JP4199368B2 - Method for producing ferritic stainless steel - Google Patents

Description

【００３６】
【表２】

【００３７】
【表３】

【００３８】
【発明の効果】
本発明によって、リジングとｒ値を向上させる熱延鋼帯の巻取り保定温度やボックス焼鈍温度を確保し、これらの過程や冷却途中に生じるリンの粒界偏析と硫酸デスケール酸洗との組合わせにより生じる粒界腐食溝の発生を防止して、その後の冷間圧延表面の優れた最終製品の製造を可能としたものであり、その工業的意義は極めて大きい。
【図面の簡単な説明】
【図１】熱延巻き取り保定あるいは高温保定により硫酸との組み合わせで粒界腐食の生ずる熱延鋼帯について、更に昇温した場合の昇熱温度と鋼帯のＡｌ含有量の間の、昇温処理後の硫酸酸洗時に粒界腐食の生成との関係を示す図。
【図２】熱延巻き取り保定あるいは高温保定により硫酸との組み合わせで粒界腐食の生ずる熱延鋼帯について、更に昇温した場合の昇熱温度と冷却速度の間の、昇温処理後の硫酸酸洗時に粒界腐食の生成との関係を示す図。
【図３】熱延鋼帯の巻き取り保定温度，保定時間と、冷延焼鈍した後のリジング高さとの関係を示す図。
【図４】熱延鋼帯の巻き取り保定温度，保定時間と、冷延焼鈍した後のｒ値との関係を示す図。
【図５】ボックス焼鈍の保定温度，時間を変更し冷延焼鈍た場合のリジング高さとの関係を示す図。
【図６】ボックス焼鈍の保定温度，時間を変更し冷延焼鈍た場合のｒ値との関係を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention improves the ridging and r value of a product when hot rolling or further annealing ferritic stainless steel, descaling pickling, cold rolling annealing to produce a cold rolled steel strip, and improving the product surface. The present invention relates to a method for producing a steel sheet having good properties.
[0002]
[Prior art]
Generally, ferritic stainless steel is descaled by combining mechanical descaling and sulfuric acid after hot rolling or annealing.
About the heat history after this hot rolling, in order to reduce the ridging of the product plate after the subsequent cold rolling / annealing and the anisotropy at the time of molding, by heating and holding in the state wound around the coil after hot rolling Annealing, continuous annealing in which the coil is rewound and heated, and further, coiling is performed at a high temperature after hot rolling, and annealing using the retained heat of the coil itself is performed.
[0003]
However, when the hot-rolled ferritic stainless steel sheet that has been hot-rolled is cooled after being coiled at 700 ° C. or more, or when the coil that has been hot-rolled is box-annealed at about 800 ° C., it continues. There was a problem that the grain boundaries were dissolved in the sulfuric acid in the descaling process, resulting in the formation of intergranular corrosion grooves, followed by sparkling (gold dust) on the final product surface after cold rolling annealing, and the surface quality was significantly impaired. .
[0004]
Such a problem is that phosphorus, which is an impurity element in steel, segregates at grain boundaries during cooling after hot rolling, or during box annealing and during the cooling process, corresponding to the phosphorus concentration in the vicinity of the grain boundaries. This is due to the fact that the intergranular corrosion grooves are formed because the dissolution in the inside proceeds. In order to avoid such a problem, nitric acid hydrofluoric acid pickling that is expensive in both cost and equipment cost has to be used instead of sulfuric acid.
[0005]
On the other hand, for example, Japanese Patent Application Laid-Open No. 61-199036 discloses that cooling after hot rolling or after hot rolling-annealing is performed from 830 ° C. to 400 ° C. within 30 minutes. Japanese Patent Laid-Open No. 62-174349 discloses that small amounts of elements such as Zr, Ti, Nb, Mo, Y, La, and Ce, which have strong interaction with phosphorus in steel, are added. Various gazettes have been disclosed such as regulating the contents of Cu and Mn in steel in the gazette, and adding a small amount of Ca and Mg in JP-A-2-190450. I couldn't say it was going.
[0006]
[Problems to be solved by the invention]
The present invention controls the coil cooling conditions after hot rolling of ferritic stainless steel or the coil annealing conditions after cooling to promote the recrystallization of the hot-rolled steel strip, and the phosphorus generated during these heat treatments. It is an object of the present invention to provide a method for producing a ferritic stainless steel having a new steel component composition in which grain boundary segregation does not generate a grain boundary corrosion groove after the subsequent sulfuric acid pickling step.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the gist of the present invention is as follows.
(1) By weight%
C: 0.12% or less, Si: 1.00% or less,
Mn: 1.00% or less, P: 0.100% or less,
S: 0.030% or less, Cr: 10-20%,
Ni: 0.75% or less, Cu: 0.005 to 0.20%,
Al: 0.001 to 0.5%
Steel, the balance of Fe and the inevitable impurities are hot-rolled, the steel strip winding temperature after hot rolling is kept at 700 ° C or higher for 30 minutes or more , and after cooling, the relationship with the amount of Al After raising the temperature to the temperature of the following formula <1> or formula <2> determined by the following, the temperature range from the heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 degC / sec or more, and after mechanical descaling A method for producing a ferritic stainless steel, characterized by obtaining a steel strip by sulfuric acid pickling or hydrochloric acid pickling.
