KR100320958B1 - Method for manufacturing free cutting hot tool steel - Google Patents

Abstract

PURPOSE: A method for manufacturing free cutting hot tool steels excellent in machinability, heat fatigue resistance at high temperatures and impact properties is provided. CONSTITUTION: The method for manufacturing free cutting hot tool steels includes step of preparing a molten steel comprising C 0.1 to 1.0 wt.%, Si 0.2 to 0.8 wt.%, Mn 0.3 to 4.0 wt.%, Ni 0.001 to 4.0 wt.%, Cr 1.5 to 6.0 wt.%, Mo to 4.0 wt.%, V 0.1 to 3.5 wt.%, Al 0.005 to 0.1 wt.%, at least one element selected from W 0.005 to 0.2 wt.%, Zr 0.005 to 0.2 wt.%, Nb 0.005 to 0.2 wt.%, Te 0.001 to 0.25 wt.%, Ti 0.01 wt.% or less and Co 0.001 wt.% or less, a balance of Fe and incidental impurities; performing vacuum oxygen decarburization at a vacuum condition of less than 1x10¬-1 torr for both desulfurization and decarburization; injecting either one element from light rare earth metal, light rare earth metal together with Ca, one element from mischmetal or mischmetals together with Ca into the pretreated molten steel at either one moment right after vacuum oxygen decarburization, one moment right before molten steel pouring, on the course of molten steel pouring into bottom or on the course of molten steel pouring into mold; cooling a molten steel including light rare earth metal and Ca at 1,600±50°C to prepare ingots; hot rolling the ingots at 1,000 to 1,300°C followed by forging; spheroidizing the forged tool steel at 840 to 870°C for more than 60 minutes 60/25.4mm; quenching the spheroidized tool steel at 1,000 to 1050°C for more than 30 minutes/25.4mm; and tempering the quenched tool steel at 200 to 650°C for more than 60 minutes/25.4mm.

Description

Manufacturing method of free cutting hot oral steel with excellent high temperature thermal fatigue characteristics and impact characteristics

[Field of Invention]

The present invention relates to a method for manufacturing a high-quality hot-rolled oral cavity, and more particularly, to a method for manufacturing a high-temperature hot-rolled section having excellent cleanability, high temperature fatigue fatigue characteristics and impact characteristics, and low anisotropy of impact value, as well as excellent cutting property. will be.

[Prior art]

As the usage conditions of hot oral cavity become more and more severe and high speed, the mold material tends to be damaged prematurely due to thermal fatigue crack generation, propagation, and high temperature softening due to severe thermal fatigue shock, so the improvement is strongly required. In addition, due to the characteristics of the hot-hole oral cavity, cutting or machining is difficult, and thus, it is difficult to produce a mold.

In addition, it is impossible to satisfy all the opposing characteristics such as thermal fatigue, impact and machinability with the existing hot-drilled STD61 or STD61. Gave.

The present invention has been made in order to solve the above problems, to improve the problem that the mold material is damaged early in hot extrusion, press, forging, etc. to provide a high-life and excellent cutting hot work oral to the company It aims to do it.

1 is a photograph comparing the chip treatability of the inventive steel and the comparative steel according to an embodiment of the present invention

In order to achieve the object of the present invention as described above, the free cutting hot oral cavity of the present invention has a chemical composition in weight% of C: 0.1% to 1.0%, Si: 0.2% to 0.8%, Mn: 0.3% to 4.0%, Ni: 0.001% to 4.0%, Cr: 1.5% to 6.0%, Mo: 0.3% to 4.0%, V: 0.1% to 3.5%, Al: 0.005% to 0.1%, and W: 0.005% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, Ti: 0.01% or less, Co: 0.01% or less, Te: 0.001% to 0.25%, and the balance of Fe and electricity Preparing a molten steel including trace impurities that may be contained during steelmaking; Vacuum refining the molten molten steel in a vacuum atmosphere to perform preliminary desulfurization and preliminary deoxidation; Adding light rare earth elements (Ce, La, Nd, Pr, Y, etc., hereinafter referred to as REM) to the deoxidized and desulfurized molten steel alone or adding a combination of light rare earth elements and Ca; Preparing a steel ingot by cooling the molten steel to which the light rare earth element and Ca are added; Forging, spheroidization, quenching, and tempering the prepared ingot is characterized in that it comprises a step.

