JP2001032023A - Manufacture of ferritic stainless steel excellent in surface characteristic and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the ridging resistance, r-value and surface characteristic of a product by performing cooling at specific velocity after a specific heat-up temperature is reached at the time of subjecting a steel having a specific composition consisting of C, Si, P, S, Cr, Ni, Al and Fe to hot rolling, to holding at specific temperature and then to rapid cooling. SOLUTION: The ferritic stainless steel, which has a composition consisting of, by weight, <=0.12% C, <=1.00% Si, <=0.100% P, <=0.030% S, 10-20% Cr, <=0.75% Ni, 0.001-0.5% Al and the balance Fe with inevitable impurities, and further containing, if necessary, 0.005-0.20% Cu and <=1.00% Mn, is hot rolled. The resultant hot rolled plate is coiled at >=700 deg.C and held or the coil is cooled, annealed at 700-800 deg.C, and held at the temperature. The coil is uncoiled, regulated to the required heat-up temperature represented by inequalities I or II, and cooled down to 400 deg.C at a rate of >=0.2 deg.C/sec. The resultant steel strip is subjected to mechanical descaling and then to pickling with sulfuric acid or hydrochloric acid and formed into picked steel strip.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a ridging resistance of a ferritic stainless steel when hot-rolled or further annealed, descale pickled, cold-rolled and annealed to produce a cold-rolled steel strip. The present invention relates to a method for producing a steel sheet having improved product and surface properties while improving the r value.

[0002]

2. Description of the Related Art Generally, ferritic stainless steels include:
After hot rolling or annealing of a hot rolled sheet, descaling is performed by combining mechanical descaling and sulfuric acid pickling. Regarding the heat history after this hot rolling, in order to reduce the anisotropy at the time of ridging and forming of the product sheet after the subsequent cold rolling and annealing, heat and keep it in a state wound around a coil after hot rolling. , Continuous annealing in which the coil is rewound and heated, and annealing using the heat contained in the coil itself after winding the coil at a high temperature after hot rolling.

However, when a hot-rolled high-temperature ferritic stainless steel sheet is wound into a coil at 700 ° C. or higher and then cooled, or the hot-rolled coil is box-annealed at about 800 ° C. In the case, the crystal grain boundaries are particularly dissolved in sulfuric acid in the subsequent descaling process to generate intergranular corrosion grooves, and subsequently, glittering defects (gold dust) are generated on the surface of the final product after cold rolling and annealing, and the surface quality is significantly impaired. There was a problem.

[0004] Such a problem occurs during cooling after hot rolling, or during box annealing and its cooling process.
Phosphorus, an impurity element in steel, segregates at the grain boundaries, and dissolution in sulfuric acid proceeds according to the phosphorus concentration near the grain boundaries,
This is because grain boundary corrosion grooves are formed. In order to avoid such a problem, nitric acid pickling, which is expensive in terms of both cost and equipment cost, may have to be used instead of sulfuric acid.

On the other hand, for example, Japanese Patent Laid-Open No. 61-199
No. 036 discloses that cooling after hot rolling or after hot rolling and annealing is performed from 830 ° C. to 400 ° C. within 30 minutes. Japanese Patent Application Laid-Open No. 62-174349
In the publication, Zr, Ti, which have strong interaction with phosphorus in steel,
The addition of a small amount of element such as Nb, Mo, Y, La, Ce, and the like.
regulating the contents of u and Mn.
Japanese Patent No. 90450 discloses various preventive measures such as adding a small amount of Ca and Mg, but cannot be said to be sufficiently satisfactory.

[0006]

SUMMARY OF THE INVENTION The present invention promotes recrystallization of a hot-rolled steel strip by controlling the coil cooling conditions after hot rolling of a ferritic stainless steel or the coil annealing conditions once cooled. In addition, an object of the present invention is to provide a method for producing a ferritic stainless steel having a new steel component composition, in which grain boundary segregation of phosphorus generated during these heat treatments does not generate intergranular corrosion grooves after a subsequent sulfuric acid pickling step.

