A rolling reduction method for improving the segregation of bloom crankshaft steel

Abstract

The present invention relates to a reduction method for improving segregation of billet crankshaft steel. The method comprises: casting molten steel through a continuous casting machine with full protection and producing the billet at a constant drawing speed; controlling the reduction amount of each straightening roller position of the continuous casting machine according to the solid phase ratio of the billet in a lower region; determining a maximum reduction rate at a region where the solid phase ratio of the billet is 65%-95% at a rate close to a critical strain rate, serving as the reduction rate at the entrance of this region; gradually decreasing the reduction rate along the billet drawing direction; reducing the average reduction rate as the drawing speed increases, adapting to timely reduction as the drawing speed increases, and making maximum use of the reduction effect; avoiding the reduction region being too biased toward the liquid phase region, which results in failure to improve the center segregation; avoiding the reduction region being biased toward the solid phase region, which results in adverse effects on the internal quality of the billet; and coordinating the total deformation of the reduction region to avoid excessive reduction, which results in negative segregation of a bright white band, inverse V-shaped segregation, or cracks. The billet center segregation index is 0.93-1.05, and the carbon segregation extreme fluctuation is ≤0.03%.

Description

Technical Field

[0001] The invention belongs to the technical field of crankshaft steel, and in particular relates to a pressing method for improving segregation of square billet crankshaft steel. Background Art

[0002] The crankshaft performs work under the combined action of constantly changing loads, reciprocating and rotational inertia forces and torques, and slides and rubs against the bearings at a relatively high relative speed, generating high temperatures and wear, and the service conditions are harsh. Therefore, crankshaft steel requires high tensile strength, fatigue strength, hardness, wear resistance and core toughness. However, the uneven segregation of alloy components during the solidification process will affect the organizational structure of the steel, resulting in performance differences between different sections and damaging the mechanical properties, crack resistance, plasticity and high-temperature properties of the crankshaft steel.

[0003] To achieve higher strength and toughness in larger cross-sections, crankshaft steel can be manufactured by increasing the C and Mn contents and adding certain microalloying elements. However, during billet solidification, the precipitation of a large amount of harmful alloying element-rich phases will affect the element diffusion rate. As the billet shell bulges during solidification, the resulting negative pressure differential within the billet exacerbates the flow of dendrite-enriched solute molten steel, causing localized enrichment and increasing the difficulty of controlling segregation. Existing methods for improving segregation primarily reduce segregation by increasing molten steel purity, strictly controlling endpoint carbon, reducing carbon in mold slag, and using low-temperature pouring and electromagnetic stirring during continuous casting to expand the equiaxed crystal zone. However, these methods cannot reduce or prevent the centerward flow of solute-rich molten steel caused by the shrinkage of the two-phase zone at the end of solidification, which still causes central segregation. While reduction can compensate for solidification shrinkage, existing techniques employ a light reduction technique at the end of continuous casting to reduce solute molten steel accumulation.

[0004] However, its main defect is that as the pulling speed increases, the area with severe solute segregation gradually approaches the narrow wall, which can reduce solute segregation, but the heat flow on the surface of the square billet is small, the solidification rate of the square billet is slow, and the two-phase area is large. If the reduction area is too biased towards the liquid phase area, the reduction rate is less than the solidification rate, and the liquid core thickness is too large, resulting in untimely reduction, the improvement effect on the central segregation cannot be exerted. If the reduction area is biased towards the solid phase area, the reduction rate is greater than the solidification rate, resulting in strain exceeding the critical strain, it is detrimental to the internal quality of the square billet, increases the drawing resistance, and affects the life of the equipment. The strain rate of the solid-liquid interface exceeds the critical strain rate, resulting in negative segregation of the white bright band or internal cracks, and the reduction effect cannot be maximized. Excessive reduction will also cause negative segregation of the white bright band, inverse V-shaped segregation or cracks.

[0005] Secondly, excessive superheating is conducive to the growth of columnar crystals and causes central segregation. The large temperature difference between the surface and the core of the billet leads to increased fluctuations in carbon segregation on the center line of the billet. Frequent changes in the pulling speed cause changes in the position of the end of solidification, making it difficult to exert the effect of light reduction, further aggravating the segregation.

