KR20070067325A - A method of manufacturing a ferritic stainless steel for improving ridging resistance - Google Patents

Abstract

A stainless steel manufacturing method for suppressing defects of a product by controlling manufacturing process variables according to the equiaxed crystal ratio, thereby improving ridging resistance of a final cold rolled workpiece is provided. In a ferritic stainless steel manufacturing method comprising performing hot rolling, hot annealing, cold rolling and cold annealing processes on a stainless steel slab comprising, by weight percent, 0.05% or less of C, 0.7% or less of Ti, 1.0% or less of Si, 1.0% or less of Mn, 0.04% or less of P, 0.03% or less of S, 16.0 to 22% of Cr, 0.5% or less of Ni, 1.0% or less of Mo, 0.05% or less of N, 1.0% or less of Cu, 0.15% or less of Al, and the balance of Fe and other ordinary impurities to manufacture a steel sheet with a thickness of 1.5 mm or less, a method of manufacturing a ferritic stainless steel with improved ridging resistance comprises controlling a ridging prediction index to 20 or less using the following mathematical expression 1 to improve ridging resistance of the ferritic stainless steel: -25.6+0.7K+0.35xCRR+0.05xCRA, where K: -9.4xln[equiaxed crystal ratio(%)](logarithmic equiaxed crystal ratio influence on ridging), CRR: (hot rolling thickness-cold rolling thickness)x100/hot rolling thickness, and CRA: cold annealing temperature(deg.C).

Description

A method of manufacturing a ferritic stainless steel for improving ridging resistance

Figure 1 is a graph comparing the leasing height predicted in consideration of the actual leasing height measurement value and the slab microstructure and manufacturing process parameters after stretching the cold rolled annealing material.

The present invention relates to a method of manufacturing ferritic stainless steel with improved leasing resistance, and particularly, in manufacturing cold rolled sheet containing 16-20% chromium (Cr), which is mainly used in building materials, kitchen containers, and home appliances. It relates to a manufacturing method for improving.

In general, ferritic stainless steel has wrinkled surface defects during molding, which is called ridging. The cause of leasing is primarily due to the development of columnar tablets in the casting structure. That is, when columnar tablets having a certain orientation remain undestructed in the rolling or annealing process, they exhibit deformation behavior in a width and thickness direction different from the recrystallized structures around the tensile processing, and are expressed as ridging defects. This ridging defect not only deteriorates the appearance of the product, but also causes the increase of the manufacturing cost of the final product, because if the ridging occurs badly, an additional polishing process is required after molding.

Many researchers have suggested various manufacturing methods for improving the ridging property of ferritic stainless steel. Basically, Sawatani's research report (Nippon Steel Tech. Report, 21 (1983), p.275) has a way to improve the ridging ability by reducing the fraction of columnar tablets by improving the equiaxed ratio ratio. Such equiaxed rate control is a method of solving the underlying cause of leasing, and in order to obtain a steel sheet excellent in ridging resistance by ordinary rolling, the lower limit of equiaxed rate should be 60%.

As a representative example of ridging suppression by adjusting process variables in the manufacturing process, in order to promote recrystallization, the appropriateness of hot rolling temperature (JP1975-016616, JP2000-256748), hot rolling reduction rate (JP2000-256748), annealing temperature ( JP1983-199822) and various methods of improving leasing resistance have been known, such as the addition of an intermediate annealing process during cold rolling to increase the number of recrystallizations of cold rolling (JP1989-118341).

As described above, in order to improve the ridging property, it is important not only to improve the casting structure such as slab equiaxed crystal but also to control the manufacturing process variables. In particular, the microstructural factors such as equiaxed crystallinity during casting of Ti-added ferritic stainless steel slab through steelmaking and continuous casting are highly dependent on the Ti content change, so the ridging property of the final cold rolled product may be caused by low equiaxed crystallinity. In order to suppress defects, it is necessary to change the process conditions according to equiaxed crystals in the sub-hot rolling process.

Accordingly, the present invention has been made in accordance with the above requirements, and in order to reduce the ridging defect rate of the final cold rolled material produced from a material having a low equiaxed crystallization rate, by adjusting the manufacturing process variable according to the equiaxed crystallization rate, It is an object of the present invention to provide a stainless steel manufacturing method for suppressing defects of a product.

In order to achieve the above object, the present invention, in weight%, C 0.05% or less, Ti 0.7% or less, Si 1.0% or less, Mn 1.0% or less, P 0.04% or less, S 0.03% or less, Cr 16.0 to 22%, Stainless steel slab composed of Ni 0.5% or less, Mo 1.0% or less, N 0.05% or less, Cu 1.0% or less, Al 0.15% or less, balance Fe and other common impurities is heated to a temperature range of 1200 to 1300 ^° C. After sufficiently aging, an object of the present invention is to provide a manufacturing method in which the ridging height is maintained at less than 20 mm by adjusting the hot rolled thickness, cold rolled thickness, and cold rolled annealing temperature according to the equiaxed rate.

