JP2018083980A - Hot die steel used for die casting having an excellent thermal conductivity at high temperature and a long life, and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot die steel used for die casting having an excellent thermal conductivity at high temperature and a long life, and a manufacturing method thereof.SOLUTION: The hot die steel contains, relative to the total amount, 0.35 to 0.45 wt.% of carbon(C), 0.20 to 0.30 wt.% of silicon(Si), 0.30 to 0.40 wt.% of manganese(Mn), 0.50 to 1.20 wt.% of nickel(Ni), 1.5 to 2.2 wt.% of chromium(Cr), 2.0 to 2.6 wt.% of molybdenum(Mo), 0.0001 to 1.0 wt.% of tungsten(W), more than 0 and 0.40 wt.% or less of titanium(Ti), 0.30 to 0.50 wt.% of vanadium(V), 0.0001 to 0.003 wt.% of boron(B), 0.005 to 0.02 wt.% of copper(Cu) and the balance being Fe and inevitable impurities. Furthermore, the hot die steel preferably contains 0.02 to 0.08 wt.% of aluminum(Al), 0.005 to 0.06 wt.% of nitrogen(N), 0.001 to 0.006 wt.% of phosphorus(P) and 0.0001 to 0.002 wt.% of sulfur(S).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a long-life die-casting hot die steel excellent in high-temperature thermal conductivity and a method for producing the same, and more specifically, can be applied to die-casting used in producing automobile parts and the like. In particular, the present invention relates to a hot mold steel excellent in high-temperature thermal conductivity and durability and a method for producing the same.

  Hot mold steel is a mold steel of an alloy containing different amounts of carbon, chromium, tungsten, silicon, nickel, molybdenum, manganese, vanadium and cobalt in addition to iron as an alloy element. Hot mold steel objects suitable for forming materials can be produced from hot mold steel, in particular during die casting, extrusion or die forging. Examples of such dies are extrusion dies, forging dies, die casting molds, press dies or the like that must have special mechanical strength properties at high processing temperatures.

  The important function of mold steels, in particular hot mold steels and the steel objects made with them, is that when used in a technical process, sufficient release of heat previously introduced or generated in the process itself. It is to guarantee.

  Hot molds made of hot mold steel must have good thermal conductivity and high thermal wear resistance along with high mechanical stability at high processing temperatures. Other important properties of hot mold steel are excellent hardness and wear resistance at high service temperatures as well as sufficient hardness and strength.

  The high thermal conductivity of the hot mold steel used for mold manufacture is very important in many applications because it leads to significant cycle time reduction. Since the operation of hot forming equipment for hot forming of workpieces is relatively expensive, much cost can be saved by shortening the cycle time. Also, the high thermal conductivity of hot mold steel is preferred during die casting because the mold used for it has a much longer life due to the very high thermal durability. Mold steels often used to make molds typically have a thermal conductivity of about 18-24 W / mK at room temperature.

  On the other hand, STD61 used as a hot forming mold steel has a high thermal conductivity of less than 28 W / mK and a low thermal conductivity. Therefore, from the difference in expansion coefficient due to the temperature difference according to the part of the material during the high temperature operation. The frequency of heat check cracks is high, which reduces the life of the mold, does not sufficiently increase the cooling rate of the product, and reduces the quality and productivity of hot stamping products that require a high cooling rate. There was a problem to induce. In addition, since the precipitation phase that maintains wear resistance at high temperatures is a chromium carbide system having low hardness, there is a problem that wear resistance is low at high temperatures.

  Recently, the use of lightweight non-ferrous metals has been increasing in order to achieve weight reduction due to environmental considerations and high fuel consumption trends in the automobile industry, and the demand for mold steel for die casting to form this has increased. The hot die steel market for die casting used in Korea is occupied by overseas advanced companies such as Hitachi and the price range is high, but when manufacturing long-life die casting, the thermal conductivity of die steel Or there is a problem that durability does not reach a satisfactory level.

  The present invention provides a hot mold steel that is capable of producing a long-life die casting having excellent high temperature thermal conductivity and high temperature durability by optimizing the composition of the hot mold steel and its manufacturing conditions.

  According to one embodiment of the present invention, carbon (C) 0.35 to 0.45 wt%, silicon (Si) 0.20 to 0.30 wt%, manganese (Mn) 0. 30 to 0.40 wt%, nickel (Ni) 0.50 to 1.20 wt%, chromium (Cr) 1.5 to 2.2 wt%, molybdenum (Mo) 2.0 to 2.6 wt%, Tungsten (W) 0.0001 to 1.0 wt%, Titanium (Ti) 0 excess 0.40 wt% or less, Vanadium (V) 0.30 to 0.50 wt%, Boron (B) 0.0001 to 0 Relates to hot mold steel containing 0.003 wt% and copper (Cu) 0.005 to 0.02 wt%, the balance containing Fe and inevitable impurities.

In the present invention, the hot mold steel may further include 0.02 to 0.08 wt% of aluminum (Al).
In the present invention, the hot mold steel may further include 0.005 to 0.06 wt% of nitrogen (N).
In the present invention, the hot mold steel may further include 0.001 to 0.006 wt% phosphorus (P) and 0.0001 to 0.002 wt% sulfur (S).
In the present invention, when the content values of carbon, silicon, manganese, chromium, molybdenum, and nickel forming the hot mold steel are substituted into the following formula (1), the value may be 25 or more:

F (C) × F (Si) × F (Mn) × F (Cr) × F (Mo) × F (Ni) Formula (1)
(However, in the above formula (1),
F (C) = 0.37-0.39 × (0.12 ^ carbon content (%));
F (Si) = 0.7 × silicon content (%) + 1;
F (Mn) = 3.35 × manganese content (%) + 1;
F (Cr) = 2.16 × chromium content (%) + 1;
F (Ni) = 0.36 × nickel content (%) + 1; and F (Mo) = 3 × molybdenum content (%) + 1. ).

