KR20010112887A - BILLETS FOR Bi-S SYSTEM FREE CUTTING STEEL WITHOUT SPLIT END DURING WIRE-ROD ROLLING AND A METHOD THEREFOR - Google Patents

Abstract

The present invention relates to a Bi-S-based free-cutting steel billet, and a method of manufacturing the same, which does not have a crack at the tip when the wire is rolled; The purpose is to control the size and distribution of MnS inclusions in free-cutting steel and to control the amount of Al and [O] in molten steel to eliminate gaps at the tip, thereby producing Bi-S free cutting steel with excellent hot rolling. The purpose is to provide.

The present invention for achieving the above object in the Bi-S-based free-cutting steel billet, by weight%, C: 0.03 ~ 0.15%, Si: 0.03% or less, Mn: 0.50 ~ 2.0%, S: 0.15 ~ 0.40%, Al: 0.002% or less, [O]: 30-100 ppm, Bi: 0.10-0.20%, B: 30-150 ppm, residual Fe and other unavoidable impurities, and are composed of MnS inclusions having a size in the range of 30-80 μm. Re-heated Bi-S-based free-cutting steel billets having a shear surface area ratio of 1.3-1.9% and a bloom formed as described above with a distance in the range of 10-30 μm in a temperature range of 1250-1350 ° C. Next, the technical gist of the method for manufacturing a bi-S-based free-cutting steel billet which is roughly rolled after the reheated bloom is roughly rolled at a section reduction rate of 50 to 60%.

Description

BILETS FOR Bi-S SYSTEM FREE CUTTING STEEL WITHOUT SPLIT END DURING WIRE-ROD ROLLING AND A METHOD THEREFOR}

The present invention relates to the production of general-purpose Bi-S-based free-cutting steel, and more particularly, to a billet for producing Bi-S-based free-cutting steel excellent in hot rolling without leading edges open.

General purpose Bi-S free cutting steel has good machinability and machinability, so it is widely used in automobile hydraulic parts as well as small parts such as office OA equipment, cameras and watches. The machinability of the Bi-S-based free cutting steel is caused by the low melting point metal, Bi, and the non-metallic inclusions, MnS inclusions, acting as stress concentration sources during cutting. Therefore, in order to improve the machinability of free-cutting steel, appropriate control such as MnS inclusions is required, and appropriate control is required in manufacturing processes such as a continuous casting process and a hot rolling process.

The manufacturing of Bi-S-based free-cutting steel by continuous casting is carried out by reheating a bloom obtained after continuous casting of molten steel, and then rolling the reheated bloom with a billet to produce a wire rod, and rolling the manufactured wire rod. Is done. So far, various methods have been proposed for the manufacturing method of Bi-S-based free cutting steel.

For example, according to Korean Patent Application No. 99-29038, a technique of manufacturing a billet having a surface defect is reduced by appropriately controlling a steelmaking process when manufacturing Bi-S-based free cutting steel. Specifically, this method prevents center segregation by setting the casting speed in the range of 0.6 to 0.7 m / min while adding an appropriate amount of B and controlling the end point oxygen [O] to 25 to 100 ppm in the tundish. The present invention relates to the production of a free cutting steel wire which reduces scratches.

In addition, Korean Patent Publication No. 98-29705 discloses a method for manufacturing Bi-S-based free-cutting steel, in which B is added to minimize particle defects and billet internal defects, and the amount of dissolved oxygen is limited to the range of 5 to 20 ppm. Continuous casting is known.

As another example, according to Korean Patent No. 213334, the surface flaw and the scab are generated during wire rolling by limiting the shear surface fraction, length and width of MnS inclusions and Bi-adsorbed inclusions during Bi-S-based free cutting steel production. A method for producing a wire rod having excellent heat embrittlement resistance is proposed.

However, these methods are the biggest problem in the production of Bi-S-based free-cutting steel is very effective in preventing the surface scratches and center segregation of the billet, but the problem that the hot rolling does not proceed smoothly due to the deepening phenomenon of the billet tip is still present Doing.

In other words, Bi-S-based free-cutting steel has excellent machinability because MnS inclusions are continuously distributed in the matrix structure unlike general carbon steels, but the presence of the inclusions itself causes the embrittlement of the material during hot rolling. The leading edge of the split appears. The split end phenomenon refers to a phenomenon in which the tip end portion of the billet is opened during hot wire rolling. When the end opening phenomenon occurs during wire rolling, it becomes difficult to insert the material into the mill stand, thereby leaving the rolling line. In addition to equipment damage, the material itself has to be scrapped.