Hereinafter, the chemical composition range of the inventive steel is limited to the weight percent, and the reason for limitation is described.

C: C is an essential element to secure high temperature wear resistance and high temperature softening resistance in hot oral cavity by increasing the hardness and strength by solid solution at the base. In order to secure the above effects, it is essential to add more than 0.1%, and limited to less than 0.1% because the impact properties and machinability is lowered at the time of excessive addition.

Si: Effectively acts as a deoxidizer and increases A ₁ transformation point and imparts oxidation resistance. However, it does not exceed 0.8% because it impairs the machinability and impact characteristics of the material when excessively added, and 0.2% or more since the desired effect cannot be obtained when a small amount is added. It is limited to.

Mn: In addition to deoxidation and desulfurization effects during steelmaking, it is desirable to actively add in order to secure hardenability and to form an insulating film. However, when excessively added, the A ₁ transformation point is lowered, so that the annealing hardness is increased, the machinability is lowered, the residual austenite is increased, and the hot workability is deteriorated. In addition, since the quenchability decreases when a small amount is added, it is limited to 0.3% or more.

Ni: As an element that strengthens the matrix structure and enhances the hardenability, it is less than 4.0% because it impairs the impact characteristics and machinability when it is added excessively, and it is limited to 0.001% or more because it is difficult to secure the uniform hardness and hardenability when the small amount is added.

Cr: An element that forms fine carbonitrides and imparts quenching and oxidation resistance. It increases high temperature wear resistance, high temperature softening resistance, and high temperature strength, but excessive addition of Cr carbide damages cutting and high temperature strength. It should be less than%, and it is limited to more than 1.5% because the above effects cannot be obtained when a small amount is added.

Mo: It promotes bainite transformation and produces stable residual austenite and forms carbide, and it is under 4.0% because it increases high temperature softening resistance and high temperature wear resistance, but damages cutting and impact toughness when added excessively. Since effects cannot be expected, it is limited to 0.3% or more.

V: Combines with C and N to form fine carbonitrides to improve high temperature abrasion resistance, high temperature softening resistance and impact characteristics, but it is not economical compared to the effects of excessive addition, and it forms macrocarbide (VC) to form impact characteristics and high temperature The strength is lowered to 3.5% or less, and it is limited to 0.1% or more because it is difficult to secure the above effects when a small amount is added.

W: 0.005% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, Ti: 0.01% or less, all of which are carbide forming elements to ensure high temperature wear resistance and high temperature softening resistance. It is important to add at least one kind.

In more detail, the reason for the numerical limitation of the carbonization forming elements is explained in detail. W: is dissolved in a matrix to improve high temperature strength, form WC carbide to improve high temperature wear resistance, and control austenite grain growth at high temperature. It is limited to 0.005% to 0.2% because it contributes to grain refinement and improves impact toughness. When 0.2% or more is added, a large primary MC (WC) type process carbide is formed during solidification, and the WC process carbides are clustered to form a band layer by hot plastic processing, which adversely affects the impact toughness, and is added below 0.005%. You can not expect the above effects.

In addition, Nb: Nb is easy to combine with C, such as V, in the present invention to create a light carbide in the form of V and complex carbide (Nb · V) C and (Nb) CN carbon nitride combined with residual N to improve wear resistance Since it is an effective element in controlling austenite grain growth at high temperature and improving toughness, it is limited to 0.005%-0.2% of range. The addition of 0.2% or more results in the formation of a large primary MC (NbC) type eutectic carbide upon solidification and its NbC eutectic carbides in the toughness caused by the band layer, and when added below 0.005%, the above effect cannot be expected.

