JPH0558080B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing high Cr-Mo stainless steel strip, and in particular to cold-rolled annealed pickled and tempered (No. 2B finish) steel that maintains high corrosion resistance and is free from surface defects. The present invention relates to a method of manufacturing a belt. [Conventional technology] Recently, ferritic stainless steel containing high Cr-Mo (super ferritic stainless steel) has been developed.
Various types of stainless steel have been developed, and SUSXM27 (26%Cr
-1%Mo) is standardized. 26 more
%Cr-4%Mo steel, 30%Cr-2%Mo steel, 26%Cr
-3.5%Mo-2%Ni steel etc. are well known.
These high Cr-Mo stainless steels do not have to worry about stress corrosion cracking and have excellent pitting corrosion resistance, crevice corrosion resistance, and chemical resistance, so they are used in chemical plants and other applications instead of conventional austenitic stainless steels. Demand is increasing as a component in fields where high corrosion resistance is required. High Cr-Mo stainless steel sheets used for these purposes are manufactured into products with a predetermined thickness by being repeatedly rolled and annealed during the manufacturing process. However, as can be inferred from the fact that steel itself is a highly corrosion-resistant material, the oxidation scale that occurs during this annealing cannot be removed from conventional stainless steel sheets as disclosed in JP-A No. 59-59899. There is a problem in that even if a pickling process is applied, sufficient descaling is not possible. In particular, the problem of remaining scale not only reduces the corrosion resistance of the material, but also causes indentation and crimping defects due to the scale in the subsequent temper rolling process, significantly reducing the commercial value. Conventionally, as a countermeasure to this type of scale remaining problem, a polishing step was added after pickling to remove the scale by polishing, but this method inevitably increased costs due to consumption of polishing material and lower material yield. was. [Problems to be Solved by the Invention] The present invention is directed to the production of a high Cr-Mo stainless steel strip that can eliminate any scale remaining as a result of annealing and pickling and at the same time prevent surface defects during skin pass rolling. The present invention provides a method. [Means for Solving the Problems] The present inventors have conducted various studies in order to make the various properties of high Cr-Mo stainless steel sufficiently excellent and at the same time to prevent the formation of scale residue. . In particular, in the annealing process, it is important to sufficiently soften and recrystallize, which is the original purpose, and then to facilitate descaling in the subsequent process. In addition, in the pickling process, the type of acid and its usage are high Cr-
It is necessary to effectively adapt it to the annealing scale removal of Mo-based stainless steel. The present invention has been completed based on the discovery of a method capable of realizing the above. That is, in the present invention, in terms of weight ratio, C: 0.030% or less N: 0.020% or less Si: 0.50% or less Mn: 0.50% or less P: 0.030% or less S: 0.008% or less Cr: 20.0 to 35.0% Ni: 3.0% A high Cr-Mo stainless steel containing Mo: 0.5 to 6.0%, one or two of Nb or Ti to 0.60% of the above (C+N)%, and unavoidable impurities is cooled. After rolling, annealing and pickling;
The steel strip is then subjected to temper rolling.
In the manufacturing process of Cr-Mo stainless steel strip, (1) the annealing temperature is 950℃ to 1050℃, (2) molten alkali salt treatment as pre-pickling treatment.
(3) Then, as the first pickling step, perform the following () ()
Perform electrolytic pickling in which the plate itself is anodized and then cathodized at a current density in a sulfuric acid bath under conditions that satisfy the following formula (), 5≦C≦50...() 20≦T ₁ ≦100 ... () 0.5≦d≦50 ... () Where, C: Concentration of sulfuric acid (H ₂ SO ₄ ) (W/V%) T ₁ : Temperature of sulfuric acid bath (°C) d: Sulfuric acid electrolysis Current density (A/dm ² ) (4) As the second pickling step, use the following (), (), ()
Immersion in a nitric-fluoric acid bath under conditions that satisfy the following formula: 0.01N≦H≦10N …() 5≦(N+H)≦30 …() 20≦T ₂ ≦80 …() Here, N : Concentration of nitric acid (HNO ₃ ) (W/V%) H: Concentration of hydrofluoric acid (HF) (W/V%) T ₂ : High Cr-Mo characterized by liquid temperature of nitric-fluoric acid bath (°C) This is a method for producing stainless steel strips. [Function] The annealing treatment in the present invention is performed for the purpose of facilitating softening recrystallization and descaling treatment of the material. The reason why the annealing temperature is 950°C to 1050°C is that at temperatures below 950°C, softening and recrystallization are insufficient, resulting in hardness and poor workability. Further, at temperatures exceeding 1050°C, the structure tends to become coarse grained, roughening occurs on the machined surface, and corrosion resistance tends to deteriorate to some extent. Figures 1 and 3 are graphs specifically showing this. In Figure 1, 26%Cr-4%Mo steel having the composition shown in Table 1 was cold rolled to a thickness of 0.8mm.
This figure shows the results of examining changes in hardness and recrystallization state when the specimens were annealed at various temperatures in a gas combustion atmosphere.

