KR20010086183A - A method for manufacturing ferritic stainless steel having improvable formability and ridging resistance - Google Patents

Abstract

PURPOSE: A method is provided that manufactures ferritic stainless steel having superior forming property and ridging resistance by adding B and a stabilizing element Nb, and optimizing casting conditions, hot and cold rolling conditions and hot and cold annealing conditions for securing an equiaxed grain ratio during continuous casting. CONSTITUTION: The method for manufacturing ferritic stainless steel having superior forming property and ridging resistance comprises the processes of casting a steel slab comprising 0.02 wt.% or less of C, 1.0 wt.% or less of Si, 1.0 wt.% or less of Mn, 0.02 wt.% or less of P, 0.003 wt.% or less of S, 14 to 20 wt.% of Cr, 1.0 wt.% or less of Mo, 0.02 wt.% or less of N, 0.6 wt.% or less of Cu, 0.6 wt.% or less Nb, 0.01 wt.% or less of B and a balance of Fe and inevitably added impurities, wherein C+N is 0.03 wt.% or less, and Nb/(C+N) is controlled to 8 to 15 by controlling a casting temperature to 1538 deg.C or less and ΔT(degree of superheat) to 30 deg.C or less; reheating the slab in the temperature range of 1150 to 1220 deg.C, and hot rolling the reheated slab at a finishing rolling temperature ranging from 750 to 900 deg.C; hot annealing the hot rolled slab in the temperature range of 980 to 1020 deg.C, and rapidly cooling the hot annealed slab in a cooling rate of 15 deg.C/sec or more before pickling and cold rolling the rapidly cooled slab; cold annealing the cold rolled slab at a temperature of 950 to 970 deg.C, rapidly cooling the cold annealed slab in a cooling rate of 15 deg.C/sec or more, and then pickling the rapidly cooled slab.

Description

A method for manufacturing ferritic stainless steel with excellent formability and resistance to stiffness {A METHOD FOR MANUFACTURING FERRITIC STAINLESS STEEL HAVING IMPROVABLE FORMABILITY AND RIDGING RESISTANCE}

The present invention relates to a method of manufacturing ferritic stainless steel used for various kitchen appliances, washing tank drums and automobile exhaust systems, and more particularly, to produce ferritic stainless steel exhibiting excellent moldability and ridging resistance. It is about a method.

In general, ferritic stainless steel does not add Ni, which is advantageous in terms of cost than STS304 austenitic stainless steel, but has lower corrosion resistance and elongation and worse moldability than STS304 steel. In addition, there is a problem in that the use is limited when used for deep processing, because of the rim phenomenon that the curved portion occurs on the surface which is a problem of ferritic stainless steel during molding.

However, with recent advances in steelmaking refining technology, C and N, which are invasive elements, are managed extremely low, and stabilization elements (Ti, Nb, Al, Zr, etc.) are added to increase corrosion resistance and workability, and Mo is added to STS304 steel and Efforts have been actively made to obtain equal corrosion resistance and processability. As such a trend, the demand and use of stabilized ferritic stainless steel used for kitchen appliances, washing machine drums and automobile exhaust systems tend to increase day by day.

Japanese Patent Laid-Open No. 51-149116 and Japanese Patent Laid-Open No. 56-158850 are typical examples of the production of ferritic stainless steel by adding the stabilizing elements as described above.

Japanese Patent Application Laid-Open No. 51-149116 discloses C: 0.03% or less, Si: 1% or less, Mn: 0.5% or less, S: 0.005% or less, Cr: 14-20%, Mo: 1-3%, Ti: 0.1 ˜1.2%, Nb: 0.1-1.2%, N: 0.02% or less, residual Fe and other unavoidably contained impurities, wherein C + N is 0.04% or less and (Ti + Nb) / (C + N) The present invention relates to a method for producing ferritic stainless steel by annealing a steel maintained at 8 to 30 at a temperature range of 970 to 1170 ° C., and has an effect of improving the reforming properties.

