Graphite free-cutting steel and graded control method of annealing graphitization process thereof

Abstract

The invention provides graphite free-cutting steel and a grading control method of an annealing graphitization process thereof. The graphite free-cutting steel comprises the following chemical components in percentage by weight: 1.2-1.5% of C, 1.5-1.9% of Si, 0.2-0.5% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, less than or equal to 0.10% of Cr, less than or equal to 0.10% of Ni, 0.005-0.007% of B, less than or equal to 0.005% of Al, 0.003-0.007% of N and the balance of Fe and inevitable impurities. A casting blank containing the components is subjected to rolling and online heat treatment to obtain different steel wire rods, metallographic samples are taken from the steel wire rods and corroded, observation and photographing are conducted through an electron microscope, an obtained microscopic structure picture is compared with a formulated microscopic structure evaluation standard and a formulated microscopic structure guidance map, and the microscopic structure evaluation standard and the formulated microscopic structure guidance map are obtained. And confirming an annealing process needing to be carried out and a graphitized structure obtained after annealing.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and in particular to a hierarchical control method of graphite free-cutting steel and an annealing and graphitization process thereof. Background Art

[0002] Globally, the market size of free-cutting steel is stable at about 1 million tons per year, of which lead-containing free-cutting steel products account for more than 20%. Usually, lead is added to free-cutting steel to improve the cutting performance of the material. However, as a toxic metal, lead is dispersed in the air in the form of micron-sized particles during production, processing and recycling, posing a potential threat to the health of operators in multiple systems. In view of this, the RoHS Directive and ELV regulations for the restriction of hazardous substances limit the lead content in products to a maximum of 0.1%. Nevertheless, for free-cutting steels where there is a lack of alternative materials, special regulations allow the lead content to be capped at 0.35%. In recent years, as countries around the world have increasingly tightened restrictions on the use of lead-containing parts, it is expected that more stringent restrictions will be introduced in the future.

[0003] In view of this, the development of new free-cutting steel has become the focus of research and development in various countries. Compared with traditional lead alloy steel, the new free-cutting steel has more obvious technical advantages during the machining process due to the presence of graphite inclusions. In order to ensure its economic competitiveness, the steel design, production process and cutting performance must be further optimized. Therefore, the industry has proposed a solution to replace lead alloy steel with graphite steel, which shows certain potential. For this type of steel, it is crucial to shorten the graphitization time while ensuring cutting performance.

[0004] The introduction of graphite free-cutting steel is in line with the development trend of free-cutting steel to be lead-free and low-sulfur. The cutting performance of graphite free-cutting steel is better than that of lead-containing free-cutting steel, and it meets the requirements of environmental protection regulations. Its market positioning is to replace lead-containing free-cutting steel. Due to its excellent cutting performance, graphite free-cutting steel can not only improve processing efficiency, but also replace other free-cutting steels, so it has broad market prospects. However, for graphite free-cutting steel with hypoeutectoid or hypereutectoid structure characteristics, its graphitization process has always been difficult. Therefore, the key to the development of this steel grade is to promote the graphitization process. At present, companies such as Posco in South Korea and JFE in Japan are exploring their own organizational evaluation and guidance maps, aiming to provide process guidance for the graphitization process in industrial production, but have not yet formed a complete organizational rating and guidance map before and after annealing.

[0005] In summary, the design and development of steel composition based on adding graphite core to accelerate the graphitization process, as well as the establishment of microstructure evaluation and guidance maps before and after annealing, have become urgent issues to be solved in the industrial production of graphite free-cutting steel. Summary of the invention

[0006] The present invention proposes a hierarchical control method for graphite free-cutting steel and its annealing and graphitization process, aiming to obtain different hot-rolled structures through different cooling processes before wire rod annealing, and determine the final degree of graphitization by formulating organizational evaluation and guidance maps corresponding to corresponding annealing processes.

