A method for producing a graphite free-cutting steel having excellent properties

Abstract

The application discloses a preparation method of graphite free-cutting steel with excellent performance and relates to the technical field of graphite free-cutting steel quality control. r1 After the casting blank is heated, rough rolling and finish rolling are performed; controlled cooling: after the finish rolling is completed, the steel is cooled at a rate of 10-30 DEG C / s for 4-12 s, and is rapidly cooled to below A r1 - (30-40) DEG C, the finish rolling temperature is A r1 - (50-60) DEG C, and the rolling deformation amount is 30%-60%; cooling: after the warm rolling is completed, the steel is immediately water-cooled to room temperature to obtain a ferrite and martensite dual-phase structure; annealing: the annealing temperature is 650-680 DEG C, the holding time is 2-3 h, the steel is cooled to room temperature after the annealing, and the graphite free-cutting steel is obtained; by adopting appropriate cooling rates, warm rolling deformation amounts, warm rolling temperatures and other parameters, the C atoms in the steel can be rapidly graphitized in a short time of 2-3 h, and the mechanical properties and cutting performance can be greatly improved.

Description

Technical Field

[0001] This invention relates to the field of quality control technology for graphite free-cutting steel, and specifically to a method for preparing graphite free-cutting steel with excellent properties. Background Technology

[0002] With the rapid development of my country's automotive and machinery manufacturing industries in recent years, the demand for free-cutting steel has been increasing. Existing free-cutting steels mainly improve their machinability by adding free-cutting elements such as S, P, Pb, Se, Te, and Bi, either alone or in combination. However, the addition of these elements increases the impurity of the steel, making its basic properties lower than those of the base steel, and thus difficult to meet the requirements of mechanical structural steels with strict strength and toughness requirements. For example, commonly used sulfur-based cutting steels suffer from severe air pollution during smelting, and high sulfur content can lead to a significant reduction in the steel's mechanical properties (especially transverse impact toughness), increased surface scrap rate and hot brittleness, and poor hot working performance. While lead-based free-cutting steels have excellent machinability, their application is limited due to lead pollution and the harmful effects of lead in scrap steel recycling and smelting. For instance, the European Community Directive on End of Life Vehicles has stopped the recycling of lead-containing automotive parts. It is evident that existing free-cutting steels all suffer from the aforementioned problems to varying degrees. Therefore, it is necessary to develop a free-cutting steel with excellent cutting performance, good mechanical properties, and environmental friendliness.

[0003] Graphitized free-cutting steels have attracted considerable attention from metallurgical researchers and manufacturers worldwide in recent years due to their excellent machinability, cold formability, high fatigue strength, and environmental friendliness. Japan began research in this area in the 1980s; for example, Japanese patent JP01198452 describes hot-rolling steel of a certain composition, followed by quenching and tempering at 600-750℃. However, because the steel was not processed or deformed before tempering, the tempering time was long, resulting in high production costs. US patent US5830285A involves adding a small amount of boron (B) to the steel, followed by direct quenching after hot rolling and then tempering. This method requires a high starting cooling temperature, a high cooling rate, and a tempering time of over 10 hours, making it difficult to implement in actual production. In recent years, domestic research on graphite free-cutting steel has gradually increased. For example, the patent "A method for preparing graphitized free-cutting steel high-speed wire rod (CN106947907B)" applied for by Zhang Yongjun et al. in 2017 mainly uses graphitization annealing at 620℃~Ac1, but the annealing time is relatively long. The patent "A method for improving the cold heading performance of graphitized free-cutting steel (CN107904377B)" mainly uses graphitized free-cutting steel wire rod that has been graphitized to heat to 550~710℃, and after homogenization, it is rolled in a pass with an elongation of 18~38% to improve the performance. The patent "A Grading Control Method for Graphite Free-Cutting Steel and its Annealing Graphitization Process" invented by Zuo Jinzhong et al. mainly relies on comparing the microstructure photographs of the samples with established microstructure evaluation standards and guide spectra to confirm the required annealing process and the graphitized structure obtained after annealing. This method has a relatively long annealing time and high cost. While the inventors' earlier patents, "A Production Method of Graphitized Free-Cutting Steel" (CN101899555B) and "A Heat Treatment Method to Improve the Mechanical Properties of Graphite Free-Cutting Steel" (CN102660666B), can achieve graphitization in a shorter time and produce steels with good strength, toughness, and fatigue performance, the cutting performance needs further improvement because graphite particles mainly precipitate along grain boundaries and the ferrite size is relatively large. Therefore, a method for preparing graphite free-cutting steel with excellent properties is needed to solve the above problems. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing graphite free-cutting steel with excellent performance, so as to solve the problems existing in the prior art mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing graphite free-cutting steel with excellent properties includes the following steps: S1: After heating the billet, rough rolling and finish rolling are performed; S2: Controlled cooling: After finishing rolling, cool at a rate of 10-30℃ / s for 4-12 seconds, rapidly cooling to A. r1 Below the temperature; S3: Warm rolling: Rolling temperature A r1 - (30-40)℃ temperature, final rolling temperature A r1 - (50-60)℃, rolling deformation is 30%-60%; S4: Cooling: After warm rolling, immediately water cool to room temperature to obtain a dual-phase structure of ferrite and martensite; S5: Annealing: Annealing temperature 650-680℃, holding time 2-3h, cool to room temperature after annealing to obtain graphite free-cutting steel.

