Method for improving cleanliness of rare earth treated die steel

Abstract

The invention belongs to the technical field of steel smelting, and particularly relates to a method for improving the cleanliness of rare earth treated die steel. In order to solve the problems that inclusions exceed the standard and rare earth inclusions are easy to gather at a nozzle to cause nozzle nodulation during rare earth treatment of die steel, Mg is added before rare earth treatment to reduce the oxygen content and deformed inclusions in steel, Ca is added after rare earth treatment and before molten steel pouring, and the content of impurities in the molten steel is reduced. The production of aluminum oxide and rare earth aluminate inclusions in the cooling and solidification process is reduced, the B-type inclusions are finally controlled, the problem of water gap blockage after rare earth treatment is solved, and therefore the steel cleanliness and the product performance are improved. The method not only can be popularized and applied to rare earth treatment die steel, but also can be applied to other types of rare earth treatment steel, solves the problems of water gap blockage and difficulty in stable control of inclusions after rare earth treatment, and has good popularization and application prospects.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, specifically relating to a method for improving the cleanliness of rare earth treatment mold steel. Background Technology

[0002] Rare earth elements can significantly purify molten steel, effectively modify inclusions, improve the corrosion resistance of stainless steel, and effectively control the carbide level in steel, thereby improving the toughness of steel. However, rare earth treatment of steel has many drawbacks: (1) Rare earth inclusions are large in size and difficult to float and remove, thus affecting the cleanliness and performance of steel; (2) Uneven distribution of rare earth elements will affect the inclusion refinement effect; (3) Rare earth inclusions are prone to accumulate at the nozzle, leading to nozzle nodules; (4) Class B inclusions are easily generated, resulting in reduced steel cleanliness and unstable product quality.

[0003] Therefore, to overcome the aforementioned defects after rare earth treatment, patent CN114058767A discloses a method for refining rare earth inclusions in super stainless steel. This method involves Ca treatment before rare earth treatment and proposes a formula for calculating the amount of silicon-calcium wire fed into the steel to increase the rare earth recovery rate and improve the inclusion refinement effect. However, it requires checking the Ca content in the steel before feeding, which is difficult to implement in practice; it also does not specify a range for rare earth content. For example, patent CN111593252A discloses a rare earth steel smelting method. After RH refining and vacuum treatment, before adding rare earth, Ca wire is fed in. A specific container is designed to hold the rare earth and add it to the molten steel to ensure the rare earth recovery rate and uniform distribution in the steel, thereby refining inclusions and improving steel cleanliness. However, it requires the rare earth addition to be between 0.08 and 0.1 kg / t of molten steel, resulting in a narrow range for rare earth content control and limiting its application. Summary of the Invention

[0004] To address the issues of excessive inclusions and the tendency of rare earth inclusions to accumulate at the sprue gate during rare earth treatment of mold steel, leading to sprue gate blockage, this invention reduces the oxygen content and deformation inclusions by adding Mg before rare earth treatment. Furthermore, after rare earth treatment and before pouring molten steel, Ca is added to reduce the formation of alumina and rare earth aluminate inclusions during cooling and solidification, ultimately controlling Class B inclusions and resolving the sprue gate blockage problem after rare earth treatment, thereby improving steel cleanliness and product performance.

[0005] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows: In a first aspect, the present invention provides a method for improving the cleanliness of rare earth treated mold steel, comprising the following steps: (1) Electric arc furnace smelting: The steelmaking raw materials are smelted in an electric arc furnace, and the steel is tapped when the P content meets the requirements of the steel composition; the mold steel is Cr12 or Cr18 series steel; (2) LF refining: After the molten steel ladle enters the LF, the composition of the molten steel is adjusted and then fed into the Al wire to ensure that the Al content of the molten steel meets the requirements of 0.01-0.03%. Then, slag additives are added to ensure that the composition of the LF slag meets the requirements of: Al2O3 26-35%, CaO 45-55%, MgO ≤6%, SiO2 15-25%. At the same time, the composition of the refining slag is controlled to ensure that the Al content in the steel after vacuum treatment is ≤0.01%. (3) VD vacuum treatment: VD vacuum treatment is carried out after LF refining; (4) Mg treatment: After VD is broken, Mg-containing alloy is added for Mg treatment; (5) RE treatment: After Mg treatment, La / Ce composite rare earth treatment is added; (6) VD soft blowing: After adding rare earth elements, control the temperature and perform soft blowing of argon; (7) Ca treatment: Take a sample and measure the temperature during soft blowing. When the temperature is 10-20°C higher than the pouring temperature, feed in the Ca wire. (8) Pouring: After feeding in the Ca line, let it stand and then pour it.