[0007]

To achieve the above object, the present invention has the following gist. (1) By weight%, C: 0.12% or less, Si: 1.00% or less, P: 0.100% or less, S: 0.030% or less, Cr: 10 to 20%, Ni: 0. 75% or less, Al: 0.001 to 0.5%, balance of steel consisting of Fe and unavoidable impurities is hot-rolled, and the steel strip winding temperature after the completion of hot rolling is set to 700 ° C or higher, and the winding temperature is set. When the coil is unwound and rapidly cooled after being held, the required heating temperature is equal to or higher than the required heating temperature of the formula <1> or the formula <2>, which is determined by the relationship with the amount of Al when the winding temperature is held or unwound. After that, the temperature range from the required heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 deg C / sec or more, and the steel strip thus manufactured is washed with sulfuric acid or hydrochloric acid after mechanical descaling, Good surface properties and workability characterized by obtaining pickled steel strip Good ferritic stainless steel manufacturing method. Formula <1> Required heating temperature (° C.) ≧ 1250 × Al (%) +
In the case of 650... Al (%) ≦ 0.16 Formula <2> Required Heating Temperature (° C.) ≧ 850... Al (%)>
In the case of 0.16

(2) By weight%, C: 0.12% or less, Si: 1.00% or less, P: 0.100% or less, S: 0.030% or less, Cr: 10 to 20%, Ni : 0.75% or less, Al: 0.001 to 0.5%, balance of steel consisting of Fe and unavoidable impurities is hot-rolled, and the coil after hot rolling is cooled and then subjected to an annealing temperature of 700 to 8
Annealing in the range of 80 ° C., holding at the annealing temperature, unwinding the coil after annealing from a high temperature state, and quenching the coil. When the annealing temperature is held or unwinding, the above equation determined by the relationship with the amount of Al. After the temperature is raised to the required heating temperature of <1> or the formula <2> or more, the temperature range from the required heating temperature to 400 ° C. is cooled at a cooling rate of 0.2 degC / sec or more. A sulfuric acid pickling or a hydrochloric acid pickling after mechanical descaling to obtain an acid-washed steel strip.

(3) The steel according to the above (1) or (2), wherein the steel having the composition described in the above (1) or (2) further contains Cu: 0.005 to 0.20% by weight. ) A method for producing a ferritic stainless steel having excellent surface properties and workability as described above.

(4) The above (1), (2) or (3)
Ferrite with good surface properties and workability according to the above (1), (2) or (3), wherein the steel is further limited to Mn: 1.00% or less by weight%. Method for producing stainless steel.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have focused on the winding conditions and heating conditions of a hot-rolled steel strip in order to find out the production conditions of ferritic stainless steel with less anisotropy during ridging and forming on a product plate. And studied. The details will be described below based on experiments. As the chemical components (% by weight, hereinafter abbreviated as%) of the test steel for this examination, C: 0.02%, Si: 0.50
%, Mn: 0.50%, P: 0.030%, S: 0.0
15%, Cr: 16%, Ni: 0.35%, Cu: 0.1%
At 06%, the Al content was changed particularly in the range of 0.010 to 0.5%, and a steel having the balance Fe was prepared.

This steel ingot was hot-rolled, wound at a temperature of 700 ° C. or more, held for 30 minutes or more, and then cooled to room temperature to obtain a hot-rolled steel strip. After subjecting this hot-rolled steel strip to mechanical descaling by shot blast, 300 g / L
(L) Sulfuric acid was pickled at 90 ° C. for 60 sec. As a result, intergranular corrosion grooves were formed on the surface of the pickled steel strip in all the test steels irrespective of the Al content.

On the other hand, hot-rolled in the same manner as in the above-mentioned hot-rolled steel strip,
After winding at a temperature of 00 ° C or more and holding for 30 minutes or more,
After unwinding from the winding temperature, high-speed cooling is performed, or a heat treatment of raising the temperature to 650 to 1100 ° C. with a retention time of 10 sec while unwinding is performed, followed by mechanical descaling by shot blasting and sulfuric acid pickling. Then, the intergranular decay grooves of the steel thus formed were investigated.
The results are shown in FIG.

