JPH05320772A - Production of ferritic stainless steel - Google Patents

Abstract

(57) [Summary] [Objective] To provide a method for producing a ferritic stainless steel sheet having excellent workability. [Constitution] C 0.015% or less, N 0.015% or less, Cr 10 to 14%, Si 1% or less, Mn 1% or less, P 0.04% or less, S 0.010% or less, Ti
0.05-0.3%, Al 0.1% or less, if necessary Cu 0.5% or less, V 0.5% or less, Nb
0.5% or less, Ni 1% or less and Mo 1% or less, and the balance Fe and unavoidable impurities, and the product of effective Ti% (= Ti% -4C% -3.4N%) and P%. A ferritic stainless steel having a composition in which the content of Ti, C, N, and P is limited to 0.007 or less is hot-rolled, the obtained steel sheet is water-cooled, and wound on a coil at a temperature of 750 ° C or less. This hot-rolled coil is pickled without being annealed, cold-rolled as usual, and then continuously finish annealed and pickled.

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 ferritic stainless steel sheet having excellent workability.

[Prior Art]

It is known that a fully ferritic stainless steel containing low C and N and stabilizing elements Ti, Nb, etc. exhibits excellent workability. Among them, the Cr content is 10-14
%, The low Cr ferritic stainless steel sheet whose main stabilizing element is Ti is soft and has excellent elongation and deep drawability, so workability of automobile exhaust gas materials is required.
Used in materials where the corrosive environment is relatively mild.

Unlike ordinary low-carbon steel, ordinary ferritic stainless steel anneals a hot-rolled steel sheet by box annealing or continuous annealing, but various steps have been attempted without annealing. The problem with such a method is a method of improving the workability of a cold-rolled steel sheet, and the method of hot rolling is mainly devised. Japanese Unexamined Patent Publication (Kokai) No. 52-95527 discloses a method of lowering the yield strength and increasing the elongation by winding at a temperature of 850 ° C. or higher.

In Japanese Patent Laid-Open No. 57-70231, A
A method by adding l and low temperature winding up to 700 ° C. is shown. These methods are intended for steel types that have a two-phase mixed structure of ferrite-austenite during hot rolling, and the ferrite single-phase Ti targeted by the present invention is Ti.
The situation is slightly different from the low Cr ferritic stainless steel.

[Problems to be Solved by the Invention]

When the hot rolled steel sheet is not annealed in the manufacturing process of the Ti-containing low Cr ferritic stainless steel sheet, there is a problem that the strength of the cold rolled steel sheet increases and the elongation decreases. .. In this type of ferritic stainless steel sheet, elongation rather than strength is often emphasized, so there is a strong demand for development of a method for obtaining a low Cr ferritic stainless steel sheet with excellent workability without annealing the hot rolled steel sheet. It has been done.

[0006]

Therefore, the inventor of the present invention
Regarding the i-containing low Cr ferritic stainless steel, first, the precipitates in the hot-rolled steel plate and the cold-rolled steel plate were investigated, and it was found that the fine precipitates of Ti, P and Fe increase the strength of the cold-rolled steel plate when hot-rolled sheet annealing is omitted And that it is affecting the decline in growth. Further, as a result of investigating the amount of precipitation, it was possible to sort out by the following formula.

[0007]

[Equation 1] log ([solid solution Ti%] · [solid solution P%]) = 4.1-
7500 / T (T is absolute temperature)

The Ti PFe precipitates are mainly precipitated in the cooling process after the completion of hot rolling. On the other hand, TiC and TiN that started to precipitate at a higher temperature are almost completely precipitated in such a temperature range of the cooling process after the above-mentioned hot rolling is completed, and the precipitates of TiC and TiN are It is stable. Therefore, the solid solution Ti that can contribute to the precipitation of TiPFe is the total T
It has been found that it may be defined by the balance obtained by subtracting Ti% spent for forming carbides and nitrides from i%.

