KR100285520B1 - METHOD FOR RESTRAINING FINE CRACKS ON SURFACE OF BISMUTH (Bi)-SULFUR (S) BASED FREE-CUTTING STEEL WIRE ROD - Google Patents

Abstract

PURPOSE: A method is provided which effectively restrains fine cracks on the surface of a bismuth (Bi)-sulfur (S) based free-cutting steel wire rod. CONSTITUTION: In a method for restraining fine cracks on the surface of the wire rod by hot rolling into a wire rod a Bi-S based free-cutting steel billet containing 0.05 to 0.15 wt.% of C, 0.5 to 2.0 wt.% of Mn, 0.15 to 0.40 wt.% of S, 0.01 to 0.10 wt.% of P, 0.003 to 0.020 wt.% of O, 0.03 to 0.30 wt.% of Bi, 0.01 wt.% or less of Si, 0.0009 wt.% of Al and a balance of Fe, a section area fraction of a MnS inclusion into which MnS and Bi are adsorbed is from minimum 0.5% to maximum 2.0%, a length of the inclusion is from 5 to 20 microns, a width of the inclusion is from 1 to 10 microns, and a section area fraction of a metallic Bi inclusion is minimum 0.03% to maximum 0.30%, the method for restraining fine cracks on the surface of a bismuth (Bi)-sulfur (S) based free-cutting steel wire rod comprises the process of rolling the heated billet at a temperature of 900 deg.C or more into a wire rod so as to reduce fine cracks on the surface of the wire rod after heating the billet at the temperature range from 1050 to 1150 deg.C for one hour to one hour and thirty minutes so that a formation layer of FeS-FeO in grain boundary on the surface of the billet is maintained to a thickness of less than 0.08 mm.

Description

Fine crack suppression method of bismuth (Bi) -sulfur (S) based free cutting steel wire surface

The present invention relates to a method for suppressing fine cracks on the surface of a Bi-S based free cutting steel wire.

Free-cutting steel is a general-purpose material for processing small precision parts such as cameras and watches that require excellent machinability. Moreover, in recent years, lead (P) -sulfur (S) -based free-cutting steel has been developed in which sulfur (S) -based free-cutting steel has been added to elements such as lead (Pb) to improve machinability. It has been put to practical use as a steel suitable for high speed and automation in cutting. However, because lead oxide is toxic, the steel developed to solve it is a bismuth (Bi) -sulfur (S) -based free cutting steel.

Cracks on the surface of hot rolled wire can be classified into two types. One of them is a large scab-shaped crack, 5-10 mm in length, with a uniform spacing of more than a few hundred millimeters. Another defect is a fine crack slanted obliquely in the longitudinal direction of the wire rod, which is 1 millimeter (mm) in length, and its spacing is about several hundred micrometers (µm), and is somewhat regular. Scab-shaped cracks are grown through pin / blow holes of some of the many fine cracks. Nevertheless, in the past, the mechanism of generating fine cracks and growing micro cracks into large scab-shaped cracks has not been known, and the only method was to suppress the formation of cracks due to hot embrittlement by managing the rolling temperature high. .

It is an object of the present invention to provide a method for effectively suppressing fine cracking of a Bi-S based free cutting steel wire surface.

An object of this invention is to heat the billet at a grain boundary of less than 1050, at least 1 hour and at most 1 hour and 30 minutes at a temperature of 1150 ° C or less so that the formation layer of FeS-FeO is less than 0.08 millimeters at the grain boundary. A method of reducing fine cracks on the surface of the wire by rolling the wire at 950 ° C. or higher, and removing the surface of the billet by 5 mm or more so that the formation layer of FeS-FeO is less than 0.08 mm at the grain boundary at the billet surface. And heating at a temperature of 1250 ° C. for 1 hour or more and for 2 hours 30 minutes or less and then rolling the wire at 950 ° C. or more to reduce fine cracks on the surface of the wire.

