Medium carbon steel and production method for controlling surface decarburization and oxidation of medium carbon steel billet

Abstract

The invention relates to medium carbon steel and a production method for controlling surface decarburization and oxidation of a medium carbon steel billet. The mass content of Cr in the medium carbon steel is 0.20%-0.25%. The production method comprises the following steps: (1) forced cooling and slow cooling of a cast hot-state steel billet; (2) rolling heating, wherein a temperature zone in a heating furnace is divided into five sections, namely a preheating section, a heating section I, a heating section II, a heating section III and a soaking section; (3) controlling the air-fuel ratio of the heating furnace; and (4) controlling the oxygen content and the moisture content in the heating furnace. According to the method, the oxidation burning loss of the steel billet in the heating furnace can be obviously reduced, the thickness reduction amplitude of the generated primary oxide scale is close to 40%, the thickness reduction amplitude of the decarburized layer of the rolled steel is close to 35%, and the influence of oxidation and decarburization on the small-specification square billet with short in-furnace time is obvious.

Description

Technical Field

[0001] The invention relates to medium carbon steel and a production method for controlling surface decarburization and oxidation of the medium carbon steel billet, belonging to the technical field of metal thermal deformation. Background Art

[0002] In various manufacturing fields of society such as construction machinery, automobile manufacturing, marine ships and engineering, carbon steel with a carbon content of 0.25% to 0.50% accounts for a large proportion. Therefore, the present invention takes the carbon steel within this range as the research object. For the convenience of research, the present invention defines steel with a carbon content in the range of 0.25% to 0.50% as medium carbon steel.

[0003] Generally, rolling is a pressure processing process under hot state, that is, the billet needs to be heated before rolling. For billets with different sections, there are large differences in the time and temperature of the billet in the heating furnace. The larger the cross-sectional area of ​​the billet, the higher the temperature in the heating furnace, and the longer the melting time. During the heating process of the billet in the heating furnace, two main reactions will occur on the surface of the billet: one is that the iron on the surface of the billet is oxidized by the oxidizing substances in the furnace to produce primary iron oxide; the other is that the carbon atoms in the steel diffuse to the surface of the billet after heating, and then combine with oxygen, hydrogen, etc. to form gas, causing decarburization on the surface of the billet. The oxidation and decarburization of iron on the surface of the steel billet will have adverse effects on the subsequent process. The oxidation of iron will cause the yield rate of the steel billet to decrease. The decarburization of the steel billet surface in the heating furnace is the first decarburization (the corresponding second decarburization is the decarburization of the steel billet and steel in the air during the rolling process). The full decarburization of the normal steel surface is generated and formed at this time. This decarburization will be inherited by the steel, which will greatly reduce the carbon content on the surface of the steel. This reduction in carbon content will deteriorate the mechanical properties and process properties of the steel, resulting in the mechanical properties of the processed products not meeting the requirements, poor wear resistance, insufficient fatigue performance, etc. The large difference in carbon content between the surface and the inside will cause the deformation of the steel surface and the internal organization to be inconsistent, resulting in surface defects such as cracks on the steel surface. Therefore, it is very necessary to study how to control the decarburization of the steel billet inside the heating furnace.

