Steel for nitriding pot and production method and use thereof

Abstract

The application provides a nitriding pot steel and a production method and application thereof, and the composition is as follows: C: 0.0015%-0.0035%, Mn: 0.09%-0.20%, S: <=0.012%, P: <=0.015%, Si: <=0.030%, Al: 0.020%-0.040%, N: <=0.0030%, Ti: 0.035%-0.050%, and the rest is Fe and other inevitable impurities. Compared with the prior art, by controlling the content of carbon, manganese, titanium and the like in the steel, the steel can have certain strength, the problem of brittle cracking of the nitriding pot can be eliminated when the nitriding pot is prepared, and meanwhile, the steel can have good formability while having certain strength. By controlling the temperature parameter of the cold rolling process and cooperating with the hot rolling coiling temperature, the metallographic structure of the grain size of 8-10 levels is obtained, the stamping performance of the steel is improved, and the corrosion resistance of the steel is improved to a certain extent.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel smelting technology, specifically relating to a nitriding boiler steel, its production method, and its application. Background Technology

[0002] Since the preparation of nitriding pots usually involves spinning steel plates, cleaning the surface oil, and then heating them in a pit furnace to 500-550℃ for nitriding, the steel plates are required to have certain stamping properties to ensure that they can be spun. Too high a strength is not conducive to spinning, while too low a strength tends to result in larger grains, which can easily cause brittleness and make the surface more prone to rust spots after nitriding.

[0003] Currently, the strength, plasticity, and grain size of the steel used to prepare nitriding pots cannot simultaneously meet the manufacturing requirements. Patent CN 106222570 A, published on December 14, 2016, discloses a substrate for nitriding steel with excellent corrosion resistance and its production method. The composition is as follows: wt% C 0.010–0.10%, Mn 0.10–1.00%, Si 0.010–0.10%, P ≤0.02%, S ≤0.01%, Als 0.50–1.50%, N ≤0.005%, Cr 0.10–1.50%, Cu 0.01–1.00%, Ni 0.01–1.00%. The preparation steps are: smelting and continuous casting into a billet; heating the billet; hot rolling; coiling; cold rolling; annealing; leveling. Using traditional iron pots as the base material, this nitriding treatment effectively prevents rusting, resulting in high surface hardness, excellent corrosion resistance, stable high-temperature performance, and high fatigue strength. Low-carbon steel plates, after nitriding, can replace stainless steel and functional coated steel plates in the manufacture of woks, corrosion-resistant containers, etc. However, its low elongation affects its processing and forming performance.

[0004] Therefore, there is an urgent need to develop a type of steel that meets the requirements for nitriding pot preparation, possessing high strength, plasticity, and grain size to meet market demands. Summary of the Invention

[0005] The purpose of this invention is to provide a steel for nitriding pots and its production method. By controlling the content of carbon, manganese, titanium, etc., in the steel, it is possible to ensure that the steel has a certain strength, eliminate the problem of brittle fracture in nitriding pots during manufacturing, and simultaneously ensure that the steel has good formability while maintaining a certain strength. By controlling the temperature parameters of the cold rolling process and coordinating them with the hot rolling coiling temperature, a microstructure with a grain size of 8-10 is obtained, improving the stamping performance of the steel and, to a certain extent, enhancing its corrosion resistance.

[0006] Another objective of this invention is to provide an application of steel for nitriding pots, used in the manufacture of nitriding pots.

[0007] The specific technical solution of this invention is as follows:

[0008] A nitriding pot steel comprises the following components by weight percentage:

[0009] C: 0.0015%–0.0035%, Mn: 0.09%–0.20%, S: ≤0.012%, P: ≤0.015%, Si: ≤0.030%, Al: 0.020%–0.040%, N: ≤0.0030%, Ti: 0.035%–0.050%, with the remainder being Fe and other unavoidable impurities.

[0010] Preferably, the steel used for the nitriding pot comprises the following components by weight percentage:

[0011] C: 0.0018%–0.0028%, Mn: 0.09%–0.15%, S: ≤0.012%, P: ≤0.015%, Si: ≤0.030%, Al: 0.020%–0.035%, N: ≤0.0030%, Ti: 0.035%–0.045%, with the remainder being Fe and other unavoidable impurities.

[0012] The metallographic structure of the nitriding pot steel includes ferrite and cementite, wherein the volume percentage of ferrite is 95-99%, the volume percentage of cementite is 1-5%, and the grain size of ferrite is 8.0-10.

[0013] The steel used for the nitriding boiler has a yield strength of 140–175 MPa, a tensile strength of 290–310 MPa, and an elongation of A. 80 ≥44%, surface roughness Ra is 0.70~1.60μm.

[0014] The present invention provides a method for producing steel for nitriding boilers, including hot rolling of continuously cast billets, pickling and cold rolling, annealing, leveling and tension straightening.