Co: Co is a fully solid solution element in the base and does not cause austenitic grain coarsening. It is possible to increase the temperature, so it is effective to increase the heat resistance and improve the wear resistance in the high-speed wear zone, and to secure high impact toughness even at high hardness. It is limited to 0.01% or less because it is not economical when 0.01% or more is added as an expensive alloy element and there is almost no formation of a second phase as a complete solid solution element in a matrix.

Ti: Ti is an element effective in preventing grain boundary embrittlement due to segregation of the glass N to grain boundaries during heat treatment by fixing the glass N as a grain refinement and a strong carbonitride (TiCN) forming element. If it is 0.01% or more, it forms a huge TiC and forms a band layer, thereby reducing the toughness, so it is limited to 0.01% or less.

Zr: Zr is added as trace alloy element rather than deoxidizer. It improves toughness and ductility by grain refinement, improves hot strength, improves machinability as a compound with S, and above all, is effective in preventing red brittleness. If it is added below 0.005%, the effect cannot be expected and it is not economical when added above 0.2%. It is limited to 0.005% to 0.2% because it forms two-phase particles such as excessive oxides and thus adversely affects toughness and ductility.

Al: As an element with deoxidation effect, grain refining effect, and nitriding effect, it is added to maximize REM and Ca addition effect. In case of excessive addition, a large amount of Al ₂ O ₃ is formed to decrease the effect of adding REM and Ca. Therefore, the addition amount is limited to 0.1% or less.

Te: It is a major element that does not change the shape of sulfides but forms a process with sulfides to lower the melting point of nonmetallic inclusions, thereby improving machinability. However, the addition of more than 0.25% can not expect a great effect compared to the addition amount and is not economical.

Ca: A clean element and a non-metallic inclusion shape control element, which is a major element for improving machinability, and is limited to 0.001% to 0.1%. However, compared with the final amount of S, Ca% / S% = 1 or less. If the value is less than 0.001%, the shape control effect and the cutting property improvement effect of the non-metallic inclusions are not seen. If the content is more than 0.1% and Ca% / S% = 1 or more, the REM additive effect is inhibited, so the limit value is set.

Light Rare Earth Element (REM): As a clean element of steel and shape control element of non-metallic inclusions, it improves solidification structure, improves impact characteristics, machinability, high temperature thermal fatigue characteristics, and reduces anisotropy of impact value. However, when the addition amount is not sufficient, the addition effect is not obtained as well as the economical amount, so the final REM amount is limited to 0.25% or less, and when the small amount is added, it completely controls the sole generation of harmful nonmetallic inclusions such as Al ₂ O ₃ and MnS. Since it is impossible, it should be over 0.0002%. However, compared with the final amount of S, it is limited to REM% / S% = 2 to 90 and [REM%] [S%] = 3 × 10 ⁻⁵ -100 × 10 ⁻⁵ .

When REM% / S% = 2 or less and [REM%] [S%] = 3 × 10 ^-5 or less, the shape control of nonmetallic inclusions is not sufficient, REM% / S% = 90 or more, [REM%] [S When%] = 100 × 10 ^{-5 or} more, the deoxidation and desulfurization effect as large as the amount of addition is not seen, it is not economical, and it adversely affects the characteristics. The remaining amount is trace impurities that may be contained during steelmaking with Fe and electricity.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention is to change the chemical composition of the existing hot oral cavity (our hot oral cavity species) by adding two or more components of the rare earth element (REM) and REM + Ca by using a specific addition time and method, O, S, P, Improving the cleanliness of the material by reducing impurities such as N and total nonmetallic inclusions, and fully controlling the formation of hazardous nonmetallic inclusions such as Al ₂ O ₃ and MnS existing in existing hot work materials and at the same time less than 5㎛ The present invention relates to a high-clean hot-hole oral cavity which improves all of the opposing properties such as high temperature thermal fatigue properties, impact characteristics, and machinability by generating and remaining a small amount of RE-based composite inclusions and Ca-containing RE-based composite inclusions in molten steel.

Hereinafter, the high-cleaning hot pore of the present invention will be described in more detail.