[0004] At present, the control of the primary decarburization layer of steel billets at home and abroad mainly focuses on the following aspects: first, coating the surface of the steel billet with a coating, using the coating to isolate the surface of the steel billet from the reaction substance as much as possible, and reducing the hot chemical reaction between the coating and the surface of the steel billet. For example, in the patent application publication number CN115870332A "A production method for eliminating the full decarburization of Si-Cr spring steel wire rods and improving the fatigue life", there is a measure to prevent decarburization, which is to spray anti-decarburization coating on the surface of the hot steel billet. Although this type of anti-decarburization method has a certain effect, it increases the production process and cost, and also brings certain harm to the working environment of workers; second, control the temperature and time, reduce the time of the steel billet in the furnace and reduce the heating temperature. For example, the patent application publication number CN112404130A "A method for controlling S45C decarburization" requires that the heating temperature must be controlled at 1120℃~1150℃, and the total time of the second heating stage and the soaking stage must be controlled at 150~190 minutes, and the decarburization layer can be controlled within 1%; third, change the atmosphere in the furnace, The air-fuel ratio in the furnace is set very small or very large. For example, in the patent application publication number CN114405991A "Method for reducing the surface decarburization depth of medium and high carbon steel billets", the air-fuel ratio in the heating furnace is controlled at 0.45-0.65. Normally, the air-fuel ratio in the heating furnace is set in this range, which is very likely to cause gas leakage. The patent application publication number CN106521120A "A method for controlling heating and decarburization of bearing steel" sets the air-fuel ratio as high as 2.4-2.6. The method adopted in this patent is different from that in the patent application publication number CN114405991A "Method for reducing the surface decarburization depth of medium and high carbon steel billets" The idea of ​​patent application publication number CN116790860A "A control method for full decarburization of silicon spring flat steel" is similar to that of preventing and controlling the decarburization layer by strong oxidation of the steel billet in the heating furnace. This method will cause a serious reduction in the yield rate of the steel billet, or introduce a protective gas into the heating furnace to reduce the oxidizing atmosphere in the furnace in disguise. For example, the measure in patent application publication number CN110760653A "A control method for preventing decarburization of bearing steel" introduces a nitrogen protective atmosphere into the furnace to control the oxidizing atmosphere in the furnace.

[0005] The decarburization and oxidation on the surface of medium carbon steel billets have not been reported in relevant patents or literature. In view of the impact of decarburization and oxidation on steel quality and yield rate, as well as the wide application of medium carbon steel in various fields, it is very necessary to study the decarburization and oxidation on the surface of the billet and form a mature control method. Summary of the invention

[0006] In order to solve the above-mentioned problems, the present invention discloses a medium carbon steel and a production method for controlling surface decarburization and oxidation of a medium carbon steel billet. The purpose of the present invention is to develop an economical and effective production method for controlling surface decarburization and oxidation of a medium carbon steel billet in view of the deficiencies in the prior art and actual production needs, thereby controlling the surface decarburization layer of the medium carbon steel and improving the yield rate of the billet.

[0007] The specific technical solution is as follows:

[0008] A medium carbon steel is composed of the following elements in percentage by mass: C 0.25% to 0.50%, Si≤0.50%, Mn≤1.50%, Cr0.20% to 0.25%, Cu≤0.10%, Ni≤0.20%, Mo≤0.15%, V≤0.10%, Nb≤0.04%, As≤0.01%, Al 0.010% to 0.020%, P≤0.025%, S≤0.015%, B≤0.0010%, Ti≤0.03%, N≤0.0050%, [O]≤20ppm, [H]≤2ppm, and the balance is Fe and unavoidable impurity elements.

[0009] Furthermore, the element [O]≤15ppm.

[0010] Furthermore, the mass content of the element Cr is 0.22% to 0.25%.

[0011] A production method for controlling the surface decarburization and oxidation of the above-mentioned medium carbon steel billet comprises the following steps:

[0012] Step 1) Strong cooling and slow cooling of cast hot billet:

[0013] After the surface temperature of the hot billet in casting drops to 730°C, the billet enters the forced air cooling process. Through air cooling, the billet is quickly cooled down. The cooling rate is controlled at 0.30-0.35°C / S, and the surface temperature of the billet is quickly reduced to 520-570°C. Then the billet is slowly cooled in the pit.

[0014] Step 2) Rolling heating process:

[0015] The temperature zone in the heating furnace is divided into 5 sections: preheating section, heating section 1, heating section 2, heating section 3 and soaking section. The temperature and time of each section are controlled as follows:

[0016] Preheating section: The preheating section is controlled at 550-570℃;

[0017] Heating stage: The temperature of heating stage is controlled at 650-700℃;

[0018] Heating stage 2: The temperature of heating stage 2 is controlled at 800~1050℃;

[0019] Heating stage three: The temperature of heating stage three is controlled at 1050~1250℃;

[0020] Soaking section: The temperature of the soaking section is controlled at 1120~1260℃;

[0021] Step 3) Control of air-fuel ratio of heating furnace:

[0022] Preheating section: The air-fuel ratio in the preheating section is controlled at 1.5 to 1.7;