[0015] The hot rolling includes rough rolling, and the thickness of the intermediate billet obtained by rough rolling is 15-20% of the thickness of the continuously cast billet, preferably 16-18%.

[0016] The process before rough rolling also includes heating the continuously cast billet at a temperature of 1200–1240°C.

[0017] Preferably, the horizontal rolling mill for roughing includes a first horizontal rolling mill and a second horizontal rolling mill, wherein the first horizontal rolling mill has 3 rolling passes and the second horizontal rolling mill has 3 or 5 rolling passes.

[0018] Preferably, the reduction parameters of the first horizontal rolling mill include: a first pass reduction rate of 20-24%, a second pass reduction rate of 18-22%, and a third pass reduction rate of 15-18%.

[0019] Preferably, the exit temperature of the rolled piece from the first horizontal rolling mill is 1030–1080°C.

[0020] Preferably, the rolled piece includes a descaling operation both when entering and leaving the first horizontal rolling mill;

[0021] Preferably, the reduction parameters when the second horizontal rolling mill has 3 rolling passes include: 29-35% reduction rate for the first pass, 32-38% reduction rate for the second pass, and 16-20% reduction rate for the third pass.

[0022] Preferably, the reduction parameters when the second horizontal rolling mill has 5 rolling passes include: 20-24% reduction rate for the first pass, 20-25% reduction rate for the second pass, 20-25% reduction rate for the third pass, 20-24% reduction rate for the fourth pass, and 10-15% reduction rate for the fifth pass.

[0023] Preferably, when the rolling width of the workpiece is >1300mm and / or the width reduction of the workpiece is >50mm, the second horizontal rolling mill has 5 rolling passes;

[0024] Preferably, the outlet temperature of the rolled piece in the second horizontal rolling mill is 1000–1030°C;

[0025] Preferably, the rolled piece includes a descaling operation before entering the second horizontal rolling mill.

[0026] The hot rolling also includes finishing rolling, which includes rolling the intermediate billet obtained from the rough rolling. The final rolling temperature of the rolled piece during finishing rolling is 860-900℃, preferably 865-895℃.

[0027] Preferably, the finishing rolling process further includes laminar flow cooling of the rolled piece, wherein the laminar flow cooling method is front-stage cooling, and the opening degree of the upper and lower manifolds of the front-stage cooling is 80-100%, more preferably 100%.

[0028] Preferably, the laminar cooling process further includes coiling the rolled piece at a temperature of 660–700°C, more preferably 665–695°C.

[0029] The pickling cold continuous rolling includes pickling and rolling the rolled piece obtained after hot rolling;

[0030] Preferably, the parameters of the pickling and cold rolling process include: pickling temperature 75-85℃, stretch leveler elongation 1.5-2.5%, cold rolling reduction 70-80%, roughness of the pickling and rolling S5 stand rolls 3.5-4.5μm, and unit rolling force of the pickling and rolling S5 stand rolls 5-7MN / m.

[0031] The temperature of the soaking zone during annealing is 780–820°C;

[0032] Preferably, the pickled and cold-rolled workpiece is placed in a vertical continuous annealing furnace for continuous annealing;

[0033] Preferably, the annealing is a continuous annealing process, including a preheating section, a heating section, a soaking section, a slow cooling section, a rapid cooling section, an over-aging section, and a final cooling section;

[0034] More preferably, the preheating section temperature is 140-160°C, the heating section temperature is 805-820°C, the soaking section temperature is 810-820°C, the slow cooling section temperature is 640-660°C, the rapid cooling section temperature is 390-440°C, the over-aging section temperature is 300-400°C, and the final cooling section temperature is 150-180°C.

[0035] The production method further includes leveling and tension straightening the annealed rolled piece, wherein the surface roughness of the leveling work roll is 3.0 to 3.50 μm and the leveling elongation is 0.3 to 0.5%.

[0036] The elongation of the tension straightener is 0.03–0.1%.

[0037] The production method of nitriding boiler steel provided by the present invention includes the following steps in sequence: desulfurization of molten iron, smelting in a converter, argon blowing, refining in an LF furnace, and vacuum decarburization in an RH furnace; continuous casting of refined molten steel into a continuous casting billet; hot rolling of the continuous casting billet, which includes rough rolling and finish rolling, and laminar cooling and coiling after finish rolling to obtain a hot-rolled coil; pickling of the hot-rolled coil, followed by cold continuous rolling of the pickled coil; leveling and tensioning to obtain nitriding boiler steel.

[0038] The steel used to produce nitriding pots according to the above method is either steel strip or steel plate.

[0039] This invention provides an application of steel for nitriding pots, used in the manufacture of nitriding pots. It exhibits good formability, does not show brittleness after nitriding, and shows no oxidation or rust spots in smoke tests after nitriding.

