JPH0448866B2 - - Google Patents

Description

[Detailed description of the invention]

Field of Industrial Application This invention relates to a method for manufacturing martensitic stainless steel sheets used for Western tableware, etc., and in particular, the invention relates to a method for manufacturing martensitic stainless steel sheets used for Western tableware, etc., and in particular, it is possible to achieve excellent oxidation resistance and workability by short-time hot-rolled sheet annealing (softening annealing). The present invention relates to a method for obtaining a martensitic stainless steel sheet. Conventional technology Martensitic stainless steel is generally used for applications that require relatively mild corrosion resistance, such as Western tableware such as knives and forks, and its composition is 11.5 to 14.0% Cr, with a maximum of 0.40% or less of C. ,
Generally, it contains 1.0% or less of Si and 1.0% or less of Mn. In addition, the manufacturing method involves hot rolling a continuous casting slab or a slab obtained by the ingot-blowing method, followed by batch annealing to soften the hot rolled plate, followed by pickling and cold rolling. It is usually made into a product by rolling and finish rolling. Problems to be solved by the invention Batch annealing for softening after hot rolling in the conventional manufacturing method described above generally takes a long time of several tens of hours. If hot-rolled sheet annealing is carried out over such a long period of time, a Cr-depleted layer will be formed on the surface of the hot-rolled sheet, which poses a major problem, especially in martensitic stainless steels with relatively low Cr content. In other words, if a Cr-free layer is generated in the surface layer due to hot-rolled sheet annealing,
Since the oxidation resistance of the surface deteriorates, a thick defective scale is generated on the surface of the steel sheet during finish annealing in the cold rolled steel strip production process after hot rolled sheet annealing, and the descaling performance becomes a problem. On the other hand, martensitic stainless steel is normally used after finishing annealing and then being processed into a beautiful surface by buffing, etc. However, the above-mentioned defective scales are generated by finishing annealing, and If scale remains,
There is a problem that makes polishing work extremely difficult. To solve the above-mentioned problem of descaling layer formation, it has been conventional to remove Cr by sufficiently removing the surface layer in the descaling process after hot-rolled sheet annealing, for example by making the acid tip time sufficiently long. remove the layer,
Measures have been taken to prevent deterioration of oxidation resistance during final annealing, but when such methods are applied, the pickling time becomes longer and the amount of chemicals used increases, resulting in increased costs. Furthermore, new problems arise, such as difficulty in disposing of the acid tip waste liquid from which a large amount of metal has been eluted. Therefore, the present inventors solved the problem by shortening the hot-rolled sheet annealing time and lowering the annealing temperature.
We will consider ways to prevent the formation of the Cr layer itself, and
We conducted an experiment and found that if we simply shortened the hot-rolled plate annealing time or lowered the annealing temperature for conventional martensitic stainless steel, desorption occurred.
Although the effect of reducing the Cr layer was recognized, it was found that the softening during hot-rolled sheet annealing was insufficient, and the mechanical properties of the cold-rolled product, particularly the workability, were significantly inferior. Therefore, the present invention makes it possible to sufficiently soften the hot rolled sheet even though the hot rolled sheet annealing is performed for a very short time in order to prevent the formation of a Cr-free layer during hot rolled sheet annealing. Therefore, there are problems caused by the formation of a Cr-free layer, typically the problem of oxidation resistance in cold-rolled steel sheets, and problems with conventional steels when hot-rolled sheets are annealed for a short time, typically problems with cold-rolled steel sheets. It is an object of the present invention to provide a method for manufacturing a martensitic stainless steel sheet that can solve the problem of deterioration of mechanical properties, particularly deterioration of workability. Means for Solving the Problems In order to achieve the above-mentioned objective, the inventors of the present invention have investigated the composition of martensitic stainless steel and conducted repeated experiments.
By containing 0.025 to 0.060% of N,
Even when hot-rolled steel sheets are annealed for a short time of 300 seconds or less, cold-rolled steel sheets can be obtained that have workability that is equal to or better than when the conventional long-time batch annealing is performed. I discovered that. In addition, in the surface layer of hot-rolled martensitic stainless steel,
A Cr-free layer with a thickness of about 3 to 6 μm is usually formed at the end of hot rolling, but when hot-rolled sheets are annealed for a short time of about 300 seconds or less, the Cr-free layer does not increase further during annealing, and the Cr-free layer does not increase further during annealing. It was also found that the rolled sheet had excellent oxidation resistance. Therefore, the method for producing a martensitic stainless steel sheet of the present invention requires C0.04% or less, Si1.0% or less, Mn1.0% or less, Ni0.6% or less, Cr10-15%,
A steel containing 0.025-0.30% Al and 0.025-0.060% N, with the balance consisting of Fe and unavoidable impurities is used as a material, and after hot rolling the material into a hot-rolled sheet,
A method for manufacturing a martensitic stainless steel sheet comprising a series of steps of softening annealing, followed by pickling, cold rolling, and final annealing, wherein the softening annealing is performed within 300 seconds within a temperature range of 800 to 1000°C. It is characterized by being carried out by short-time annealing. Detailed Description of the Invention In this invention, as mentioned above, by actively containing Al and N in the steel,
The hot rolled sheet can be sufficiently softened by an extremely short annealing time of 300 seconds or less. That is, by including Al in the steel, the A ₁ transformation point increases, and as a result, the annealing temperature can be increased without increasing the amount of martensitic phase precipitation to promote recrystallization of the steel sheet. Furthermore, if Al and N are simultaneously contained in the steel, a large amount of fine OlN will precipitate in the mesh plate during hot rolling, and the steel plate will recrystallize around these precipitates during high-temperature short-time annealing. is activated, recrystallization and softening are promoted. Here, the content of Al and N is 0.025% each
When the temperature is less than 1, the amount of OlN precipitation during hot rolling is small;
