A process for reducing residual stress in high-carbon steel

Abstract

The present application discloses a process for reducing the residual stress of high carbon steel, including a billet heating process, wherein the billet heating process comprises: heating the billet from room temperature to 900-950°C at a heating rate of 150-200°C / h, and continuing to heat the billet to 1200-1220°C at a heating rate of 280-300°C / h after the billet transforms into a fully austenitized state, and keeping the temperature for 3-5 hours; and solving the problem of low efficiency and high energy consumption of the currently used spheroidizing annealing treatment method due to the high residual stress of high carbon steel.

Description

Technical Field

[0001] The invention relates to a controlled rolling and controlled cooling method for steel materials, and in particular to a process for reducing residual stress of high carbon steel. Background Art

[0002] Generally, steel with a carbon content exceeding 0.6% is called high carbon steel. High carbon steels represented by 9SiCr, GCr15, GCr15SiMn, and T10 all have a carbon content of around 1%. They have the characteristics of high strength, high hardness, good wear resistance, and good tempering stability. The addition of alloying elements can also significantly improve hardenability, so they are widely used. For example, 9SiCr can be used as drill bits, cold working dies, etc.; GCr15 and GCr15SiMn are high carbon chromium bearing steels, widely used as bearing materials for various purposes, and can ensure stable operation under various harsh environments and alternating stresses; T10 is suitable for use as a mold material with high wear resistance requirements.

[0003] Steel will generate residual stress during the heating, cooling and deformation processes, but the residual stress of high carbon steel is much higher than that of medium and low carbon steel. First, thermal stress caused by thermal expansion and contraction will be generated in steel during the heating and cooling process. The thermal conductivity of high carbon steel (for example, the thermal conductivity of GCr15SiMn ranges from 40 to 45W / m·K, compared with the thermal conductivity of 45 steel of 51.9W / m·K) is lower, which will lead to a greater temperature difference between the center and edge of the steel cross section. The deformation caused by thermal expansion and contraction will naturally be greater, and the stress will be higher. Secondly, there is the tissue stress generated by the phase transformation process. This is caused by the volume change caused by different tissue specific volumes (specific volumes of common tissues in steel: austenite < ferrite < pearlite < cementite). High carbon steel contains more cementite, and the volume change during the phase transformation process is greater, so the stress is higher. The last step is the deformation process, which is generally the steel rolling process. During the rolling process of high-carbon steel, carbon is dissolved in austenite, which has greater deformation resistance and higher energy storage. At room temperature, cementite is hard and brittle and cannot undergo plastic deformation. Therefore, it cannot release more stored energy, and will also produce higher residual stress.

[0004] The residual stress of high-carbon steel poses a significant threat to subsequent processing of the steel, reducing its toughness. If not removed promptly, cracks often occur during processing. For example, in the past, users typically used band saws or toothless saws to cut high-carbon steel. However, due to the high hardness of high-carbon steel, this method has a slow cutting speed, resulting in high saw blade wear and low production efficiency. To improve production efficiency, users currently use induction heating and hot shearing for cutting. Induction heating has a fast heating rate, and combined with shearing, it can improve processing efficiency. However, due to the strong skin effect of induction heating, the heating is uneven, resulting in the interaction between the thermal stress during induction heating and the original residual stress of the high-carbon steel, ultimately causing cracks in the steel.

[0005] The current solution is to spheroidize anneal the steel after rolling. Although this method can reduce the residual stress of high-carbon steel, the spheroidizing annealing temperature is high and the cycle is long (generally spheroidizing annealing needs to be heated to about 800°C and the total time is about 24 hours). It also sacrifices cost and efficiency. Therefore, it is necessary to invent a new way to solve the above problem. Summary of the Invention

[0006] In view of this, in view of the high residual stress of high carbon steel, the present invention provides a process for reducing the residual stress of high carbon steel, thereby solving the problems of low efficiency and high energy consumption of the currently used spheroidizing annealing treatment method.

[0007] To achieve the above-mentioned object of the invention, a process for reducing residual stress in high carbon steel includes a steel billet heating step, wherein the steel billet heating step includes:

[0008] The steel billet is heated from room temperature to 900-950°C at a heating rate of 150-200°C / h. After the steel billet is transformed into a fully austenitized state, it is further heated to 1200-1220°C at a heating rate of 280-300°C / h and kept warm for 3-5 hours.

