JPH0232332B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a ferritic stainless thin steel sheet with good surface properties without surface grinding before cold rolling. (Prior Art) When a ferritic stainless thin steel sheet, typified by 17Cr steel, is cold rolled without surface grinding after pickling, surface defects called cold dust occur. These surface flaws are caused by the unevenness that exists on the surface of the pickled plate.During cold rolling, the convex parts of the surface collapse into the concave parts, creating an overlapping part, and this part becomes the gold dust in the final product. This is a surface flaw called. In order to prevent the generation of gold dust, special surface grinding called "gold grinding" is performed as a pretreatment process before the cold rolling process in the normal manufacturing process. This surface grinding process requires polishing in several stages using abrasives with different grain sizes, and is an extremely costly process. A method for omitting this step is to eliminate unevenness on the surface of the pickling material, and there is a prior art method typified by the method described in JP-A-54-72728, for example. The technique involves removing the surface by removing it by several tens of micrometers using an acid mainly composed of H ₂ SO ₄ that does not cause intergranular corrosion, and smoothing the surface. This is not necessarily an economical method as there is a large amount of loss. (Problems to be Solved by the Invention) The present invention provides a gold dust prevention technology that does not require the surface grinding process before cold rolling as described above and has a high production yield. This problem was solved by increasing the C content concentration near the surface of the rolled sheet. The present invention will be explained in detail below. (Means to solve the problem) Ferritic stainless thin steel sheets, typically 17Cr steel, usually have a carbon content in the range of 0.04 to 0.08% due to mechanical properties, intergranular corrosion characteristics of welded parts, etc. Limited. That is, if the C content is lower than about 0.04%, the welded part is susceptible to intergranular corrosion, and if it exceeds 0.08%, the elongation decreases, leading to deterioration of mechanical properties such as a decrease in r value. However, the inventor's research has revealed that the steel sheet in the as-hot-rolled state and after annealing the hot-rolled sheet to a depth of about 100 μm inside from the outermost surface is mainly decarburized during the slab heating stage or the hot-rolled sheet annealing process. It was found that a C concentration gradient was often formed. Figure 1 shows an example of the C distribution survey results in the depth direction from the surface immediately after the slab is cast and after the slab is heated.In the as-cast state, the C distribution in the depth direction is almost uniform. , it can be seen that the surface layer is decarburized by heating the slab. Furthermore, FIG. 2 shows an example of the results of an investigation of the distribution of C in the thickness direction of a hot-rolled sheet. Figure 3 shows the amount of C at the time of tapping.
0.052% stainless steel slab is heated to 1200°C for 3 hours, hot-rolled, hot-rolled plate annealed at 1000°C for 20 seconds, and water-cooled. This is a metallurgical micrograph showing an example of the cross-sectional structure. . The analysis value of surface layer C is
The analysis value of the central layer C is 0.048%, indicating that about 100 μm of the surface has been decarburized and has become coarse grains. In such a region, as shown in FIG. 3, the crystal grains are coarse, about 100 μm, and are soft and easily deformed. Therefore, when a hot rolled sheet having such surface properties is subjected to mechanical descaling such as shot blasting or liquid honing, or pickling, irregularities are formed in the surface layer. When this material is rolled, the soft, protruding portions of the material's surface layer collapse and overlap, resulting in surface flaws called gold dust. Therefore, in order to remove the gold dust, the decarburized portion may be removed by pickling or grinding, but such treatment is not economical. Therefore, in the present invention,
At least 50μm inside from the outermost surface of the hot-rolled sheet after pickling
The amount of C in the process is adjusted to 0.04% or more and 0.1% or less, thereby reducing surface irregularities after pickling and preventing gold dust due to collapse during the cold rolling process. In the present invention, the lower limit of the C content in the depth range of at least 50 μm below the surface is limited to 0.04% because
This is because if the C content is less than this, collapse will occur and gold dust will be generated.
The reason for setting the C content to 0.1% is that if the C content exceeds this value, the surface layer will become too hard and small cracks will occur perpendicular to the rolling direction during cold rolling, which is not preferable. By the method of the present invention, C of the surface layer
By adjusting the amount within this range, surface defects called heat streaks that occur when the rolling speed increases in the cold rolling process can also be prevented. Heat streaks occur due to adhesion between the steel plate surface and the roll when there is insufficient lubrication, but as in the present invention, by increasing the C content of the stainless steel surface in contact with the roll to 0.04% or more, Adhesion can also be prevented, and heat streaks can also be prevented. In addition, the amount of C in the surface layer is 0.04%~
The thickness that should be maintained within 0.1% is 50μm from the surface.
Since the depth is sufficient, in the present invention, from the surface
Although the C content is limited to a depth of 50 μm, it is of course possible to enrich the C content throughout the entire decarburized layer, that is, up to 100 μm from the surface. Next, the reason why the amount of C during tapping in the present invention is limited to 0.002% to 0.08% will be described. In general ferritic stainless steel thin plates, the C content is 0.04
% or less, the formation of the γ phase during welding is insufficient, making the welded part susceptible to intergranular corrosion. Lowering C improves workability (r value, elongation) and has the advantage of requiring less rolling power in the rolling process, but since there is a problem of intergranular corrosion in welds, C cannot be lowered too much. However, if the C content of the surface layer is made high as in the present invention, α' phase will be formed on the surface part that comes into direct contact with the environment during welding, so there will be no problem of intergranular corrosion, so there will be no problem of intergranular corrosion during casting. C was set to be 0.002% or more, which is the lowest value in normal melting technology. The upper limit of C during tapping is 0.08%
The reason for this is that if it exceeds this value, the r value and elongation will be poor, and the power will also increase in the rolling process, which is not preferable. Specific measures to set the C value of the surface layer before cold rolling to 0.04% to 0.1% include (a) carburizing from the surface layer during casting, (b) carburizing during slab heating, and (c) hot-rolled sheet. There are measures such as carburizing during annealing and (d) preventing decarburization in each process.For this purpose, commonly used solid carburizing agents, such as powder C, are mixed with "glue" such as polyvinyl alcohol. For example, powder C may be mixed with 2% of soda carbonate, barium carbonate, calcium carbonate, etc., and then mixed with "glue" such as polyvinyl alcohol. To prevent carburization or decarburization during annealing of hot-rolled sheets, it is necessary to apply such a solid solution carburizing material before annealing, or to add hydrocarbon gas such as propane, butane, methane, or pentane to the furnace atmosphere. It may be mixed. Note that unlike the as-cast surface, the surface of a hot-rolled sheet has a scale layer that is less likely to react with the carburizing agent.
It is effective to apply an alkali salt such as K ₂ CO ₃ to the surface in advance to make it easier to carburize. (Example) A carburizing material was applied to the surface of a slab with a thickness of 200 mm having the components shown in Table 1, and then hot-rolled to obtain a hot-rolled plate with a thickness of 3.0 mm. The c value of the as-hot rolled surface layer is 0.06%
It was expensive. After pickling with sulfuric acid and nitric acid, the sheet was made into a cold-rolled sheet with a thickness of 0.4 mm without surface grinding, and recrystallized annealing was performed, but no gold dust, heat streaks, etc. were generated, and a good surface quality was obtained. Indicated. For comparison, in a hot-rolled sheet without carburizing, the C content in the surface layer was as low as 0.01%, and even within 50 μm from the surface, it was as low as 0.02%. When this hot rolled sheet was treated in exactly the same process as the present invention, gold dust was generated and heat streaks were also generated during cold rolling.

