High-temperature-resistant enamelled steel for high-formability water heater inner container and production method of high-temperature-resistant enamelled steel

Abstract

The invention relates to the technical field of strip steel production, and discloses high-temperature-resistant enamel steel for a high-formability water heater liner, which comprises the following chemical components in percentage by mass: 0.035 to 0.065 percent of C, 0.6 to 1.0 percent of Mn, 0.03 to 0.09 percent of Ti, 0.003 to 0.008 percent of S, 0.004 to 0.008 percent of N, less than or equal to 0.05 percent of Si, less than or equal to 0.015 percent of P and the balance of Fe and inevitable impurities. The invention further discloses a production method of the high-temperature-resistant enamelled steel for the high-formability water heater inner container. According to the enamelled steel for the high-temperature-resistant enameling-firing high-formability water heater inner container and the production method of the enamelled steel, the enamelled steel is good in formability, high in strength after enameling firing, excellent in fatigue life in the service process and good in adherence performance and fish scaling resistance at the same time.

Description

Technical Field

[0001] The invention relates to the technical field of strip steel production, and in particular to a high-temperature resistant enameling high-formability enameled steel for a water heater inner tank and a production method thereof. Background Art

[0002] Enameled steel for water heater tank is mainly used to manufacture water heater tank, which can effectively prevent scale and bacteria from growing and improve the sanitation and safety of water quality. At the same time, enamel is not easily corroded and oxidized, which can extend the service life of the water heater. Enamel material also has good thermal insulation and heat conduction properties, which can achieve rapid heating and long-term heat preservation.

[0003] The manufacturing process of the water heater tank usually includes forming, welding, pretreatment, enameling, enameling and other processes. The enameling process requires high-temperature firing to make the enamel material and the iron matrix closely bonded, and the enameling temperature is generally above 800°C. Therefore, the enameled steel for the water heater tank is required to have good comprehensive properties such as processing and forming performance, welding performance, and enameling performance. In addition, the water heater tank needs to have a certain strength after enameling, be able to withstand the pressure and impact of water, and have a good fatigue life. Due to the high enameling temperature, the yield strength of the steel base will decrease during the enameling process. The lower the strength of the enameled steel matrix, the lower the strength after enameling. In order to ensure the strength of the water heater tank after enameling, it is usually necessary to increase the strength of the enameled steel plate, which deteriorates the forming performance. Quality defects such as large rebound, local bulging, and cracking are prone to occur during the forming process. How to obtain good forming performance, high strength and excellent fatigue performance at the same time has become a key technical problem in the water heater tank industry.

[0004] After searching, the patent "A kind of low-carbon hot-rolled high-strength enameled steel with a yield strength of 360MPa and a production method thereof" (202310750782) provides a low-carbon hot-rolled high-strength enameled steel with a yield strength of 360MPa and a production method thereof, C: 0.050%~0.100%, Si0.010%~0.030%, Mn: 0.350~0.550%, P≤0.015%, S≤0.010%, Al: 0.025~0.070%, N≤0.010%, Ti: 0.020~0.070%, Nb: 0.010~0.050%, Mo: 0.050~0.190%, 2Ti+Nb=Mo, the remainder is Fe and unavoidable inclusions. The yield strength of the product is 400-440Mpa, the tensile strength can reach 500Mpa, the elongation after fracture is greater than 20%, and the yield strength after enameling can be maintained at more than 85% of the yield strength before enameling. However, the yield strength of the patented product is too high, the forming performance is poor, and the addition of Mo element leads to high cost.