[0009] In some embodiments and possible embodiments of the present disclosure, the final rolling temperature of the steel is 850-1000°C.

[0010] In some embodiments and possible embodiments of the present disclosure, after the rolling is completed, the steel is water-cooled.

[0011] In some embodiments and possible embodiments of the present disclosure, the cooling rate of the water cooling treatment is ≥15°C / s, and the water cooling treatment is performed to 650-700°C.

[0012] In some embodiments and possible embodiments of the present disclosure, after the water cooling treatment is completed, the steel is placed in a furnace at 600-650° C. and kept at this temperature for 3-4 hours;

[0013] After the heat preservation is completed, the steel is cooled to 500-550° C. in the furnace and then air-cooled after being taken out of the furnace.

[0014] In some embodiments and possible embodiments of the present disclosure, the steel billet is a continuous casting billet or a mold casting ingot.

[0015] In some embodiments and possible embodiments of the present disclosure, the carbon content of the high carbon steel is between 0.6% and 1.3%.

[0016] The present invention has the following beneficial effects:

[0017] The process of reducing the residual stress of high carbon steel of the present invention is as follows: during the billet heating process, the billet is first heated from room temperature to 900-950°C at a heating rate of 150-200°C / h. The relatively slow heating rate of 150-200°C / h is adopted mainly because the slow heating rate is conducive to the release of residual stress in the billet, thereby preventing thermal stress cracks caused by excessively fast heating rate in the early stage. At 900-950°C, the steel has basically completed the austenitization phase transformation and the tissue stress has also been basically released. When the steel is in the fully austenitized state, the steel is heated to 900-950°C. Under this condition, the present invention increases the heating rate, specifically increases the rate to 280-300°C / h. At this time, due to the good plasticity of austenite itself, it has a good buffering effect on the thermal stress of rapid heating. At this heating rate, the steel billet is heated to 1200-1220°C and kept warm for 3-5h to fully carry out high-temperature diffusion, thereby improving the microsegregation of carbides in high-carbon steel and achieving the purpose of reducing the residual stress of high-carbon steel. Obviously, the process of the present invention has low energy consumption and high efficiency, thereby effectively solving the problems existing in the currently used spheroidizing annealing treatment method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0019] Figure 1 is a process flow chart for reducing residual stress in high carbon steel according to Example 1 of the present invention;

[0020] Figure 2 is a cross section of the steel material of Example 2 of the present invention after induction heating and shearing;

[0021] Figure 3 This is the cross section of the steel material of Example 2 of the present disclosure that is air-cooled to room temperature after conventional rolling, and then induction heated and sheared. DETAILED DESCRIPTION

[0022] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts not described in detail, those skilled in the art can also fully understand the present disclosure.

[0023] Furthermore, those skilled in the art should understand that the drawings are only provided to illustrate the purpose, features and advantages of the present disclosure, and are not actually drawn to scale.

[0024] At the same time, unless the context clearly requires otherwise, words such as "include", "comprising" and the like throughout the specification and claims should be interpreted as inclusive rather than exclusive or exhaustive; that is, as "including but not limited to".

[0025] The following embodiments of the present invention are based on Figure 1 The process is carried out as shown in the process flow diagram.

[0026] Example 1:

[0027] The steel grade produced is 9SiCr, the cross-sectional size of the continuous casting billet is 350×470mm, the rolled product specification is φ100mm, and the composition by mass percentage is C: 0.89%, Si: 1.40%, Mn: 0.49%, P: 0.013%, S: 0.009%, Cr: 1.09%, and the remainder is iron and unavoidable impurities.