[0040] Compared with existing technologies, this invention, by controlling the content of carbon, manganese, and titanium in steel, ensures that the steel possesses a certain strength and eliminates the problem of brittle cracking during the preparation of the nitriding pot. Simultaneously, it maintains both strength and good formability. By controlling the temperature parameters of the cold rolling process and coordinating them with the hot rolling coiling temperature, a microstructure with a grain size of 8-10 is obtained, improving the steel's stamping performance and, to a certain extent, its corrosion resistance. No brittle cracking occurs after nitriding, and smoke tests after nitriding show no oxidation discoloration or rust spots. Attached Figure Description

[0041] Figure 1The metallographic structure of the nitriding boiler steel provided in Embodiment 1 of the present invention is shown in the figure.

[0042] Figure 2 This is a metallographic diagram of the steel used for nitriding boilers provided in Embodiment 4 of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0044] The present invention provides a nitriding pot steel comprising the following components by mass percentage:

[0045] C: 0.0015%–0.0035%, Mn: 0.09%–0.20%, S: ≤0.012%, P: ≤0.015%, Si: ≤0.030%, Al: 0.020%–0.040%, N: ≤0.0030%, Ti: 0.035%–0.050%, with the remainder being Fe and other unavoidable impurities.

[0046] Preferably, the steel used for the nitriding pot comprises the following components by weight percentage:

[0047] C: 0.0018%–0.0028%, Mn: 0.09%–0.15%, S: ≤0.012%, P: ≤0.015%, Si: ≤0.030%, Al: 0.020%–0.035%, N: ≤0.0030%, Ti: 0.035%–0.045%, with the remainder being Fe and other unavoidable impurities.

[0048] The design concept of this invention is as follows:

[0049] C: A strengthening element for steel. To control the strength of steel, its content is controlled between 0.0015% and 0.0035%.

[0050] Si: Degrades the surface properties of steel. As the Si content increases, the iron oxide scale on the steel becomes difficult to remove, affecting the effectiveness of the pickling process. Therefore, the silicon content must be controlled within a low range.

[0051] Mn: Appropriate amounts of Mn are used to supplement the strength of steel. Mn can also act as a deoxidizer, reducing the oxygen content in the product and thus improving its processability. Mn can also combine with S to form high-melting-point MnS, reducing the content of low-melting-point FeS and preventing the product from becoming hot brittle. This invention controls Mn to be 0.09% to 0.20%.

[0052] P: P is a harmful element in steel. Although it increases the strength of the material, it also increases the cold brittleness of the steel and reduces the plasticity of the material. Generally, the lower the better. In order to ensure the excellent performance of the product, the P content is controlled at ≤0.015%.

[0053] S: S is a harmful element in steel, which can form inclusions and reduce the ductility and toughness of steel. Generally, the lower the better. In order to ensure the excellent performance of the product, the S content is controlled to be ≤0.012%.

[0054] Al: Al is a strong deoxidizer and has a strong binding force with N. The AlN generated during the reaction process can reduce the aging of the finished product, and AlN can inhibit grain growth in steel. By controlling its proportion within the range of Al: 0.020% to 0.040% in this invention, it can refine the grains during steel heating and annealing.

[0055] Ti: Ti has a strong affinity for elements such as N, C, and S. In steel, it combines with carbon and nitrogen to form Ti(C,N) compounds, thereby fixing C and N elements in the steel, reducing their precipitation along austenite grain boundaries, and improving the mechanical properties and aging properties of the steel plate. At the same time, the fine and dispersed Ti(C,N) compounds precipitated at high temperatures can inhibit the growth of austenite grains at high temperatures and play a role in refining the grains. However, as the Ti content increases, TiN precipitates at the grain boundaries during nitriding, increasing the risk of nitriding cracking. In this invention, the Ti content in the steel is controlled to completely fix the C, N, and S elements in the steel, which is calculated to be 0.035% to 0.050%.

[0056] N: A strengthening element for steel. To improve the strength of steel, a small amount of N can be added. In this invention, N is controlled to be ≤0.0030%.

[0057] The production method of the steel used in the nitriding pots mentioned above includes hot rolling of continuously cast billets, pickling and cold rolling, annealing, leveling and tension straightening.

[0058] By controlling the elemental composition of the continuously cast billet within the aforementioned range and combining this with temperature control during the annealing process, the steel coil is held at a temperature of 780–820℃ in the soaking zone, resulting in a flattening elongation of 0.30–0.50%. This ensures the steel possesses sufficient strength and eliminates the problem of brittle fracture during nitriding pot preparation. Furthermore, it maintains both strength and good formability. By controlling the temperature parameters of the cold rolling process in conjunction with the hot rolling coiling temperature, a microstructure with a grain size of 8–10 is obtained, improving the steel's stamping performance and, to some extent, its corrosion resistance.

[0059] In an optional implementation, annealing includes a preheating section, a heating section, a soaking section, a slow cooling section, a rapid cooling section, an over-aging section, and a final cooling section.

