Steel bar for ball screw blank with high structure uniformity and machining method of steel bar

Abstract

The invention discloses steel for a ball screw blank with high structure uniformity and a preparation method of the steel, and belongs to the technical field of steel for ball screws. The technical problem that an existing steel for the ball screw blank is poor in structure uniformity is solved. The steel for the ball screw blank with the high structure uniformity comprises the following chemical elements in percentage by weight: 0.47 to 0.53 percent of C, less than or equal to 0.35 percent of Si, 0.70 to 1.00 percent of Mn, less than or equal to 0.020 percent of S, less than or equal to 0.015 percent of P, 0.80 to 1.10 percent of Cr, 0.15 to 0.25 percent of Mo, less than or equal to 0.30 percent of Ni, less than or equal to 0.2 percent of Cu, less than or equal to 0.05 percent of Nb, less than or equal to 0.05 percent of V, less than or equal to 10ppm of O, less than or equal to 0.00015 percent of H and the balance of Fe and inevitable impurities. The steel for the ball screw blank, prepared by the method, is fine and uniform in microstructure and uniform in performance, and can provide technical support for manufacturing of a high-precision retentivity ball screw.

Description

Technical Field

[0001] The invention relates to the technical field of steel for ball screws, and in particular to a steel bar for ball screw blanks with high structural uniformity and a processing method thereof. Background Art

[0002] Ball screws are the most commonly used transmission components on "industrial mother machines" such as tool machinery and precision machinery. They convert rotary motion into linear motion or linear motion into rotary motion. They are usually used in combination with linear guides and have the advantages of high transmission efficiency, low friction, contact rigidity, low temperature rise, low thermal deformation, no transmission gap, no creep, etc. The core components of high-end ball screw pairs required by machine tools are heavily dependent on imports. One of the important reasons is that the steel bars used for ball screw blanks have poor uniformity of organization, are easily deformed during processing, and have low precision life during use, affecting the processing accuracy and service life of high-end machine tools.

[0003] Ball screw steel is often made of high carbon steel (typical grade GCr15), medium carbon steel (typical grade 50CrMo), stainless steel (30Cr15Mo1N steel) and other materials. At present, 50CrMo steel is gradually replacing GCr15 for ball screw manufacturing due to its advantages in high strength and toughness, good processing performance and manufacturing cost, etc., in high-end machine tool equipment. However, medium carbon steel 50CrMo has problems such as component segregation and uneven structure control metallurgical problems, which in turn affect the size and precision retention in the manufacturing process of screws and guide rails. It is necessary to improve the uniformity of materials under economic cleanliness and ensure the batch stability of industrial production.

[0004] In addition, the medium-carbon alloy steel for ball screws prepared by the existing preparation methods cannot guarantee high structural uniformity of the material. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a steel for ball screw blank with high structural uniformity and a preparation method thereof, so as to solve the technical problem that the existing ball screw steel 50CrMo has poor structural uniformity and cannot meet the use requirements.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] On the one hand, the present invention provides a steel for a ball screw blank, wherein the components of the steel for a ball screw blank include, by mass percentage: C: 0.47-0.53%, Si≤0.35%, Mn: 0.70%-1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.80%-1.10%, Mo: 0.15%-0.25%, Ni≤0.30%, Cu≤0.2%, Nb: 0.01-0.1%, V: 0.01-0.05%, O≤10ppm, H≤0.00015%, and the balance is Fe and unavoidable impurities.

[0008] In one possible design, the components of the steel bar for ball screw blank include, by mass percentage: C: 0.49%-0.53%, Si≤0.34%, Mn: 0.80%-1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.85%-1.10%, Mo: 0.18%-0.25%, Ni≤0.25%, Cu≤0.2%, Nb: 0.01%-0.09%, V: 0.03-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0009] In one possible design, the components of the steel bar for ball screw blank include, by mass percentage: C: 0.50%~0.52%, Si≤0.32%, Mn: 0.85%~1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.95%~1.10%, Mo: 0.20%~0.25%, Ni≤0.22%, Cu≤0.19%, Nb: 0.01%-0.04%, V: 0.04-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0010] In one possible design, the microstructure of the ball screw blank steel is tempered troostite.