In the present invention, when the values of the contents of carbon, silicon, manganese, chromium, molybdenum and nickel forming the hot mold steel are substituted into the above formula (1), the value may be 30 or more.
In the present invention, when the respective content values of molybdenum and tungsten constituting the hot mold steel are substituted into the following formula (2), the value may be 2 or more and 3 or less:

Molybdenum content (%) + 0.5 x tungsten content (%) Formula (2)
In the present invention, when the respective content values of titanium and vanadium forming the hot mold steel are substituted into the following formula (3), the value may be 0.4 or more and 0.5 or less:
Titanium content (%) + vanadium content (%) Formula (3)
In the present invention, when the values of the contents of chromium, molybdenum and tungsten constituting the hot mold steel are substituted into the following formula (4), the value may be 9 or more:

Chromium content (%) + 3.3 × {Molybdenum content (%) + 0.5 × Tungsten content (%)} Equation (4)
In the present invention, the mold steel may be used for die casting.

According to another embodiment of the present invention, carbon (C) 0.35 to 0.45 wt%, silicon (Si) 0.20 to 0.30 wt%, manganese (Mn) 0 based on the total weight. .30 to 0.40 wt%, nickel (Ni) 0.50 to 1.20 wt%, chromium (Cr) 1.5 to 2.2 wt%, molybdenum (Mo) 2.0 to 2.6 wt% Tungsten (W) 0.0001 to 1.0 wt%, Titanium (Ti) 0 excess 0.40 wt% or less, Vanadium (V) 0.30 to 0.50 wt%, Boron (B) 0.0001 to Producing a steel ingot comprising 0.003% by weight and copper (Cu) 0.005-0.02% by weight, the remainder comprising Fe and inevitable impurities;
Forging the steel ingot to produce a mold material;
Quenching the mold material;
And a step of tempering after the quenching.

In the present invention, the steel ingot may further include 0.02 to 0.08 wt% of aluminum (Al).
In the present invention, the steel ingot may further include 0.005 to 0.06% by weight of nitrogen (N).
In the present invention, the steel ingot may further include 0.001 to 0.006% by weight of phosphorus (P) and 0.0001 to 0.002% by weight of sulfur (S).

In the present invention, an electro-slag remelting (ESR) process may be further performed before forging the steel ingot.
In the present invention, the electroslag remelting step may be performed in an argon gas atmosphere.
In the present invention, the steel ingot may be further preliminarily heat-treated at a temperature of 800 to 1300 ° C. before forging the steel ingot.

In the present invention, the forging may be performed at a forging ratio of 5S or more.
In the present invention, the forging may be performed at a temperature of 850 to 1300 ° C.
In the present invention, the quenching may be performed at a temperature of 900 to 1030 ° C.
In the present invention, the tempering may be performed at a temperature of 500 to 630 ° C.
In the present invention, the tempering includes primary tempering at a temperature of 580 to 600 ° C., and
Secondary tempering at a temperature of 550 to 590 ° C.
In the present invention, after the secondary tempering, a step of tertiary tempering at a temperature of 610 to 630 ° C. may be further included.

The hot mold steel provided in the present invention is excellent in heat-checking characteristics because it has excellent high-temperature thermal conductivity and generates a small temperature difference between materials at high temperatures. Therefore, when the hot die steel according to the present invention is used for die casting, the product produced using the die casting has a high cooling rate, the physical properties of the produced product are improved, and the cooling time is shortened to increase the productivity. Can be improved.
Furthermore, the hot mold steel provided by the present invention is excellent in high temperature durability, and a die cast produced using the hot mold steel can have a long life characteristic.

In Experimental Example 1, the thermal conductivity of the hot die steel produced in Example 3 is measured at each temperature and the result is shown. In Experimental example 2, the value of the yield strength with respect to the hardness of the hot metal mold | die manufactured by Example 2, 3-5, and 10 and the comparative examples 2 and 4 is shown with a graph. In Experimental example 2, the value of the tensile strength with respect to the hardness of the hot mold steel manufactured in Examples 2, 3-5, and 10 and Comparative Examples 2 and 4 is shown by a graph. In Experimental example 2, the value of the impact energy with respect to the hardness of the hot mold steel manufactured in Examples 2, 3 to 5 and 10 and Comparative Examples 2 and 4 is shown by a graph. In Experimental example 3, the result of having measured the change of the heat conductivity by temperature about the hot die steel manufactured in Example 12 and Comparative Examples 7 and 8 is shown with a graph. In Experimental example 3, the value of the Charpy U-notch impact energy with respect to hardness is shown with a graph about the hot die steel manufactured in Example 12 and Comparative Examples 7 and 8. FIG. (A) and (b) show the results of taking surface particle photographs of the hot mold steel produced in Example 3 and Comparative Example 9 in Experimental Example 5, respectively. In Experimental Example 6, although hot mold steel is manufactured in the same process as in Example 3, the results of evaluating the strength of the hot mold steel by the quenching temperature by variously adjusting the quenching temperature are shown in a graph. Is. In Experimental Example 7, although hot mold steel is manufactured in the same process as in Example 3, as a result of various adjustments of quenching temperature and tempering temperature, a structural photograph of the finally manufactured hot mold steel is shown. Is. In Experimental Example 8, although hot mold steel is manufactured in the same process as in Examples 1 and 3, the results of evaluating the strength of the hot mold steel according to the tempering temperature by variously adjusting the tempering temperature in a graph. It is shown.

