KR101302693B1

KR101302693B1 - Plastic mold steel with uniform hardness and workability by reducing segregation

Info

Publication number: KR101302693B1
Application number: KR1020120115548A
Authority: KR
Inventors: 이광로; 김병옥
Original assignee: 주식회사 세아베스틸
Priority date: 2012-10-17
Filing date: 2012-10-17
Publication date: 2013-09-03

Abstract

The present invention relates to a plastic mold steel excellent in uniform hardness and workability by reducing internal macro segregation (A-type segregation) of an ingot which is a raw material of a mold steel, and a manufacturing method thereof.
Plastic mold steel according to the present invention by weight% C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 ~ 0.84%, P: 0.020% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 ~ 0.50%, Cr: 1.00 ~ 1.50%, Mo: 0.20 ~ 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ 0.0050%, Zr: 0.0050 ~ 0.0500% And remainder Fe and impurities which are inevitably mixed during steelmaking. The present invention controls the length and spacing of the dendrite produced during ingot solidification by regulating Si, which increases the hardenability and serves as a deoxidizer, as compared to the ingot fabrication method that has been used in the prior art. Securing uniformity through segregation reduction, which is inevitably generated in the part, and reducing unnecessary inclusions generated by using deoxidizer (Si, Al), improves cleanliness and machinability.

Description

Plastic Mold Steel with Uniform Hardness and Workability By Reducing Segregation

The present invention relates to a plastic mold steel having excellent hardness uniformity and workability.

In general, the properties required for plastic mold steel require uniform hardness distribution, weldability, cleanliness, workability, and specularity.

The required characteristics of the plastic mold steel material to satisfy the above conditions should be less porosity after forging, clean and homogeneous structure and uniform hardness distribution.

Conventional plastic mold steel manufacturing methods include Mn, Cr, Ni, Mo, etc. for the fatigue strength and life extension of the plastic mold steel, and Ca, Te, Ti, Zr, Ce, Nb, V, La, The addition of Pb, Bi, Se, etc. has been developed in the direction of spheroidizing or complexing non-metallic inclusions such as oxides and sulfides, and reducing impurities to reduce cutting resistance. Moreover, the technique which adds hardenability improvement elements, such as a boron (B), in order to prevent the hardenability fall by mass effect is developed.

The conventional method for manufacturing plastic mold steel undergoes a process such as dissolution, refining, ingot, forging, etc., in which molten steel is deoxidized using Si or Al, which is a strong deoxidizing agent, in order to secure hardenability. It is common to contain Si content in 0.10 to 0.35%.

Korean Patent Application No. 10-2009-0099853 describes the reason for limiting the Si content in the above range as follows.

Si is used as an effective deoxidizer in steelmaking, and it is an element that is added to the base to secure strength by strengthening ferrite reinforcement. If the content is lower than 0.10% by weight, deoxidation will be insufficient, and more than 0.35% by weight. Higher amounts promote ferrite production, embrittlement, and lower forgeability. Therefore, the content of Si was limited to the range of 0.10% to 0.35% by weight.

However, in the case of the steel having the Si content as described above, segregation of carbon (C) generated inside the ingot during solidification cannot be controlled, and therefore, a region having a heterogeneous structure and hardness exists even after quenching and / or annealing. Therefore, workability and specularity are reduced.

Si also promotes "A" type segregation due to the effect of segregating or acting as an embrittlement element with nickel at the austenite grain boundaries in steels containing nickel and expanding the solid-liquid coexistence zone during solidification. Therefore, since the poor hardness site which does not satisfy the required hardness range is cut and discarded, the process recovery rate at the time of forging production is low.

Therefore, mold steels used for general household goods, office automation equipment, TV and home appliance housings, and automobile bumpers require homogeneous hardness inside and outside and excellent workability. Therefore, ingot "A" type segregation is required. There is an urgent need for plastic mold materials that minimize and minimize macro segregation through alloy design and manufacturing process invention for uniform structure and uniform hardness distribution.

An object of the present invention is to provide a plastic mold steel having a homogeneous hardness and excellent workability of the interior and exterior, which is a required characteristic of the mold steel.