[0011] On the other hand, the present invention also provides a method for preparing a steel bar for a ball screw blank, which is used to prepare the above-mentioned steel bar for a ball screw blank, and the preparation method comprises the following steps:

[0012] Step 1, molten steel smelting;

[0013] Use smelting raw materials to smelt molten steel, then perform refining after smelting, and then perform vacuum degassing after refining to ensure that the O content in the molten steel is ≤10ppm;

[0014] Step 2, preparing an ingot;

[0015] The molten steel smelted in step 1 is poured into an ingot casting device to prepare an ingot;

[0016] Step 3, heating the ingot;

[0017] The ingot heating temperature is 1200-1250°C and the residence time is ≥120min.

[0018] Step 4, rolling the ingot to obtain a steel bar;

[0019] The rolling of ingots includes two stages: billet rolling and bar rolling; the rolling temperature is ≥1150℃;

[0020] Step 5: annealing the steel bar after rolling;

[0021] Step 6: straighten the steel bar;

[0022] A straightening machine is used to make the fiber length inside the steel rod uniform through external force;

[0023] Step 7, quenching and tempering the steel bar;

[0024] Step 8: Perform subsequent processing on the steel rod;

[0025] Subsequent processing refers to the peeling, straightening and annealing of the steel bars after quenching and tempering.

[0026] Furthermore, in step 4, the final rolling temperature of the steel bar is 800-950°C.

[0027] Further, in step 4, during the rolling process of the bar, it includes rough rolling and finish rolling;

[0028] The rough rolling adopts reciprocating rolling. During the rough rolling, the diameter of the bar is rolled from 240mm to 120mm, and the deformation of each rolling pass is 28%-32%.

[0029] Furthermore, in step 5, the hot-rolled steel bar is placed in a trolley furnace for annealing at a temperature of 600 to 780°C.

[0030] Furthermore, in step 6, during straightening, the straightness of the steel bar is controlled within the range of curvature ≤ 1 mm / m using a straightening device.

[0031] Furthermore, in step 8, during the annealing treatment, the annealing temperature is ≤ the tempering temperature of the quenching and tempering treatment.

[0032] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0033] (1) The steel rod for ball screw blank with high organizational uniformity provided by the present invention introduces a combination of Nb and V elements. Nb microalloying precipitates MC type carbides in 50CrMo steel. The precipitated phase not only refines the cast structure, but also plays a role in hindering grain growth during the heating process of the blank, thereby refining the microstructure of the steel and improving the organizational uniformity of the steel; V microalloying plays a precipitation strengthening role of MC type carbides and / or a fine grain strengthening role of hindering grain growth, thereby improving wear resistance. The MC type refers to a crystal structure formed by the combination of metal or metalloid elements and carbon.

[0034] (2) The present invention performs an annealing treatment after the steel rod is peeled and straightened, which can effectively eliminate the stress generated by the straightening, thereby reducing the deformation of the screw during the processing.

[0035] (3) The present invention performs high-temperature homogenization treatment on the billet before the ingot is forged or hot-rolled. The high temperature and long-term heat preservation at a temperature above 1200° C. can improve the microsegregation behavior of the cast structure.

[0036] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the embodiments of the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0038] Figure 1 Tempered martensite structure after quenching and tempering treatment;

[0039] Figure 2 Fine and uniform martensite structure after quenching;

[0040] Figure 3 Photo of the original austenite grain boundary of the steel bar after quenching at 860℃, the average grain size reaches level 10;

[0041] Figure 4 Low-magnification tissue photographs;

[0042] Figure 5 Hardenability curve. DETAILED DESCRIPTION

[0043] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0044] The invention provides a steel rod for a ball screw blank with high organizational uniformity. The components of the steel rod for the ball screw blank include, by mass percentage, C: 0.47-0.53%, Si≤0.35%, Mn: 0.70%-1.00%, S:≤0.020%, P≤0.015%, Cr: 0.80%-1.10%, Mo: 0.15%-0.25%, Ni≤0.30%, Cu≤0.2%, Nb: 0.01-0.1%, V: 0.01-0.05%, O≤10ppm, H≤0.00015%, and the balance is Fe and unavoidable impurities.

[0045] The functions of the above components and their control within their respective ranges are as follows:

[0046] C: The lead screw is usually subjected to overall quenching and tempering heat treatment to obtain high-temperature tempered martensite structure, and then subjected to surface induction quenching treatment to form a performance match of high core toughness and surface strength. C is a key factor in determining the strength of martensite. If the content is too low, the quenching hardness cannot be guaranteed, while if it is too high, it will affect the martensite phase transformation behavior and obtain too much needle-shaped martensite. When it is 0.47-0.53%, the strength level of steel can be guaranteed, and the fluctuation range of element composition in industrial production can also be met.

