A method for inhibiting graphitization of high carbon steel and high carbon steel

Abstract

The present invention provides a method for inhibiting graphitization of high carbon steel and high carbon steel. The preparation method comprises the following steps: coiling a high carbon steel strip at a target coiling temperature to obtain a high carbon steel coil, heating the coil to a target spheroidizing annealing temperature in a closed space under the condition of introducing a hydrogen gas flow, keeping the temperature for 12 to 20 hours, cooling the coil, and then taking it out of the closed space. The above-mentioned method for preparing high carbon steel selects a lower target coiling temperature and a lower target spheroidizing annealing temperature. A lower coiling temperature is conducive to making the pearlite in the steel thin and the carbide small and dispersed. A lower target spheroidizing annealing temperature is conducive to restricting the diffusion of carbon elements in the high carbon steel coil, and preventing the carbon elements in the steel from being enriched in the spheroidized cementite nucleus to form a graphitized structure.

Description

Technical Field

[0001] The invention relates to the field of steel production, and in particular to a method for inhibiting graphitization of high-carbon steel and high-carbon steel. Background Art

[0002] High carbon steel (C≥0.60%) is prone to graphitization during spheroidizing annealing due to its high carbon content. Graphite is equivalent to small cracks in steel, which causes stress concentration inside the steel, low strength, poor plasticity, and increased brittleness, which significantly reduces the strength and plasticity of the steel and causes brittle fracture of steel parts. This affects the mechanical properties of the steel. Graphitized steel is prone to uneven hardness and cracks during quenching, and the material is prone to brittle fracture during processing and use, so the tool life is short.

[0003] Graphitization is the process of decomposition of cementite in pearlite into free carbon under high temperature and stress: Fe 3 C The phenomenon of 3Fe+C (graphite) and gradual precipitation in the form of graphite is the graphitization of steel materials.

[0004] The conventional method of inhibiting graphitization of high carbon steel is mainly to adjust the content of elements, such as increasing the amount of P and reducing the amount of C. However, when adjusting the content of elements, it is necessary to consider the impact of the change in the element content on other elements and the impact on other properties of steel. Therefore, the scope of application is greatly limited. Summary of the invention

[0005] The main purpose of the present invention is to provide a method for inhibiting graphitization of high carbon steel and high carbon steel, so as to solve the technical problem that the application scope of the method for inhibiting graphitization of high carbon steel by adjusting the change of element content is limited.

[0006] To achieve the above object, the present application provides a method for preparing high carbon steel, wherein the mass percentage of carbon element in the high carbon steel is ≥0.60%, and the preparation method comprises the following steps:

[0007] The high carbon steel strip is coiled at a target coiling temperature to obtain a high carbon steel coil, wherein the lower limit of the theoretical coiling temperature range of the high carbon steel ≤ the target coiling temperature ≤ the lower limit of the theoretical coiling temperature range + 18°C.

[0008] Under the condition of passing hydrogen gas flow, the steel coil is heated to the target spheroidizing annealing temperature in a closed space and kept warm for 12 to 20 hours, wherein the lower limit of the theoretical spheroidizing annealing temperature range of high carbon steel is ≤ the target spheroidizing annealing temperature ≤ the lower limit of the theoretical spheroidizing annealing temperature range + 10°C.

[0009] After the steel coil is cooled, it is taken out from the confined space.

[0010] Wherein, the step of heating the high carbon steel coil to the target spheroidizing annealing temperature in a closed space comprises a first heating stage, a second heating stage and a third heating stage performed sequentially;

[0011] The first heating stage comprises heating the steel coil to 200° C. by free heating and keeping the temperature for 1 hour;

[0012] The second heating stage comprises heating the steel coil after the first heating stage to 400°C at a heating rate of 30-50°C / h and keeping the temperature for 2 hours;

[0013] The third heating stage includes further heating the steel coil after the second heating stage at a heating rate of 30-50° C. / h to a target spheroidizing annealing temperature;

[0014] During the first heating stage until the temperature of the steel coil reaches 600°C, the flow rate of the hydrogen gas flow is 10-20 m / s. 3 / h.

[0015] According to the embodiment of the present application, during the process from when the temperature of the steel coil is greater than 600°C to when it is taken out of the confined space, the flow rate of the hydrogen gas flow is 2-5 m / s. 3 / h.

[0016] According to an embodiment of the present application, the target spheroidizing annealing temperature is the lower limit of the theoretical spheroidizing annealing temperature range of high carbon steel, and the target coiling temperature is the lower limit of the theoretical coiling temperature range of high carbon steel.

[0017] According to an embodiment of the present application, the chemical composition of the high carbon steel, by mass fraction, includes C: 0.8-1.3%; Si: 0.1-2.0%; Mn: 0.1-2.0%; Cr: ≤1.5%; P: ≤0.03; S: ≤0.03, and the rest is Fe and unavoidable impurities.

[0018] According to an embodiment of the present application, before introducing the hydrogen gas flow, the method further comprises:

[0019] Steel coils are placed in confined spaces using N 2 Pre-purge 10~25min, N 2 The volume is 30~50m 3 .