[0006] Furthermore, the mold steel is at least one of Cr12MoV, Cr12MoV1, D2, SKD11 or 95Cr18.

[0007] Preferably, the composition of the D2 steel, by mass percentage, includes: C: 1.40-1.60%, Si≤0.60%, Mn≤0.60%, S≤0.03%, P≤0.03%, Cr: 11.5-13.00%, V≤1.00%, Mo≤0.70-1.20%, with the remainder being Fe and unavoidable impurities.

[0008] Preferably, the composition of the Cr12MoV steel, by mass percentage, includes: C: 1.45-1.70%, Si: ≤0.40%, Mn ≤0.40%, S ≤0.03%, P ≤0.03%, Cr: 11.00-12.50%, V: 0.15-0.30%, Mo: 0.40-1.60%, with the remainder being Fe and unavoidable impurities.

[0009] Furthermore, in step (1), the smelting is carried out using the return method.

[0010] Furthermore, in step (1), the tapping temperature is controlled at 1620-1660℃.

[0011] Furthermore, in step (2), the slag additives contain refining slag, lime and quartz sand.

[0012] Furthermore, in step (3), the thickness of the ladle slag during vacuum treatment is 60–100 mm.

[0013] Further, in step (3), the VD vacuum treatment conditions are: ultimate vacuum ≤ 66 Pa, vacuum maintained for 25 to 45 min, and argon flow rate 150 to 250 L / min.

[0014] Further, in step (4), the Mg-containing alloy is a Ni-Mg alloy with 15% content, and the addition amount is 0.02 to 0.06 kg / t.

[0015] Further, in step (5), the amount of composite rare earth added is 0.2–0.7 kg / t. Preferably, the composite rare earth is a La / Ce composite rare earth with a content >95%.

[0016] Furthermore, in step (6), a steel ladle covering agent is used to control the temperature. Preferably, the steel ladle covering agent is carbonized rice husk, and the amount added is 1.0 to 1.5 kg / t.

[0017] Furthermore, in step (6), the argon flow rate is 30-50 L / min and maintained for more than 40 min.

[0018] Further, in step (7), the amount of Ca added is 0.01 to 0.04 kg / t.

[0019] Furthermore, in step (8), the settling time is 3 to 5 minutes.

[0020] Beneficial Effects: This invention reduces the oxygen content and deformation inclusions in steel by adding Mg before rare earth treatment, and further reduces the formation of alumina and rare earth aluminate inclusions during cooling and solidification by adding Ca after rare earth treatment and before pouring the molten steel, ultimately controlling Class B inclusions. This method can be widely applied not only to rare earth treated die steels but also to other types of rare earth treated steels, solving the problems of nozzle blockage and difficulty in stable inclusion control after rare earth treatment, and has good prospects for widespread application. Attached Figure Description

[0021] Figure 1 The image shows the typical inclusion morphology (top) and energy dispersive spectral composition (bottom) in Example 1. Figure 2 The image shows the morphology (top) and energy dispersive spectral composition (bottom) of typical inclusions in Comparative Example 1. Detailed Implementation

[0022] To make the technical problems, solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with the embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as understood by one of ordinary skill in the art.