As shown in FIG. 1 (a), it is possible to eliminate intergranular corrosion by setting the winding temperature according to the Al content or the heating temperature after the heating from the winding temperature. It has been found. When the Al content is 0.16% or less, the required heating temperature increases as the Al content increases. However, when the Al content exceeds 0.16%, the required heating temperature saturates to 85%.
0 ° C. was sufficient.

[0015] The film is wound at a temperature of 700 ° C.
After holding for at least 1 minute, the hot rolled steel strip cooled to room temperature is subjected to a heat treatment of raising the temperature to 650 to 1100 ° C. with a holding time of 10 sec, followed by rapid cooling (FIG. 1).
As compared with (b)), the required heat-up temperature could be reduced for the same amount of Al.

Further, a steel having an Al content fixed at 0.08%, and a cooling rate in a temperature range from a winding temperature or a temperature rising temperature after heating from the winding temperature to 400 ° C. is changed. The band was subjected to mechanical descaling by shot blasting and then washed with sulfuric acid, and the results of investigation of intergranular corrosion grooves are shown in FIG. As shown in FIG. 2, when the cooling rate is set to 0.2 degC / sec or more, a pickled steel strip having no intergranular corrosion grooves can be obtained.

The pickled steel strip having no intergranular corrosion grooves is continuously cold-rolled and annealed, and the product sheet does not have any fine covering flaws caused by the intergranular corrosion grooves, and the product is not subjected to a rust test. The evaluation was good and the tape was peeled off after the protective tape was applied to the surface and deep drawn, and no scratches called glittering scratches or gold dust were generated.

It has been found that the same effects can be obtained by applying the knowledge obtained during the holding during the winding after hot rolling to the holding during the box annealing.

Next, the ridging and anisotropy were investigated for each of the steel strips after the completion of the hot rolling prior to the heat treatment, which had been processed by changing the winding temperature, the holding time, the box annealing temperature and the holding time. . Regarding the retention of the winding temperature of the steel strip after the completion of the hot rolling, the coil after winding was covered with a lid and heated to maintain the winding temperature at 1 to 5 hours. On the other hand, as for the heat history as it is wound, in the case of a 10 m to 20 m coil having a normal width of 1 m, the coil was cooled after being at a winding temperature for about 30 minutes.

As a result, as shown in FIGS. 3 and 4,
By raising the winding temperature to 700 ° C. or higher and holding the temperature for 1 hour or longer, the ridging height can be reduced, and r
The values also improved, and it was found that both could be commonly improved.

Further, the relationship between temperature and time when the coil once cooled after the hot rolling was wound around the coil and subjected to box annealing was also examined. As a result, as shown in FIG. 5 and FIG. 6, by raising the coil heating temperature to 700 ° C. or higher, the ridging and the r value can be improved in the same manner as the coil winding temperature. It was found to improve.

After the completion of hot rolling, the steel strip is kept at a constant winding temperature of 70.
0 ° C or higher, or the box annealing temperature is set to 70 ° C.
When the temperature is set to 0 ° C. or higher, the reason why intergranular corrosion grooves are formed in combination with sulfuric acid at the time of descaling is that P segregates at the grain boundaries during holding at these temperatures or during cooling, and the sulfuric acid solution This is because the property of itself dissolves the P-enriched portion more, thereby generating intergranular corrosion grooves. By raising the temperature of the steel strip to a temperature higher than the temperature determined by the relationship with the amount of Al and rapidly cooling the steel strip, it is possible to prevent generation of intergranular corrosion.

The heating temperature required in relation to the amount of Al is increased because the solid solution temperature of P is increased by increasing the amount of Al. This is because it is possible to freeze a state in which the solid solution of P at high temperature does not have the grain boundary segregation. In addition, compared to the case where the temperature is once cooled to room temperature and then raised, the required heating temperature after the heating from the winding temperature or the winding temperature, and the required heating after the heating from the box annealing temperature or the box annealing temperature. The reason why the thermal temperature is low is that the degree of P segregation is small and a large driving force is not required to eliminate the P segregation.