The present invention has been proposed to solve such a conventional problem, by appropriately controlling the size and distribution of MnS inclusions in free-cutting steel, and by appropriately adjusting the amount of Al and [O] in the molten steel to eliminate the flaring of the tip and accordingly hot rolling properties An object of the present invention is to provide a billet and a method for manufacturing the excellent Bi-S free cutting steel.

Figure 1a and Figure 1b is a photograph showing the opening phenomenon of the tip portion during the wire rod rolling of the comparative steel and the invention steel billet, respectively

The present invention for achieving the above object in the Bi-S-based free cutting steel billet,

By weight%, C: 0.03 to 0.15%, Si: 0.03% or less, Mn: 0.50 to 2.0%, S: 0.15 to 0.40%, Al: 0.002% or less, [O]: 30 to 100 ppm, Bi: 0.10 to 0.20 %, B: 30 ~ 150ppm, balance Fe and other unavoidable impurities, and the surface area ratio of 1.3 ~ 2.0% with the distance between MnS inclusions in the range of 30 ~ 80㎛ in the range of 10 ~ 30㎛ It relates to a Bi-S free cutting steel billet with no open end portion distributed in the range of.

In addition, the present invention is a manufacturing method of Bi-S-based free cutting steel billet,

Re-heating the bloom having the composition as above in the temperature range of 1250 ~ 1350 ℃, then roughly rolling the reheated bloom at 50 ~ 60% sectional reduction rate, and then free rolling the Bi-S-based free cutting steel It relates to a method for producing a billet.

Hereinafter, the billet of the present invention will be described in detail.

The C contained in the free cutting steel billet of the present invention is preferably limited to the range of 0.03 to 0.15%, because it is difficult to sufficiently secure the strength of the material below 0.03%, and the ductility of the final free cutting steel material is reduced by 0.15% or more. This is because it is not preferable.

In addition, the Si produces SiO ₂ forming MnS inclusions and composite inclusions, and the resulting composite inclusions have a very poor plastic deformation ability, which interferes with the formation of the MnS inclusion layer at the end of the free cutting steel cutting tool, thus adversely affecting the surface roughness. Therefore, it is desirable to manage Si content to 0.03% or less as low as possible.

The Mn is a component that secures the amount of the MnS inclusions and suppresses hot embrittlement due to the formation of FeS at grain boundaries during hot rolling, and at least 0.50% or more should be contained therein. However, when more than 2.0%, the hardness of the steel is increased, leading to a decrease in machinability, which is not preferable.

S is a component that forms the MnS inclusions to improve the surface roughness of the workpiece by the growth inhibition of the build-up edge during cutting of the material, the content should be added at least 0.15%. However, if the S content is more than 0.40%, it is difficult to secure hot workability and cold workability, and the S content is preferably managed in the range of 0.15 to 0.40%.

In the continuous casting of general carbon steel, deoxidizer such as Al is added to remove residual oxygen in steel refining process, but in high-cutting steel, oxygen in molten steel must remain, so oxygen remains in molten steel as much as possible without performing complete deoxidation work. . In the Bi-S-based free cutting steel billet of the present invention, Al is preferably limited to 0.002% or less. When the Al content is added at 0.002% or more, it is not preferable because the phenomenon of opening of the billet tip occurs due to the fine distribution and coalescence of MnS inclusions during wire rod rolling of the billet prepared by the reduction of the oxygen content.

In addition, the oxygen content [O] in the molten steel when manufacturing the billet of the present invention is preferably managed in the range of 30 ~ 100ppm. If the oxygen content in the billet is less than 30ppm, the MnS inclusions are small and finely distributed, and then the tip flare occurs due to the rolling reduction during hot wire rolling of the billet. However, when the oxygen content in the billet exceeds 100 ppm, it is not preferable to secure the plastic deformation capacity of the MnS inclusions during cutting.

The Bi is present around the Bi grains or MnS inclusions to melt and lubricate when friction heat is generated at the tip of the tool during cutting, thereby improving cutting properties. Preferably, at least 0.10% or more of Bi is added. However, if the Bi content is more than 0.20%, hot ductility is lowered, causing hot brittleness, which is not preferable.

B is a component that preferentially segregates around grain boundaries and non-metallic inclusions to suppress void formation and thus improve high temperature ductility, and at least 30 ppm or more should be added thereto. However, when the content of B exceeds 150ppm, precipitates of B are formed in the austenite grain boundary, so that the austenite grain boundary is vulnerable even if Bi, S and the like are not segregated.

