A kind of 20MnCrS5 steel and its preparation method and application

Abstract

The invention discloses a 20MnCrS5 steel, belonging to the technical field of preparation of large-size gear steel. The invention is composed of the following chemical elements in percentage by weight: C: 0.17% to 0.22%, Si: 0.15% to 0.40%, Mn: 1.10% to 1.30%, P≤0.035%, S: 0.035% to 0.045%, Cr: 1.00% to 1.30%, Alt: 0.020% to 0.060%, and the remainder is iron and inevitable impurities. The invention adopts a die casting process to improve the cooling conditions of the steel ingot, so that the temperature gradient and cooling intensity during the solidification of the ingot are weaker than those of the continuous casting process, thereby avoiding the overlap of the dendrites of the solidified structure, reducing the element segregation, and improving the compactness of the steel.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of large-size gear steel, and specifically relates to 20MnC rS5 steel and a preparation method and application thereof. Background Art

[0002] 20MnCr5 is an alloy structural steel, carburizing steel, with high strength, toughness and good hardenability. 20MnCrS5 is an improved steel of 20MnCr5. 20MnCrS5 belongs to alloy structural steel, carburizing steel, and can also be used as quenched and tempered steel. It has good hardenability, small heat treatment deformation, good wear resistance, good low temperature toughness, and good cutting performance. It can be used as carburized parts, quenched and tempered parts with large specifications and cross-sections and high loads, such as large wind power gears, shafts, worms, sleeves, friction wheels, etc.

[0003] As mentioned above, 20MnCrS5 can be used for steel parts with larger specifications and cross-sections, such as large wind power gears, etc., which requires the original 20MnCrS5 steel to have larger specifications, and the parameters related to solidification, such as the density, segregation, looseness and shrinkage of the steel, must meet the use requirements. However, the 20MnCrS5 prepared by continuous casting technology cannot fully meet this requirement, especially for large-size steels with specifications above φ180mm. The reason is that continuous casting is equivalent to the solidification of a steel ingot with a particularly large height and width ratio. Since the solidification of the billet is subjected to strong forced cooling by water spray, the temperature gradient and cooling intensity during solidification are much greater than those of the steel ingot under slow cooling conditions, so the columnar crystals are relatively developed, especially in the late stage of solidification of the billet, due to the overlap of columnar dendrites, insufficient or reduced volume of equiaxed dendrites, and the solidification and crystallization phenomenon of small steel ingots is formed, which is easy to form a solidification bridge. There is a central shrinkage cavity and looseness at the bottom of the solidification bridge, which aggravates the segregation of elements and causes the billet to be brittle and fractured during hot working. In addition, the cross section of the continuous casting billet is generally small, and it solidifies first from the side, which may form a liquid phase gap as deep as tens of meters inside, which is easy to cause defects such as central shrinkage due to insufficient steel liquid filling. It can be seen that the solidification characteristics of the continuous casting billet determine that it will inevitably produce internal defects such as central segregation and shrinkage.

[0004] At present, 20MnCrS5 produced by continuous casting process has a certain degree of castability due to the continuity of production. However, since 20MnCrS5 is a sulfur-containing steel, sulfur exists in the steel in the form of sulfide inclusions. When the inclusions increase, they will gather and form nodules at the continuous casting nozzle and block the nozzle, resulting in poor castability. In the publicly published document "Technology Practice for Improving the Castability of Low-Silicon 20MnCrS5 Steel Molten Steel", "Hebei Metallurgy" introduced that during the smelting of 20MnCrS5, oxygen and sulfur with high content in molten steel react to generate more CaS inclusions, which deteriorates the purity of molten steel. A large number of inclusions are formed during the smelting and continuous casting process. High melting point inclusions accumulate and block the nozzle during the continuous casting process, which deteriorates the castability of molten steel. Although the process has been improved, the number of continuous casting furnaces is also low. Summary of the invention

[0005] In view of the above-mentioned deficiencies, the present invention provides a 20MnCrS5 steel and a preparation method and application thereof, which avoids dendrite overlap in solidification structure, reduces element segregation, and improves the compactness of the steel.