After the completion of hot rolling, the steel strip is kept at a constant winding temperature of 70.
Using a hot-rolled steel strip that has been annealed at 0 ° C or higher or box-annealed with the coil cooled once after hot-rolling and the coil has been coiled and the annealing temperature has been set to 00 ° C or higher, de-scaled, ridging with The reason why the anisotropy at the time of molding is reduced is that 700 ° C
This is because recrystallization of the hot-rolled steel strip progressed by the above-mentioned retention, and the crystal orientation could be randomized.

Next, the reasons for limiting the components will be described. C
The lower the better from the viewpoint of pitting corrosion resistance and corrosion resistance, the upper limit is set to 0.12% in consideration of manufacturability.

[0026] Si is a useful component having a special deoxidizing effect. However, if it is added in an amount exceeding 1.00%, hot workability is degraded, and surface flaws are frequently generated during hot rolling. 1.00% or less.

[0027] P indicates the winding temperature of the steel strip after the completion of hot rolling at 7
In the case where the temperature is kept at 00 ° C or higher, or once cooled after hot rolling, the box is annealed at an annealing temperature of 700 ° C or higher while keeping the coil wound, and in the case where the temperature is kept or during subsequent cooling, the solid solubility of P decreases. It is a harmful element that is concentrated at the target grain boundary, segregates at the grain boundary, and forms intergranular corrosion grooves in combination with sulfuric acid. The lower the better, the better.
The following is assumed.

S is desirably as low as possible for improving corrosion resistance, and is set to 0.030% or less.

Cr is an important element as a stainless steel for improving corrosion resistance and oxidation resistance. For that purpose, at least 10% is required. On the other hand, if it exceeds 20%, toughness is deteriorated. The range was limited to 10 to 20%.

Ni has an action of imparting excellent corrosion resistance and acid resistance, and also has an action of improving pitting corrosion resistance and crevice corrosion resistance. However, since adding more than 0.75% impairs economic efficiency, it was set to 0.75% or less.

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 exceeds 0.5%, it is not preferable in terms of reduction in hot workability. Therefore, it is limited to the range of 0.001 to 0.5%. .

Cu is useful for improving corrosion resistance and mechanical strength, and is added as necessary. 0.00
If it is less than 5%, the desired effect cannot be obtained, while 0.20%
If it exceeds 0.005% to 0.20%, it promotes grain boundary segregation of P and is not preferable in terms of reduction in hot workability.
Limited to the range.

Mn effectively contributes to mechanical strength and deoxidation. However, if it exceeds 1.00%, it promotes the segregation of P at the grain boundaries and causes a disadvantage of toughness deterioration, and the surface of hot-rolled steel is deteriorated. Since it causes frequent flaws, 1.0%
It must be 0% or less.

Next, a method for producing the stainless steel of the present invention will be described. The ferritic stainless steel having the above-described component composition is hot-rolled after being smelted in a smelting furnace such as a converter, an electric furnace, a VOD, an AOD, or the like, and then subjected to ingot making / bulking or continuous casting.

By keeping the winding temperature of the steel strip after the completion of the hot rolling at 700 ° C. or higher, the ridging and r-value of the product after the cold rolling annealing are remarkably improved. , The temperature was set to 700 ° C or higher.

After the coil has been cooled once after hot rolling, the annealing temperature in the case of box annealing is 700 ° C. or higher, and the ridging and r value of the product after cold rolling annealing are remarkably improved as in the case of the winding temperature. Further improvement is recognized by keeping the temperature at this temperature for a long time. This temperature is more effective as the temperature is increased, but if the γ phase exceeds 880 ° C., ear cracks occur during cold rolling and the rolling load increases.

The holding and box annealing after these windings are as follows:
It is necessary to maintain the set temperature for at least 30 minutes.
Below this, no effect is observed. The upper limit is not particularly defined, but if the length is longer than necessary, the effect is saturated and the cost increases, so that the upper limit is appropriately limited.