[Table] (Effects of the invention) As detailed above, the present invention improves the carbon content of the surface layer by enriching the carbon content in the surface layer and adjusting the carbon content before cold rolling to a range of 0.04% to 0.1%. It is possible to produce thin stainless steel sheets that do not generate gold dust without special surface grinding beforehand.
It is extremely useful industrially.

[Brief explanation of drawings]

Figure 1 shows the slab heating temperature and C in the thickness direction of the slab.
Figure 2 is a diagram showing an example of the distribution relationship. Figure 2 is a diagram showing an example of C distribution investigation in the thickness direction of a hot rolled sheet. Figure 3 is a metallurgical microscope showing an example of the cross-sectional structure of a hot rolled sheet. It is a photographic diagram.

Claims (1)

[Claims] 1. A ferritic stainless steel slab with a C content of 0.002% to 0.08% at the time of tapping is heated and hot rolled in a rolling mill consisting of a rough rolling mill and a finishing mill,
In a manufacturing method in which a hot rolled sheet is made into a thin steel sheet, the hot rolled sheet is annealed or pickled without annealing, and then cold rolled and finish annealed to make a thin steel sheet. A method for producing a ferritic stainless thin steel sheet with good surface quality, comprising cold rolling a hot rolled sheet with a C content in the range of 0.04 to 0.1% from the surface layer to a depth of at least 50 μm.