled to a thickness of 4 mm to obtain a final annealed product at 850 ° C. × 60 seconds. Table 2 also shows the results of measuring the ridging height when 16% tensile strain was applied to each product sheet in the cold.

[0030]

[Table 1]

In Table 2, no. 1 to 11 follow the method of the present invention. Steels B, C, D and F, in which either the Si content or the Al content was reduced, had a slab heating temperature of 1
By setting the temperature to 150 ° C. or lower, the ridging resistance is greatly improved, and the lower the slab heating temperature, the greater the effect of improving the ridging resistance (Nos. 1, 2, 3, 4, and 5). Si content,
Regarding steels D and E in which both the Al contents were reduced, when the slab heating temperature was 1150 ° C. or lower, excellent ridging resistance was exhibited, and this effect increased as the slab heating temperature became lower (No. 6, 7,10), slab heating temperature is 115
Even when the temperature exceeds 0 ° C., the ridging height can be kept low (No. 9, 11). Further, when hot-rolled sheet annealing is performed, the ridging resistance is further improved (No. 8).

On the other hand, the steel A, in which neither the Si content nor the Al content decreases, is inferior in ridging resistance regardless of the slab heating temperature (No. 12, 13, 14). Although slight improvement is observed, the ridging resistance is still inferior (No. 15). Also Si
For steels B and C in which either the amount of Al or the amount of Al was decreased, the ridging resistance was inferior when the slab heating temperature exceeded 1150 ° C (Nos. 16, 1).
7) Even after further hot-rolled sheet annealing, the ridging resistance is still inferior (No. 18).

[0033]

[Table 2]

[0034]

According to the present invention, 11 having excellent ridging resistance can be obtained.
% Cr high-purity ferritic stainless steel sheet can be manufactured.

[Brief description of the drawings]

FIG. 1 shows a sample of 11% Cr high-purity ferritic stainless steel having different amounts of Si and Al at 850 ° C. to 1200 ° C.
It is the result of having measured the austenite phase ratio in steel at the time of holding for 0 minute.

FIG. 2 is a diagram showing the results of measuring the ridging height of 11% Cr high-purity ferritic stainless steel sheet with varied amounts of Si.

Claims (6)

[Claims] 1. Mass%, C: 0.015% or less, N: 0.020% or less, Mn: 1% or less, Cr: 10.5 to 12%, Ti: 8 × (C + N) to 0. Continuity of high-purity ferritic stainless steel, which satisfies one of the following: Si: 0.2% or less, or Al: 0.03% or less, with the balance being Fe and unavoidable impurities. The cast slab is heated to 1150 ° C. or lower and hot-rolled,
A method for producing a high-purity ferritic stainless steel sheet having excellent ridging resistance, characterized by cold rolling and annealing. 2. In mass%, C: 0.015% or less, N: 0.020% or less, Mn: 1% or less, Cr: 10.5-12%, Ti: 8 × (C + N) -0. Continuity of high-purity ferritic stainless steel, which satisfies one of the following: Si: 0.2% or less, or Al: 0.03% or less, with the balance being Fe and unavoidable impurities. The cast slab is heated to 1150 ° C. or lower and hot-rolled,
The obtained hot-rolled steel strip is heated to a temperature of 800 ° C. to 900 ° C. for 2 hours.
A method for producing a high-purity ferritic stainless steel sheet having excellent ridging resistance, which comprises annealing for at least one hour, followed by cold rolling and annealing. 3. A high-purity ferritic stainless steel having excellent ridging resistance according to claim 1, wherein the high-purity ferritic stainless steel further contains B: 0.005% or less by weight%. Method of manufacturing stainless steel. 4. In mass%, C: 0.015% or less, N: 0.020% or less, Mn: 1% or less, Cr: 10.5 to 12%, Ti: 8 × (% C +% N) -0.4%, Si: 0.2% or less, Al: 0.03% or less, the balance being Fe and unavoidable impurities, the high purity ferritic stainless steel having excellent ridging resistance. steel. 5. In mass%, C: 0.015% or less, N: 0.020% or less, Mn: 1% or less, Cr: 10.5 to 12%, Ti: 8 × (% C +% N) -0.4%, Si: 0.1% or less, Al: 0.03% or less, the balance being Fe and unavoidable impurities, the high purity ferritic stainless steel excellent in ridging resistance. steel. 6. The high-purity ferritic stainless steel excellent in ridging resistance according to claim 4, further comprising B: 0.005% or less by weight%.