The necessary heat-up condition after the steel strip winding temperature after the completion of the hot rolling is maintained at 700 ° C. or more must be set so as not to cause intergranular corrosion at the time of sulfuric acid pickling.
As for the required heating temperature, as shown in FIG.
If l (%) ≦ 0.16, the required heating temperature (° C.) ≧ 125
When 0 × Al (%) + 650 and Al (%)> 0.16, the required heating temperature (° C.) ≧ 850 is required. The same applies to the case where the temperature is increased from the annealing temperature of 700 to 880 ° C. or the annealing temperature.

As for the cooling rate, as shown in FIG. 2, the temperature range from the winding temperature or the temperature after heating up from the winding temperature to 400 ° C. is cooled at a cooling rate of 0.2 deg C / sec or more. There is a need. The same applies to the case where the temperature is increased from the annealing temperature of 700 to 880 ° C. or the annealing temperature. The retention time of the temperature rise only needs to reach the temperature, and it works effectively at 0 sec or more.

These heat-treated steel sheets are subjected to mechanical descaling by shot blasting, then descaled by pickling with sulfuric acid, cold-rolled and annealed to produce ferritic stainless steel. It is also possible to produce the same 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 (liter) or more of hydrochloric acid and 0 to 300 g / L of nitric acid.

[0041]

EXAMPLE A ferritic stainless steel composed of the components shown in Table 1 was hot-rolled to a thickness of 4 mm and heated at 700 to 850 ° C.
Coil whose winding retention time has been changed in the above, or 700-8
The temperature of the box-annealed coil was unwound at 80 ° C. and the temperature was raised and high-speed cooling was performed according to the amount of Al, or the temperature was raised according to the amount of Al at the end of winding and holding or at 700 to 880 ° C. Cooling was performed. After mechanically descaling the coil with the oxide scale by shot blasting, the intergranular corrosion groove in 100 ° C. immersion in a 30% sulfuric acid solution for 100 seconds was evaluated. Subsequently, it was cold rolled to 0.5 mm and annealed to obtain a product. Cover flaw, ridging, r
Tables 2 and 3 show the results of evaluating the values.

The steel strip winding temperature after the completion of hot rolling is maintained at 700 ° C. or higher, and even in a steel strip annealed at an annealing temperature of 700 to 880 ° C., the temperature is determined to be higher than the temperature determined by the relationship with the amount of Al. The steel strip of the present invention, which was cooled at a high cooling rate after the temperature was raised, did not show intergranular corrosion grooves after sulfuric acid pickling compared to the comparative steel strip. Further, the steel strip of the present invention is continuously cold-rolled, and has no flaws on the surface subjected to bright annealing.Even in product evaluation of peeling off the tape after applying a protective tape and deep drawing, flaws called glitter flaws and gold dust. There was no occurrence, and a good product surface was obtained. In addition, the ridging and the r-value also exhibited considerably better characteristics than the comparative steel strip.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

According to the present invention, the winding retention temperature and the box annealing temperature of the hot-rolled steel strip for improving the ridging and the r value are sulfur-preserved, and the grain boundary segregation of phosphorus and sulfuric acid generated during these moderate and cooling processes. Prevents the generation of intergranular corrosion grooves caused by the combination with descale pickling, and enables the production of a final product with an excellent cold-rolled surface after that. Utilizing the heat at the time of holding after annealing, it is also advantageous in terms of energy, and its industrial significance is extremely large.

[Brief description of the drawings]

FIG. 1 (a) shows the relationship between the hot-rolling winding temperature or the heating temperature after heating from the winding temperature and the Al content of the steel strip, and sulfuric acid after high-speed cooling treatment from these temperatures. It is a diagram showing the relationship with the generation of intergranular corrosion when pickling,
(B) shows the relationship between the heat-up temperature when the hot-rolling is wound up and then cooled to room temperature, and then the heat-up temperature, and the Al content of the steel strip. It is a figure which shows the relationship with generation | occurrence | production of intergranular corrosion at the time of pickling.

