JPH0325487B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention contains 5 to 18% Cr and 0.2% C by weight.
The present invention relates to a new method for producing martensitic or ferritic stainless steel thin plates containing the following: (Prior art) Conventionally, in the production of thin sheets of martensitic stainless steel such as 13% Cr and 0.08C, or ferritic stainless steel sheets such as 11% Cr and Ti-containing steel, continuous casting slabs ( CC slab) is hot rolled,
After winding, the hot-rolled structure and martensitic structure are softened and recrystallized by annealing, followed by pickling and descaling, followed by grinding to remove surface defects, and then cold rolling. It has been manufactured by a process that includes rolling, final annealing, and pickling. In particular, martensitic stainless steel turns into martensitic material during cooling after hot rolling and becomes hard. As disclosed, an inefficient manufacturing method called batch annealing, in which the stainless steel strip is kept in a coiled state at a temperature of 800°C or more for several hours or more, and then cooled, has been forced. When heat treated for a long time in this way, a thick oxide scale is formed and the descaling properties are deteriorated, and the surface of the steel strip after descaling has a large roughness, so the surface is roughened during the coil grinding process (process of grinding the steel strip surface). Cold rolling was necessary after removing the defects. As described above, in the prior art, hot-rolled plate annealing processes and coil grinding processes have been indispensable in the production of ferritic or martensitic stainless steel thin sheets in the low Cr range. (Problems to be Solved by the Invention) The present invention is a new method for producing stainless steel thin sheets, which, like ordinary steel thin sheets, omits the annealing process after hot rolling and further performs pickling and descaling. Omitting the coil grinding process that is carried out later,
The purpose of this work was to provide a highly efficient method for manufacturing thin stainless steel sheets using a tandem cold rolling method using large diameter rolls. (Means for solving the problems) The gist of the present invention is that Cr: 5% by weight
A method for manufacturing a thin martensitic or ferritic stainless steel sheet containing ~18% as a main alloying element, the coiling temperature of a longitudinal and widthwise central portion of a hot rolled steel strip being 800 to 600%. ℃, and the amount of cooling water for the steel strip at the runout table during hot rolling is controlled individually according to the longitudinal and widthwise parts of the steel strip, and the recuperation during and after winding of the steel strip is carried out. By uniformizing the progress of softening or tempering of the entire coil, the hot-rolled plate annealing process can be omitted, and then in the descaling of the steel strip, a pickling solution mainly composed of sulfuric acid or hydrochloric acid is used. The steel strip surface layer is melt-cut by 10 micrometers or more per side, the coil grinding process is omitted or simplified, and then cold-rolled.
It is in the manufacturing method of Cr stainless steel thin plate. The present invention will be explained below. The inventors omitted the hot-rolled plate annealing process and coil grinding process in the manufacturing process of martensitic or ferritic stainless steel thin sheets containing 5 to 18% Cr by weight, and achieved high efficiency. We conducted research on processes that allow cold rolling. As a result, it was found that the following problem had to be solved. In other words, in order to omit the current batch-type hot-rolled plate annealing process, the steel strip must be softened in the steel strip winding process after hot rolling to an extent that does not interfere with the subsequent cold rolling process. is necessary. Furthermore, even if the hot-rolled plate annealing process is omitted,
After the steel strip is wound up in a hot rolling process, it is subjected to mechanical descaling and then pickled to perform descaling, but depending on the pickling solution, intergranular corrosion may occur on the surface of the steel strip. In this way, the portions where intergranular corrosion has occurred overlap the surface of the folded steel strip during the cold rolling process, resulting in defects (gold dust) on the product surface and deteriorating the gloss. Therefore, in order to make it possible to omit the coil grinding process, prevention of intergranular corrosion is essential. On the other hand, the surface of the steel strip after hot rolling has an average value of
