HP295 steel coil with low yield ratio and preparation method thereof

Abstract

The invention belongs to the technical field of steel coil materials, and relates to a low-yield-ratio HP295 steel coil and a preparation method thereof. According to the preparation method and component design of the low-yield-ratio HP295 steel coil, the requirement of the GB 5842-2023 standard is met, the thickness of the steel coil is set to range from 2.0 mm to 3.5 mm, the lower yield strength of the steel coil is larger than or equal to 295 MPa, the tensile strength meets the requirement of 440-560 MPa, the percentage elongation after fracture is larger than or equal to 26%, and the key yield ratio is smaller than or equal to 0.75. The steel coil is particularly suitable for application scenes with strict requirements on material performance.

Description

Technical Field

[0001] The invention belongs to the technical field of steel coil materials and relates to a low-yield ratio HP295 steel coil and a preparation method thereof. Background Art

[0002] With the vigorous development of my country's petroleum industry and the continuous improvement of people's living standards, household bottled liquefied petroleum gas has become popular in every corner of urban and rural areas and has become an indispensable part of daily life. Welded gas cylinders have gained widespread application due to their significant advantages in safety, energy saving, sanitation, and ease of use and transportation. This has not only driven the prosperity of the liquefied petroleum gas market, but also greatly promoted the rapid development of the production of hot-rolled steel plates and steel strips used to manufacture these welded gas cylinders. In particular, the demand for materials used in the manufacture of steel welded gas cylinders that can be used at normal ambient temperatures (-40°C to 60°C), have a test pressure of 3.2MPa, and can be repeatedly filled with liquefied petroleum gas is growing significantly. Such materials must possess good welding and stamping properties to ensure the quality and safety of the final product.

[0003] The specifications of liquefied gas cylinders mainly include four models: 2kg, 5kg, 15kg and 50kg. Among them, the output of 15kg cylinders commonly used in households accounts for about 90% of the total output, with a quantity of about 18 million. They are mainly made of HP295 material that complies with the provisions of GB / T 6653-2017 "Steel Plates and Steel Strips for Welded Gas Cylinders", and their thickness is usually between 2.0mm and 3.0mm. According to the GB / T6653-2017 standard, the mechanical properties of HP295 material are as follows: the lower yield strength is not less than 295MPa, and the tensile strength is between 440MPa and 560MPa; for products with a thickness of less than 3mm, the elongation after fracture should be at least 20%, while for products with a thickness equal to or greater than 3mm, the elongation after fracture must be guaranteed to be above 26%; in addition, when conducting a 180° bending test, no cracks should occur when using a bending core diameter of D=2.0a; the requirements for the yield strength ratio are based on negotiations between the supply and demand parties and are clearly marked in the contract. In principle, the yield strength ratio of steel plates and steel strips should not exceed 0.80.

[0004] However, to further improve the safety of liquefied petroleum gas (LPG) cylinders, China began formulating and implementing a new mandatory standard, GB 5842-2023, "Liquefied Petroleum Gas Cylinders," starting in 2023. This standard mandated the yield ratio requirement, previously a non-mandatory clause in GB / T 6653-2017, and set specific limits based on different tensile strength ranges: when the tensile strength is less than 490 MPa, the yield ratio must not exceed 0.75; if the tensile strength is equal to or greater than 490 MPa, the upper limit on the yield ratio is relaxed to 0.85. Previously, HP295 products produced in accordance with GB / T 6653-2017 had yield ratios ranging from 0.76 to 0.80. This meant that to meet the requirements of the new standard, especially for products with a tensile strength below 490 MPa, production processes needed to be optimized to effectively reduce the yield ratio and meet more stringent safety regulations, thereby ensuring that LPG cylinders could safely and reliably serve a wide range of users. Summary of the Invention

[0005] The purpose of the present invention is to address the above-mentioned problems existing in the prior art and propose a low yield ratio HP295 steel coil. The thickness of the steel coil is between 2.0mm and 3.0mm. The mechanical properties of the product meet the requirements of GB 5842-2023, with a lower yield strength ≥295MPa, a tensile strength of 440MPa-560MPa, an elongation after fracture ≥26%, and a yield ratio ≤0.75.