That is, the balance (Ti% -4C% -3.4N%) obtained by subtracting the Ti content (4C% + 3.4N%) used for forming carbides and nitrides from the total Ti content is the effective Ti%.
When defined as, the Ti content required for precipitation of TiPFe in the steel sheet is the effective Ti% from the temperature at which hot rolling is performed to the temperature at which cooling and winding are performed.

Since the equilibrium relationship between solid solution Ti and solid solution P (Ti and P not forming precipitates) relating to the precipitation of TiPFe is given by the equation (1), it is large if the product of effective Ti% and P% is large. Since TiPFe begins to precipitate even at a much higher temperature, and the transition introduced during hot rolling serves as precipitation nuclei,
TiPFe will be finely precipitated.

On the basis of such knowledge, first, the finish annealing temperature after cold rolling is usually in a relatively low temperature range, for example, 8
After fixing at one point within 60 to 900 ° C., various hot rolling conditions were changed, and the workability of the cold rolled steel sheet was investigated.
In order to prevent precipitation of TiPFe during hot rolling, Ti
It was found that a cold-rolled steel sheet having excellent workability can be obtained by a method of controlling the contents of P and P and suppressing precipitation during cooling by water cooling after hot rolling.

[0012] Next, as a result of investigating the workability of the cold rolled steel sheet by varying the hot rolling conditions and the finish annealing temperature after cold rolling, it was found that the finish after cold rolling at a high temperature of 930 ° C or higher. It has been found that when the annealing temperature is performed, a cold-rolled steel sheet having excellent workability can be obtained even when the Ti and P contents are higher. The method of the present invention was completed based on these findings.

That is, 1. C 0.015% or less, N 0.015% or less, C
r 10-14%, Si 1% or less, Mn 1% or less, P
0.04% or less, S 0.010% or less, Ti0.05
.About.0.3%, Al 0.1% or less, balance Fe and unavoidable impurities, and more effective Ti% (= Ti% -4
C% -3.4N%) and P% are obtained by hot rolling a ferritic stainless steel having a composition in which the content of Ti, C, N and P is limited so that the product is 0.007 or less. The steel sheet is water-cooled and wound around a coil at a temperature of 750 ° C. or lower, and the hot-rolled coil is pickled without cold annealing and cold-rolled, and then subjected to continuous finish annealing and pickling. A method for producing a low Cr ferritic stainless steel sheet having excellent workability.

2. The method for producing a ferritic stainless steel sheet according to the above 1, wherein the continuous finish annealing is performed at a temperature of 930 ° C. or higher.

3. The product of effective Ti% and P% is 0.00
The content of Ti, C, N, and P is limited to 4 or less, and the water-cooled steel sheet after hot rolling is wound around a coil at a temperature of 650 ° C or less, and further subjected to continuous finish annealing at 930 ° C.
The method for producing a ferritic stainless steel sheet according to the above 1, characterized in that it is carried out at a temperature of 860 ° C. or higher not exceeding 100 °

4. Cu 0.5 for ferritic stainless steel
% Or less, V 0.5% or less, Nb 0.5% or less, Ni
1% or less, Mo 1% or less 1 type or 2 types or more are contained, The manufacturing method of the ferritic stainless steel plate of said 1 thru | or 3 characterized by the above-mentioned.

[Work]

(1) The reason why the content of each constituent element of the stainless steel described in the claims is numerically limited will be described below.

C and N are preferably as small as possible in order to improve workability and corrosion resistance, and are 0.015% or less in consideration of manufacturing cost.

Cr is a main element responsible for the corrosion resistance of stainless steel, and from this aspect, it is necessary to contain 10% or more. On the other hand, it is preferable that the amount is small from the viewpoint of workability. Therefore, it is limited to 10% or more and 14% or less.

Si and Mn are basic elements involved in deoxidation of steel and need to be added. However, excessive addition may impair workability, so the amounts are limited to 1% or less.

When P and S exceed a certain amount, they are harmful to the corrosion resistance, so 0.04% or less and 0.010% respectively.
Limited to:

Ti is added to improve workability. With the addition of less than 0.05%, C and N in steel are changed to Ti C,
The effect of fixing as Ti N is not sufficient and the workability improving effect is not remarkable. On the other hand, if the content exceeds 0.3%, the steel is hardened and the workability is deteriorated. Therefore, the Ti content is limited to the range of 0.05% to 0.3%.