Japanese Patent Application Laid-Open No. 56-158850 is C: 0.0015% or less, Si: 0.6% or less, Mn: 0.5% or less, S: 0.005% or less, Cr: 15-20%, Mo: 0.5% or less, Ni: 1.0% Cu: 0.1-1.0%, Ti: 0.1-0.5%, Nb: 0.1-0.5%, N: 0.02% or less, Al: 0.1%, W or V alone or residual Fe and other unavoidable impurities In the method for producing a ferritic stainless steel, the C + N is 0.04% or less and (Ti + Nb) / (C + N) is annealed in the temperature range of 970 ~ 1170 ℃ to maintain a temperature of 8 ~ 30 It is related to, and has an effect of reforming.

However, in the ferritic stainless steel as described above, since the Mo and Cr content, W, V alloy ferrous input is increased, the manufacturing cost is increased, the moldability is lowered, and the problem is not satisfactory in the improvement of the rim, which is the biggest problem of the ferritic stainless steel There is this. In particular, when Ti is added among the stabilizing elements, nozzle clogging during continuous casting, surface defects caused by Ti oxide, and tempered color during light annealing occur. The elongation is higher than that of Nb-added steel, but the moldability is good. However, orange peel occurs on the surface, and new etching occurs when the sheet is stretched in the rolling direction and 45 ° direction. As a result of coarsening, the orange peel phenomenon and the rhythm act as a complex, resulting in a higher curvature of the surface after molding.

Therefore, the present inventors conducted research and experiments to solve the above problems, and proposed the present invention based on the results. The present invention adds B and a stabilizing element, Nb, and casts to secure equiaxed crystallization during playing. An object of the present invention is to provide a method for manufacturing ferritic stainless steel having excellent formability and crush resistance by optimizing conditions, hot rolling and cold rolling conditions, and hot rolling and cold rolling annealing conditions.

In the present invention, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.02% or less, S: 0.003% or less, Cr: 14-20%, Mo: 1.0% or less, N : 0.02% or less, Cu: 0.6% or less, Nb: 0.6% or less, B: 0.01% or less, but C + N is controlled to 0.03% or less, and Nb / (C + N) is controlled to 8 to 15, A steel slab composed of the remaining Fe and inevitably added impurities is cast at a temperature of ΔT (superheat) of 30 ° C. or less by controlling the casting temperature to 1538 ° C. or lower, and reheating and finishing the slab at a temperature range of 1150 to 1220 ° C. After hot rolling with a rolling temperature in the range of 750 to 900 ° C., hot rolling annealing at 980 to 1020 ° C., pickling and cold rolling, and then cold rolling at 950 to 970 ° C., the cooling rate after the hot rolling and cold rolling annealing is performed. The present invention relates to a method for producing ferritic stainless steel having excellent moldability and resistance to quenching at 15 ° C / sec or more.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The C and N are present as the carbonitride-forming element in an invasive type, and the lower the C and the N, the higher the strength and the impact toughness, the corrosion resistance and the moldability. Therefore, in the present invention, the content was set to 0.02% or less, and C + N was set to 0.03% or less.

The Si is a ferrite forming element, the content is increased to increase the stability of the ferrite phase and the oxidation resistance is also improved, but when added to more than 1.0% to increase the hardness and strength and lower the elongation, which is disadvantageous to formability, it is set to 1.0% or less It is preferable.

The Mn is preferably limited to 1.0% or less because the content of Mn elutes MnS and lowers pitting resistance.

Since P and S form MnS and inhibit corrosion resistance and hot workability, P and S are preferably controlled as low as possible, so that P is limited to 0.02% or less, and S to 0.003% or less.

When the Cr is added at 14% or less, the corrosion resistance decreases, and when a large amount is added, the corrosion resistance is improved, but when 20% or more is used, the strength is high and the elongation is low, so that the moldability is lowered, and in particular, there is a high risk of precipitation of sigma phase. The content is preferably 14-20%.

Mo is an element having not only improved corrosion resistance but also grain refinement effect at the center of the plate thickness, and the content thereof is preferably 1.0% or less. The reason is that when 1% or more is added, the elongation decreases due to the increase in hardness and strength, resulting in poor moldability.

The Nb improves the corrosion resistance due to the control of the texture of the annealing material crystal by the precipitation of carbonitride, and also forms the carbonitride by combining with C and N to suppress the precipitation of Cr carbide to improve the corrosion resistance. However, when excessive addition of Nb to 0.6% or more reduces elongation and moldability, it is preferable to limit the content to 0.6% or less.