[0006] Preferably, in step S1, the billet heating process is as follows: The first heating temperature is 1020±50℃, the second heating temperature is 1100±50℃, the third heating temperature is 1180±20℃, the heat spread temperature is 1160±20℃, and the heating time is 200-400min.

[0007] Preferably, in step S1, the initial rolling temperature of the roughing mill is ≥1050℃ and the final rolling temperature is ≥980℃; the initial rolling temperature of the finishing mill is ≤950℃ and the final rolling temperature is 800-830℃.

[0008] Preferably, in step S3, the rolling deformation of the warm rolling is controlled such that the ferrite volume fraction after warm rolling is 20%-30%.

[0009] Preferably, in step S5, the specific steps for cooling to room temperature after annealing are as follows: after annealing, the furnace is cooled to 300°C, and then air-cooled to room temperature.

[0010] A graphite free-cutting steel with excellent properties, wherein the chemical composition of the graphite free-cutting steel is: 0.40%~0.50%C, 1.40%~1.60%Si, 0.50%~0.90%Mn, P≤0.01%, S≤0.005%, and the balance is Fe and unavoidable impurities.

[0011] Preferably, the chemical composition of the graphite free-cutting steel further includes at least one of the following elements: 0.6% to 1.0% Cr, 0.1% to 0.3% Mo, and 0.3% to 0.7% Al.

[0012] Preferably, the metallographic structure of the graphite free-cutting steel is ultrafine ferrite and dispersed graphite, wherein the ferrite grain size is 1-4 μm.

[0013] Compared with the prior art, the beneficial effects of the present invention are: 1. The microstructure of the graphite free-cutting steel prepared by this invention consists of ultrafine-grained ferrite and dispersed graphite particles. Due to the recovery and recrystallization of the martensite formed by water cooling after warm rolling during tempering, the matrix structure is refined. At the same time, graphite particles preferentially precipitate at the ultrafine grain boundaries, improving the uniformity of graphite distribution. Thus, the steel possesses both excellent mechanical properties and machinability. Data from the examples show that the tensile strength can reach 650~770MPa, and the elongation is ≥24.5%, which is far higher than the elongation requirement of Y45 steel with the same carbon content. Moreover, the cutting chips are mostly C-shaped, and the cutting surface is smooth with good roughness. This effectively solves the problems of low mechanical properties and environmental pollution of traditional sulfur-based and lead-based free-cutting steels, as well as the insufficient machinability of existing graphite steels. It is particularly suitable for mechanical structural steel applications with high requirements for strength, toughness, and machining accuracy.