[0028] Figure 1 : is a process flow chart for reducing residual stress in high carbon steel according to Example 1 of the present invention; Figure 1 As shown:

[0029] During the billet heating process, the billet is first heated in the first stage, specifically, from room temperature to 900-950°C at a heating rate of 150-200°C / h. The relatively slow heating rate of 150-200°C / h is adopted mainly because slow heating is conducive to the release of residual stress in the billet, thereby preventing thermal stress cracks caused by excessively fast heating rate in the early stage. At 900-950°C, the steel has basically completed the austenitization phase transformation and the tissue stress has also been basically released. When the steel is in a completely austenitized state, the present invention increases the heating rate for the second stage heating, specifically increasing the rate to 280-300°C / h. At this time, due to the good plasticity of austenite itself, it has a good buffering effect on the thermal stress of rapid heating. At this heating rate, the billet is heated to 1200-1220°C and kept warm for 3-5h to fully carry out high-temperature diffusion, thereby improving the carbide microsegregation of high carbon steel.

[0030] Among them, how to judge whether the steel has reached a fully austenitized state is a conventional technical means in this field, and this application will not describe it in detail.

[0031] In this embodiment, the continuous casting billet is heated to 930°C at a heating rate of 160°C / h in the first stage, and is then heated to 1220°C at a heating rate of 280°C / h in the second stage, and is kept warm for 4 hours.

[0032] After the billet is heated, it is rolled and deformed according to the specifications of the steel to be produced. The final rolling temperature after rolling is generally controlled at 850-1000°C. The stress generated during the rolling process can be fully removed through dynamic recovery and dynamic recrystallization mechanisms. The specifications of the rolled material to be rolled and deformed in this embodiment are φ

[0033] 70mm, final rolling temperature 930℃.

[0034] Since a large part of high carbon steel is hypereutectoid steel, secondary network cementite will first precipitate at the grain boundary during the phase transformation of supercooled austenite. Since cementite is a hard and brittle phase, precipitation at the grain boundary reduces the toughness of the steel. The processed parts are also prone to fatigue cracks and failure in the network cementite during service, which must be removed. Usually, rapid cooling after rolling is generally used to suppress the precipitation of secondary network cementite. Figure 1 As shown, the present invention adopts a post-rolling water cooling method with a cooling rate of ≥15°C / s and a cooling termination temperature of 650-700°C where the phase transformation is completed. In this embodiment, the cooling rate of the water cooling operation for the rolled steel is 18°C / s and the cooling termination temperature is 680°C.

[0035] During the water cooling process, the temperature of the steel changes very quickly, and the phase transformation from austenite to pearlite also occurs during this temperature change. Therefore, the thermal stress and structural stress generated in this stage are very high and must be removed in time, otherwise stress cracks will occur. In the prior art, the stress is usually removed by spheroidizing annealing after the steel cools to room temperature. Figure 1 As shown, the present invention directly transports the steel cooled to 650-700°C into an annealing furnace that has been preheated to 600-650°C, and directly performs a heat preservation operation for 3-4 hours to fully remove the thermal stress and structural stress in the steel. After the heat preservation is completed, the steel is cooled to 500-550°C with the furnace and then air-cooled out of the furnace.

[0036] In this embodiment, after water cooling, the steel is directly loaded into an annealing furnace which has been adjusted to 610° C. in advance and kept at this temperature for 3 hours. After the steel is cooled to 530° C. in the furnace, it is taken out of the furnace and air-cooled.

[0037] Example 2:

[0038] The steel grade produced is GCr15SiMn, the cross-sectional size of the continuous casting billet is 235×265mm, the rolled product specification is φ85mm, and the composition by mass percentage is C: 0.98%, Si: 0.50%, Mn: 1.02%, P: 0.010%, S: 0.005%, Cr: 1.44%, and the balance is iron and unavoidable impurities.

[0039] In the first stage, the continuous casting slab is heated to 925°C at a rate of 180°C / h. In the second stage, it is heated to 1220°C at a rate of 290°C / h and held at this temperature for 5 hours. The steel is then removed from the furnace for rolling and deformation. The rolled product size is φ85mm, and the final rolling temperature is 890°C. The steel is then water-cooled at a rate of 25°C / s to a final cooling temperature of 650°C. The steel is then directly charged into an annealing furnace pre-set to 620°C and held at this temperature for 3.5 hours. After this, the steel is cooled in the furnace to 500°C and then air-cooled.