On the other hand, the MnS inclusions in the Bi-S free cutting steel billet of the present invention manage the distribution of the MnS inclusions in an appropriate range so as to satisfy the machinability and hot rolling property of the material at the same time. Specifically, the shear surface area ratio is distributed in the range of 1.3 to 2.0% at a distance of 10 to 30 μm in the size of the MnS inclusions in the range of 30 to 80 μm.

If the size of MnS inclusion is too small, the role of stress concentration is reduced, and the cutting effect is decreased. On the contrary, if too coarse, the surface of the product is easily embrittled because it acts as a large volume of flaw during hot rolling. I can't.

In addition, when the inclusions are distributed too closely, cracks due to embrittlement between the inclusions and the base material are generated to promote the spreading of the material. When the inclusions are too far, chips are cut under stress by a tool during cold processing of the product. It worsens and machinability falls and is unpreferable.

In addition, when the area ratio of shear surface of inclusions is less than 1.3%, the amount of inclusions is small, and the effect of improving machinability is insufficient due to grain boundary embrittlement due to stress concentration during machining, and when it exceeds 2.0%, hot workability is weakened by defects. It is not preferable because it can be made.

Hereinafter, the manufacturing method of the present invention will be described in detail.

Although the blown formed as described above is obtained in the continuous casting process, the MnS inclusions in the bloom cause stretching along with the reduction of the cross section during the rolling of the steel sheet. If the stretching is insufficient during this process, cracking occurs at the interface between the MnS and the base metal. Or the phenomena of the tip end are generated. The continuous failure of MnS inclusions in free-cutting steel is different depending on the temperature and section reduction rate during rolling.

In the present invention, in order to form an appropriate range of MnS inclusions, first, the reconstituted bloom is reheated at a temperature range of 1250 to 1350 ° C., and then the reheated bloom is roughly rolled at a cross-sectional reduction rate of 50 to 60%. Roll.

When the reheating temperature is 1250 ° C. or less, the ductility of the material rapidly decreases due to the temperature drop during the rough rolling and finishing rolling, thereby facilitating bursting or spreading of the surface portion, making it difficult to produce the billet. In other words, when the Bi-adsorbed MnS inclusions in the free-cutting steel billet are deformed at low temperatures, the stretching is insufficient and causes embrittlement with the surroundings. Therefore, the higher the reheating temperature is better, but there is no problem when heating up to the austenite region, but it is not preferable to raise the temperature to too high in the specification of the steel sheet furnace.

In addition, it is preferable to perform the rough rolling of the slab in the range of 50 to 60%, but if the reduction of the cross section is less than 50%, at least 40% of the reduction in the finishing rolling after the rough rolling is not good. If the reduction rate of the cross section is more than 60%, it is not only difficult to maintain the dimensions in accordance with the exact shape of the billet, but also becomes unreasonable to the equipment.

Through this manufacturing process, the size of MnS inclusions in free-cutting steel billets can be adjusted in the range of 30 to 80 μm, and the distribution of MnS inclusions is in the range of 1.3 to 2.0% with a shear surface area of 10 to 30 μm. You can do it by mine.

Hereinafter, the present invention will be described in detail through examples.

Example 1

The molten steel was refined to have a composition as shown in Table 1 and continuously cast the refined molten steel, followed by rolling the cast bloom to prepare 160 square billets. At this time, the steel rolling re-heated the size of the 250 × 330mm bloom at 1310 ℃, and reduced the reheated bloom at a rate of reduction of 56.2% in the steel-rolled rolling, and produced 160 angle billet through finishing rolling. After cutting each billet tip prepared in this way, the cross section of the tip was observed, and the size, average distance, and shear area ratio of the MnS inclusions were examined, and the results are shown in Table 1.

