JPS6214005B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a ferritic stainless steel sheet with excellent corrosion resistance and surface quality. Ferritic stainless steel sheets, typified by 17Cr steel (SUS430), are used in automobile parts, kitchen products, home appliances due to their beautiful surfaces, and hot water supply parts due to their stress corrosion cracking resistance, which austenitic stainless steels do not have. Widely used. However, ferritic stainless steels generally have inferior corrosion resistance compared to austenitic stainless steels, and have the disadvantage that fine uneven patterns called roping or ridging appear during cold rolling or press working, which impairs their aesthetic appearance. conventionally,
For this reason, expensive austenitic stainless steel had to be used due to its slight lack of corrosion resistance and deterioration of surface quality. To this end, the present invention provides a method for producing a ferritic stainless steel sheet with excellent corrosion resistance and surface quality. The present inventors have developed a method for reducing the amount of C and N in ferritic stainless steel and increasing the amount of C and N in the steel.
We have found that the corrosion resistance of stainless steel can be improved by adding Nb. Conventionally, it is well known that adding Nb to stainless steel, whether ferritic or austenitic, improves intergranular corrosion resistance, especially in welded areas, which requires welding. It is used for a purpose. And the reason for this is that C, N in steel
It is interpreted that this is due to the fixation of Nb by Nb. However, the findings of the present inventors suggest that Nb dissolved in solid solution in the steel rather than fixation of C and N is effective, and as a result of easy passivation rather than intergranular corrosion resistance, corrosion resistance This will improve the results. These relationships will be explained below based on the survey results. The excellent corrosion resistance of stainless steel is due to the fact that a passive film is formed on the surface, and the rate of dissolution of the film, that is, the rate of corrosion, is so slow that it practically does not corrode. Therefore, the corrosion resistance of stainless steel can be evaluated by whether it is easy or difficult to form a passive film, that is, how easy it is to passivate. The ease of passivation can be compared by the passivation current density (arrows 1 and 2 in the figure) of the anode polarization curve shown in Figure 1; the smaller the value, the easier it is to passivate, the better the corrosion resistance. It can be evaluated as excellent. In the example shown, steel B has better corrosion resistance than steel A. The present inventors investigated the influence of Nb on the ease of passivation of ferritic stainless steel, and found that the existence of Nb other than those precipitated by C, N, etc., that is, Nb dissolved in the steel. It was therefore observed that the passivation current density decreased. Figure 2 shows various
5% H ₂ SO ₄ (25
FIG. 3 is a diagram showing the results of investigating the passivation current density in a solution (° C.). In the figure, Nb is C and N in steel.
The effective Nb shown in equation (1) by subtracting the amount precipitated as
Shown in quantity. As shown in the figure, the effective Nb amount is 0.05
% or more, it is recognized that the passivation current density decreases and the corrosion resistance improves. Effective Nb amount (%) = [%Nb] -92.9/12.0 [%C] -92.
9/14.0 [%N]... (1). In addition, as a conventional stainless steel with excellent corrosion resistance,
Steels containing Ti are known (for example, Japanese Patent Application Laid-Open No. 52-123917), but this steel has relatively large angular
The drawback was that TiN crystals precipitated, significantly degrading the surface quality. However, the inventors
It has been found that the addition of Nb improves corrosion resistance without such deterioration of surface properties. The reason for this is
This is thought to be because Nb carbides and nitrides precipitate in solid steel at temperatures below about 1200°C and are very fine. Based on such knowledge, the present invention has developed a method for manufacturing a ferritic stainless steel sheet that has superior corrosion resistance and surface quality compared to conventional ferritic stainless steel.The gist of the invention is as follows. It is. (1) C: 0.03% or less, N: 0.025% or less, Si: 1.0
% or less, Mn: 1.0% or less, Cr: 11.5-22.0%,
Nb: Contains 0.05 to 0.8% in the effective Nb amount of formula (1),
When hot rolling a ferritic stainless steel slab whose composition consists of Fe and unavoidable impurities in a continuous hot rolling mill consisting of a rough rolling mill and multiple hot rolling mills, the rough rolling start temperature is
Rough rolling at 1250℃ or lower and at 850℃ or higher
After performing one or more passes of 40% or more rolling, the final hot rolling start temperature is adjusted in the continuous finishing hot rolling process.