  Hereinafter, preferred embodiments of the present invention will be described. However, the embodiment of the present invention can be modified into various different forms, and the scope of the present invention is not limited to the embodiment described below. In addition, the embodiments of the present invention are provided to more fully explain the present invention to those having average knowledge in the art.

Hereinafter, hot mold steel according to an embodiment of the present invention will be described.
The hot mold steel according to the present embodiment includes carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W) as essential elements. Titanium (Ti), vanadium (V), boron (B) and copper (Cu) are contained, and the remainder is composed of Fe (iron), fine elements and inevitable impurities. The hot die steel according to the present embodiment contains phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N) and oxygen (O) as inevitable impurities. In this specification, all percentages defined by mass are the same as those defined by weight. Hereinafter, the component composition of the hot mold steel and the reason for the numerical limitation will be described.

Carbon (C)
Carbon (C) is an essential element necessary for adjusting the strength of steel, and is an element that increases the strength of the base material by solid solution strengthening and affects the hardenability. Also, carbide is formed by heat treatment. When the carbon content is less than 0.35% by weight, the hardness and strength are low, the curability is reduced, and uniform cross-sectional hardness cannot be obtained, and the carbon content exceeds 0.45% by weight. In this case, the hardness tends to saturate, and at the same time, the amount of carbide becomes excessive to deteriorate the fatigue strength and impact value. Therefore, it is preferable that the hot mold steel of the present invention contains 0.35 to 0.45% by weight of the carbon.

Silicon (Si)
Silicon (Si) is an essential element necessary to adjust the machinability of steel, and is an element that suppresses the formation of cementite and promotes the formation of carbides at high temperatures to greatly increase the thermal conductivity. . If the silicon content is less than 0.20 wt%, it becomes difficult to ensure the same or higher machinability as STD61, and if the silicon content exceeds 0.30 wt%, the thermal conductivity A big decrease occurs. Therefore, it is preferable that the hot die steel of the present invention contains the silicon in an amount of 0.20 to 0.30% by weight.

Manganese (Mn)
Manganese is an essential element for improving the transformation operation (hardening ability), and is the element having the highest effect on hardenability. If the Mn content is less than 0.30% by weight, the effect of reducing the transformation point and refining the microstructure is insufficient and it is difficult to secure hardness and impact value. When the Mn content exceeds 0.40% by weight, not only the impact value is further reduced, but also high thermal conductivity can hardly be maintained. Therefore, it is preferable that the hot die steel of the present invention contains 0.30 to 0.40% by weight of the manganese.

Nickel (Ni)
The nickel (Ni) is an element that not only improves toughness and hardenability, but also improves stability at high temperatures. If the nickel content is less than 0.50% by weight, the effect of improving toughness that can prevent toughness reduction can be reduced as the hardness and strength increase, and the nickel content is reduced to 1.20% by weight. If it exceeds, retained austenite is generated, the structure is unstable, and deformation may occur during use, cutting workability is reduced and it is uneconomical. Therefore, it is preferable that the hot die steel of the present invention contains the nickel at 0.50 to 1.20% by weight.

Chrome (Cr)
The chromium (Cr) is an element that improves the hardenability and produces a composite carbide to improve hardness, strength, temper softening resistance, and wear resistance. If the chromium content is less than 1.5% by weight, the effect of improving the hardenability is reduced, it is difficult to obtain uniform cross-sectional hardness, and the formation of composite carbides with molybdenum, vanadium, etc. is reduced and temper softening resistance. Decreases, and the improvement effect on strength and oxidation resistance decreases. On the other hand, when the chromium content exceeds 2.2% by weight, the thermal conductivity is decreased, and the properties of hardness, tensile strength and yield strength are rapidly decreased. Therefore, it is preferable that the hot die steel of the present invention contains 1.5 to 2.2% by weight of the chromium.

Molybdenum (Mo)
The molybdenum forms carbides such as molybdenum carbide to improve high temperature hardness and strength, and during tempering, it causes secondary hardening phenomenon at high temperature to increase high temperature strength, and phosphorus existing at grain boundaries. It is an element that combines to prevent temper embrittlement by phosphorus during tempering heat treatment. If the molybdenum content is less than 2.0% by weight, the effect of suppressing temper embrittlement is reduced, the secondary hardening phenomenon is lowered, and the hardness and strength at high temperature are reduced. If it exceeds 2.6% by weight, not only the effect of molybdenum is reduced, but also uneconomical. Therefore, it is preferable that the hot die steel of the present invention contains 2.0 to 2.6% by weight of the molybdenum.

Tungsten (W)
Tungsten is an optional element that can be added to increase the strength of carbide precipitation (precipitation hardening). If the tungsten content is less than 0.0001% by weight, the effect of increasing the strength is small, and if the tungsten content exceeds 1.0% by weight, the effect is saturated and the cost is greatly increased. Therefore, it is preferable that the hot mold steel of the present invention contains the tungsten in an amount of 0.0001 to 1.0% by weight.

Titanium (Ti)
Titanium (Ti) is an element that generates the strongest precipitation phase, and is an austenite that has a low solubility and shows an effect of refining the structure. In the hot die steel of the present invention, it is preferable to contain the titanium in excess of 0 to 0.40% by weight or less in order to improve the physical properties of hardness and strength, and more preferably 0.10 to 0.40% by weight. And more preferably 0.15 to 0.40% by weight.