[0006] The purpose of the present invention can be achieved through the following technical solutions:

[0007] A low-yield ratio HP295 steel coil has a thickness of 2.0 mm to 3.5 mm, a lower yield strength of ≥295 MPa, a tensile strength of 440 MPa to 560 MPa, an elongation after fracture of ≥26%, and a yield ratio of ≤0.75.

[0008] In the above-mentioned low yield ratio HP295 steel coil, the low yield ratio welding bottle steel composition and its mass percentage content are: 0.14-0.18% C, 0.05-0.10% Si, 0.82-0.90% Mn, ≤0.015% P, ≤0.009% S, 0.025-0.040% Als, and the balance is Fe and unavoidable impurities.

[0009] The present invention also provides a method for preparing the above-mentioned low yield ratio HP295 steel coil, the method comprising the following steps:

[0010] S1, heating the ingot and then descaling it with high-pressure water;

[0011] S2, then rolling in a roughing mill, and then rolling in a finishing mill;

[0012] S3. After rolling, laminar cooling is carried out and coiling is carried out. The coiled steel coil is placed in a warehouse for slow cooling.

[0013] In the above-mentioned method for preparing a low-yield ratio HP295 steel coil, the temperature of the ingot out of the heating furnace is 1200±20° C., and the heating time of the ingot is 160-180 minutes.

[0014] In the above-mentioned method for preparing a low-yield ratio HP295 steel coil, the pressure of the descaling water should be ≥18 MPa.

[0015] In the above-mentioned method for preparing a low yield ratio HP295 steel coil, the rough rolling and finishing rolling temperatures should be controlled at 1050±20°C.

[0016] In the above-mentioned method for preparing a low yield ratio HP295 steel coil, the final rolling temperature after continuous rolling using a 7-stand finishing mill is 840° C.-880° C.

[0017] In the above-mentioned method for preparing a low yield ratio HP295 steel coil, laminar cooling is performed in a two-stage cooling mode, with the first stage cooling temperature being 730-770°C and the second stage cooling temperature being 580-620°C.

[0018] Preferably, the cooling rate in the first stage is 1-5°C / s, and the cooling rate in the second stage is 10-25°C / s.

[0019] During the production of low-yield ratio HP295 steel coils, this invention utilizes a unique two-stage cooling pattern for post-rolling cooling, designed to precisely control the internal microstructure of the steel to achieve ideal mechanical properties. The first stage, set at 730-770°C, reduces the cooling rate during this process, allowing the deformed austenite grains to precipitate high-temperature ferrite, which reduces the yield strength of the product.

[0020] The subsequent second cooling stage is controlled at a temperature of 580°C-620°C. Compared with the first cooling stage, this cooling process increases the cooling rate of the steel coil to suppress the precipitation of ferrite. Increasing the cooling rate also helps to enhance the eutectic phase transformation dynamics of the austenite structure, obtaining pearlite with finer interlamellar spacing (pearlite with finer interlamellar spacing can effectively enhance the tensile strength of the steel coil and significantly improve the plasticity of the steel coil) and ferrite hardened by the high cooling rate. Completing the cooling at 580°C-620°C mainly ensures that the final phase transformation products are ferrite and pearlite.

[0021] If the second stage cooling temperature is further lowered, abnormal structures such as bainite will appear in the phase transformation structure. These structures can significantly improve the strength, but will seriously damage the plasticity of the steel coil and are not suitable for welding bottle steel. Further increasing the second stage cooling temperature will not achieve a sufficient cooling rate, and the effect of refining the pearlite lamellar spacing is not obvious, which is not conducive to improving the tensile strength.

[0022] In this way, the two-stage cooling mode regulates the ferrite and pearlite structures within the steel coil, while retaining a portion of the soft phase (ferrite precipitates first) while increasing the strength of the hard phase (pearlite + hardened ferrite), thereby reducing the coil's yield strength ratio. Furthermore, the finely spaced ferrite and pearlite lamellar spacing improve the plasticity of the welded steel, enhancing the product's machinability.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. This invention proposes a preparation method and composition design for a low-yield ratio HP295 steel coil to meet the requirements of GB5842-2023. The coil thickness is set between 2.0mm and 3.5mm, with a lower yield strength of 295MPa or higher, a tensile strength of 440MPa-560MPa, an elongation of 26% or higher, and a critical yield ratio of 0.75 or lower. This coil is particularly suitable for applications with strict material performance requirements.