Formula <1>: Heating temperature (° C.) ≧ 1250 × Al (%) + 800 (Al (%) ≦ 0.16)
Formula <2>: Heating temperature (° C) ≧ 1000 ……………… (Al (%)> O.16)
[0008]
(2) By weight%
C: 0.12% or less, Si: 1.00% or less,
Mn: 1.00% or less, P: 0.100% or less,
S: 0.030% or less, Cr: 10-20%,
Ni: 0.75% or less, Cu: 0.005 to 0.20%,
Al: 0.001 to 0.5%
Hot-rolled steel consisting of Fe and unavoidable impurities, and after cooling the coil after hot rolling, after cooling by performing box annealing for 30 minutes or more at an annealing temperature of 700 to 880 ° C, After raising the temperature to the temperature of the following formula <1> or formula <2> determined by the relationship with the amount of Al, the temperature range from the heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 degC / sec or more. A method for producing a ferritic stainless steel, characterized in that a steel strip is obtained by sulfuric acid pickling or hydrochloric acid pickling after mechanical descaling.
Formula <1>: Heating temperature (° C.) ≧ 1250 × Al (%) + 800 (Al (%) ≦ 0.16)
Formula <2>: Heating temperature (° C) ≧ 1000 ……………… (Al (%)> O.16)
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In order to find out the production conditions of ferritic stainless steel with little anisotropy during ridging and forming on the product plate, the present inventors have studied by focusing on the winding conditions and heating conditions of the hot-rolled steel strip. This will be described in detail below based on experiments.
[0011]
As chemical components (weight%, hereinafter abbreviated as%) of the test steel for this study, C: 0.02%, Si: 0.50%, Mn: 0.50%, P: 0.030%, S: 0.015%, Cr: 16%, Ni: 0.35%, Cu: 0.06%, Al content is changed particularly in the range of 0.001 to 0.5%, and the balance is Fe Steel was prepared.
This steel ingot was hot rolled, wound at a temperature of 700 ° C. or higher and held for 30 minutes or more, and then cooled to room temperature to obtain a hot rolled steel strip.
This hot-rolled steel strip was mechanically descaled by shot blasting and then subjected to sulfuric acid pickling at 300 g / L sulfuric acid at 90 ° C for 60 seconds. Intergranular corrosion grooves occurred.
[0012]
On the other hand, the hot-rolled steel strip was subjected to a heat treatment in which the holding time was set to 10 seconds and the temperature was raised to 700 to 1100 ° C. and then rapidly cooled, and then mechanical descaling by shot blasting and sulfuric acid pickling were performed. The intergranular corrosion grooves of the resulting steel were investigated. The result is shown in FIG.
As shown in FIG. 1, it was found that intergranular corrosion can be eliminated by setting the heating temperature in accordance with the Al content. If the Al content is 0.16% or less, the necessary heating temperature increases as the Al content increases, but if it exceeds 0.16%, the necessary heating temperature is saturated and 1000 ° C. is sufficient.
[0013]
Furthermore, with regard to a steel strip in which the Al content is fixed to 0.12% and the heating temperature and the cooling rate in the temperature range from the heating temperature to 400 ° C. are changed, sulfuric acid is added after mechanical descaling by shot blasting. The results of pickling and investigating intergranular corrosion grooves are shown in FIG.
As shown in FIG. 2, when the cooling rate is 0.2 degC / sec or more, a pickled steel strip without intergranular corrosion grooves can be obtained.
[0014]
These pickled steel strips without intergranular corrosion grooves are continuously cold-rolled and annealed, and the product plate has no fine covering caused by intergranular corrosion grooves, and the product has a good start test. Even when evaluated by peeling off the tape after deep-drawing with a protective tape applied to the surface, there was no generation of wrinkles called sparkle flaws or gold dust.
[0015]
Next, ridging and anisotropy were investigated for those treated by changing the steel strip winding temperature and holding time, and the box annealing temperature and holding time after completion of the hot rolling prior to these heat treatments.
About steel strip winding temperature maintenance after the end of hot rolling, the coil after winding was covered with a lid and heated to maintain the winding temperature at the winding temperature for 1 to 5 hours. On the other hand, in the case of a normal 1 m width and 10-20 ton coil, the heat history as wound was cooled after being at the winding temperature for about 30 minutes.
