JP5167314B2 - Method for producing ferritic stainless steel with excellent ridging resistance - Google Patents

Description

  The present invention relates to a method for producing a ferritic stainless steel in which surface defects called “ridging” or “roping” that occur in the direction parallel to the rolling direction during forming of ferritic stainless steel are reduced. Note that “riding” is also referred to as “roping”, but is hereinafter collectively described as “riding”.

  Ferritic stainless steel represented by SUS430 is generally cheaper than austenitic stainless steel, and excellent in corrosion resistance, heat resistance, and moldability compared to martensitic stainless steel. For this reason, ferritic stainless steel is widely used in applications such as kitchen equipment, tableware, home appliances and building hardware.

  However, when a ferritic stainless steel is rolled, surface defects called “ridging”, which are striped or streaky undulations, occur in a direction parallel to the rolling direction generated on the surface of the steel sheet. When ridging occurs, the aesthetics of the molded product are impaired, so a polishing step is required to remove ridging, but if this `` riding '' is large, the amount of polishing increases, which increases the load of polishing work. Rebounded to increased cost of molded products.

  As described above, the “riding” is a striped or streak-like undulation in the direction parallel to the rolling direction on the surface of the steel sheet. If this is molded as it is, the “riding” remains on the surface of the molded product and appears. Detract from aesthetics. For this reason, a polishing step for removing it after forming is necessary. However, if the “riding” on the surface of the steel sheet is excessive, the amount of polishing increases accordingly, and the polishing load in the forming step increases.

  "Ridging" is said to be caused by columnar crystals of the slab structure produced by continuous casting, and attempts have been made to increase the equiaxed crystal ratio by miniaturizing the slab structure or electromagnetic stirring. It was not enough. In the cold rolling process, “riding” can be improved by repeatedly performing rolling and annealing, but an increase in cost becomes a problem.

  In order to destroy the columnar crystals of the slab structure that cause “riding”, it is necessary to repeatedly cause recrystallization. For example, it is conceivable to use hot rolling (hereinafter abbreviated as hot rolling). . However, in ferritic stainless steel, dynamic recrystallization hardly occurs, and on the other hand, since a recovery phenomenon is likely to occur, it is necessary to provide a large working strain and a recrystallization time in hot rolling. However, if it does so, productivity will fall correspondingly.

  As described above, “riding” is a factor that lowers the competitiveness in the molded product market. Therefore, various techniques have been conventionally developed in order to prevent or reduce the occurrence of ridging in ferritic stainless steel. Since “riding” is caused by columnar crystals of a slab structure manufactured by continuous casting as described above, for example, “equiaxial crystal ratio is reduced by refining a slab structure or electromagnetic stirring” described in Patent Document 1. Can be mentioned. Further, “Technology for increasing austenite potential” described in Patent Document 2, “Slab cooling control” described in Patent Document 3, “Rough rolling conditions” described in Patent Documents 4 and 5, and Patent Document 6 "rolled sheet annealing" and the like.

Japanese Patent Publication No. 64-149 (Increase of equiaxed crystal ratio) JP-A-7-118754 (Increase in A.P.) JP 7-11595 (Slab cooling control) JP-A-7-126757 (rough rolling conditions) JP-A-7-310122 (rough rolling conditions) Japanese Patent Publication No. 6-94575 (rolled sheet annealing)

  In view of such circumstances, the present invention provides a method for producing ferritic stainless steel having excellent ridging resistance.