[0023] Heating stage: The air-fuel ratio in the heating stage is controlled at 1.2-1.4;

[0024] Heating stage 2: The air-fuel ratio of heating stage 2 is controlled at 0.8-1.0;

[0025] Heating stage 3: The air-fuel ratio of heating stage 3 is controlled at 0.6-0.7;

[0026] Soaking section: The air-fuel ratio in the soaking section is controlled at 0.6-0.65;

[0027] Step 4) Control of oxygen content and moisture content in the atmosphere of the heating furnace:

[0028] Preheating section: The oxygen content in the atmosphere of the preheating section is controlled at 4.0% to 5.0%, and the water content is controlled at <0.06%;

[0029] Heating stage: The oxygen content in the atmosphere of the heating stage is controlled at 2.0% to 3.0%, and the water content is controlled at <0.06%;

[0030] Heating stage 2: The oxygen content in the atmosphere of the heating stage 2 furnace is controlled at 1.0% to 1.5%, and the water content is controlled at <0.04%;

[0031] Heating stage 3: The oxygen content in the atmosphere of the heating stage 3 furnace is controlled at 0.5% to 1.0%, and the water content is controlled at <0.03%;

[0032] Soaking section: The oxygen content in the atmosphere of the furnace in the soaking section is controlled at 0.5% to 0.8%, and the water content is controlled at <0.03%.

[0033] Furthermore, in the step 1), the steel billet is in the slow cooling pit for ≥ 72 hours.

[0034] Furthermore, in the step 2), the time ratio of the steel billet in the five temperature zones of preheating section, heating section 1, heating section 2, heating section 3 and soaking section is 3:2:2:1.5:1.

[0035] The working principle of the present invention is:

[0036] The patent of the present invention realizes the control of surface decarburization and oxidation of medium carbon steel billet by adding appropriate amount of specific chemical elements, slow cooling of the billet at a specific temperature and innovative heating process. In terms of chemical composition, an appropriate amount of chromium is added to the steel. With the cooperation of a special heating process, an oxide layer is formed on the surface of the billet by utilizing the oxidation passivation effect of chromium, which prevents the diffusion and penetration of oxidizing elements into the billet surface and weakens the surface decarburization and oxidation of the medium carbon steel billet. In terms of slow cooling of the billet, the cast hot billet is first subjected to a period of forced air cooling to reduce the oxidation of the billet in the air, and then the billet is quickly put into the pit for slow cooling. A dense oxide layer is formed on the surface of the billet at a specific temperature of 520-570°C to prevent further oxidation and decarburization. In terms of heating process, the water content of the material entering the furnace is strictly controlled, a low-temperature and high-oxidizing atmosphere is formed in the preheating section and the first section, and a medium-temperature reducing atmosphere is used in the second heating section and the uniform section to complete low-temperature high oxidation and medium-temperature complete austenitization. Through these two innovative means, effective control of surface decarburization and oxidation of medium carbon steel billets is achieved. The present invention can significantly reduce the oxidation and burning of steel billets in a heating furnace, with the thickness of the primary iron oxide scale reduced by nearly 40%, and the thickness of the decarburization layer of the rolled steel reduced by nearly 35%. For small-sized square billets with a shorter furnace time, the oxidation and decarburization effects are more significant.

[0037] The innovative features and beneficial effects of the present invention are:

[0038] First, the invention innovatively uses the cheap alloy element Cr to control the surface decarburization and oxidation of medium carbon steel billets. Considering the wide applicability of this patent and its related impact on steel performance and cost, the amount of alloy element Cr added is appropriately controlled.

[0039] Secondly, the production method of the present invention controls the aluminum content in the steel of the present invention to be relatively low, thereby reducing its influence on the activity of oxygen element and further weakening the oxidation of oxygen on the surface of the steel billet.

[0040] Thirdly, the present invention innovatively utilizes the oxidation characteristics of the hot steel billet during slow cooling at a specific temperature to form a dense oxide film on the surface of the steel billet entering the furnace, thereby isolating the steel billet from the external atmosphere, thereby reducing the reaction between the steel billet and oxidizing substances.

