KR0146798B1 - Method for manufacturing ferritic stainless steel - Google Patents

Abstract

The present invention relates to a method for manufacturing ferritic stainless steel used for kitchen appliances, washing tanks, automobile exhaust systems, etc., and is capable of reducing moldability by optimizing (Ti + Nb) / (C + N) and Nb / Ti. And to provide a method for producing a ferritic stainless steel excellent in Risping resistance, the object is.

In the present invention for achieving the above object, in the manufacturing method of ferritic stainless steel, in weight%, C: 0.02% or less, Si: 0.6% or less, Mn: 0.5% or less, P: 0.02% or less, S: 0.003 % Or less, Cr: 15-20%, Mo: 0.6% or less, N: 0.02% or less, Ti: 0.6% or less, Nb: 0.6% or less, remaining Fe and inevitably added impurities, and the C + N : Stainless steel satisfying 0.03% or less, (Ti + Nb) / (C + N): 12-20, and Nb / Ti: 2 or more in a temperature range of 1170-1250 ° C. after hot rolling, and then 750-900 It is an object of the present invention to provide a method for producing ferritic stainless steel having excellent formability and gritty resistance, including finishing rolling in a temperature range of C, hot rolling and cold annealing in a temperature range of 900 to 970 ° C, and quenching.

Description

Ferritic stainless steel manufacturing method with excellent formability and stiffness resistance

1 to 5 show the hardness, yield strength, tensile strength, yield ratio ([YS x Ts] x 100), elongation, according to Mo content when Ti and Nb are added alone or in combination in ferritic stainless steel. Graph showing changes in, and Ericsen values.

6 shows the change of Ti + Nb content of the invention and the comparative material to which the method of the present invention is applied Graph showing the change in.

7 shows the change according to the change of (Ti + Nb) / (C + N) of the invention and the comparative material to which the method of the present invention is applied Graph showing the change in.

8 to 11 are graphs showing changes in the Ericsen value, CCV, work hardening index, and Rishik height according to the Ti + Nb content change of the inventive material to which the method of the present invention is applied and the comparative material.

The present invention relates to a method for manufacturing ferritic stainless steel used for kitchen appliances, laundry tubs and automobile exhaust systems, and more particularly, to a method for producing ferritic stainless steel excellent in formability and rigid resistance. .

In general, ferritic stainless steel is advantageous in terms of cost than STS 304 austenitic stainless steel because Ni is not added. However, when the ferritic stainless steel is used for deep processing, the corrosion resistance and elongation are lowered and the moldability is poor.

However, recently, steelmaking and refining technology has been developed to manage the invasive elements C and N extremely low, and the addition of stabilizing elements, such as Ti, Nb, Al, and Zr, improves the corrosion resistance and workability, and at the same time, the addition of Mo is equivalent to STS 304. And research to obtain workability is actively attempted. With this trend, the demand and use of ferritic stainless steels used for kitchen appliances, washing tanks and automobile exhaust system parts tend to increase day by day.

Japanese Patent Laid-Open No. 51-149116 and Japanese Patent Laid-Open No. 56-158850 are typical among the conventional proposals for producing ferritic stainless steel by adding the stabilizing element as described above.

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

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%, Ni: 1.0% or less, Mo: 0.5% or less , Cu: 0.1-1.0%, Ti: 0.1-0.5%, Nb: 0.1-0.5%, N: 0.02% or less, Al: 0.1% or less, W or V alone or in combination, 0.3% or less, the rest: Fe and The present invention relates to a ferritic stainless steel, which is composed of other unavoidable impurities, and is maintained at C + N: 0.03% or less and (Ti + Nb) / (C + N): 10 or more, and has an effect of improving corrosion resistance.

However, in the ferritic stainless steel as described above, the corrosion resistance is improved when the Mo and Cr content is increased, but the input cost of the iron, Cr, W and V alloy is increased, the manufacturing cost increases, the formability is lowered, and the most of the ferritic stainless steel There is a problem such as being unsatisfactory in the improvement of the riching which is a big problem.

In particular, steel containing Ti as a stabilizing element generates nozzle clogging during surface casting, surface defects caused by Ti oxide, and tempered color during bright annealing. In addition, the elongation is good, the moldability is good, but the orange peel phenomenon occurs on the surface after molding, and in particular, when it is stretched in the rolling direction and in the 35 ° direction, a new rhythm is generated. There is a problem in that the bending height of the surface after molding is higher due to the combination of the phenomenon and the rhythm.

Accordingly, the present inventors have conducted studies and experiments to solve the above problems and improve the formability and stiffness resistance of ferritic stainless steel, and based on the results, the present invention lowers the Mo content. An object of the present invention is to provide a method of manufacturing ferritic stainless steel having excellent moldability and rhythm resistance by optimizing (Ti + Nb) / (C + N) and Nb / Ti.