[0006] The first aspect of the present invention protects a 20MnCrS5 steel, which is composed of the following chemical elements in percentage by weight: C: 0.17% to 0.22%, Si: 0.15% to 0.40%, Mn: 1.10% to 1.30%, P≤0.035%, S: 0.035% to 0.045%, Cr: 1.00% to 1.30%, Alt: 0.020% to 0.060%, and the balance is iron and inevitable impurities.

[0007] The second aspect of the present invention protects a method for preparing 20MnCrS5 steel, comprising the following steps: electric furnace smelting, LF furnace refining, VD furnace refining, die casting, heating and rolling;

[0008] The mold casting includes introducing a first inert gas to protect the pouring before pouring, and the first inert gas is preferably argon. The amount of molten steel drained is set to be ≥100kg; the time from the ladle being lifted out of the VD furnace to the start of pouring is controlled to be ≤10min, and the superheat is controlled at 25℃~35℃; during pouring, the ladle water outlet is 150~200mm higher than the funnel brick.

[0009] Furthermore, the electric furnace smelting includes controlling the final carbon of steel tapping to be 0.05%≤C≤0.13%, the final phosphorus to be P≤0.020%, and the electric furnace steel tapping temperature to be 1640°C~1680°C.

[0010] Furthermore, the LF furnace refining includes white slag time ≥ 25 min, a second inert gas, which is preferably argon, a pressure of 0.2 MPa to 0.4 MPa, and the LF furnace bag temperature is controlled at 1665° C. to 1695° C.

[0011] Furthermore, the VD furnace refining includes feeding high calcium wire, vacuum treatment, the vacuum degree of the vacuum treatment is ≤100Pa, the time of the vacuum treatment is ≥15min, breaking the vacuum, feeding the sulfur wire, static blowing of the third inert gas, the third inert gas is preferably argon, the static blowing time of the third inert gas is 15min~35min, the static blowing pressure is 0.1Mpa~0.3Mpa, and the VD furnace bag temperature is 1579℃~1611℃.

[0012] Furthermore, the heating includes heating in a soaking furnace, keeping the temperature at 200°C for 0.5h, then heating to 1200°C-1220°C for 3h-3.5h, and finally keeping the temperature at 1200°C-1220°C for 1.5h-2h to ensure uniform temperature of the ingot and thorough burning.

[0013] Furthermore, the rolling includes a starting rolling temperature of 1100°C to 1200°C, a final rolling temperature of 850°C to 1000°C, a rolling specification range of φ180mm to φ250mm, and after rolling, the steel is placed in an insulation pit for slow cooling, the pit entry temperature is ≥550°C, the pit exit temperature is ≤200°C, and the slow cooling time is ≥24h.

[0014] A third aspect of the present invention protects the use of the above-mentioned 20MnCrS5 steel in gears.

[0015] In order to achieve the above object, the present invention adopts the following technical solutions:

[0016] Beneficial effects:

[0017] (1) The present invention aims at the deficiencies of the existing continuous casting process of 20MnCrS5 in terms of the density, segregation, looseness and shrinkage of the steel. By adopting a die casting process, the cooling conditions of the steel ingot can be improved, so that the temperature gradient and cooling intensity during the solidification of the ingot are weaker than those of the continuous casting process, thereby avoiding the overlap of the dendrites in the solidification structure, reducing the segregation of elements, and improving the density of the steel.

[0018] (2) The existing technology 20MnCrS5 has the characteristic of nozzle clogging due to the continuous casting process itself. In addition, 20MnCrS5 is a sulfur-containing steel. Sulfur exists in the steel in the form of sulfide inclusions, which is more likely to accumulate and form nodules at the continuous casting nozzle and block the nozzle, resulting in poor castability and ultimately affecting the quality of the steel. The die casting process is different from the continuous casting process and requires continuous casting in multiple furnaces, which can effectively avoid product fluctuations caused by continuous casting interruptions.