A decarburized layer of about 10 μm may be formed. If a steel strip is cold rolled, annealed, and pickled with this decarburized layer left in place, gloss defects in the form of intergranular corrosion occur in the final pickling step. Therefore, it is necessary to prevent the formation of a decarburized layer on the surface of the steel strip or to remove it. In this way, the hot-rolled plate annealing process of stainless steel containing 5 to 18% Cr, especially martensitic stainless steel, and the coil grinding process can be omitted or simplified to achieve sufficient cold rollability and In order to ensure the material quality and surface properties, the winding conditions for hot-rolled steel strip to omit the hot-rolled plate annealing process. Pickling conditions and hot-rolled steel strip to prevent intergranular corrosion in the steel strip. The winding conditions for preventing the formation of a decarburized layer on the surface of the hot rolled steel strip and the cutting conditions must be clarified. As a result of repeated research involving experiments, the inventors discovered that
First, the conditions for winding the hot-rolled steel strip to omit the hot-rolled plate annealing process are as follows: The steel strip is wound in a temperature range of 800 to 600°C at the central part in the length and width directions of the steel strip. The inside of the wound coil regenerates heat,
Although softening and tempering proceed sufficiently, the head end, tail end, or widthwise side edges of the steel strip remain hardened due to insufficient recuperation even after the steel strip is wound into a coil. This hardened portion becomes a problem in the subsequent cold rolling process. Therefore, in the process of the present invention, the steel strip is wound with the head and tail portions and both side edges of the steel strip kept in a relatively high temperature state. Therefore, in the process of the present invention, when the steel strip is water-cooled on a runout table after hot rolling, the cooling strength of the head, tail, and both side edges of the steel strip is lower than that of the center. In other words, the head and tail of the steel strip are 100°C lower than the center.
The hot-rolled sheet annealing process is performed by controlling the cooling so that the temperature is approximately 50°C higher than the center on both sides, and then winding the sheet in a coiled manner. Even if omitted, it is possible to obtain a strip coil that does not cause any problems in subsequent cold rolling. Furthermore, in the case of steel strips that have particularly high martensite hardness depending on the C content and N content, after being wound under the above conditions, they should be immediately placed in a thermal annealing furnace or a thermal insulation furnace with a built-in heat source. It is effective to charge the coil and gradually cool it by making the heat recovery of the coil more uniform. Even when the heat recovery of the coil is made to proceed more uniformly using the heat insulating furnace, when cooling the steel strip on the run-out table, the head, tail, and both side edges of the steel strip are relatively smaller than the center. It is effective to control the cooling so that the temperature reaches a high temperature during winding, and the time required in the insulating furnace can be significantly shortened. The hot-rolled strip coil thus obtained is mechanically descaled and then pickled. In ferritic stainless steel and martensitic stainless steel, if the hot-rolled plate annealing process is omitted, Cr carbides are precipitated at the grain boundaries, resulting in Cr-deficient metals at the grain boundaries. Therefore, the use of acids that tend to cause intergranular corrosion must be avoided. From this point of view, HNO ₃ /HF pickling is dangerous. Pickling must be carried out using an acid mainly composed of H ₂ SO ₄ or HCl. Furthermore, when removing smuts that occur during pickling, the use of HNO ₃ /HF or HNO ₃ is harmful, and mechanical action such as brushing and underwater smut treatment are preferable. It is desirable that the surface roughness of the steel strip after pickling be as low as possible, and from this point of view, high-pressure water descaling using fine grained iron sand is effective as an abrasive agent in mechanical descaling. It is also effective to melt the surface of the steel strip with an acid solution containing mainly H ₂ SO ₄ or HCl. Decarburized grains generated during hot rolling may be present immediately below the surface of the hot-rolled steel strip after pickling. The decarburized layer in a hot-rolled sheet is approximately 10 μm thick, and after pickling, cold rolling is performed with the decarburized grains remaining, final annealing, and pickling. Significant precipitation of carbides occurs during the process, which leads to desorption.