[0005] The patent "A hot-rolled pickled enameled steel with a yield strength of ≥300MPa after enameling and a production method thereof" (CN202211254049.0) provides a hot-rolled pickled enameled steel with a yield strength of ≥300MPa after enameling and a production method thereof, with the composition: C: 0.04% to 0.07%; Si ≤ 0.03%; Mn: 0.50% to 0.65%; P ≤ 0.020%; S: 0.0 10%~0.025%; Al: 0.010%~0.060%; Ti: 0.030%~0.060%; Cu: 0.001%~0.050%, Cr: 0.001%~0.050%, V: 0.001%~0.040%, Mo: 0.001%~0.030%, Ni: 0.001%~0.030%, the remainder is Fe and unavoidable impurities. Through the production process design, the hot-rolled pickled enameled steel with excellent enameling performance is produced, and the yield strength reduction value before and after enameling is ≤50MPa, with excellent anti-scale explosion performance and adhesion performance. However, this patent mainly improves the thermal stability of steel by adding Cr, Ni, Mo, and V, and reduces the strength reduction during the enameling process, and the product alloy design cost is relatively high. Summary of the invention

[0006] The purpose of the present invention is to address the deficiencies of the above-mentioned technology and to provide a high-temperature resistant enameled steel with high formability for water heater inner tank and a production method thereof, which has good formability, high strength after enameling, excellent fatigue life during service, and good adhesion and anti-scale explosion performance.

[0007] To achieve the above-mentioned purpose, the high-temperature resistant enameled high-formability enameled steel for water heater inner tank designed by the present invention has the following chemical components in percentage by weight: C: 0.035-0.065%, Mn: 0.6-1.0%, Ti: 0.03-0.09%, S: 0.003-0.008%, N: 0.004-0.008%, Si≤0.05%, P≤0.015%, and the rest are Fe and unavoidable impurities.

[0008] The role and reasons for control of the main components and processes in the present invention:

[0009] C: Solid solution C will increase the yield strength and tensile strength of steel, and combine with Ti to form TiC as an effective hydrogen trap to improve the anti-scale explosion performance of enameled steel. However, as the carbon content increases, the plasticity will decrease. At the same time, solid solution C is not good for the enameling performance of enameled steel. On the one hand, the higher the carbon content, the higher the pearlite content. Pearlite is a two-phase structure of ferrite and lamellar cementite. Cementite does not react with the enamel layer, which reduces the bonding force between the matrix and the enamel layer. On the other hand, the higher the solid solution carbon content, the more CO and CO2 gases are generated during the enameling process. These gases destroy the compactness of the enamel layer, and pinhole defects will be generated in severe cases. In addition, when the C content exceeds 0.07%, it enters the peritectic steel range, and cracks are easily generated during the casting process, and even steel leakage in severe cases. In order to balance the mechanical properties, enameling performance and easy castability, the present invention controls the C content in the steel to be 0.035-0.065%.

[0010] Si: The main component of enamel powder is SiO2. During the enameling process, FeO formed by oxidation of the substrate combines with SiO2 in the enamel layer to form Fe2SiO4, forming a chemical bond. This chemical bond is one of the main mechanisms for the adhesion between the substrate and the enamel layer. Si in steel will form SiO2, which slows down the chemical reaction between the substrate and the enamel layer, which is not good for the adhesion between the substrate and the enamel layer. The weight percentage of Si is controlled to ≤0.05%.

[0011] Mn: Mn is dissolved in ferrite and austenite, which can improve strength and reduce plasticity. Too low manganese content will cause FeS precipitation and reduce the thermoplasticity of steel. Too high manganese content will make the strength too high and the formability worse. Taking all factors into consideration, the weight percentage of Mn is controlled to be 0.6-1.0%.

[0012] Ti: Ti can combine with C, N, S and other elements in steel to form second phase particles, becoming a good hydrogen storage trap, improving the anti-scale explosion performance of the steel plate. At the same time, Ti precipitates during the enameling process can hinder the growth of austenite grains and reduce the strength drop during the enameling process. However, too high Ti content will increase the strength of the material, reduce the plasticity of the material, and make the material's forming and processing performance worse. Taking all factors into consideration, the weight percentage of Ti is controlled to be 0.03-0.09%.