Hereinafter, the present invention will be described.

In the manufacturing method of the ferritic stainless steel, the present invention is, in weight%, C: 0.02% or less, Si: 0.6% or less, Mn: 0.5% or less, P: 0.02% or less, S: 0.003% or less, Cr: 15- 20%, Mo: 0.6% or less, N: 0.02% or less, Ti: 0.6% or less, Nb: 0.6% or less, remaining Fe and inevitably contained impurities, and C + N: 0.03% or less, (Ti After stainless steels satisfying + Nb) / (C + N): 12-20 and Nb / Ti: 2 or more are heated to a temperature range of 1170-1250 ° C and hot rolled to a finish rolling temperature of 750-900 ° C In the manufacturing method of ferritic stainless steel having excellent moldability and rhythm resistance, comprising hot-rolled annealing and quenching at a temperature range of 900-970 ℃, followed by cold rolling, followed by cold-rolling and quenching at a temperature range of 900-970 ℃. It is about.

Hereinafter, the present invention will be described in more detail.

In order to achieve the above object, in the present invention, it is preferable to form the steel as described above, and the reason thereof is as follows.

When the C and N are in the intrusion type as the carbonitride-forming element, the strength is increased, and the lower the impact strength, the lower the impact resistance, the lower the moldability and the lower the moldability, so the content is preferably 0.02% or less, and C + N: 0.03%, respectively. It limits to the following.

Si is a ferrite forming element which increases the content and increases the safety of ferrite phase and improves the oxidation resistance. However, when it is added more than 0.6%, Si is disadvantageous to formability because it increases hardness, yield strength, tensile strength and decreases elongation. .

Mn is limited to 0.5% 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 managed as low as possible, so they are limited to P: 0.02% or less and S: 0.003% or less.

If Cr is too low (15% or less), the corrosion resistance is lowered. If the content is too high, the corrosion resistance is improved, but if it is 20% or more, the strength is high and the elongation is low, thus the moldability is lowered. In particular, there is a high possibility of precipitating sigma phase. Therefore, the content is limited to 15-20%.

Mo not only improves the corrosion resistance significantly but also reduces the grain size at the center of the plate thickness and improves the rhythm. As shown in FIGS. , Tensile strength, yield strength and yield ratio, Value and Ericsen value fall, and moldability worsens. Therefore, considering the corrosion resistance and moldability, Mo content is limited to 0.6% or less.

Ti combines with C and N to form carbonitrides to improve leasing properties and to inhibit the precipitation of Cr carbides to improve corrosion resistance. On the other hand, when Ti is added in excess of 0.6%, the same problem as in the above-described Ti-added steel occurs and the surface quality is worsened, so the content is limited to 0.6% or less.

Nb is added to improve the ridging property by controlling the annealing material crystal texture by carbonitride precipitation. In addition, Nb combines with C and N to form carbonitrides, thereby inhibiting the precipitation of Cr precipitates, thereby improving corrosion resistance. However, since excessively adding Nb to 0.6% or more reduces workability, Nb limits its content to 0.6% or less.

And (Ti + Nb) / (C + N) is the ratio of 10 or more to prevent the intergranular corrosion, showing a deep drawing property in the range of 12-20 Since the value is the highest, it is preferable to limit the range to 12-20 which satisfies the grain boundary corrosion and formability at the same time. Nb / Ti is limited to two or more because the height is effective for improving moldability and stiffness.

In addition, in the present invention, after forming the steel as described above, it is preferable to produce the ferritic stainless steel of the present invention by heating, hot rolling, hot rolling, cold rolling, cold rolling and cooling under the following conditions.

The lower the heating temperature, finish rolling temperature, and winding temperature of the cast and ingot for hot rolling, the better the formability and the crush resistance. However, when the hot rolling temperature is lowered, there is a problem of sticking, which is a surface defect of the slab surface layer buried in the rolling roll, so that defects occur frequently on the surface of the hot rolled coil, and considering the capability and workability of the hot rolling mill in an industrial site. The cast heating temperature is preferably 1170-1250 ° C., and the finish rolling temperature is 750-900 ° C.

In addition, in the Nb-added steel which delays recrystallization when the hot rolled and cold rolled annealing temperatures are too low below 900 ° C, recrystallization and grain growth are insufficient, resulting in deterioration of impact characteristics, elongation and formability, and high grains above 970 ° C. The hot rolling and cold annealing treatment temperature is preferably in the range of 900-970 ° C., because the impact characteristics are worse and the orange peel generation and crushing property are poor after molding.