[0047] Si: Si element is not added as a main alloy element in the present invention, but is added as a deoxidizer during the smelting process, so its content is limited. Considering the relatively low cost of Si deoxidation and the level of industrial production control, Si is specified to be ≤0.35%.

[0048] Mn: It plays a role of solid solution strengthening in steel. After high-temperature tempering, it can participate in the precipitation of alloy cementite and play a precipitation strengthening role. The cost of using Mn element to strengthen alloy is relatively low. It is often used in civilian products. However, Mn element is easy to form element segregation during the solidification process of molten steel. If it is not properly controlled, it is easy to form banded structure in the steel. Considering the strengthening effect of Mn element and the control of segregation behavior, Mn: 0.70%~1.00%.

[0049] S: S is an element that needs to be controlled. When some screw steels are developed, S elements are appropriately contained to form easy-to-cut MnS, thereby improving the cutting performance of the steel. The present invention does not consider cutting performance, and the lower the S content, the better while ensuring economy. Therefore, S is specified to be ≤ 0.020%.

[0050] P: exists in steel as an impurity element. Under the premise of ensuring economy, it is hoped that the S content is as low as possible. Based on production experience, P is specified to be ≤ 0.015%.

[0051] Cr: Cr in the present invention plays the role of increasing the hardenability of steel and precipitating alloy carbides. A content of about 1% can meet the requirements. Considering the fluctuation of elements during the smelting process, the Cr element is specified to be 0.80% to 1.10%.

[0052] Mo: Mo is a strong carbide-forming element, which significantly improves the hardenability of steel. The inventor's research results show that when 0.2% of Mo is added to a steel containing 0.5% C and 1% Cr, the hardenability of the steel can be significantly improved. When the end quenching obtains a hardness of ≥58HRC, the depth of the hardened layer can be increased from 5mm to 9mm. Therefore, the Mo content of the present invention is 0.15% to 0.25%.

[0053] Ni: Ni is not the main element added in the present invention, but its proper presence is beneficial to the performance of the steel. When scrap steel is used as raw material for smelting, Ni element will remain in the steel. Therefore, in the present invention, Ni≤0.30%.

[0054] Cu: plays a role of precipitation strengthening, improves the corrosion resistance of steel, has little effect on the main properties of steel, and when scrap steel is used as raw material for smelting, Cu element will remain in the steel. In the present invention, Cu≤0.2%.

[0055] Nb: Nb microalloying precipitates MC type carbides in 50CrMo steel. The precipitated phase not only refines the cast structure, but also hinders the growth of grains during the heating process of the billet, thereby refining the microstructure of the steel and improving the uniformity of the steel structure. A small amount of Nb can produce a strong precipitation strengthening effect, but when the content is too high, continuous casting ingot breakage and other production accidents are prone to occur. After research by the inventor, it is determined that Nb: 0.01% ~ 0.05%.

[0056] V: V is a strong carbide-forming element. V microalloying plays a role in the precipitation strengthening of MC-type carbides and / or the fine grain strengthening effect of hindering grain growth, thereby improving wear resistance. The inventor's research results show that 0.008-0.050% can play a strengthening role. When the addition amount is too high, it not only increases the cost of steel but also increases the difficulty of machining.

[0057] O: Non-metallic inclusions such as manganese oxide and aluminum oxide are easily formed in steel, which can easily become crack sources during the service of the screw and cause rolling contact fatigue damage. Therefore, the O content should be strictly controlled, but deoxidation after O≤10ppm will increase the smelting cost, so O≤10ppm.

[0058] It should be pointed out that the existing ball screw steel contains element B, which will affect the hardenability of the ball screw steel, causing the steel to become brittle and have poor toughness. Therefore, in order to avoid the toughness of the ball screw steel rod, the present invention avoids adding element B.

[0059] In a specific embodiment, the components of the steel rod for ball screw blank include, by mass percentage: C: 0.49-0.53%, Si≤0.34%, Mn: 0.80%-1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.85%-1.10%, Mo: 0.18%-0.25%, Ni≤0.25%, Cu≤0.2%, Nb: 0.01%-0.09%, V: 0.03-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0060] In a specific embodiment, the components of the steel rod for ball screw blank include, by mass percentage: C: 0.50%-0.52%, Si≤0.32%, Mn: 0.85%-1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.95%-1.10%, Mo: 0.20%-0.25%, Ni≤0.22%, Cu≤0.19%, Nb: 0.01%-0.04%, V: 0.04-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0061] In a specific embodiment, the components of the steel rod for ball screw blank include, by mass percentage: C: 0.48%, Si: 0.31%, Mn: 0.82%, S: ≤0.020%, P≤0.015%, Cr: 0.90%, Mo: 0.19%, Ni: 0.24%, Cu≤0.18%, Nb: 0.02%, V: 0.04%, O≤10ppm, and the remainder is Fe and unavoidable impurities.