As a result of various studies on the relationship between hot rolling conditions and the recrystallization behavior of ferritic stainless steel, the present inventor has clarified the relationship between the rolling temperature, the rolling reduction, the holding time after rolling, and the recrystallization behavior. It came. That is, the present invention
% By mass, 0.040% ≦ C ≦ 0.100%, 0.20% ≦ Si ≦ 1.00%, 0.30% ≦ Mn ≦ 1.00%, P ≦ 0.040%, S ≦ 0 0.010%, Ni ≦ 0.45%, 16.0% ≦ Cr ≦ 18.0%, Mo ≦ 0.50%, Cu ≦ 0.30%, N ≦ 0.050%, the balance being inevitable with Fe Consisting of impurities, and
After heating the slab having a component of the following formula 1 with 55% ≦ austenite potential ≦ 65% in the range of 1,000 ° C. to 1,200 ° C.,
With a roughing mill, perform hot rough rolling with a reduction rate of 30% or more per pass for 2 passes or more,
Then hold it for more than 1 minute,
After that, in a reversible rolling mill equipped with a heat-retaining furnace on both sides of the finish rolling mill, with a steel plate temperature maintained at 850 ° C. or higher in the heat-retaining furnace, a high-pressure finish with a reduction rate of 30% or more per pass. In rolling, perform 3 passes or more to perform hot finish rolling,
Then, after performing batch annealing at a temperature of 900 ° C. or higher under soaking for 4 hours or longer, the steel sheet is naturally cooled until the steel plate temperature reaches 600 ° C., and is a manufacturing method for manufacturing a ferritic stainless steel. .
Austenite potential [A. P. (%)
= 288 (% C) +350 (% N) +22 (% Ni) +7.5 (% Mn) -18.75 (% Cr) -54 (% Si) + 338.5 …… Formula 1

  According to the invention of claim 1, it has the chemical composition of claim 1 and A. P. By subjecting a slab having a content of 55% to 65% to hot rough rolling under the conditions described in claim 1, the ferritic stainless steel sheet structure could be recrystallized and the ridging resistance could be improved. . Thereafter, finish rolling under high pressure is performed by a reversible rolling mill (for example, a Steckel mill).

  By performing finish rolling under high pressure by using a reversible rolling mill, it is possible to provide a method for producing ferritic stainless steel having further excellent ridging resistance without reducing productivity. More specifically, in a reversible rolling mill, a heat-retaining furnace is provided on both sides of the finish rolling mill, so that the heat-retaining time in the heat-retaining furnace for a few minutes generated between one pass and the next pass is recrystallized. Therefore, it is not necessary to provide a separate recrystallization time for the structure, and no special means is required for improving ridging resistance, so that productivity is not impaired.

  And by annealing at a high temperature (austenite + ferrite region), improving ridging and controlling the cooling conditions after annealing does not produce a martensite structure and produces ferritic stainless steel with good corrosion resistance. I can do it.

  According to the present invention, by appropriately selecting the composition, austenite potential, rough rolling, and finish rolling of ferritic stainless steel, ridging generated on the surface of ferritic stainless steel can be significantly reduced without increasing costs. And the application of ferritic stainless steel to the molded product can be made easier.

It is a graph which shows the relationship between rolling temperature, holding time, and recrystallization index at the time of rough rolling with a rolling reduction of 20%, 30% and 40%. It is a graph which shows the relationship with the rolling reduction, holding time, and Vickers (Hv) hardness at the time of finish rolling of rolling temperature 700 degreeC, 800 degreeC, 850 degreeC, and 900 degreeC. It is a ridging external appearance photograph replacing the drawing of the surface of comparative steel and development steel.

  Hereinafter, the reasons for limiting the chemical composition of the slab in the present invention will be described. The proportions of these components are all mass%.

0.040% ≦ C ≦ 0.100%: In C, when ferritic stainless steel (hereinafter simply referred to as “stainless steel”) is heated to 1100 ° C., the entire structure of the stainless steel The content of at least 0.04% is necessary because it has the effect of increasing the austenite potential (hereinafter referred to as “AP”) indicating the proportion of the austenite structure. On the other hand, a large amount of carbon lowers the corrosion resistance of stainless steel due to an increase in precipitation of carbides, and increases the proof stress of stainless steel and adversely affects the press formability, so the upper limit of the content is 0.100%. did.

  0.20% .ltoreq.Si.ltoreq.1.00%: When dissolved, Si has the effect of increasing the corrosion resistance of the ferritic stainless steel as well as acting as a deoxidizer for the molten metal, and 0.20% or more is necessary. On the other hand, Si is A.I. P. In order to lower the value, the upper limit is made 1.00%.

  0.35% ≦ Mn ≦ 1.00%: Mn is A.I. It has the effect of increasing P. 0.35% is necessary. Even if the manganese content exceeds 0.35%, the material properties are not particularly affected. P. In order not to exceed the upper limit of 1.00%, the upper limit was made 1.00%.

  P ≦ 0.040%: If the P content increases, the hot workability of stainless steel is adversely affected, so the upper limit was made 0.040%.

  S ≦ 0.010%: As with P, if the S content increases, the hot workability is adversely affected, so the upper limit was made 0.010%.