Thus, the cooling at the time of hot-rolling annealing and cold-rolling annealing is limited to the prevention of high temperature embrittlement and quenching with good 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. The ingot thus prepared was heated at 1230 ° C. for 150 minutes and hot rolled at 810 ° C. finish rolling temperature to produce a 4 mm thick hot rolled sheet, wound at 700 ° C., and the Nb alone and Nb + Ti composite additive steel were 950 ° C. In, the Ti-added steel was quenched and pickled after 5 minutes of continuous annealing treatment at 900 ° C.

The hot rolled sheet was cold rolled at a rolling rate of 70% to prepare a cold rolled sheet having a thickness of 1,2 mm, followed by cold rolling for 4 minutes at the same temperature as the hot rolled sheet annealing temperature, followed by quenching and pickling, followed by cold rolling at 2%. The specimen was prepared by rough rolling at a rate.

In order to evaluate the deep-rowing properties of the specimens prepared as described above, the tensile specimens were processed by JIS 13B and subjected to a 15% tensile test. Three values were measured, and the average value (r) according to the Ti + Nb complex addition amount change was shown in FIG. 6, and the average value ((+) ) Is shown in FIG.

As shown in FIG. 6, in the case of the invention material (A, B) which satisfies the condition of Nb / Ti: 2 or more of the present invention by adding more Nb than Ti, a comparative material in which Ti and Nb were added alone ( 4-12) and Nb / Ti: comparative materials (2,3) that do not satisfy the condition of 2 or more It can be seen that the characteristic is equal to or higher than the value. In the case of the comparative material (1) which satisfies the Nb / Ti condition of the present invention but does not satisfy the (Ti + Nb) / (C + N) condition, the invention material (A, B) It can be seen that the values are inferior.

As shown in Figure 7, It can be seen that the value (Ti + Nb) / (C + N) ratio is the highest in the range of 12-20. Therefore, in the case of the inventive materials (A, B), the (Ti + Nb) / (C + N) ratio was adjusted within the range of 12-20 to improve the deep drawing property. The value is high.

In order to evaluate the elongation properties of the specimens, the Erichsen test was conducted by JIS Z 2247B method, and the average value of the three specimens was shown in FIG. 8.

As can be seen in Figure 8, the invention material (A, B) that satisfies the scope of the present invention it can be seen that the Eriksen value is greater than the comparative material (1-12) is excellent in elongation formation.

Conical cup test to evaluate the deep drawing property and elongation property formation of the specimens was conducted in accordance with JIS Z 2249 type 21 standards, and after forming, the average value of the maximum and minimum diameters of the specimens was expressed as the CCV value, and the result was ninth. It is shown in the figure.

As can be seen from FIG. 9, in the case of the invention materials (A, B) satisfying the scope of the present invention, it can be seen that the composite moldability is excellent because the CCV value is lower than that of the comparative materials (1-12).

In order to investigate the reason why the inventive material (A, B) is superior in formability to the comparative material (1-12), the work hardening index n value obtained from the 8% to 16% elongation section of the specimens was measured and the result was measured. It is shown in the figure.

As can be seen in Figure 10, the invention material (A, B) that satisfies the scope of the present invention because the n value is higher than that of the comparative material (1-12), the necking is delayed during molding and excellent moldability It can be seen.

In order to evaluate the cold rolling annealing, the tensile tester was processed according to JIS No. 5 standard in parallel with the rolling direction. After grinding the parallel part of the test specimen, it was stretched by 20% and the surface roughness was used as the maximum height of the surface profile (Rt). The height of the ricin was measured and the result is shown in FIG.

As can be seen in Figure 11, the invention material (A, B) that satisfies the scope of the present invention can be seen that the rimability is improved compared to the comparative material (1-12) lower the rhythm height.

As described above, the present invention enables the production of ferritic stainless steel having excellent moldability and stiffness resistance by appropriately controlling the steel content, Nb / Ti, and (Nb + Ti) / (C + N), and manufacturing conditions. It works.

Claims (1)

In the manufacturing method of ferritic stainless steel, in weight%, C: 0.02% or less, Si: 0.6% or less, Mn: 0.5% or less, P: 0.02% or less, S: 0.003% or less, Cr: 15-20%, Mo: 0.6% or less, N: 0.02% or less, Ti: 0.6% or less, Nb: 0.6% or less, remaining Fe and inevitably contained impurities, and C + N: 0.03% or less, (Ti + Nb) / (C + N): 12-20, and Nb / Ti: stainless steel that satisfies 2 or more is heated to a temperature range of 1170-1250 ℃ and hot-rolled to the finish rolling temperature conditions of 750-900 ℃, 900- Hot-annealed and quenched at a temperature range of 970 ° C., followed by cold rolling, followed by cold-annealed and quenched at a temperature range of 900-970 ° C. .