JPS6216252B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a manufacturing method aimed at uniformizing the quality of so-called 13% Cr martensitic stainless steel. 13% Cr stainless steel (hereinafter abbreviated as 13Cr steel), including SUS 410 steel, has sufficient corrosion resistance in relatively mild corrosive environments and is inexpensive, so it is widely used in Western tableware and the like. in general
13Cr steel is manufactured using a process in which hot-rolled steel strip is heated and held at 800 to 850°C for over an hour using a box-type annealing furnace, then cooled to room temperature for a long time, and then cold-rolled. be done. The reason for annealing 13Cr steel is to avoid the fact that if annealing is omitted, edge cracking will occur during cold rolling, making it impossible to roll well and resulting in insufficient ductility of the final product. The reason for this is that the hot-rolled structure of 13Cr steel is different from ordinary steel and has an optical microscopic structure similar to that of cold-rolled steel, and has high hardness, and the main purpose of annealing is to soften it. . However, the important point is that it is not only important to simply soften the material, but also to secure enough softening, or hardening, to withstand hardening during cold rolling. In this sense, the standard for softening usually differs depending on the cold rolling mill and the required rolling reduction, but it is generally 88 or less on the H _R B scale.
(above) are stipulated. However, although box annealing has the advantage that the equipment is easy to design and there are no difficulties in operation, it has the disadvantage that it is inefficient because it takes a long time to heat and cool the equipment. Even more fatally, decarburization and deCr removal of the surface layer are unavoidable due to prolonged heating, and since the temperature at each location in the furnace is not uniform, differences in annealing temperature and time occur within the steel strip, resulting in quality In other words, only extremely heterogeneous products could be produced. The present invention was made to solve various problems of box annealing all at once, and provides a method for producing martensitic stainless steel strip of uniform quality. The findings and ideas that form the basis of the present invention are shown below. The present inventors thought that it was impossible to make the quality uniform with box annealing, which inherently has a variation factor, and aimed at continuous annealing. Continuous annealing naturally involves high-temperature, short-time annealing. However, in the case of 13Cr steel, when exposed to high temperatures, an austenite phase appears, which transforms into a martensite phase upon cooling, and there is a risk of hardening (quench hardening). Of course, even in the state of low-temperature short-time annealing, ie, insufficient annealing, the hardness can be easily reduced to the above-mentioned level or less by reducing the interstitial solid solution elements such as C and N. However, the present inventors have found that even if a steel strip with reduced C and N content and insufficient annealing has a satisfactory hardness (H _R B ≦88), elongation occurs during the rolling process and the rolled shape deteriorates. discovered. When C and N are low, the level of hardness that hardens through work hardening is also low, so when cold rolling starts on an original sheet with a similar hardness level, maximum hardening occurs at a small rolling reduction level, even if the rolling reduction cannot be achieved. Regardless, it was thought that the shape would be defective because further rolling reduction would be applied. That is, it was discovered that in addition to the absolute hardness condition (H _R B≦88), a hardening allowance up to maximum work hardening is required. Based on this knowledge, we investigated the components and annealing conditions. FIG. 1 is a diagram showing the hardness of 13Cr stainless steel containing 0.05% of the sum of weight percentages of C and N (hereinafter abbreviated as C+N) when heated and air cooled at each temperature for 2 minutes.
The reason why the material hardens when the temperature exceeds 900°C is due to the so-called quench hardening, which occurs because the material transforms into an austenite phase during heating and undergoes martensitic transformation during cooling. Although quench hardening can occur at equilibrium temperatures of 850°C or higher, the transformation does not occur up to 900°C when heated for a short period of less than 5 minutes. Conversely, high hardness below 700°C is due to insufficient annealing. In order to control the hardness, it is common practice among those skilled in the art to control C and N. Figure 2 shows the hardness and C+ after heating and air cooling at 850℃ for 2 minutes and 650℃ for 2 minutes.
It is a figure showing the relationship of N. Of course C
If +N is reduced, the hardness will decrease regardless of the heating temperature. For H _R B to be 88 or less, which enables cold rolling, C+N is required to be 0.09% or less when heated to 850°C, but 0.04% or less is required when heated to 650°C. As shown above, if we focus only on the hardness that allows cold rolling, it can be seen that if C+N is sufficiently reduced, sufficient cold rolling can be achieved even if annealing is incomplete. However, if annealing is incomplete, the cold rolled shape will be poor. FIG. 3 is a diagram illustrating the evaluation of cold-rolled shapes after annealing under various C+N conditions and various annealing conditions. When C+N was less than 0.02%, cold rolling was possible and the shape was good regardless of the annealing conditions, and in some cases even if annealing was omitted.
However, if it is more than 0.02%, the shape will be poor even if it can be cold rolled on the low annealing temperature side (insufficient side).
It was recognized that annealing at 750°C or higher is necessary regardless of hardness. That is, in order to find conditions for a good cold-rolled shape, it is not appropriate to simply evaluate the hardness alone, but an appropriate combination of C+N components and annealing conditions is required. Next, the limiting conditions of the present invention are shown. The upper limit of C+N was determined to be 0.08%, which was in the range of H _R B <88 from FIG. 2 and in which good rolling was possible in FIG. 3. If C+N is less than 0.02%, cold rolling is possible regardless of the annealing conditions and the shape is good, but C,
Since reducing N is costly, the lower limit was set at 0.03%. From FIGS. 1 and 3, the annealing temperature is H _R B<
88 and over 700℃, which is an excellent range for cold rolled shape.
The temperature was set to 900℃ or less. If the annealing time is too long, there is a risk of quenching and hardening, especially at temperatures above 850°C, which is disadvantageous in terms of cost, so the upper limit was set to 5 minutes. The fact that the material is extremely uniform under the conditions determined based on the above results will be explained using examples. The 13Cr stainless steel strips shown in Table 1 were annealed under various conditions. The hardness, material, and cold-rolled shape of the steel strip are shown in Table 2, and an example of their distribution is shown in FIG. According to the method of the present invention, it is recognized that the hardness and cold rolled shape are at the same level as conventional materials and are extremely homogeneous. As shown above, the present invention not only enables continuous annealing of 13Cr steel, but also makes it possible to significantly reduce the variation in quality within the steel strip.
The industrial benefits are enormous.

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between annealing temperature and hardness, Figure 2 is a diagram showing the relationship between C+N content and hardness as the annealing temperature is varied, and Figure 3 is a diagram showing the relationship between C+N content and annealing temperature. FIG. 4 is a columnar diagram showing the distribution of Y and P in the steel strip. Figure 3: ○ mark: cold rolling possible and good shape after cold rolling, △ mark: cold rolling possible but poor shape after cold rolling, × mark: cold rolling impossible.

Claims (1)

[Claims] 1 The sum of the weight of C and N is 0.03% or more and 0.08% or less,
A hot-rolled martensitic stainless steel strip containing 10% to 14% Cr is heated and held in a temperature range of 750°C to 900°C for 5 minutes or less, then cooled to room temperature, and then cold rolled. A method for producing martensitic stainless steel strip with uniform quality.