[0023] In one embodiment of the present invention, a method for improving the cleanliness of rare earth treated mold steel is provided, comprising the following steps: (1) Electric arc furnace smelting: The steelmaking raw materials are smelted in an electric arc furnace, and the steel is tapped when the P content meets the requirements of the steel composition; the mold steel is Cr12 or Cr18 series steel; (2) LF refining: After the molten steel ladle enters the LF, the composition of the molten steel is adjusted and then fed into the Al wire to ensure that the Al content of the molten steel meets the requirements of 0.01-0.03%. Then, slag additives are added to ensure that the composition of the LF slag meets the requirements of: Al2O3 26-35%, CaO 45-55%, MgO ≤6%, SiO2 15-25%. At the same time, the composition of the refining slag is controlled to ensure that the Al content in the steel after vacuum treatment is ≤0.01%. (3) VD vacuum treatment: VD vacuum treatment is carried out after LF refining; (4) Mg treatment: After VD is broken, Mg-containing alloy is added for Mg treatment; (5) RE treatment: After Mg treatment, La / Ce composite rare earth treatment is added; (6) VD soft blowing: After adding rare earth elements, control the temperature and perform soft blowing of argon; (7) Ca treatment: Take a sample and measure the temperature during soft blowing. When the temperature is 10-20°C higher than the pouring temperature, feed in the Ca wire. (8) Pouring: After feeding in the Ca line, let it stand and then pour it.

[0024] In one specific embodiment of the present invention, the mold steel is at least one of Cr12MoV, Cr12MoV1, D2, SKD11 or 95Cr18.

[0025] Preferably, the composition of the D2 steel, by mass percentage, includes: C: 1.40-1.60%, Si≤0.60%, Mn≤0.60%, S≤0.03%, P≤0.03%, Cr: 11.5-13.00%, V≤1.00%, Mo≤0.70-1.20%, with the remainder being Fe and unavoidable impurities.

[0026] Preferably, the composition of the Cr12MoV steel, by mass percentage, includes: C: 1.45-1.70%, Si: ≤0.40%, Mn ≤0.40%, S ≤0.03%, P ≤0.03%, Cr: 11.00-12.50%, V: 0.15-0.30%, Mo: 0.40-1.60%, with the remainder being Fe and unavoidable impurities.

[0027] In one specific embodiment of the present invention, in step (1), the smelting is carried out by the return method.

[0028] In one specific embodiment of the present invention, in step (1), the tapping temperature is controlled at 1620-1660℃.

[0029] In step (2) of this invention, 0.01-0.03% Al is fed in to ensure that the oxygen content in the steel is removed as much as possible, and the composition of the slag additives, especially the refining slag, is controlled to ensure that Als in the steel after vacuum treatment is ≤0.01%.

[0030] In one specific embodiment of the present invention, in step (3), the thickness of the ladle slag during vacuum treatment is 60-100 mm.

[0031] In a specific embodiment of the present invention, in step (3), the VD vacuum treatment conditions are: ultimate vacuum ≤ 66 Pa, vacuum maintained for 25 to 45 min, and argon flow rate 150 to 250 L / min.

[0032] In step (4) of this invention, Mg treatment can further modify the inclusions in the steel and perform deep deoxidation.

[0033] In one specific embodiment of the present invention, the Mg-containing alloy is a Ni-Mg alloy with 15% content, and the addition amount is 0.02 to 0.06 kg / t.

[0034] In one specific embodiment of the present invention, in step (5), the content of the composite rare earth is >95%, and the amount added is 0.2 to 0.7 kg / t.

[0035] In one specific embodiment of the present invention, in step (6), a ladle covering agent is used to control the temperature.

[0036] In this invention, the addition of a ladle covering agent to fully cover the molten steel ensures that the temperature does not drop too quickly, allowing rare earth inclusions to effectively float to the surface and be absorbed by the slag. Preferably, the ladle covering agent is carbonized rice husk, and the addition amount is 1.0–1.5 kg / t.

[0037] In one specific embodiment of the present invention, in step (6), the argon flow rate is 30-50 L / min and is maintained for more than 40 minutes.

[0038] In one specific embodiment of the present invention, in step (7), the amount of Ca added is 0.01 to 0.04 kg / t.

[0039] In this invention, feeding in Ca wire can further suppress alumina or rare earth aluminate inclusions generated during casting and solidification.

[0040] In one specific embodiment of the present invention, in step (8), the settling time is 3 to 5 minutes.