[0060] The preheating section, heating section, soaking section, and slow cooling section constitute the first stage of annealing. Their main function is to recrystallize the ferrite in the steel and to grow the ferrite grains. However, as described above, due to the control of the temperature of the soaking section in this invention, the grain growth in the first stage will not be unlimited, but rather limited and controllable, thus ensuring the strength of the steel while avoiding brittle fracture.

[0061] The rapid cooling section, the over-aging section, and the final cooling section are the second stage of annealing. Their main function is to determine the distribution of cementite in the microstructure and the content of supersaturated solid solution carbon, ensuring that the quality of the steel meets the manufacturing requirements of the nitriding pot.

[0062] In this invention, the annealing process includes a preheating zone temperature of 140–160°C, a heating zone temperature of 805–820°C, a soaking zone temperature of 810–820°C, a slow cooling zone temperature of 640–660°C, a rapid cooling zone temperature of 390–440°C, an over-aging zone temperature of 300–400°C, and a final cooling zone temperature of 150–180°C.

[0063] In an optional embodiment, hot rolling includes rough rolling, wherein the thickness of the intermediate billet obtained by rough rolling is 15-20% of the thickness of the continuously cast billet, preferably 16-18%. For example, when the thickness of the continuously cast billet is 230 mm, the thickness of the intermediate billet obtained by rough rolling is 34.5-46 mm, more preferably 36.8-41.4 mm.

[0064] Preferably, the horizontal rolling mill for roughing includes a first horizontal rolling mill and a second horizontal rolling mill, wherein the first horizontal rolling mill has 3 rolling passes and the second horizontal rolling mill has 3 or 5 rolling passes.

[0065] It is understandable that the roughing process of hot rolling generally consists of a horizontal rolling mill and a vertical rolling mill. The horizontal rolling mill is used to adjust the thickness of the rolled piece, and the vertical rolling mill is used to adjust the width of the rolled piece. There are usually two horizontal and two vertical rolling mills, which are set up alternately. During rolling, the rolled piece passes through the first vertical rolling mill, the first horizontal rolling mill, the second vertical rolling mill, and the second horizontal rolling mill in sequence, and then returns in reverse along the above path for multiple rolling passes to ensure that the intermediate billet obtained after roughing meets the required specifications.

[0066] Preferably, in order to control the shape and thickness accuracy of the intermediate billet, the reduction rate of each roughing pass decreases sequentially. The reduction parameters of the first horizontal rolling mill include: 20-24% reduction rate for the first pass, 18-22% reduction rate for the second pass, and 15-18% reduction rate for the third pass.

[0067] Preferably, the exit temperature of the rolled piece from the first horizontal rolling mill is 1030–1080°C.

[0068] Preferably, in order to improve the surface quality of the rolled piece, descaling operations are included both when the rolled piece enters and leaves the first horizontal rolling mill. Specifically, the descaling operation in roughing can be performed using high-pressure water descaling.

[0069] Preferably, in order to control the shape and thickness accuracy of the intermediate billet, the reduction rate of each roughing pass decreases sequentially. The reduction parameters when the second horizontal rolling mill has 3 rolling passes include: 29-35% reduction rate for the first pass, 32-38% reduction rate for the second pass, and 16-20% reduction rate for the third pass.

[0070] Preferably, in order to control the shape and thickness accuracy of the intermediate billet, the reduction rate of each roughing pass decreases sequentially. The reduction parameters when the second horizontal rolling mill has 5 rolling passes include: 20-24% reduction rate for the first pass, 20-25% reduction rate for the second pass, 20-25% reduction rate for the third pass, 20-24% reduction rate for the fourth pass, and 10-15% reduction rate for the fifth pass.

[0071] Preferably, the reduction parameters for the second horizontal rolling mill when the rolling passes are 5 include: 20-23% reduction for the first pass, 22-25% reduction for the second pass, 21-24% reduction for the third pass, 21-23% reduction for the fourth pass, and 12-15% reduction for the fifth pass.

[0072] Preferably, in order to ensure that the thickness of the intermediate billet is within the aforementioned range, when the width of the continuously cast billet is large or the width reduction is large, it is necessary to increase the number of rolling passes. In some embodiments of the present invention, the above problem is solved by increasing the number of rolling passes of the second horizontal roll. Specifically, when the rolling width of the rolled piece is >1300mm and / or the width reduction of the rolled piece is >50mm, the number of rolling passes of the second horizontal roll mill is 5.

[0073] Preferably, the exit temperature of the rolled piece in the second horizontal rolling mill is 1000–1030°C.

[0074] Preferably, in order to ensure the rolling temperature and final rolling temperature of the second horizontal rolling mill, the workpiece is descaled only before entering the second horizontal rolling mill.

[0075] Because the steel used in nitriding pans has high strength, a 3+3 or 3+5 pattern is used for rough rolling. This means the first horizontal roll is used for three passes, and the second horizontal roll is used for three or five passes. This method ensures that the resulting steel has higher strength. The present invention controls the rough rolling parameters within the above range because the workpiece temperature is high during rough rolling, resulting in low deformation resistance, good plasticity, and a larger total reduction rate in the rough rolling mill, thus reducing the load on the finishing mill.