As a result, as shown in FIGS. 3 and 4, by raising the coiling temperature to 700 ° C. or higher and holding it for 1 hour or longer, the ridging height can be reduced, the r value is improved, It turned out that it can improve in common.
[0016]
In addition, the relationship between temperature and time when a box annealed with a coil once cooled after hot rolling was also examined. As a result, by increasing the coil heating temperature to 700 ° C. or more as shown in FIGS. 5 and 6, the ridging and the r value can be improved similarly to the coil winding temperature. It turned out to improve.
[0017]
Intergranular corrosion grooves are formed in combination with sulfuric acid at the time of descaling when the winding holding temperature of the steel strip after hot rolling is 700 ° C. or higher, or when the annealing temperature of box annealing is 700 ° C. or higher. The reason is that P is segregated at the grain boundaries while being held at these temperatures or during cooling, and the nature of the sulfuric acid solution itself dissolves more of the P-concentrated portion, resulting in grain boundary corrosion grooves. by. By raising the temperature of the steel strip to a temperature determined by the relationship with the amount of Al and quenching it, it becomes possible to prevent the formation of intergranular corrosion.
The reason why the required heating temperature is increased in relation to the Al amount is that the solid solution temperature of P is increased due to an increase in the Al amount. This is because it is possible to freeze the solid solution of P without grain boundary segregation.
[0018]
Using a hot-rolled steel strip that has been subjected to box annealing at a temperature of 700 ° C. or higher while the coil is once coiled after hot-rolling and the annealing temperature is 700 ° C. or higher while the coil is being wound, After descaling, the reason why the ridging and the anisotropy at the time of forming decrease in the cold-rolled annealed product plate is that the recrystallization of the hot-rolled steel strip proceeds by holding at 700 ° C or higher from the observation of the structure of the material before descaling. This is because the crystal orientation can be randomized.
[0019]
Next, the reasons for limiting the components will be described. The content of each component is% by weight.
C is preferably lower in terms of pitting corrosion resistance and corrosion resistance, but the upper limit is set to 0.12% in consideration of manufacturability.
[0020]
Si is a useful component having a deoxidizing action, but if it exceeds 1.00%, hot workability deteriorates, and surface flaws during hot rolling frequently occur. It was 1.00% or less.
[0021]
P is the case where the coiling temperature of the steel strip after hot rolling is maintained at 700 ° C. or higher, or the case where the annealing temperature is kept at 700 ° C. or higher while being coiled and cooled once after hot rolling, During the subsequent cooling, it is a harmful element that concentrates at the grain boundary with the decrease in P solid solubility and causes segregation at the grain boundary, and creates a grain boundary corrosion groove in combination with sulfuric acid. Considering economic efficiency, it is made 0.100% or less.
S is desirably as low as possible for improving corrosion resistance, and is 0.030% or less.
[0022]
Cr is an important element as stainless steel for improving corrosion resistance and oxidation resistance. For this purpose, at least 10% is necessary. On the other hand, if it exceeds 20%, toughness is deteriorated. % Range.
[0023]
Ni has an effect of imparting excellent corrosion resistance and acid resistance, and also has an effect of improving pitting corrosion resistance and crevice corrosion resistance. However, since addition exceeding 0.75% impairs economic efficiency, it was made 0.75% or less.
[0024]
Al is an element useful for promoting recrystallization. If it is less than 0.001%, the desired effect cannot be obtained. On the other hand, if it is added in excess of 0.5%, it is not preferable in terms of a decrease in hot workability, so it is limited to a range of 0.001 to 0.5%.
[0025]
Cu is an element useful for improving corrosion resistance and mechanical strength. However , if it is less than 0.005%, a desired effect cannot be obtained, while if it exceeds 0.20%, it promotes grain boundary segregation of P. In addition, since it is not preferable in terms of a decrease in hot workability, it is limited to a range of 0.005 to 0.20%.
[0026]