The effect of accelerating recrystallization and softening during annealing of the hot rolled sheet due to AlN precipitation was not observed, so the lower limits of Al and H were each set at 0.025%. On the other hand, the Al content
If the N content exceeds 0.30%, the effect will not increase any further, and if the N content exceeds 0.06%, the steel plate will become softer due to the increase in N content, leading to the occurrence of edge cracks during hot rolling and poor mechanical properties. This may lead to problems such as deterioration. Therefore, the upper limit for Al is 0.30% and the upper limit for N is 0.030%.
And so. Regarding the steel components other than Al and N, it is sufficient if they are almost the same as those of conventional ordinary martensitic stainless steel, and the reasons for these limitations will be described below. C is an element necessary to ensure strength, but if it exceeds 0.40%, the steel plate will become hard, so the upper limit was set at 0.40%. Although Si is effective as a deoxidizing agent, if it exceeds 1.0%, toughness deteriorates, so the upper limit was set at 1.0%.
Mn is effective in improving strength and toughness, but 1.0
%, the mechanical properties of the steel sheet will deteriorate.
The upper limit was set at 1.0%. Ni is an element that improves corrosion resistance, but since it is an expensive element, the upper limit was set at 0.6% in consideration of cost. Cr is a basic element in martensitic stainless steel, and 10% or more is required to obtain the necessary corrosion resistance.Although increasing the amount of Cr added improves corrosion resistance, the corrosion resistance for martensitic stainless steel applications is 10 because less than 15% is sufficient
It was set within the range of ~15%. In the manufacturing method of the present invention, a steel material having the above-mentioned composition, that is, a continuous cast slab or a slab obtained by an ingot-blowing rolling method, is hot-rolled in accordance with a conventional method. , Softening annealing is performed on the obtained hot rolled sheet by holding it in a temperature range of 800 to 1000°C for a short time of 300 seconds or less. Thereafter, pickling, cold rolling, and final annealing are performed in that order according to conventional methods to obtain a cold rolled steel sheet. By subjecting the hot-rolled sheet to high-temperature annealing for a short period of time as described above, a cold-rolled steel sheet having the above-mentioned composition can be obtained with excellent both oxidation resistance and workability. Here, to explain the reasons for limiting the hot-rolled sheet annealing conditions for softening, first of all, regarding the temperature conditions, if the annealing temperature is less than 800°C, sufficient recrystallization will not occur even with short annealing of 300 seconds or less. On the other hand, annealing at a temperature exceeding 1000°C has a significant recrystallization effect, but the crystal grains become coarser, leading to deterioration of mechanical properties, and even within a short time annealing, the Cr-free layer is removed. This may cause harm such as the formation of Therefore, the temperature range for softening annealing of hot rolled sheets is 800 to 1000℃.
was within the range of Also, the reason why the holding time in the above temperature range was set to within 300 seconds is that
In short-time annealing of less than 2 seconds, the steel sheet recrystallizes and becomes soft enough to make it unnecessary to hold it any longer, and if it is held for more than 300 seconds, a Cr-free layer is generated, making the cold-rolled sheet oxidation resistant. This is because the quality deteriorates. Example Steel A as a conventional steel having the chemical composition shown in Table 1, Steel B and Steel C as inventive steels were used as test materials, and continuous casting slabs of each steel were hot rolled according to a conventional method. , a hot-rolled sheet with a thickness of 3.5 mm. Subsequently, the hot-rolled sheets of each steel were subjected to softening annealing in which the hot-rolled sheets were held at various temperatures at 50°C intervals within the temperature range of 750 to 1050°C for 100 seconds. Here, for steel B, in addition to the above-mentioned hot rolled sheet softening annealing held for 100 seconds,
A hot-rolled sheet softening annealing was also performed at 850°C for 200 or 400 seconds. Steel A was also subjected to long-time batch annealing at 800°C for 8 hours, which is a conventional method. Subsequently, the annealed hot-rolled sheet was subjected to two-step pickling under the conditions shown in Table 2, and then cold rolled to a thickness of 1.8 mm and final annealed at 800° C. for 1 minute. FIG. 1 shows the Cr concentration distribution in the thickness direction of the steel sheets under typical conditions among the hot-rolled steel sheets after softening annealing. As shown in Figure 1, when the hot-rolled sheet was softened for a long time exceeding 300 seconds, the depth of the Cr-free layer reached approximately 14 μm or more from the surface of the steel sheet. When annealing was performed for a short time (100 seconds) on A steel, B steel, and C steel, the depth of the Cr-free layer was only 6 to 7 μm;
It is clear that the formation of a Cr-free layer is suppressed by short-time annealing. In addition, an oxidation resistance test was conducted on the hot-rolled annealed sheets shown in Figure 1 by pickling them under the conditions shown in Table 2, cold-rolling them, and then heating them at 800°C to examine the weight increase due to oxidation. The results are shown in Figure 2. As shown in Figure 2, when a hot-rolled sheet is softened for a long period of time exceeding 300 seconds, the weight increases significantly due to oxidation, and the oxidation resistance of the cold-rolled sheet is therefore poor, making it difficult to finish annealing. As a result, it is clear that poor scale forms thickly.On the other hand, when each steel is annealed for a short time, there is almost no increase in weight due to oxidation, and therefore the oxidation resistance of cold rolled sheets is reduced. It is clear that the steel has excellent properties and that poor scale is not produced by final annealing. Furthermore, Fig. 3 shows the results of examining the mechanical properties of the product cold-rolled sheet obtained by finish annealing at 800°C for 1 minute after cold rolling, in correspondence with the annealing temperature during softening annealing of the hot-rolled sheet. show. From Figure 3, when the conventional steel A steel is annealed for a short time, the elongation is small at any annealing temperature and the workability is inferior, compared to the conventional batch annealed A steel. It is clear that the workability is worse than if it were done. On the other hand, when steel B and steel C, which are the steels of the present invention, are annealed for a short time, the annealing temperature is 800 to 1000.
It is clear that excellent workability is obtained when the temperature is 0.degree.