[0006] In addition, with the increase of the solid phase ratio of the billet, the front edge of the reduction end is unevenly pressed, the straightening roller is easily damaged by the excessive reaction force, and the strain generated by the small deformation reduction penetrating into the center area of the billet is too small, which fails to compensate for the local pressure drop caused by the solidification shrinkage of the billet, further aggravating the segregation. Summary of the Invention

[0007] The present invention aims to solve at least one of the above-mentioned technical problems to a certain extent. The present invention provides a reduction method for improving the segregation of square billet crankshaft steel, which can maximize the use of the reduction effect, reduce the segregation index, and avoid negative segregation of the bright band, V-shaped segregation or cracks.

[0008] The technical solution adopted by the present invention to solve its technical problem is:

[0009] A method for improving segregation of billet crankshaft steel by pressing down, the method comprising:

[0010] The molten steel is poured through the continuous casting machine with full protection and constant casting speed to produce billets to prevent secondary oxidation;

[0011] The continuous casting speed is 0.79-1.62m / min. The constant casting speed reduces the frequent changes in the liquid phase hole and the bulging phenomenon, and avoids the frequent changes in the casting speed causing the position change of the solidification end, which makes it difficult to exert the light reduction effect and causes segregation;

[0012] The reduction area controls the reduction amount of each straightening roll position of the continuous casting machine according to the solid phase ratio of the billet in the center of the billet. The reduction amount is controlled in the two-phase area enriched with segregation elements at the end of solidification, thereby reducing or avoiding the flow of solute-rich molten steel to the center due to the contraction of the liquid-solid two-phase area at the end of solidification, and preventing the flow and accumulation of solute-rich molten steel, thereby forming a dense, segregation-free resolidification structure in the center area.

[0013] In the area where the solid phase fraction fs of the square billet is 65%-95%, the critical strain rate of the molten steel corresponding to the steel grade is obtained ,but , the above formula where is the solid-liquid interface strain rate, %; is a constant; is the reduction rate, mm / m; is the billet thickness, mm; is the distance between the straightening rolls, mm; the maximum reduction rate is determined based on the critical strain rate. As the reduction rate at the entrance of this area, it is gradually reduced by 0.02-0.03mm / m along the drawing direction. This can maximize the use of the reduction effect and avoid the strain rate at the solid-liquid interface exceeding the critical strain rate, which may lead to negative segregation or internal cracks in the white bright band. Since the liquid phase area in the cross section of the two-phase region at the end of solidification gradually decreases, the solidification shrinkage per unit length of the billet decreases along the drawing direction;

[0014] As the drawing speed increases, the area with severe solute segregation gradually approaches the narrow wall, which can reduce solute segregation. At the same time, when the drawing speed increases, the heat flow on the billet surface is small, the solidification speed of the billet is slow, the two-phase area is larger, the solid phase ratio fs of the billet moves in the cutting direction, the solidification shrinkage per unit length of the billet along the drawing direction decreases, and the reduction rate decreases accordingly. For every 0.1m / min increase in the continuous casting drawing speed, the average reduction rate decreases by 0.4-0.6mm / m. It can adapt to the increase in drawing speed and timely reduction, avoid the reduction area being too biased towards the liquid phase area, which will not play a role in improving the central segregation, and the reduction area being biased towards the solid phase area, which is unfavorable to the internal quality of the billet, and reduce the billet drawing resistance;

[0015] The total deformation in the pressing area is 4.5%-7.5% to avoid excessive pressing that may cause negative segregation of the white bright band, reverse V-shaped segregation or cracks.

[0016] Furthermore, the mass fraction of easily segregated elements in molten steel is strictly controlled, P≤0.01%, S≤0.005%, and easily segregated elements include S, P, C, Mn, Sb, N, As, H, and Sn. This avoids grain boundary segregation of easily segregated elements during heating or solid-state phase transformation, thereby further improving segregation.

[0017] Furthermore, the superheat of the molten steel in the tundish is controlled at 15-25°C, and the temperature of the molten steel in the tundish is 1460-1495°C, to avoid too low superheat which is detrimental to preventing nozzle blockage and promoting the floating of inclusions, and to avoid too high superheat which is conducive to the growth of columnar crystals and causes central segregation.

[0018] Furthermore, a uniform and gentle control method is adopted in the secondary cooling area, the secondary cooling water ratio is 0.59-0.71kg / t steel, the maximum heat recovery rate of the billet surface is ≤100℃ / m, the temperature difference in the width direction of the billet is ≤45℃, and the surface temperature of the billet in the pressing area is 905-1025℃. The surface temperature recovery and temperature difference of the billet are controlled to avoid the large temperature difference between the billet surface and the core, which leads to increased fluctuation of carbon segregation on the center line of the billet, thereby further improving segregation.