[0076] In an optional implementation, since roughing is a hot rolling process, the continuous casting billet needs to be heated before roughing to ensure effective deformation of the continuous casting billet into the rolled product. Heating is carried out in a heating furnace with a heating temperature of 1200-1240°C.

[0077] In an optional embodiment, hot rolling further includes finish rolling, which involves rolling the intermediate billet obtained from rough rolling. The final rolling temperature of the rolled piece during finish rolling is 860–900°C, preferably 865–895°C. Controlling the final rolling temperature within the above range allows for more complete recrystallization of ferrite, enabling the ferrite grains to grow to a suitable grain size.

[0078] Preferably, after finishing rolling, the rolled piece is further subjected to laminar flow cooling, which is front-stage cooling. The opening degree of the upper and lower manifolds for front-stage cooling is 80-100%, more preferably 100%. Controlling the opening degree of the upper and lower manifolds for cold flow cooling within the above range can effectively refine the grains.

[0079] Preferably, after laminar cooling, the rolled piece is coiled to obtain a hot-rolled coil. In order to meet the high strength requirements of the steel for nitriding pots, it is necessary to control the grain growth during the coiling process to prevent the grain growth from affecting the steel's properties due to excessively high coiling temperature. Therefore, the coiling temperature is 660-700℃, preferably 665-695℃.

[0080] In an optional embodiment, pickling cold rolling includes pickling the hot-rolled piece and then placing it in a pickling and rolling unit for rolling.

[0081] Preferably, the parameters of the pickling and cold rolling process include: pickling temperature 75-85℃, stretch leveler elongation 1.5-2.5%, cold rolling reduction 70-80%, roughness of the pickling and rolling S5 stand rolls 3.5-4.5μm, and unit rolling force of the pickling and rolling S5 stand rolls 5-7MN / m.

[0082] In an optional embodiment, the annealed rolled piece is further subjected to leveling and tension straightening, wherein the surface roughness of the leveling work roll is 3.0 to 3.50 μm, the elongation of the leveling is 0.30 to 0.50%, and the elongation of the tension straightening is 0.03 to 0.1%.

[0083] In an optional embodiment, the preparation of the continuously cast billet sequentially includes hot metal desulfurization, converter smelting, argon blowing of the smelted steel, refining through an RH furnace, and then continuous casting of the steel into a continuously cast billet. The above preparation process yields purer steel, eliminates surface inclusions and sand holes, and can be performed using existing smelting methods, as long as the elemental composition of the continuously cast billet is within the range described in this invention.

[0084] The following are several specific embodiments of the present invention:

[0085] Example 1

[0086] A method for producing steel for nitriding boilers includes the following steps:

[0087] S01. Preparation of continuous casting billet

[0088] The existing smelting method involves sequential desulfurization of molten iron, converter smelting, argon blowing of the molten steel, refining in an LF furnace, decarburization in an RH furnace, and then continuous casting of the molten steel into continuously cast billets. The composition of the continuously cast billets is: C: 0.0027%, Mn: 0.16%, S: 0.010%, P: 0.012%, Si: 0.021%, Al: 0.035%, N: 0.0022%, Ti: 0.045%, with the remainder being Fe and other unavoidable impurities.

[0089] S02, Hot-rolled:

[0090] S021. Heating: Place the continuously cast billet in a heating furnace and heat it to 1226℃.

[0091] S022, Rough Rolling: The heated continuously cast billet is placed in a rough rolling mill to produce an intermediate billet. The thickness of the continuously cast billet is 230 mm, and the thickness of the intermediate billet is 40 mm. Specific rough rolling process parameters are as follows:

[0092] The roughing horizontal rolling mill includes a first horizontal rolling mill and a second horizontal rolling mill.

[0093] The first horizontal rolling mill has three rolling passes: 24% reduction in the first pass, 21% reduction in the second pass, and 16% reduction in the third pass. The exit temperature of the rolled piece from the first horizontal rolling mill is 1040℃. To improve the surface quality of the rolled piece, high-pressure water descaling is used both when the rolled piece enters and leaves the first horizontal rolling mill.

[0094] Since the rolling width of the workpiece is 1250mm, the second horizontal rolling mill adopts three passes: the first pass has a reduction rate of 34%, the second pass has a reduction rate of 36%, and the third pass has a reduction rate of 18.37%.

[0095] The exit temperature of the rolled piece is 1000℃ at the second horizontal rolling mill. In order to ensure the rolling temperature and final rolling temperature of the second horizontal rolling mill, the rolled piece is only descaled before entering the second horizontal rolling mill.

