Low-sulfur lead-free free-cutting steel and manufacturing method thereof

Abstract

The invention discloses low-sulfur lead-free free-cutting steel and a manufacturing method thereof, and belongs to the technical field of metal materials. According to the scheme provided by the invention, the composition design containing copper and low B and N is adopted, so that the obtained steel has good mechanical properties while meeting excellent free cutting, and the machining efficiency and the machining precision are improved; the yield strength is greater than or equal to 400MPa, the tensile strength is greater than or equal to 550MPa, and A50 is greater than or equal to 25%. Compared with Q345 steel with the same strength grade, the steel provided by the invention has the advantages that the finish degree of the machined surface of the steel is higher under the same turning condition, Ra is about 1.5 microns (Ra of the Q345 steel with the same strength grade is about 3 microns), and meanwhile, the wear degree of a cutter is lower than that of the Q345 steel which is turned.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal materials, and particularly relates to a low-sulfur, lead-free free-cutting steel and a manufacturing method thereof. BACKGROUND

[0002] With the development of high-precision numerical control machine tools, the machining precision can be further improved in theory, but the free-cuting property of steel materials is also required to be more and more strict. The steel plate is required to have high free-cuting property while meeting the mechanical property, to meet the requirement of high-precision machining, improve the machining efficiency, and prolong the service life of the tool. Therefore, the development of the steel material with high mechanical property and excellent free-cuting property can not only meet the requirement of high-precision machining, but also bring considerable economic benefits to the steel enterprises.

[0003] A kind of copper-containing BN type free-cutting steel with small anisotropy is disclosed in CN201310345138.0, and the component and weight percentage content are as follows: C: 0.10~0.70%, Si: 0.10~0.60%, Mn: 0.20~1.50%, P<0.025%, S<0.010%, Al: 0.02~0.08%, B: 0.021~0.028%, N: 0.020~0.040%, Cu: 0.70~2.00%, and the balance is iron and inevitable impurities.

[0004] A preparation method of high-strength lead-free free-cutting steel is disclosed in CN102477515A, and the powder sintering method is used for production. The mass fraction of various powders and binders is as follows: graphite powder 0.5%-0.8%, copper powder 0.9%-1.2%, binder zinc stearate 0.5%-1.0%; dispersant PVA 0.3%-0.5%; and the balance is iron powder.

[0005] A high-phosphorus sulfur free-cutting steel and a production method thereof are disclosed in CN119433349A, and the chemical composition of the steel is as follows: C0.51-0.56%, Si0.06-0.13%, Mn0.80-1.0%, P0.041-0.065%, S0.08-0.10%, Als≤0.010%, Ca0.0015-0.0030%, Mo0.16-0.22%, Cr1.90-1.98%, and Mn / S: 8-12.

[0006] The publication number CN119506720A describes a low-carbon free-machining steel wire rod capable of large deformation cold heading. Its chemical composition, by weight percentage, is: C≤0.09%, Si≤0.01%, Mn 1.00-1.30%, P 0.035-0.060%, S 0.30-0.60%, Cr≤0.10%, Ni≤0.10%, Cu 0.05-0.20%, Al≤0.005%, Nb 0.003-0.010%, with the balance being Fe and unavoidable impurities. The publication number CN116426710A discloses a converter smelting method for phosphorus-containing free-cutting steel. The elemental composition of the phosphorus-containing free-cutting steel, by weight percentage, is: C: ≤0.09%, Si: ≤0.08%, Mn: 1.10-1.40%, P: 0.04-0.09%, S: 0.33-0.42%, Cr, Ni, Cu ≤0.20%, with the remainder being iron and unavoidable impurities.

[0007] Most of the inventions mentioned above are designed with high sulfur content, or high B and N content, and are manufactured using powder metallurgy methods.