[0007] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme: a graphite free-cutting steel, whose chemical composition in weight percentage is C: 1.2-1.5%, Si: 1.5-1.9%, Mn: 0.2-0.5%, P≤0.025%, S≤0.025%, Cr≤0.10%, Ni≤0.10%, B: 0.005-0.007%, Al≤0.005%, N: 0.003-0.007%, and the balance is Fe and unavoidable impurities.

[0008] The present invention also provides a method for grading and evaluating the annealing graphitization of graphite free-cutting steel wire rods, the method comprising the following steps in sequence:

[0009] S1. rolling and online heat treatment of the ingot with the above composition to obtain different wire rod structures;

[0010] S2. Take a metallographic sample at a representative position of the obtained wire rod structure, perform hot mounting and sample preparation, then grind and polish the metallographic sample, and then use a 4% nitric acid alcohol solution to corrode it, observe it under an optical microscope, and take a photo at the horizontal 1 / 4 position of the wire rod to obtain a microstructure photo;

[0011] S3. According to the obtained microstructure photos, the annealing process to be performed and the graphitized structure obtained after annealing are compared with the formulated microstructure evaluation standards and guidance maps. The microstructure evaluation standards are: if the microstructure is pearlite + ferrite, the structure grade is determined to be level 1;

[0012] If the microstructure is troostite + pearlite + a small amount of ferrite, the organizational grade is determined to be level 2;

[0013] If the microstructure is granular bainite, the organizational grade is determined to be level 3;

[0014] If the microstructure is granular bainite + rod-shaped bainite, the organizational grade is determined to be 4;

[0015] If the microstructure is rod-shaped bainite, the organizational grade is determined to be 5;

[0016] If the microstructure is martensite, the organizational grade is determined to be 6;

[0017] S4. For the wire rod of grade 1, the subsequent annealing process is annealing holding temperature of 800°C and holding time of 12h, and the graphite particle size can be obtained to be 3-10μm and the cementite residue is 6-12%;

[0018] For the wire rod of grade 2, the subsequent annealing process is annealing holding temperature of 800℃, holding time of 10h, and the graphite particle size can be obtained to be 3-9μm, and the cementite residue is 5-10%;

[0019] For grade 3 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 6h, and the graphite particle size can be 3-6μm, and the cementite residue is ≤1.5%;

[0020] For grade 4 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 5.5h, and the graphite particle size can be 2-6μm, and the cementite residue is ≤1.1%;

[0021] For grade 5 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 5h, and the graphite particle size can be 2-5μm, and the cementite residue is ≤0.7%;

[0022] The subsequent annealing process for grade 6 wire rod is annealing holding temperature of 720°C and holding time of 4.5h, which can obtain graphite particle size of 2-5μm and cementite residue ≤0.5%.

[0023] Preferably, the representative position of the wire rod in step 1 is to cut off the defective positions at the head and tail of the coil, randomly select 2 coils of wire rod, and mark the edge position and the middle position of the roller.

[0024] Preferably, the optical microscope in step 2 is magnified to 500 times to take pictures of the sample.

[0025] Compared with the prior art, the advantages and positive effects of the present invention are:

[0026] Only by inspecting the structure of the wire rod that has been heat treated online after hot rolling and comparing it with the microstructure evaluation standards and guidance maps, the subsequent graphitization annealing process and the morphology of the optimal graphitized structure can be confirmed, avoiding repetitive experiments and shortening the inspection cycle. This method can be used for the production process control standards of graphite free-cutting steel products in steel plants and the process guidance of subsequent annealing heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The metallographic structure diagram of the first level guidance diagram of the present invention;

[0028] Figure 2 It is the metallographic structure diagram of the second-level guidance diagram of the present invention;

[0029] Figure 3 It is the metallographic structure diagram of the third-level guidance diagram of the present invention;

[0030] Figure 4 It is the metallographic structure diagram of the 4-level guidance diagram of the present invention;