After hot rolling at 850°C or higher, with one or more rolling passes with a finish hot rolling end temperature of 500°C or higher and 850°C or lower and a rolling reduction of at least 25%, 850°C or higher and 1100°C. Annealing for a short time within 10 minutes at the following temperature, followed by cold rolling or cold rolling including intermediate annealing,
A method for producing a ferritic stainless steel sheet with excellent corrosion resistance and surface quality, which comprises performing final annealing. Effective Nb amount (%) = [%Nb] -92.9/12.0 [%C] -92.
9/14.0 [%N]... (1) (2) C: 0.03% or less, N: 0.025% or less, Si: 1.0
% or less, Mn: 1.0% or less, Cr: 11.5-22.0%,
Nb: Contains 0.05 to 0.8% in the effective Nb amount of formula (1),
Furthermore, 1 of Mo: 1.5% or less, Ni: 1.5% or less
When hot rolling a ferritic stainless steel slab having a composition of Fe and unavoidable impurities, the rough rolling mill consists of a rough rolling mill and multiple hot rolling mills. The starting temperature is 1250℃ or less and 850℃
After performing one pass or more of rolling of 40% or more in the above rough rolling, the finishing hot rolling start temperature is set to 850°C or higher in the continuous finish hot rolling process, and the finishing hot rolling end temperature is set to 500°C or higher to 850°C. After hot rolling by adding one or more passes of rolling with a rolling reduction rate of at least 25% and a rolling reduction of at least 25%, annealing is performed for a short time of 10 minutes or less at a temperature of 850°C or higher and 1100°C or lower, followed by cold rolling. A method for producing a ferritic stainless steel sheet with excellent corrosion resistance and surface quality, which comprises rolling or cold rolling including intermediate annealing and final annealing. Effective Nb amount (%) = [%Nb] -92.9/12.0 [%C] -92.
9/14.0 [%N]... (1) Next, the reason for limiting the alloy components in the present invention will be explained below. Although C and N themselves have a small effect on corrosion resistance, they consume Nb as carbide nitrides, and not only is it necessary to add a large amount of Nb to obtain a certain amount of effective Nb, but also precipitated carbides are generated. Due to negative effects such as becoming the starting point for rust, the upper limit of C was set at 0.03%.
The upper limit of N was set at 0.025%. Si and Mn also have a small effect on corrosion resistance, but if added in large amounts they become hard and disadvantageous in terms of cost, so the upper limit for both was set at 1.0%. If Cr is less than 11.5%, it will not become passivated in the environment of automobile parts for which the steel of the present invention is used, so 11.5% is the lower limit.If it exceeds 22.0%, workability will deteriorate and it will be disadvantageous in terms of cost, so 22.0 %
was set as the upper limit. Nb is the main element constituting the present invention and the second
According to the figure, the lower limit is 0.05%, at which a decrease in the passivation current density is observed, and 0.8%.
The effect remains the same even if added in excess of 0.8%, which is disadvantageous in terms of cost. The technology of the present invention can also be applied to items requiring a high level of corrosion resistance, such as automobile exterior parts, and one or both of Mo and Ni can be added depending on the required level of corrosion resistance. In that case, adding a large amount is not only disadvantageous in terms of cost but also deteriorates hot workability, so the upper limit for Mo was set at 1.5% and the upper limit for Ni was set at 1.5%. The stainless steel sheet of the present invention is usually manufactured through the steps of hot rolling, hot rolling sheet annealing, cold rolling (including cases where intermediate annealing is performed during the process), and final annealing.
However, during the cold rolling process or press working, uneven striped patterns called roping or ridging sometimes occur, which seriously impairs the appearance. The degree of occurrence of this roping varies depending on the manufacturing conditions of the stainless steel sheet, but it is recognized that there is a significant correlation with the processing conditions of the hot rolling process and the annealing conditions. For example, there is knowledge that lowering the finish hot rolling temperature reduces roping, but in this case, there is a drawback that surface flaws, so-called scale flaws, are more likely to occur. Also,
There is knowledge that roping can be reduced by short-time annealing by heating and rapidly cooling to 950℃ or higher, but the effect is not always consistent and in some cases, roping can be reduced by heating and rapidly cooling to 950℃ or higher.