[0008] Therefore, how to provide a free-cutting steel containing copper and low levels of boron and nitrogen while ensuring mechanical properties and machinability has become an important issue that urgently needs to be addressed. Summary of the Invention

[0009] Therefore, the purpose of this invention is to provide a low-sulfur, lead-free free-cutting steel and its manufacturing method, which adopts a copper-containing and low-B and N composition design, so as to satisfy excellent free-cutting performance, good mechanical properties, and improve machining efficiency and machining accuracy.

[0010] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a low-sulfur, lead-free free-cutting steel, comprising the following chemical composition by mass percentage: C 0.1%-0.2%, Si ≤0.01%, Mn 0.07%-0.1%, Al 0.02%-0.08%, Cu 0.6%-1.2%, P<0.012%, S 0.005%-0.01%, N ≤0.003%, balance being Fe and unavoidable impurities.

[0011] C is a common element in steel. It can expand the austenite phase region and improve the strength of steel, but it significantly reduces the plasticity of steel. This invention selects an ultra-low carbon design to improve the plasticity of steel. Considering the smelting cost and mechanical properties, the carbon content is determined to be 0.1%-0.2%.

[0012] Among them, nitrogen, as a common gaseous element in steel, can form aluminum nitride to refine the grains, but it increases the aging sensitivity of steel and reduces its machinability. It should be controlled at ≤0.003%.

[0013] Si is also a common element in steel, which improves the strength of steel. Based on cost factors and the mechanical properties of steel, the silicon content is designed to be Si ≤0.01%.

[0014] Mn is a common element in steel. It forms MnS with sulfur, which improves the machinability of steel. However, MnS reduces the mechanical properties of steel and increases its susceptibility to hot brittleness. Therefore, a low manganese design of 0.07%-0.1% is adopted.

[0015] Al has the effect of solid solution strengthening. When it forms AlN with nitrogen, it can refine grains and precipitation strengthening, but it increases the content of non-metallic inclusions in steel and reduces the plasticity of steel. The aluminum content of this invention is designed to be 0.02%-0.08%.

[0016] Cu expands the austenite phase region, which has solid solution strengthening and precipitation strengthening effects, improving the weather resistance of steel. At the same time, the softer copper precipitates and CuS can improve the machinability of steel. Considering the strength and machinability of steel, the copper content is designed to be 0.6%-1.2%.

[0017] Among them, sulfur (S) is a harmful impurity that deteriorates the cold and hot working properties and plasticity of steel, but sulfides can improve the machinability of steel. The present invention is designed to contain 0.005%-0.01% sulfur.

[0018] While P can improve strength and machinability, it severely deteriorates the cold working properties of steel. This invention controls P to be <0.012%.

[0019] Secondly, the present invention provides a method for manufacturing a low-sulfur, lead-free free-cutting steel, comprising the following steps: S1: After heating a qualified steel billet and homogenizing it, it is rolled to obtain steel. S2: The steel is cooled using a two-stage cooling method to obtain low-sulfur, lead-free free-cutting steel; In this process, when the molten steel composition meets the design requirements through vacuum smelting in an electric furnace, it is injected into a steel ingot mold at a pouring temperature of 1580℃ to obtain a steel billet with qualified composition.

[0020] The steel billet is rolled to a thickness of 5.5 mm in four passes.

[0021] In S2, after the second-stage cooling is completed, the steel is cooled to room temperature along with the furnace.

[0022] Based on the above technical solution, further, in S1, the steel billet is heated to 1100℃-1150℃ and then homogenized for 30min-40min.

[0023] Based on the above technical solution, the final rolling temperature in S1 is 860℃-900℃.

[0024] Based on the above technical solution, further, the second-stage cooling in S2 specifically involves: cooling the steel to 810℃-830℃ at a cooling rate of 15℃ / s-25℃ / s and holding it at that temperature for 15min-20min; then cooling it to 560℃-600℃ at a cooling rate of 30℃ / s-45℃ / s and holding it at that temperature for 20min-40min.

