JP2000246408A - Manufacture of high sulfur steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of crack on the surface of a steel slab having low hot woakability and to obtain a high quality steel material with a rolling into a product thereafter by rapidly cooling the cast slab surface which is obtd. by continuously casting a high sulfur steel to make fine of the crystal grain, heating in a heating furnace after forming the fine structure and bloom-rolling. SOLUTION: The molten high sulfur steel 2 containing >0.10 wt.% S and if necessary, 0.10-0.35% Pb and further, <0.05% P, is poured into a water- cooling mold 4 through a tundish 3 from a ladle 1 and a produced outer shell- solidified cast slab 5 is cooled in a spray zone 6, and after drawing out and straightening with pinch rolls 7, the solidified cast slab 8 is cut off into a prescribed length with a cutting device 9. The obtd. cast slab at about 680 deg.C is water-cooled and the surface is rapidly cooled at about 420 deg.C to make fine of the crystal grain on the surface. Successively, this cast slab is charged into the heating furnace and extracted at >=1230 deg.C and the bloom-rolling is executed. In this way, the generation of crack on the surface of the steel slab is prevented, and even in the case of generating the crack, the high sulfur steel material having low hot workability, in which the depth of crack is shallow, is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high S steel material suitable for producing a high S steel material having low hot workability by continuous casting, heating and slab rolling.

[0002]

2. Description of the Related Art When manufacturing a high S steel having low hot workability, molten steel of a high S content steel is poured and solidified into a large mold placed on a surface plate to form an ingot. Ingot casting method obtained by ingot rolling, continuous casting method in which molten steel of high S content steel is poured into a water-cooled mold, continuously drawn out from the lower part of the water-cooled mold to form slabs, and then slab-rolled Is adopted.

[0003] Among them, the continuous casting method has an advantage that the process is simplified as compared with the ingot making method, so that the yield can be improved and energy can be saved.

FIG. 1 shows an example of a continuous casting facility used for carrying out this type of continuous casting method, in which molten steel 2 contained in a ladle 1 is transferred into a tundish 3 and molten steel in the tundish 3 2 is flowed down into the water-cooled mold 4, the outer shell solidified slab 5 from the lower part of the water-cooled mold 4 is spray-cooled in a spray zone 6, and then drawn and corrected by a pinch roll 7 to form a solidified slab 8, and a cutting device 9 And then heated in a heating furnace, and then slab-rolled the heated slab to obtain a slab.

[0005]

When a high S steel material having low hot workability is to be manufactured by such a continuous casting method, a relatively deep crack is generated on the surface of the slab after slab rolling. Since this crack may not be completely removed even by scarfing, the steel material after billet rolling may have flaws that appear to be residual flaws, resulting in a decrease in surface quality. There was a problem that could be.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and has been made in consideration of the problems described above.
Even when a steel material is manufactured by continuous casting-heating-bulking rolling, it is possible to prevent cracks on the steel slab surface after lumping rolling or to keep the steel slabs in a crack that can be easily removed by subsequent scarfing. It is an object of the present invention to make it possible to prevent a residual flaw from occurring in a steel material after billet rolling and to further improve surface quality.

[0007]

According to a first aspect of the present invention, there is provided a method of manufacturing a high S steel material having a low hot workability by continuously casting, heating and separating a high S steel material. In the production by ingot rolling, the surface of the slab is quenched after continuous casting and before charging into a heating furnace, whereby cracks on the surface of the slab after slab rolling are prevented.

In the embodiment of the method for producing a high S steel material according to the present invention, as described in claim 2, the high S steel has an S content exceeding 0.10% by weight. The feature is that there is.

Similarly, in an embodiment of the method for producing a high-S steel according to the present invention, as described in claim 3, the high-S steel has an S content exceeding 0.10% by weight and a Pb content. The amount is 0.10 to 0.35% by weight.

[0010] Similarly, in an embodiment of the method for producing a high-S steel material according to the present invention, as described in claim 4, the high-S steel has a P content of less than 0.05% by weight. The feature is that there is.

[0011] Similarly, in an embodiment of the method for producing a high S steel material according to the present invention, as set forth in claim 5, the heating furnace extraction temperature at the time of heating is set to 1230 ° C or higher. It is characterized by:

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The method for producing a high-S steel according to the present invention has the above-mentioned structure. As a high-S steel having a low hot workability, for example, the S content is 0.10% by weight. %, More than 0.20% by weight or more than 0.25% by weight depending on the case, or one having a Pb content of about 0.10 to 0.35% by weight. .