Hereinafter, the present invention will be described in detail.

Method for producing a ferritic stainless steel according to the present invention to achieve the above object comprises the steps of heating the slab to a temperature range of 1200 ~ 1300 ^° C sufficiently aging; Manufacturing a sheet having a thickness of 25 to 35 mm by hot rough rolling; Adjusting the thickness of the hot rolled sheet to a thickness of 2.5 to 5.5 mm by hot finishing rolling; Heating the hot rolled sheet formed by the hot rolling to 950 to 1050 ^° C. for 10 seconds to 5 minutes, and then quenching and continuous annealing; Cold rolling the continuously annealed hot rolled annealing plate to produce a thickness of 0.6 to 1.5 mm; And maintaining the cold rolled plate at a temperature of 850 to 1050 ^° C. for 10 seconds to 3 minutes, followed by quenching and continuous annealing.

The inventors have found that the leaching resistance increases as the thickness of the hot roll is smaller, the cold rolling reduction is lower, and the cold rolling temperature is lower at the same equiaxed crystal. In addition, we found that leasing height has equiaxed rate and logarithmic dependence. Therefore, the main feature of the present invention is that the correlation between the aforementioned factors and the leasing height can be described by the following formula.

[Equation 1]

-25.6 + 0.7 K + 0.35 CRR + 0.05 CRA

Where K is -9.4 * ln [% equiaxed rate (%)], which is an index indicating the logarithmic isometric effect on leasing, and CRR is (hot rolled-cold rolled thickness) x100 / hot rolled thickness, and CRA ( ^ㅇ C) It means cold annealing temperature. Equation 1 corresponds to a case in which the rough rolling finish thickness is 25 to 35 mm. In this case, when the CRR is large, the hot rolling reduction ratio is relatively small.

In general, in order to be a steel sheet having excellent surface quality in terms of quality control of stainless steel, it is required to maintain the leasing height of 20 mm or less, so the leasing height calculated by Equation 1 is 20 in the present invention. The smaller case means the condition satisfying the above condition.

(Example)

The present invention is explained using the following examples.

To illustrate the characteristics of the present invention, alloys of STS439 steel composition, which are representative steels of stainless steel containing 16 to 20% Cr, were prepared as shown in Table 1 below. These alloys were produced by the production method according to the present invention in the laboratory by taking a slab of 200mm thickness continuously cast in the actual production line.

Table 1 shows the equiaxed crystallinity of the ferritic stainless steel slabs used in this example and the ridging height measurement results of the cold rolled annealing plate. The ridging height is a measure for evaluating the ridging resistance of the manufactured stainless steel, and was measured through the surface roughness after the tensile test using the manufactured stainless steel evaluation specimen.

1 is a graph comparing the measured ridging height shown in Table 1 with the ridging height predicted by Equation 1.

As described above, in order to manufacture a high-quality steel sheet having excellent surface properties in terms of quality control from a slab having a low equiaxed crystallization rate, the ridging height is required to be maintained at 20 mm or less. This means that a condition having a leasing resistance of 20 mm or less can be obtained when the leasing resistance index predicted by Equation 1 is 20 or less. Inventive Examples and Comparative Examples are illustrated in Table 2 below.

ingredient C Si Mn Cr Ti Ni N Alloy 1 0.0049 0.148 0.228 18.50 0.26 0.13 0.0094 Alloy 2 0.0053 0.476 0.443 18.34 0.36 0.12 0.0096 Alloy 3 0.0054 0.327 0.446 18.29 0.35 0.14 0.0101 Alloy 4 0.0053 0.271 0.414 18.37 0.333 0.12 0.0103 Alloy 5 0.0056 0.224 0.302 18.25 0.327 0.13 0.0092 Alloy 6 0.0049 0.148 0.228 18.50 0.261 0.13 0.0094

