JPH08295941A - Production of ferritic stainless steel sheet excellent in ridging resistance - Google Patents

Abstract

PURPOSE: To provide a method for producing a ferrite single phase stainless steel added with Ti and Nb in which the generation of ridging at the time of forming can be suppressed to a level free from problems in practical use with hardly changing the coventional producing process. CONSTITUTION: In hot rolling for a ferritic stainless steel having a compsn. contg., by weight, 11 to 18% Cr, 4N to 1% Ti, satisfying Ti+Nb: 8(C+N) or above and contg. 0.02% C, 1% Si, 1% Mn, 0.04% P, 0.02% S, 0.02% N, <=0.5% Nb, and the balance Fe with inevitable impurities, it is executed in such a manner that, in the temp. range of 1100 to 950 deg.C, the passes satisfying the draft (R)>=1.4Cr+94TiE+16Nb-201ogt-30; where t: the time (sec) between the rolling passes, TiE: the effective Ti content (wt.%) and TiE=Ti-3.4N are executed for >=three times, and next, cold rolling is executed.

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 ridging resistance.

[0002]

2. Description of the Related Art Ferritic stainless steel sheets such as JIS-SUS430 are less expensive than austenitic stainless steel sheets and are used in a wide range of fields such as automobile parts and household appliances. In addition, reduction of C + N amount by improvement of refining technology of ferritic stainless steel, Ti, Nb
By designing the components such as carbonitride forming elements, ferritic stainless steel sheets have come to be produced in the same level of corrosion resistance and workability as austenitic stainless steel sheets.

However, it is known that the ferritic stainless steel sheet has a drawback that wrinkles called ridging are formed on the surface of the steel sheet during press forming, and the appearance of the steel sheet is significantly impaired.

Therefore, if this ridging occurs, it must be removed by an operation such as surface polishing after the molding process, which leads to an increase in manufacturing cost.

The basic idea for improving the ridging resistance is to improve the structure formed during casting and solidification, or hot rolling,
In the manufacturing process such as annealing and cold rolling, how to destroy the solidified structure to promote the random orientation of the metal crystal structure has been studied from that viewpoint.

Regarding the concrete measures against ridging,
The following proposals have already been made. (A) Refining of solidification structure, equiaxing (b) Recrystallization promotion during hot rolling, and (c) refining of crystal grains by forming a two-phase structure.

As specific means of (a), electromagnetic stirring during solidification in continuous casting, lowering of casting temperature, Ti, Nb,
Although there are additions of crystal refining elements such as Zr, it is difficult to completely transform the solidification structure of continuous casting into a fine equiaxed crystal structure. Even if a perfect equiaxed crystal solidification structure is obtained, the ridging resistance can greatly change depending on the subsequent hot rolling conditions, so that it is not a sufficient improvement measure.

As the above (b), for example, JP-A-57-57
Japanese Patent No. 70234 discloses a method of performing recrystallization rolling in which continuous rolling hot rolling is started from 900 ° C. or higher and the rolling pass has a rolling reduction of at least 25%. This method is intended to promote recrystallization under cumulative pressure, but this is also a sufficient measure for improving ridging resistance for ferritic stainless steel to which Ti, Nb, etc. having a high recrystallization temperature are added. The current situation is not.

Further, in Japanese Patent Application Laid-Open No. 58-199822, Nb-containing ferritic stainless steel is 100
Disclosed is a method of performing hot rolling including a reduction of a reduction rate of 30% or more per pass at 0 ° C. or more once or more, followed by hot-rolled sheet annealing, and further performing intermediate annealing during cold rolling to perform cold rolling. ing. This is effective for ridging resistance because recrystallization is repeated many times, but it is difficult to adopt from the viewpoint of many annealing steps and inexpensive manufacturing.

Next, as the means of (c) above, there is a method of hot rolling or annealing in the α + γ two-phase temperature region with a component design that increases the γ potential. Steel types applicable to such a method (For example, SUS430) has a high content of C and N and is inferior in corrosion resistance, and has drawbacks such as intergranular corrosion during pickling.

[0011]

Carbonitride forming elements such as Ti and Nb are often added to ferritic stainless steel plates for the following purposes.

(1) Suppress formation of Cr carbonitride and improve corrosion resistance.

(2) By precipitating as carbonitrides during solidification or hot rolling, the amounts of solute C and N are reduced and the formability of the cold rolled steel sheet is improved.