[0020] According to an embodiment of the present application, after the steel coil is cooled, the step of taking it out from the confined space includes:

[0021] When the target spheroidizing annealing temperature of the steel coil is 660~800℃, the steel coil is naturally cooled to 640~660℃ in a closed space, then cooled to 380℃ by air cooling, and then cooled to below 100℃ by water cooling before being taken out from the closed space.

[0022] According to an embodiment of the present application, after the steel coil is cooled, the step of taking it out from the confined space includes:

[0023] When the target spheroidizing annealing temperature of the steel coil is 600-660°C, the steel coil is cooled to 380°C by air cooling, and then cooled to below 100°C by water cooling, and then taken out from the closed space.

[0024] According to an embodiment of the present application, the method for determining the theoretical spheroidizing annealing temperature range includes:

[0025] According to the chemical composition or CCT curve of the high carbon steel, combined with the Ac1 temperature line, the Acm temperature line and at least one of the target performance indicators of the high carbon steel, the theoretical spheroidizing annealing temperature range of the high carbon steel is determined.

[0026] The method for determining the theoretical coiling temperature range includes:

[0027] The theoretical coiling temperature range is determined according to the chemical composition or CCT curve of the high carbon steel and in combination with the target performance index of the high carbon steel.

[0028] The present application also provides a high carbon steel obtained by the above-mentioned preparation method.

[0029] In the above-mentioned method for preparing high carbon annealed steel, a lower temperature is selected as the target coiling temperature within the theoretical coiling temperature range of high carbon steel. A lower coiling temperature is beneficial for making the pearlite in the steel thin and the carbides small and dispersed, and can also prepare the structure for the subsequent spheroidizing annealing at a lower temperature and in a shorter time. Within the theoretical spheroidizing annealing temperature range of high carbon steel, a lower temperature is selected as the target spheroidizing annealing temperature. A lower target spheroidizing annealing temperature is beneficial for restricting the diffusion of carbon elements in high carbon steel coils, greatly reducing the probability of carbon elements in the steel being enriched in the spheroidized cementite core to form graphitized structure, and even inhibiting the carbon elements in the steel from being enriched in the spheroidized cementite core to form graphitized structure. Under the combined effect of these two aspects, the graphitization of high carbon steel can be better inhibited. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0031] Figure 1 is a flow chart of a method for preparing high carbon steel according to an embodiment of the present application;

[0032] Figure 2 is a cross-sectional metallographic image of the SK85 steel of Example 1 of the present application without nitric acid alcohol corrosion;

[0033] Figure 3 is a cross-sectional metallographic image of the SK85 steel of Example 1 of the present application after being corroded by nitric acid alcohol;

[0034] Figure 4 is a cross-sectional metallographic image of the SK85 steel of Comparative Example 1 of the present application without nitric acid alcohol corrosion;

[0035] Figure 5 is a cross-sectional metallographic image of the SK85 steel of Comparative Example 1 of the present application after being corroded by nitric acid alcohol;

[0036] Figure 6 This is a scanning electron microscope photograph of the annealed graphitized structure of SK85 steel in Comparative Example 1 of the present application;

[0037] Figure 7 is a cross-sectional metallographic image of the GCr15 steel of Example 3 of the present application without being corroded by nitric acid alcohol;

[0038] Figure 8 This is a cross-sectional metallographic image of the GCr15 steel of Example 3 of the present application after being corroded by nitric acid alcohol.

[0039] The realization of the purpose, functional features and advantages of the present invention will be further explained in conjunction with the implementation methods and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0040] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0041] It should be noted that all directional indications (such as up, down, etc.) in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

[0042] In addition, in the present invention, the descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0043] Furthermore, the technical solutions between the various embodiments of the present invention may be combined with each other, but this must be based on the fact that they can be implemented by ordinary technicians in the field. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0044] The present application provides a method for preparing high carbon steel, wherein the mass percentage of carbon element in the high carbon steel is ≥0.60%, and the preparation method comprises the following steps:

[0045] Step S100: Coil a high carbon steel strip at a target coiling temperature to obtain a high carbon steel coil, wherein the lower limit of the theoretical coiling temperature range of the high carbon steel ≤ the target coiling temperature ≤ the lower limit of the theoretical coiling temperature range + 18°C.

[0046] In this step, the coiling temperature range (i.e., theoretical coiling temperature range) that can be used for high carbon steel can be determined first, and then the target coiling temperature can be taken as the actual coiling temperature within the theoretical coiling temperature range. Among them, the lower limit of the theoretical coiling temperature range of high carbon steel ≤ target coiling temperature ≤ lower limit of the theoretical coiling temperature range + 18°C. The method for determining the theoretical coiling temperature range includes: determining the theoretical coiling temperature range of high carbon steel according to the chemical composition or CCT curve of high carbon steel, combined with the target performance index of high carbon steel.

[0047] The target coiling temperature takes a smaller value within the theoretical coiling temperature range, and the actual coiling temperature of the steel coil is lower. A lower coiling temperature is conducive to making the pearlite in the steel thinner and the carbides smaller and more dispersed. The finer the original pearlite grains, the thinner the lamellar structure, and the cementite is relatively easier to dissolve during annealing. At the same time, it can also prepare the organization for the subsequent spheroidizing annealing at a lower temperature and in a shorter time.