Mn is an element that effectively contributes to the maintenance of mechanical strength and deoxidation. However , if it exceeds 1.00%, it promotes grain boundary segregation of P and causes a disadvantage of toughness deterioration. Frequently occurs, it is necessary to make it 1.00% or less.
[0027]
Next, a method for producing the stainless steel of the present invention will be described.
Ferritic stainless steel having the above-described composition is hot-rolled after being melted in a melting furnace such as a converter, electric furnace, VOD, AOD, etc., and then ingot / bundled or continuously cast.
[0028]
The winding and holding temperature of the steel strip after this hot rolling is set to 700 ° C. or more, so that the ridging and r value of the product after cold rolling annealing are remarkably improved, and the holding at this temperature for a long time is further improved. Since it was recognized, it was set to 700 ° C. or higher.
The annealing temperature in the case of box annealing after cooling the coil once after hot rolling is not less than 700 ° C, and the ridging and r value of the product after cold rolling annealing are remarkably improved as in the case of the coiling temperature. Since further improvement was recognized by holding time, it was set to 700 ° C. or higher. The higher the temperature, the more effective. However, when the γ phase is generated at a temperature exceeding 880 ° C., ear cracking occurs during cold rolling and the rolling load increases.
[0029]
These holding and box annealing after winding must be held at a prescribed temperature for 30 minutes or more, and if it is less than this, no effect is recognized. Although the upper limit is not particularly defined, the effect is saturated and the cost becomes high even if it is made longer than necessary, so that it is appropriately limited.
[0030]
The steel strip winding temperature after hot rolling is kept at 700 ° C. or higher and box annealing is performed at 700 to 880 ° C., and the temperature is increased after cooling so as not to cause intergranular corrosion during sulfuric acid pickling. Must be set.
As shown in FIG.
When Al (%) ≦ 0.16,
Heating temperature (° C.) ≧ 1250 × Al (%) + 800
When Al (%)> 0.16,
Heating temperature (° C) ≧ 1000
is required.
[0031]
As for the cooling rate, as shown in FIG. 2, it is necessary to cool the temperature range from the heating temperature to 400 ° C. at a cooling rate of 0.2 degC / sec or more. It is sufficient that the temperature raising retention time reaches the temperature, and it works effectively at 0 sec or longer.
[0032]
These heat-treated steel plates are manufactured as ferritic stainless steel by mechanical descaling by shot blasting, descaling by pickling with sulfuric acid, cold rolling and annealing. Further, it can be produced in the same manner by pickling using hydrochloric acid instead of sulfuric acid. Hydrochloric acid can be achieved by using a hydrochloric acid-based solution containing 100 g / L or more of hydrochloric acid and 0 to 300 g / L of nitric acid.
[0033]
【Example】
About the ferritic stainless steel shown in Table 1, the coil which was hot-rolled to a thickness of 3 mm and changed the winding holding time at 700 to 850 ° C., or the coil which was subjected to box annealing at 700 to 880 ° C. for changing the holding time was once cooled. . Thereafter, the temperature was further raised and cooled. When this oxide-scaled coil is mechanically descaled by shot blasting and then evaluated for intergranular corrosion grooves after 90 sec immersion in 90 ° C, 25% sulfuric acid solution, and then cold rolled to 0.5 mm and annealed into a product Table 2 and Table 3 show the evaluation results of the covering wrinkle, ridging, and r value.
[0034]
Even when the steel strip winding temperature after hot rolling is kept at 700 ° C. or higher, or even in the steel strip box-annealed at an annealing temperature of 700 to 880 ° C., the temperature is rapidly increased after the temperature rises above the temperature determined in relation to the Al amount. Invention steel strips Nos. 1 to 8 cooled at a cooling rate do not show intergranular corrosion grooves after sulfuric acid washing as compared with the comparative steel strip.
In addition, the steel strip of the present invention has no surface wrinkles on the surface that has been cold-rolled and brightly annealed, and even in product evaluations in which the tape is peeled off after deep drawing after applying a protective tape, There was no occurrence and a good product surface was obtained. Also, ridging and r values were considerably better than those of the comparative steel strip.
[0035]
[Table 1]