1 shows the chemical composition (weight percentage) of a bismuth (Bi) -sulfur (S) based free cutting material used in the present invention;

FIG. 2 shows an optical micrograph showing a fine crack (,!) Formed on the surface of a bismuth-sulfur free cutting steel wire; FIG.

3 shows electron micrographs showing scales (SCALE ,, "), FeS-FeO process (, #) and MnS (, $) formed on the surface of bismuth-sulfur free cutting steel material immediately after extraction of a furnace;

4 is a view showing an optical micrograph showing the (scale ,, ") and FeS-FeO process (, #) formed on the surface of the Bi-S-based free cutting steel immediately after the heating furnace extraction;

(a) is a surface non-abrasive, heating conditions: the drawing showing the case of extraction after 2 hours 30 minutes heating at 1250 ℃;

(b) is a view showing a case of extraction after heating for 1 hour and 30 minutes at a surface 5 mm abrasive, heating conditions: 1250 ℃;

(c) is a surface non-abrasive, heating conditions: the drawing showing the case after extraction for 1 hour 30 minutes at 1150 ℃;

(d) is a surface 5mm abrasive, heating conditions: the drawing showing the case after extraction for 1 hour 30 minutes heating at 1150 ℃.

5 is a table showing the depth of the FeS-FeO process (, #) formed on the surface of the bismuth-sulfur free cutting steel material according to the surface grinding and heating conditions and the degree of occurrence of minute cracks on the hot-rolled wire surface.

In the present invention, carbon (C): 0.05-0.15, manganese (Mn): 0.5-2.0%, sulfur (S): 0.15-0.40, phosphorus (P): 0.01-0.10, oxygen (O): 0.003- Shear face fraction of MnS inclusions adsorbed with MnS and bismuth containing 0.020%, bismuth (Bi): 0.03-0.30, silicon (Si): 0.01% or less, aluminum (Al): 0.0009% and the remaining iron (Fe) (section area fraction%) with a minimum of 0.5%, a maximum of 2.0%, a length of 5-20 micrometers, and a width of 1-10 micrometers, and a shear fraction of metallic bismuth inclusions at a minimum of 0.030% and a maximum of 0.30% In hot rolling a Bi-S-based free-cutting steel billet containing the inclusion shape and distribution of the wire into a wire rod, at a temperature of 1050 ° C. or less and 1150 ° C. or more so that the formation layer of FeS-FeO is less than 0.08 mm at the grain boundary at the billet surface. A method of reducing fine cracks on the surface of a wire by heating the billet for a time of more than 1 hour and 30 minutes or less and then rolling the wire at 950 ° C. or more. After removing 5 mm or more of the surface of the billet so that the formation layer of FeS-FeO is less than 0.08 mm at the grain boundary at, it is heated at a temperature of 1050 ° C. or more and 1250 ° C. or more for 1 hour or more and 2 hours and 30 minutes or less. The present invention relates to a method of reducing fine cracks on the surface of a wire by rolling the wire over 950 ° C.

Free-cutting steel is a non-metallic or metallic inclusion in the steel to improve the machinability. The representative non-metallic inclusion is MnS, and the metallic inclusions have low melting points such as lead (Pb) and bismuth (Bi), which have little solidity in the steel. Metal. These inclusions act as stress concentrators during cutting, which facilitates the generation of voids and cracks at the interface, reducing the force required for cutting, and softening or melting by cutting. It acts as a lubricant at the interface between the chip and the cutting tool, thus reducing tool wear and reducing cutting forces.