[0096] S023. Finishing, Laminar Flow Cooling, and Coiling: The intermediate billet obtained from roughing is rolled to a thickness of 5.4 mm after finishing. The final rolling temperature during finishing is 880℃. After finishing, the billet is subjected to laminar flow cooling, specifically front-stage cooling, with both the upper and lower manifolds having a 100% opening. After laminar flow cooling, the billet is coiled at 680℃ to obtain a hot-rolled coil.

[0097] S03, pickling and cold continuous rolling

[0098] The hot-rolled coils obtained after hot rolling are pickled and then placed in a pickling and rolling unit for rolling. The rolling parameters include: the concentration of hydrochloric acid for pickling is 182 g / L, the pickling temperature is 81℃, the elongation of the tension leveler is 1.50%, the cold rolling reduction is 77.63%, the surface roughness of the pickling and rolling S5 stand rolls is 4.0 μm, the unit rolling force of the pickling and rolling S5 stand rolls is 5 MN / m, and the pickling and rolling thickness is 1.208 mm.

[0099] S04, Annealing

[0100] Annealing is performed in a vertical continuous annealing furnace. The pickled and cold-rolled workpiece is placed in the vertical continuous annealing furnace and sequentially passes through the preheating section, heating section, soaking section, slow cooling section, rapid cooling section, over-aging section and final cooling section.

[0101] The preheating section has a temperature of 150℃, the heating section has a temperature of 810℃, the soaking section has a temperature of 815℃, the slow cooling section has a temperature of 650℃, the rapid cooling section has a temperature of 430℃, the over-aging section has a temperature of 380℃, and the final cooling section has a temperature of 160℃.

[0102] S05, leveling and straightening

[0103] The parameters for leveling and tension straightening include: the surface roughness of the leveling work roll is 3.0 μm, the elongation of leveling is 0.35%, and the elongation of tension straightening is 0.1%.

[0104] In this embodiment, the nitriding pot steel obtained through the above steps is a steel strip. The surface roughness of the steel strip, as measured by a roughness tester, is 1.125 μm. The steel strip is then observed under a scanning electron microscope to obtain the following... Figure 1The results shown are from Figure 1 It is known that the steel strip structure provided in the embodiments of the present invention consists of ferrite and cementite, with the ferrite content being 97.4%, the cementite content being 2.6%, and the ferrite grain size being grade 8.0.

[0105] Example 2

[0106] This embodiment provides a method for producing steel for nitriding boilers. The specific steps are similar to those in Embodiment 1, with the only difference being:

[0107] In step S021, the heating temperature of the furnace is 1230℃.

[0108] In step S023, the final rolling temperature of the workpiece during finishing rolling is 890℃, and the coiling temperature is 690℃.

[0109] The temperature of the soaking zone in step S04 is 812℃.

[0110] The elongation rate of the smoothing in step S05 is 0.45%.

[0111] In this embodiment, the steel used for the nitriding pot prepared by the above steps is a steel strip, and the surface roughness of the steel strip is 1.08 μm as measured by a roughness tester.

[0112] Example 3

[0113] This embodiment provides a method for producing steel for nitriding boilers. The specific steps are similar to those in Embodiment 1, with the only difference being:

[0114] In step S021, the heating temperature of the furnace is 1228℃.

[0115] In step S023, the final rolling temperature of the workpiece during finishing rolling is 880℃, and the coiling temperature is 665℃.

[0116] The temperature of the soaking zone in step S04 is 814℃.

[0117] The elongation rate of the smoothing in step S05 is 0.40%.

[0118] In this embodiment, the steel used for nitriding pots prepared through the above steps is a steel strip, and the surface roughness of the steel strip is measured to be 1.093 μm by a roughness tester.

[0119] Example 4

[0120] This embodiment provides a method for producing steel for nitriding boilers, including the following steps:

[0121] S01. Preparation of continuous casting billet

[0122] The existing smelting method involves sequential desulfurization of molten iron, converter smelting, argon blowing, refining in an LF furnace, vacuum decarburization in an RH furnace, and then continuous casting of the molten steel into a continuously cast billet. The composition of the continuously cast billet is: C: 0.0032%, Mn: 0.18%, S: 0.009%, P: 0.013%, Si: 0.023%, Al: 0.030%, Ti: 0.042%, N: 0.0025%, with the remainder being Fe and other unavoidable impurities.

[0123] S02, Hot-rolled:

[0124] S021. Heating: Place the continuously cast billet in a heating furnace and heat it to 1232℃.

[0125] S022, Rough Rolling: The heated continuously cast billet is placed in a rough rolling mill to produce an intermediate billet. The thickness of the continuously cast billet is 230 mm, and the thickness of the intermediate billet is 42 mm. Specific rough rolling process parameters are as follows:

[0126] The roughing horizontal rolling mill includes a first horizontal rolling mill and a second horizontal rolling mill.