[0048] Step S200: Under the condition of introducing hydrogen gas flow, the steel coil is heated to the target spheroidizing annealing temperature in a closed space and kept warm for 12 to 20 hours, wherein the lower limit of the theoretical spheroidizing annealing temperature range of high carbon steel is ≤ the target spheroidizing annealing temperature ≤ the lower limit of the theoretical spheroidizing annealing temperature range + 10°C.

[0049] Wherein, the step of heating the high carbon steel coil to the target spheroidizing annealing temperature in a closed space comprises a first heating stage, a second heating stage and a third heating stage performed sequentially;

[0050] The first heating stage comprises heating the steel coil to 200° C. by free heating and keeping the temperature for 1 hour;

[0051] The second heating stage comprises heating the steel coil after the first heating stage to 400°C at a heating rate of 30-50°C / h and keeping the temperature for 2 hours;

[0052] The third heating stage includes further heating the steel coil after the second heating stage at a heating rate of 30-50° C. / h to a target spheroidizing annealing temperature;

[0053] During the first heating stage until the temperature of the steel coil reaches 600°C, the flow rate of the hydrogen gas flow is 10-20 m / s. 3 / h.

[0054] In this step, the steel coil is annealed. The steel coil is usually placed in a relatively closed space, such as an annealing furnace, specifically, a bell-type annealing furnace. Take the bell-type annealing furnace as an example. The steel coil is loaded into the bell-type annealing furnace, and then the inner cover is pressed. After the hydrogen valve, inner cover and furnace table sealing tests are completed in sequence, N 2 Pre-purge.

[0055] In some embodiments, the steel coil is in a confined space using N 2 Pre-purge 10~25min, N 2 The volume is 30~50m 3 .

[0056] After the purging is completed, the hydrogen blowing and heating operations are started. The steel coil is heated to the target spheroidizing annealing temperature in the bell annealing furnace.

[0057] The annealing temperature range (i.e., theoretical spheroidizing annealing temperature range) that can be used for high carbon steel can be determined first, and then the target spheroidizing annealing temperature can be taken as the actual annealing temperature within the theoretical spheroidizing annealing temperature range. Among them, the lower limit of the theoretical spheroidizing annealing temperature range of high carbon steel is ≤ the target spheroidizing annealing temperature ≤ the lower limit of the theoretical spheroidizing annealing temperature range + 10°C. The method for determining the theoretical spheroidizing annealing temperature range includes: the chemical composition or CCT curve of the high carbon steel, combined with the Ac1 temperature line, Acm temperature line and at least one of the target performance indicators of the high carbon steel, to determine the theoretical spheroidizing annealing temperature range of the high carbon steel. Exemplarily, the spheroidizing annealing heating temperature is Ac1+(20~40)°C or Acm-(20~30)°C, and the annealing temperature range that can be used for high carbon steel is generally 600~800°C.

[0058] The target spheroidizing annealing temperature takes a smaller value within the theoretical spheroidizing annealing temperature range, and the actual annealing temperature of the steel coil is lower. The lower annealing temperature is beneficial to restrict the diffusion of carbon elements in the high-carbon steel coil and prevent the carbon elements in the steel from being enriched in the spheroidizing cementite core to form graphitized structure.

[0059] Since the target coiling temperature is low, the target spheroidizing annealing temperature can be lower and the time can be shorter. When the hot-rolled steel coil adopts a lower coiling temperature, the annealing holding temperature can be reduced by more than 20°C compared with the higher coiling temperature, and the annealing holding time can be reduced by about 2 hours, which is conducive to reducing the annealing holding temperature to prevent material graphitization.

[0060] During the first heating stage until the temperature of the steel coil reaches 600°C, the flow rate of the hydrogen gas flow is relatively large, ranging from 10 to 20 m / s. 3 / h. Specifically in the annealing step, after the steel coil is loaded into the furnace, it is first heated freely to 200°C and kept warm for 1 hour. At this time, the water adsorbed on the surface of the steel coil is vaporized into water vapor. Under the condition of large flow of hydrogen blowing, the water vapor can be quickly taken away. Then continue to heat up to 400°C at the set heating rate and keep warm for 2 hours. At this time, the oil and grease that may exist on the surface of the hot-rolled steel coil will volatilize due to heat and can also be quickly discharged from the hood furnace. At the same time, the overall temperature of the steel coil can be uniformized at a temperature below the recrystallization temperature, making the annealing structure of the steel coil more uniform. Then heat to the holding temperature at the set heating rate for insulation.

[0061] When the hot-rolled steel coil adopts a lower coiling temperature, the annealing and holding temperature can be reduced by more than 20°C compared with the higher coiling temperature, and the annealing and holding time can be reduced by about 2 hours, which is beneficial to reduce the annealing and holding temperature to prevent material graphitization.

[0062] Step S300: After the steel coil is cooled, it is taken out from the enclosed space.

[0063] In this step, the steel coil is cooled along with the furnace and can be taken out from the bell annealing furnace after cooling.