During the final pickling with _HNO3 , a pattern appears in the form of intergranular corrosion, which deteriorates the luster of the steel strip. Therefore, when pickling hot-rolled sheets, it is necessary to dissolve _and remove decarburized grains of around 10 μm that have formed on the surface of the steel strip _.
Machining is done using a pickling solution mainly containing HCl. Of course, in addition to lowering the heating temperature of the steel strip prior to hot rolling, it is also effective to make the decarburized layer on the surface of the steel strip as shallow as possible by controlling the heating atmosphere and furnace time. Yes, direct rolling from continuous casting process, hot charge (directly rolling high temperature cast slabs, etc.)
An effective means is to shorten the time in the furnace by charging (charging into the heating furnace). Next, the reasons for limiting the components and process conditions in the present invention will be described. The present invention is directed to ferritic or martensitic stainless steels having a Cr content of 5 to 18% by weight. If Cr is less than 5%, corrosion resistance is insufficient,
In ferritic or martensitic stainless steel with Cr: 5 to 18%, a γ phase appears, and the cooling control in the runout table after hot rolling and decomposition during steel strip winding according to the present invention function effectively. Because it does. When the Cr content exceeds 18%, the γ phase hardly appears and the process of this invention does not function effectively. The coiling temperature of the steel strip after hot rolling is 800 to 600°C in the longitudinal and widthwise central portions of the steel strip. If the temperature exceeds 800°C, the amount of scale generated increases, which is harmful, and if the temperature is lower than 600°C, the elongation of the product decreases. After hot rolling, the steel strip is placed on a run-out table.
The amount of cooling water applied is controlled to cool the head and tail or both side edges so that the head and tail are about 100°C hotter and the side edges are about 50°C hotter than the center. After that, it is wound into a coil. This is reheating in a coiled state, which makes the tempering and softening uniform throughout the coil. For steel types with a high content of martensite, after the steel strip is wound into a coil, it is immediately placed in a heat retention furnace to promote more uniform tempering and softening. When pickling a steel strip in which the hot-rolled sheet annealing process is omitted in this way, the steel strip is pickled with a pickling solution mainly containing H ₂ SO ₄ or HCl in order to prevent intergranular corrosion. HNO ₃ /HF and HNO ₃ that cause intergranular corrosion are
do not use. Furthermore, in order to remove decarburized structures scattered on the surface of the steel strip after hot rolling, at least a surface layer of 10 μm thick on one side was added.
Requires fusing. If the cutting thickness is insufficient (less than 10 μm), decarburized structures will remain and impede the gloss of the product surface. (Example) Below, an example of the method for manufacturing a Cr-based stainless steel thin plate according to the present invention will be described. Table 1 shows 6% Cr to 18 Cr produced as usual.
Shows the alloy composition up to. Both are low C ferritic and martensitic stainless steels. These molten stainless steels are cast into slabs using a known continuous casting process, which is then cooled, partially treated, and reheated, or hot-charged into a heating furnace during the hot rolling process. It was hot rolled into hot strips with a thickness of 3-4 mm in a strip mill. When cooling the hot-rolled steel strip on the run-out table at the latter stage of the finishing mill row, the amount of cooling water applied to the head of the steel strip, approximately 30 m, and the tail, approximately 100 m, is lower than that to the central portion in the longitudinal direction. The steel strip was wound at a temperature approximately 100° C. higher at the head and tail than at the center (shown as pattern A in Table 2).