[0040] The cross section of the steel produced by the steps described in Example 2 after induction heating and shearing is as follows Figure 2 As shown, the shear surface is smooth and has no cracks. Among them: the frequency of induction heating is 780HZ~820HZ, the heating time is 1.5min~2min, and the steel surface is heated to 600℃~700℃. For comparison, Figure 3 For steel that is air-cooled to room temperature after conventional rolling, stress cracks can be seen in the cross section after the same induction heating and shearing conditions.

[0041] pass Figure 2 and Figure 3 It can be seen from the comparison that the process of the present invention can effectively reduce the residual stress of high carbon steel, is not prone to cracking during induction heating and shearing, and can improve production efficiency and reduce production costs.

[0042] Example 3:

[0043] The steel grade produced is T10, the cross-sectional size of the continuous casting billet is 210×210mm, the rolled product specification is φ40mm, and the composition by mass percentage is C: 0.97%, Si: 0.28%, Mn: 0.31%, P: 0.011%, S: 0.008%, and the remainder is iron and unavoidable impurities.

[0044] In the first stage, the continuous casting billet is heated to 950°C at a rate of 200°C / h. In the second stage, it is heated to 1200°C at a rate of 300°C / h and held at this temperature for 3 hours. The steel is then removed from the furnace for rolling and deformation. The rolled product size is φ40mm and the final rolling temperature is 880°C. The steel is then water-cooled at a rate of 20°C / s to a final cooling temperature of 660°C. The steel is then directly charged into an annealing furnace pre-set to 640°C and held at this temperature for 3 hours. After cooling to 550°C, the steel is removed from the furnace and air-cooled.

[0045] Based on the above embodiments, the beneficial effects of the present invention are:

[0046] (1) The present invention can effectively remove the residual stress of high carbon steel, so that the steel will not produce delayed stress cracking and processing cracks due to excessive stress during transportation, storage and processing, thereby reducing the amount of objections for the enterprise and increasing the efficiency of the enterprise.

[0047] (2) Currently, induction heating has become the mainstream heating method used by users. This heating method is fast, low-cost, and highly efficient, but it places extremely high demands on the residual stress of steel. This is especially true for high-carbon steel, as even the slightest miscontrol can cause stress cracks. The process for reducing the residual stress of high-carbon steel provided by the present invention can effectively reduce the stress of high-carbon steel and fully meet the user's needs for this process.

[0048] (3) Compared with the traditional method of using spheroidizing annealing to improve the residual stress of high carbon steel, this process does not need to wait for the steel to completely cool before entering the annealing furnace for insulation, saving energy costs. The insulation time is only 3 to 4 hours, saving time costs and improving production efficiency.

[0049] (4) The process method of the present invention saves energy and fuel gas consumption.

[0050] The above-described embodiments are merely examples of implementation methods of the present disclosure, and their descriptions are relatively specific and detailed. However, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art may make various modifications, equivalent substitutions, and improvements without departing from the scope of the present disclosure, and these modifications are all within the scope of protection of the present disclosure. Therefore, the scope of protection of the present invention shall be based on the appended claims.

Claims

1. A process for reducing residual stress in high carbon steel, comprising a billet heating process, characterized in that: The billet heating process includes: heating the billet from room temperature to 900-950°C at a heating rate of 150-200°C / h, and heating the billet to 1200-1220°C at a heating rate of 280-300°C / h after the billet is transformed into a fully austenitized state, and then keeping the temperature for 3-5 hours; The final rolling temperature of steel is 850-1000℃; After the rolling is completed, the steel is subjected to water cooling treatment; The cooling rate of the water cooling treatment is ≥15°C / s, and the water cooling treatment is performed to 650-700°C; After the water cooling treatment is completed, the steel is placed in a furnace at 600-650° C. and kept warm for 3-4 hours.

2. The process for reducing residual stress in high carbon steel according to claim 1, characterized in that: After the heat preservation is completed, the steel is cooled to 500-550° C. in the furnace and then air-cooled after being taken out of the furnace.

3. The process for reducing residual stress in high carbon steel according to any one of claims 1 to 2, characterized in that: The steel billet is a continuous casting billet or a mold casting ingot.

4. The process for reducing residual stress in high carbon steel according to claim 3, wherein: The carbon content of the high carbon steel is between 0.6% and 1.3%.

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