The present invention is to reduce the segregation inside the ingot in order to obtain a plastic mold steel having a homogeneous hardness in the inside and outside and also having excellent workability, so as to satisfy the homogeneous structure and hardness,

1) By controlling the Si content of the alloy components of the conventional steel grades to suppress the dendrite growth affected by the Si content during solidification of the ingot to reduce the segregation generated between the dendrite to ensure a homogeneous hardness distribution ,

2) Since the use of deoxidizers such as Si is suppressed, Si-based inclusions are reduced, leading to improved workability and specularity, thereby obtaining mold steel capable of satisfying the properties required for plastic mold steel production.

3) In the present invention, deoxidation was implemented by deoxidation using carbon (C) instead of oxygen (O) removal in molten steel using Si through vacuum degassing during general refining.

The present invention is a plastic mold steel excellent in uniform hardness and workability, C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 ~ 0.84%, P: 0.020% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 to 0.50%, Cr: 1.00 to 1.50%, Mo: 0.20 to 0.45%, V: 0.03 to 0.12%, Al: 0.020 to 0.050%, B: 0.0008 to 0.0050%, Zr: It contains 0.0050-0.0500% and contains the balance of Fe and impurities which are inevitably incorporated during steelmaking.

According to another aspect, the manufacturing method of the plastic mold steel of the present invention is by weight% C: 0.25-0.35%, Si: 0.02-0.08%, Mn: 0.65-0.84%, P: 0.020% or less, S: 0.020% or less , Cu: 0.30% or less, Ni: 0.20 ~ 0.50%, Cr: 1.00 ~ 1.50%, Mo: 0.20 ~ 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ 0.0050%, Zr : Preparing a molten metal containing 0.0050 to 0.0500% and containing a remainder Fe and impurities which are inevitably mixed during steelmaking using a vacuum carbon deoxidation process; Casting the molten metal to form an ingot; Heating the ingot to 1200 to 1250 ° C. to perform first upsetting; Performing second upsetting and free forging after reheating again at 1200 to 1250 ° C .; Performing heat treatment at 600-700 ° C. for homogenization of the tissue and diffusion of hydrogen after the forging; And quenching at 850-950 ° C. and soaking at 550-650 ° C.

According to the present invention, the invention steel can ensure uniform hardness by reducing the segregation of carbon (C) deepening when the ingot of the large plastic mold steel is solidified by adjusting the Si content and optimizing the alloying components compared to the conventional steel, In addition, by limiting the use of deoxidizers (Al, Si) can reduce the absolute amount of non-metallic inclusions to improve the cleanliness and processability of the steel.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the cross-sectional schematic of the ingot of the conventional plastic mold steel, and the A type segregation component segregation value of the conventional ingot.
Figure 2 is a view showing the segregation distribution of the ingot cross-section of the invention steel and conventional steel according to the present invention.
3 is a view showing the cross-sectional hardness after hardening heat treatment of the invention steel forgings of the invention steel and conventional steel.
4 is a view showing the mechanical properties after quenching heat treatment of the forged steel of the invention steel and conventional steel.
5 is a view showing the drilling workability after the quenching heat treatment of the forgings of the invention steel and conventional steel.

The present invention is a plastic mold steel excellent in uniform hardness and workability, by weight% C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 ~ 0.84%, P: 0.020% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 ~ 0.50%, Cr: 1.00 ~ 1.50%, Mo: 0.20 ~ 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ .0050%, Zr : 0.0050 ~ 0.0500%, remainder Fe and impurity that is inevitably mixed during steelmaking.

Hereinafter, the reason for the addition of the alloy component of the present invention and the limitation of the component range will be described.

C: 0.25 wt% ~ 0.35 wt%

C is an austenite stabilizing element and is an important element which is dissolved in a matrix at the time of hardening to increase strength and hardness. In the present invention, as an element that increases the hardness and wear resistance, when it is 0.25 wt% or less, the curing ability is drastically reduced, and the strength is lowered. In addition, 0.25% to 0.35% by weight is ideal in order to meet the target strength of the present invention, and more than 0.35% by weight of the core segregation, resulting in abnormal tissue formation, and workability according to heterogeneous hardness Decreases. Therefore, the content of C is limited to 0.25 wt% to 0.35 wt%.