FIG. 2 shows the relationship between the hot-rolling winding temperature or the heating temperature after heating from the winding temperature and the cooling rate during high-speed cooling, intergranular corrosion when sulfuric acid was washed after these heat treatments. FIG. 9 is a diagram showing a relationship with generation of a.

FIG. 3 is a diagram showing a relationship between a winding retention temperature and a retention time of a hot-rolled steel strip and a ridging height after cold rolling annealing.

FIG. 4 is a diagram showing the relationship between the winding retention temperature and retention time of a hot-rolled steel strip and the r-value after cold rolling annealing.

FIG. 5 is a diagram showing the relationship between the holding temperature and time of box annealing and the ridging height when cold rolling annealing is performed.

FIG. 6 is a diagram showing the relationship between the holding temperature and the time of box annealing and the r-value when cold rolling annealing is performed.

Continuation of front page (72) Inventor Masayuki Abe 1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture F-term in Nippon Steel Corporation Yawata Works (reference) 4K037 EA01 EA05 EA06 EA12 EA13 EA15 EA20 EA23 EA25 EA27 FE03 FF03 HA05 4K043 AA01 AB01 AB03 AB04 AB12 AB13 AB15 AB22 AB25 AB26 AB27 BA05 BA06 DA05 FA01 FA13 HA02

Claims (4)

[Claims] C: 0.12% or less, Si: 1.00% or less, P: 0.100% or less, S: 0.030% or less, Cr: 10 to 20%, Ni: 0.75% or less, Al: 0.001 to 0.5%, balance of steel consisting of Fe and unavoidable impurities is hot-rolled, and the steel strip winding temperature after hot rolling is set to 700 ° C. or more, and the winding is performed. When keeping the coil at the take-up temperature, unwinding the coil after holding, and cooling rapidly, the required heat-up of the formula <1> or <2>, which is determined by the relationship with the amount of Al, at the time of keeping the take-up temperature or unwinding After the temperature has been increased, the temperature range from the required heat-up temperature to 400 ° C is cooled at a cooling rate of 0.2 degC / sec or more, and the steel strip thus manufactured is subjected to mechanical descaling, followed by sulfuric acid pickling or hydrochloric acid pickling. The surface properties and characteristics of obtaining pickled steel strip. Method for manufacturing a sexual good ferritic stainless steel. Formula <1> Required heating temperature (° C.) ≧ 1250 × Al (%) +
In the case of 650... Al (%) ≦ 0.16 Formula <2> Required Heating Temperature (° C.) ≧ 850... Al (%)>
In the case of 0.16 2. In% by weight, C: 0.12% or less, Si: 1.00% or less, P: 0.100% or less, S: 0.030% or less, Cr: 10 to 20%, Ni: 0.75% or less, Al: 0.001 to 0.5% The balance of steel consisting of Fe and unavoidable impurities is hot-rolled, and the coil after hot rolling is cooled, and then an annealing temperature of 700 to 8 is used.
Annealing in the range of 80 ° C., holding at the annealing temperature, and unwinding the coil after annealing from a high temperature state and rapidly cooling it, the equation < After the temperature is raised to the required heating temperature of 1> or the formula <2> or more, the temperature range from the required 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 having good surface properties and workability, wherein a sulfuric acid pickling or a hydrochloric acid pickling is performed after mechanical descaling to obtain a pickled steel strip. Formula <1> Required heating temperature (° C.) ≧ 1250 × Al (%) +
In the case of 650... Al (%) ≦ 0.16 Formula <2> Required Heating Temperature (° C.) ≧ 850... Al (%)>
In the case of 0.16 3. The steel according to claim 1 or 2,
3. The method for producing a ferritic stainless steel having good surface properties and workability according to claim 1, further comprising Cu: 0.005 to 0.20% by weight. 4. The surface texture according to claim 1, wherein the steel having the composition according to claim 1, 2, or 3 is further limited to Mn: 1.00% or less by weight%. Method for producing ferritic stainless steel with good workability.