More specifically, for example, JIS G
Soil and sulfur combined free-cutting steel specified in 4804 are applied, C: 0.48% by weight or less, Mn: 0.30 to 0.30% by weight.
1.65% by weight, P: 0.12% by weight or less, sometimes less than 0.05% by weight, S: 0.08 to 0.40% by weight, and in some cases, Pb: 0.10 to 0.35% weight%
Things apply.

In the present invention, the slab surface is rapidly cooled after the continuous casting and before the heating furnace is inserted. In this rapid cooling, water cooling by immersion, water cooling by spraying, water cooling by mist, and the like are used. by performing the rapid cooling of the slab surface, such as, with can be a tissue crystal grains of the surface is miniaturized ones fine, it is possible to suppress the precipitation of α tissues by a ₃ transformation Since the effect of suppressing crack growth is sufficiently large due to the refinement of the structure, even if cracks are generated on the billet surface after slab rolling, the cracks are shallow (about 0.30 mm or less). This is the extent to which scars that can be easily and sufficiently removed by scarfing remain.

Then, the cast slab after the scarfing is charged into a heating furnace, and heated at a heating furnace extraction temperature of preferably 1230 ° C. or higher, followed by slab rolling of the heated slab.
Appropriate product rolling is performed on the obtained slab to obtain a high S steel material. In this case, if the heating furnace extraction temperature is low, corner cracking tends to occur.

The high-S steel material produced in this way has high quality with a low appearance defect rate without any abnormal inspection on the surface of the steel material despite its inherently low hot workability.

[0017]

Hereinafter, embodiments of the present invention will be described.
The present invention is not limited to only such an embodiment.

Example 1 In the continuous casting equipment shown in FIG. 1, as the molten steel 2, C: 0.07% by weight, Si:
0.30% by weight, Mn: 0.90% by weight, P: 0.04
4% by weight, S: 0.312% by weight, solid cast slab 8 was obtained by continuous casting using the balance of Fe and impurities, and then cut by a frame cutting device 9 to cut the slab surface. Water was quenched by spraying with a spray nozzle.

At this time, the temperature of the slab before water cooling was 680 ° C., the cooling water temperature was 28 ° C., the water cooling time was 40 seconds, and the slab temperature after water cooling was 420 ° C.

Observation of the surface and the corner of the water-cooled slab obtained here showed that the surface had fine crystal grains and a microstructure as shown in FIG. 2 (plane) and FIG. 3 (corner). In addition to the refinement, bainite precipitation was partially observed in the surface portion, and bainite precipitation was hardly observed in the corner portion.

Next, the slab after cooling with water is cooled to a furnace inlet temperature of 50%.
The mixture was charged into a heating furnace at 0 ° C., and the heating furnace extraction temperature was set at 1250.
After extracting at a temperature of ° C., slab rolling was performed, and the appearance of the slab obtained by slab rolling was examined. As a result, no corner cracks opened in the surface of the slab were found.

Next, eddy current inspection was carried out on the high S steel material obtained by further rolling the slab obtained by slab rolling, and no abnormal surface was found. It was 0%.

On the other hand, in the observation of the surface portion and the corner portion of the slab which was not subjected to the water cooling before charging the heating furnace after the continuous casting, as shown in FIG. 4 (surface portion) and FIG. 5 (corner portion), Had slightly larger crystal grains, and both the face part and the corner part had almost an α structure.

Next, the slab which is not water-cooled is cooled to a furnace inlet temperature of 5%.
The mixture was charged into a heating furnace at a temperature of
After extraction at 0 ° C., slab rolling was performed, and the appearance of the slab obtained by slab rolling was examined. As a result, corner cracks opened on the surface of the slab were observed.

Next, a vortex test was performed on the high S steel material obtained by further rolling the steel slab obtained by slab rolling, and the occurrence of surface abnormalities was recognized.
The appearance defect rate was about 18%.

Example 2 In the continuous casting facility shown in FIG. 1, as the molten steel 2, C: 0.08% by weight, Si:
0.30% by weight, Mn: 1.00% by weight, P: 0.04
Continuous casting using 4% by weight, S: 0.301% by weight, Pb: 0.152% by weight, balance Fe and impurities to obtain a solidified slab 8, and then cut by a frame cutting device 9 Then, the surface of the cut slab was rapidly cooled by spraying water with a spray nozzle.

At this time, the temperature of the slab before water cooling was 662 ° C., the cooling water temperature was 28 ° C., the water cooling time was 40 seconds, and the slab temperature after water cooling was 430 ° C.

Observation of the surface and the corner of the water-cooled cast slab obtained here showed that the crystal grain was fine on the surface and the structure was similar to that shown in FIG. 2 (the surface) and FIG. 3 (the corner). Was refined, and bainite precipitation was partially observed in the surface portion, and almost no bainite precipitation was observed in the corner portion.

