JPH11179493A - Production of steel strip excellent in surface quality with twin roll type continuous casting machine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To basically prevent the fall-down flaw caused by the transferring of dimples on a steel strip and to shorten the descaling treatment time of the steel strip in a twin roll type continuous casting method. SOLUTION: In the method for producing the cast strip by casting the thin cast strip with the twin roll type continuous casting machine, in which the cast strip is shifted synchronously with the wall surfaces of the cooling rolls with dispersed many dimples, after forming the surface roughness to <=30 μm in Rmasx by mechanically grinding the thin cast strip surface, this strip is hot-rolled, anneled and coiled as a hot-rolled annealing steel strip and thereafter, a descaling treatment is executed. After the above bushing-grinding, the hot- rolling is executed with rolling reduction ratio regulated to >=15%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a steel strip by casting a thin cast slab by a twin-roll continuous caster in which the slab moves in synchronization with the wall surface of a cooling drum in which a large number of dimples are scattered. It is a technique about the method.

[0002]

2. Description of the Related Art In a twin-roll continuous casting method, as disclosed in Japanese Patent Publication Nos. 4-13053 and 4-33537, the diameter is 0.1 to 1.2 mm and the depth is 5 mm. Molten steel is poured between cooling drums having dimples of １００100 μm to obtain a thin strip slab. These dimples are formed in order to prevent casting cracks stably, but a large number of fine projections to which the dimples have been transferred remain on the surface of the cast piece cast in this way.

As a method of eliminating such minute projections, a method disclosed in Japanese Patent Application Laid-Open No. 8-300107 has been proposed. That is, according to the present invention, the thin cast slab obtained by casting with the cooling drum in which the dimples are scattered is hot-rolled at a rolling reduction of 15% or more, thereby transferring the fine slab transferred to the surface of the thin cast slab. This method is intended to reduce and eliminate the protrusions. By this method, the surface roughness of the cast piece can be reduced, and the poor gloss due to the remaining unevenness of the transfer dimple can be improved.

In Japanese Patent Application Laid-Open No. Hei 8-165523, a slab cast by a cooling drum in which a large number of random dimples are scattered by a shot blast method is subjected to hot rolling at a rolling reduction of 10% or more. A method is disclosed in which dimples transferred onto a sheet are eliminated under pressure.

[0005]

However, as a result of further detailed examination, the following has been found. JP-A-8-
In the case where a hot-rolled annealed steel strip is manufactured by the method disclosed in Japanese Patent Publication No. 300107 and pickled after mechanical descaling by shot blasting, the dimple-transferred projections roll the scale during hot rolling to form a circular shape. Alternatively, it has been found that the descaling property of the half-moon-shaped fall-down flaw is extremely reduced.

On the other hand, Japanese Patent Application Laid-Open No. Hei 8-277423 discloses a method for preventing a dimple transfer portion from falling down, etc., in which the hot rolling ratio is limited to 15% or less. In the method of reducing the hot rolling rate, roughening called roping occurs during cold rolling due to insufficient reduction.

Accordingly, an object of the present invention is to fundamentally prevent collapse flaws caused by a dimple transfer portion of a steel strip, which could not be achieved by the prior art, to shorten the time required for the descaling of the steel strip and to reduce the surface of the cold-rolled steel strip. An object of the present invention is to provide a method for producing a steel strip having excellent surface quality by using a twin-roll continuous caster that improves the quality.

[0008]

In order to achieve the above object, the present invention has the following constitution. (1) A thin-walled cast piece is cast by a twin-roll continuous casting machine in which the cast piece moves in synchronization with the wall surface of a cooling drum in which a large number of dimples are scattered, subsequently hot-rolled, annealed, and hot-rolled annealed steel In a method of manufacturing a steel strip to be wound as a strip and then subjected to a descaling treatment, the surface of the thin cast slab is excellent in surface quality by a twin-roll continuous casting machine characterized by mechanically grinding before hot rolling. Steel strip manufacturing method.

(2) The surface quality of the twin-roll continuous caster according to the above (1), wherein the surface roughness is reduced to 30 μm or less in Rmax by mechanical grinding before hot rolling. Method of producing excellent steel strip.

(3) The hot rolling after the mechanical grinding is performed with a rolling reduction of 1
The method for producing a steel strip having excellent surface quality by using a twin-roll continuous caster according to the above (1) or (2), wherein the content is 5% or more.

(4) The twin-roll continuous caster according to the above (1), (2) or (3), wherein the thin cast slab produced by the twin-roll continuous caster is stainless steel. For producing steel strip with excellent surface quality.

(5) The method for producing a steel strip having excellent surface quality using a twin-roll continuous caster according to the above (4), wherein the stainless steel is austenitic stainless steel.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various detailed studies, the present inventors have found that a twin roll type continuous casting method is used as a method for fundamentally preventing a falling flaw caused by a dimple transfer portion of a steel strip. It has been found that it is most effective to mechanically grind the slab surface immediately before hot rolling. Hereinafter, a description will be given according to experimental results.