[0127] The first horizontal rolling mill has three rolling passes: 21% reduction in the first pass, 19% reduction in the second pass, and 17% reduction in the third pass. The exit temperature of the rolled piece from the first horizontal rolling mill is 1050℃. To improve the surface quality of the rolled piece, high-pressure water descaling is used both when the rolled piece enters and leaves the first horizontal rolling mill.

[0128] Since the rolling width of the workpiece is 1350mm, the second horizontal rolling mill adopts a five-pass rolling process. The reduction rate is 21% for the first pass, 22% for the second pass, 21% for the third pass, 21% for the fourth pass, and 10.60% for the fifth pass. The exit temperature of the workpiece in the second horizontal rolling mill is 1020℃. In order to ensure the rolling temperature and finishing temperature of the second horizontal rolling mill, the workpiece is only descaled before entering the second horizontal rolling mill.

[0129] S023. Finishing, Laminar Flow Cooling, and Coiling: The intermediate billet obtained from roughing is rolled to a thickness of 4.75 mm after finishing. The final rolling temperature during finishing is 880℃. After finishing, the billet is subjected to laminar flow cooling, specifically front-stage cooling, with both the upper and lower manifolds of the front-stage cooling system having a 100% opening.

[0130] After laminar cooling, the rolled piece is coiled at a temperature of 680℃ to obtain a hot-rolled coil.

[0131] S03, pickling and cold continuous rolling

[0132] The hot-rolled coils obtained after hot rolling are pickled and then placed in a pickling and rolling unit for rolling. The rolling parameters include: the concentration of hydrochloric acid for pickling is 165 g / L, the pickling temperature is 84℃, the elongation of the tension leveler is 1.5%, the cold rolling reduction rate is 79.01%, the surface roughness of the pickling and rolling S5 stand rolls is 3.5 μm, the unit rolling force of the pickling and rolling S5 stand rolls is 5.5 MN / m, and the pickling and rolling thickness is 0.997 mm.

[0133] S04, Annealing

[0134] Annealing is performed in a vertical continuous annealing furnace. The pickled and cold-rolled workpiece is placed in the vertical continuous annealing furnace and sequentially passes through the preheating section, heating section, soaking section, slow cooling section, rapid cooling section, over-aging section and final cooling section.

[0135] The preheating section has a temperature of 155℃, the heating section has a temperature of 809℃, the soaking section has a temperature of 810℃, the slow cooling section has a temperature of 660℃, the rapid cooling section has a temperature of 435℃, the over-aging section has a temperature of 380℃, and the final cooling section has a temperature of 160℃.

[0136] S05, leveling and straightening

[0137] The parameters for leveling and tension straightening include: the surface roughness of the leveling work roll is 3.5 μm, the elongation of leveling is 0.40%, and the elongation of tension straightening is 0.1%.

[0138] In this embodiment, the steel strip used in the nitriding pot, prepared through the above steps, has a surface roughness of 1.03 μm as determined by a surface roughness tester. The steel strip is then observed under a scanning electron microscope to obtain the following... Figure 2 The results shown are from Figure 2 It is known that the steel strip structure provided in the embodiments of the present invention is ferrite and cementite, and the volume content of ferrite is 98.42%, the volume content of cementite is 1.58%, and the grain size of ferrite is grade 8.5.

[0139] Example 5

[0140] This embodiment provides a method for producing steel for nitriding boilers. The specific steps are similar to those in Embodiment 1, with the only difference being:

[0141] In step S021, the heating temperature of the furnace is 1226℃.

[0142] In step S023, the final rolling temperature of the workpiece during finishing rolling is 885℃, and the coiling temperature is 685℃.

[0143] The temperature of the soaking zone in step S04 is 812℃.

[0144] In this embodiment, the steel used for the nitriding pot prepared by the above steps is a steel strip, and the surface roughness of the steel strip is 1.06 μm as measured by a roughness tester.

[0145] Example 6

[0146] This embodiment provides a method for preparing steel for nitriding boilers. The specific steps are similar to those in Embodiment 1, with the only difference being:

[0147] In step S021, the heating temperature of the furnace is 1231℃.

[0148] In step S023, the final rolling temperature of the workpiece during finishing rolling is 870℃, and the coiling temperature is 682℃.

[0149] The temperature of the soaking zone in step S04 is 809℃.

[0150] The elongation rate of the smoothing in step S05 is 0.40%.

[0151] In this embodiment, the steel used for nitriding pots prepared through the above steps is a steel strip, and the surface roughness of the steel strip is 1.103 μm as measured by a roughness tester.