[0041] Fourthly, the present invention innovatively utilizes the characteristics of iron-oxide reaction to divide the heating furnace into five temperature zones. In the high oxidizing atmosphere in the low temperature zone, a dense oxide layer is further formed on the surface of the steel billet, so as to cut off the contact between the steel billet and the oxidizing atmosphere in the medium and high temperature zones, thereby preventing oxidation and decarburization of the steel billet surface.

[0042] Fifth, the present invention jointly controls the oxygen content and moisture content in the atmosphere of the heating furnace, controls the oxidizing atmosphere in different temperature zones in different ranges, and reduces oxidation and decarburization of the steel billet surface in the medium and high temperature zones.

[0043] Sixth, the patent of the present invention does not require additional equipment, nor does it require the steel billet to be peeled. It only achieves the control of oxidation and decarburization on the surface of the steel billet through technological innovation, appropriately improves the yield rate of steel, and reduces the decarburization layer on the surface of the steel.

[0044] Seventh, the production method of the present invention significantly reduces the oxidation and burning of steel billets in the heating furnace. The thickness of the primary oxide scale produced is reduced by nearly 40%, and the thickness of the decarburization layer of the rolled steel is reduced by nearly 35%. For small-sized square billets with a shorter furnace time, the effects of oxidation and decarburization are more significant. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a microscopic image of the decarburized layer produced by the prior art in Example 1.

[0046] Figure 2 This is a microscopic view of the decarburized layer produced by the present invention in comparison 1.

[0047] Figure 3 This is a microscopic image of the decarburized layer produced by the prior art in Example 2.

[0048] Figure 4 This is a microscopic view of the decarburized layer produced by the present invention in comparison 2.

[0049] Figure 5 This is a microscopic image of the decarburized layer produced by the prior art in Example 3.

[0050] Figure 6 This is a microscopic view of the decarburized layer produced by the present invention in comparison 3. DETAILED DESCRIPTION

[0051] The present invention is further explained below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0052] The medium carbon steel disclosed by the invention is composed of the following elements by mass percentage: C 0.25%-0.50%, Si≤0.50%, Mn≤1.50%, Cr 0.20%-0.25%, Cu≤0.10%, Ni≤0.20%, Mo≤0.15%, V≤0.10%, Nb≤0.04%, As≤0.01%, Al 0.010%-0.020%, P≤0.025%, S≤0.015%, B≤0.0010%, Ti≤0.03%, N≤0.0050%, [O]≤20ppm, [H]≤2ppm, and the balance is Fe and unavoidable impurity elements.

[0053] As a further preference, the element [O]≤15 ppm.

[0054] As a further preference, the mass content of element Cr is 0.22% to 0.25%.

[0055] The reasons for limiting the specific chemical composition of medium carbon steel in the present invention are described in detail below:

[0056] Cr: Generally, chromium has the characteristics of anti-oxidation, anti-corrosion, improving wear resistance, strength and hardness, but a high Cr content will deteriorate toughness. At the same time, GB / T699 "High-quality Carbon Structural Steel" requires that the Cr content is ≤0.25% when the carbon content is ≥0.25%. For the wide applicability of this patent, this patent involves adding a small amount of Cr to steel grades. After comprehensive consideration, the Cr content is determined to be 0.20%~0.25%.

[0057] Al: Aluminum is mainly used for grain refinement and smelting deoxidation in steel. In order to prevent high aluminum content from being detrimental to surface oxidation and decarburization of steel billets, the aluminum content is controlled in the present invention. However, too low aluminum content is not conducive to deoxidation in the smelting process, control of oxygen content in steel, and requirements for grain refinement. In the composition design of this patent, the Al content is designed to be Al0.010% to 0.020%.

[0058] In order to measure the effect of this patent, the following examples are evaluated by the thickness of the primary iron oxide scale during high-pressure water dephosphorization of steel billets and the depth of the decarburized layer on the surface of the steel.

[0059] Example 1

[0060] Steel type: 35

[0061] Steel billet chemical composition (wt%, smelting analysis): C 0.37%, Si 0.25%, Mn 0.68%, Cr 0.23%, Cu0.04%, Ni 0.02%, Mo 0.0049%, V 0.0036%, Nb≤0.0028%, As 0.0021%, Al 0.015%, P0.018%, S 0.007%, B 0.0005%, Ti 0.0035%, N 0.0032%, [O] 20ppm, [H] 1.4ppm, the balance is Fe and unavoidable impurity elements.