Example ingredient Equivalence rate (%) Hot rolled thickness (mm) Cold rolled thickness (mm) K CRR CRA (℃) Ridging (μm) Leasing prediction index Inventive Example Alloy 5 31 2.5 1.5 -32.25 40.00 880 14.20 9.82 Inventive Example Alloy 2 42 3.7 1.5 -35.10 59.5 880 15.20 14.64 Inventive Example Alloy 3 100 5.4 One -43.25 81.5 880 15.80 16.64 Inventive Example Alloy 6 16 2.5 1.5 -26.04 40.00 880 15.90 14.17 Inventive Example Alloy 2 42 3.1 1.5 -35.10 51.6 880 16.00 11.89 Inventive Example Alloy 3 100 5.4 0.6 -43.25 88.9 880 16.30 19.24 Inventive Example Alloy 3 100 3.3 1.5 -43.25 54.5 880 16.40 7.22 Inventive Example Alloy 3 100 5.4 1.5 -43.25 72.2 880 16.60 13.40 Inventive Example Alloy 5 31 2.5 One -32.25 60.00 880 17.10 16.82 Inventive Example Alloy 3 100 5.4 1.5 -43.25 72.2 980 17.50 18.40 Inventive Example Alloy 6 16 2.5 1.5 -26.04 40.00 980 17.90 19.17 Inventive Example Alloy 3 100 3.3 1.5 -43.25 54.5 980 18.10 12.22 Inventive Example Alloy 2 42 3.7 1.5 -35.10 59.5 980 18.40 19.64 Inventive Example Alloy 3 100 3.3 0.6 -43.25 81.8 880 18.40 16.76 Inventive Example Alloy 3 100 3.3 One -43.25 69.7 880 18.50 12.52 Inventive Example Alloy 4 55 5.5 1.5 -37.64 72.73 880 18.90 17.51 Inventive Example Alloy 2 42 3.7 One -35.10 73.0 880 19.30 19.37 Inventive Example Alloy 2 42 3.1 One -35.10 67.7 880 19.50 17.54 Inventive Example Alloy 2 42 3.1 1.5 -35.10 51.6 980 19.80 16.89 Inventive Example Alloy 3 100 3.3 One -43.25 69.7 980 20.00 17.52 Comparative example Alloy 3 100 3.3 0.6 -43.25 81.8 980 20.10 21.76 Comparative example Alloy 3 100 5.4 0.6 -43.25 88.9 980 20.40 24.24 Comparative example Alloy 3 100 5.4 One -43.25 81.5 980 21.80 21.64 Comparative example Alloy 5 31 2.5 0.6 -32.25 76.00 880 22.10 22.42 Comparative example Alloy 1 16 5.4 1.6 -26.04 70.4 880 22.50 24.80 Comparative example Alloy 4 55 5.5 One -37.64 81.82 980 22.50 25.69 Comparative example Alloy 5 31 4.0 1.5 -32.25 62.50 980 22.90 22.70 Comparative example Alloy 2 42 3.1 0.6 -35.10 80.6 980 24.10 27.05 Comparative example Alloy 6 16 2.5 One -26.04 60.00 980 24.30 26.17 Comparative example Alloy 4 55 5.5 0.6 -37.64 89.09 980 26.10 28.24 Comparative example Alloy 2 42 3.7 0.6 -35.10 83.8 880 26.30 23.15 Comparative example Alloy 2 42 3.7 One -35.10 73.0 980 26.60 24.37 Comparative example Alloy 1 16 5.4 0.6 -26.04 88.9 880 28.00 31.28 Comparative example Alloy 1 16 5.4 One -26.04 81.5 880 28.00 28.69 Comparative example Alloy 1 16 5.4 One -26.04 81.5 980 28.80 33.69 Comparative example Alloy 6 16 2.5 0.6 -26.04 76.00 880 29.40 26.77 Comparative example Alloy 1 16 5.4 1.6 -26.04 70.4 980 29.50 29.80 Comparative example Alloy 6 16 2.5 0.6 -26.04 76.00 980 29.80 31.77 Comparative example Alloy 5 31 4.0 One -32.25 75.00 980 30.10 27.07 Comparative example Alloy 2 42 3.7 0.6 -35.10 83.8 980 30.90 28.15 Comparative example Alloy 5 31 4.0 0.6 -32.25 85.00 980 32.00 30.57 Comparative example Alloy 1 16 5.4 0.6 -26.04 88.9 980 41.40 36.28

As described above, the ferritic stainless steel manufacturing method provided by the present invention lowers the sensitivity of equiaxed crystallization to the ridging by improving the thermal ridging resistance of a material having a low equiaxed crystallization through process control, thereby reducing the sensitivity of the equiaxed crystallization on the surface of the product. This has the effect of reducing the quality problem.

Claims (2)

By weight%, C 0.05% or less, Ti 0.7% or less, Si 1.0% or less, Mn 1.0% or less, P 0.04% or less, S 0.03% or less, Cr 16.0-22%, Ni 0.5% or less, Mo 1.0% or less, Plates up to 1.5 mm thick are produced from a stainless steel slab consisting of N 0.05% or less, Cu 1.0% or less, Al 0.15% or less, balance Fe and other common impurities through hot rolling, hot annealing, cold rolling and cold annealing. The method of manufacturing a ferritic stainless steel having improved leasing resistance by controlling the leaching prediction index to 20 or less using the following equation. [Equation 1] -25.6 + 0.7 K + 0.35 CRR + 0.05 CRA (K: -9.4 * ln [% isotropic rate (%)]) (the effect of logarithmic isometric rate on leasing), CRR: (hot rolled thickness-cold rolled thickness) x 100 / hot rolled thickness, CRA: cold rolled annealing temperature (℃)) The method of claim 1, A method of manufacturing ferritic stainless steel with improved leasing resistance, characterized in that the ridging height controlled by the leaching prediction index is 20 μm or less.

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