[0064] In the above-mentioned method for preparing high carbon annealed steel, a lower temperature is selected as the target coiling temperature within the theoretical coiling temperature range of high carbon steel. A lower coiling temperature is beneficial for making the pearlite in the steel thin and the carbides small and dispersed, and can also prepare the structure for the subsequent spheroidizing annealing at a lower temperature and in a shorter time. Within the theoretical spheroidizing annealing temperature range of high carbon steel, a lower temperature is selected as the target spheroidizing annealing temperature. A lower target spheroidizing annealing temperature is beneficial for restricting the diffusion of carbon elements in high carbon steel coils, greatly reducing the probability of carbon elements in the steel being enriched in the spheroidized cementite core to form graphitized structure, and even inhibiting the carbon elements in the steel from being enriched in the spheroidized cementite core to form graphitized structure. Under the combined effect of these two aspects, the graphitization of high carbon steel can be better inhibited.

[0065] In some embodiments, during the process from when the temperature of the steel coil is greater than 600°C to when the steel coil is taken out of the confined space, the flow rate of the hydrogen gas flow is 2-5 m / s. 3 / h.

[0066] In the hydrogen blowing system, the flow rate of hydrogen gas will be dynamically adjusted according to the temperature of the steel coil. Since the steel coil is placed in a bell-type furnace, the temperature of the steel coil can also be understood as the temperature of a closed space, such as the temperature of a bell-type annealing furnace.

[0067] The initial hydrogen flow is relatively large, for example, in some embodiments, the first 15 to 20 m 3 / h for purging, and the purging time is until the temperature inside the bell-type furnace is greater than 400℃. The use of large-flow purging in the early stage of bell-type annealing is conducive to removing the moisture adsorbed on the surface of the steel coil in the bell-type furnace, the oil and grease that may exist on the surface of the steel coil, and the moisture generated by the reaction of iron oxide scale and hydrogen. 3 / h flow rate of hydrogen blowing to further remove the remaining oil vapor and water vapor in the furnace.

[0068] When the temperature in the furnace reaches above 600°C, the flow rate of hydrogen gas is 2~5m 3 / h, that is, a very small flow rate of hydrogen blowing mode is used, similar to the furnace suffocation treatment, which lasts until the steel coil is taken out of the furnace. When the temperature is above 600℃, the carbon in the steel begins to react with hydrogen to generate methane, resulting in decarburization. According to the principle of chemical equilibrium shift, stopping hydrogen blowing and retaining the methane generated by the decarburization reaction in the gas can inhibit the reaction and prevent decarburization.

[0069] In some embodiments, the target spheroidizing annealing temperature is the lower limit of the theoretical spheroidizing annealing temperature range of high carbon steel. In this embodiment, taking the minimum value can restrict the diffusion of carbon elements in the high carbon steel coil to a greater extent, and prevent the carbon elements in the steel from being enriched in the spheroidized cementite core to form graphitized structure.

[0070] In some embodiments, the target coiling temperature is the lower limit of the theoretical coiling temperature range of high carbon steel. In this embodiment, taking the minimum value can make the pearlite in the steel thinner and the carbides smaller and more dispersed to a greater extent, and can also prepare the structure for the subsequent spheroidizing annealing with a lower temperature and a shorter time.

[0071] In some embodiments, the chemical composition of the high carbon steel, by mass fraction, includes C: 0.8-1.3%; Si: 0.1-2.0%; Mn: 0.1-2.0%; Cr: ≤1.5%; P: ≤0.03; S: ≤0.03, and the rest is Fe and unavoidable impurities. The high carbon steel of this composition adopts the above-mentioned preparation method to suppress the occurrence of graphitization of the high carbon steel.

[0072] In some embodiments, after the steel coil is rapidly cooled, the step of removing the steel coil from the confined space includes:

[0073] When the target spheroidizing annealing temperature of the steel coil is 660~800℃, the steel coil is naturally cooled to 640~660℃ in a closed space, then cooled to 380℃ by air cooling, and then cooled to below 100℃ by water cooling before being taken out from the closed space.

[0074] In this embodiment, after the heat preservation is completed, the steel coil can be cooled slowly with a hood and then powered off for 3 to 4 hours, and then replaced with a cooling hood for air cooling to a certain temperature to avoid graphitization of carbon steel. Then, the steel coil is cooled with water to a lower temperature, such as below 100°C, and then taken out of the furnace. A faster cooling rate can prevent the diffused carbon elements from enriching and graphitizing on the cementite nucleus, thereby inhibiting the occurrence of graphitization of high carbon steel.

[0075] In some embodiments, after the steel coil is rapidly cooled, the step of removing the steel coil from the confined space includes:

[0076] When the target spheroidizing annealing temperature of the steel coil is 600-660°C, the steel coil is cooled to 380-400°C by air cooling, and then cooled to below 100°C by water cooling before being taken out from the closed space.

[0077] In this embodiment, after the heat preservation is completed, the cooling cover can be directly replaced for air cooling to a certain temperature to avoid graphitization of carbon steel. Then, the steel coil is water cooled to a lower temperature, such as below 100°C, and then taken out of the furnace. A faster cooling rate can prevent the diffused carbon elements from enriching and graphitizing on the cementite nucleus, thereby inhibiting the occurrence of graphitization of high carbon steel.