[0062] Billet specifications: 210mm×210mm

[0063] Rolled steel specifications:

[0064] Follow these steps to produce:

[0065] 1) Strong cooling and slow cooling of hot steel billets

[0066] After the surface temperature of the cast hot billet drops to 730℃, the billet enters the forced air cooling process. The billet is cooled rapidly by air cooling. The cooling rate is controlled at 0.30-0.35℃ / S, and the surface temperature of the billet is quickly reduced to 545℃. The billet is then put into the pit for slow cooling. The billet stays in the slow cooling pit for 96 hours.

[0067] 2) Rolling heating process

[0068] The temperature zone in the heating furnace is divided into 5 sections: preheating section, heating section 1, heating section 2, heating section 3 and soaking section. The temperature and time of each section are controlled as follows:

[0069] Preheating section: The preheating section is controlled at 553-567°C, and the ratio of the billet time in the preheating section to the soaking section is 3:1;

[0070] Heating stage: The temperature of the heating stage is controlled at 660-700℃, and the ratio of the time of the billet in the heating stage to the soaking stage is 2:1;

[0071] Heating stage 2: The temperature of heating stage 2 is controlled at 820-863°C, and the ratio of the time of the billet in heating stage 2 to the time of the soaking stage is 2:1;

[0072] Heating stage three: The temperature of heating stage three is controlled at 1052-1070℃, and the ratio of the time of heating stage one to the time of soaking stage is 1.5:1;

[0073] Soaking section: The temperature of the soaking section is controlled at 1120~1132℃.

[0074] 3) Control of air-fuel ratio of heating furnace

[0075] Preheating section: The air-fuel ratio in the preheating section is controlled between 1.53 and 1.60;

[0076] Heating stage: The air-fuel ratio of heating stage is controlled at 1.25-1.3;

[0077] Heating stage 2: The air-fuel ratio of heating stage 2 is controlled at 0.8-0.9;

[0078] Heating stage 3: The air-fuel ratio of heating stage 3 is controlled at 0.6-0.65;

[0079] Soaking section: The air-fuel ratio in the soaking section is controlled between 0.6 and 0.65.

[0080] 4) Atmosphere control in heating furnace

[0081] The focus is on controlling the oxygen content and moisture content in the atmosphere of the heating furnace.

[0082] Preheating section: The oxygen content and moisture content in the atmosphere of the preheating section are controlled at 4.0% to 4.8% and <0.04% respectively;

[0083] Heating stage: The oxygen content and moisture content in the atmosphere of the heating stage furnace are controlled at 2.2% to 2.8% and <0.03% respectively;

[0084] Heating stage 2: The oxygen content and moisture content in the atmosphere of the heating stage 2 furnace are controlled at 1.0% to 1.3% and <0.04% respectively;

[0085] Heating stage 3: The oxygen content and moisture content in the atmosphere of the heating stage 3 furnace are controlled at 0.7% to 1.0% and <0.03% respectively;

[0086] Soaking section: The oxygen content and moisture content in the atmosphere of the soaking section furnace are controlled at 0.5% to 0.6% and <0.03% respectively.

[0087] The decarburized layer of the steel billet is shown in Figure 1 .

[0088] Comparative Example 1

[0089] The steel grade in Example 1 was produced according to the conventional method. The decarburized layer of the obtained steel billet is shown in FIG. Figure 2 .

[0090] Example 2

[0091] Steel type: 45 (C 0.47%)

[0092] Steel billet chemical composition (wt%, smelting analysis): C 0.47%, Si 0.30%, Mn 0.75%, Cr 0.25%, Cu 0.05%, Ni 0.01%, Mo 0.0055%, V 0.0063%, Nb 0.0045%, As 0.0038%, Al 0.015%, P0.020%, S 0.006%, B 0.0008%, Ti 0.0031%, N 0.0039%, [O] 17ppm, [H] 1.9ppm, the balance is Fe and unavoidable impurity elements.