(3) Ti and Nb carbonitrides, which have the effect of refining the solidified structure and are present in the precipitated state during solidification during casting and heating of the slab, described in (3) Measures to prevent ridging,
It acts as a barrier to the movement of ferrite grain boundaries, which leads to refinement of crystal grains and leads to improved ridging resistance.

As described above, the Ti and Nb elements are indispensable for the ferritic stainless steel in improving the corrosion resistance and the formability, and have a close relationship with the ridging resistance.

The present invention, in the ferritic single phase stainless steel to which Ti and Nb, which are important elements from the viewpoint of corrosion resistance and formability as described above, are added, ridging during forming without substantially changing the conventional manufacturing process. It is an object of the present invention to provide a manufacturing method capable of suppressing the occurrence of the above to a level at which there is no practical problem.

[0017]

The inventors of the present invention destroyed the solidification structure at the time of casting, which greatly influences the occurrence of ridging, in the hot rolling in the stage before the hot rolling to obtain a recrystallized structure. The following findings were obtained as a result of intensive experiments and studies on the effect of elements on recrystallization during hot rolling and the recrystallization of rolling conditions during hot rolling and the effect on ridging. .

A) In hot rolling, by recrystallizing 50% or more of the worked structure during three or more passes,
The ridging resistance is significantly improved.

B) Cr, Ti, and Nb in the steel have an action of suppressing recrystallization during hot rolling.

C) The main cause is not the carbonitrides precipitated during hot rolling, but rather the solid solution Ti, Nb and Cr.

D) 50 of the work structure between passes during hot rolling
%, The Cr, Ti, and Nb contents are related to the rolling reduction condition, and the rolling reduction R of each pass is 1.4.
_{Cr + 94Ti E + 160Nb-20log} t-30 may be if more.

The present invention has been completed on the basis of the above findings, and the gist of the invention is "C: 0.02% or less, Si: 1% or less, Mn: 1% by weight.
% Or less, P: 0.04% or less, S: 0.02% or less,
N: 0.02% or less, Cr: 11-18%, Ti: 4N
% To 1%, Nb: 0 to 0.5% included, Ti + Nb: 8
In the hot rolling of ferritic stainless steel containing (C + N)% or more and the balance Fe and unavoidable impurities, 11
In rolling in the temperature range of 00 to 950 ° C., hot rolling is performed so that the pass of the rolling reduction R (%) satisfying the following formula (1) is three times or more, and then cold rolling is performed. and manufacturing method excellent ferritic stainless steel sheet ridging resistance _{R ≧ 1.4Cr + 94Ti E + 160Nb} -20log t-30 ···· (1) However, t: rolling path time (in seconds) Ti _E: effective Ti Amount (wt%), Ti _E = Ti-3.4
Let N. "It is in.

[0023]

Next, the reasons and effects of limiting the component composition and hot rolling conditions in the present invention will be explained.

A 25 kg round steel ingot having the composition shown in Table 1 was smelted in a vacuum melting furnace, a cylindrical forging test piece having a diameter of 8 mm and a height of 12 mm was sampled, and a hot rolling simulation test by two-stage compression was performed. I went.

[0025]

[Table 1]

The test conditions were as follows: the test piece was heated to 1150 ° C., held for 3 minutes, and then isothermal two-stage compression (height 12 mm → 9 mm → 6 mm) at 800 to 1100 ° C. 1 to 6 after each compression.
After holding for 00 seconds, the recrystallization rate of the cross section was measured with a microscope.

FIG. 1 is a view in which the above test results are arranged in relation to the compression temperature and the time after processing required for 50% recrystallization of the structure of compression processing.

From the figure, it is easier to recrystallize as it is compressed at a higher temperature, and it takes time to recrystallize when Ti and Nb are added, and recrystallization takes longer to suppress recrystallization when the amount of Cr increases. You can see that it works.

When a part of the test piece was observed with a transmission electron microscope (TEM) and the electrolytic extraction residue was analyzed,
It was found that Ti and Nb inclusions precipitated during hot rolling were precipitated at about 950 ° C or lower, and were hardly precipitated in the temperature range higher than 950 ° C. Therefore, it was found that the retardation of recrystallization in the rolling region above 950 ° C. was due to Ti, Nb, and Cr in the solid solution state.

As a result of quantitative analysis of the inhibitory effect of the solid solution Ti, Nb and Cr by using the logarithm of the time required for recrystallization after processing, the coefficient of Cr, Ti and Nb recrystallization inhibition per weight% was found. It became 0.07: 4.7: 8.0.