[0025] 2. This invention innovates the conventional post-rolling cooling process for steel coils, designing a two-stage cooling mode based on the steel coil's composition and mechanical property requirements. Combining the temperatures of this two-stage cooling mode with austenite deformation, this produces a microstructure composed of proeutectoid ferrite, pearlite with fine lamellar spacing, and hardened ferrite. This microstructure control reduces yield strength and increases tensile strength, thereby lowering the coil's yield-to-tensile ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a microstructure photograph of the steel coil prepared in Example 1;

[0027] Figure 2 is a microstructure photograph of the steel coil prepared in Example 2;

[0028] Figure 3 is a microstructure photograph of the steel coil prepared in Example 3;

[0029] Figure 4 is a microstructure photograph of the steel coil prepared in Example 4;

[0030] Figure 5 is a microstructure photograph of the steel coil prepared in Example 5;

[0031] Figure 6This is a microstructure photograph of the steel coil prepared in Comparative Example 1;

[0032] Figure 7 This is a microstructure photograph of the steel coil prepared in Comparative Example 2. DETAILED DESCRIPTION

[0033] The following are specific embodiments of the present invention, which further describe the technical solutions of the present invention, but the present invention is not limited to these embodiments.

[0034] Example 1:

[0035] S1, HP295 steel ingot composition and its mass percentage content are: 0.14% C, 0.06% Si, 0.86% Mn, 0.014% P, 0.006% S, 0.025% Als, and the balance is Fe and unavoidable impurities; the raw materials are smelted and cast into ingots in a continuous casting machine, the ingot thickness is 230 mm, and the steel coil thickness is 2.0 mm;

[0036] S2. The temperature of the billet out of the furnace is 1233°C. The billet is heated in the heating furnace for 165 minutes. The billet is treated with high-pressure descaling water (pressure is about 18 MPa) to remove the iron oxide scale on the surface.

[0037] S3, the billet is rolled through the roughing and finishing mills, and the thickness is reduced from 230mm to 2.0mm, with the final rolling temperature being 880℃;

[0038] S4. After rolling, laminar cooling is carried out and coiling is carried out. The laminar cooling adopts a two-stage cooling mode. The first stage cooling temperature is 770℃, the cooling rate is 3℃ / s, the second stage cooling temperature is 620℃, and the second stage cooling rate is 15℃ / s. After the steel coil is coiled, it is placed in a warehouse for slow cooling to room temperature.

[0039] Example 2:

[0040] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.17% C, 0.07% Si, 0.85% Mn, 0.012% P, 0.007% S, 0.036% Al, with the balance being Fe and unavoidable impurities; the ingot thickness is 230 mm, and the coil thickness is 2.3 mm;

[0041] S2. The temperature of the billet out of the furnace is 1224℃. The billet is heated in the heating furnace for 160min. The billet is treated with high-pressure descaling water (pressure is about 18MPa) to remove the iron oxide scale on the surface.

[0042] S3, the billet is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, the thickness is reduced from 230mm to 2.3mm, and the final rolling temperature is 867℃;

[0043] S4. After rolling, laminar cooling is carried out and coiling is carried out. The laminar cooling adopts a two-stage cooling mode. The first stage cooling temperature is 763°C, the cooling rate is 3°C / s, the second stage cooling temperature is 615°C, and the second stage cooling rate is 15°C / s. After the steel coil is coiled, it is placed in a warehouse for slow cooling to room temperature.

[0044] Example 3:

[0045] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.18% C, 0.08% Si, 0.88% Mn, 0.011% P, 0.007% S, 0.030% Al, with the balance being Fe and unavoidable impurities; the ingot thickness is 230 mm, and the coil thickness is 2.5 mm;

[0046] S2. The temperature of the billet out of the furnace is 1229°C. The billet is heated in the heating furnace for 168 minutes. The billet is treated with high-pressure descaling water (pressure is about 18 MPa) to remove the iron oxide scale on the surface.

[0047] S3, the slab is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, with the thickness reduced from 230mm to 2.5mm, and the final rolling temperature is 860℃;

[0048] S4. After rolling, laminar cooling is carried out and coiling is carried out. The laminar cooling adopts a two-stage cooling mode. The first stage cooling temperature is 750℃, the cooling rate is 3℃ / s, the second stage cooling temperature is 600℃, and the second stage cooling rate is 15℃ / s. After the steel coil is coiled, it is placed in the warehouse for slow cooling to room temperature.