[0152] Comparative Example 1

[0153] This comparative example provides a method for producing steel for nitriding boilers. The specific steps are the same as in Example 1, except that the annealing soaking temperature in step S04 includes: 760℃。

[0154] Comparative Example 2

[0155] This comparative example provides a method for producing steel for nitriding boilers. The specific steps are the same as in Example 1, except that the annealing soaking temperature in step S04 includes: 835℃。

[0156] Comparative Example 3

[0157] This comparative example provides a method for producing steel for nitriding boilers. The specific steps are the same as in Example 1, except that the final rolling temperature in step S023 is [not specified]. 850℃。

[0158] Comparative Example 4

[0159] This comparative example provides a method for producing steel for nitriding boilers. The specific steps are the same as in Example 1, except that the final rolling temperature in step S023 is [not specified]. 945℃。

[0160] Comparative Example 5

[0161] This comparative example provides a method for preparing nitriding pot steel, with the specific steps being the same as in Example 1, except that the coiling temperature in step S023 is... 620℃。

[0162] Comparative Example 6

[0163] This comparative example provides a method for preparing nitriding pot steel, with the specific steps being the same as in Example 1, except that the coiling temperature in step S023 is... 740℃。

[0164] Experimental Example 1

[0165] The nitriding boiler steels prepared by the methods of Examples 1-6 and Comparative Examples 1-6 were subjected to mechanical property testing according to GB / T228. Among these tests, yield strength, tensile strength, and elongation A were obtained through tensile testing. 80 The results are shown in Table 1.

[0166] Table 1 Mechanical properties of steel used in nitriding boilers

[0167]

[0168]

[0169] As shown in Table 1, the embodiments of the present invention, by controlling the composition and production method of the steel used in nitriding pots, produce steel with excellent mechanical properties that fully meet the requirements for nitriding pots. When used to prepare nitriding pots, the steel is first spun into a pot shape, then cleaned, and then subjected to gas nitriding in a pit furnace at a temperature generally between 500 and 550°C. After nitriding, its formability is good, and no brittle cracking occurs. Salt spray tests after nitriding show no oxidation or rust spots. In Comparative Example 1, the low temperature of the soaking zone during annealing leads to insufficient ferrite grain growth, resulting in a higher yield strength and affecting stamping performance. In Comparative Example 2, the high temperature of the soaking zone during annealing leads to excessive ferrite grain growth, resulting in lower yield strength and tensile strength, making it prone to cracking after nitriding and failing to meet the requirements for nitriding steel. In Comparative Example 3, the low final rolling temperature easily leads to hot rolling in a two-phase state, causing a mixed-grain structure and making stamping prone to cracking. Comparative Example 4 had an excessively high final rolling temperature, resulting in overly large hot-rolled ferrite grains and a low yield strength, failing to meet the requirements for nitriding steel. Comparative Example 5 had an excessively low coiling temperature, resulting in fine hot-rolled ferrite grains and a high yield strength, affecting stamping performance and failing to meet the requirements for nitriding steel. Comparative Example 6 had an excessively high coiling temperature, resulting in large hot-rolled ferrite grains, which easily caused brittle fracture and also failed to meet the requirements for nitriding steel.

[0170] The data underlined above do not meet the requirements of this invention.

[0171] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A type of steel for nitriding boilers, characterized in that, The steel used for the nitriding boiler comprises the following components by mass percentage: C: 0.0015%~0.0035%, Mn: 0.09%~0.20%, S: ≤0.012%, P: ≤0.015%, Si: ≤0.030%, Al: 0.020%~0.040%, N: ≤0.0030%, Ti: 0.035%~0.050%, with the remainder being Fe and other unavoidable impurities; The production method of the steel for nitriding boilers includes hot rolling of continuously cast billets, coiling, pickling and cold continuous rolling, annealing, leveling and tension straightening; The metallographic structure of the nitriding pot steel includes ferrite and cementite, wherein the volume percentage of ferrite is 95-99%, the volume percentage of cementite is 1-5%, and the grain size of ferrite is 8.0-10. The steel used for the nitriding boiler has a yield strength of 140~175MPa, a tensile strength of 290~310MPa, and an elongation of A. 80 ≥44%, surface roughness Ra is 0.70~1.60µm.

2. A method for producing nitriding boiler steel according to claim 1, characterized in that, The production method includes: hot rolling of continuously cast billets, coiling, pickling and cold rolling, annealing, leveling and tension straightening.

3. The production method according to claim 2, characterized in that, The hot rolling of the continuously cast billet includes finishing rolling, with a finishing rolling temperature of 860~900℃.

4. The production method according to claim 2, characterized in that, The winding temperature is 660~700℃.

5. The production method according to claim 2, characterized in that, The annealing process involves controlling the temperature of the soaking zone to be 780~820℃.

6. The production method according to claim 2 or 5, characterized in that, The annealing process includes a preheating zone temperature of 140-160℃, a heating zone temperature of 805-820℃, a soaking zone temperature of 810-820℃, a slow cooling zone temperature of 640-660℃, a rapid cooling zone temperature of 390-440℃, an over-aging zone temperature of 300-400℃, and a final cooling zone temperature of 150-180℃.

7. The production method according to claim 2, characterized in that, The flatness has a flatness elongation of 0.30~0.50%.

8. An application of the nitriding boiler steel according to claim 1, characterized in that, Used in the manufacture of nitriding pots.

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