[0078] In general, the technical effect of the high carbon steel preparation method of the present application is that it can significantly inhibit the graphitization of high carbon steel (C ≥ 0.60%) coils during spheroidizing annealing, which is mainly manifested in that there is basically no obvious graphitized structure in the high carbon steel annealed using this process, and the proportion of graphitized material is basically below 0.005%, and can be kept below 0.001% in better cases. In contrast, in the ordinary annealing process, graphitization problems occur in nearly 1% of cases, and some manufacturers even have graphitization defects in a certain batch of continuous production.

[0079] On the other hand, the graphitization level produced by this method is below 0.5, and the graphitized structure size is below 5μm, which does not affect processing and use.

[0080] Therefore, for applications requiring fatigue performance, strength, plasticity and impact toughness, such as tool steel, spring steel, bearing steel, etc., this application can significantly improve the performance of finished parts, prevent brittle failure of materials, and achieve better use effects.

[0081] In general, the core of the preparation method of high carbon steel includes at least the following points:

[0082] 1. Fine pearlite is obtained by low-temperature rolling, which is convenient for dissolving cementite during annealing, and at the same time, extremely fine carbide nuclei are obtained to prevent the nuclei from being too large during spheroidization to form larger spherical cementite (i.e. spheroidized pearlite, because pearlite is actually a mixture of cementite and ferrite, and the carbon element is almost all in cementite, while ferrite contains less carbon).

[0083] 2. A lower spheroidizing annealing temperature, about Ac1+20~40℃, is selected to prevent the carbide nucleation from excessive growth when Ac1+20~30℃ is selected.

[0084] 3. Faster cooling rate: According to the experimental situation, a cooling rate of 20-25℃ / h is required, with air cooling at a transition point of 640-660℃, water cooling at a transition point below 400℃, and then water cooling to below 100℃. Otherwise, carbides will be enriched in cementite during the cooling process, causing the nucleus to grow excessively and lead to graphitization.

[0085] 4. The cooling conditions of this application are quite different from those of the related technology. In the related technology, the furnace cooling is generally below 600°C.

[0086] In addition, it should be noted that in the production process of high carbon steel, there are usually multiple annealing-cold rolling processes. For example, the strip steel undergoes the first annealing and is rolled to a 2.0nn specification, and then undergoes the second annealing and rolling to a 1.0mm specification. After the third annealing, it is rolled to a 0.5nn specification, and then undergoes the fourth annealing.

[0087] The purpose of each annealing in the first annealing, the second annealing, the third annealing, and the fourth annealing is different, such as controlling the spheroidization rate (spheroidizing annealing), adjusting the hardness (softening annealing, incomplete annealing), recovering recrystallization (recrystallization annealing), eliminating residual stress (stress relief annealing), etc., and the annealing is arranged reasonably. The embodiments of this application all refer to spheroidizing annealing. The temperature of spheroidizing annealing in the related art is greater than the spheroidizing annealing temperature of this application.

[0088] The present application also provides a high carbon steel obtained by the above-mentioned preparation method. The high carbon steel has a low graphitized material ratio, a graphitized grade below 0.5, and a graphitized structure size below 5 μm, which does not affect processing and use. The high carbon steel can be used in occasions with requirements for fatigue performance, strength, plasticity, and impact toughness, such as tool steel, spring steel, bearing steel, etc.

[0089] Example 1: Spheroidizing annealing of SK85 cold-rolled steel coil

[0090] SK85 is a carbon tool steel used to make parts such as art blades and air conditioning valve plates. Its chemical composition is: 0.87%C, 0.18%Si, 0.47%Mn, 0.009%P, 0.002%S, 0.20%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of SK85 is set to 640℃, and the hot rolled coil is slowly cooled to room temperature off the line. Four rolls of SK85 cold rolled steel coils with a coil weight of 17 tons, a specification of 2.0×1250mm and a cold rolling reduction rate of 30% are loaded into the hood annealing furnace for annealing. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0091] The annealing process is first carried out at 10m 3 / h hydrogen blowing for 7h, then high flow mode for 20m 3 / h hydrogen blowing for 10 hours, at which time the temperature inside the inner hood reaches about 450℃, and then 10m 3 / h hydrogen blowing for 3 hours, then blow hydrogen in automatic pressure maintaining mode (about 2m 3 / h) until the annealing is completed.

[0092] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 720℃ in another 8 hours and keeping warm for 16 hours; the fourth stage: after the insulation is completed, cooling with the hood for 4 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 80℃ with water before taking out of the furnace.

[0093] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 0.85%, the spheroidization rate was above 90%, and no graphitized structure appeared in the material.

[0094] Example 2: Spheroidizing annealing of 65Mn cold-rolled steel coil

[0095] 65Mn is a high-quality carbon structural steel, widely used in hardware, home appliances, automobiles and other parts. Its chemical composition is: 0.66%C, 0.21%Si, 0.96%Mn, 0.007%P, 0.006%S, 0.14%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of 65Mn is set to 620℃, and the hot rolled coil is slowly cooled to room temperature off the line. Four rolls of 65Mn cold rolled steel coils with a coil weight of 24 tons and a specification of 2.5×1250mm and a cold rolling reduction rate of 22% are loaded into the hood annealing furnace for annealing. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0096] The annealing process is first carried out at 10m 3 / h hydrogen blowing for 7h, then high flow mode for 20m 3 / h hydrogen blowing for 10 hours, at which time the temperature inside the inner hood reaches about 450℃, and then 10m 3 / h hydrogen blowing for 3 hours, then blow hydrogen in automatic pressure maintaining mode (about 2m 3 / h) until the annealing is completed.