Vanadium (V)
Vanadium (V) is an element that increases the tensile strength by making a solid solution in iron, increases the high-temperature hardness by forming insoluble carbides, and increases the tempering resistance. This shows the effect of suppressing the growth of austenite particles by finely forming a precipitated phase. If the vanadium content is less than 0.30% by weight, the effect may be slight. If the vanadium content exceeds 0.50% by weight, the crystal grain refinement phenomenon is conspicuous and the curability decreases. Therefore, uniform cross-sectional hardness cannot be obtained, which is uneconomical. Therefore, it is preferable that the hot die steel of the present invention contains vanadium at 0.30 to 0.50% by weight.

Boron (B)
Boron has the effect of greatly improving the quenching characteristics by grain boundary segregation even when added in a very small amount, and also improving the quenching characteristics of other elements such as manganese, chromium, and nickel. In contrast, the effect of improving the quenching characteristics of boron tends to decrease as the carbon content increases. The hot mold steel of the present invention preferably contains 0.0001 to 0.003% by weight of the boron.

Copper (Cu)
The copper (Cu) is an element contained in scrap iron, and if added over 0.02% by weight, surface cracking phenomenon occurs during hot forging and the forgeability is reduced. In the hot die steel of the invention, the copper is preferably contained in a content of 0.02% by weight or less, preferably 0.005 to 0.02% by weight.

Phosphorus (P)
Phosphorus partially contributes to increasing the strength, but if it exceeds 0.006% by weight, there is a problem that weldability deteriorates. Therefore, in this invention, it limits to 0.006 weight% or less, Preferably 0.001-0.006 weight% is preferable.

Sulfur (S)
Sulfur (S) is a typical inevitable impurity, and in the present invention, 0.0001 to 0.002% by weight is preferable.

Aluminum (Al) and Nitrogen (N)
Aluminum and nitrogen are impurities contained in steelmaking and must be reduced to the maximum. However, when boron is added to obtain grain boundary segregation, the range does not harm steel properties. When an appropriate amount is added below, a buffering action between aluminum, nitrogen and boron can occur. As described above, since the amount of solid solution boron segregating at the grain boundaries contributing to the improvement of hardenability is secured, in the hot die steel of the present invention, aluminum is 0.02 to 0.08% by weight. Preferably, nitrogen is contained at 0.005 to 0.06% by weight.

In the present invention, except for the components of the hot mold steel described above, the remainder is substantially made of iron (Fe).
That the remainder consists essentially of iron (Fe) means that it includes the inevitable impurities and other trace elements as long as the effects of the present invention are not impaired. To do.
In the present invention, the hot mold steel has a high thermal conductivity at a high temperature and a very excellent strength and hardness by adjusting the chromium content to a specific range in the composition. Shape steel can be provided.
On the other hand, in the present invention, when the content values of carbon, silicon, manganese, chromium, molybdenum, and nickel forming the mold steel are substituted into the following formula (1), the value is preferably 25 or more.

F (C) × F (Si) × F (Mn) × F (Cr) × F (Mo) × F (Ni) Formula (1)
However, the definition of each factor used by said Formula (1) is as follows.
F (C) = 0.37-0.39 × (0.12 ^ carbon content (%));
F (Si) = 0.7 × silicon content (%) + 1;
F (Mn) = 3.35 × manganese content (%) + 1;
F (Cr) = 2.16 × chromium content (%) + 1;
F (Ni) = 0.36 × nickel content (%) + 1; and F (Mo) = 3 × molybdenum content (%) + 1.

  In the present invention, the value obtained from the above formula (1) is the ideal critical diameter (unit is inch), which means the maximum diameter that is quenched when cooled as quickly as possible. Since the size of the product that can be made from martensite to the deep part at a cooling rate increases, it is advantageous for production of the product. Therefore, in the present invention, when considering the thermal conductivity, strength, hardness and the like of the hot mold steel to be manufactured, and considering the productivity of the product, the value obtained from the above formula (1) is 25 or more. Preferably, it may be 26 or more, more preferably 30 or more.

  Moreover, in this invention, when substituting the value of each content of molybdenum and tungsten in following formula (2) in the composition of the said hot die steel, it is preferable that the value is 2 or more and 3 or less.

Molybdenum content (%) + 0.5 x tungsten content (%) Formula (2)
The new formula (2) provided by the present invention is a factor for adjusting the high temperature strength and corrosion resistance. When the value of the above formula is less than 2 or more than 3, sufficient high temperature strength and corrosion resistance are obtained. It is difficult to ensure, and when die casting is produced using the hot mold steel, the life can be shortened.

  Moreover, in this invention, when the value of each content of titanium and vanadium is substituted into following formula (3) in the composition of the said hot die steel, the value may be 0.4 or more and 0.5 or less. preferable.

Titanium content (%) + vanadium content (%) Formula (3)
The new formula (3) provided in the present invention is a factor for adjusting the high temperature thermal conductivity related to carbide, and when the value of the formula is less than 0.4 or exceeds 0.5, It is difficult to ensure conductivity, and it may affect the quality and production speed of products using die casting manufactured with the hot die steel according to the present invention.

  Moreover, in this invention, when substituting the value of each content of chromium, molybdenum, and tungsten in following formula (4) in the composition of the said hot die steel, it is preferable that the value is 9 or more.

Chromium content (%) + 3.3 × {Molybdenum content (%) + 0.5 × Tungsten content (%)} Equation (4)
The above formula (4) provided by the present invention is a factor for adjusting the corrosion resistance at high temperatures. When the value of the above formula is less than 9, it is difficult to ensure sufficient high temperature corrosivity. Therefore, in the present invention, the value obtained from the above formula (4) is preferably 9 or more, more preferably 9.5 or more, and even more preferably 10 or more.