Sulfur added to form MnS is supersaturated when there is not enough manganese content or if the content of manganese is sufficiently solidified or cooled. In the continuous casting, the main surface has a solidified surface with a constant thickness of a certain thickness, and then a columnar tablet is formed therein, and MnS is determined between the columnar tablets. But in a rapidly solidified organization called chill, sulfur is not defined as MnS and is supersaturated. Sulfur supersaturated in Ichijungjung precipitates as MnS during cooling or reheating, and the remainder precipitates as FeS causing segregation at grain boundaries and hot brittleness. In addition, the seven tablets supersaturate not only sulfur but also gas components with little solid solubility in the steel, and these gas components exceeding the solid solubility in solids remain as fine bubbles (pin / blow holes). On the other hand, low-melting metals with little solid solubility in steel tend to segregate at grain boundaries in general carbon steels that do not intentionally produce MnS inclusions, resulting in grain embrittlement, whereas in free cutting steels that intentionally produce MnS inclusions, they are adsorbed to MnS inclusions. As it is crystallized, hot embrittlement occurs at the interface between the MnS inclusion and the iron.

Defects on the hot rolled wire surface include micro cracks and scab-shaped large defects (cracks) in which these micro cracks have grown. In order to suppress the generation of such defects, first, the production of microcracks must be suppressed. This fine crack occurs because sulfur, which has been supersaturated in the settlement of the billet surface, segregates to the grain boundary, and the segregated sulfur precipitates as FeS of low melting point and develops into a crack during hot rolling. Therefore, it is possible to suppress the formation of fine cracks by suppressing the production of FeS or by removing the seventh tablet in which FeS is generated.

Hereinafter, the chemical composition in terms of securing high temperature ductility and machinability of Bi-S-based free cutting steel, the shape of MnS inclusions adsorbed by MnS and Bi, and the polishing conditions of the surface of the billet for hot rolling are defined. The reason and the reason which limited the heating conditions of a billet are demonstrated.

The reasons for limiting chemical composition are as follows.

Carbon (C): To secure the surface roughness and mechanical properties of the workpiece, it should be added at least 0.05%. However, if it exceeds 0.15%, the machinability is inferior due to the increase of hard pearlite phase.

Manganese (Mn): At least 0.5% should be added to secure the required amount of Mns inclusions and to suppress hot embrittlement due to the formation of FeS at grain boundaries during hot rolling. However, it should not exceed 2.0%. This is because the hardness of the steel is increased, resulting in a decrease in machinability.

Phosphorus (P): Must be 0.01% or more to improve the surface roughness of the workpiece. To ensure mechanical properties and cold workability, it should not exceed 0.1%.

Sulfur (S): 0.15% or more should be added to form Mns inclusions to improve the surface roughness of the workpiece by inhibiting growth of BUE (build-up-edge). However, in order to secure hot workability and cold workability, it should not exceed 0.4%.

Oxygen (O): It should be added at least 0.003% to prevent machinability deterioration by stretching MnS inclusions during hot rolling. However, in order to ensure plastic deformation of MnS inclusions during cutting, it should not exceed 0.020%.

Bismuth (Bi): Bismuth is adsorbed in bismuth alone or in MnS in steel and thus acts as a stress concentration source during cutting, thereby reducing chip curvature and improving chip treatability. Moreover, bismuth has the effect of improving the surface roughness of the workpiece by increasing the area of the MnS inclusion layer on the cutting tool surface. In addition, bismuth is melted in the cutting heat to reduce friction between the chip and the cutting tool and to suppress wear of the cutting tool. Therefore, less than 0.02%, the machinability is inferior. However, if it is more than 0.030%, hot workability will become very bad.

Silicon (Si): Produces SiO ₂ to form MnS inclusions and composite inclusions. Since plastic deformation of such composite inclusions is very bad, the formation of the MnS-based inclusion layer at the end of the tool easily causes the BEU to adversely affect the surface roughness of the workpiece. Therefore, the concentration of silicon should be kept as low as possible to not exceed 0.003%.

Aluminum (Al): Al ₂ O ₃ is easy to form Mns inclusions and composite inclusions. Since the composite inclusions have very bad plastic strains as in the SiO ₂ composite inclusions of MnS, the composite inclusions interfere with the formation of the MnS-based inclusion layer at the end of the tool, thereby easily forming the BEU, which adversely affects the surface roughness of the workpiece. Al ₂ O ₃ is also very hard, which promotes wear of cutting tools. Therefore, it should not exceed 0.0009%.