Si: 0.02 wt% ~ 0.08 wt%

As the ingot solidifies, the dendrite grows on the solidification front and "A" type segregation is produced by the density difference between the molten steel and the bulk molten steel between the dendrites. However, when the Si content is low, the side-arms of the dendrite formed on the solidification front grow rapidly, thereby preventing the molten steel in which the alloying component is concentrated from flowing between the dendrite and forming “A” type segregation. This property is especially pronounced in the NiCrMoV steel series.

In the manufacture of plastic mold steel and other steel products, Si uses effective deoxidizer and hardenability increasing element during steelmaking, but in the present invention, Si content is limited to 0.02 ~ 0.08% to reduce “A” type segregation, and other alloy components By optimizing, hardness uniformity and cleanliness were secured.

Mn 0.65 wt% to 0.84 wt%

Mn was set to 0.65% by weight or more in order to improve the hardenability and strength, and if it is 0.84% by weight or more, the micro segregation and hydrogen crack sensitivity are increased, so the upper limit thereof is limited.

P: 0.020 wt% or less

P is limited to 0.020% by weight or less because impurity lowers the impact resistance and promotes temper brittleness when segregating in the material.

S: 0.020 wt% or less

S is an element that forms FeS in steel, which makes the material brittle and causes cracking during processing, so the content is limited to 0.02% by weight or less.

Cu 0.30 wt% or less

Cu slightly increases tensile strength and corrosion resistance in steel, but reduces elongation, causing cracks in processing and limiting its content to less than 0.30% by weight since it is a temper brittle element.

Ni : 0.20 wt% to 0.50 wt%

Ni is an element that increases toughness and promotes graphitization, and when added at 0.20% by weight or less, Ni has no effect of improving toughness. When Ni is added at an amount higher than 0.50% by weight, Ni generates residual austenite and causes embrittlement. Therefore, the content of Ni is limited to 0.20% by weight to 0.50% by weight.

Cr : 1.00 wt% to 1.50 wt%

Cr is an element that increases the hardenability and makes the carbide to increase the impact resistance. The lower limit is 1.00% by weight to compensate for the hardenability by reducing the Mn content and to increase the temper resistance by forming complex compounds with Mo, V and the like. In consideration of corrosion resistance and economy, the upper limit is limited to 1.50% by weight.

Mo : 0.20 wt% to 0.45 wt%

Mo is an element that forms carbide to give high temperature hardness and strength, and Mo included in Mo ₂ C combines with phosphorus (P) of grain boundary to alleviate temper brittleness by P and improve temper secondary hardenability. The content range is limited to 0.20% by weight to 0.45% by weight.

V: 0.03 wt% to 0.12 wt%

V refines the grains by fine carbide formation to improve strength and toughness. If the added amount is 0.03% by weight or less, the effect of increasing strength is small, and when it is added more than 0.12% by weight, the strength is increased, but toughness is not only lowered, but it is not preferable because there is no economic effect due to the increase in manufacturing cost. Therefore, the V content is limited to 0.03% by weight to 0.12% by weight.

Al : 0.020 wt% ~ 0.050 wt%

Al acts as a strong deoxidizer and binds with N to refine the grains, but at 0.020% by weight or less, deoxidation or grain refining is less desirable, and when excessively added, rather than nonmetallic inclusions such as Al ₂ O ₃ The increase can have a rather detrimental effect. Therefore, the appropriate content range of Al is limited to 0.020 ~ 0.050% by weight.

B: 0.0008 wt% to 0.0050 wt%

With the addition of trace amounts, B hardly improves the hardenability due to grain boundary segregation, while B has an effect of increasing the hardenability of the basic components of steel such as Mn, Cr, Mo, etc. It tends to decrease with a small increase. When the B amount is less than 8 ppm, so the boron mouth gyepyeon seats difficult hardenability improving effect is sharply reduced was set as the lower limit, 50 ppm or more when the grain boundary precipitates of BN and Fe ₂₃ of boron from (C, B) ₆ Was formed, and hot brittleness caused the red brittleness, which adversely affects the product characteristics, so it was set as the upper limit.