  In the present invention, the hot die steel having the composition as described above is excellent in high temperature durability and high temperature thermal conductivity, and generates a small temperature difference of the material at high temperature, so that it has excellent heat-checking characteristics. . Therefore, when the hot mold steel according to the present invention is applied to die casting, the life of the die casting is prolonged, and the physical properties of the product produced by increasing the cooling rate of the product produced using the die casting. As a result, the cooling time is shortened and productivity can be improved.

  Hereinafter, the manufacturing method of the hot die steel using the above steel components will be described.

  In the present invention, first, a steel ingot having the above composition can be produced. As an artificial heat source, for example, one of an electric furnace, a vacuum induction furnace, and an atmospheric induction furnace is used to melt metal, and then effectively remove gases such as oxygen, hydrogen, and nitrogen that are generated during steelmaking operations. Thus, a steel ingot can be manufactured.

  In the present invention, the steel ingot includes, as essential elements, carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), tungsten (W), titanium ( Ti), vanadium (V) and boron (B), the remainder being Fe (iron), fine elements and inevitable impurities such as phosphorus (P), sulfur (S), aluminum (Al), nitrogen (N) And oxygen (O). Preferably, the steel ingot is carbon (C) 0.35 to 0.45 wt%, silicon (Si) 0.20 to 0.30 wt%, manganese (Mn) 0.30 to 0.40 wt%, Nickel (Ni) 0.50-1.20 wt%, Chromium (Cr) 1.5-2.2 wt%, Molybdenum (Mo) 2.0-2.6 wt%, Tungsten (W) 0.0001- 1.0 wt%, titanium (Ti) 0 excess 0.4 wt% or less, vanadium (V) 0.30 to 0.50 wt%, boron (B) 0.0001 to 0.003 wt% and copper (Cu ) 0.005-0.02% by weight, the remainder may contain Fe and inevitable impurities, in addition, aluminum (Al) 0.02-0.08% by weight, nitrogen (N) 0.005 -0.06 wt%, phosphorus (P) 0.001-0.006 wt% and sulfur S) 0.0001 to 0.002 may further comprise a weight percent.

  In the present invention, in the composition of the steel ingot, the reason for limiting the content for each component overlaps with that described in the composition of the hot mold steel, and the detailed description thereof will be omitted below.

  In the present invention, when the steel ingot is prepared as described above, the steel ingot can be refined selectively by performing an electro-slag remelting (ESR) process.

  Further, in the present invention, the electroslag remelting step can be performed in an inert gas atmosphere, and preferably performed in an argon gas atmosphere to form nitride by nitrogen dissolved from the air. Therefore, it can prevent that the toughness of a raw material falls. However, in the present invention, the electroslag remelting step may be performed within a range that can be changed from a method generally used in the technical field, or a normal engineer can obviously change from the method, and a gas atmosphere. Specific process conditions other than are not particularly limited.

  In the present invention, preliminary heat treatment can be performed before forging, which is a process for obtaining a standard shape to be manufactured of the electroslag remelting steel ingot obtained after the electroslag remelting process.

  In the present invention, the temperature of the preliminary heat treatment is not particularly limited, but may preferably be 800 to 1300 ° C. When the temperature of the preliminary heat treatment is less than 800 ° C., the operation is difficult due to a temperature drop during forging, and when the temperature of the preliminary heat treatment exceeds 1300 ° C., a high temperature embrittlement phenomenon may occur due to overheating.

  In the present invention, the preliminary heat treatment includes a step of primary heat treatment at a temperature of 1150 to 1300 ° C. for 15 to 25 hours and a step of secondary heat treatment at a temperature of 1100 to 1200 ° C. for 8 to 13 hours. Can do.

  In the present invention, after the preliminary heat treatment, the steel ingot can be forged to produce a mold material. Specifically, the heat-treated steel ingot is forged at a temperature of 850 to 1300 ° C. to destroy the cast structure of the steel ingot, and the internal quality of the steel ingot is reduced by pressure bonding and removal to improve the internal quality. The shape of the mold material can be made. In the present invention, when the execution temperature of the forging process is less than 850 ° C., deformation is difficult during the forging operation and cracks occur. When the temperature exceeds 1300 ° C., high temperature embrittlement due to overheating occurs and cracks occur. Sometimes.

  In the present invention, in the forging step, the forging ratio is preferably 5S or more, and more preferably 5 to 10S. In the present invention, by forging the steel ingot at a forging ratio of 5S or more, the pores existing in the steel ingot are reduced and eliminated, and finally the microstructure of the mold steel can be formed finely. it can. However, when the forging ratio is less than 5S, the structure of the mold steel becomes coarse and the toughness becomes weak, and when such mold steel is applied to die casting, the quality of the produced product may deteriorate. On the other hand, when the forging ratio exceeds 10S, it may cause a problem in the limited size of the cast steel ingot and the working range of the forging press. Therefore, in the present invention, it is preferable to perform the forging process at a forging ratio of 5 to 10S. .

  In the present invention, the spheroidizing heat treatment of the mold material obtained by the forging process as described above can be performed. The forging process makes the microstructure and crystal grains of the mold material coarse and non-uniform. Therefore, in the present invention, by recrystallizing the non-uniform crystal grains and microstructure of the mold material by spheroidizing heat treatment, and making it fine and uniform, good required properties in the subsequent quenching and tempering Can be obtained.

  In this invention, it is preferable that the execution temperature of the said spheroidization heat processing is 650-850 degreeC. When the execution temperature of the spheroidizing heat treatment is less than 650 ° C., recrystallization and crystal grains become non-uniform, and the microstructure is non-uniform even after the spheroidizing heat treatment, and the execution temperature of the spheroidizing heat treatment is 850 ° C. If it exceeds, the crystal grains become coarse and it may be difficult to obtain the desired properties in the subsequent quenching and tempering steps.