【table】

[Table] Effects of the invention As is clear from the above examples, according to the method for manufacturing a martensitic stainless steel sheet of the present invention, by containing appropriate amounts of Al and N as steel components, It is possible to sufficiently recrystallize and soften the hot-rolled sheet with an extremely short softening annealing of 300 seconds or less, and as a result, it is possible to ensure sufficient workability of the cold-rolled sheet, and at the same time, it is possible to By shortening the sheet softening annealing time, it is possible to suppress the formation of a Cr-free layer during the annealing and significantly improve the oxidation resistance of the cold rolled sheet, resulting in excellent workability and oxidation resistance at the same time. Cold-rolled martensitic stainless steel sheets can actually be produced.

[Brief explanation of the drawing]

Figure 1 is a graph showing the Cr concentration distribution in the thickness direction of the surface layer of hot rolled sheets after softening annealing for hot rolled sheets under various annealing conditions, and Figure 2 is a graph showing the Cr concentration distribution in the thickness direction of the surface layer of hot rolled sheets after softening annealing. A graph showing the relationship between time and oxidation weight gain for various hot-rolled sheet softening annealing conditions. Figure 3 shows the relationship between the elongation and hardness of a cold-rolled sheet after finish annealing and the hot-rolled sheet softening annealing conditions. It is a graph shown about various steels A, B, and C.

Claims (1)

[Claims] 1 C0.40% (weight%, same below) or less, Si1.0%
Below, Mn1.0% or less, Ni0.6% or less, Cr10-15%,
After hot rolling a steel material containing 0.025 to 0.30% Al and 0.025 to 0.060% N, with the balance consisting of Fe and unavoidable impurities to form a hot rolled sheet, it is subjected to softening annealing,
In the method for manufacturing a martensitic stainless steel sheet, which comprises a series of steps of pickling, cold rolling, and final annealing, the softening annealing is performed at a temperature of 800 to 1000.
A method for manufacturing a martensitic stainless steel sheet with excellent oxidation resistance and workability, which is characterized by short-time heating within a temperature range of 300 seconds or less.