The reasons for limiting the content and shape of the inclusions are as follows.

If the shear surface fraction of the MnS inclusions adsorbed with MnS and Bi is 0.5% or less, the machinability is poor, and the high temperature embrittlement mechanism transitions from interfacial embrittlement of grain iron and MnS to grain boundary embrittlement. If it is 2.0% or more, hot workability will fall. If the metallic bismuth (Bi) inclusions have a shear surface fraction of 0.030% or less, the machinability is poor, and if it is 0.30% or more, hot workability is inferior. If the size of the inclusion is too small, the role as a stress concentration source is reduced, so that there is no cutting effect and also does not serve as a source of voids during hot working. If too coarse, machinability and hot workability will be inferior. Also, if the aspect ratio (length / wide) is too large or too small, the machinability is poor.

When deformed, the void becomes a source of generation.

The reason for limiting polishing conditions of the billet surface is as follows.

In the casting state, the surface of the cast steel (chill) is supersaturated with sulfur and manganese, and fine pores are formed therein. Some of the supersaturated sulfur (S) in the chiljung is combined with supersaturated manganese to precipitate fine MnS, and the remaining supersaturated sulfur is segregated at grain boundaries and precipitated as FeS of low melting point. This FeS is oxidized during reheating to a FeS-FeO process of lower melting point. Since the FeS-FeO process has a low melting point, it acts like a crack during hot rolling. Ichijung is reduced to about 5mm thick when hot rolled into billets. Therefore, when the surface of the billet is polished to 5 mm or more, the FeS-FeO layer and the fine pores, which acted as fine cracks, are removed, thereby suppressing the formation of fine cracks on the hot-rolled wire surface.

The reason for limiting the heating conditions of the billet is as follows.

Unpolishing agent does not completely crack the billet when the heating conditions are 1050 ℃ or less, less than 1 hour, so the rolling temperature falls below 950 ℃ and the ductility of the base material is easy to cause surface defects. If the heating time is higher than 1150 ° C. or longer than 1.5 hours, the FeS—FeO layer on the surface of the billet becomes deep. In the case of 5mm abrasive, the FeS-FeO layer on the surface of the billet becomes deeper at 1250 ℃ or more and 2.5 hours or more, and at 1.0 hours or less at 1050 ℃ or less, the billet is not completely cracked. .

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

The material used in the present invention is a billet containing 1.09% of manganese (Mn), 0.32% of sulfur (S), and 0.14% of bismuth (Bi) as a representative component, as shown in FIG. 1, and a volume fraction of MnS and bismuth inclusions. The volume fraction of only 1.5% and bismuth (Bi) inclusions is 0.2%. The inclusions were 5-15 micrometers long and 5-15 micrometers wide, with a length-to-width ratio of 1-2.

FIG. 2 is an optical micrograph showing a fine crack (,!) Formed on a conventional Bi-S-based free cutting steel wire surface. FIG. Fine cracks slanted on the surface of the wire rod are 0.2 mm long and their spacing is about 0.4-0.5 mm. Such cracks are closely related to the grain boundary FeS—FeO layer as shown in FIG. 3. Figure 3 is an electron micrograph showing the scale (","), FeS-FeO process (, #) and MnS (, $) formed on the surface of the Bi-S-based free-cutting steel material immediately after extraction of the heating furnace. The melting point of the FeS-FeO process is very low, about 900 ° C, and thus plays a crack-like role at a normal hot rolling temperature (900-1100 ° C).

Figure 4 is an optical micrograph showing the scale (",") and FeS-FeO process (, #) formed on the surface of the Bi-S-based free-cutting steel material immediately after extraction of the heating furnace. 2 hours and 30 minutes at 1250 ° C., (b) 1 hour and 30 minutes at 1250 ° C. after grinding 5 mm of the billet surface, (c) 1 hour and 30 minutes at 1150 ° C., (d) ) Shows the depth of scale (, ") and FeS-FeO (, #) formed on the surface when the billet surface was polished 5 mm and then heated at 1150 ° C. for 1 hour and 30 minutes. When heated at 1250 ° C. for 2 hours and 30 minutes without polishing the surface, the FeS-FeO layer appears as deep as about 0.15 millimeters, but when the temperature is lowered and the time is shorter, the FeS-FeO layer becomes thinner. In addition, the surface of the billet is polished to remove the supersaturated lacquer of sulfur, resulting in a thinner FeS-FeO layer.