[0093] Billet specifications:

[0094] Rolled steel specifications:

[0095] Follow these steps to produce:

[0096] 1) Strong cooling and slow cooling of hot steel billets

[0097] After the surface temperature of the cast hot billet drops to 730℃, the billet enters the forced air cooling process. The billet is cooled rapidly by air cooling. The cooling rate is controlled at 0.30-0.35℃ / S, and the surface temperature of the billet is quickly reduced to 565℃. The billet is then put into the pit for slow cooling. The billet stays in the slow cooling pit for 150 hours.

[0098] 2) Rolling heating process

[0099] The temperature zone in the heating furnace is divided into 5 sections: preheating section, heating section 1, heating section 2, heating section 3 and soaking section. The temperature and time of each section are controlled as follows:

[0100] Preheating section: The preheating section is controlled at 560-570℃, and the ratio of the time of the billet in the preheating section to the soaking section is 3:1;

[0101] Heating stage: The temperature of the heating stage is controlled at 680-698°C, and the ratio of the time of the billet in the heating stage to the soaking stage is 2:1;

[0102] Heating stage 2: The temperature of heating stage 2 is controlled at 1000-1050℃, and the ratio of the time of the billet in heating stage 2 to the time of soaking stage is 2:1;

[0103] Heating stage three: The temperature of heating stage three is controlled at 1240-1250℃, and the ratio of the time of heating stage one to the time of soaking stage is 1.5:1;

[0104] Soaking section: The temperature of the soaking section is controlled at 1245~1255℃.

[0105] 3) Control of air-fuel ratio of heating furnace

[0106] Preheating section: The air-fuel ratio of the preheating section is controlled at 1.60-1.70;

[0107] Heating stage: The air-fuel ratio of heating stage is controlled at 1.26-1.3;

[0108] Heating stage 2: The air-fuel ratio of heating stage 2 is controlled at 0.9-1.0;

[0109] Heating stage 3: The air-fuel ratio of heating stage 3 is controlled at 0.64-0.70;

[0110] Soaking section: The air-fuel ratio in the soaking section is controlled at 0.62-0.65.

[0111] 4) Atmosphere control in heating furnace

[0112] The focus is on controlling the oxygen content and moisture content in the atmosphere of the heating furnace.

[0113] Preheating section: The oxygen content and moisture content in the atmosphere of the preheating section are controlled at 4.4% to 5.0% and <0.05% respectively;

[0114] Heating stage: The oxygen content and moisture content in the atmosphere of the heating stage furnace are controlled at 2.3% to 2.8% and <0.05% respectively;

[0115] Heating stage 2: The oxygen content and moisture content in the atmosphere of the heating stage 2 furnace are controlled at 1.3% to 1.5% and <0.04% respectively;

[0116] Heating stage 3: The oxygen content and moisture content in the atmosphere of the heating stage 3 furnace are controlled at 0.8% to 1.0% and < 0.03% respectively;

[0117] Soaking section: The oxygen content and moisture content in the atmosphere of the soaking section furnace are controlled at 0.6% to 0.7% and <0.03% respectively.

[0118] The decarburized layer of the steel billet is shown in Figure 3 .

[0119] Comparative Example 2

[0120] The steel grade in Example 2 was produced according to the conventional method, and the decarburized layer of the obtained steel billet was Figure 4 .

[0121] Implementation 3

[0122] Steel type: 45 (C 0.45%)

[0123] Steel billet chemical composition (wt%, smelting analysis): C 0.45%, Si 0.35%, Mn 0.71%, Cr 0.23%, Cu 0.02%, Ni 0.03%, Mo 0.0067%, V 0.0058%, Nb 0.0033%, As 0.0040%, Al 0.014%, P 0.015%, S 0.009%, B 0.0006%, Ti 0.0029%, N 0.0044%, [O] 14ppm, [H] 1.3ppm, the balance is Fe and unavoidable impurity elements.

[0124] Billet specifications: 210mm×210mm

[0125] Rolled steel specifications:

[0126] Follow these steps to produce:

[0127] 1) Strong cooling and slow cooling of hot steel billets

[0128] After the surface temperature of the cast hot billet drops to 730℃, the billet enters the forced air cooling process. The billet is cooled rapidly by air cooling. The cooling rate is controlled at 0.30-0.35℃ / S, and the surface temperature of the billet is quickly reduced to 530℃. The billet is then put into the pit for slow cooling. The billet stays in the slow cooling pit for 102 hours.