[0041] The following specific embodiments will be provided to explain the solution of the present invention. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0042] Example 1 In this embodiment, the steel smelted is D2 steel, with C: 1.40-1.60%, Si≤0.60%, Mn≤0.60%, S≤0.03%, P≤0.03%, Cr: 11.5-13.00%, V≤1.00%, Mo≤0.70-1.20%, and the remainder being Fe and unavoidable impurities.

[0043] 1. Electric arc furnace smelting: The raw materials are mainly D2 Benxi Steel return material, Benxi Steel similar alloy return material and industrial pure iron. The electric arc furnace adopts the return method for smelting. The composition of the electric furnace requires that the P content is less than 0.03% and the tapping temperature is 1630 ℃.

[0044] 2. LF furnace refining: After the ladle enters the LF furnace, alloys are added to adjust the composition of the molten steel, and then electricity is supplied to raise the temperature. Once the composition of the molten steel meets the requirements of the steel grade (C: 1.45%, Si: 0.56%, Mn: 0.55%, S: 0.002%, P: 0.01%, Cr: 12.1%, V: 0.98%, Mo: 0.81%), it is fed into the Al line for deoxidation. When the Al content of the molten steel reaches 0.03%, slag additives (refining slag (Al2O3: 80%, MgO: 12%, CaO: 4%, SiO2: 2%), lime (CaO≥90%), and quartz sand (SiO2≥90%) are added. The resulting LF slag composition includes: Al2O3 26.31%, CaO 48.12%, MgO 5.13%, SiO2 18.12%, with the remainder being impurities such as CaF2 and FeO.

[0045] 3. Vacuum Treatment (VD): After LF refining, VD treatment is performed, with the slag thickness in the ladle being 60–100 mm. After ladle refining, VD vacuum treatment is performed, maintaining a limiting vacuum of 66 Pa for 32 min at an argon flow rate of 180 L / min.

[0046] 4. Mg treatment: After VD breaks the void, the Al content is 0.008%. 0.06 kg / t of Ni-Mg alloy containing 15% Mg is added to further modify the inclusions in the steel and to carry out deep deoxidation.

[0047] 5. RE treatment: After adding Mg for 5 min, 0.3 kg / t of La / Ce composite rare earth (rare earth content greater than 95%, La:Ce ratio of 2:3) is added to the molten steel.

[0048] 6. VD soft blowing: After adding rare earth elements, add 1.5 kg / t of carbonized rice husk as a ladle covering agent to fully cover the molten steel and ensure that the temperature does not drop too quickly; at the same time, perform soft blowing of argon at a flow rate of 32 L / min for 45 min.

[0049] 7. Ca treatment: During the soft blowing process, the temperature was measured at 1470℃, and 0.02 kg / t of Ca line was fed in.

[0050] 8. Casting: After feeding in the Ca wire, let it stand for 3 minutes. Use a protective device to safeguard the casting process. After casting, pour the molten steel into shape to obtain the finished steel. Rating is performed according to national standard GB / T 10561-2023 (coarse and fine refer to the width of inclusions, 0 and 1 refer to the inclusion rating). The inclusion ratings are shown in Table 1, and typical inclusions are shown in... Figure 1 .

[0051] Depend on Figure 1 As shown in Table 1, the inclusions in the finished steel D2 of this embodiment are spherical and mainly consist of rare earth sulfides, indicating a high degree of cleanliness.

[0052] Table 1: Grade of Inclusions in Finished Steel in Example 1 Example 2 In this embodiment, the steel smelted is Cr12MoV steel, with C: 1.45-1.70%, Si≤0.40%, Mn≤0.40%, S≤0.03%, P≤0.03%, Cr: 11.00-12.50%, V: 0.15-0.30%, Mo: 0.40-0.60%, and the remainder being Fe and unavoidable impurities.

[0053] 1. Electric arc furnace smelting: The raw materials are mainly Cr12MoV steel recycled material, similar alloy recycled material and industrial pure iron. The electric arc furnace adopts the recycled method for smelting. The composition of the electric furnace requires that the P content be less than 0.03% and the tapping temperature be 1635 ℃.