FIG. 5 shows the degree of fine cracks occurring on the surface of the wire when the surface of the billet is polished, extracted after heating while being heated, and then rolled into the wire in hot. If the surface of the billet is heated at 1250 ° C for at least 1 hour and 30 minutes without rolling, then the wire is rolled into the wire rod, and the fine crack is reduced enough to be accepted as a genuine product. After polishing the surface of the billet by 5 millimeters to remove the sulfur-saturated lacquer, even though the heating temperature was as high as 1250 ° C. and the heating time was about 2 hours 30 minutes, there were almost no minute cracks on the surface of the hot rolled wire.

Therefore, when polishing the surface of the billet to control the coating or heating at low temperature and short time to ensure the lowest rolling temperature, the FeS-FeO layer becomes thin and fine cracks are suppressed on the surface of the hot rolled wire. Can be.

According to the present invention, there is an effect of suppressing fine cracks on the surface of the Bi-S free cutting steel wire.

Claims (2)

By weight percentage, carbon (C): 0.05 to 0.15%, manganese (Mn): 0.5 to 2.0%, sulfur (S): 0.15 to 0.40%, phosphorus (P): 0.01 to 0.10%, oxygen (O): 0.003 to 0.020%, bismuth (Bi): 0.03 to 0.30%, silicon (Si): 0.01% or less, aluminum (Al): 0.0009% and the remaining cross-sectional fraction of MnS inclusions containing MnS and bismuth adsorbed (section area fraction%) with a minimum of 0.5%, a maximum of 2.0%, a length of 5-20 micrometers and a width of 1-10 micrometers, and a cross-sectional fraction of metallic bismuth (Bi) inclusions with a minimum of 0.030% and a maximum of 0.30% A method of hot rolling a Bi-S based free cutting steel billet containing inclusion shapes and distributions with wire rods to suppress fine cracks on the wire surface, wherein the formation layer of FeS-FeO is less than 0.08 millimeters at grain boundaries at the billet surface. The billet is heated at a temperature of not less than 1 ° C. and not more than 1,150 ° C. for at least 1 hour and not more than 1 hour 30 minutes, and then at By rolling on a wire to reduce the fine cracks in the pre-existing surface method for inhibiting the micro cracks of the Bi-S based free cutting steel wire surface. By weight percentage carbon (C): 0.05 to 0.15%, manganese (Mn): 0.5 to 2.0%, sulfur (S): 0.15 to 0.40%, phosphorus (P): 0.01 to 0,10%, oxygen (O): 0.003% to 0.020%, bismuth (Bi): 0.03% to 0.30%, silicon (Si): 0.01% or less, aluminum (Al): 0.0009% and the remaining iron (Fe), and MnS and bismuth adsorbed MnS inclusions Section area fraction is at least 0.5%, at most 2.0%, length is 5-20 micrometers, width is 1-10 micrometers, and cross-sectional fraction of metallic bismuth inclusions is at least 0.030%, at most 0.30% In the method of hot rolling a Bi-S based free cutting steel billet containing the inclusion shape and distribution of the wire rod to suppress fine cracks on the surface of the wire rod, the formation layer of FeS-FeO at the grain boundary at the billet surface is less than 0.08 mm. After removing the surface of the billet 5 mm or more, at least 1 hour, 2 hours 30 minutes at a temperature of 1050 ℃ or more, 1250 ℃ or less Method under heating for hour, and then rolled on a wire at least 950 ℃ to suppress the micro cracks of the Bi-S based free cutting steel wire surface to reduce fine cracks in the pre-existing surface.