As a result of analysis of the relationship between the rolling reduction and the recrystallization time, the coefficient per 1% of the reduction is -0.0.
It was determined that it would be 5.

Based on the above knowledge, the time t50% for recrystallization by 50% in volume ratio after hot rolling is set to Cr, Ti, Nb.
The expression using the amount and the rolling reduction R is as follows.

Logt 50% = 0.07Cr + 4.7Ti
_E + 8.0Nb-0.05R-D where, Ti _E is calculated by Ti _E = Ti% -3.4N% effective Ti amount (subtracting TiN precipitation amount). This is because it was experimentally found that TiN was entirely precipitated in a size of 1 μm or more during slab heating and had little effect on recrystallization during hot rolling. It is also known that TiN rather refines the solidified structure and promotes hot recrystallization.
For steel types of 2% or less, there is almost no difference in ridging resistance due to the effect. Further, D in the above formula is a constant obtained by the test.

Based on the above equation, from the viewpoint of the length of time required for 50% of the work structure to be recrystallized in each pass during rolling and the rolling load, an appropriate minimum temperature in hot rolling is set to 950. As a result of conducting various experiments to obtain conditions for recrystallizing 50% or more of the structure after rolling when hot-rolling at a temperature higher than ℃ and t seconds after rolling, the following formula (1)
I got

R ≧ 1.4Cr + 94Ti _E + 160Nb−20log t−30 (1) (R is reduction ratio%) In the ferritic stainless steel containing Ti and Nb, the above formula (1) should be satisfied. In addition, it is possible to promote recrystallization during hot rolling and improve ridging resistance by appropriately controlling the time between passes during hot rolling, the reduction ratio in each pass, and the amounts of Cr, Ti, and Nb. Become.

The hot rolling temperature is usually 1250 to 900 ° C.
Annealing of hot-rolled sheet and cold-rolled sheet is carried out if necessary. In addition, cold rolling after hot rolling may be an ordinary method.

Next, the reasons for limiting the component composition and the hot rolling conditions and the action in the present invention will be explained.

C, N: The lower the content of C, N, the better. If the content exceeds 0.02% by weight, the amount of solid solution in the cold-rolled sheet will increase due to the relationship with the amounts of Ti and Nb, resulting in corrosion resistance and forming. The upper limit was set to 0.02% because both the properties deteriorate.

Si, Mn: These are elements for deoxidation, but if each exceeds 1%, the formability deteriorates, so the upper limit was made 1% each.

P: If the content of P as an impurity exceeds 0.04%, the formability is significantly deteriorated, so the upper limit is made 0.04%.

S: If S of impurities exceeds 0.02%, the corrosion resistance deteriorates, so the upper limit is made 0.02%.

Cr: Cr is an element for ensuring the corrosion resistance, and if it is less than 11%, the corrosion resistance as stainless steel cannot be maintained, and if it exceeds 18%, the C and N activities in the steel decrease and the hot rolling Carbonitrides of Ti and Nb are likely to be finely precipitated during the process, and the progress of recrystallization during hot rolling is extremely suppressed. C
This is because the recrystallization suppressing effect of r is also increased, and the desired recrystallization after rolling cannot be sufficiently obtained.

Ti: The lower limit of Ti is set to 4 times the amount of N. The reason for this is that Ti is precipitated as TiN so that solid solution N can be substantially eliminated at the stage before hot rolling with the formation of NbN.
If it is less than 0.1%, TiN is precipitated during hot rolling and sufficient recrystallization, which is the object of the present invention, cannot be obtained. On the other hand, if it exceeds 1%, the formability is deteriorated, recrystallization is suppressed between rolling passes, and the ridging resistance is deteriorated, so the upper limit is made 1%.

Nb: Nb has an effect of improving corrosion resistance and is added if necessary, but if it exceeds 0.5%, it has an effect of suppressing recrystallization between rolling passes and deteriorates ridging resistance, so the upper limit is 0. 0.5%.

Ti + Nb: The lower limit of Ti + Nb is 8 (C +
The reason why N)% is less than 8 (C + N)% is that C and N at the time of slab heating cannot be fixed sufficiently as Ti and Nb carbonitrides, which also leads to suppression of recrystallization and corrosion resistance,
This is because the moldability is also adversely affected.