[0019] Furthermore, in the area where the solid phase fraction fs of the square billet is 15%-45%, the single-roll reduction is 1-3mm, and the reduction rate is 1.6-2mm / min, so that the upstream molten steel flows downstream under static pressure, further improving segregation.

[0020] Furthermore, with the increase of billet solid fraction, the concentration of enriched solute elements in the intergranular residual liquid phase increases. In the area where the billet solid fraction fs is 15%-45%, the liquidus temperature, solidus temperature, and tundish billet size parameters of the corresponding steel grade are obtained, and the initial reduction rate is , where Billet thickness, mm; is the billet width, mm; is the liquidus temperature, °C; is the solidus temperature, ℃; it can transfer the reduction to the two-phase region to compensate for the local pressure drop caused by the solidification shrinkage of the billet, improve the uniformity of the reduction at the front end of the reduction, and avoid damage to the straightening roller due to excessive reaction force.

[0021] Furthermore, in the area where the solid phase fraction fs of the square billet is 45%-65%, the single-roll reduction is 2-5mm, which prevents the deformation caused by small deformation from penetrating into the central area of the square billet and causes the strain to be too small. The speed at which the upstream molten steel flows downstream under static pressure is slowed down, and the reduction rate is 2.5-4.8mm / min, which can compensate for the local pressure drop of the molten steel and prevent the local pressure drop from causing the molten steel between the dendrites to flow to the center, which is further beneficial to improving the central defects.

[0022] Furthermore, in the area where the solid phase fraction fs of the square billet is 65%-95%, the reduction rate is 3-5mm / min to balance the reduction rate with the solidification rate, avoid the reduction rate being less than the solidification rate, and the liquid core thickness being too large, resulting in untimely reduction, and avoid the reduction amount and reduction rate being greater than the solidification rate, resulting in strain exceeding the critical strain, causing internal cracks in the square billet, or the reduction area becoming narrower, which is not conducive to improving segregation.

[0023] Furthermore, the total reduction in the reduction area is 10-19 mm to avoid too small a reduction, which is insufficient to compensate for the volume shrinkage of the molten steel in the reduction area during the solidification process, resulting in the formation of periodic macroscopic V-shaped segregation in the solidification area at the center of the billet thickness, and to avoid excessive reduction, which will produce negative segregation of the white bright band, reverse V-shaped segregation or cracks, and further improve the segregation.

[0024] Furthermore, the center segregation index of the billet is 0.93-1.05, and the extreme fluctuation of carbon segregation is ≤0.03%, which can reduce the performance difference between different sections of the billet and is beneficial to improving the mechanical properties, crack resistance, plasticity and high-temperature performance of the crankshaft steel.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] (1) According to the solid phase ratio of the billet in the center of the billet, the reduction amount at each straightening roller position of the continuous casting machine is controlled. The reduction amount is controlled in the two-phase region where the segregation elements are enriched at the end of solidification. In the region where the solid phase ratio fs of the billet is 65%-95%, the maximum reduction rate is determined at a rate close to the critical strain rate. As the reduction rate at the entrance of this region, it is gradually reduced along the billet drawing direction. The average reduction rate is reduced as the drawing speed increases. The reduction can be timely adapted to the increase in the drawing speed, and the reduction effect can be maximized. It is avoided that the reduction area is too biased towards the liquid phase region, resulting in the inability to play the role of improving the central segregation. The reduction area is biased towards the solid phase region, which is detrimental to the internal quality of the billet. The billet drawing resistance is reduced, and the total deformation of the reduction area is coordinated to avoid excessive reduction, which causes negative segregation of the white bright band, inverse V-shaped segregation or cracks.

[0027] (2) Strictly control the mass fraction of elements that are prone to segregation in molten steel to avoid excessive overheating that is conducive to the growth of columnar crystals and causes central segregation. Control the surface temperature recovery and temperature difference of the billet to avoid a large temperature difference between the billet surface and the core, which leads to increased fluctuations in carbon segregation on the center line of the billet. In the area where the solid phase fraction fs of the billet is 15%-45%, press down to make the upstream molten steel flow downstream under static pressure, further improving segregation.