  In the present invention, after the spheroidizing heat treatment as described above, a step of cooling to a cooling end temperature of 200 to 300 ° C. can be performed at a cooling rate of 10 to 30 ° C./hr. The cooling method is not particularly limited, and may be any one of oil cooling, air cooling, and water cooling.

  In the present invention, after the cooling step, a quenching step of heat-treating the mold material can be performed. In this invention, it is preferable that the said quenching execution temperature is 900-1030 degreeC, More preferably, 940-1030 degreeC may be sufficient. When the quenching execution temperature is less than 900 ° C., the solid solution effect of the added alloying element is small and the homogenization effect of the structure may be reduced. When the quenching execution temperature exceeds 1030 ° C., the particles become coarse As a result, the hardness of the mold material may be drastically reduced.

  In the present invention, after the quenching step, a high-pressure accelerated cooler is used, and the cooling rate is 0.5 ° C./s or more, preferably 0.5 to 3.0 ° C./s, and 80 to 100 By additionally performing the step of rapidly cooling to the cooling end temperature of 0 ° C., the strength of the finally produced mold steel can be further improved.

  Furthermore, in the present invention, a step of tempering the mold material that has been quenched as described above can be performed. In the present invention, when the tempering execution temperature is 500 to 630 ° C., the brittleness of the steel can be improved, the residual stress can be removed, and the predetermined strength and impact toughness can be obtained by the formation of fine carbides. . When the tempering execution temperature is less than 500 ° C., the temperature is low, residual stress remains, the toughness improvement effect of martensite having brittleness is small, and when the tempering execution temperature exceeds 630 ° C., the hardness decreases rapidly. Sometimes.

  In the present invention, the rapidly cooled mold material is first tempered at a temperature of 580 to 600 ° C. for 3 to 6 hours, and then secondarily tempered at a temperature of 550 to 590 ° C. for 3 to 6 hours. Tertiary tempering can be performed at a temperature of 630 ° C. for 1 to 4 hours.

In the present invention, residual austenite in the structure of the mold material is removed by the primary tempering, fine carbide is formed, and the strength of the mold material can be improved by tempering of martensite.
In the present invention, fine carbides in the structure of the mold material are formed by the secondary tempering, and the strength of the mold material can be further improved by tempering of fresh martensite.
Furthermore, in the present invention, the hardness of the mold material can be precisely adjusted by the tertiary tempering.

  However, after the primary, secondary, and tertiary tempering steps are performed, each of the oil cooling, air cooling, and water cooling is used to cool to a temperature of 80 ° C. or less, thereby uniform. And a fine carbide or martensite structure can be formed.

In the present invention, the mold material tempered as described above can be selectively inspected. It is possible to inspect whether or not there is an unhealthy part in the mold material obtained by the process as described above. If there is an unhealthy part, it can be removed and shipped.
When the step of inspecting as described above is completed, the hot mold steel according to the present invention can be obtained. The hot die steel manufactured by the present invention can be used for die casting used for manufacturing automobile parts.

  The present invention aims at producing a hot die steel under a specific process condition for a steel ingot having a specific composition, and finally, a hot metal excellent in high temperature thermal conductivity and high temperature durability. Shape steel can be manufactured. The hot mold steel manufactured according to the present invention can be used for a long period of time and is environmentally friendly, and can improve all the production quality and production speed of products manufactured from the mold steel.

  Hereinafter, the present invention will be described more specifically through specific examples. The following examples are merely illustrative for understanding of the present invention, and the scope of the present invention is not limited thereto.

Examples [Examples 1 to 12 and Comparative Examples 1 to 8]
First, after preparing a steel ingot having the composition shown in Table 1 and Table 2 below, an electroslag remelting step is performed in an argon gas atmosphere, and forging is performed at a forging ratio of 5.2 S at 1180 ° C. After manufacturing, spheroidizing heat treatment was performed at 800 ° C. Thereafter, hot die steel was manufactured by performing quenching, rapid cooling and tertiary tempering processes under the conditions shown in Table 3 below.

[Experimental Example 1] Thermal conductivity evaluation The thermal conductivity of the mold steel produced in Example 3 was measured at each temperature, and the results are shown in Table 4 and FIG.

As shown in Table 4 and FIG. 1 above, the hot die steel produced according to the present invention exhibits a thermal conductivity of 30 W / mK or higher at room temperature or higher, and approximately 35 W / mK or higher at a high temperature of 100 ° C. or higher. The thermal conductivity is 31 W / mK or higher even at a very high temperature of 600 to 700 ° C.
Thereby, it turns out that the hot metal mold | die steel which concerns on this invention is excellent in high temperature thermal conductivity.

[Experimental Example 2] Evaluation of physical properties In order to confirm the change in strength due to the change in the chromium and titanium contents, the hardness of the hot die steels produced in Examples 2, 3 to 5 and 10 and Comparative Examples 2 and 4 were compared. The yield strength and tensile strength values with respect to are shown graphically in FIGS. 2 and 3, and the impact energy values with respect to hardness are graphically shown in FIG.

  As shown in FIGS. 2 and 3, in Examples 2, 3 to 5 and 10, in which the chromium content falls within the scope of the present invention, the hardness, the yield strength, and the tensile strength are very high. In Comparative Examples 2 and 4, where the chromium content is outside the scope of the present invention, the hardness and the yield strength are remarkably low. Moreover, in the case of Example 10, it turns out that hardness and yield strength are very higher than Examples 2 and 3-5.