  Ni ≦ 0.45%: Ni is A.I. P. However, since the present invention is ferritic stainless steel, the upper limit was made 0.45%.

  16.0% ≦ Cr ≦ 18.0%: Since Cr is an element effective for corrosion resistance, 16.0% or more is necessary. However, Cr is A.I. P. Therefore, the upper limit of the content is set to 18.0%.

  Mo ≦ 0.50%: Mo is an element effective for the corrosion resistance of stainless steel, but since it is expensive, the upper limit was made 0.50%.

  Cu ≦ 0.30%: Since Cu adversely affects the hot workability of stainless steel, 0.30% was made the upper limit of the content.

  N ≦ 0.050%: N is A.I. P. However, when a large amount is added, the corrosion resistance is reduced due to the formation of nitrides and the yield strength is increased, so that the formability is deteriorated. Therefore, 0.050% was made the upper limit of the content.

  55% ≦ A. P. ≦ 65%: A.I. P. Represents the proportion of the austenite structure in the structure when heated to 1100 ° C. Since high temperature processing of ferritic stainless steel is easy to recover, large processing strain and recrystallization time are required to cause recrystallization. If an austenite structure is present in the heated structure, the ferrite is divided by austenite harder than ferrite during hot rolling. As a result, the slab structure is divided, and the strain concentrates at the grain boundaries of austenite and ferrite, forming recrystallization nuclei and promoting recrystallization. However, in order to obtain the effect, 55% or more of A.I. P. is necessary. On the other hand, a large amount of A.I. P. In the final component, it is difficult to obtain a ferrite structure in the final product, so 65% was made the upper limit.

  In the present invention, a slab containing such a composition component is heated to a predetermined rough rolling temperature and then subjected to high pressure rough rolling, and the high pressure rough rolled steel sheet is held for a predetermined time. In this state, ferritic stainless steel having excellent ridging properties is produced by subjecting the steel sheet to a finish hot rolling in a state where it is maintained and further annealing the steel sheet at a predetermined temperature. In order to improve ridging, it has been known that it is effective to destroy the continuous cast structure and promote recrystallization. However, the conventional rolling method showed the recrystallization index (recrystallization rate in stages). However, improvement of ridging was limited. Therefore, the inventors further examined hot rolling conditions and subsequent annealing conditions as measures for improving ridging. In other words, rough slab rolling conditions, finish hot rolling conditions, and subsequent annealing conditions necessary for producing stainless steel with excellent ridging resistance were determined. Hereinafter, this basis will be described.

  In the present invention, it is assumed that ferritic stainless steel is hot-rolled by a hot-rolling facility composed of a rough rolling mill and a reversible rolling mill (for example, a Steckel mill). The reversible rolling mill includes a heat-retaining furnace before and after the finish rolling mill, and keeps the temperature in order to avoid a decrease in the steel plate temperature during the finish rolling.

  First, in a roughing mill, the relationship between rolling temperature, rolling reduction, holding time, and recrystallization rate was examined. FIG. 1 shows the relationship between the recrystallization index and the rolling temperature, rolling reduction and holding time at the research stage. This is done by heating to a predetermined temperature, rolling for one pass at a predetermined rolling reduction in a four-stage rolling mill for research experiments, holding in a predetermined temperature furnace for a predetermined time, and then re-cooling the water-cooled test piece. The result obtained by measuring the crystal ratio is shown. As a result, it has been clarified that recrystallization occurs when the rolling is performed at a rolling temperature of 1,000 to 1,200 ° C. at a rolling reduction of 30% or more and held for a predetermined time.

  As described above, it is known that a higher recrystallization rate is preferable for improving ridging. However, in the conventional method, the recrystallization index could be realized only about 1 or 2, and the reduction rate was 30%, 1, When held at 200 ° C. for 1 minute or longer, a recrystallization index of 3 or higher (that is, a recrystallization ratio of 50% or higher) could be achieved. In actual production, in order to reliably achieve a recrystallization index of 3 or more, rough rolling under high pressure with a reduction rate of 30% or more is performed for 2 passes or more, and then held for 1 minute or more. Realization of 3 or more (recrystallization rate of 50% or more) was confirmed.

  In addition, in rough rolling in actual production, it was confirmed that recrystallization proceeds with the heat of the material even if the material to be rolled is thick, even if it is not heated and held.