[0028] (3) In the area where the solid phase fraction fs of the square billet is 15%-45%, the initial reduction rate is determined by the liquidus temperature, solidus temperature of the corresponding steel grade of the molten steel, and the size parameters of the tundish square billet. This can improve the uniformity of the reduction at the front end of the reduction and avoid damage to the straightening roller due to excessive reaction force. In the area where the solid phase fraction fs of the square billet is 45%-65%, the local pressure drop of the molten steel can be compensated to prevent the local pressure drop from causing the molten steel between the dendrites to flow to the center, which is further beneficial to improve the center defect. DETAILED DESCRIPTION

[0029] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0030] As the pulling speed increases, the area with severe solute segregation gradually approaches the narrow wall, which can reduce the solute segregation. However, the heat flow on the surface of the square billet is small, the solidification rate of the square billet is slow, and the two-phase area is large. In order to avoid the pressing area being too biased towards the liquid phase area, the pressing rate is less than the solidification rate, and the liquid core thickness is too large, which leads to an improvement effect on the central segregation. To avoid the pressing area being biased towards the solid phase area, the pressing rate is greater than the solidification rate, resulting in strain exceeding the critical strain, which is detrimental to the internal quality of the square billet. The segregation index is reduced, and negative segregation of the white bright band, inverse V-shaped segregation or cracks are avoided. It is necessary to adapt to the pulling speed and press in time to maximize the use of the pressing effect. To this end, the present invention provides a preferred implementation method of a pressing method for improving segregation of square billet crankshaft steel, which includes:

[0031] Molten steel is poured through the continuous casting machine with full protection and constant casting speed to produce billets. The continuous casting casting speed is 0.79-1.62m / min. The reduction area controls the reduction amount of each straightening roller position of the continuous casting machine according to the solid phase ratio of the billet in the center of the billet.

[0032] In the area where the solid phase fraction fs of the square billet is 65%-95%, the critical strain rate of the molten steel corresponding to the steel grade is obtained ,but , the above formula where is the solid-liquid interface strain rate, %; is a constant; is the reduction rate, mm / m; is the billet thickness, mm; is the distance between the straightening rollers, mm; the maximum reduction rate is determined based on this, and the reduction rate at the entrance of this area is gradually reduced by 0.02-0.03mm / m along the throwing direction;

[0033] For every 0.1m / min increase in continuous casting speed, the average reduction rate decreases by 0.4-0.6mm / m, and the total deformation in the reduction area is 4.5%-7.5%.

[0034] Furthermore, the method includes:

[0035] Molten steel is cast through a continuous casting machine under full protection to produce billets. The mass fraction of easily segregated elements in the molten steel is strictly controlled. The easily segregated elements include S, P, C, Mn, Sb, N, As, H, and Sn. P in the molten steel is ≤ 0.01% and S ≤ 0.005%. The ladle filled with molten steel is transported to a turntable. After the turntable rotates to the pouring position, the molten steel in the ladle is poured into a tundish and then distributed to a crystallizer. The crystallizer is cooled by water to form the billet, which is quickly cooled and solidified to form a primary shell. The billet in the crystallizer is pulled out of the crystallizer through the combined action of a pulling speed controlled by a straightening machine and an oscillating device. It is then cooled in the secondary cooling zone of the continuous casting machine. Water is sprayed on the billet with core liquid to completely solidify it. The billet is cut to a fixed length to obtain a billet.

[0036] The superheat of the molten steel in the tundish is controlled at 15-25°C, and the temperature of the molten steel in the tundish is 1460-1495°C. A uniform and gentle control method is adopted in the secondary cooling area. The secondary cooling water volume is 0.59-0.71kg / t steel. The maximum heat recovery rate of the billet surface is ≤100°C / m, the temperature difference in the width direction of the billet is ≤45°C, and the continuous casting speed is 0.79-1.62m / min.

[0037] The surface temperature of the billet in the reduction zone is 905-1025℃. The reduction amount of each straightening roll of the continuous casting machine is controlled according to the solid phase ratio of the billet center. In the area where the solid phase ratio fs of the billet is 15%-45%, the reduction amount of a single roll is 1-3mm.