According to the present invention, the amount of oxygen (O) in the molten steel is reduced to 0.005% to 0.1% of Al in the molten steel before the addition of REM and REM + Ca to the molten steel which changes the components of the existing hot-hole oral cavity and the vacuum degree is 1 × 10 ^-1 torr or less. The amount is controlled to 15 ppm or less and the sulfur (S) amount is 100 ppm or less. If the amount of O in the molten steel is more than 15ppm, the formation of RE-Oxide, which is a high hardness oxide, becomes free after the addition of REM, and the amount of the high hardness oxide is increased, which adversely affects the material, and when the amount of S in the steel is 100ppm or more, This also adversely affects the material as RE-based composite inclusions are created.

When adding REM alone or REM + Ca complex in molten steel, the addition form is first added in the form of mischmetal (Ce, La, Nd, Pr, Fe), and the second light rare earth elements Ce, La, Nd, Pr, Y, etc. In this case, the monoaddition, the third mischmetal + Ca complex addition, and the fourth Ce, La, Nd, Pr, Y, and Ca are added in two to three components.

The addition time of REM and REM + Ca in the molten steel is added after vacuum refining during the steelmaking process. It is added immediately after vacuum refining, in the inlet injection pipe just before injection of molten steel, and in the mold during molten steel injection. It is not added before vacuum refining in the furnace to minimize duplex reaction and RE-Oxide formation due to the time delay from the addition of REM to the ingot as in the following equation.

2RES (S) + 20 = RE ₂ O ₂ S (S) + S

RE ₂ O ₂ S + O = RE ₂ O ₃ (S) + S

The more REM and REM + Ca is added, the greater the control effect of the generation of harmful nonmetallic inclusions such as Al ₂ O ₃ and MnS in molten steel, and the impurities can be greatly reduced. Although removed by separation, some remain in the molten steel and remain in the steel after molten steel solidification. That is, when excessive addition, there is a large deoxidation, desulfurization effect, but rather adverse effects on the characteristics. In order to achieve both deoxidation and desulfurization effects and a small amount of the resultant RE-based composite inclusion and Ca-containing RE-based composite inclusion in the steel at the same time, an appropriate amount of REM and Ca are required. Deoxidation and desulfurization reaction by adding REM and Ca in molten steel are as follows.

2RE + 2O + S = RE ₂ O ₂ S

xRE + yS = RE _X S _Y

Ca + (x + 1/3) Al ₂ O ₃ = CaO _X Al ₂ O ₃ +2/3 Al

CaO + 2 / 3Al + S = CaS = 1/3 Al ₂ O ₃

The addition of REM and REM + Ca in molten steel is based on two methods: adding appropriate REM and Ca at a certain time or adding the appropriate REM and Ca at a fixed time interval. There are various methods such as adding REM added at regular time intervals, for example, by dividing a plurality of times at regular time intervals, and adding the same amount each time. The most effective addition method is to add 40% at the beginning of the injection time and divide the remaining REM by a certain interval and add 100% until 3 minutes before the completion of injection.

EXAMPLE

Hereinafter, the present invention will be described in more detail with reference to Examples.

6 ton steel ingots of STD61, KH90, KH71, KH80, which are our hot work steels, were prepared, and scraps were prepared in the middle of each steel ingot and remelted using 50kg vacuum induction furnace. Was made into 18kg grade ingot at 1600 ℃ ± 50 ℃.

Table 1 and Table 2 shows the chemical composition and the production method of the inventive steel and comparative steel in detail. After the molten steel is prepared in the same way as the comparative steel, the chemical composition of the invention steel is adjusted to the components except REM and Ca, and then the amount of S in the molten steel is controlled to 100ppm or less, and the amount of Al in the molten steel is 0.005. Al is added so as to be within the range of% to 0.1%, and during vacuum refining, the vacuum degree is 1 × 10 ^-1 torr or less, preliminary deoxidation is carried out so that the amount of O in the molten steel is 15 ppm or less, and then REM and REM + Ca are added. More than two ingredients were added using a specific addition method. And molten steel was made into 18kg steel ingot with molten steel temperature of 1600 ℃ ± 50 ℃.