[Table] As is known from Figure 1, an annealing temperature of 950°C or higher is required for softening and recrystallization. Figure 2 shows the results of investigating the relationship between workability and annealing temperature using a conical cup (CCVmm) test.
Workability is excellent at the softening and recrystallization temperature of 950°C to 1050°C, but when the temperature exceeds 1050°C, roughness is observed on the surface after processing. This rough skin is thought to be caused by the remarkable grain growth of the ferrite grain structure due to the high temperature. In short, from the viewpoint of processing characteristics, it is necessary to avoid increasing the temperature higher than necessary (over 1050°C), and the temperature should be in the range of 950°C to 1050°C. Figure 3 shows the results of investigating the relationship between pitting corrosion resistance and annealing temperature. When the annealing temperature exceeds 1050°C, pitting corrosion resistance tends to deteriorate to some extent. From the results shown in FIGS. 1, 2, and 3, it can be seen that the annealing temperature is preferably in the range of 950° C. to 1050° C. from the viewpoint of material properties. Next, the steel plates that had been annealed at various temperatures and had oxide scale attached to their surfaces were subjected to descaling treatment under various descaling conditions shown in Table 2. The degree of descaling was evaluated by counting the amount of oxygen on the surface using an X-ray microanalyzer while keeping the beam diameter (30 μm) and analysis time constant, since it is difficult to distinguish the scale using a light microscope or the like. In other words, the more significant the scale remains, the greater the amount of oxygen detected on the surface. The amount of surface oxygen that can ensure sufficient corrosion resistance and prevent surface flaws during temper rolling is 70 CPS or less. Table 2 also shows the results of measuring the amount of surface oxygen. Based on this result, the annealing temperature was 1030℃, and the molten alkali salt treatment as pickling pretreatment was carried out at 400℃~
After treatment at 500°C, sulfuric acid and nitrofluoric acid treatments were performed sequentially, followed by electrolytic treatment with sulfuric acid and anode treatment.
It was found that descaling was significantly promoted when cathodically treated. (No. 9, 11, 12, 13, 14,
18, 19, 20, 21). Even when these plates were subsequently subjected to temper rolling, no surface flaws such as indentation occurred, and the surface gloss was very good. This is due to the anodization in sulfuric acid, which first dissolves a little of the base iron directly under the scale and peels off some of the scale, and then the cathode treatment activates the partially scaled surface, allowing the nitric fluoride to dissolve. It is thought that this is because the descaling reaction in acid is promoted. As in the examples No. 8 and 10, descaling is incomplete even if electrolysis is performed in a sulfuric acid bath or anode treatment is performed after cathode treatment. However, when the annealing temperature is as high as 1100°C, significant scale remains even after the sulfuric acid electrolytic treatment described above, and it has been found that increasing the annealing temperature unnecessarily makes descaling extremely difficult. Ta.

【table】

〔Example〕

A hot-rolled steel plate of high Cr-Mo stainless steel having the composition shown in Table 3 was cold-rolled into a steel plate with a thickness of 0.7 mm. These plates are placed in a furnace that burns LPG.
Annealing was performed by holding at 1000°C for 30 seconds and then cooling in air. Next, descaling was performed under the pickling conditions shown in No. 13 in Table 2, and the surface was analyzed for oxygen using an X-ray microanalyzer.
It was found that the scale was within the range of 63CPS, scale was sufficiently removed, and the surface gloss was good. When temper rolling was subsequently performed, no indentation or crimping flaws due to residual scale occurred at all. Furthermore, when we conducted press forming, weldability tests, and corrosion resistance tests on the heat-rolled plate (2B finish), we confirmed that it was sufficiently suitable as a No. 2B finish plate.

〔Effect of the invention〕

According to the present invention, there is no problem of scale remaining during the annealing and pickling process of high Cr-Mo stainless steel strips, and surface defects such as indentation defects and crimping defects occur during subsequent temper rolling. It became possible to manufacture excellent stainless steel strips. Furthermore, since the obtained product has excellent corrosion resistance and workability, it can be put to practical use as a highly corrosion-resistant stainless steel without any problem, and brings great industrial benefits.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between annealing temperature and hardness of a 26% Cr-Mo cold rolled steel sheet, Figure 2 is a graph showing the relationship between annealing temperature and conical cup test value,
FIG. 3 is a graph showing the relationship between annealing temperature and corrosion loss due to ferric chloride.

Claims (1)

[Claims] 1 In terms of weight ratio, C: 0.030% or less N: 0.020% or less Si: 0.50% or less Mn: 0.50% or less P: 0.030% or less S: 0.008% or less Cr: 20.0 to 35.0% Ni: 3.0 % or less Mo: 0.5 to 6.0%, and further contains one or two of Nb or Ti to 0.60% of the above (C+N)% and unavoidable impurities. After inter-rolling, the steel strip is subjected to annealing and pickling, and then the steel strip is temper-rolled.
In the manufacturing process of Cr-Mo stainless steel strip,
The annealing temperature was set at 950°C to 1050°C, and molten alkali salt treatment as pickling pretreatment was performed in the range of 400°C to 550°C, followed by the following () as the first pickling step.
Electrolytic pickling is performed in a sulfuric acid bath under conditions that satisfy the equation () below at a current density within the range of the equation () below. () () A high-temperature method characterized by being immersed in a nitric-fluoric acid bath under conditions that satisfy the following formulas:
Method for manufacturing Cr-Mo stainless steel strip. Note 5≦C≦50 …() 20≦T ₁ ≦100 …() 0.5≦d≦50 …() 0.01N≦H≦10N …() 5≦(N+H)≦30 …() 20≦T ₂ ≦80 ... () Where, C: Concentration of sulfuric acid (H ₂ SO ₄ ) (W/V%) T ₁ : Temperature of sulfuric acid bath (℃) d: Current density of sulfuric acid electrolysis (A /dm ² ) N: Concentration of nitric acid (HNO ₃ ) (W/V%) H: Concentration of hydrofluoric acid (HF) (W/V%) T ₂ : Liquid temperature of nitric-fluoric acid bath (°C).