  As shown in FIG. 4, in Examples 2, 3 to 5 and 10 in which the chromium content is within the scope of the present invention, the impact energy is very low, whereas the chromium content is outside the scope of the present invention. In Comparative Examples 2 and 4, it can be seen that the impact energy has a very high value. Moreover, in the case of Example 10, it turns out that impact energy is still lower than Example 2 and 3-5.

[Experimental Example 3] Physical Property Evaluation The change in thermal conductivity with temperature was measured for the hot mold steels produced in Example 12 and Comparative Examples 7 and 8, and the results are shown in FIG. The results are shown in Table 5 below, and the toughness evaluation results are shown in FIG.

However, the heat check evaluation conditions were that after heating at 700 ° C. for 13 seconds and cooling for 12 seconds 1,000 times, the average crack length (mm) and the maximum crack length (mm) were measured. In addition, the toughness was evaluated according to the standard NADCA with a U-notch impact toughness of 20 J / cm ² or more.

As shown in FIGS. 5 and 6, it can be seen that the hot mold steel of Example 12 has significantly superior thermal conductivity and impact toughness characteristics than the hot mold steels of Comparative Examples 7 and 8.
Further, as shown in Table 5 above, it can be seen that the heat check characteristics of Example 12 are remarkably superior to those of Comparative Example 7 and have a level equal to or higher than that of Comparative Example 8.

[Experimental Example 4] Evaluation of electroslag remelting condition The steel ingot of the steel example 1 was subjected to an electroslag remelting step under air (20 ton injection) or argon gas (100 kg injection), respectively. The steel ingot dissolved material and the content of hydrogen, oxygen and nitrogen in the steel ingot obtained after the electroslag remelting step in air or argon gas atmosphere were evaluated. Table 6 shows.

  As shown in Table 6, when the electroslag remelting process is performed in an argon gas atmosphere to produce hot mold steel, the nitrogen content in the steel ingot material is higher than that in the molten state. Although it increased slightly, it was found that when air was injected, a considerable amount of nitrogen dissolved from the air formed nitrides.

[Experimental Example 5] Evaluation of Forging Ratio Conditions In order to compare the difference in effect due to the forging ratio, the hot mold steel manufactured in Example 3 was prepared, and the hot metal was manufactured in the same process as in Example 3. Although a mold steel was manufactured, a hot mold steel manufactured by forging with a forging ratio of 3.2 S was prepared as Comparative Example 9. The results of taking surface particle photographs of the hot mold steels of Example 3 and Comparative Example 9 are shown in FIGS. 7A and 7B as follows.

  As shown in FIG. 7 (a), it can be seen that the hot mold steel of Example 3 has a dense structure with a particle size of ASTM # 7 or more, but the hot mold steel of Comparative Example 9 is It was confirmed that the particle size was about ASTM # 2.5 and the particles became coarse.

[Experimental Example 6] Evaluation of quenching temperature condition In order to evaluate the strength change due to the quenching temperature condition, a hot mold steel was manufactured in the same process as in Example 3, but the quenching temperature was 940 ° C, 970. ℃, 1000 ℃, 1030 ℃ and 1060 ℃。 The strength of the finally produced hot mold steel according to the quenching temperature was evaluated, and the result is shown in FIG. 8 below.

  As shown in FIG. 8, as the quenching temperature increased to 940 ° C., 970 ° C., and 1000 ° C., the strength also tended to increase, and the most excellent strength was exhibited at 1030 ° C., but the strength was increased at 1060 ° C. It turns out that it decreases rapidly.

[Experimental Example 7] Evaluation of quenching temperature and tempering temperature conditions In order to evaluate the strength change due to the quenching temperature and tempering temperature conditions, a hot mold steel was manufactured in the same process as in Example 3, but quenching was performed. The temperature was adjusted to 1000 ° C., 1020 ° C. and 1040 ° C., and tempering was performed in one step, but the running temperatures were adjusted to 400 ° C., 550 ° C. and 625 ° C., respectively. The structure photograph of the finally produced hot die steel according to the quenching temperature and the tempering temperature is shown in FIG. 9 below.

As shown in FIG. 9, when quenching is performed at 1000 ° C. and 1020 ° C. and tempering is performed at 550 ° C., it can be seen that the structure particles of the hot mold steel are fine and have a stable structure. When the ching is performed at 1040 ° C., it can be seen that even when tempering is performed at 550 ° C., the particles of the hot mold steel are coarsened and the structure has an irregular structure.
Therefore, according to Experimental Examples 6 and 7, it is understood that quenching is preferably performed at 940 to 1030 ° C. because the structure of the hot mold steel is made dense and strength and toughness are improved.

[Experimental Example 8] Evaluation of Tempering Temperature Conditions In order to evaluate the strength change due to the tempering temperature conditions, hot mold steel is manufactured in the same process as in Examples 1 and 3, but the tempering is performed in one step. The running temperature was adjusted to 0 ° C, 400 ° C, 500 ° C, 550 ° C, 565 ° C, 580 ° C, 600 ° C, 625 ° C and 650 ° C. The strength of the finally produced hot die steel according to the tempering temperature was evaluated, and the result is shown in FIG. 10 below.

  As shown in FIG. 10, it can be seen that when tempering is performed at a temperature of 500 to 600 ° C., the strength is maximized. However, when the tempering execution temperature exceeds 600 ° C., the hardness may rapidly decrease. I understand.

  Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited to these embodiments, and various modifications are possible within the scope of the technical idea of the present invention described in the claims. It will be apparent to those skilled in the art that various modifications and variations are possible.