[0097] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 660℃ in another 7 hours and keeping warm for 16 hours; the fourth stage: after the insulation, change to air cooling to 380℃, change to water cooling to 80℃ and then take out of the furnace.

[0098] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 0.645%, the spheroidization rate was above 90%, and there was no obvious graphitized structure.

[0099] Example 3: Spheroidizing annealing of GCr15 hot-rolled steel coil

[0100] GCr15 is a representative steel grade of high carbon chromium bearing steel. It is used to manufacture steel balls, rollers, bushings and other parts on the transmission shafts of internal combustion engines, electric locomotives, machine tools, tractors, steel rolling equipment, drilling machines, railway vehicles and mining machinery. Some users also use it to make wear-resistant parts such as super wear-resistant cement delivery pump pipes and textile machine cams. The chemical composition is: 1.01% C, 0.22% Si, 0.33% Mn, 0.014% P, 0.002% S, 1.51% Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of GCr15 is set to 670℃, and the hot rolled coil is slowly cooled to room temperature off the line. Four rolls of GCr15 hot rolled steel coils with a coil weight of 16 tons and a specification of 4.0×1150mm are loaded into the hood annealing furnace for annealing. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace table sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0101] According to the high flow mode 20m 3 / h hydrogen blowing for 14 hours, at which time the temperature inside the inner cover reaches about 450℃, and then hydrogen is blown in automatic pressure-maintaining mode (about 2m 3 / h) until the annealing is completed.

[0102] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 760℃ at 40℃ / h and keeping warm for 20 hours; the fourth stage: after the insulation is completed, cooling with the hood for 7 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 90℃ with water before taking out of the furnace.

[0103] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 1.01%, the spheroidization rate was above 90%, and there was no obvious graphitized structure.

[0104] Example 4: Spheroidizing annealing of SK4 cold-rolled steel coil

[0105] SK4 is a carbon tool steel used to make parts such as art blades and air conditioning valve plates. Its chemical composition is: 0.96%C, 0.18%Si, 0.37%Mn, 0.009%P, 0.002%S, 0.15%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of SK4 is set to 640℃, and the hot rolled coil is slowly cooled to room temperature after it comes off the line. Four rolls of SK4 cold rolled steel coils with a coil weight of 22 tons and a specification of 0.5×1250mm and a reduction rate of 35% after three cold rolling are loaded into the hood annealing furnace for annealing. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0106] First press 10m 3 / h hydrogen blowing for 7h, then high flow mode for 20m 3 / h hydrogen blowing for 10 hours, at which time the temperature inside the inner hood reaches about 450℃, and then 10m 3 / h hydrogen blowing for 3 hours, then blow hydrogen in automatic pressure maintaining mode (about 2m 3 / h) until the annealing is completed.

[0107] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 720℃ at 60℃ / h and keeping warm for 15 hours; the fourth stage: after the insulation is completed, cooling with the hood for 4 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 80℃ with water before taking out of the furnace.

[0108] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 0.95%, the spheroidization rate was above 90%, and no graphitized structure appeared in the material.

[0109] Example 5: Spheroidizing annealing of 9SiCr hot-rolled steel coil

[0110] 9SiCr is a low alloy steel for cutting tools, mainly used to make cutting tools, but also often used to make cold working die parts. Its chemical composition is: 0.91%C, 1.36%Si, 0.45%Mn, 0.009%P, 0.001%S, 1.07%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of 9SiCr is set to 670℃, and the hot rolled coil is slowly cooled to room temperature after it comes off the line. Four rolls of 9SiCr hot rolled steel coils with a coil weight of 17 tons and a specification of 6.0×1100mm are loaded into the hood annealing furnace for annealing. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0111] According to the high flow mode 20m 3 / h hydrogen blowing for 14 hours, at which time the temperature inside the inner cover reaches about 450℃, and then hydrogen is blown in automatic pressure-maintaining mode (about 2m 3 / h) until the annealing is completed.

[0112] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 760℃ at 40℃ / h and keeping warm for 16 hours; the fourth stage: after the insulation is completed, cooling with the hood for 7 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 90℃ with water before taking out of the furnace.

[0113] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 0.893%, the spheroidization rate was above 90%, and there was no obvious graphitized structure.

[0114] Example 6: Spheroidizing annealing of 75Cr1 hot-rolled steel coil

[0115] 75Cr1 is an alloy tool steel used in circular saw blades and marble saw blades. Its chemical composition is: 0.75%C, 0.32%Si, 0.72%Mn, 0.017%P, 0.002%S, 0.42%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of 75Cr1 is set at 630℃. The hot rolled coil is slowly cooled to room temperature off the line. Four rolls of 75Cr1 hot rolled steel coils with a coil weight of 22 tons and a specification of 4.5×1250mm are loaded into the hood annealing furnace. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace table sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0116] According to the high flow mode 20m 3 / h hydrogen blowing for 14 hours, at which time the temperature inside the inner cover reaches about 450℃, and then hydrogen is blown in automatic pressure-maintaining mode (about 2m 3 / h) until the annealing is completed.