[0019] (3) The 20MnCrS5 of the present invention has a specification range of φ180mm~φ250mm, general porosity ≤0.5 level, central porosity ≤1.0 level, ingot segregation ≤0.5 level, ultrasonic flaw detection ≥A level, and grain size ≥7 level, all of which are superior to continuous casting materials of the same specifications. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a low-magnification inspection sample image of Example 1 of the present invention.

[0021] Figure 2 This is a low-magnification inspection sample diagram of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0022] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0023] The 20MnCrS5 in this application is prepared by adding sulfur to 20MnCr5 to increase the cutting performance of the steel. Sulfur exists in the steel in the form of sulfides, which can play a role in solid lubrication and wrapping to reduce wear, so that the steel can obtain good cutting performance. Good cutting properties can make the product surface smooth, and easy chip breaking during cutting makes the machining tool less stressed and energy-consuming, and the tool life is long.

[0024] The 20MnCrS5 of the present invention is composed of the following chemical elements in percentage by weight: C: 0.17% to 0.22%, Si: 0.15% to 0.40%, Mn: 1.10% to 1.30%, P≤0.035%, S: 0.035% to 0.045%, Cr: 1.00% to 1.30%, Alt: 0.020% to 0.060%, and the remainder is iron and inevitable impurities.

[0025] The production process of 20MnCrS5 is: electric furnace smelting → LF furnace refining → VD furnace refining → die casting → heating → rolling, which specifically includes the following steps:

[0026] (1) Electric furnace smelting

[0027] The electric furnace smelting adopts an eccentric bottom steel-tapping electric furnace, oxygen oxidation, foam slag throughout the process, decarburization ≥ 0.25%, the carbon at the end of steel-tapping is controlled at 0.05% ≤ C ≤ 0.13%, the phosphorus at the end is controlled at P ≤ 0.020%, and the steel-tapping temperature of the electric furnace is controlled at 1640 ℃ ~ 1680 ℃. During the steel-tapping process, 2.5 ~ 3.5 kg / t of aluminum-iron alloy (composition: Al: 43 ~ 48%, C < 0.05%, Si < 1.0%, P < 0.08%, S < 0.05, the balance is Fe) is added as a pre-deoxidizer. When 1 / 4 of the steel is tapped, 0.8 ~ 1.0 kg / t of calcium carbide is added. t, using medium carbon ferromanganese (composition: Mn: 75-82%, C≤2.0%, Si≤1.5%, P≤0.20%, S≤0.03%, the balance is Fe) and low carbon ferrochrome (composition: Cr: 60-70%, C≤0.25%, Si≤1.5%, P≤0.03%, S≤0.025%, the balance is Fe) alloying, medium carbon ferromanganese addition amount 11.5-12.5kg / t, low carbon ferrochrome addition amount 13.0-14.5kg / t, slag added lime 8.0-10.0kg / t, fluorite 2.0-3.0kg / t. Add alloy and calcium carbide first, then add slag.

[0028] (2) LF furnace refining

[0029] LF furnace adopts diffusion deoxidation to make white slag, the amount of aluminum powder added as deoxidizer is 1.0~1.3kg / t, lime 14.0~16.0kg / t and alumina balls 5.0~6.0kg / t are added to the slag, the white slag time is ≥25min, high carbon ferromanganese (composition: Mn: 65~72%, C≤7.0%, Si≤2.5%, P≤0.25%, S≤0.03%, the balance is Fe) 3.0~4.0kg / t and low carbon ferrochrome (composition: Cr : 60~70%, C≤0.25%, Si≤1.5%, P≤0.03%, S≤0.025%, the remainder is Fe) 4.0~5.0kg / t to adjust the composition of the molten steel, argon is blown at the bottom of the ladle for stirring to facilitate uniform composition and removal of non-metallic inclusions, the argon pressure is controlled at 0.2MPa~0.4MPa, 2.0~2.5m / t of aluminum wire is fed to fine-tune the composition and temperature of the molten steel, and the temperature of the LF furnace hanging ladle is controlled at 1665℃~1695℃.