[0036]
[Table 2]

[0037]
[Table 3]

[0038]
【The invention's effect】
According to the present invention, the coiling retention temperature and the box annealing temperature of the hot-rolled steel strip for improving the ridging and the r value are ensured, and the combination of the grain boundary segregation of phosphorus generated during these processes and cooling and the sulfuric acid descale pickling The occurrence of intergranular corrosion grooves caused by the above is prevented, and the subsequent end product having an excellent cold-rolled surface can be produced, and its industrial significance is extremely great.
[Brief description of the drawings]
[Fig. 1] For hot-rolled steel strips where intergranular corrosion occurs in combination with sulfuric acid by hot-rolling holding or high-temperature holding, the rise between the heating temperature when the temperature is further raised and the Al content of the steel strip The figure which shows the relationship with the production | generation of intergranular corrosion at the time of the sulfuric acid pickling after temperature processing.
FIG. 2 shows a hot-rolled steel strip that undergoes intergranular corrosion in combination with sulfuric acid by hot-rolling holding or high-temperature holding, and between the heating temperature and the cooling rate when the temperature is further raised, after the heating process. The figure which shows the relationship with the production | generation of intergranular corrosion at the time of sulfuric acid pickling.
FIG. 3 is a diagram showing the relationship between the winding and holding temperature and holding time of a hot-rolled steel strip and the ridging height after cold rolling annealing.
FIG. 4 is a diagram showing the relationship between the winding and holding temperature and holding time of a hot-rolled steel strip and the r value after cold rolling annealing.
FIG. 5 is a diagram showing the relationship with the ridging height when cold rolling annealing is performed by changing the holding temperature and time of box annealing.
FIG. 6 is a graph showing a relationship with r value when cold rolling annealing is performed by changing the holding temperature and time of box annealing.

Claims (2)

% By weight
C: 0.12% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.100% or less,
S: 0.030% or less,
Cr: 10 to 20%,
Ni: 0.75% or less,
Cu: 0.005 to 0.20%,
Al: 0.001 to 0.5%
Steel, the balance of Fe and the inevitable impurities are hot-rolled, the steel strip winding temperature after hot rolling is kept at 700 ° C or higher for 30 minutes or more , and after cooling, the relationship with the amount of Al After raising the temperature to the temperature of the following formula <1> or formula <2> determined by the following, the temperature range from the heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 degC / sec or more, and after mechanical descaling A method for producing a ferritic stainless steel, characterized by obtaining a steel strip by sulfuric acid pickling or hydrochloric acid pickling.
Formula <1>: Heating temperature (° C.) ≧ 1250 × Al (%) + 800 (Al (%) ≦ 0.16)
Formula <2>: Heating temperature (° C) ≧ 1000 ……………… (Al (%)> O.16) % By weight
C: 0.12% or less,
Si: 1.00% or less,
Mn: 1.00% or less,
P: 0.100% or less,
S: 0.030% or less,
Cr: 10 to 20%,
Ni: 0.75% or less,
Cu: 0.005 to 0.20%,
Al: 0.001 to 0.5%
Hot-rolled steel consisting of Fe and unavoidable impurities, and after cooling the coil after hot rolling, after cooling by performing box annealing for 30 minutes or more at an annealing temperature of 700 to 880 ° C, After raising the temperature to the temperature of the following formula <1> or formula <2> determined by the relationship with the amount of Al, the temperature range from the heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 degC / sec or more. A method for producing a ferritic stainless steel, characterized in that a steel strip is obtained by sulfuric acid pickling or hydrochloric acid pickling after mechanical descaling.
Formula <1>: Heating temperature (° C.) ≧ 1250 × Al (%) + 800 (Al (%) ≦ 0.16)
Formula <2>: Heating temperature (° C) ≧ 1000 ……………… (Al (%)> O.16)

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