JPH0694573B2 - Method for producing ferritic stainless steel sheet having excellent surface properties and press formability - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a ferritic stainless steel sheet having excellent surface properties and formability. The ferritic stainless steel sheet represented by JIS430 is widely used for household appliances, automobile parts, kitchen supplies and the like. However, when a ferritic stainless steel sheet is press-formed in order to commercialize these products, peculiar irregularities along the rolling direction called ridging are likely to occur, which considerably impairs the surface beauty of the formed product. Therefore, prevention of ridging has become a major issue in producing ferritic stainless steel sheets. The ferritic stainless steel sheet is a continuously cast slab, or a slab obtained by slab-rolling an ingot,
Is hot-rolled, then subjected to batch annealing or continuous annealing,
It is manufactured in the process of continuous cold rolling and finish annealing.

Annealing following hot rolling in the manufacturing process is a process necessary for improving ridging resistance and formability, but when performing annealing by batch type annealing, a treatment of several tens of hours is required, resulting in a significant decrease in production efficiency. Invite. Therefore, continuous annealing is being carried out for the purpose of shortening the annealing processing time.

(Prior Art) Japanese Patent Publication No. 59-43978 discloses a ferritic stainless steel slab containing Al: 0.01 to 0.2 wt% (hereinafter simply referred to as%) and N: 0.025% or less at 900 to 1200 ° C. It is disclosed that after heating and holding at the above temperature, hot rolling is performed with a reduction of 20% / pass or more for one pass or more, and then continuous annealing is performed in which it is heated to a temperature of 700 to 1050 ° C. and rapidly cooled.

(Problems to be Solved by the Invention) In the continuous annealing method disclosed in the above-mentioned prior art, since the annealing time is short, sufficient recrystallization and softening are not performed, and ridging resistance and moldability are deteriorated. The problem remains.
Therefore, it is conceivable to heat the slab in the low temperature range for the purpose of improving the ridging resistance, or to promote recrystallization during annealing of the hot-rolled sheet by strong reduction rolling, but the effect is not sufficient. Moreover, such a proposal for improvement rather increases the load on the roll during hot rolling, leading to problems such as roughening of the surface of the steel sheet, deterioration of the surface properties of the steel sheet such as generation of bald defects, and reduction of roll life.

The present invention advantageously solves the above problems, and a ferritic stainless steel sheet having excellent ridging resistance and press formability can be produced by annealing for a short time without lowering the heating temperature of the slab. To aim.

(Means for Solving Problems) The purpose of annealing a hot-rolled steel sheet is mainly to improve ridging resistance after cold-rolling finish annealing, improve formability and restore sensitization. However, when batch annealing is continuously annealed for the purpose of improving the productivity of the hot rolled sheet annealing process,
Since annealing takes a short time, recrystallization is not complete, and deterioration of ridging resistance and moldability becomes a serious problem. On the other hand, reduction of the slab heating temperature before the hot rolling described above, characteristics can be improved by activating recrystallization during annealing by high-pressure rolling, etc., but ridging resistance equivalent to that of the conventional batch-type annealing material, In order to obtain formability, it is inevitable that the surface quality of the steel sheet is significantly deteriorated due to an excessive load on the roll during hot rolling.

Therefore, in order to obtain excellent press formability even in the continuous annealing step without increasing the load on the rolling rolls caused by the reduction of the slab heating temperature or the high-pressure reduction rolling, Al in steel, It has been found that a method in which N is positively contained and is not recrystallized in hot-rolled sheet annealing, cold rolling is performed in an unrecrystallized state, and then finish annealing is extremely effective.

The present invention is derived from the above findings.

That is, the present invention, a ferritic stainless steel slab containing Cr: 16.0 ~ 18.0%, Al: 0.05 ~ 0.3% and N: 0.025 ~ 0.07%, after heating above 1200 ℃ to 1300 ℃ or less Rolled and continued to 650 ℃ or higher 8
Hot rolling annealed sheet consisting of unrecrystallized structure obtained by performing hot rolling sheet annealing within 300 seconds in a temperature range of less than 50 ° C., cold rolling followed by finish annealing, surface texture and press It is a method for producing a ferritic stainless steel sheet having excellent formability.

The hot-rolled sheet annealing is intended to recover the sensitization, and the recovery of the sensitization at a lower temperature improves the productivity in the hot-rolled sheet annealing step. In this invention, Al, N is finely precipitated in the crystal grains during hot rolling by containing a large amount of Al, N in the steel, and the Al, N is a nucleation site of Cr carbonitride precipitation. Therefore, the precipitation of Cr carbonitrides at the grain boundaries is reduced, so that the sensitization can be easily recovered at a low temperature.

For example, Al, for ferritic stainless steel slabs that do not positively contain N, hot-rolled sheet annealing is carried out at a low temperature as in the present invention, cold rolling with unrecrystallized, finish annealing, Since the (100) crystal orientation, which causes ridging, remains non-uniform and the (111) orientation is not well developed, sufficient press formability cannot be obtained. However, when a large amount of Al and N is precipitated in the hot-rolled sheet as in the present invention, the strain of cold rolling is accumulated in the unrecrystallized state, and the growth of the (111) orientation becomes uniform during the finish annealing. , Uneven (10
0) Orientation does not remain and remarkably excellent press formability is obtained.