[0025] Based on the above technical solution, the low-sulfur, lead-free free-cutting steel has a yield strength ≥400MPa and a tensile strength ≥550MPa.

[0026] Among them, low-sulfur, lead-free free-cutting steel A 50 ≥25%.

[0027] Compared with the prior art, the present invention has the following beneficial effects: 1. The solution provided by this invention adopts a copper-containing and low B and N composition design, resulting in steel that, while satisfying excellent machinability, possesses good mechanical properties, improving machining efficiency and accuracy; yield strength ≥ 400 MPa, tensile strength ≥ 550 MPa, A 50 ≥25%, applicable to free-cutting hot-rolled steel plates.

[0028] 2. Compared with Q345 steel of the same strength grade, the steel provided by the present invention has a higher surface finish under the same turning conditions, with Ra of about 1.5μm (Q345 steel of the same strength grade has Ra of about 3μm), and the wear of the tool is lower than that when turning Q345 steel. Detailed Implementation

[0029] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0030] Unless otherwise specified, all of the following materials can be purchased commercially, and the testing methods are all conventional methods.

[0031] The low-sulfur, lead-free free-cutting steels of Examples 1 to 6 were manufactured using the manufacturing method provided by the present invention. The steel billet compositions of Examples 1 to 6 are shown in Table 1, and the hot rolling process parameters are shown in Table 2.

[0032] The low-sulfur, lead-free free-cutting steels obtained in Examples 1 to 6 were tested by tensile testing to obtain the yield strength (Rel) and tensile strength (Rm); the low-sulfur, lead-free free-cutting steels obtained in Examples 1 to 6 that fractured after the tensile test were measured to obtain A. 50 The surface roughness of the low-sulfur, lead-free free-cutting steels obtained in Examples 1 to 6 was directly measured using a surface roughness measuring instrument to obtain the turning surface Ra (μm); the results are shown in Table 3.

[0033] Table 1. Chemical composition (wt, %) of embodiments of the present invention.

[0034]

[0035] Table 2 shows the hot rolling process parameters of this invention.

[0036]

[0037] Table 3 shows the performance of the steel plates in the embodiments of the present invention.

[0038]

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A low-sulfur, lead-free free-cutting steel, characterized in that, Composed of the following chemical components by mass percentage: C 0.1%-0.2%, Si ≤0.01%, Mn 0.07%-0.1%, Al 0.02%-0.08%, Cu 0.6%-1.2%, P <0.012%, S 0.005%-0.01%, N ≤0.003%, balance being Fe and unavoidable impurities.

2. The method for manufacturing a low-sulfur, lead-free free-cutting steel as described in claim 1, characterized in that, Includes the following steps: S1: After heating a qualified steel billet and homogenizing it, it is rolled to obtain steel. S2: The steel is cooled using a two-stage cooling method to obtain low-sulfur, lead-free free-cutting steel.

3. The method for manufacturing a low-sulfur, lead-free free-cutting steel according to claim 2, characterized in that, In S1, the steel billet is heated to 1100℃-1150℃ and then homogenized for 30min-40min.

4. The method for manufacturing a low-sulfur, lead-free free-cutting steel according to claim 2, characterized in that, The final rolling temperature in S1 is 860℃-900℃.

5. The method for manufacturing a low-sulfur, lead-free free-cutting steel according to claim 2, characterized in that, The second-stage cooling in S2 specifically involves: cooling the steel to 810℃-830℃ at a cooling rate of 15℃ / s-25℃ / s and holding it at that temperature for 15min-20min; then cooling it to 560℃-600℃ at a cooling rate of 30℃ / s-45℃ / s and holding it at that temperature for 20min-40min.

6. The method for manufacturing a low-sulfur, lead-free free-cutting steel according to claim 2, characterized in that, The low-sulfur, lead-free free-cutting steel has a yield strength ≥ 400 MPa and a tensile strength ≥ 550 MPa.

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