By a twin-roll type continuous casting method, a diameter of 0.1 mm is obtained.
Inject molten steel between cooling drums having dimples having a diameter of 1 to 1.2 mm and a depth of 5 to 100 μm to prevent casting cracks.
A 5 mm thick austenitic stainless steel strip was obtained. On the surface of the cast slab after casting, microprojections having an Rmax of 65 to 100 μm to which dimples were transferred on the surface of the cooling drum were found. Immediately after casting on these slabs, brushing grinding was performed with a roll in which a 1 mm diameter steel wire was planted. On this occasion,
Adjust the rolling force of the brushing within an appropriate range to obtain Rmax
Was varied between 5 and 100 μm to produce cast pieces. This slab is continuously reduced at 1150 ° C. and 20%,
It was annealed at 1070 ° C. and wound up as a hot-rolled annealed steel strip.
Immediately after the grinding, the surface roughness (Rmax) measured by sampling was measured. The rolled steel strip was cooled to room temperature, mechanically descaled by shot blasting, and then 100 g / L (liter) nitric acid, 15 g.
The solution was pickled with a nitric acid solution of 0 g / L hydrofluoric acid and 70 ° C., and the time required for completing the descaling was measured. FIG. 1 shows the relationship between the surface roughness and the descale completion time.

Even in the case of a slab that has not been subjected to brushing grinding or a slab having a large surface roughness even after the grinding, most scale on the slab surface can be removed in a short time of about 30 sec. It is difficult to remove the portion where the scale remains in a circular shape or a half-moon shape of 0.5 to 1.5 mm, and a long immersion time is required to descale this portion. On the other hand, by brushing the height of the fine projections to reduce the surface roughness Rmax to 30 μm or less, a circular or crescent-shaped scale remaining portion is prevented.
Descale was completed within 0 seconds.

[0016] As described above, the reason why the circular or semi-lunar scale remaining portion is not generated by brushing the height of the minute projections is that the minute projections exceeding the limit height are
This is because, while the subsequent hot rolling takes the form of a cover, the minute projections having a height less than the limit height are not covered.

The method of mechanical grinding is performed at a high temperature of 900 to 1200 ° C. immediately before hot rolling.
It is preferable to use brushing grinding using a roll in which a steel wire having a diameter of 2 mm is planted, a polisher provided with ceramic blades, a grinding disk of a grindstone, or the like.

The reason why the hot rolling after the mechanical grinding is set to a rolling reduction of 15% or more is as follows. Annealing by applying a reduction of 15% or more is intended to prevent the occurrence of roping (surface roughening) due to the coarse crystal grains of the slab on the surface after cold rolling. This is a condition for imparting strain to the slab and heating it to make the structure equiaxed and fine. If the rolling reduction is less than this, a good recrystallized structure cannot be obtained. Further, regarding the hot rolling temperature, 900
If the temperature is lower than 0 ° C, the roll is damaged. If the temperature is higher than 1200 ° C, the wear of the roll becomes large.
A temperature range of 200 ° C. is preferred.

If the annealing temperature is lower than 1000 ° C., recrystallization does not proceed, and if it exceeds 1150 ° C., grain growth occurs, making it impossible to make the structure into appropriate equiaxed and fine grains. preferable. Further, after annealing, in order to prevent intergranular corrosion from occurring during descal pickling using nitric hydrofluoric acid pickling, it is immediately cooled with water and wound at 600 ° C. or less.

As a pre-treatment for cold rolling the hot-rolled steel strip thus obtained, the hot-rolled sheet produced by the above method is subjected to a descaling treatment. The descaling process is performed, for example, by immersing in an acid solution after mechanical descaling. The mechanical descaling is performed by shot blasting, a method of blowing iron sand with high-pressure water, a method of grinding after the shot blasting with a heavy grinding brush with a grindstone or a brush roll containing abrasive grains. As the acid solution, as a nitric hydrofluoric acid solution, a solution in a range of 30 to 200 g / L of nitric acid and 50 to 200 g / L of hydrofluoric acid is immersed or sprayed at a liquid temperature of 20 to 80 ° C.

After pickling with a sulfuric acid solution, treatment with a hydrochloric acid / nitric acid solution is performed. In this case, the sulfuric acid has a concentration of 300 g / L or less and a temperature of 95 ° C. or less. For hydrochloric acid / nitric acid solution, add 1 hydrochloric acid
It is immersed or sprayed in a liquid at 50 to 100 ° C. containing 10 to 300 g / L, nitric acid 60 to 110 g / L, and iron ions 20 g / L or more.