And it is preferable to make Nb / (C + N) into 8-15 from the relationship of said Nb and C + N. This is because when the weight ratio is 8 or less, grain boundary corrosion occurs, and when 15 or more, moldability and elongation are lowered, and thus corrosion resistance and moldability are not satisfied at the same time.

The B is an element for minimizing the annealing structure to improve the formability and the rhythm resistance, the content is preferably limited to 0.01% or less in consideration of elongation and formability.

In producing the ferritic stainless steel of the present invention using the steel slab prepared as described above, ΔT is preferably controlled to 30 ° C. or lower and the casting temperature is controlled to 1538 ° C. or lower when the slab is played. The reason is that ΔT must be adjusted to 30 ° C. or lower to ensure an equiaxed crystal rate in the slab for improving the formability and resistance to the ferritic stainless steel to 50% or more.

In the above process, the lower the slab reheating temperature and the finish rolling temperature, the better the formability and the crushing resistance, but if the hot rolling temperature becomes too low, the slab surface layer is buried on the hot rolled roll and sticking phenomenon. Cause. Accordingly, the slab reheating temperature and the finish rolling temperature are preferably in the range of 1150 to 1220 ° C. and 750 to 900 ° C., in consideration of defect occurrence problems on the hot rolled coil surface and shape and hot rolling workability in the industrial field.

After hot rolling as described above, hot rolling annealing is preferably performed at a temperature range of 980 to 1020 ° C. The reason is that when hot-annealed at 980 ℃ or lower, Nb-added steels have insufficient recrystallization and grain growth, and elongation and formability are deteriorated. If hot-annealing temperature is 1020 ℃ or higher, grains are coarse to reduce elongation and formability. This is because the orange peel occurs and the erosion worsens.

Next, in cold rolling after pickling, the cold rolling may be performed in one or two stages. When the cold rolling is carried out in two stages, the cold rolling ratio of each stage is preferably 50 to 70%. The reason is that if the cold rolling ratio is less than 50%, the stiffness and surface properties are not good. If it is higher than%, the ear generation value increases after molding.

After hot-rolling annealing and cold-rolling annealing under the above conditions, cooling is performed, and the cooling at this time is preferably quenched at a cooling rate of 15 ° C / sec or more. This is because quenching at the cooling rate as described above can prevent high temperature embrittlement and obtain good corrosion resistance and mechanical properties.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

The stainless steel composition as shown in Table 1 was dissolved in a vacuum induction melting furnace to prepare a 30 kg ingot. At this time, in order to see the change of equiaxed rate, cast the ingot by changing the ΔT temperature and the casting temperature as shown in Table 2, and then the heating temperature is changed to heat for 150 minutes at each temperature, the finish rolling temperature is constant at 800 ℃ After rolling, a hot rolled sheet having a thickness of 3 mm was manufactured, wound at 700 ° C., and then annealed for 5 minutes by varying the hot rolling temperature, and quenched at a cooling rate of 15 ° C./sec, followed by pickling treatment.

The hot rolled sheet was cold rolled at a 83.3% rolling rate to prepare a cold rolled sheet having a thickness of 0.5 mm, cold-rolled annealing temperature was changed at 3 ° C for 3 minutes, quenched and pickled, and then tempered at 1% cold rolling rate. Each specimen was prepared by rolling. In addition, in order to investigate the effect of cold rolling in two stages, cold rolling was carried out at 60% of the first stage, cold-rolled annealing and quenched, followed by pickling, and then 60% of the second stage was cold rolled. Cold-rolled, cold-annealed, quenched, pickled, and temper-rolled to prepare each specimen with a final thickness of 0.48 mm.

In order to evaluate the formability of the specimen prepared as described above, by using a 13% tensile test by processing the tensile specimen in the 0 °, 45 ° and 90 ° direction parallel to the rolling direction using JIS 13B to measure the width change The values were measured and the results are shown in Table 2 below. In addition, after the 15% tensile test by processing JIS No. 5 tensile test specimen parallel to the rolling direction for evaluation of the refrigerating property, using the surface roughness was measured by the surface roughness height Rt value of the maximum height of the surface shape It is shown in Table 2 below.