[0129] 2) Rolling heating process

[0130] The temperature zone in the heating furnace is divided into 5 sections: preheating section, heating section 1, heating section 2, heating section 3 and soaking section. The temperature and time of each section are controlled as follows:

[0131] Preheating section: The preheating section is controlled at 551-565°C, and the ratio of the billet time in the preheating section to the soaking section time is 3:1;

[0132] Heating stage: The temperature of the heating stage is controlled at 652-680℃, and the ratio of the time of the billet in the heating stage to the soaking stage is 2:1;

[0133] Heating stage 2: The temperature of heating stage 2 is controlled at 834-860℃, and the ratio of the time of the billet in heating stage 2 to the time of soaking stage is 2:1;

[0134] Heating stage three: The temperature of heating stage three is controlled at 1057-1075℃, and the ratio of the time of heating stage one to the time of soaking stage is 1.5:1;

[0135] Soaking section: The temperature of the soaking section is controlled at 1128~1140℃.

[0136] 3) Control of air-fuel ratio of heating furnace

[0137] Preheating section: The air-fuel ratio of the preheating section is controlled at 1.51-1.60;

[0138] Heating stage: The air-fuel ratio of heating stage is controlled at 1.23-1.3;

[0139] Heating stage 2: The air-fuel ratio of heating stage 2 is controlled at 0.8-0.87;

[0140] Heating stage 3: The air-fuel ratio of heating stage 3 is controlled at 0.62-0.70;

[0141] Soaking section: The air-fuel ratio in the soaking section is controlled between 0.6 and 0.65.

[0142] 4) Atmosphere control in heating furnace

[0143] The focus is on controlling the oxygen content and moisture content in the atmosphere of the heating furnace.

[0144] Preheating section: The oxygen content and moisture content in the atmosphere of the preheating section are controlled at 4.2% to 4.9% and <0.04% respectively;

[0145] Heating stage: The oxygen content and moisture content in the atmosphere of the heating stage furnace are controlled at 2.1% to 2.7% and <0.03% respectively;

[0146] Heating stage 2: The oxygen content and moisture content in the atmosphere of the heating stage 2 furnace are controlled at 1.2% to 1.5% and <0.04% respectively;

[0147] Heating stage 3: The oxygen content and moisture content in the atmosphere of the heating stage 3 furnace are controlled at 0.6% to 0.9% and <0.03% respectively;

[0148] Soaking section: The oxygen content and moisture content in the atmosphere of the soaking section furnace are controlled at 0.5% to 0.7% and <0.03% respectively.

[0149] The decarburized layer of the steel billet is shown in Figure 5 .

[0150] Comparative Example 3

[0151] The steel grade in Example 3 was produced according to the conventional method. The decarburized layer of the obtained steel billet is shown in FIG. Figure 6 .

[0152] The primary oxide scale and decarburized layer of the medium carbon steel produced by the above example are compared with those of the prior art as shown in Tables 1, 2 and Figure 1-6 By comparison, it can be seen that the primary oxide scale and decarburized layer of the medium carbon steel produced by the present invention are significantly reduced.

[0153] Table 1 Primary oxide scale thickness

[0154]

[0155] Table 2 Decarburized layer

[0156]

[0157] In summary, the present invention proposes a production method for controlling the decarburization and oxidation of the surface of a medium carbon steel billet, and adds an appropriate amount of Cr element and the like to the steel grade while meeting the content requirements of the standard. A dense oxide layer is formed on the surface of the billet by using the Cr element, different heating temperatures and furnace atmosphere, and further oxidation and decarburization of the billet surface are prevented by forming a dense oxide layer.

[0158] The present invention innovatively proposes the atmosphere in the furnace to achieve the formation of an oxide layer on the surface.

[0159] The present invention and the patent have strong adaptability to production process.

[0160] The process of the present invention is simple, and the control of decarburization and oxidation on the surface of medium carbon steel can be achieved without changing the existing production line or adding equipment.