TABLE 1

TABLE 2

The steel ingots made in Table 1 and Table 2 are upgraded to 33Ψ and 1.8U within the range of 1000 ° C to 1300 ° C. After the upset, the forging was performed in two dimensions of 60 × 60 mm. The spheroidizing heat treatment was cooled slowly after holding at 25.4 mm for more than 60 minutes in the range of 840 ° C. to 870 ° C., and the 33Ψ material was left for machinability test. 60 × 60mm material was quenched by air-cooling, oil-cooling, and water-cooling after holding at 30 ℃ / 25.4mm for more than 30 minutes at 1000 ℃ ～ 1050 ℃, followed by tempering treatment. Although it may be performed in the range of 500 ° C, preferably in this embodiment, two or more tempering processes of air cooling after holding at 25.4 mm for 60 minutes or more at 500 ° C to 650 ° C were carried out at least twice. Materials subjected to tempering were prepared for impact and thermal fatigue evaluation. Forging size and heat treatment conditions of the comparative steel are the same as the invention steel.

TABLE 3

Table 3 shows the final REM% / S%, [REM%] [S%], Ca% / S% of the present invention to show that it satisfies the limits of the present invention method.

In Table 4, after the forging of the impact characteristics of the inventive steel and the comparative steel at 60 × 60 mm, each specimen was subjected to tempering treatment, and specimens were taken from the forging direction and the direction of the monostructure, and then the KS No. 3 impact test specimen was manufactured. The test was performed at room temperature, and the result was determined by the impact absorption energy. The results were also divided into similar components, and the test specimens having a diameter of 25Ψ and a length of 20 mm were prepared for the same heat treatment conditions, respectively. The fatigue characteristics are compared and shown.

TABLE 4

In the above-mentioned thermal fatigue characteristic evaluation, after the test piece is manufactured, it is heated to 700 ° C by burner torch heating at room temperature and maintained until the inside and outside of the test piece are at a uniform temperature. The deepest thermal crack crack length generated and propagated in the test piece was quantified.

Compared to the heat check tester in the fourth table, the high temperature heat fatigue crack depth was reduced to 83.1% and 67.3%, respectively. Therefore, it can be seen that the thermal fatigue property is superior to the inventive steel compared to the comparative steel. In addition, it can be seen that the impact absorption energy of the inventive steel is superior to the comparative steel. In addition, the impact absorption energy of the inventive steel increased for both the abscissa and the longitudinal axis, and the anisotropy ratio of the impact absorption energy (the abscissa / vertical axis) also increased.

Machinability test is performed after turning the test material forged and spheroidized to 33Ψ into 30Ψ and using a carbide tool coated with TiCN on an automatic lathe, after the machinability test with a rotational speed of 1010rpm, feed rate of 0.314mm / rev, depth of cut of 1mm and 2mm. It was determined to compare the maximum cutting resistance with the chip throughput, and the results are shown in Table 5 and FIG.

TABLE 5

Table 5 compares the maximum machinability resistance after the machinability test for the invention steel and the comparative steel. At the cutting depth of 1mm, the comparative steel showed the highest 240kgf and the lowest 200kgf, and the inventive steel showed the highest 120kgf and the lowest 76kgf. At the cutting depth of 2mm, the comparative steel showed the highest 405kgf and the lowest 350kgf, and the inventive steel showed the highest 245kgf and the lowest 200kgf. Therefore, the maximum cutting resistance of the inventive steel was significantly reduced at both cutting depths of 1mm and 2mm compared to the comparative steels.

TABLE 6

Table 6 shows the results of measuring the amount of impurities, total amount of Al, soluble Al, and total amount of non-metallic inclusions of O, S, P, and N remaining in the inventive steel and the comparative steel. O, S, P, N content, total Al content, and soluble Al content were measured at the middle 1/2 position of each ingot, and total nonmetallic inclusions were measured according to KS D0204 at 1/2 position of forged material at 33Ψ. It is.