[0030] (3) VD furnace refining

[0031] Before vacuuming the VD furnace, calcium treatment is carried out, high calcium wire is fed in at 2.4-3.2m / t, and then vacuum treatment is carried out, the vacuum degree is ≤100Pa, and the vacuum holding time is ≥15min. After breaking the vacuum, sulfur wire is fed in at 2.5-3.5m / t according to the sulfur composition, and then argon is blown for 15min-35min. The argon pressure is maintained at 0.1Mpa-0.3Mpa, and the slag surface is slightly moved and the molten steel is not exposed. It is strictly forbidden to add alloys after degassing. The temperature drop value during vacuum refining should be considered to ensure that the temperature before degassing is appropriate. Large argon blowing and cooling operations shall not be carried out after vacuum refining. The temperature of the VD furnace hanging bag is controlled at 1579℃-1611℃.

[0032] (4) Die Casting

[0033] The dimensions of the mold casting ingots are shown in Table 1.

[0034] Table 1 Ingot size

[0035]

[0036] The time from the ladle being hoisted out of the VD to the start of pouring should be ≤10min, and the superheat should be controlled at 25℃~35℃. During pouring, the ladle water outlet should be 150~200mm higher than the funnel brick. Before pouring, argon should be introduced in time to ensure argon protection pouring, and the amount of molten steel drained should be ≥100kg. Strengthen the filling operation of the cap mouth, and the trickle filling method or the stamping filling method can be used according to the specific conditions of the steel type and pouring. When pouring with lightweight insulation lining and composite protective slag, start adding riser insulation agent when each plate is cast to 2 / 3 of the riser. The insulation agent is carbon powder, and the addition amount is 1.5kg / t~2.0kg / t. After pouring, the funnel brick should be pried off immediately to prevent it from falling out of the center injection pipe after cooling.

[0037] (5) Heating

[0038] Use a soaking furnace for heating and a cold charging process, keep warm at 200℃ for 0.5h, then raise the temperature to 1200℃~1220℃ for 3h~3.5h, and finally keep at 1200℃~1220℃ for 1.5h~2h to ensure uniform temperature of the ingot and thorough burning.

[0039] (6) Rolling

[0040] The rolling adopts φ1150mm primary rolling mill and continuous rolling mill, the starting rolling temperature is 1100℃~1200℃, the final rolling temperature is 850℃~1000℃, the rolling specification range is φ180mm~φ250mm, and the steel is put into the insulation pit for slow cooling after rolling. The pit entry temperature is ≥550℃, the pit exit temperature is ≤200℃, and the slow cooling time is ≥24h.

[0041] The following are specific embodiments.

[0042] Example:

[0043] The present invention provides 3 embodiments, and the chemical compositions of embodiments 1-3 are shown in Table 2.

[0044] Table 2 Chemical composition of the present invention by weight%

[0045]

[0046] The electric furnace smelting process of Examples 1-3 is shown in Table 3.

[0047] Table 3 Electric furnace smelting process of the embodiment of the present invention

[0048]

[0049] The LF furnace refining process of Examples 1-3 is shown in Table 4.

[0050] Table 4 Electric furnace smelting and LF refining process of the embodiment of the present invention

[0051]

[0052] The VD furnace refining process of Examples 1-3 is shown in Table 5.

[0053] Table 5 VD furnace refining process of the embodiment of the present invention

[0054]

[0055] The molding process of Examples 1-3 is shown in Table 6.