[0014] 2. This invention achieves deformation-induced ferrite phase transformation of supercooled austenite by rapidly cooling it to below Ar1 temperature at a rate of 10~30℃ / s after precision rolling, combined with warm rolling with a deformation of 30%~60% within the Ar1-(30~60)℃ temperature range. Subsequently, water cooling is used to obtain a two-phase microstructure of ferrite and martensite. This process not only avoids the problem of insufficient ferrite content control precision in traditional slow cooling processes, but also utilizes the large number of dislocations introduced by warm rolling as rapid diffusion channels for carbon atoms during subsequent annealing, significantly accelerating the graphitization process. This allows for the acquisition of a microstructure with fine and dispersed graphite particles in just 2~3 hours of holding at 650~680℃, greatly shortening the annealing time, reducing production energy consumption and costs, and avoiding grain coarsening caused by prolonged high-temperature annealing. It has the potential for efficient and energy-saving industrial applications. Attached Figure Description

[0015] Figure 1 This is a CCT curve diagram of Embodiment 1 of the present invention.

[0016] Figure 2 This is a tissue diagram obtained in Embodiment 1 of the present invention.

[0017] Figure 3 This is a tissue diagram obtained in Embodiment 2 of the present invention.

[0018] Figure 4 This is a tissue diagram obtained in Embodiment 3 of the present invention.

[0019] Figure 5 This is a tissue diagram obtained in Embodiment 4 of the present invention.

[0020] Figure 6 This is a surface quality diagram of the cutting sample in Embodiment 1 of the present invention.

[0021] Figure 7 This is a diagram showing the shape of the debris after cutting in Embodiment 2 of the present invention.

[0022] Figure 8 This is a diagram showing the shape of the debris after cutting in Embodiment 3 of the present invention. Detailed Implementation

[0023] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0024] Please see Figure 1-8 The present invention provides the following technical solutions: A method for preparing graphite free-cutting steel with excellent properties includes the following steps: S1: After heating the billet, rough rolling and finish rolling are carried out; the billet heating process is as follows: first heating temperature 1020±50℃, second heating temperature 1100±50℃, third heating temperature 1180±20℃, soaking temperature 1160±20℃, heating time 200-400min.

[0025] The initial rolling temperature of roughing is ≥1050℃ and the final rolling temperature is ≥980℃; the initial rolling temperature of finishing rolling is ≤950℃ and the final rolling temperature is 800-830℃.

[0026] S2: Controlled cooling: After finishing rolling, cool at a rate of 10-30℃ / s for 4-12 seconds, rapidly cooling to A. r1 Below the temperature, where A r1 The temperature is the temperature at which austenite begins to decompose into ferrite and cementite during continuous cooling.

[0027] S3: Warm rolling: Rolling temperature A r1 - (30-40)℃ temperature, final rolling temperature A r1 - (50-60)℃, rolling deformation is 30%-60%; the rolling deformation of warm rolling is controlled so that the ferrite volume fraction after warm rolling is 20%-30%.

[0028] S4: Cooling: After warm rolling, immediately water cool to room temperature to obtain a dual-phase structure of ferrite and martensite; S5: Annealing: Annealing temperature 650-680℃, holding time 2-3h, after annealing, furnace cooling to 300℃, then air cooling to room temperature, to obtain ultrafine ferrite and dispersed graphite structure.

[0029] This invention also provides a graphite free-cutting steel with excellent properties. The chemical composition of the graphite free-cutting steel is: 0.40%–0.50% C, 1.40%–1.60% Si, 0.50%–0.90% Mn, P ≤ 0.01%, S ≤ 0.005%, with the balance being Fe and unavoidable impurities. The chemical composition of the graphite free-cutting steel also includes at least one of the following elements: 0.6%–1.0% Cr, 0.1%–0.3% Mo, and 0.3%–0.7% Al.