[0161] The present invention develops a production method for controlling the decarburization and oxidation on the surface of a medium-carbon steel billet by appropriately adding Cr elements and innovating the process. The present invention has good economic and social benefits.

[0162] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as those generally understood by those skilled in the art to which this application belongs. It should also be understood that terms such as those defined in common dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless defined as herein.

[0163] Based on the above ideal embodiments of the present invention, the relevant staff can make various changes and modifications without departing from the technical concept of the present invention through the above description. The technical scope of the present invention is not limited to the contents of the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A medium carbon steel, characterized in that: It is composed of the following elements in mass percentage: C 0.25% to 0.50%, Si≤0.50%, Mn≤1.50%, Cr 0.20% to 0.25%, Cu≤0.10%, Ni≤0.20%, Mo≤0.15%, V≤0.10%, Nb≤0.04%, As≤0.01%, Al 0.010% to 0.020%, P≤0.025%, S≤0.015%, B≤0.0010%, Ti≤0.03%, N≤0.0050%, [O]≤20ppm, [H]≤2ppm, and the balance is Fe and unavoidable impurity elements.

2. The medium carbon steel according to claim 1, characterized in that: The element [O]≤15ppm.

3. The medium carbon steel according to claim 1, characterized in that: The mass content of the Cr element is 0.22% to 0.25%.

4. A production method for controlling the surface decarburization and oxidation of a medium carbon steel billet as claimed in any one of claims 1 to 3, characterized in that: The following steps are involved: Step 1) Strong cooling and slow cooling of cast hot billet: After the surface temperature of the hot billet in casting drops to 730°C, the billet enters the forced air cooling process. Through air cooling, the billet is quickly cooled down. The cooling rate is controlled at 0.30-0.35°C / S, and the surface temperature of the billet is quickly reduced to 520-570°C. Then the billet is slowly cooled in the pit. Step 2) Rolling heating process: The temperature zone in the heating furnace is divided into 5 sections: preheating section, heating section 1, heating section 2, heating section 3 and soaking section. The temperature and time of each section are controlled as follows: Preheating section: The preheating section is controlled at 550-570℃; Heating stage: The temperature of heating stage is controlled at 650-700℃; Heating stage 2: The temperature of heating stage 2 is controlled at 800~1050℃; Heating stage three: The temperature of heating stage three is controlled at 1050~1250℃; Soaking section: The temperature of the soaking section is controlled at 1120~1260℃; Step 3) Control of air-fuel ratio of heating furnace: Preheating section: The air-fuel ratio in the preheating section is controlled at 1.5 to 1.7; Heating stage: The air-fuel ratio of heating stage is controlled at 1.2-1.4; Heating stage 2: The air-fuel ratio of heating stage 2 is controlled at 0.8-1.0; Heating stage 3: The air-fuel ratio of heating stage 3 is controlled at 0.6-0.7; Soaking section: The air-fuel ratio in the soaking section is controlled at 0.6-0.65; Step 4) Control of oxygen content and moisture content in the atmosphere of the heating furnace: Preheating section: The oxygen content in the atmosphere of the preheating section is controlled at 4.0% to 5.0%, and the water content is controlled at <0.06%; Heating stage: The oxygen content in the atmosphere of the heating stage is controlled at 2.0% to 3.0%, and the water content is controlled at <0.06%; Heating stage 2: The oxygen content in the atmosphere of the heating stage 2 furnace is controlled at 1.0% to 1.5%, and the water content is controlled at <0.04%; Heating stage 3: The oxygen content in the atmosphere of the heating stage 3 furnace is controlled at 0.5% to 1.0%, and the water content is controlled at <0.03%; Soaking section: The oxygen content in the atmosphere of the furnace in the soaking section is controlled at 0.5% to 0.8%, and the water content is controlled at <0.03%.

5. The method for controlling the surface decarburization and oxidation of medium carbon steel billet according to claim 4, characterized in that: In the step 1), the steel billet is in the slow cooling pit for ≥ 72 hours.

6. The method for controlling surface decarburization and oxidation of medium carbon steel billet according to claim 4, characterized in that: In the step 2), the time ratio of the steel billet in the five temperature zones of the preheating section, the first heating section, the second heating section, the third heating section and the soaking section is 3:2:2:1.5:1.

Images