[0038] Obtain the liquidus temperature, solidus temperature, and tundish billet size parameters of the corresponding steel type, then the initial reduction rate , where Billet thickness, mm; is the billet width, mm; is the liquidus temperature, °C; is the solidus temperature, °C; the reduction rate is selected to be 1.6-2 mm / min;

[0039] =1536-(78*C%+7.6*Si%+4.9*Mn%+34.4P%+38*S%+4.7*Cu%+3.1*Ni%+1.3*Cr%+3.6*Al%);

[0040] =1536-(415.5*C%+12.3*Si%+6.8*Mn%+124.5P%+183.9*S%+4.3*Ni%+1.4*Cr%+4.1*Al%);

[0041] In the area where the solid phase fraction fs of the billet is 45%-65%, the single-roll reduction is 2-5mm, and the reduction rate is 2.5-4.8mm / min;

[0042] In the area where the billet solid fraction fs is 65%-95%;

[0043] Obtain the critical strain rate of molten steel corresponding to the steel grade ,but , the above formula where is the solid-liquid interface strain rate, %; is a constant; is the reduction rate, mm / m; is the billet thickness, mm; is the distance between the straightening rollers, mm; the maximum reduction rate is determined based on this, and the reduction rate is the largest at the entrance of this area, and gradually decreases by 0.02-0.03mm / m along the throwing direction;

[0044] For every 0.1m / min increase in continuous casting speed, the average reduction rate decreases by 0.4-0.6mm / m;

[0045] Pressing rate , in the above formula is the reduction rate, mm / m; The continuous casting speed is m / min, preferably 3-5 mm / min;

[0046] The total reduction in the pressing area is 10-19 mm, and the total deformation in the pressing area is 4.5%-7.5%;

[0047] The center segregation index of the billet is 0.93-1.05, and the extreme fluctuation of carbon segregation is ≤0.03%.

[0048] The steel type of Examples 1-3, Comparative Example 1, Comparative Example 3, and Comparative Example 6 is 38MnVS6, and its composition is: C: 0.37%, Si: 0.52%, Mn: 1.43%, P: 0.005%, S: 0.005%, Cr: 0.2%, Mo: 0.05%, V: 0.11%, and the balance is Fe and unavoidable impurities. The specification is 160*200mm.

[0049] The steel type of Examples 4-6, Comparative Example 2, Comparative Example 4, Comparative Example 5, Comparative Example 7, and Comparative Example 8 is 49MnVS3, and its composition is: C: 0.46%, Si: 0.44%, Mn: 0.85%, P: 0.005%, S: 0.005%, Cr: 0.2%, Ni: 0.1%, Mo: 0.05%, V: 0.12%, and the balance is Fe and unavoidable impurities. The specification is 240mm*280mm.

[0050] The difference between Comparative Example 1 and Example 3 is that the continuous casting speed is relatively low, and in the area where the solid phase fraction fs of the square billet is 65%-95%, the reduction rate at the entrance of this area is relatively large, the reduction rate remains unchanged along the casting direction, and the total deformation in the reduction area is relatively large.

[0051] The difference between Comparative Example 2 and Example 6 is that the continuous casting speed is higher, and in the area where the solid phase fraction fs of the square billet is 65%-95%, the reduction rate at the entrance of this area is smaller, the reduction rate remains unchanged along the casting direction, and the total deformation in the reduction area is smaller.

[0052] The continuous casting speeds of Examples 1-6 and Comparative Examples 1-2, the reduction parameters in the region where the billet solid fraction fs is 65%-95%, and the total deformation in the reduction region are shown in Table 1 below:

[0053] Table 1

[0054]

[0055] From the comparison results of Comparative Example 1 and Example 3, it can be seen that if the reduction area is too biased towards the liquid phase area, the reduction rate is lower than the solidification rate, and the liquid core thickness is too large, resulting in untimely reduction, the improvement effect on the center segregation cannot be achieved.

[0056] From the comparison results of Comparative Example 2 and Example 6, it can be seen that as the pulling speed increases, the area with severe solute segregation gradually approaches the narrow wall, which can reduce solute segregation. However, the heat flow on the surface of the square billet is small, the solidification rate of the square billet is slow, and the two-phase area is large. If the reduction area is biased towards the solid phase area, the reduction rate is greater than the solidification rate, resulting in strain exceeding the critical strain, and the solid-liquid interface strain rate exceeds the critical strain rate, resulting in negative segregation of the white bright band or internal cracks, and the reduction effect cannot be maximized. Excessive reduction will also cause negative segregation of the white bright band, inverse V-shaped segregation or cracks.