As can be seen from the table, the amount of O and S was significantly reduced compared to the comparative steel, and the amount of P and N was not significantly reduced, but was slightly reduced in the invention steel. The amount of soluble Al was significantly increased compared to that of the comparative steel due to the reduction of O content in the invention steel below 10 ppm, and the total Al was also increased due to the cause of not producing Al ₂ O ₃ . In comparison with the total Al content and the soluble Al content, the comparative steel showed less than about 50%, while the inventive steel showed more than 80%. The total amount of non-metallic inclusions was invented due to the preliminary deoxidation, desulfurization effect before the addition of REM and Ca, and the addition of a small amount of RE-based composite inclusions and Ca-containing RE-based composite inclusions after REM addition, and part of which was removed by floating separation effect. The steel has been reduced to a maximum of 75.9% and a minimum of 22.9% compared to the comparative steel.

1 is a comparison of the chipability after machinability test with respect to the invention steels A, H, I, K and comparative steel M. For both the depth of cut 1mm and 2mm, the invention steel is considerably better than the comparative steel. In general, the inventive steel showed better chip treatment than the comparative steel.

As described above, the inventive steel exhibited higher cleanliness and higher hardness than the comparative steel under the same heat treatment conditions. In addition, by satisfying all of the opposing characteristics such as improvement of impact characteristics, improvement of anisotropy of impact value, improvement of high temperature thermal fatigue, and improvement of cutting property, it is possible to supply a high-quality, high- machinability cutting tool having a long life and excellent machinability.

Claims (5)

By weight% C: 0.1%-1.0%, Si: 0.2%-0.8%, Mn: 0.3%-4.0%, Ni: 0.001%-4.0%, Cr: 1.5%-6.0%, Mo: 0.3%-4.0% , V: 0.1% to 3.5%, Al: 0.005% to 0.1%, W: 0.005% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, Te: 0.001% Preparing a molten steel containing at least one of? 0.25%, Ti: 0.01% or less, and Co: 0.001% or less, and the balance of Fe and trace impurities that may be contained during steelmaking; Desulfurizing and deoxidizing the molten steel by vacuum refining under a vacuum atmosphere of 1 × 10 −1 torr or less; Addition of light rare earth elements (Ce, La, Nd, Pr, Y, etc.) alone, complex addition of light rare earth elements and Ca, and addition of mischmetal (Ce, La, Nd, Pr, Fe) to preliminary deoxidation and desulfurized molten steel Alternatively, any one of four dosing periods may be performed by directly adding vacuum, metal, and Ca mixed into the lower injection pipe during molten steel, immediately before molten steel injection, into molten steel injection, or into the mold during molten steel injection. Doing; Preparing a steel ingot by cooling the molten steel to which the light rare earth element and Ca are added at 1,600 ± 50 ° C .; The steel ingots were hot rolled and forged at 1,000 to 1,300 ° C, spheroidized at a temperature of 840 to 870 ° C for 60 minutes or more and 25.4 mm, and quenched at 1,000 to 1050 ° C for 30 minutes or more and 25.4 mm. A high-temperature hot fatigue and impact characteristics excellent manufacturing method of high-temperature hot cavity, comprising the step of tempering at 650 ℃ 60 minutes / 25.4mm conditions. The method of claim 1, wherein the oxygen content and the sulfur content after the deoxidation and desulfurization step are 15 ppm or less and 100 ppm or less, respectively. The method of claim 1 or 2, wherein the light rare earth element is 0.0002% to 0.25%, Ca is 0.001% to 0.1% in the addition step of the mesh metal, light rare earth element and calcium production Way. The method of claim 1 or 2, wherein the method of adding the mismetal, light rare earth element, or calcium is added in the same amount every time by adding 100% at the time of addition or by dividing a plurality of times at a predetermined time interval from the beginning to the completion of the injection. 40% at the beginning of the molten steel injection, the remaining 60% is one of the input divided into three minutes before the completion of injection method for producing a high-quality hot-air cavity. 3. The final rare earth element amount and calcium amount as defined in claim 1 or 2 are REM% / St = 2 to 90, [REM] [S%] = 3 × 10 −5 to 100 × 10 −, relative to the sulfur element amount. 5, Ca% / S% = 1 or less satisfies the manufacturing method of high-quality hot-rolled oral cavity.