Al is an element having a very high deoxidizing ability of steel, and S is added to enhance the deoxidizing and Ti addition yields.
It is added together with i and Mn. It also has the effect of increasing the oxidation resistance. On the other hand, excessive addition causes hardening of the steel and may deteriorate workability, so the addition amount is limited to 0.1% or less.

Cu, V, Ni and Mo are elements added as necessary in order to additionally enhance the corrosion resistance of ferritic stainless steel. 0.5 for Cu and V
%, Ni, Mo, if contained in excess of 1%, the degree of corrosion resistance improvement depending on the added amount will be reduced, and the strength of the steel sheet will be increased and the cost will be increased. It was set to 0.5% and 1%.

In order to prevent the precipitation of TiPFe during hot rolling, the Ti content and the P content are regulated by the formula of [effective Ti%] · [P%] ≦ 0.007. Here, the effective Ti% = Ti% -4C% -3.4N% is Ti
Ti content in the hot rolling temperature range (900 ° C or higher)
It means a value obtained by subtracting the amount of Ti spent for forming C and TiN from the total amount of Ti. [Effective Ti%]
By setting [P%] ≦ 0.007, fine precipitates of TiPFe are dissolved by finish annealing at 930 ° C. or higher, and a ferritic stainless steel sheet having good characteristics can be obtained.

On the other hand, when the finish annealing is carried out in a lower temperature range than usual, for example, 860 to 900 ° C., TiP is used.
Since Fe easily precipitates, the product of [effective Ti%] and [P%] is 0.00 in order to prevent precipitation of TiPFe.
It must be limited to 4 or less.

(2) The reason why the coiling temperature is set to 750 ° C. or lower by performing water cooling after the hot rolling is completed is to prevent fine precipitation of Ti PFe on the hot rolled sheet. The cooling rate at this time may be approximately 10 ° C./s or more, and can be achieved by ordinary water cooling. When it is wound up at 750 ° C or higher, the precipitates become large, and even if the finish annealing temperature is 930 ° C or higher, the TiPFe precipitates do not dissolve, which adversely affects the recrystallization start in the annealing process, and the elongation and depth of the cold rolled steel sheet. The drawability may be deteriorated. By winding at 750 ° C. or lower, good workability can be obtained as shown in Table 2 of the example, and thus it is defined as described above.

On the other hand, when finish annealing is performed in a lower temperature range than usual, for example, 860 to 900 ° C., it is necessary to set the coiling temperature after water cooling to 650 ° C. or less. If it is wound up above 650 ° C, precipitation will occur,
It may adversely affect the start of recrystallization in the finish annealing step and may lead to elongation of the cold rolled steel sheet and deterioration of deep drawability. When the finish annealing temperature is 860 to 900 ° C, 65
By winding at 0 ° C. or lower, good workability was obtained as shown in Table 3 of the examples.

(3) The reason why the hot rolled coil is pickled without being annealed is that the economical effect is extremely large as described above. Pickling of hot rolled coil can be done by ordinary means,
Generally, mechanical shot blasting, brush roll grinding, bender, tension leveler, etc. are combined with immersion in a pickling solution such as hydrochloric acid, sulfuric acid, nitric hydrofluoric acid, or electrolytic pickling. ..

(4) Cold rolling is carried out by a general cold rolling mill such as a Zenzimer mill or a tandem mill.

(5) Finish annealing is carried out continuously.
According to the box annealing, the temperature rising rate is small, and the workability may not be good even if the composition and the hot rolling conditions are defined in the above ranges, and thus it is excluded from the scope of the claims of the present invention.

At a finish annealing temperature of 930 ° C. or higher, low C
Since the high temperature strength of r-ferritic stainless steel is extremely small, it is necessary to design equipment so as to reduce the tension of the steel strip in the heating zone and the soaking zone in order to continuously anneal the steel sheet in such a temperature range.