Hot rolling temperature, 1100 to 950 ° C .: The reason for limiting the upper limit to 1100 ° C. and the lower limit to 950 ° C. is 11
This is because hot rolling in a temperature range of higher than 00 ° C does not lead to destruction of the solidified structure due to a structure mainly for recovery rather than recrystallization, and if the temperature is lower than 950 ° C, the time required for recrystallization becomes long, This is because a sufficient recrystallized structure cannot be obtained unless the hot-rolled sheet is reheated.

R ≧ 1.4Cr + 94Ti _E + 160Nb
-20log t-30: The reason why the rolling reduction R in the rolling of three or more passes of hot rolling is specified is to recrystallize 50% of the worked structure between the rolling passes as described above, and this formula This is because when rolling is carried out at a rolling reduction smaller than the value given on the right side of, the recrystallization structure of 50% or more cannot be obtained between the passes in the temperature range of the hot rolling temperature of 1100 to 950 ° C.

Rolling with a rolling pass of a rolling reduction R of 3 times or more: If the rolling pass of a rolling reduction satisfying the above formula is performed 3 times or more, the solidified structure is completely destroyed and the crystal orientation is randomized if the rolling is performed less than 3 times. This is because it is not possible to improve ridging resistance. In addition, the rolling reduction satisfying this equation is 1100 to 95.
The rolling may be performed in the temperature range of 0 ° C., and the rolling reduction in the hot rolling temperature range outside this temperature range may be arbitrary.

[0049]

EXAMPLES Next, the effects of the present invention will be specifically described by way of examples.

A 25 kg ingot having the components shown in Table 2 was melted and forged into a slab having a thickness of 40 mm to obtain a sample steel. After heating this slab to 1150 ° C., hot rolling and hot rolling were performed according to the pass schedule shown in Table 3 to 4 mm.
A hot rolled steel sheet was used.

[0051]

[Table 2]

[0052]

[Table 3]

This hot-rolled sheet was annealed by holding it at 950 ° C. for 6 minutes and then air-cooling, pickling and descaling, cold-rolling to 0.8 mm, and then holding it at 980 ° C. for 2 minutes and air-cooling finish annealing. The ridging resistance and moldability were evaluated. The results are shown in Table 4.

[0054]

[Table 4]

The ridging resistance was evaluated by processing the cold-rolled steel sheet into JIS No. 5 tensile test pieces (two pieces under each condition), mirror-polishing the surface, and after 20% tensile deformation, the surface of the test piece had a three-dimensional roughness. The ridging height was measured with a meter, and the grade was determined as follows according to the ridging height.

Grade A: less than 5 μm Grade B ′: 20 to less than 30 μm 〃 A ′: less than 5 to 10 μm 〃 C: 30 μm or more 〃 B: less than 10 to 20 μm where A and A ′ are practically no problem is there. Table 3
From the above, it can be seen that in order to secure the ridging resistances A and A ′, hot rolling needs to be performed three or more passes with a rolling reduction satisfying the formula of the present invention. For example, rolling number V in Table 4
In the case of, the rolling satisfying the rolling reduction specified in the present invention is an example of two times of 3 and 4 passes, but it is understood that the ridging resistance is inferior. In addition, the r-value increases as well as ridging resistance
It has sufficient characteristics in terms of moldability.

[0057]

As shown in the above examples, according to the present invention, a ferritic stainless steel having excellent ridging resistance can be used under the optimum conditions in which the amounts of Ti and Nb added and the hot rolling reduction conditions are combined. Can be manufactured below.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of Cr, Ti, and Nb on the recrystallization time.

Claims (1)

[Claims] 1. By weight%, C: 0.02% or less, Si: 1
% Or less, Mn: 1% or less, P: 0.04% or less, S:
0.02% or less, N: 0.02% or less, Cr: 11 to 1
In the hot rolling of ferritic stainless steel containing 8%, Ti: 4 N% to 1%, Nb: 0 to 0.5%, Ti + Nb: 8 (C + N)% or more and the balance Fe and unavoidable impurities, 1100 When rolling in the temperature range of up to 950 ° C., there are 3 passes with a rolling reduction R (%) that satisfies the following formula (1).
A method for producing a ferritic stainless steel sheet having excellent ridging resistance, which comprises performing hot rolling so that the number of times is equal to or more than one, and then performing cold rolling. R ≧ 1.4Cr + 94Ti _E + 160Nb-20log t-30 ... - (1) where, t: rolling path time (in seconds) Ti _E: effective Ti amount _{(wt%), Ti E = Ti-} 3.4
Let N.