[0031] Figure 5 It is the metallographic structure diagram of the 5-level guidance diagram of the present invention;

[0032] Figure 6 It is the metallographic structure diagram of the 6-level guidance diagram of the present invention;

[0033] Figure 7 500X metallographic structure diagram of the hot-rolled and online heat-treated wire rod produced in Example 1 of the present invention;

[0034] Figure 8 500X metallographic structure diagram of the annealed wire rod produced in Example 1 of the present invention;

[0035] Fig. 9 500X metallographic structure diagram of the hot-rolled and online heat-treated wire rod produced in Example 2 of the present invention;

[0036] Fig.10 This is a 500X metallographic structure diagram of the annealed wire rod produced in Example 2 of the present invention. DETAILED DESCRIPTION

[0037] In order to more clearly understand the above-mentioned purpose, features and advantages of the present invention, the present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0038] In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention may also be implemented in other ways than those described herein. Therefore, the present invention is not limited to the specific embodiments of the following disclosure.

[0039] Example 1. This embodiment provides a graphite free-cutting steel, whose chemical composition in weight percentage is C: 1.32%, Si: 1.69%, Mn: 0.26%, P: 0.015%, S: 0.005%, Cr: 0.01%, Ni: 0:007%, B: 0.0062%, Al: 0.002%, N: 0.0054%, and the remainder is Fe and unavoidable impurities.

[0040] This embodiment also provides a method for grading and evaluating the annealing graphitization of graphite free-cutting steel wire rods, the method comprising:

[0041] S1. The ingots with the above composition are rolled and subjected to online heat treatment to obtain different wire rod structures. The online heat treatment adopts an air cooling process with an air cooling intensity of 23°C / s. After air cooling to 600°C, air cooling to room temperature is performed.

[0042] S2. Cut off the defective positions at the head and tail of the obtained wire rod, randomly select 2 circles of wire rod, mark the edge position and the middle position of the roller, respectively select 2 samples from the edge and the middle position of each circle of the roller, a total of 8 samples, perform hot mounting and sample preparation, then grind and polish the 8 metallographic samples, and then use a 4% nitric acid alcohol solution to corrode, observe under an optical microscope, and take a photo at a magnification of 500 times at the horizontal 1 / 4 position of the wire rod to obtain a microstructure photo;

[0043] S3, according to the obtained microstructure photos (such as Figure 7 ) and the established microstructure evaluation standards and guidance diagrams (such as Figure 1-6 The structure obtained is pearlite + troostite + a small amount of ferrite, and the confirmed grade is level 2. The control annealing process is: 800℃ for 10h, and F+ granular graphite structure can be obtained, and the graphite particle size is 3-10μm. Figure 8 shown.

[0044] Example 2. This embodiment provides a graphite free-cutting steel, whose chemical composition in weight percentage is C: 1.45%, Si: 1.85%, Mn: 0.22%, P: 0.014%, S: 0.003%, Cr: 0.01%, Ni: 0:006%, B: 0.0055%, Al: 0.001%, N: 0.0081%, and the remainder is Fe and unavoidable impurities.

[0045] This embodiment also provides a method for grading and evaluating the annealing graphitization of graphite free-cutting steel wire rods, the method comprising:

[0046] S1. After rolling and online isothermal heat treatment of the ingot with the above composition, different wire rod structures are obtained. The isothermal heat treatment temperature of the online isothermal heat treatment process is 320° C., and after keeping the temperature for 10 minutes, the ingot is air-cooled to room temperature.