[0013] N: N combines with Ti to form TiN and Ti(C,N), which hinder the growth of austenite grains during the enamelling process and reduce the strength drop. The precipitation of TiN has little effect on the strength, but too high a nitrogen content will consume the Ti element, reducing the amount of TiC precipitation, which is not good for the scale explosion performance of the steel plate. At the same time, too high a nitrogen content will lead to poor plasticity of the material, so the N content is controlled to 0.004-0.008%.

[0014] S: S in steel combines with Ti to form TiS and Ti4C2S2, which hinder the growth of austenite grains during the enamelling process and reduce the strength drop. The precipitates of TiS and Ti4C2S2 have little effect on the strength, but S will consume the Ti element, reducing the amount of TiC precipitation, which is not good for the scale explosion performance of the steel plate. Therefore, S is controlled to 0.003-0.008%.

[0015] P: P is a harmful impurity element in steel. P in steel is easy to form segregation in the steel, reducing the toughness and welding performance of the steel. Therefore, the lower the P content, the better. Taking all factors into consideration, the P content is ≤0.015%.

[0016] A method for producing high-temperature resistant enameled steel with high formability for water heater inner tank, comprising the following steps:

[0017] A) Casting: The pulling speed is 4-5.5 m / min, and the thickness of the ingot is 50-80 mm. The formation of liquid TiN is reduced by rapid solidification of the thin slab. The size of the liquid TiN is too coarse, and the inhibitory effect on the growth of austenite grains during the enameling process is weak. In addition, the large-sized liquid TiN is also easy to cause stress concentration, which endangers the fatigue life of the liner. Therefore, the pulling speed of the ingot needs to be controlled to be ≥4 m / min, and the thickness of the ingot needs to be ≤80 mm. It should be noted that too high a pulling speed is not conducive to the stability of the molten steel during continuous casting, and it is easy to cause slag defects, which endangers the surface quality and steel purity of the product. Too thin a ingot makes the rolling reduction rate insufficient, which is easy to cause uneven organization, which is not conducive to the comprehensive quality of the product. Taking all factors into consideration, the pulling speed is controlled to be 4-5.5 m / min, and the thickness of the ingot is 50-80 mm;

[0018] B) Soaking: The soaking temperature is 1150-1190°C, which promotes the precipitation of TiN, TiS and Ti4C2S2 in the solid phase. At this time, the precipitated TiN, TiS and Ti4C2S2 are coarse in size, which has a certain inhibitory effect on the growth of austenite grains during the enamelling process, and will not significantly increase the room temperature strength of the steel. If the soaking temperature is too high, the precipitation thermodynamic driving force is small, and if the soaking temperature is too low, the deformation resistance of the material is easily increased significantly, which is not conducive to the control of plate shape and rolling stability. Considering comprehensively, the soaking temperature is controlled to be 1150-1190°C;

[0019] C) Finish rolling: 7-stand rolling, the start rolling temperature is 1030-1090℃, the first three passes reduction rate is 45-65%, and Ti(C,N), TiC, and Ti4C2S2 are precipitated and coarsened by high temperature and large reduction. These precipitate particles will not significantly increase the room temperature strength of the steel, but will inhibit the growth of austenite grains during the enameling process and reduce the strength drop during the enameling process. Based on the set soaking temperature and considering the temperature drop during the descaling process, the start rolling temperature is ≤1090℃. If the start rolling temperature is too low, the deformation resistance of the material will increase significantly, which is not conducive to the control of plate shape and rolling stability. Therefore, the start rolling temperature needs to be controlled at 1030-1090℃, and the first three passes reduction rate is too high, which is easy to The load distribution of the finishing rolling stand 7 is unbalanced, which is not good for the plate shape. The reduction rate of the first three passes is too low, resulting in an excessively high reduction rate of the rear stand, which promotes the dispersion and precipitation of Ti(C, N), TiC, and Ti4C2S2 in a large amount in a lower temperature area, resulting in a fine structure of the room temperature steel, high material strength, and poor formability. Therefore, the reduction rate of the first three passes is controlled to be 45-65%, and the final rolling temperature is 880-920℃, which promotes the coarsening and growth of Ti(C, N), TiC, Ti4C2S2 and other particles precipitated during the rolling process, and at the same time promotes the coarsening of austenite grains, reduces the nucleation position of ferrite precipitation during the cooling process, obtains coarse ferrite grains, reduces the room temperature strength of the steel, and improves the formability;