[0030] To verify the superior properties of the graphite free-cutting steel prepared by this invention, the following embodiments are provided: Example 1: The selected steel grade has the following chemical composition: 0.45% C, 1.50% Si, 0.8% Mn, ≤0.01% P, ≤0.005% S, 0.8% Cr, 0.2% Mo, with the balance being Fe. The specific roughing, finishing, warm rolling, and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows: Figure 2 As shown in Table 2. The surface quality of the cut specimen is as follows: Figure 6 As shown.

[0031] Example 2: The selected steel grade has the following chemical composition: 0.45% C, 1.50% Si, 0.6% Mn, ≤0.01% P, ≤0.005% S, with the balance being Fe. The specific roughing, finishing, warm rolling, and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows: Figure 3 As shown in Table 2. After tempering, a cutting test was conducted, and the cutting chips were mostly C-shaped, as detailed in Table 2. Figure 7 .

[0032] Example 3: The selected steel grade has the following chemical composition: 0.45% C, 1.50% Si, 0.6% Mn, 0.5% Al, ≤0.01% P, ≤0.005% S, with the balance being Fe. The specific roughing, finishing, warm rolling, and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows: Figure 4 As shown in Table 2. After tempering, a cutting test was conducted, and the cutting chips were mostly C-shaped, as detailed in Table 2. Figure 8 .

[0033] Example 4: The selected steel grade has the same chemical composition as in Example 2. The specific roughing, finishing, warm rolling, and tempering processes are shown in Table 1. After tempering, the microstructure and mechanical properties of the steel plate are as follows: Figure 5 As shown in Table 2.

[0034] Example 5: The chemical composition of the selected steel is the same as in Example 3. The specific roughing, finishing, warm rolling, and tempering processes are shown in Table 1. The mechanical properties of the steel plate after tempering are shown in Table 2.

[0035]

[0036] Table 1. Steel Plate Production Process in Examples Table 2 Mechanical properties of steel plates in the examples

[0037] (Y45 steel refers to the performance requirements of Y45 with the same C content in GB / T 8731-2008) To ensure that the steel has good mechanical and machinability, this invention adopts a coordinated control of the cooling rate before warm rolling, the deformation during warm rolling, and the tempering process parameters to achieve the refinement of the steel microstructure and the fine dispersion of graphite particles.

[0038] In this invention, cooling is performed at a rate of 10-30℃ / s after finishing rolling. This is to prevent the undercooled austenite from transforming into ferrite or pearlite due to a slow cooling rate after finishing rolling. As can be seen from the CCT curve of steel 1 in this invention, the critical cooling rate required for the transformation of undercooled austenite into ferrite is 10℃ / s, and the time required for the ferrite transformation at 700℃ is 223s, corresponding to a cooling rate of 0.7℃ / s. Therefore, when the cooling rate is ≥10℃ / s, it can be ensured that the microstructure is undercooled austenite.

[0039] In this invention, the initial rolling temperature A is... r1 - (30-40)℃ temperature, final rolling temperature A r1 - (50-60)℃ is chosen because, on the one hand, it ensures that the austenite is completely supercooled at this temperature, while retaining a certain processing window and avoiding excessively low rolling temperatures that could increase tensile strength. The rolling deformation is controlled at 30%-60%, primarily because supercooled austenite induces ferrite phase transformation during rolling, and the ferrite content increases with the deformation. This means there is a certain correlation between the ferrite volume fraction and the deformation, avoiding the inadequacy of traditional processes that rely on slow cooling after rolling to control ferrite content precision. After warm rolling deformation, the microstructure consists of ferrite and untransformed supercooled austenite. During subsequent water quenching, the austenite transforms into martensite, and the existing ferrite, as a soft phase, can offset some of the structural stress generated by the martensitic phase transformation, thus preventing excessive phase transformation stress and cracking.