[0117] The set temperature system of the hood furnace is as follows: the first stage: free heating to 200℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 5 hours and keeping warm for 2 hours; the third stage: heating to 730℃ at 40℃ / h and keeping warm for 16 hours; the fourth stage: after the insulation is completed, cooling with the hood for 4 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 90℃ with water before taking out of the furnace.

[0118] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 0.728%, the spheroidization rate was above 90%, and there was no obvious graphitized structure.

[0119] Example 7: Spheroidizing annealing of SK2 hot-rolled steel coil

[0120] SK2 is an alloy tool steel used in circular saw blades and marble saw blades. Its chemical composition is: 1.19%C, 0.21%Si, 0.45%Mn, 0.013%P, 0.002%S, 0.55%Cr, and the rest is Fe and unavoidable impurities. The hot rolling coiling temperature of SK2 is set to 670℃. The hot rolled coil is slowly cooled to room temperature off the line. Four SK2 hot rolled steel coils with a coil weight of 22 tons and a specification of 2.0×1250mm are loaded into the hood annealing furnace. After pressing the inner cover and completing the hydrogen valve, inner cover and furnace table sealing experiments in turn, N 2 Pre-purge 20min, volume 50m 3 After purging, start the hydrogen blowing and heating operations.

[0121] According to the high flow mode 20m 3 / h hydrogen blowing for 14 hours, at which time the temperature inside the inner cover reaches about 450℃, and then hydrogen is blown in automatic pressure-maintaining mode (about 2m 3 / h) until the annealing is completed.

[0122] The set temperature system of the hood furnace is as follows: the first stage: free heating to 100℃ and keeping warm for 1 hour; the second stage: heating to 400℃ in 7.5 hours and keeping warm for 2 hours; the third stage: heating to 750℃ at 40℃ / h and keeping warm for 16 hours; the fourth stage: after the insulation is completed, cooling with the hood for 5 hours, at which time the temperature reaches about 650℃, and then cooling to 380℃ with air, and cooling to 90℃ with water before taking out of the furnace.

[0123] After spheroidizing annealing, the steel coil was sampled and inspected, and the carbon content was 1.17%, the spheroidization rate was above 90%, and there was no obvious graphitized structure.

[0124] Comparative Example 1

[0125] A domestic manufacturer has used the following process to produce annealed SK85. The chemical composition of the material is the same as that of the material in Example 1, and the hot rolling coiling temperature is 650°C. The material specification is 0.7×380mm. It is put into the furnace at room temperature, and it takes 3.5 hours to rise to 380°C. After keeping it warm for 4 hours, it takes 5 hours to rise to 680°C, and keeps it warm for 10 hours. Then it is cooled with the furnace for 5 hours to 580°C, and air-cooled for 5 hours to about 410°C. The cooling cover is removed, and air-cooling is continued for 14 hours to 80°C to obtain a steel coil.

[0126] Test Example 1

[0127] The SK85 steel of Example 1, the SK85 steel of Comparative Example 1, and the GCr15 steel of Example 3 were tested. The three steels were subjected to metallographic tests before and after nitric acid corrosion.

[0128] in, Figure 2 It is a cross-sectional metallographic image of the SK85 steel of Example 1 of the present application, and the plate surface has no strip-shaped graphitization defects when not corroded by nitric acid alcohol. Figure 3 This is a cross-sectional metallographic image of the SK85 steel of Example 1 of the present application. After being corroded by nitric acid alcohol, there is no strip-shaped graphitization defect on the plate surface. The image shows a spheroidized pearlite structure with a spheroidization grade of 2.5.

[0129] Figure 4 This is a cross-sectional metallographic image of SK85 steel in comparative example 1 of the present application. The plate surface has strip-shaped graphitization defects without nitric acid corrosion. Among them, the annealing graphitization level of the material is 2.5, and the graphitized structure in the metallographic phase can be clearly observed. After the carbon element gathers in large quantities on the cementite nucleus to produce graphitization, the size of the graphitized structure is between 30-300μm, which seriously affects the normal use of the material.

[0130] Figure 5 This is a cross-sectional metallographic diagram of the SK85 steel of comparative example 1 of the present application. After being corroded by nitric acid alcohol, the plate surface has strip-shaped graphitization defects. The figure shows that the matrix parent material is a spheroidized pearlite structure with a spheroidization grade of 2.5.

[0131] Figure 6 This is a scanning electron microscope photo of the annealed graphitized structure of SK85 steel in Comparative Example 1 of the present application, wherein: Figure 6 The black spots are graphitization defects. Figure 6 Chinese spectrum Figure 1 The energy spectrum test results of the graphitized structure at the position shown are shown in Table 1. The specific process is that the SEM tunnel scanner is in working state, and the element composition of the conductive image under the electron microscope field of view is detected. The energy spectrum analysis test results show that the C content is 15.78%, which is much higher than the target C content of 0.87%, indicating that the carbon element is enriched and graphitization occurs at this location.