[0020] D) Laminar cooling: Laminar cooling is divided into three stages of cooling, cooling to 790-840°C at 8-30°C / s, then cooling at 5-10°C / s for 1-4s, and then cooling to 500-580°C at 60-120°C / s. The first stage of slow cooling increases the ferrite phase transformation temperature, and the ferrite phase transformation begins at 790-840°C. By cooling at 5-10°C / s for 1-4s, the austenite is completely transformed into coarse ferrite, and then cooled to the coiling temperature of 500-580°C at 60-120°C / s, the precipitation of TiC in ferrite is inhibited, and the precipitation strengthening effect of TiC is reduced;

[0021] E) Coiling: The coiling temperature is 500-580°C.

[0022] Preferably, in the step C), the starting rolling temperature is 1050-1090°C, the reduction rate of the first three passes is 55-65%, and the final rolling temperature is 900-920°C. The higher the starting rolling temperature, the greater the reduction rate of the first three passes, the more complete the precipitation of Ti(C,N), TiC, and Ti4C2S2, and the coarser the precipitate particles.

[0023] Preferably, in the step D), laminar cooling is performed at 8 to 30°C / s to 820 to 840°C, followed by cooling at 5 to 10°C / s for 3 to 4s, and then cooling at 90 to 120°C / s to 500 to 530°C. The higher the cooling temperature in the first stage is between 790 and 840°C, the longer the cooling time in the second stage is, the coarser the ferrite grains obtained are, the lower the coiling temperature is, the less TiC precipitated in the ferrite is, the lower the room temperature strength of the material is, and the better the forming performance is. When the cooling temperature in the first stage is >840°C, due to insufficient traction force for austenite to transform into ferrite, a complete phase transformation of austenite to ferrite fails to occur during the second cooling process, which will cause the austenite to form fine non-equiaxed ferrite or even bainite structure during the third rapid cooling process, resulting in a significant increase in material strength, decreased plasticity, and deterioration in forming performance. If the coiling temperature is lower than 500℃, the cooling rate in the third stage will be too high, which will easily cause uneven cooling and uneven stress distribution inside the material, which is not conducive to controlling the wave shape of the steel plate.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. Use Ti precipitates to hinder grain coarsening during the enameling process, reduce the strength drop of the water heater tank during the enameling process, and combine the solid solution strengthening of Mn to improve the strength of the tank after enameling;

[0026] 2. Promote the precipitation and coarsening of Ti precipitates in the high-temperature austenite stage through high temperature and high pressure reduction, and at the same time induce the formation of ferrite in a higher temperature area, inhibit the precipitation of TiC in ferrite, obtain coarse ferrite grains and Ti precipitate particles, reduce the room temperature strength of enameled steel, and improve the elongation. Ti precipitate particles can also serve as an effective hydrogen trap to improve the anti-scale performance;

[0027] 3. No need to add Mo, Ni, Nb and other elements, low alloy cost and good economic applicability;

[0028] 4. The product yield strength is 240-360MPa, the tensile strength is 340-450MPa, the product elongation is ≥35%, the strength drop after enameling is ≤10%, and it has good adhesion and anti-scale burst performance. The adhesion level is ≥1, and the anti-scale burst performance sensitive TH value is ≥10min / mm 2 It not only has good forming properties, but also has high strength and excellent fatigue performance after enameling. DETAILED DESCRIPTION

[0029] The technical solution of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than 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.