[0040] In this invention, the austenite is immediately water-cooled to room temperature after warm rolling. This is because a large number of dislocations are formed in the supercooled austenite during warm deformation. Due to the low deformation temperature, the dislocations do not undergo violent annihilation, and the supercooled austenite undergoes a martensitic transformation. This results in a large number of dislocations being inherited by the martensitic structure after the transformation. These dislocations, on the one hand, act as diffusion channels for carbon atoms during the subsequent tempering process, accelerating the graphitization of carbon atoms. On the other hand, the bonding ability between carbon and dislocations is higher than that between carbon and alloying elements in carbides. The segregation of carbon atoms in dislocations also facilitates the graphitization of carbon atoms during tempering. Therefore, the graphitization of carbon atoms in the steel can be achieved in a relatively short time.

[0041] In this invention, the high-temperature tempering process involves first cooling the steel in the furnace to 300°C and then air-cooling it to room temperature. This is primarily to avoid excessively rapid temperature drops that could generate significant residual stress, reducing the risk of cracking and improving machinability. The free-cutting steel prepared using this method exhibits excellent mechanical and machinability properties. This is mainly because, during tempering, the ferrite formed during warm rolling recrystallizes, forming ultrafine-grained ferrite. Simultaneously, the fine martensite with high-density dislocations also recovers and recrystallizes, forming ultrafine ferrite, thereby refining the matrix structure and improving mechanical properties. Furthermore, the large number of residual carbon atoms expelled from the martensite precipitate as graphite particles at the ultrafine ferrite grain boundaries, thus improving the distribution of graphite particles and enhancing machinability.

[0042] The graphite free-cutting steel prepared by the process of this invention has a microstructure mainly composed of ultrafine-grained ferrite and dispersed graphite. The ferrite grain size is between 1-4 μm, the tensile strength is ≥650 MPa, the elongation is ≥24.5%, the chip removal during cutting is smooth, the chips are mostly C-shaped, the surface after cutting is smooth, the surface roughness is good, and it has excellent comprehensive properties.

[0043] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing free-cutting graphite steel with excellent properties, characterized in that, Includes the following steps: S1: After heating the billet, rough rolling and finish rolling are performed; S2: Controlled cooling: After finishing rolling, cool at a rate of 10-30℃ / s for 4-12 seconds, rapidly cooling to A. r1 Below the temperature; S3: Warm rolling: Rolling temperature A r1 - (30-40)℃ temperature, final rolling temperature A r1 - (50-60)℃, rolling deformation is 30%-60%; S4: Cooling: After warm rolling, immediately water cool to room temperature to obtain a dual-phase structure of ferrite and martensite; S5: Annealing: Annealing temperature 650-680℃, holding time 2-3h, cool to room temperature after annealing to obtain graphite free-cutting steel.

2. The method for preparing a free-cutting graphite steel with excellent properties according to claim 1, characterized in that, In step S1, the billet heating process is as follows: The first heating temperature is 1020±50℃, the second heating temperature is 1100±50℃, the third heating temperature is 1180±20℃, the heat spread temperature is 1160±20℃, and the heating time is 200-400min.

3. The method for preparing a free-cutting graphite steel with excellent properties according to claim 2, characterized in that, In step S1, the initial rolling temperature of the roughing mill is ≥1050℃ and the final rolling temperature is ≥980℃; the initial rolling temperature of the finishing mill is ≤950℃ and the final rolling temperature is 800-830℃.

4. The method for preparing a free-cutting graphite steel with excellent properties according to claim 1, characterized in that, In step S3, the rolling deformation of the warm rolling is controlled so that the ferrite volume fraction after warm rolling is 20%-30%.

5. The method for preparing a free-cutting graphite steel with excellent properties according to claim 1, characterized in that, In step S5, the specific steps for cooling to room temperature after annealing are as follows: after annealing, the furnace is cooled to 300°C, and then air-cooled to room temperature.

6. A graphite free-cutting steel with excellent properties, prepared by the method according to any one of claims 1-5, characterized in that, The metallographic structure of the graphite free-cutting steel is ultrafine ferrite and dispersed graphite, and the ferrite grain size is 1-4 μm.

7. The graphite free-cutting steel according to claim 6, characterized in that, The graphite free-cutting steel has a tensile strength ≥650MPa, an elongation ≥24%, smooth chip removal during cutting, and a smooth surface with good roughness after cutting.

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