[0047] S2. Cut off the defective positions at the head and tail of the obtained wire rod, randomly select 2 circles of wire rod, mark the edge position and the middle position of the roller, respectively select 2 samples from the edge and the middle position of each circle of the roller, a total of 8 samples, perform hot mounting and sample preparation, then grind and polish the 8 metallographic samples, and then use a 4% nitric acid alcohol solution to corrode, observe under an optical microscope, and take a photo at a magnification of 500 times at the horizontal 1 / 4 position of the wire rod to obtain a microstructure photo;

[0048] S3, according to the obtained microstructure photos (such as Fig. 9) and the established microstructure evaluation standards and guidance diagrams (such as Figure 1-6 The obtained structure is martensite, and the confirmed grade is 6. The control annealing process is: 720℃ for 4.5h, and F+ granular graphite structure can be obtained, and the graphite particle size is 2-5μm. Fig.10 shown.

[0049] The above description is only a preferred embodiment of the present invention and does not limit the present invention in other forms. Any technician familiar with the profession may use the technical content disclosed above to change or modify it into an equivalent embodiment with equivalent changes and apply it to other fields. However, any simple modification, equivalent change and modification made to the above embodiment based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A graphite free-cutting steel, characterized in that: The chemical composition of the graphite free-cutting steel is C: 1.2-1.5%, Si: 1.5-1.9%, Mn: 0.2-0.5%, P≤0.025%, S≤0.025%, Cr≤0.10%, Ni≤0.10%, B: 0.005-0.007%, Al≤0.005%, N: 0.003-0.007%, and the balance is Fe and unavoidable impurities.

2. A hierarchical control method for the annealing and graphitization process of graphite free-cutting steel, characterized in that: The method comprises the following steps in sequence: S1. Rolling and online heat treatment of the ingot with the composition described in claim 1 to obtain different wire rod structures; S2. Take a metallographic sample at a representative position of the obtained wire rod structure, perform hot mounting and sample preparation, then grind and polish the metallographic sample, and then use a 4% nitric acid alcohol solution to corrode it, observe it under an optical microscope, and take a photo at the horizontal 1 / 4 position of the wire rod to obtain a microstructure photo; S3. According to the obtained microstructure photos, the annealing process to be performed and the graphitized structure obtained after annealing are compared with the formulated microstructure evaluation standards and guidance maps. The microstructure evaluation standards are: if the microstructure is pearlite + ferrite, the structure grade is determined to be level 1; If the microstructure is troostite + pearlite + a small amount of ferrite, the organizational grade is determined to be level 2; If the microstructure is granular bainite, the organizational grade is determined to be level 3; If the microstructure is granular bainite + rod-shaped bainite, the organizational grade is determined to be 4; If the microstructure is rod-shaped bainite, the organizational grade is determined to be 5; If the microstructure is martensite, the organizational grade is determined to be 6; S4. For the wire rod of grade 1, the subsequent annealing process is annealing holding temperature of 800°C and holding time of 12h, and the graphite particle size can be obtained to be 3-10μm and the cementite residue is 6-12%; For the wire rod of grade 2, the subsequent annealing process is annealing holding temperature of 800℃, holding time of 10h, and the graphite particle size can be obtained to be 3-9μm, and the cementite residue is 5-10%; For grade 3 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 6h, and the graphite particle size can be 3-6μm, and the cementite residue is ≤1.5%; For grade 4 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 5.5h, and the graphite particle size can be 2-6μm, and the cementite residue is ≤1.1%; For grade 5 wire rod, the subsequent annealing process is annealing holding temperature 750℃, holding time 5h, and the graphite particle size can be 2-5μm, and the cementite residue is ≤0.7%; The subsequent annealing process for grade 6 wire rod is annealing holding temperature of 720°C and holding time of 4.5h, which can obtain graphite particle size of 2-5μm and cementite residue ≤0.5%.

3. The hierarchical control method for the annealing and graphitization process of graphite free-cutting steel according to claim 2, characterized in that: The representative position of the wire rod in step 1 is to cut off the defective positions at the head and tail of the coil, randomly select 2 coils of wire rod, and mark the edge position and the middle position of the roller.

4. The hierarchical control method for the annealing and graphitization process of graphite free-cutting steel according to claim 2, characterized in that: In step 2, the optical microscope is magnified to 500 times to take pictures of the sample.

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