[0054] 2. LF furnace refining: After the ladle enters the LF furnace, alloys are added to adjust the composition of the molten steel, and then electricity is supplied to raise the temperature. After the composition of the molten steel meets the requirements of the steel grade (C: 1.50%, Si: 0.28%, Mn: 0.36%, S: 0.002%, P: 0.01%, Cr: 11.8%, V: 0.28%, Mo: 0.51%), it is fed into the Al line for deoxidation. When the Al content of the molten steel reaches 0.03%, slag additives (refining slag (Al2O3 80%, MgO 12%, CaO 4%, SiO2 2%), lime (CaO ≥ 90%) and quartz sand (SiO2 ≥ 90%)) are added to obtain the LF slag composition including: Al2O3 27.28%, CaO 46.01%, MgO ≤ 5.21%, SiO2 20.12%, and the remainder being impurities such as CaF2 and FeO.

[0055] 3. Vacuum Treatment (VD): After LF refining, VD treatment is performed, with the slag thickness in the ladle being 60–100 mm. After ladle refining, VD vacuum treatment is performed, maintaining a limiting vacuum of 66 Pa for 35 min at an argon flow rate of 180 L / min.

[0056] 4. Mg treatment: After VD breaks the void, the Al content is 0.006%. 0.06 kg / t of Ni-Mg alloy containing 15% Mg is added to further modify the inclusions in the steel and to carry out deep deoxidation.

[0057] 5. RE treatment: After adding Mg for 5 min, 0.4 kg / t of La / Ce composite rare earth (rare earth content greater than 95%, La:Ce ratio of 2:3) is added to the molten steel.

[0058] 6. VD soft blowing: After adding rare earth elements, add 1.2 kg / t of carbonized rice husk as a ladle covering agent to fully cover the molten steel and ensure that the temperature does not drop too quickly; at the same time, perform soft blowing of argon at a flow rate of 32 L / min for 45 min.

[0059] 7. Ca treatment: During the soft blowing process, the temperature was measured at 1465℃, and 0.03 kg / t of Ca line was fed in.

[0060] 8. Casting: After feeding in Ca wire, let stand for 3 minutes. Use a protective device to protect the casting process in the ladle. After the ladle is lifted, the molten steel is cast into shape to obtain the finished steel.

[0061] Table 2: Grade of Inclusions in Finished Steel from Example 2 Comparative Example 1 The comparative example steel used was the same as that in Example 2, namely Cr12MoV steel, with C: 1.45-1.70%, Si≤0.40%, Mn≤0.40%, S≤0.03%, P≤0.03%, Cr: 11.00-12.50%, V: 0.15-0.30%, Mo: 0.40-0.60%, and the remainder being Fe and unavoidable impurities.

[0062] 1. Electric arc furnace smelting: The raw materials are mainly Cr12MoV steel recycled material, similar alloy recycled material and industrial pure iron. The electric arc furnace adopts the recycled method for smelting. The composition of the electric furnace requires that the P content be less than 0.03% and the tapping temperature be 1638℃.

[0063] 2. LF furnace refining: After the ladle enters the LF furnace, alloys are added to adjust the composition of the molten steel, and then electricity is supplied to raise the temperature. After the composition of the molten steel meets the requirements of the steel grade (C: 1.51%, Si: 0.31%, Mn: 0.34%, S: 0.002%, P: 0.01%, Cr: 12.1%, V: 0.29%, Mo: 0.49%), it is fed into the Al line for deoxidation. When the Al content of the molten steel reaches 0.03%, slag additives (refining slag (Al2O3: 80%, MgO: 12%, CaO: 4%, SiO2: 2%), lime (CaO≥90%) and quartz sand (SiO2≥90%)) are added. The resulting LF slag composition includes: Al2O3: 31.14%, CaO: 53.11%, MgO: 6.23%, SiO2: 8.89%, with the remainder being impurities such as CaF2 and FeO.

[0064] 3. Vacuum Treatment (VD): After LF refining, VD treatment is performed, with the slag thickness in the ladle being 60–100 mm. After ladle refining, VD vacuum treatment is performed, maintaining a limiting vacuum of 66 Pa for 38 min at an argon flow rate of 180 L / min.