In addition, after cutting each billet by operating the shear between the rolling stand (crop shear) (roll shear) to cut the tip end, the phenomenon of the open end of the cut wire was observed and the results are shown in Table 1. In addition, the front-end | tip part of the billet of a comparative steel and an invention steel is respectively shown in FIG.

division Chemical composition (% by weight) Experiment result C Mn S Si Bi B Al [O] Average shear area fraction (%) Average distance from nearest inclusions (㎛) Average size of inclusions (㎛) Opening of the end of wire rod Comparative steel 12345678 0.0870.0600.1000.0800.0910.0930.0830.087 0.940.981.000.951.121.111.131.20 0.320.290.270.300.290.300.310.30 0.010.010.010.020.030.030.020.01 0.140.150.110.150.120.100.150.04 3146492650413975 0.0040.0030.0030.0040.0040.0030.0030.003 212920722111512 0.70.91.01.10.80.71.21.0 5.02.53.02.04.55.02.515 19.612.315.611.315.814.614.916.4 ＯＯＯＯＯＯＯＯ Invention steel 12345678 0.0570.0550.0710.0600.0700.0710.0680.049 1.301.401.341.351.311.321.321.34 0.300.280.300.300.290.290.290.29 0.010.010.020.030.010.010.020.01 0.130.140.140.120.120.090.100.11 41221008097796063 0.002-0.0010.0010.0020.002-0.001 4433354538344574 1.81.51.61.91.31.61.81.7 13.822.120.713.113.814.511.026.2 45.337.454.938.178.066.441.351.3 ×××××××× O: billet tip flaring occurs, x: not generated

As shown in Table 1, in the present invention steel (1-8) having a steel composition and inclusions according to the present invention there is no phenomenon in the distal end of the wire rods, while hot wire rolling is smoothly performed, while comparative steel (1-8) ), It can be seen that the phenomenon of opening at the tip of the wire rod is observed. This phenomenon has also been confirmed in Figs. 1A and 1B, which show the leading edges of the comparative steel and the inventive steel, respectively.

Example 2

160 square billets were prepared by rolling the steel sheet using the inventive steel 4 and the comparative steel 5 of Example 1 while changing the manufacturing conditions as shown in Table 2 below. For each billet manufactured, the surface condition was visually observed along with the bulging phenomena of the billet, and the results are shown in Table 2 below.

division Steel grade used Rough rolling Sasang Rolling Experiment result Billet's tip bulge Surface condition Reheating Temperature (℃) Rough rolling length (mm) Reduction rate (%) Reduction rate (%) Final dimension (mm) Rough rolling Sasang Rolling Invention 1234 Inventive Steel 4 1310127013201260 190188193183 56.257.254.859.4 29.127.631.323.6 160 ×××× ×××× Good good good good good Comparatives Comparative Steel 5 12201200124512301300125513101290 191190184183215219220211 55.856.259.059.444.041.941.346.0 29.829.124.423.644.646.647.142.5 100 ×××× ×××× 0000 Good Bad Good Bad Good Good Good Good 0: tip flare occurs, x: not generated

As shown in Table 2, in the case of the inventive material (1-4) obtained according to the manufacturing conditions of the present invention, it was found that not only the phenomenon of opening at the tip of the billet but also the surface state of the billet was very good.

On the other hand, in the case of the comparative material (1-8), even if the tip of the billet does not open, or the tip does not open, the surface state is very poor.

As described above, according to the present invention, the tip opening of the free-cutting steel billet does not occur by appropriately controlling the size and distribution of MnS inclusions in the Bi-S-based free-cutting steel billet, and by appropriately adjusting the amount of Al and [O] in the molten steel. Accordingly, there is a great effect of producing a Bi-S-based free-cutting steel having excellent hot rolling property very easily.

Claims (2)

In Bi-S free cutting steel billet, By weight%, C: 0.03 to 0.15%, Si: 0.03% or less, Mn: 0.50 to 2.0%, S: 0.15 to 0.40%, Al: 0.002% or less, [O]: 30 to 100 ppm, Bi: 0.10 to 0.20 %, B: 30 ~ 150ppm, balance Fe and other unavoidable impurities, and the surface area ratio of 1.3 ~ 2.0% with the distance between MnS inclusions in the range of 30 ~ 80㎛ in the range of 10 ~ 30㎛ Bi-S free-cutting steel billet with no openings at the end of wire rods, characterized in that it is distributed within the range of In the manufacturing method of Bi-S-based free cutting steel billet, By weight%, C: 0.03 to 0.15%, Si: 0.03% or less, Mn: 0.50 to 2.0%, S: 0.15 to 0.40%, Al: 0.002% or less, [O]: 30 to 100 ppm, Bi: 0.10 to 0.20 %, B: 30 ~ 150ppm, remainder Fe and other unavoidable impurities, reheat the boil in the temperature range of 1250 ~ 1350 ℃, then roughly roll the reheated boil at 50 ~ 60% of section reduction rate Method for producing Bi-S free-cutting steel billet without gap at the end of wire rod rolling