(Function) Each limitation reason in the present invention will be described below.

Cr: 16.0 to 18.0% Cr is a basic element in ferritic stainless steels, at least 16.0% is necessary to obtain the desired corrosion resistance, and an increase in the content improves the corrosion resistance, but 18.0% If it exceeds 1.0, molding processability will deteriorate, so 16.0-
The range was 18.0%.

Al: 0.05-0.3% N: 0.025-0.07% Al is an effective ferrite phase forming element and its content is 0.
If it is less than 05%, there is no effect of promoting the γ → α transformation during hot rolling, and a large amount of martensite phase remains before cold rolling. If martensite phase exists during cold rolling, the ridging resistance of the cold rolled sheet will be Although improved, the value deteriorates and sufficient press formability cannot be obtained. Further, when the Al content is less than 0.05% and the N content is less than 0.025%, the amount of Al and N precipitation is small, the effect of promoting recrystallization during finish annealing and the development of (111) orientation are not observed, and press formability is not observed. Therefore, the lower limit of Al is set to 0.05% and the lower limit of N is set to 0.025%.

On the other hand, if the Al content exceeds 0.3%, the effect does not increase and conversely the ridging resistance deteriorates. On the other hand, if the N content exceeds 0.07%, the N content increases and the steel plate becomes hard and during hot rolling. The upper limit of Al is set to 0.30% and the upper limit of N is set to 0.07% because problems such as occurrence of ear cracks and deterioration of mechanical properties are caused.

Next, the reasons for limiting the slab heating temperature during hot rolling will be described.

Generally, lowering the slab heating temperature improves ridging resistance, but when cold rolling a hot-rolled annealed sheet with a non-recrystallized structure, there is no particular improvement in properties even if the slab heating temperature is 1200 ° C or less, and conversely. The slab heating temperature was limited to over 1200 ° C as well as causing a serious problem of deterioration of the surface quality of the steel sheet due to the reduction of rolling temperature.

On the other hand, if the slab heating temperature exceeds 1300 ° C, ferrite crystal grains become abnormally coarse during heating of the slab, and the ridging resistance deteriorates in particular, so the upper limit of the slab heating temperature was limited to 1300 ° C.

Regarding the hot-rolled sheet annealing conditions, if the annealing temperature does not reach 650 ° C, the sensitization of the hot-rolled sheet will not be recovered and grain boundary corrosion will occur in the next pickling step, so the lower limit was made 650 ° C. On the other hand, short-time annealing at a temperature above 850 ° C causes partial recrystallization, and the (111) orientation develops slightly in the state of the hot-rolled annealed sheet, but it does not reappear during the subsequent finish annealing after cold rolling. Since the crystals are insufficient and many (100) orientations remain, the ridging resistance and the value deteriorate, so the upper limit was made 850 ° C.

Furthermore, the reason for limiting the treatment time in the above temperature range to 300 seconds or less is that if the temperature range is 300 seconds or less, the sensitization is sufficiently recovered by short-time annealing within 300 seconds.
This is because if it exceeds 2 seconds, partial recrystallization will occur and productivity will be reduced.

(Example) A continuously cast slab containing each of the chemical components shown in Table 1 as A to E was hot-rolled, annealed, and then 0.
It was cold-rolled to a thickness of 7 mm and subsequently subjected to finish annealing.

The values and the ridging waviness height of the finish annealed plate thus obtained are shown in Table 2 together with the production conditions. The value and the ridging waviness height were measured as follows.

That is, the value was measured by a three-point method after applying a 15% tensile prestrain using a JIS No. 5 test piece cut out from the obtained annealed sheet, and the L direction (rolling direction), C direction (right angle to the rolling direction). Direction) and the D direction (45 ° direction to the rolling direction), the average value in three directions = (r _L + r _C + 2r _D ) / 4.

The ridging undulation height was measured using a JIS No. 5 test piece cut from the rolling direction of the finished annealed sheet, with a 20% tensile prestrain applied and the surface roughness measured using a surface roughness meter.

As is clear from Table 2, ferritic stainless steel slabs containing appropriate amounts of Cr, Al and N were subjected to hot rolling at an appropriate slab heating temperature and hot rolled sheet annealing at an appropriate treatment temperature and time. By applying, a cold-rolled sheet having remarkably excellent press formability was obtained.

(Effect of the Invention) In the present invention, a ferritic stainless steel sheet having excellent surface properties and press formability can be produced by annealing for a short time without lowering the slab heating temperature,
It is possible to realize a significant reduction in manufacturing cost and an improvement in productivity as compared with the past.

Claims (1)

[Claims] 1. A ferritic stainless steel slab containing Cr: 16.0 to 18.0 wt% Al: 0.05 to 0.3 wt% and N: 0.025 to 0.07 wt% at 1200 ° C.
Over 1300 ℃ and then hot rolling,
A hot-rolled annealed sheet consisting of an unrecrystallized structure obtained by performing hot-rolled sheet annealing within 300 seconds in a temperature range of 650 ° C or higher and lower than 850 ° C is followed by cold rolling followed by finish annealing. A method for producing a ferritic stainless steel sheet having excellent surface properties and press formability.