[0030] A high-temperature resistant enameled steel with high formability for water heater inner tank, characterized in that its chemical composition by mass percentage is: C: 0.035-0.065%, Mn: 0.6-1.0%, Ti: 0.03-0.09%, S: 0.003-0.008%, N: 0.004-0.008%, Si≤0.05%, P≤0.015%, and the rest is Fe and unavoidable impurities.

[0031] The production method of the above-mentioned high-temperature resistant enameled high-formability enameled steel for water heater inner tank comprises the following steps:

[0032] A) Casting: the casting speed is 4-5.5 m / min, and the thickness of the casting is 50-80 mm;

[0033] B) Soaking: Soaking temperature is 1150~1190℃;

[0034] C) Finish rolling: 7-stand rolling, the starting rolling temperature is 1030-1090°C, the first three passes have a reduction rate of 45-65%, and the final rolling temperature is 880-920°C;

[0035] D) Laminar cooling: Laminar cooling is divided into three stages of cooling, cooling at 8-30°C / s to 790-840°C, then cooling at 5-10°C / s for 1-4s, and then cooling at 60-120°C / s to 500-580°C;

[0036] E) Coiling: The coiling temperature is 500-580°C.

[0037] In the above step C), the more optimal start rolling temperature is 1050-1090°C, the reduction rate of the first three passes is 55-65%, and the final rolling temperature is 900-920°C. In step D), the more optimal laminar cooling is cooling to 820-840°C at 8-30°C / s, then cooling at 5-10°C / s for 3-4s, and then cooling to 500-530°C at 90-120°C / s.

[0038] Ten groups of examples are listed to illustrate the implementation of this process, as shown in Table 1, which is a chemical composition table of the ten examples:

[0039] Table 1 Chemical composition of each example (wt%)

[0040] Example C Si Mn P S Ti N 1 0.058 0.032 0.6 0.009 0.0030 0.083 0.0071 2 0.036 0.050 0.65 0.014 0.0050 0.046 0.004 3 0.055 0.014 0.84 0.015 0.0080 0.081 0.0074 4 0.053 0.012 0.9 0.011 0.0072 0.051 0.0055 5 0.062 0.017 1.0 0.009 0.0063 0.03 0.0048 6 0.053 0.033 0.74 0.008 0.0075 0.048 0.0054 7 0.035 0.042 0.88 0.011 0.0048 0.03 0.0072 8 0.065 0.014 0.89 0.013 0.0036 0.071 0.0048 9 0.047 0.045 0.66 0.012 0.0041 0.09 0.0065 10 0.043 0.03 0.93 0.014 0.0037 0.071 0.008

[0041] As shown in Table 2, the rolling process parameters of 10 embodiments are as follows:

[0042] Table 2 Rolling process parameters of each embodiment

[0043]

[0044]

[0045]

[0046] As shown in Table 3, typical performances of 10 embodiments are shown:

[0047] Table 3 Cooling process parameters and typical performance of each embodiment

[0048]

[0049] It can be seen that the high-formability enameled steel for water heater inner tank with high temperature resistance of the present invention utilizes Ti precipitates to hinder the coarsening of grains during the enameling process, reduces the strength drop of the water heater inner tank during the enameling process, and improves the strength of the inner tank after enameling in combination with the solid solution strengthening of Mn; by promoting the precipitation and coarsening of Ti precipitates in the high-temperature austenite stage through high temperature and high pressure reduction, while inducing the formation of ferrite in a higher temperature region, inhibiting the precipitation of TiC in ferrite, obtaining coarse ferrite grains and Ti precipitate particles, and reducing the enameling strength. The room temperature strength of porcelain steel is improved, the elongation is improved, and the Ti precipitate particles can also serve as an effective hydrogen trap to improve the anti-scale explosion performance; there is no need to add elements such as Mo, Ni, and Nb, the alloy cost is low, and the economic applicability is good; the product yield strength is 240-360MPa, the tensile strength is 340-450MPa, the product elongation is ≥35%, and the strength reduction after enameling is ≤10%. At the same time, it has good adhesion and anti-scale explosion performance, the adhesion level is ≥1, and the anti-scale explosion performance sensitive TH value is ≥10min / mm 2 It not only has good forming properties, but also has high strength and excellent fatigue performance after enameling.