[0022]

[Example 1] For austenitic stainless steel, a thin-walled slab having a thickness of 3 to 5 mm was formed by a twin-roll continuous casting machine in which the slab moves in synchronization with the wall surface of a cooling drum in which a large number of dimples are dispersed. Immediately after casting, the surface of the obtained steel strip is brushed with a roll having a 1 mm diameter steel wire implanted, or ground with a grinding disk, hot rolled, annealed, and wound into a coil at the winding temperature. I took it. In order to remove the scale on the surface of the steel strip, mechanical descaling was performed and pickling was performed. Table 1 shows the results of immersion in a nitric hydrofluoric acid solution and pickling, and Table 2 shows the results of immersion in a sulfuric acid solution followed by immersion in a hydrochloric acid-nitric acid-Fe ion solution.

As can be seen from the pickling time of nitric hydrofluoric acid and the pickling time of sulfuric acid at which descale is completed, mechanical grinding is performed immediately after casting to reduce the surface roughness to 30 μm or less, and the hot rolling reduction ratio is 1 mm.
According to the method of the present invention in which the treatment is performed at 5% or more, both short-time pickling and roping of the cold-rolled steel strip were improved. In the case where the conditions are out of this range, the pickling descaling time becomes long or the roping of the cold-rolled steel strip becomes poor due to the rate control of the dimple fall-down flaws. Further, a combination of a winding temperature of 600 ° C. or lower and nitric hydrofluoric acid, or hydrochloric acid-nitric acid-F after sulfuric acid.
When immersed in e-ion, the surface after pickling had no intergranular corrosion, and a good surface was obtained even if it was continuously cold-rolled and brightly annealed. On the other hand, intergranular corrosion occurred in the steel strip which was wound at a winding temperature exceeding 600 ° C. and washed with nitric acid and hydrofluoric acid, and the surface of the cold-rolled and bright annealed surface had flaws, thereby significantly impairing the product quality.

[0024]

[Table 1]

[0025]

[Table 2]

Example 2 A ferritic stainless steel was cast from 3 to 3 by a twin-roll continuous caster in which a slab moves in synchronization with the wall surface of a cooling drum on which a large number of dimples are dispersed.
As shown in Table 3, a 5 mm-thick thin cast slab was cast, and the surface of the obtained steel strip was immediately brushed with a roll of 1 mm diameter steel wire and brushed or ground with a grinding disk as shown in Table 3. Then, it was annealed and wound into a coil at a winding temperature. In order to remove the scale on the surface of the steel strip, mechanical descaling was performed, and the steel strip was immersed in a sulfuric acid solution and pickled for descaling.

As shown in the pickling time at which the descaling is completed, mechanical grinding is performed immediately after casting to reduce the surface roughness to 30 μm.
m or less, and according to the method of the present invention in which the hot rolling reduction rate is 15% or more, the pickling time, roping of the cold-rolled steel strip, and ridging of the cold-rolled bright annealed product sheet are also improved. . If the conditions were not satisfied, the descale pickling time was prolonged due to the rate control of the dimple falling flaws, or the roping of the cold-rolled steel strip and the ridging of the cold-rolled bright annealed product sheet were also defective.

[0028]

[Table 3]

[0029]

According to the present invention, it is possible to fundamentally prevent the falling flaw caused by the dimple transfer portion of the steel strip, which cannot be achieved by the prior art. And the time of the subsequent descaling process can be greatly reduced. For example, in the case of austenitic stainless steel,
The time required to complete the descaling can be reduced by about 60% from about 80 seconds to about 30 seconds, and the collapse flaw peculiar to the twin-roll continuous casting method can be fundamentally eliminated.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the surface roughness after brushing grinding of a slab and the pickling time until descale is completed.

Claims (5)

[Claims] 1. A thin-walled slab is cast by a twin-roll continuous casting machine in which the slab moves in synchronization with a wall of a cooling drum in which a large number of dimples are dispersed, subsequently hot-rolled, annealed, and hot-rolled. In the method for producing a steel strip, which is wound as an annealed steel strip and then subjected to a descaling treatment, the surface quality of the thin cast slab is mechanically ground before hot rolling by a twin-roll continuous casting machine. Method of producing excellent steel strip. 2. The twin-roll type continuous casting machine according to claim 1, wherein the surface roughness is reduced to 30 μm or less in Rmax by mechanically grinding before hot rolling. Steel strip manufacturing method. 3. The hot rolling after mechanical grinding is performed with a reduction of 15%.
A method for producing a steel strip having excellent surface quality using the twin-roll continuous caster according to claim 1 or 2. 4. The steel with excellent surface quality by a twin-roll continuous caster according to claim 1, wherein the thin cast slab produced by the twin-roll continuous caster is stainless steel. The production method of the belt. 5. The method for producing a steel strip having excellent surface quality by using a twin-roll continuous caster according to claim 4, wherein the stainless steel is austenitic stainless steel.