In some cases, it was no different from long-time annealing at ℃. The present inventors investigated in detail the effects of hot rolling conditions and annealing conditions on the surface quality of cold rolled sheets, and found that high reduction was performed in the early stage of hot rolling, that is, rough rolling, and that in the final stage, that is, finishing rolling, Contrary to the previous findings, it was found that the roping of cold-rolled sheets was significantly reduced by increasing the starting temperature, followed by short-time annealing with rapid heating above 850°C and rapid cooling. Based on the above findings, we have decided to wait a certain period of time before starting finish rolling after finishing rough rolling, which was conventionally done to improve roping properties, and to lower the finish hot rolling start temperature to a low temperature of, for example, 800 to 850°C. The production method of the present invention has been completed, since delayed hot rolling, which involves rolling from the start, is no longer necessary, which not only increases the productivity of hot rolling, but also reduces scale defects since finish rolling can be started at a high temperature. First, the metallurgical reason for the manufacturing method of the present invention will be explained. The controlling factor for the roping characteristics in the hot rolling process is the extent to which recrystallization progresses in the hot rolling process and the crystal grains of the slab are refined. However, in the case of the stainless steel sheet of the present invention, recrystallization hardly progresses at low temperatures below 850°C, so the final hot rolling start temperature is
Recrystallization can be promoted by increasing the temperature to 850°C or higher. Also, for the same reason, in the rough rolling process, it is effective to carry out rolling with a large reduction of 40% per pass for one or more passes to increase the resulting strain and promote recrystallization during hot rolling. It is. The controlling factor for the roping characteristics in the hot-rolled sheet annealing process is the extent to which recrystallization progresses during the hot-rolled sheet annealing process, grains are refined, and the band-like structure that occurs during hot rolling is destroyed. This depends on the amount of strain accumulated in the hot rolling finishing process, and the larger the amount of strain accumulated, the finer the recrystallized grains become. That is, in the method of the present invention, in the process of hot rolling the ferritic stainless steel slab described above, a reduction of 40% or more is performed at least one pass in the rough rolling process at 850°C or higher, and then the final hot rolling is started. After hot rolling at a temperature of 850°C or higher and a finishing temperature of 850°C or lower and applying a reduction of 25% or more for at least one pass, it is rolled at a temperature of 850 to 1100°C for 10
As a result, it has become possible to stably produce ferritic stainless steel sheets with excellent corrosion resistance and surface quality. Next, the reasons for limiting the constituent elements of the method of the present invention will be described. The reason why the starting temperature for rough rolling is set at 1250°C or lower is that if hot rolling is started at a temperature exceeding this temperature, the crystal grains will become coarse and the strain that will cause further recrystallization will not be able to accumulate, resulting in insufficient recrystallization. This is because. In addition, in the rough rolling process at 850℃ or higher, 40
The reason why at least one pass of pressure reduction of 850°C or more was carried out was because if it was less than 40%, the amount of strain to promote recrystallization would be small and recrystallization would be insufficient. This is because even if strain is applied, sufficient recrystallization cannot be expected and the temperature falls within the finish hot rolling temperature range. Next, the reason why the start temperature of finish hot rolling is set to 850℃ or higher is that if finish hot rolling can be started from this temperature, recrystallization can be promoted even in finish hot rolling. Unlike the rough rolling process, inter-rolling is continuous hot rolling, so
This is because by performing multiple pass rolling with a pass reduction ratio of 25% or more, it is possible to expect a recrystallization effect metallurgically equivalent to one-pass large reduction rolling. It is also possible to apply a reduction of 25% or more at a temperature below 850℃ in the rough rolling process, but in this case, the time between passes is long, so the effect of accelerating recrystallization due to cumulative reduction, which is a characteristic of finish hot rolling, is expected. However, there is a drawback that the start temperature of finish hot rolling inevitably decreases and scale flaws occur. In addition, the reason why the finish hot rolling end temperature is set to 500℃ or more and 850℃ or less is because strain accumulates in the hot-rolled sheet due to rolling at 850℃ or less, and promotes recrystallization in the subsequent hot-rolled sheet annealing process. This is because if hot rolling is finished at a temperature exceeding 850°C, no strain remains in the hot rolled sheet, so recrystallization during hot rolled sheet annealing is insufficient and the surface properties of the cold rolled sheet do not improve. It is. However, rolling at extremely low temperatures causes disadvantages such as unnecessarily high driving power of the rolling mill and increased surface defects, so the temperature at which the finishing hot rolling ends is
The temperature was set at 500℃ or higher. Next, the hot rolled sheet annealing conditions were set at 850 to 1100℃.