  Subsequently, finish hot rolling is performed. In this embodiment, a stickel mill, which is a reversible rolling mill, is used as a hot rolling mill for finishing hot rolling. And before and after this Steckel mill, a steel plate insulation furnace (coiler furnace) is provided. By using such a stickel mill, the steel sheet can be kept at a predetermined temperature in a heat-retaining furnace in a waiting time of several minutes generated between one pass and the next pass. It can be used as the recrystallization time.

  Using such a finish reversible rolling mill (Steckel mill), the recrystallization behavior was examined from the relationship between the rolling temperature, rolling reduction and holding time during finish rolling, and the changes in structure and hardness. FIG. 2 shows the relationship between rolling temperature, rolling reduction, holding time and hardness. The test method was the same as in FIG. 1 except that the temperature was different, and the hardness was measured with a Vickers hardness meter at the center of the plate thickness. This measurement result will be described below.

  In rolling at 700 ° C., hardly any change in hardness was observed even if the rolling reduction and holding time were changed.

  Next, at 800 ° C., when the rolling reduction was 40%, the hardness decreased with the lapse of the holding time, but under the other conditions, almost no change was observed in the hardness. As a result of the structure observation, the 40% rolling reduction test piece in which the change in hardness was recognized had a recrystallized structure as the holding time passed.

  Further, at 850 ° C., when the rolling reductions were 10% and 20%, no change in hardness was observed even after the holding time had elapsed, but other than that (ie, when the rolling reductions were 30% and 40%). ) Decreased in hardness with the lapse of the holding time, and increased in degree of softening as the rolling reduction increased. A recrystallized structure was obtained as the holding time passed. As a result of the structure observation, it has been found that the degree of softening is caused by an increase in the recrystallized structure as the rolling reduction and holding time increase.

  At 900 ° C., the change in hardness was large, and regularity was not observed in the rolling reduction, holding time and hardness. As a result of the structure observation, a lot of martensite structure was observed. Therefore, it seems that the rolling was performed in a two-phase structure of ferrite and austenite in this temperature range.

  From these results, in finish rolling, if the rolling temperature is 850 ° C. and the rolling reduction of one pass (three passes for safety in actual production) is 30% or more, recrystallization proceeds during holding in the heat-retaining furnace. It turns out that there is a high possibility of doing. In particular, it was estimated that the finish rolling can achieve the same recrystallization effect as that during rough rolling even at 900 ° C. or higher. In addition, it was confirmed that the recrystallization surely progressed by using 3 passes or more in the actual machine manufacture.

  Finally, the annealing conditions for hot-rolled sheets were examined. In a ferrite system, particularly SUS430 in which austenite is generated by annealing in a high temperature region, annealing is usually performed at a transformation point temperature or lower. This is because when austenite is generated during annealing and martensite is generated during cooling, a problem may occur in the corrosion resistance of the final product.

  On the other hand, the austenite-ferrite transformation is effective for breaking the slab structure or making the ferrite finer. Therefore, we decided to avoid the generation of martensite by lowering the cooling rate after batch annealing.

  That is, in the conventional cooling method, after batch annealing in the temperature range of 800 to 850 ° C., immediately after the forced cooling by a dedicated cooler, batch annealing is performed in the temperature range of 900 to 950 ° C. in the present invention. After the cooling, forced cooling was not performed, natural cooling was performed until the steel plate temperature reached 600 ° C., and forced cooling was performed when the steel plate temperature reached 600 ° C. or lower. As a result, martensite did not occur in the cooled structure.

  A. Stainless steel P. Developed steels 1-6, which are within the scope of the present invention; P. However, the comparative steels 1 to 4 that fall outside the scope of the present invention were compared, and the manufacturing process and conditions were compared between the conventional method and the present invention method, and the ridging resistance effect of the stainless steel according to the present invention was verified.

  Comparative steels 1 to 4 and developed steels 1 to 6 having the chemical components shown in Table 1 are respectively melted in an electric furnace, refined in an AOD furnace (argon-oxygen-decarburization refining furnace), and a slab is produced with a continuous casting machine. did. These slabs were processed based on the manufacturing process and manufacturing conditions shown in Table 2, and “Ridging” was evaluated for Comparative Steels 1-4 and Development Steels 1-6.