[0065] 5. RE treatment: After VD cavitation, the Al content is 0.015%; 0.4 kg / t of La / Ce composite rare earth (rare earth content greater than 95%, La:Ce ratio of 2:3) is added to the molten steel.

[0066] 6. VD soft blowing: After adding rare earth elements, add 0.8 kg / t of carbonized rice husk as a ladle covering agent; at the same time, perform soft blowing of argon at a flow rate of 32 L / min for 35 min.

[0067] 7. VD hoisting: Temperature was measured during the soft blowing process, and the temperature was 1455℃, after which preparations were made for pouring.

[0068] 8. Casting: A protective device is used during casting. After the ladle is lifted, the molten steel is poured into shape to obtain the finished steel. The inclusions are rated according to the national standard GB / T 10561-2023. The inclusion rating is shown in Table 3, and typical inclusions are shown in... Figure 2 .

[0069] Depend on Figure 2 As shown in Table 3, the inclusions in the comparative example finished steel are elongated and mainly consist of rare earth aluminates. The B-class inclusion level is higher than that in the example, and the overall cleanliness is poor.

[0070] Table 3: Rating of Inclusions in Finished Steel Products of Comparative Example 1 It should be noted that the specific features, structures, materials, or characteristics described in this specification may be combined in any suitable manner in one or more embodiments. Furthermore, those skilled in the art can combine and integrate the different embodiments described in this specification and the features of those embodiments without contradiction.

Claims

1. A method for improving the cleanliness of rare earth treated mold steel, characterized in that, Includes the following steps: (1) Electric arc furnace smelting: The steelmaking raw materials are smelted in an electric arc furnace, and the steel is tapped when the P content meets the requirements of the steel composition; the mold steel is Cr12 or Cr18 series steel; (2) LF refining: After the molten steel ladle enters the LF, the composition of the molten steel is adjusted and then fed into the Al wire to ensure that the Al content of the molten steel meets the requirements of 0.01-0.03%. Then, slag additives are added to ensure that the composition of the LF slag meets the requirements of: Al2O3 26-35%, CaO 45-55%, MgO≤6%, SiO2 15-25%. At the same time, the composition of the refining slag is controlled to ensure that the Al content in the steel after vacuum treatment is ≤0.01%. (3) VD vacuum treatment: VD vacuum treatment is carried out after LF refining; (4) Mg treatment: After VD is broken, Mg-containing alloy is added for Mg treatment; (5) RE treatment: After Mg treatment, La / Ce composite rare earth treatment is added; (6) VD soft blowing: After adding rare earth elements, control the temperature and perform soft blowing of argon; (7) Ca treatment: Take a sample and measure the temperature during soft blowing. When the temperature is 10-20°C higher than the pouring temperature, feed in the Ca wire. (8) Pouring: After feeding in the Ca line, let it stand and then pour it.

2. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, The mold steel is at least one of Cr12MoV, Cr12MoV1, D2, SKD11 or 95Cr18.

3. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (1), the smelting is carried out by the return method, and the tapping temperature is controlled at 1620-1660℃.

4. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (2), the slag auxiliary material contains refining slag, lime and quartz sand.

5. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (3), the VD vacuum treatment conditions are: ultimate vacuum ≤ 66 Pa, vacuum maintained for 25 to 45 min, and argon flow rate 150 to 250 L / min.

6. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (4), the Mg-containing alloy is a 15% Ni-Mg alloy, and the amount added is 0.02 to 0.06 kg / t.

7. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (5), the amount of composite rare earth added is 0.2 to 0.7 kg / t.

8. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (6), a steel ladle covering agent is used to control the temperature. The steel ladle covering agent is carbonized rice husk, and the amount added is 1.0 to 1.5 kg / t.

9. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (6), the argon flow rate is 30-50 L / min and maintained for more than 40 min.

10. The method for improving the cleanliness of rare earth treated mold steel according to claim 1, characterized in that, In step (7), the amount of Ca added is 0.01 to 0.04 kg / t.

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