[0062] In a specific embodiment, the components of the steel rod for ball screw blank include, by mass percentage: C: 0.51%, Si: 0.28%, Mn: 0.90%, S: ≤0.020%, P≤0.015%, Cr: 1.0%, Mo: 0.24%, Ni: 0.19%, Cu≤0.14%, Nb: 0.03%, V: 0.03%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0063] It should be noted that, in the steel rod for ball screw blank with high organizational uniformity of the present invention, high organizational uniformity means that the microstructure of the steel is fine and uniform, the level of macrostructural defects is low (the level is not higher than level 1), and the hardenability bandwidth is small (the hardenability bandwidth at J3mm and J5mm points is ≤1HRC).

[0064] The invention discloses a steel bar for a ball screw blank with high structural uniformity. By introducing a small amount of Nb and V elements, Nb microalloying plays a role in the precipitation of MC type carbides, which not only refines the cast structure, but also hinders the growth of grains during the heating process of the blank, refines the microstructure, and improves the structural uniformity of the steel; V microalloying plays a role in the precipitation strengthening of MC type carbides and / or a role in the fine grain strengthening that hinders the growth of grains, thereby improving the wear resistance. Among them, MC type refers to a crystal structure formed by the combination of metal or metalloid elements and carbon.

[0065] The present invention also provides a method for preparing steel for a ball screw blank, comprising the following steps:

[0066] Step 1, molten steel smelting;

[0067] The smelting raw materials are smelted into molten steel by electric furnace or converter, and then refined by LF ladle refining furnace, and then degassed by RH vacuum circulation degassing furnace or VD vacuum degassing to ensure that the O content in molten steel is ≤10ppm;

[0068] Step 2, preparing an ingot;

[0069] The molten steel smelted in step 1 is sprayed from the ladle into the ingot mold from the bottom of the ingot mold through the casting pipe. In addition, an electromagnetic stirrer is provided outside the ingot mold; alternatively, the molten steel smelted in step 1 is cast into the ingot casting device through the top of the ingot casting device to prepare an ingot.

[0070] Step 3, heating the ingot;

[0071] Ingot heating includes hot charging and hot delivery or cold charging. The ingot heating temperature is 1200-1250°C and the residence time is ≥120min.

[0072] In the above step 3, since the homogenization of the ingot is a key process, under a certain solidification structure, high-temperature homogenization treatment can fully melt the carbides in the steel and reduce the dendrite segregation defect. The present invention requires the heating temperature of the ingot to be 1200℃-1250℃, and the residence time is ≥120min, which can ensure that the ingot has a good homogenization effect; if the temperature is too high, the production efficiency may not be guaranteed.

[0073] Step 4, rolling the ingot to obtain a steel bar;

[0074] The rolling of ingots includes two stages: blanking and bar rolling. The blanking stage is mainly carried out for ingots, adopting a high temperature and high pressure rolling route, and the starting rolling temperature is ≥1150℃. The steel billet undergoes intense recrystallization during the blanking process to form defect-free grains, which is beneficial to the homogenization process of the material.

[0075] Bar rolling is a key stage in controlling the grain size of steel bars. The rolling temperature is carried out in a lower recrystallization temperature range, so that the steel bars undergo recrystallization without producing obvious grain growth. Therefore, the final rolling temperature of the steel bars is 800-950°C.

[0076] The rolling process of the bar includes rough rolling and finish rolling, wherein the rough rolling adopts reciprocating rolling, during which the diameter of the bar is rolled from 240 mm to 120 mm, and the deformation amount of each pass is 28%-32%.

[0077] During finish rolling, the bar diameter is rolled from 120mm to 50mm, and the deformation amount of each pass is 15%-20%.

[0078] Step 5: annealing the steel bar after rolling;

[0079] The hot-rolled steel bars are placed in a trolley furnace for annealing at a temperature of 600-780°C to reduce the deformation caused by rolling stress during the cooling process. At the same time, spherical carbide precipitation phases can be formed during the annealing process, thereby reducing stress and hardness.

[0080] Step 6: straighten the steel bar;

[0081] A straightening machine is used to make the fiber length inside the steel rod consistent through external force, thereby eliminating the bending and uneven deformation in the steel rod. During straightening, the straightening equipment is used to control the straightness of the steel rod within the range of curvature ≤1mm / m.

[0082] Step 7, quenching and tempering the steel bar;

[0083] During the quenching and tempering treatment, the steel bar is heated to above 860℃ at a speed of 10℃ / s by medium frequency induction heat treatment equipment and kept warm for more than 1min, then cooled to room temperature by spraying water, and then heated to above 650℃ by medium frequency induction heat treatment equipment, and then air-cooled to room temperature. After the quenching and tempering treatment, the hardness of the steel bar from the surface to the radius 1 / 2 is 240-270HBW; heating time is 6-8min, and cooling time is 15-25min;

[0084] The above-mentioned quenching and tempering process has high temperature and short time, and the obtained rod has fine and uniform structure and small deformation of the steel plate.

[0085] Step 8: Perform subsequent processing on the steel rod;

[0086] In the above step 8, the above subsequent treatment refers to peeling, straightening and annealing the steel rod after the quenching and tempering treatment.

[0087] In the above step 8, the peeling process is used to ensure the surface quality and dimensional accuracy of the steel rod.

[0088] In the above step 8, the straightening process refers to controlling the straightness of the steel bar within a curvature of ≤1mm / m. Since the straightening process introduces stress again, it is necessary to perform stress relief annealing again. The annealing is performed in a pit furnace to ensure that the steel bar is heated in a vertical state and no deformation occurs during the heating process. The annealing temperature is ≤ the tempering temperature of the quenching and tempering treatment, such as the annealing temperature is 650℃.

[0089] It should be noted that in the above step 2, when an ingot casting device is used to prepare an ingot, the ingot casting device includes an ingot casting mold, an electromagnetic stirrer is provided on the outside of the ingot casting mold; a heat preservation riser is provided on the top of the ingot casting mold, and a double-hole pouring funnel is provided on the top of the heat preservation riser. The molten steel smelted in an electric furnace or a converter is poured into the double-hole pouring funnel through a casting pipe, and then injected into the ingot casting mold through the double-hole pouring funnel and the heat preservation riser.

[0090] Compared with the prior art, the ingot obtained by using the ingot casting device of the present invention has good structural uniformity, and the segregation of the material can be reduced by electromagnetic stirring.

[0091] In order to cool the contained molten steel to form an ingot, the ingot mold of the present invention is connected to a water cooling system, and an ingot with good metallurgical quality can be obtained by adjusting the parameters of the cooling water inlet temperature, flow rate and power-on frequency of the electromagnetic stirrer.

[0092] It is also necessary to emphasize that the present invention can rapidly solidify the molten steel by providing an ingot casting mold and connecting it to a cooling water system. When the molten steel solidifies, the high degree of supercooling and the fast cooling rate can not only increase the solidification nucleation rate, but also make it less likely to have obvious component segregation due to selective crystallization, thereby making the alloy composition more uniform. At the same time, the grain size in the rapidly solidified steel is small and the size distribution is uniform.

[0093] In order to stir the molten steel in the ingot mold, the electromagnetic stirrer of the present invention comprises a first plate-shaped electromagnetic stirrer and a second plate-shaped electromagnetic stirrer; the first plate-shaped electromagnetic stirrer and the second plate-shaped electromagnetic stirrer are both located outside the ingot mold, which can form a uniform magnetic field in the ingot mold and stir the molten steel. In addition, the first plate-shaped electromagnetic stirrer and the second plate-shaped electromagnetic stirrer are both double-sided traveling wave magnetic field stirrers, and the power-on frequency thereof is 20 to 60 Hz.

[0094] Compared with the prior art, the present invention, by arranging an electromagnetic stirrer on the outside of the ingot mold, can ensure that the magnetic field formed in the ingot mold is a uniform magnetic field with magnetic flux lines parallel to the horizontal plane of the molten steel, thereby making the solidification quality of the ingot (including grain size and degree of element homogenization) have good consistency.

[0095] In order to buffer and distribute the pouring molten steel, the double-hole pouring funnel of the present invention includes a concave soup channel and a first funnel and a second funnel located on both sides of the concave soup channel; a first funnel outlet is provided at the bottom of the first funnel; a second funnel outlet is provided at the bottom of the second funnel; during pouring, the molten steel flows through the concave soup channel and enters the cavity of the ingot mold from the first funnel outlet and the second funnel outlet.

[0096] Compared with the prior art, the double-hole pouring funnel adopted in the present invention can play a role of buffering the molten steel during the pouring of the molten steel.

[0097] It should be noted that the first funnel outlet and the second funnel outlet of the present invention are respectively located at one quarter and three quarters of the length of the double-hole pouring funnel; the concave soup channel is located in the middle of the length of the double-hole pouring funnel. It should be noted that the first funnel outlet and the second funnel outlet are in the shape of a circular hole, and their diameters are both 24-26 mm.

[0098] Compared with the prior art, the present invention pours molten steel into the ingot mold through a double-hole pouring funnel, so that the molten steel flows and solidifies quickly and evenly in the ingot mold, which can reduce segregation.

[0099] In summary, the present invention can prepare a steel rod for ball screw blank with high structural uniformity through the above-mentioned preparation method.

[0100] Example 1

[0101] The preparation process of the steel bar for the ball screw blank with high organizational uniformity in this embodiment is as follows:

[0102] Step 1, molten steel smelting;

[0103] The smelting raw materials are smelted into molten steel in a 30-ton converter, and then refined in an LF ladle refining furnace. After refining, RH vacuum circulation degassing furnace or VD vacuum degassing is used to ensure that the O content in the molten steel is ≤10ppm;

[0104] Step 2, preparing an ingot;

[0105] The molten steel smelted in step 1 is sprayed into the ingot mold from the bottom of the ingot mold through the ladle through the casting pipe. In addition, an electromagnetic stirrer is provided outside the ingot mold, and the weight of the ingot is 2.6 tons / piece. It should be explained that spraying the smelted molten steel into the ingot mold through the bottom of the ingot mold can increase the disturbance of the molten steel. In addition, the electromagnetic stirrer provided outside the ingot mold can further increase the disturbance of the molten steel to obtain an ingot with uniform structure.

[0106] Step 3, heating the ingot;

[0107] Ingot heating includes hot charging and hot delivery or cold charging. The ingot heating temperature is 1200℃ and kept warm for 2h.

[0108] Step 4, rolling the ingot to obtain a steel bar;

[0109] After rolling, the billet is formed into a billet with a diameter of 110 mm. The billet is furnace cooled to room temperature. After passing the inspection, it is heated at 1160°C for 2 hours and hot rolled into a steel bar with a diameter of 40.4 mm.

[0110] Step 5: annealing the steel bar after rolling;

[0111] The hot-rolled steel bars are placed in a trolley furnace for annealing at a temperature of 650°C to reduce the deformation caused by rolling stress during the cooling process. At the same time, spherical carbide precipitation phases can be formed during the annealing process, thereby reducing stress and hardness.

[0112] Step 6: straighten the steel bar;

[0113] A straightening machine is used to make the fiber length inside the steel rod consistent through external force, thereby eliminating the bending and uneven deformation in the steel rod. During straightening, the straightening equipment is used to control the straightness of the steel rod within the range of curvature ≤1mm / m; ultrasonic flaw detection meets Class A standards.

[0114] Step 7, quenching and tempering the steel bar;

[0115] During the quenching and tempering treatment, the steel bar is heated to 880℃ at a speed of 10℃ / s by medium frequency induction heat treatment equipment and kept at this temperature for 2 minutes. The quenching is performed by water spray cooling, and then the steel bar is heated to 700℃ by medium frequency induction heat treatment equipment. After tempering, the steel bar has a hardness of 240-270HBW from the surface to 1 / 2 of the radius; the heating time is 7 minutes and the cooling time is 18 minutes;

[0116] Step 8: Perform subsequent processing on the steel rod;

[0117] In the above step 8, the steel bar after the quenching and tempering treatment is peeled and then straightened. The straightening process refers to controlling the straightness of the steel bar to a curvature of 1 mm / m. Finally, the stress relief treatment is carried out in a pit furnace at 650°C for 4 hours.

[0118] The microstructure of the steel bar after heat treatment is tempered martensite ( Figure 1 ), after the steel bar is quenched at 860℃, a fine and uniform martensite structure is obtained ( Figure 2 ), corroding the original austenite grain boundary, and measuring its grain size by the intercept method to reach level 10 ( Figure 3 ), the hardness of the steel bar is 250-265HB, the stress level of the steel bar tested by XRD is -200MPa, and the grain size after quenching reaches above level 8. The steel bar was evaluated for macrostructure, including central looseness 0.5 level, general looseness, ingot segregation 0.5 level ( Figure 4 ); The surface of the end quenching result shows that the hardness of J3mm 3mm away from the end is 60.0HRC, the hardness of J5mm is 59.5HRC, the hardenability bandwidth of J3mm and J5mm is 0.5HRC, and the hardness of J9mm reaches 59.5HRC ( Figure 5 ).

[0119] The steel used for the ball screw blank of this embodiment has obtained a relatively high uniformity of organization, which is mainly reflected in the central looseness, general looseness, and ingot-shaped segregation of its low-magnification organization inspection results, which are all at level 0.5, which is significantly lower than the common level of 1.0 or above on the market. The low-magnification organization is the macroscopic organizational morphology observed at a low magnification state, which characterizes the defects of the solidified organization after subsequent deformation processing. The lower the level, the better the organizational uniformity. The uniformity of performance is mainly reflected in the end quenching test results. The end quenching test is a method of characterizing the hardenability of a rod-shaped sample after austenitization heating, spraying water at one end of the sample for cooling, and using the distance from the end hardness to characterize the hardenability. The smaller the hardenability bandwidth of the steel, the higher the uniformity of the steel. The hardenability bandwidth of the steel J3mm and J5mm points of the present invention is 0.5HRC, while ordinary materials are usually greater than 2HRC, indicating a higher level of uniformity.

[0120] Example 2

[0121] The preparation process of the steel bar for the ball screw blank with high organizational uniformity in this embodiment is as follows:

[0122] Step 1, molten steel smelting;

[0123] The smelting raw materials are smelted into molten steel in a 35-ton converter, and then refined in an LF ladle refining furnace. After refining, RH vacuum circulation degassing furnace or VD vacuum degassing is used to ensure that the O content in the molten steel is ≤10ppm;

[0124] Step 2, preparing an ingot;

[0125] The molten steel smelted in step 1 is poured into the ingot mold through a double-hole pouring funnel and a heat-insulating riser to complete the casting; during the casting process, the electromagnetic stirrer and the cooling system are turned on.

[0126] Step 3, heating the ingot;

[0127] Ingot heating includes hot charging and hot delivery or cold charging. The ingot heating temperature is 1230℃ and kept warm for 3h.

[0128] Step 4, rolling the ingot to obtain a steel bar;

[0129] After rolling, the billet is formed into a billet with a diameter of 110mm. The billet is furnace cooled to room temperature. After passing the inspection, it is heated at 1180℃ for 2h and hot rolled into a steel bar. The hot-rolled steel bar is annealed at 700℃ and then straightened. The ultrasonic flaw detection meets the A-level standard.

[0130] Step 5: annealing the steel bar after rolling;

[0131] The hot-rolled steel bars are placed in a trolley furnace for annealing at a temperature of 700°C to reduce the deformation caused by rolling stress during the cooling process. At the same time, spherical carbide precipitation phases can be formed during the annealing process, thereby reducing stress and hardness.

[0132] Step 6: straighten the steel bar;

[0133] A straightening machine is used to make the fiber length inside the steel rod consistent through external force, thereby eliminating the bending and uneven deformation in the steel rod. During straightening, the straightening equipment is used to control the straightness of the steel rod to a curvature of 0.8mm / m.

[0134] Step 7, quenching and tempering the steel bar;

[0135] During the quenching and tempering treatment, the steel bar is heated to 900℃ at a speed of 10℃ / s by medium frequency induction heat treatment equipment and kept at this temperature for 3 minutes. The quenching is performed by water spray cooling, and then the steel bar is heated to 720℃ by medium frequency induction heat treatment equipment. After tempering, the hardness of the steel bar from the surface to the radius 1 / 2 is 240-270HBW; heating time is 8 minutes, and cooling time is 20 minutes;

[0136] Step 8: Perform subsequent processing on the steel rod;

[0137] In the above step 8, the steel bar after the quenching and tempering treatment is peeled and then straightened. The straightening process refers to controlling the straightness of the steel bar within the curvature ≤ 1mm / m. Finally, the stress relief treatment is carried out in a pit furnace at 650°C for 4 hours.

[0138] The microstructure of the steel bar after heat treatment of the present embodiment is tempered troostite. After quenching, the steel bar obtains a fine and uniform martensitic structure, corrodes the original austenite grain boundary, and measures its grain size by the intercept method to reach 10 grades. The hardness of the steel bar is 250-265HB, and the stress level of the steel bar tested by XRD is -200MPa. After quenching, the grain size reaches more than 8 grades. The steel bar has been evaluated for macrostructure, wherein the center is loose 0.5 grade, the general looseness, and the ingot segregation is 0.5 grade. The end quenching result shows that the hardness J3mm 3mm away from the end is 61.0HRC, the hardness J5mm is 59.3HRC, the hardenability bandwidth of J3mm and J5mm is 0.5HRC, and the hardness of J9mm reaches 59.6HRC.