[0050] It should be noted that the description of the above technical solutions is exemplary, and this specification can be embodied in different forms and should not be interpreted as being limited to the technical solutions set forth herein. On the contrary, providing these descriptions will make the disclosure of the present invention thorough and complete, and will fully convey the scope disclosed in this specification to those skilled in the art. In addition, the technical solutions of the present invention are limited only by the scope of the claims.

[0051] The various aspects disclosed for describing the present specification and claims are merely examples, and therefore, the present specification and claims are not limited to the details shown. In the above description, when the detailed description of the related known functions or configurations is determined to be unnecessary to obscure the focus of the present specification and claims, the detailed description will be omitted.

[0052] In the case of using “including”, “having” and “comprising” described in the present specification, unless otherwise used, there may also be another part or other parts, and the terms used may generally be in the singular but may also represent the plural form.

[0053] Finally, it should be pointed out that the above content is a further detailed description of the invention in combination with specific implementation methods. It cannot be considered that the specific implementation of the present invention is limited to these descriptions. For ordinary technicians in the technical field to which the present invention belongs, simple replacements made without departing from the concept of the present invention should be regarded as belonging to the protection scope of the present invention. The above embodiments are only more representative examples of the present invention. Obviously, the present invention is not limited to the above embodiments, and there can be many variations. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention should be considered to belong to the protection scope of the present invention.

[0054] At the same time, it should be noted that the description of the above technical solutions is exemplary, and this specification can be embodied in different forms and should not be construed as being limited to the technical solutions set forth herein. On the contrary, providing these descriptions will make the disclosure of the present invention thorough and complete, and will fully convey the scope disclosed in this specification to those skilled in the art. In addition, the technical solutions of the present invention are limited only by the scope of the claims. The features of the various embodiments of the present invention may be partially or completely combined or spliced ​​with each other, and may be performed in various different configurations as those skilled in the art may fully understand. The embodiments of the present invention may be performed independently of each other, or may be performed together in a mutually dependent relationship.

[0055] For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, and the above structures should be regarded as belonging to the protection scope of the present invention.

Claims

1. A high-temperature resistant enameling steel with high formability for water heater inner tank, characterized in that: The mass percentage of its chemical composition is: C: 0.035-0.065%, Mn: 0.6-1.0%, Ti: 0.03-0.09%, S: 0.003-0.008%, N: 0.004-0.008%, Si≤0.05%, P≤0.015%, and the rest is Fe and unavoidable impurities.

2. A method for producing high-formability enameled steel for water heater inner tanks resistant to high temperature enameling as claimed in claim 1, characterized in that: The steps include: A) Casting: the casting speed is 4-5.5 m / min, and the thickness of the casting is 50-80 mm; B) Soaking: Soaking temperature is 1150~1190℃; C) Finish rolling: 7-stand rolling, the starting rolling temperature is 1030-1090°C, the first three passes have a reduction rate of 45-65%, and the final rolling temperature is 880-920°C; D) Laminar cooling: Laminar cooling is divided into three stages of cooling, cooling at 8-30°C / s to 790-840°C, then cooling at 5-10°C / s for 1-4s, and then cooling at 60-120°C / s to 500-580°C; E) Coiling: The coiling temperature is 500-580°C.

3. The method for producing high-temperature resistant enameled steel with high formability for water heater inner tank according to claim 2, characterized in that: In the step C), the starting rolling temperature is 1050-1090°C, the reduction ratios of the first three passes are 55-65%, and the final rolling temperature is 900-920°C.

4. The method for producing high-temperature resistant enameled steel with high formability for water heater inner tank according to claim 2, characterized in that: In the step D), the laminar cooling is performed at 8-30°C / s to 820-840°C, then at 5-10°C / s for 3-4s, and then at 90-120°C / s to 500-530°C.

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