  For the measurement of ridging, the surface waviness (= filtered centerline waviness) generated when cold-rolled sheet (thickness: 0.7 mm or 0.8 mm) is stretched 15% in the rolling direction is used with a roughness meter. And measured as ridging. In addition, although the continuous annealing pickling process is passed after batch annealing, the purpose of this is scale-off by pickling.

  Comparative steel 1 is a steel that has been subjected only to continuous annealing, and comparative steel 2 is a steel that has been subjected to batch annealing (810 ° C.). The ridging produced in these steels was 4.0 μm and 4.2 μm, respectively. Comparative steel 3 was subjected to batch annealing (900 ° C.), and its ridging was 3.8 μm, which was slightly improved. However, both comparative steels are P. Is out of the scope of the present invention, and its ridging is larger than that of the present invention.

  The developed steels 1 to 3 have the component composition and A.I. P. (= 56%). However, compared with the comparative steel 1 manufactured in the same manufacturing process, the developed steel 1 is improved to 3.5 μm in ridging, but the manufacturing process of the developed steel 1 does not satisfy the conditions of the present invention. However, the improvement of ridging is insufficient.

  Development steel 2 and development steel 3 are examples of the present invention. The developed steel 2 is the one in which the developed steel 1 is subjected to high-pressure rolling (the rolling reduction per pass is 30% or more, the same applies hereinafter) in the hot rolling rough rolling process, and the ridging is 2.6 μm. Yes, ridging resistance is greatly improved.

  The developed steel 3 was obtained by subjecting the developed steel 2 to high pressure rolling in hot rolling finish rolling, and the ridging was 2.5 μm, which was even smaller.

  The developed steels 4 to 6 also have the component composition and A.I. P. (= 63%). The ridging of the developed steel 4 is 2.0 μm, which is greatly improved. However, since the cold rolling is performed for two heats, there is a problem that the manufacturing cost increases.

  Development steel 5 and development steel 6 are examples of the present invention. The developed steel 5 was hot rough rolled under high pressure rolling and further subjected to batch annealing (900 ° C.), and the ridging was greatly improved to 1.4 μm.

  The developed steel 6 was obtained by performing high pressure rolling in the hot finish rolling on the developed steel 5, and the ridging was further improved to 1.1 μm. As described above, the developed steel 2, the developed steel 3, the developed steel 5 and the developed steel 6 as examples of the present invention have a great ridging improvement effect.

In FIG. 3, the external appearance photograph of the ridging produced in comparative steel 1 and 4 and development steel 1-6 is shown. The vertical direction in the figure is the rolling direction, and a 15% tensile process is applied in this rolling direction. The striped pattern observed on the surface of the test piece is ridging. It can be seen that the ridging of the developed steel 6 produced by the production method according to the present invention is very slight in appearance as compared with the test piece according to the comparative steel 1 having a ridging of 4 μm.

Claims (1)

% By mass, 0.040% ≦ C ≦ 0.100%, 0.20% ≦ Si ≦ 1.00%, 0.30% ≦ Mn ≦ 1.00%, P ≦ 0.040%, S ≦ 0 0.010%, Ni ≦ 0.45%, 16.0% ≦ Cr ≦ 18.0%, Mo ≦ 0.50%, Cu ≦ 0.30%, N ≦ 0.050%, the balance being inevitable with Fe Consisting of impurities, and
After heating the slab having a component of the following formula 1 with 55% ≦ austenite potential ≦ 65% in the range of 1,000 ° C. to 1,200 ° C.,
With a roughing mill, perform hot rough rolling with a reduction rate of 30% or more per pass for 2 passes or more,
Then hold it for more than 1 minute,
After that, in a reversible rolling mill equipped with a heat-retaining furnace on both sides of the finish rolling mill, with a steel plate temperature maintained at 850 ° C. or higher in the heat-retaining furnace, a high-pressure finish with a reduction rate of 30% or more per pass. In rolling, perform 3 passes or more to perform hot finish rolling,
Next, a manufacturing method for producing a ferritic stainless steel, characterized in that after batch annealing is performed at a temperature of 900 ° C. or higher under soaking for 4 hours or longer, the steel plate is naturally cooled until the steel plate temperature reaches 600 ° C.
Austenite potential (%)
= 288 (% C) +350 (% N) +22 (% Ni) +7.5 (% Mn) -18.75 (% Cr) -54 (% Si) + 338.5 …… Formula 1