Then, the slab after cooling with water is cooled to a furnace inlet temperature of 50%.
The mixture was charged into a heating furnace at 0 ° C., and the heating furnace extraction temperature was set at 1250.
After extracting at a temperature of ° C., slab rolling was performed, and the appearance of the slab obtained by slab rolling was examined. As a result, no corner cracks opened in the surface of the slab were found.

Next, eddy current inspection was carried out on the high S steel material obtained by further rolling the steel slab obtained by slab rolling, and no abnormal surface was observed. It was 0%.

On the other hand, in the observation of the surface portion and the corner portion of the slab which was not subjected to the water cooling before charging the heating furnace after the continuous casting, as shown in FIG. 4 (surface portion) and FIG. 5 (corner portion), The crystal grains on the surface are slightly larger,
Further, both the face portion and the corner portion had almost an α structure.

Next, the slab which is not water-cooled is cooled to a furnace inlet temperature of 5%.
The mixture was charged into a heating furnace at a temperature of 00 ° C., and the heating furnace extraction temperature was set to 128.
After extraction at 0 ° C., slab rolling was performed, and the appearance of the slab obtained by slab rolling was examined. As a result, corner cracks opened on the surface of the slab were observed.

Next, the slab obtained by slab rolling was subjected to eddy current inspection on a high S steel material obtained by further rolling the product, and surface abnormalities were found.
The appearance defect rate was about 20%.

[0034]

According to the method for producing a high S steel material according to the present invention, as described in claim 1, a high S steel material having low hot workability is produced by continuous casting-heating-bulking rolling. During the casting, the slab surface was quenched before charging the heating furnace after continuous casting to prevent cracks on the slab surface after slab rolling. Significantly high effect that it is possible to produce a high S steel material of

And, as described in claim 2,
By ensuring that the high S steel has an S content of more than 0.10% by weight, it is possible to further improve the surface quality of the high S steel having a low hot workability. That is a great effect.

In addition, as described in claim 3, the high S steel has an S content exceeding 0.10% by weight and a Pb content of 0.10 to 0.35% by weight. By doing so, it is possible to obtain a machinability improving effect due to Pb in addition to the machinability improving effect due to S, so that it is possible to produce a high-S steel material with excellent machinability and good surface quality. There is a remarkably excellent effect.

Furthermore, as described in claim 4, the high S steel has a P content of less than 0.05% by weight, so that the hot workability is further ensured. A remarkable effect that a high S steel material can be manufactured is provided.

Furthermore, as described in claim 5, by making the heating furnace extraction temperature at the time of heating to be 1230 ° C. or higher, a high-S steel material having more excellent surface quality can be manufactured. A significant effect of being able to do so.

[0039]

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an example of a continuous casting method.

FIG. 2 is a schematic explanatory view showing a microstructure in a surface portion of a slab subjected to water cooling after continuous casting and before charging a heating furnace.

FIG. 3 is a schematic explanatory view showing a microstructure in a corner portion of a slab subjected to water cooling after charging into a heating furnace after continuous casting.

FIG. 4 is a schematic explanatory view showing a microstructure in a surface portion of a conventional slab which is not subjected to water cooling before charging a heating furnace after continuous casting.

FIG. 5 is a schematic explanatory view showing a microstructure in a corner portion of a conventional slab where water cooling is not performed before charging a heating furnace after continuous casting.

[Explanation of symbols]

 Reference Signs List 1 ladle 2 molten steel 3 tundish 4 water-cooled mold 5 outer shell solidified slab 6 spray zone 7 pinch roll 8 solidified slab 9 cutting device

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C21D 9/00 101 C21D 9/00 101Y C22C 38/60 C22C 38/60

Claims (5)

[Claims] 1. When producing a high S steel material having low hot workability by continuous casting, heating and slab rolling, the slab surface is rapidly cooled after continuous casting and before charging in a heating furnace. A method for producing a high-S steel material characterized by preventing cracks on the surface of a billet. 2. The method for producing a high-S steel according to claim 1, wherein the high-S steel has an S content exceeding 0.10% by weight. 3. The high S steel according to claim 1, wherein the S content is more than 0.10% by weight and the Pb content is 0.10 to 0.35% by weight. Method for producing high S steel. 4. The method for producing a high-S steel according to claim 1, wherein the high-S steel has a P content of less than 0.05% by weight. 5. The heating furnace extraction temperature at the time of heating is 1230 ° C.
The method for producing a high-S steel material according to any one of claims 1 to 4, wherein: