SPHEROIDIZING-ANNEALED STEEL FOR BALL SCREW HAVING HIGH STRENGTH AND RESISTANCE TO LOW TEMPERATURES AND MANUFACTURING METHOD THEREOF

Abstract

The present disclosure relates to a spheroidizing-annealed steel for the ball screw having high strength and resistance to low temperatures, wherein the chemical composition of the steel in mass percentage is: C: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: ?0.025%, P?0.025%, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, Al?0.05%, Ca?0.0010%, Ti?0.003%, O?0.0010%, As?0.04%, Sn?0.03%, Sb?0.005%, Pb?0.002%, the balance is Fe and unavoidable impurities. In microstructure of the steel, cementite exists in a spheroidized state with a diameter of 0.1-0.5 ?m, preferably 0.3-0.5 ?m, a spheroidizing rate is 95% or more, and the rest is ferrite.

Claims

1. A method of manufacturing spheroidizing-annealed steel for ball screw having high strength and resistance to low temperatures, wherein comprises following steps: Step 1: preparing smelting raw materials according to the chemical composition of steel, the chemical composition of the steel in mass percentage is: C: 0.40-0.70%, Si: 1.20-1.80%, Mn: 1.00-1.60%, Cr: 0.80-1.20%, S: ?0.025%, P?0.025%, Ni: 0.10-0.60%, Cu: 0.30-0.80%, Mo: 0.10-0.40%, Al?0.05%, Ca?0.0010%, Ti?0.003%, O?0.0010%, As?0.04%, Sn?0.03%, Sb?0.005%, Pb?0.002%, a balance is Fe and unavoidable impurities; smelting raw materials by converter, refining and vacuum degassing in electric furnace or converting to obtain molten steel; in a refining process of electric furnace or converter, a content of C at a smelting end point is controlled within 0.05-0.25%, and a content of P at a end point is less than 0.025%; molten steel is cast into 390?510 mm or above continuous casting billets with the same chemical composition as the finished steel products by continuous casting process; a casting flow in the solidification process is subjected to electromagnetic stirring and light pressing, and a superheat of continuous casting is 15-35? C.; Step 2: the continuous casting billet is slowly cooled in a pit, a slow cooling lasts for not less than 48 hours; then the continuous casting billet is sent to a heating furnace with a neutral atmosphere or a weakly oxidative atmosphere for heating; then the continuous casting billet is rolled into an intermediate billet of 200 mm?200 mm to 300 mm?300 mm; a heating temperature of a continuous casting slab is 1000-1250? C., a heating time is more than 5 hours, in rolling a start rolling temperature is set at 1000? C.-1200? C., a final rolling temperature is ?800? C., a compression ratio in rolling is greater than 5; the intermediate billet obtained by rolling is slowly cooled in a pit, a temperature of the intermediate billet entering the pit is ?500? C., and the slow cooling lasts for ?48 hours; Step 3: heating the intermediate billet again and rolling into a target size; the intermediate billet is heated as follows: in a section of preheating, the heating temperature is 650-900? C., in a section of heating, the heating temperature is 1000-1250? C., and in a section of soaking, the heating temperature is 1000-1250? C., a total heating time is more than 2 hours; a start rolling temperature in rolling is 1000? C.-1200? C., a final rolling temperature is ?800? C.; after the rolling is completed, stack cooling; Step 4: spheroidizing annealing a rolled product; the spheroidizing annealing process is: firstly the rolled product is kept warm at 805?10? C. for more than 7 hours; then the rolled product is cooled to 745? C.?10? C. by water-mist cooling, and kept warm at this temperature for more than 5 hours; then the rolled product is cooled to 690?10? C. in the furnace, and kept warm at this temperature for more than 4.5 hours; finally the rolled product is cooled to 500?10? C. in the furnace and then taken out of the furnace; and Step 5: straightening and detecting the rolled product after spheroidizing annealing, so as to obtain a qualified product.

2. The method according to claim 1, wherein after quenching and tempering treatment, the steel has yield strength of ?1380 MPa, tensile strength?1500 MPa, elongation?9%, impact energy AKU.sub.2 at ?40? C.?27 J; when JIS G 0561 method is used to test end hardenability, J9 mm hardness?58 HRC.

3. The method according to claim 1, wherein in microstructure of the steel, cementite exists in a spheroidized state with a diameter of 0.1-0.5 ?m, preferably 0.3-0.5 ?m, a spheroidizing rate is 95% or more, and the rest is ferrite.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a structure diagram showing the spheroidizing annealing according to the embodiment 1 of the present disclosure.

[0042] FIG. 2 is a structure diagram showing the spheroidizing annealing according to the embodiment 2 of the present disclosure.

[0043] FIG. 3 is a structure diagram showing the spheroidizing annealing according to the embodiment 3 of the present disclosure.

[0044] FIG. 4 is a diagram showing the process of spheroidizing annealing.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The present disclosure is further described in detail in conjunction with embodiments.

[0046] Embodiments 1-3 give embodiments of the chemical composition and manufacturing method of the steel for the ball screw of the present disclosure respectively, and compare them with the GCr15 bearing steel which is commonly used in the market.

[0047] The chemical composition (wt %) of each embodiment is shown in the Table 3 and Table 4.

TABLE-US-00003 TABLE 3 Embodiment C Si Mn P S Cr Cu Ni Al The present disclosure 1 0.58 1.56 1.36 0.015 0.006 1.11 0.18 0.12 0.017 The present disclosure 2 0.59 1.54 1.35 0.015 0.007 1.10 0.18 0.13 0.018 The present disclosure 3 0.58 1.57 1.38 0.015 0.006 1.10 0.20 0.13 0.017 GCr15 4 0.99 0.25 0.32 0.020 0.015 1.48 0.05 0.05 0.020

TABLE-US-00004 TABLE 4 Embodiment Mo As Sn Sb Pb Ca Ti O The present disclosure 1 0.21 0.0043 0.011 0.0015 0.001 0.0001 0.0011 0.00054 The present disclosure 2 0.21 0.0044 0.011 0.0017 0.001 0.0001 0.0011 0.00057 The present disclosure 3 0.22 0.0046 0.011 0.0016 0.001 0.0001 0.0011 0.00051 GCr15 4 0.01 0.0051 0.021 0.0023 0.001 0.0001 0.0023 0.00071

[0048] The inclusions of the steel of each embodiment are shown in Table 5.

TABLE-US-00005 TABLE 5 Group Group Group Group Group Group A A B B Group Group D D Group Embodiment Fine Thick Fine Thick C Fine C Thick Fine Thick Ds The present 1 0-0.5 0-0.5 0-1.0 0-0.5 0 0 0-0.5 0-0.5 0-0.5 disclosure The present 2 0-0.5 0-0.5 0-0.5 0 0 0 0-1.0 0-0.5 0-0.5 disclosure The present 3 0-0.5 0-0.5 0-0.5 0 0 0 0-1.0 0-0.5 0-0.5 disclosure GCr15 4 0-0.5 0-0.5 0-0.5 0-0.5 0 0 0-1.0 0-1.0 1.0-1.5

[0049] The mechanical properties of embodiments (oil quenching at 880? C.+ water cooling at 450? C.) are shown and compared in Table 6.

TABLE-US-00006 TABLE 6 Impact Yield Tensile energy strength strength Elongation at ?40? C. Hardness Embodiment Rel (MPa) Rm (MPa) A5 (%) AKU.sub.2 HRC The present 1 1431 1576 11.5 38 47.8 disclosure The present 2 1427 1564 11.0 42 47.2 disclosure The present 3 1433 1590 11.0 36 48.2 disclosure GCr15 4 1006 1170 12.0 7 37.8

[0050] The data of end hardenability of each embodiment steel is shown in Table 7.

TABLE-US-00007 TABLE 7 Embodiment J9 mm (HRC) The present disclosure 1 59.48 The present disclosure 2 59.65 The present disclosure 3 59.91 GCr15 4 43.33

[0051] The microstructure of the steel in each embodiment is shown in FIGS. 1-3. Different from the thick spherical cementites in the traditional GCr15 bearing steel, the cementites of the steel in a delivery state of the present disclosure exist in a uniform and finer (generally 0.1-0.5 ?m) spheroidized state, the spheroidizing rate is over 95%, and the rest is ferrite. The structure distortion is small, the heat treatment deformation is small in the process of processing the screw product, and the dimensional accuracy is high, which can meet the accuracy requirements of the ball screw.

[0052] The production process of the steel for the ball screw in each embodiment is electric furnace or converter-refining outside the furnace-VD or RH vacuum degassing-continuous casting-continuous casting billets are squared into intermediate billets-the intermediate billets are heated and rolled into products-spheroidizing annealing-finishing-packaging for storage.

[0053] In a specific smelting, we use high-quality molten iron, scrap steel and raw/auxiliary materials, and use high-quality deoxidizer and refractory materials. In the production process of electric furnace/converter, the C content at an endpoint for tapping of the three embodiments is controlled to be 0.05-0.25%, and the P content at the endpoint is controlled to be ?0.025%. The superheat of the continuous casting is controlled to be 15-35? C.

[0054] The continuous casting billet of each embodiment is subjected to billet rolling, and the process is shown in Table 8 below.

TABLE-US-00008 TABLE 8 Rolling process Cooling process Heating process Start Final Time of Heating Heating rolling rolling Temperature cooling in temperature time temperature temperature of entering the pit Embodiment (? C.) (h) (? C.) (? C.) the pit (? C.) (h) 1 1151 3 h 30 min 1107 976 530 60 2 1153 3 h 30 min 1111 975 536 60 3 1152 3 h 28 min 1112 966 526 60

[0055] The intermediate billet is sent to the heating furnace and rolled into the target round bar. A specific rolling process is: in the preheating section, the temperature is controlled at 650-900? C.; in the heating section, the temperature is controlled at 1000-1250? C., and in the soaking section, the temperature is controlled at 1100-1200? C.; in order to ensure that the billet is fully and evenly heated, the total heating time is 2 hours or more. The start rolling temperature is controlled at 900? C.-1100? C., and the final rolling temperature is controlled above 800? C. After rolling, it should be cooled slowly, to make the AlN particles in the steel fine, uniform, and fully separated, thereby refining the grains and preventing the steel from showing mixed crystals; after rolling is completed, stack cooling. The rolled bar is subjected to spheroidizing annealing treatment, and the process is shown in the above-mentioned three-stage spheroidizing process diagram. After spheroidizing annealing, the bar products are subjected to flaw detection and finally put into storage.

[0056] From Tables 3, 4, 5, 6, and 7, it can be seen that, compared with the traditional GCr15 bearing steel, the steel for ball screw having high strength and resistance to low temperatures in each embodiment of the present disclosure has a better level in controlling the harmful elements such as oxygen, titanium, and non-metallic elements. Especially in terms of mechanical properties, after the same quenching and tempering process, the present disclosure has significantly better performances in yield strength, tensile strength, low-temperature impact, and resistance to softening during tempering, than the traditional GCr15 bearing steel, wherein the yield strength is increased by nearly 400 MPa or more, the tensile strength is increased by 300 MPa, the low-temperature impact performance is increased by nearly 30 J, and the hardness is increased by nearly 10 HRC. The hardenability is also significantly better than the traditional GCr15 bearing steel.

[0057] Although the preferred embodiments of the present disclosure have been described in detail above, it should be clearly understood that various modifications and changes of the present disclosure will occur to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present disclosure shall be included within the protection scope of the present disclosure.