This is because at temperatures below 850°C, even if the hot-rolled sheet is strained, it will not recrystallize and the roping reduction effect of hot-rolled sheet annealing will be reduced. Is the roping improvement effect unchanged even if the temperature is disadvantageous and the temperature is high?
Rather, at higher temperatures, coarse crystals are formed and the roping characteristics deteriorate. The reason why the annealing time is set to 10 minutes or less is because heating for a longer time than this will hardly improve the roping properties and will be disadvantageous in terms of cost. Examples of the present invention will be shown below. Example After heating a 250 mm thick slab of ferritic stainless steel No. 1 to 6 shown in Table 1 at 1200°C,
The material was rolled to 250 mm using a rough rolling mill, followed by finish rolling to obtain a hot rolled sheet with a thickness of 3.5 mm. Finish rolling is carried out using a 6-stand continuous finishing mill.
and 35% and 30% rolling reduction at the 5th stand.
And so. Subsequently, this hot rolled sheet was annealed, then cold rolled and annealed to obtain a cold rolled annealed sheet having a thickness of 0.5 mm.
Table 2 shows the hot rolling conditions, hot rolled sheet annealing conditions, and roping height of these thin steel sheets. As shown in Table 2, it can be seen that the ferritic stainless steel sheets produced by the method of the present invention are excellent in both surface properties and corrosion resistance.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 shows the anode polarization curve of steel, Figure 2 shows the anode polarization curve of steel.
The figure is a diagram showing the relationship between the effective amount of Nb in the steel and the passivation current density according to the present invention.

Claims (1)

[Claims] 1 C: 0.03% or less, N: 0.025% or less, Si: 1.0
% or less, Mn: 1.0% or less, Cr: 11.5-22.0%,
Nb: A ferritic stainless steel slab containing 0.05 to 0.8% of the effective Nb amount in formula (1), with the remainder consisting of Fe and unavoidable impurities, is processed by a rough rolling mill and multiple hot rolling mills. When hot rolling with a continuous hot rolling mill, the rough rolling start temperature is set to 1250°C or lower, and after one pass or more of 40% or more reduction in rough rolling at 850°C or higher, the continuous finishing hot rolling process is performed. The finish hot rolling start temperature should be 850℃ or higher, and the finish hot rolling finish temperature should be 500℃ or higher 850℃.
℃ or less and the reduction rate of the rolling pass is at least 25
After hot rolling with one or more passes of rolling,
Excellent corrosion resistance and surface texture characterized by short-time annealing for 10 minutes or less at a temperature of 850℃ or higher and 1100℃ or lower, followed by cold rolling or cold rolling including intermediate annealing, and final annealing. A method for manufacturing ferritic stainless steel sheets. Effective Nb amount (%) = [%Nb] -92.9/12.0 [%C] -92.
9/14.0 [%N]... (1) 2 C: 0.03% or less, N: 0.025% or less, Si: 1.0
% or less, Mn: 1.0% or less, Cr: 11.5-22.0%,
Nb: Contains 0.05 to 0.8% as the effective Nb amount in formula (1), further contains one or more of Mo: 1.5% or less, Ni: 1.5% or less, and the remainder is Fe and unavoidable impurities. When hot rolling a ferritic stainless steel slab in a continuous hot rolling mill consisting of a rough rolling mill and multiple hot rolling mills, the rough rolling start temperature should be 1250°C or lower, and the rough rolling temperature should be 850°C or higher.
After performing one or more passes of reduction of 40% or more, the finishing hot rolling start temperature is set to 850 in the continuous finishing hot rolling process.
℃ or above, and the finishing hot rolling end temperature.
After hot rolling at 500°C or more and 850°C or less and with one or more rolling passes with a rolling reduction of at least 25%, annealing is performed for a short time of 10 minutes or less at a temperature of 850°C or more and 1100°C or less, A method for manufacturing a ferritic stainless steel sheet with excellent corrosion resistance and surface properties, which is characterized by subsequently performing cold rolling or cold rolling including intermediate annealing, and final annealing. Effective Nb amount (%) = [%Nb] -92.9/12.0 [%C] -92.
9/14.0 [%N]... (1)