Zr : 0.0050 wt% ~ 0.0500 wt%

It is an element added for the purpose of reducing material anisotropy by spheroidizing non-metallic inclusions, and when excessively added, it strengthens matrix structure and lowers machinability, so it is 0.0050% by weight to 0.0500% depending on sulfur (S) and oxygen (O) content. It is limited to%.

[ Example ]

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

Table 1 shows the chemical components of the inventive steel and the comparative steel having the composition of the present invention.

For the development of the invention steel, the alloy content of Si is 0.02 ~ 0.08% in the chemical composition, and the manufacturing process is dissolved in 150 ton electric furnace, and then refined and vacuum carbon deoxidation process, and the weight% C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 to 0.84%, P: 0.20% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 to 0.50%, Cr: 1.00 to 1.50%, Mo: 0.20 to 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ 0.0050%, Zr: 0.0050 ~ 0.0500%, balance Fe and a molten metal containing inevitably mixed impurities during steelmaking are obtained, and the molten metal is cast Made an ingot

Here, vacuum carbon deoxidation (abbreviated as "VCD") is a deoxidation method using a property in which the bonding property of carbon (C) and oxygen (O) is larger than Si or Al at a certain atmospheric pressure below. By limiting the use of deoxidizer Si or Al during the tapping or LF refining process, it does not remove oxygen in molten steel, but it does not remove hydrogen (H) and nitrogen (N) as well as carbon (C) in molten steel components in VD (Vacuum Degassing) process. The component and oxygen (O) are reacted at a pressure of 0.001 atm or less, and changed into CO gas without solubility in molten steel to deoxidize. Segregation is reduced due to the low Si content in molten steel and deoxidation using VCD because it regulates the deoxidizer Si. The application of VCD additionally reduces inclusions and contributes to improved processability and mechanical properties. In the present invention, a VCD process is performed to reduce segregation by maintaining low Si.

The ingots were heated to 1200-1250 ° C. to carry out the first upsetting, and after reheating to 1200-1250 ° C., the second upsetting and free forging were performed. Upsetting is performed to minimize ingot dendritic tissue and prevent internal defects. In order to homogenize the structure and hydrogen diffusion after forging, heat treatment at 600-700 ° C. (normalizing) was performed, followed by quenching at 850-950 ° C., and heat treatment at 550-650 ° C. to prepare a final mold steel material.

Comparative steel shows the measured chemical composition of conventional steel in actual production.

division C Si Mn P S Cu Ni Cr Mo Al V B
(ppm) Zr
(ppm) foot
persons
River A 0.26 0.05 0.68 0.009 0.002 0.14 0.22 1.15 0.23 0.022 0.037 21 79 B 0.29 0.06 0.70 0.016 0.001 0.16 0.33 1.28 0.30 0.031 0.049 23 196 C 0.33 0.07 0.78 0.015 0.001 0.17 0.41 1.30 0.35 0.033 0.055 20 78 D 0.30 0.06 0.82 0.010 0.002 0.13 0.47 1.42 0.41 0.040 0.096 18 95 E 0.34 0.02 0.68 0.013 0.001 0.16 0.48 1.43 0.30 0.045 0.010 15 170 Bell
Ra
River One 0.28 0.20 0.81 0.014 0.001 0.13 0.34 1.28 0.29 0.036 0.056 27 81 2 0.30 0.23 0.84 0.012 0.001 0.12 0.33 1.30 0.27 0.039 0.049 27 75 3 0.33 0.19 0.72 0.013 0.001 0.14 0.33 1.29 0.31 0.036 0.050 23 157

In Table 1, the unit is weight%.