In addition to controlling the cooling of the steel strip of pattern A in Table 2, the amount of cooling water applied to both side edges of the steel strip is controlled to raise the temperature of this part to about 50°C higher than the temperature of the center in the width direction. The steel strip was then wound up (shown as pattern B in Table 2). Furthermore, the steel strip subjected to cooling control according to pattern B in Table 2 was coiled and then charged into a heat insulating furnace preheated to 750°C, kept there for 30 minutes, taken out, and cooled in its coiled state. (shown as pattern C in Table 2). For comparison, the steel strip was cooled and wound on a run-out table as in the prior art. Table 2 shows the results of measuring the hardness of each part of the steel strip obtained by each of these patterns. Second
As is clear from the table, in the case of the process according to the present invention, the hardness in all parts of the steel strip is less than 185 in H _V , and the difference in hardness between each part is small, and the steel strip is suitable for cold rolling. The material has a sufficient degree of hardening. On the other hand, those based on the prior art have high hardness and large differences in hardness between different parts, and cannot be used as a cold-rolled material as is, requiring hot-rolled plate annealing. The hot strip coil obtained by the above process according to the present invention was mechanically descaled using high-pressure water using iron sand as an abrasive material without annealing the hot-rolled sheet, and the hot strip coil was subjected to mechanical descaling using high _- pressure water using iron sand as an abrasive. SO ₄
was pickled in a pickling solution heated to 85°C. During this pickling process, the pickling time was varied to remove a 10 μm thick decarburized layer on the steel strip surface and to reduce the unevenness of the steel strip surface. After the pickling treatment, the steel strip is brushed in water, and the resulting hot strip coil is cold rolled at a cumulative reduction rate of 50% in a cold tandem mill with 400 mm diameter rolls without coil grinding. I did this. A portion of the cold rolled strip coil thus obtained was cold reverse rolled three times in a Sendzimir mill with rolls having a diameter of 60 mm. After that,
Final annealing, pickling, and temper rolling were performed to create a 2B product. Table 3 shows the evaluation of gloss as a surface characteristic of 2B products. When pickling hot strips, if the amount of abrasion was less than 10 μm, a decarburized layer remained partially on the surface and the final product had poor gloss, but if the hot strip was abraded to a depth of 10 μm or more, was good.

【table】

【table】

[Table] (Effects of the Invention) According to the present invention, in the production of ferritic or martensitic stainless steel thin sheets, it is possible to omit the hot-rolled plate annealing process and omit or simplify the coil grinding process. Since a cold rolling process using a highly efficient tandem cold rolling mill can be adopted, the present invention has extremely large industrial benefits.

Claims (1)

[Claims] A method for manufacturing a martensitic or ferritic stainless steel thin plate containing 1% by weight and 5 to 18% Cr as a main alloying element, the method comprising: a hot rolled steel strip in the longitudinal direction; and the winding temperature at the central portion in the width direction is set to 800 to 600°C, and the amount of cooling water for the steel strip at the run-out table during hot rolling is controlled individually according to the longitudinal and widthwise portions of the steel strip, By uniformizing the progress of softening or tempering of the entire coil by reheating during and after winding the steel strip, the hot-rolled sheet annealing process is omitted,
Next, in descaling the steel strip, the surface layer of the steel strip is abraded by 10 micrometers or more per side using a pickling solution containing sulfuric acid or hydrochloric acid as a main component, and the coil grinding process is omitted or simplified, and then, A method for manufacturing a Cr-based stainless steel thin plate, which is characterized by cold rolling. 2. The method for producing a Cr-based stainless steel thin plate according to claim 1, wherein after winding the hot-rolled steel strip, the coil is placed in a heat retention furnace to more uniformly recover heat from the coil. 3 A stainless steel strip whose surface layer has been melt-cut by 10 micrometers or more per side with a pickling solution containing sulfuric acid or hydrochloric acid as the main component is cooled in a tandem rolling mill with large-diameter rolls with a roll diameter of 150 mm or more. Inter-rolling according to claim 1 or 2
Method for manufacturing Cr-based stainless steel thin plate. 4. Claim No. 4 in which a stainless steel strip is cold-rolled in a tandem rolling mill having large diameter rolls with a roll diameter of 150 mm or more, and is further cold-rolled for finishing in a cold rolling mill with rolls having a diameter of 100 mm or less. A method for producing a Cr-based stainless steel thin plate according to any one of items 1 to 3.