[0056] Table 6 Molding process of the embodiment of the present invention

[0057]

[0058] The heating and rolling processes of Examples 1-3 are shown in Table 7.

[0059] Table 7 Heating and rolling process of the embodiment of the present invention

[0060]

[0061] Comparative Example:

[0062] For comparison, the comparative example has the same component system as the present invention, and the smelting adopts converter + LF furnace + RH vacuum treatment, which is a conventional process. The casting adopts 350mm×470mm rectangular billet continuous casting process, and the heating and rolling process are the same as the present invention. The rolling specification of comparative example 1 is φ180mm, and the rolling specification of comparative example 1 is also φ180mm. The low-power, ultrasonic flaw detection, end and grain size of the present invention embodiment and comparative example are shown in Table 8. The low-power inspection samples of embodiment 1 and comparative example 1 are as follows Figure 1 and Figure 2As shown, it can be seen from the indicators and the comparison pictures that the quality of the product of the present invention is better than that of the conventional process product.

[0063] Table 8 Low magnification, ultrasonic flaw detection and grain size of the embodiments of the present invention and the comparative examples

[0064]

[0065] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A 20MnCrS5 steel, characterized in that: Composed of the following chemical elements in percentage by weight: C: 0.17% to 0.22%, Si: 0.15% to 0.40%, Mn: 1.10% to 1.30%, P≤0.035%, S: 0.035% to 0.045%, Cr: 1.00% to 1.30%, Alt: 0.020% to 0.060%, the remainder being iron and unavoidable impurities; The preparation method of 20MnCrS5 steel comprises the following steps: electric furnace smelting, LF furnace refining, VD furnace refining, die casting, heating and rolling; The mold casting includes introducing a first inert gas for protective pouring before pouring, setting the drainage molten steel volume to be ≥100kg; controlling the time from the ladle being lifted out of the VD furnace to the start of pouring to be ≤10min, and controlling the superheat to be between 25°C and 35°C; during pouring, the ladle water outlet is 150-200mm higher than the funnel brick; The heating includes heating with a soaking furnace, keeping the temperature at 200°C for 0.5h, then heating to 1200°C to 1220°C for 3h to 3.5h, and finally keeping the temperature at 1200°C to 1220°C for 1.5h to 2h to ensure uniform temperature of the ingot and thorough burning; The rolling includes a starting rolling temperature of 1100°C to 1200°C, a final rolling temperature of 850°C to 1000°C, a rolling specification range of φ180mm to φ250mm, and after rolling, the steel is placed in an insulation pit for slow cooling, the pit entry temperature is ≥550°C, the pit exit temperature is ≤200°C, and the slow cooling time is ≥24h.

2. The 20MnCrS5 steel according to claim 1, characterized in that The electric furnace smelting includes controlling the final carbon of steel tapping to be 0.05%≤C≤0.13%, controlling the final phosphorus to be P≤0.020%, and controlling the steel tapping temperature of the electric furnace to be 1640℃~1680℃.

3. The 20MnCrS5 steel according to claim 1, characterized in that The LF furnace refining includes white slag time ≥ 25 min, the second inert gas pressure is 0.2 MPa ~ 0.4 MPa, and the LF furnace hanging bag temperature is controlled at 1665 ° C ~ 1695 ° C.

4. The 20MnCrS5 steel according to claim 1, characterized in that The VD furnace refining includes feeding high calcium line, vacuum treatment, the vacuum degree of the vacuum treatment is ≤100Pa, the time of the vacuum treatment is ≥15min, breaking vacuum, feeding sulfur line, static blowing of the third inert gas, the static blowing time of the third inert gas is 15min~35min, the static blowing pressure is 0.1MPa~0.3MPa, and the VD furnace hanging bag temperature is 1579℃~1611℃.

5. Use of the 20MnCrS5 steel as claimed in any one of claims 1 to 4 in gears.

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