Figure 1 shows the ingot internal schematic of the conventional steel in production and the "A" type segregation carbon (C) of the ingot produced without the application of the vacuum carbon deoxidation process (VCD) having a chemical composition of C: 0.32% by weight and Si: 0.23% by weight. The segregation degree of "A" type segregation part was found to be more than twice as high as the carbon (C) component than the surrounding normal part. That is, the content of C was 0.45% to 0.70%, which was nearly two times higher than the carbon content of 0.25% to 0.35% of the normal site. In addition, in the content of Si, the segregation is much higher than 0.35% to 0.59%, much higher than the content of 0.10 to 0.35% of the surrounding normal site.

Figure 2 shows the segregation of the cross section mark of the invention steel produced by regulating Si in the element and the conventional steel ingot manufactured by a conventional manufacturing method. The number of segregation A of the inventive steel is reduced to 50% compared to the conventional steel.

3 shows a comparison result of the cross-sectional hardness values of the forged products of the inventive steel and the conventional steel. The hardness of steel is primarily affected by the hardenability, heat treatment temperature, and austenitic grain size of the alloy composition.

Among them, the homogeneous hardness distribution to the surface and center of the forged product has the greatest influence on the sufficient hardening to the center of the material and the ability to control the homogeneous alloy composition distribution (segregation) upon solidification. Invented steels exhibit a more uniform hardness than conventional steels. In the case of conventional steels, as described above, the carbon component is high along the segregation line, so that the hardness is partially high. Obtained.

Figure 4 shows the mechanical properties of the forged steel of the invention steel and conventional steel. The tensile strength of the inventive steel, whose Si component is regulated, is equivalent to that of conventional steel, indicating no difference in tensile strength, elongation, etc. required for plastic mold steel.

Table 2 above is the data of the grades measured according to ASTM E-45 Method A. As the data comparing the cleanliness of the invention steel and the conventional steel, the cleanliness was also improved compared to the conventional steel because the use of the deoxidizer Si and Al was restricted.

Test specimen condition Processing conditions Assessment Methods

-Tool: Carbide Tool (φ9mm)
Feedrate: 0.035mm / rev.
Tool rotation: 1600 RPM
-Processing condition: Dry

Table 3 shows the test conditions for the evaluation of workability for the forged product manufactured as described above. As shown in the test conditions of Table 3, the hole life until tool breakage was evaluated as the tool life during the drilling operation to a depth of 15 mm in the cross section using f9 mm carbide tools. Table 2 shows. Invented steels show better drilling processability results than conventional steels, and as shown in FIG. 3, FIG. 4 and Table 2, the drilling experiment (processability as shown in FIG. ), The number of holes in the invention steel was 63, and the number of holes in the conventional steel was 56, indicating that the workability of the invention steel was improved by about 12.5%.

Claims

By weight% C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 ~ 0.84%, P: 0.020% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 ~ 0.50%, Cr : 1.00 ~ 1.50%, Mo: 0.20 ~ 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ 0.0050%, Zr: 0.0050 ~ 0.0500% Plastic mold steel with excellent hardness and workability, characterized by consisting of impurities mixed.

By weight% C: 0.25 ~ 0.35%, Si: 0.02 ~ 0.08%, Mn: 0.65 ~ 0.84%, P: 0.020% or less, S: 0.020% or less, Cu: 0.30% or less, Ni: 0.20 ~ 0.50%, Cr : 1.00 ~ 1.50%, Mo: 0.20 ~ 0.45%, V: 0.03 ~ 0.12%, Al: 0.020 ~ 0.050%, B: 0.0008 ~ 0.0050%, Zr: 0.0050 ~ 0.0500% Preparing a molten metal including impurities to be mixed using a vacuum carbon deoxidation process; Casting the molten metal to form an ingot; Heating the ingot to 1200 to 1250 ° C. to perform first upsetting; After reheating to 1200 ~ 1250 ℃ again, the manufacturing method of the plastic mold steel excellent in uniform hardness and workability, including the step of performing the second upsetting and free forging.

3. The method of claim 2, further comprising: performing a heat treatment at 600-700 ° C. for homogenization of the tissue and diffusion of hydrogen after the forging; And quenching at 850 to 950 ° C. and considering at 550 to 650 ° C. for a method of manufacturing plastic mold steel having excellent hardness and workability.