On the other hand, at the finish annealing temperature of 860 to 900 ° C., it can be processed by the usual equipment and process.

The pickling may be carried out by an ordinary method, and is generally carried out by a combination of salt bath treatment, electrolytic pickling in sodium sulfate or sulfuric acid, dipping in nitric acid or nitric hydrofluoric acid.

[0035]

[Examples] [Example 1] 1.2 mm was obtained by using 3.6 mm thick hot-rolled steel sheets of steels having different Ti and P contents shown in Table 1.
Cold rolling, finish annealing (850-980 ° C _X 1-
After 5 minutes) and pickling, mechanical properties were investigated by a tensile test. Table 2 shows the hot rolling finish temperature, winding temperature, finish annealing conditions, [effective Ti%] / [P%], yield strength and elongation of the cold rolled steel sheet, which are summarized in Table 2. The yield strength and elongation were averaged in three directions by JIS 13B-shaped tensile test pieces in the directions of 0 °, 45 °, and 90 ° with respect to the rolling direction. As is clear from Table 2, the method of the present invention makes it possible to obtain a low Cr ferritic stainless steel sheet having excellent workability.

Example 2 Next, using 3.6 mm hot-rolled steel sheets of steels having different Ti and P contents shown in Table 1, 1.2
The conditions of cold rolling up to mm and finish annealing were fixed at 880 ° C. for 3 minutes, pickling was performed, and then mechanical properties were examined by a tensile test. Table 3 shows the hot rolling finish temperature, winding temperature, [effective Ti%] · [P%] of the hot rolled steel sheet, yield strength and elongation of the cold rolled steel sheet.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

The yield strength and elongation are 0 in the rolling direction.
The three-direction average value by the JIS13B shape tensile test piece in the directions of degrees, 45 degrees and 90 degrees was used. As is clear from Table 2, when the effective Ti% x P% and the winding temperature are out of the limited ranges of [claim 1] to [claim 3] of the present invention, the elongation is a predetermined value (35% or more). ) Is not reached,
On the contrary, the yield strength becomes too high (230M
workability deteriorates, but when it is within the above range, a predetermined elongation and yield strength are obtained, and excellent workability is exhibited.

[0041]

Industrial Applicability According to the present invention, it is possible to obtain a low Cr ferritic stainless steel sheet having excellent workability without carrying out hot-rolled sheet annealing, and the industrial advantage is extremely high.

Claims (4)

[Claims] 1. C 0.015% or less, N 0.015
% Or less, Cr 10 to 14%, Si 1% or less, Mn 1%
Below, P 0.04% or less, S 0.010% or less,
Ti 0.05 to 0.3%, Al 0.1% or less, balance Fe and inevitable impurities, and effective Ti%
The product of (= Ti% -4C% -3.4N%) and P% is 0.
A ferritic stainless steel having a composition in which the content of Ti, C, N, and P is limited to 007 or less is hot-rolled, the obtained steel sheet is water-cooled, and wound into a coil at a temperature of 750 ° C or less, A method for producing a low Cr ferritic stainless steel sheet excellent in workability, which comprises subjecting this hot rolled coil to pickling without annealing, cold rolling, and then continuous finish annealing and pickling. 2. The method for producing a ferritic stainless steel sheet according to claim 1, wherein the continuous finish annealing is performed at a temperature of 930 ° C. or higher. 3. A steel sheet water-cooled after hot-rolling at a temperature of 650 ° C. or lower by limiting the contents of Ti, C, N and P so that the product of effective Ti% and P% is 0.004 or less. 2. The method for producing a ferritic stainless steel sheet according to claim 1, wherein the ferritic stainless steel sheet is wound on a coil in accordance with 1. 4. Ferrite stainless steel is Cu 0.5%
Below, V 0.5% or less, Nb 0.5% or less, Ni 1
% Or less, Mo 1% or less, or a mixture of two or more thereof. 4. The method for producing a ferritic stainless steel sheet according to claim 1, wherein