[0057] From the comparison results of Examples 1-6, Comparative Examples 1 and 2, it can be seen that the reduction amount at each straightening roller position of the continuous casting machine is controlled according to the solid phase fraction of the billet in the center of the billet, the reduction amount is controlled in the two-phase region where the segregation elements are enriched at the end of solidification, and in the region where the solid phase fraction fs of the billet is 65%-95%, the maximum reduction rate is determined at a rate close to the critical strain rate. The reduction rate at the entrance of this region is gradually reduced along the drawing direction, and the average reduction rate is reduced as the drawing speed increases. The reduction can be timely adapted to the increase in the drawing speed, and the reduction effect can be utilized to the maximum extent. It is avoided that the reduction area is too biased towards the liquid phase region, resulting in the inability to improve the central segregation, and the reduction area is biased towards the solid phase region, which is detrimental to the internal quality of the billet, and the billet drawing resistance is reduced. The total deformation of the reduction area is coordinated to avoid excessive reduction to cause negative segregation of the white bright band, inverse V-shaped segregation or cracks.

[0058] The center segregation index of the billet is 0.93-1.05, and the extreme fluctuation of carbon segregation is ≤0.03%, which can reduce the performance difference between each section of the billet and is beneficial to improving the mechanical properties, crack resistance, plasticity and high-temperature performance of crankshaft steel.

[0059] The difference between Comparative Example 3 and Example 1 is that the superheat degree of the molten steel in the tundish is higher, the water volume of the secondary cooling is higher, and the maximum heat recovery rate of the billet surface is higher.

[0060] The difference between Comparative Example 4 and Example 4 is that the superheat of the molten steel in the tundish is lower, the secondary cooling water ratio is lower, and the temperature difference in the width direction of the square billet is larger.

[0061] The difference between Comparative Example 5 and Example 4 is that the secondary cooling water ratio is higher and the temperature difference in the width direction of the billet is larger.

[0062] The tundish molten steel superheat, tundish molten steel temperature, secondary cooling water ratio, maximum heat recovery rate on the billet surface, and temperature difference in the billet width direction of Examples 1-6 and Comparative Examples 3-5 are shown in Table 2 below:

[0063] Table 2

[0064]

[0065] From the comparison results of Comparative Example 3 and Example 1, it can be seen that excessive superheating is conducive to the growth of columnar crystals and induces central segregation.

[0066] From the comparison results of Comparative Example 4 and Example 4, it can be seen that from the comparison results of Comparative Example 5 and Example 4, it can be seen that the large temperature difference between the surface and the core of the billet leads to increased fluctuation of carbon segregation on the center line of the billet.

[0067] From the comparison results of Examples 1-6, Comparative Examples 3, 4 and 5, it can be seen that the mass fraction of easily segregated elements in the molten steel is strictly controlled to avoid excessive superheating that is conducive to columnar crystal growth and causes central segregation, and the surface temperature recovery and temperature difference of the billet are controlled to avoid a large temperature difference between the surface and the core of the billet, which leads to increased fluctuations in carbon segregation on the center line of the billet. In the area where the solid phase fraction fs of the billet is 15%-45%, pressure is applied to cause the upstream molten steel to flow downstream under static pressure, thereby further improving segregation.

[0068] The difference between Comparative Example 6 and Example 2 is that the reduction is performed in the regions where the billet solid fraction fs is 0-15% and 95-100%, and the reduction is greater in the region where the billet solid fraction fs is 65%-95%, and the reduction parameters are different.

[0069] The difference between Comparative Example 7 and Example 5 is that no reduction is performed in the region where the billet solid fraction fs is 0%-45%, and reduction is performed in the region where the billet solid fraction fs is 95-100%, and the reduction parameters are different.

[0070] The difference between Comparative Example 8 and Example 5 is that the reduction is performed in the region where the billet solid fraction fs is 0%-15%, and the reduction is smaller in the region where the billet solid fraction fs is 65%-95%, and the reduction parameters are different.

[0071] From the comparison results of Comparative Example 6 and Example 1, it can be seen that the reduction rate is greater than the solidification rate, resulting in the strain exceeding the critical strain, causing internal cracks in the billet, or the reduction area becomes narrower, which is not conducive to improving segregation.