[0132] Table 1 Graphitization region energy spectrum analysis detection results

[0133]

[0134] Figure 7 This is a cross-sectional metallographic image of the GCr15 steel of Example 3 of the present application, which shows that the plate surface has no strip-shaped graphitization defects when not corroded by nitric acid alcohol. Figure 8 This is a cross-sectional metallographic image of the GCr15 steel of Example 3 of the present application. After being corroded by nitric acid alcohol, there is no strip-shaped graphitization defect on the plate surface. The image shows a spheroidized pearlite structure with a spheroidization grade of 2.5.

[0135] Since the causes and manifestations of graphitization of various steel types in the examples are almost the same, the graphitization of SK85 and GCr15 are typical examples that can illustrate common problems, and the test data of other examples are not provided.

[0136] It can be seen that the preparation method of high carbon annealed steel of the present application can greatly reduce the probability of carbon elements in the steel being enriched in the spheroidized cementite core to form graphitized structure, and even inhibit the carbon elements in the steel from being enriched in the spheroidized cementite core to form graphitized structure, thereby better inhibiting the graphitization of high carbon steel.

[0137] The above technical solutions of the present invention are only preferred embodiments of the present invention, and the patent scope of the present invention is not limited thereto. All equivalent structural changes made by using the contents of the present invention specification and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields are included in the patent protection scope of the present invention.

Claims

1. A method for inhibiting graphitization of high carbon steel, characterized in that: The mass percentage of carbon element in the high carbon steel is ≥ 0.60%, and the chemical composition of the high carbon steel, calculated by mass fraction, includes C: 0.8-1.3%; Si: 0.1-2.0%; Mn: 0.1-2.0%; Cr: ≤1.5%; P: ≤0.03; S: ≤0.03, and the rest is Fe and unavoidable impurities; the method comprises the following steps: Coiling the high carbon steel strip at a target coiling temperature to obtain a high carbon steel coil, wherein the lower limit of a theoretical coiling temperature range of the high carbon steel is ≤ the target coiling temperature ≤ the lower limit of the theoretical coiling temperature range + 18°C; Under the condition of introducing hydrogen gas flow, the steel coil is heated to the target spheroidizing annealing temperature in a closed space and kept at the temperature for 12 to 20 hours, wherein the lower limit of the theoretical spheroidizing annealing temperature range of the high carbon steel is ≤ the target spheroidizing annealing temperature ≤ the lower limit of the theoretical spheroidizing annealing temperature range + 10°C; After the steel coil is cooled, it is taken out from the confined space; Wherein, the step of heating the high carbon steel coil to the target spheroidizing annealing temperature in a closed space comprises a first heating stage, a second heating stage and a third heating stage performed sequentially; The first heating stage comprises heating the steel coil to 200° C. by free heating and keeping the temperature for 1 hour; The second heating stage comprises heating the steel coil after the first heating stage to 400°C at a heating rate of 30-50°C / h and keeping the temperature for 2 hours; The third heating stage includes further heating the steel coil after the second heating stage at a heating rate of 30-50° C. / h to a target spheroidizing annealing temperature; During the first heating stage until the temperature of the steel coil reaches 600°C, the flow rate of the hydrogen gas flow is 10-20 m / s. 3 / h; The step of taking the steel coil out of the confined space after cooling the steel coil comprises: The steel coil is naturally cooled to 640-660°C in the closed space, cooled to below 400°C at a cooling rate of 20-25°C / h, and then water-cooled to below 100°C.

2. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: During the process from when the temperature of the steel coil is greater than 600°C to when it is taken out of the confined space, the flow rate of the hydrogen gas flow is 2-5 m / s. 3 / h.

3. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: The target spheroidizing annealing temperature is the lower limit of the theoretical spheroidizing annealing temperature range of the high carbon steel; the target coiling temperature is the lower limit of the theoretical coiling temperature range of the high carbon steel.

4. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: The method further comprises, before introducing the hydrogen gas stream: The steel coil is pre-purged with N2 in a closed space for 10-25 minutes, and the volume of N2 is 30-50m 3 .

5. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: The step of taking the steel coil out of the confined space after cooling the steel coil comprises: When the target spheroidizing annealing temperature of the steel coil is 660-800° C., the steel coil is naturally cooled to 640-660° C. in the closed space, then cooled to 380° C. by air cooling, and then cooled to below 100° C. by water cooling before being taken out of the closed space.

6. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: The step of taking the steel coil out of the confined space after cooling the steel coil comprises: When the target spheroidizing annealing temperature of the steel coil is 600-660° C., the steel coil is cooled to 380° C. by air cooling, and then cooled to below 100° C. by water cooling, and then taken out from the closed space.

7. The method for inhibiting graphitization of high carbon steel according to claim 1, characterized in that: The method for determining the theoretical spheroidizing annealing temperature range includes: Determine a theoretical spheroidizing annealing temperature range of the high carbon steel according to the chemical composition or CCT curve of the high carbon steel, in combination with the Ac1 temperature line, the Acm temperature line and at least one of the target performance indicators of the high carbon steel; The method for determining the theoretical coiling temperature range includes: The theoretical coiling temperature range is determined according to the chemical composition or CCT curve of the high carbon steel and in combination with the target performance index of the high carbon steel.

8. A high carbon steel obtained by the method for suppressing graphitization of high carbon steel according to any one of claims 1 to 7.

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