[0139] The ball screw blank steel of this embodiment has achieved relatively high organizational uniformity, which is mainly reflected in that the central looseness, general looseness and ingot segregation of its low-magnification organizational test results are all at level 0.5, which is significantly lower than the common level of 1.0 or above on the market. The low-magnification organization is the macroscopic organizational morphology observed under low-magnification conditions, which characterizes the defects in the solidified organization after subsequent deformation processing. The lower the level, the better the organizational uniformity.

[0140] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A steel for ball screw blank, characterized in that: The components of the steel for ball screw blank include, by mass percentage, C: 0.47% to 0.53%, Si≤0.35%, Mn: 0.70% to 1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.80% to 1.10%, Mo: 0.15% to 0.25%, Ni≤0.30%, Cu≤0.2%, Nb: 0.01-0.1%, V: 0.01-0.05%, O≤10ppm, H≤0.00015%, and the balance is Fe and unavoidable impurities.

2. The steel for ball screw blank according to claim 1, characterized in that: The components of the steel bar for ball screw blank include, by mass percentage, C: 0.49% to 0.53%, Si≤0.34%, Mn: 0.80% to 1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.85% to 1.10%, Mo: 0.18% to 0.25%, Ni≤0.25%, Cu≤0.2%, Nb: 0.01%-0.09%, V: 0.03-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

3. The steel for ball screw blank according to claim 2, characterized in that: The components of the steel bar for ball screw blank include, by mass percentage, C: 0.50% to 0.52%, Si≤0.32%, Mn: 0.85% to 1.00%, S: ≤0.020%, P≤0.015%, Cr: 0.95% to 1.10%, Mo: 0.20% to 0.25%, Ni≤0.22%, Cu≤0.19%, Nb: 0.01%-0.04%, V: 0.04-0.05%, O≤10ppm, and the balance is Fe and unavoidable impurities.

4. The steel for ball screw blank according to claim 1, characterized in that: The microstructure of the steel for the ball screw blank is tempered troostite.

5. A method for preparing a steel bar for a ball screw blank, characterized in that: For preparing the steel for ball screw blank according to any one of claims 1 to 4, the preparation method comprises the following steps: Step 1, molten steel smelting; Use smelting raw materials to smelt molten steel, then perform refining after smelting, and perform vacuum degassing after refining to ensure that the O content in the molten steel is ≤10ppm; Step 2, preparing an ingot; The molten steel smelted in step 1 is poured into an ingot casting device to prepare an ingot; Step 3, heating the ingot; The ingot heating temperature is 1200-1250° C., and the residence time is ≥120 min. Step 4, rolling the ingot to obtain a steel bar; The rolling of ingots includes two stages: billet rolling and bar rolling; the rolling temperature is ≥1150℃; Step 5: annealing the steel bar after rolling; Step 6: straighten the steel bar; A straightening machine is used to make the fiber length inside the steel rod uniform through external force; Step 7, quenching and tempering the steel bar; Step 8: Perform subsequent processing on the steel rod; The subsequent treatment refers to peeling, straightening and annealing of the steel bar after the quenching and tempering treatment.

6. The method for preparing ball screw blank steel according to claim 5, characterized in that: In step 4, the final rolling temperature of the steel bar is 800-950°C.

7. The method for preparing ball screw blank steel according to claim 6, characterized in that: In step 4, the rolling process of the bar includes rough rolling and finish rolling; The rough rolling adopts reciprocating rolling. During the rough rolling, the diameter of the bar is rolled from 240 mm to 120 mm. The deformation amount of each rolling pass is 28%-32%.

8. The method for preparing ball screw blank steel according to claim 5, characterized in that: In step 5, the hot-rolled steel bar is placed in a trolley furnace for annealing at a temperature of 600-780°C.

9. The method for preparing ball screw blank steel according to claim 5, characterized in that: In step 6, during straightening, the straightening equipment is used to control the straightness of the steel bar within the range of curvature ≤ 1 mm / m.

10. The method for preparing ball screw blank steel according to any one of claims 5 to 9, characterized in that: In the step 8, during the annealing treatment, the annealing temperature is less than or equal to the tempering temperature of the quenching and tempering treatment.

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