[0072] From the comparison results of Comparative Example 7 and Example 5, it can be seen that with the increase of the solid phase ratio of the square billet, the reduction at the front end of the reduction is uneven, and the local pressure drop caused by the solidification shrinkage of the square billet cannot be compensated. The reduction rate is lower than the solidification rate, and the liquid core thickness is too large, resulting in untimely reduction and further aggravation of segregation.

[0073] From the comparison results of Comparative Example 8 and Example 5, it can be seen that the strain amount generated by the small deformation reduction penetrating into the center area of the billet is too small, which fails to compensate for the local pressure drop caused by the solidification shrinkage of the billet, further aggravating segregation.

[0074] The reduction areas of Examples 1-6 and Comparative Examples 6-8 control the reduction amounts of each straightening roll position of the continuous casting machine according to the solid phase ratio of the billet at the center of the billet, as shown in Table 3 below:

[0075] Table 3

[0076]

[0077] The billet test results of Examples 1-6 and Comparative Examples 1-8 are shown in Table 4 below:

[0078] Table 4

[0079]

[0080] From the comparison results of Examples 1-6, Comparative Examples 7, 8 and 9, it can be seen that in the area where the solid phase fraction fs of the square billet is 15%-45%, the initial reduction rate is determined by the liquidus temperature, solidus temperature and tundish billet size parameters of the molten steel corresponding to the steel grade, which can improve the uniformity of the reduction at the front end of the reduction and avoid damage to the straightening roller due to excessive reaction force. In the area where the solid phase fraction fs of the square billet is 45%-65%, the local pressure drop of the molten steel can be compensated to prevent the local pressure drop from causing the molten steel between the dendrites to flow to the center, which is further beneficial to improve the center defect.

[0081] The series of detailed descriptions listed above are only specific descriptions of feasible embodiments of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent embodiments or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for improving segregation of billet crankshaft steel, characterized in that: The methods include: Molten steel is poured through the continuous casting machine with full protection and constant casting speed to produce billets. The continuous casting casting speed is 0.79-1.62m / min. The reduction area controls the reduction amount of each straightening roller position of the continuous casting machine according to the solid phase ratio of the billet in the center of the billet. In the area where the solid phase fraction fs of the billet is 15%-45%, the single-roll reduction is 1-3mm and the reduction rate is 1.6-2mm / min; In the area where the solid phase fraction fs of the billet is 45%-65%, the single-roll reduction is 2-5mm, and the reduction rate is 2.5-4.8mm / min; In the area where the solid phase ratio fs of the billet is 65%-95%, the critical strain rate ε0 of the corresponding steel grade of the molten steel is obtained, then ε=C(r s a) / l<ε0, where ε is the solid-liquid interface strain rate, %; C is a constant; r s is the reduction rate, mm / m; a is the billet thickness, mm; l is the distance between the straightening rolls, mm; the maximum reduction rate is determined based on this, and is used as the reduction rate at the entrance of this area, and is gradually reduced by 0.02-0.03 mm / m along the billet drawing direction; For every 0.1m / min increase in continuous casting speed, the average reduction rate decreases by 0.4-0.6mm / m, and the total deformation in the reduction area is 4.5%-7.5%.

2. A method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: Strictly control the mass fraction of easily segregated elements in molten steel, P≤0.01%, S≤0.005%.

3. The method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: Control the superheat of the molten steel in the tundish at 15-25℃, and the temperature of the molten steel in the tundish at 1460-1495℃.

4. The method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: The maximum reheat rate of the billet surface is ≤100℃ / m, the temperature difference in the width direction of the billet is ≤45℃, and the surface temperature of the billet in the pressing area is 905-1025℃.

5. The method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: In the area where the solid phase fraction fs of the billet is 15%-45%, the liquidus temperature, solidus temperature and tundish billet size parameters of the corresponding steel grade of the molten steel are obtained, and the initial reduction rate is In the above formula, a is the thickness of the billet, mm; W is the width of the billet, mm; T L is the liquidus temperature, °C; T s is the solidus temperature, °C.

6. The method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: In the area where the solid phase fraction fs of the square billet is 65%-95%, the reduction rate is 3-5 mm / min.

7. The method for improving segregation of billet crankshaft steel according to claim 1, characterized in that: The total reduction in the reduction area is 10-19 mm.

8. A method for reducing the segregation of billet crankshaft steel according to any one of claims 1 to 7, characterized in that: The center segregation index of the billet is 0.93-1.05, and the extreme fluctuation of carbon segregation is ≤0.03%.

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