Steel grade C Si Mn P S Cr Mo Cu Nb N B Nb / (C + N) Inventive Steel 1 0.014 0.30 0.30 0.015 0.002 19.25 0.1 0.52 0.255 0.0115 0.002 10 Inventive Steel 2 0.013 0.31 0.28 0.014 0.002 19.20 0.1 0.50 0.258 0.0110 0.002 10.75 Invention Steel 3 0.014 0.30 0.33 0.015 0.002 19.25 0.1 0.52 0.257 0.0120 0.002 9.88 Inventive Steel 4 0.014 0.30 0.30 0.015 0.002 19.25 0.1 0.52 0.255 0.0115 0.002 10 Comparative Steel 1 0.015 0.32 0.33 0.015 0.002 19.20 0.1 0.50 0.260 0.0115 － 9.81 Comparative Steel 2 0.015 0.31 0.32 0.015 0.002 19.10 0.1 0.50 0.467 0.0125 － 17

Psalm Number Manufacture conditions Measurement result Casting temperature (℃) ΔT (℃) Slab heating temperature (℃) Hot Annealing Temperature (℃) Cold Rolling Annealing Temperature (℃) Stage 1 rolling rate (%) Two stage rolling rate (%) Equivalence rate (%) value Resizing Height (㎛) Invention 1 Inventive Steel 1 1537 29 1512 1010 960 83.3 － 52 1.39 38.2 Invention 2 29 1512 1010 60 60 52 1.6 20.1 Invention 3 Inventive Steel 2 1530 22 1210 1000 83.3 － 60 1.43 35.0 Invention 4 22 1210 1000 60 60 60 1.65 18.0 Comparative Material 1 Invention Steel 3 1545 37 1215 1010 83.3 － 32 1.17 46.4 Comparative Material 2 Inventive Steel 4 1537 29 1250 1010 52 1.15 48.0 Comparative Material 3 1537 1215 950 1.10 46.0 Comparative Material 4 1539 1030 1.17 47.0 Comparative Material 5 1539 1050 0.96 52.0 Comparative Material 6 1539 1010 1000 0.95 54.0 Comparative Material7 Comparative Steel 1 1537 960 1.20 47.5 Comparative Material 8 Comparative Steel 2 1539 0.98 50.1

As can be seen in Table 1 and Table 2, the invention materials produced by the present invention steel compared to the comparative material outside the scope of the present invention in terms of components and manufacturing conditions The value was high and the Rimow height was low. On the other hand, in the case of the invention materials (2) and (4) subjected to cold rolling in two stages, not only the comparative material but also significantly higher than the invention materials (1) and (3) of the invention which carried out cold rolling in one stage It had a value, and it was confirmed that the height of the richam was lowered.

As described above, according to the present invention, if B and Nb are added and superheat degree and various manufacturing temperatures are controlled to produce stainless steel, a stainless steel having an equiaxed crystallinity of 50% or more and excellent moldability and crushing resistance can be obtained. It works.

Claims (2)

By weight% C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.02% or less, S: 0.003% or less, Cr: 14-20%, Mo: 1.0% or less, N: 0.02% Cu: 0.6% or less, Nb: 0.6% or less, B: 0.01% or less, but C + N is 0.03% or less, Nb / (C + N) is controlled to 8-15, and the balance Fe and The steel slab, which is inevitably added to the impurity, is cast at a casting temperature of 1538 ° C. or less and ΔT (superheat) is 30 ° C. or less, and the slab is reheated at a temperature range of 1150 to 1220 ° C. and the finish rolling temperature is 750. Hot rolling in the range of ~ 900 ℃ and hot rolling annealing in the range of 980 ~ 1020 ℃, quenching at a cooling rate of 15 ℃ / sec or more, pickling and cold rolling, cold rolling annealing at 950 ~ 970 ℃ and cooling rate Is a method of manufacturing ferritic stainless steel with excellent moldability and resistance to rinsing, which is quenched at 15 ° C./sec or higher and then pickled. The method of claim 1, wherein when the cold rolling is made in two stages, one stage cold rolling is performed at a cold rolling rate of 50 to 70%, followed by cold rolling at 950 to 970 ° C., followed by quenching and pickling, followed by two stages of cold rolling. Process for producing ferritic stainless steel with excellent formability and resistance to etching, characterized in that rolling is carried out at a cold rolling rate of 50 to 70%