Claims (23)

  Carbon (C) 0.35 to 0.45 wt%, silicon (Si) 0.20 to 0.30 wt%, manganese (Mn) 0.30 to 0.40 wt%, nickel ( Ni) 0.50-1.20% by weight, chromium (Cr) 1.5-2.2% by weight, molybdenum (Mo) 2.0-2.6% by weight, tungsten (W) 0.0001-1. 0 wt%, titanium (Ti) 0 excess 0.40 wt% or less, vanadium (V) 0.30 to 0.50 wt%, boron (B) 0.0001 to 0.003 wt%, and copper (Cu) 0 Hot mold steel containing 0.005 to 0.02% by weight with the balance containing Fe and inevitable impurities.   The hot mold steel according to claim 1, wherein the hot mold steel further includes 0.02 to 0.08 wt% of aluminum (Al).   The hot mold steel according to claim 1 or 2, wherein the hot mold steel further contains 0.005 to 0.06 wt% of nitrogen (N).   4. The hot mold steel according to claim 1, further comprising 0.001 to 0.006 wt% phosphorus (P) and 0.0001 to 0.002 wt% sulfur (S). 5. Hot mold steel as described in. The values of carbon, silicon, manganese, chromium, molybdenum and nickel constituting the hot die steel are substituted into the following formula (1), and the value is 25 or more. Hot mold steel according to any one of the following:
F (C) × F (Si) × F (Mn) × F (Cr) × F (Mo) × F (Ni) Formula (1)
(However, in the above formula (1),
F (C) = 0.37-0.39 × (0.12 ^ carbon content (%));
F (Si) = 0.7 × silicon content (%) + 1;
F (Mn) = 3.35 × manganese content (%) + 1;
F (Cr) = 2.16 × chromium content (%) + 1;
F (Ni) = 0.36 × nickel content (%) + 1; and F (Mo) = 3 × molybdenum content (%) + 1).   The heat according to claim 5, wherein when the values of the contents of carbon, silicon, manganese, chromium, molybdenum and nickel constituting the hot mold steel are substituted into the above formula (1), the value is 30 or more. Inter-mold steel. When the content values of molybdenum and tungsten constituting the hot mold steel are substituted into the following formula (2), the value is 2 or more and 3 or less, according to any one of claims 1 to 6. Hot mold steel described:
Molybdenum content (%) + 0.5 × tungsten content (%) Formula (2). Any one of claims 1 to 7, wherein when the content values of titanium and vanadium forming the hot mold steel are substituted into the following formula (3), the value is 0.4 or more and 0.5 or less. Hot mold steel according to item 1:
Titanium content (%) + Vanadium content (%) Formula (3). The value according to any one of claims 1 to 8, wherein when the values of the contents of chromium, molybdenum and tungsten forming the hot mold steel are substituted into the following formula (4), the value is 9 or more. Hot mold steel described:
Chromium content (%) + 3.3 × {molybdenum content (%) + 0.5 × tungsten content (%)} Equation (4).   The hot mold steel according to any one of claims 1 to 9, wherein the hot mold steel is for die-casting. Carbon (C) 0.35 to 0.45 wt%, silicon (Si) 0.20 to 0.30 wt%, manganese (Mn) 0.30 to 0.40 wt%, nickel ( Ni) 0.50-1.20% by weight, chromium (Cr) 1.5-2.2% by weight, molybdenum (Mo) 2.0-2.6% by weight, tungsten (W) 0.0001-1. 0 wt%, titanium (Ti) 0 excess 0.40 wt% or less, vanadium (V) 0.30 to 0.50 wt%, boron (B) 0.0001 to 0.003 wt%, and copper (Cu) 0 Producing a steel ingot comprising 0.005 to 0.02 wt%, the remainder comprising Fe and inevitable impurities;
Forging the steel ingot to produce a mold material;
Quenching the mold material;
And a step of tempering after the quenching.   The method for producing hot die steel according to claim 11, wherein the steel ingot further includes 0.02 to 0.08 wt% of aluminum (Al).   The said steel ingot is a manufacturing method of the hot die steel of Claim 11 or Claim 12 which further contains nitrogen (N) 0.005-0.06 weight%.   14. The steel ingot according to claim 11, further comprising 0.001 to 0.006 wt% phosphorus (P) and 0.0001 to 0.002 wt% sulfur (S). Manufacturing method for hot mold steel.   The manufacturing of hot die steel according to any one of claims 11 to 14, further comprising performing an electro-slag remelting (ESR) process before forging the steel ingot. Method.   The method for producing hot die steel according to claim 15, wherein the electroslag remelting step is performed in an argon gas atmosphere.   The method for producing hot die steel according to any one of claims 11 to 16, further comprising a step of pre-heating the steel ingot at a temperature of 800 to 1300 ° C before forging the steel ingot.   The method for producing hot die steel according to any one of claims 11 to 17, wherein the forging is performed at a forging ratio of 5S or more.   The method for producing hot die steel according to any one of claims 11 to 18, wherein the forging is performed at a temperature of 850 to 1300 ° C.   The method for producing hot mold steel according to any one of claims 11 to 19, wherein the quenching is performed at a temperature of 900 to 1030 ° C.   The said tempering is a manufacturing method of the hot die steel as described in any one of Claim 11 to 20 performed under the temperature of 500-630 degreeC. The tempering includes primary tempering at a temperature of 580 to 600 ° C .;
The method for producing hot die steel according to any one of claims 11 to 21, comprising a step of secondary tempering at a temperature of 550 to 590 ° C.   The method for producing hot die steel according to claim 22, further comprising a step of tertiary tempering at a temperature of 610 to 630 ° C after the secondary tempering.