[0049] Example 4:

[0050] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.18% C, 0.07% Si, 0.90% Mn, 0.012% P, 0.008% S, 0.032% Al, with the balance being Fe and unavoidable impurities. The raw materials are smelted and then cast into ingots in a continuous casting machine. The ingot thickness is 230 mm, and the steel coil thickness is 2.8 mm.

[0051] S2. The temperature of the billet out of the furnace is 1227℃. The billet is heated in the heating furnace for 163min. The billet is treated with high-pressure descaling water (pressure is about 18MPa) to remove the iron oxide scale on the surface.

[0052] S3, the billet is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, the thickness is reduced from 230mm to 2.8mm, and the final rolling temperature is 847℃;

[0053] S4. After rolling, laminar cooling is carried out and coiling is carried out. The laminar cooling adopts a two-stage cooling mode. The first stage cooling temperature is 745°C, the cooling rate is 3°C / s, the second stage cooling temperature is 587°C, and the second stage cooling rate is 15°C / s. After the steel coil is coiled, it is placed in a warehouse for slow cooling to room temperature.

[0054] Example 5:

[0055] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.17% C, 0.06% Si, 0.88% Mn, 0.012% P, 0.005% S, 0.039% Al, with the balance being Fe and unavoidable impurities; the ingot thickness is 230 mm, and the coil thickness is 3.5 mm;

[0056] S2. The temperature of the billet out of the furnace is 1220℃. The billet is heated in the heating furnace for 162 minutes. The billet is treated with high-pressure descaling water (pressure is about 18MPa) to remove the iron oxide scale on the surface.

[0057] S3, the slab is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, with the thickness reduced from 230mm to 3.5mm, and the final rolling temperature is 840℃;

[0058] S4. After rolling, laminar cooling is carried out and coiling is carried out. The laminar cooling adopts a two-stage cooling mode. The first stage cooling temperature is 730℃, the cooling rate is 3℃ / s, the second stage cooling temperature is 580℃, and the second stage cooling rate is 15℃ / s. After the steel coil is coiled, it is placed in a warehouse for slow cooling to room temperature.

[0059] Comparative Example 1:

[0060] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.17% C, 0.07% Si, 0.88% Mn, 0.015% P, 0.005% S, 0.034% Al, with the balance being Fe and unavoidable impurities; the ingot thickness is 230 mm, and the coil thickness is 2.5 mm;

[0061] S2. The temperature of the billet out of the furnace is 1223℃. The billet is heated in the heating furnace for 166 minutes. The billet is treated with high-pressure descaling water (pressure is about 18MPa) to remove the iron oxide scale on the surface.

[0062] S3, the billet is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, the thickness is reduced from 230mm to 2.5mm, and the final rolling temperature is 863℃;

[0063] S4. After rolling, the coiling is carried out at a coiling temperature of 635°C. After the coil is coiled, it is placed in a warehouse for slow cooling.

[0064] Comparative Example 2:

[0065] The composition and mass percentage of S1 and HP295 ingots are as follows: 0.17% C, 0.06% Si, 0.88% Mn, 0.012% P, 0.005% S, 0.039% Al, with the balance being Fe and unavoidable impurities; the ingot thickness is 230 mm, and the coil thickness is 3.0 mm;

[0066] S2. The temperature of the billet out of the furnace is 1203℃. The billet is heated in the heating furnace for 176 minutes. The billet is treated with high-pressure descaling water (pressure is about 18MPa) to remove the iron oxide scale on the surface.

[0067] S3, the billet is subjected to rough rolling at 1050℃ and continuous rolling in a 7-stand finishing mill, the thickness is reduced from 230mm to 3.0mm, and the final rolling temperature is 859℃;

[0068] S4. After rolling, the coiling is carried out at a coiling temperature of 623°C. After the coil is coiled, it is placed in a warehouse for slow cooling.

[0069] Table 1: Performance test results of steel coils prepared in Examples 1-9 and Comparative Examples 1-2

[0070]

[0071]

[0072] Depend on Figure 1-5 As can be seen, the microstructure of the HP295 product corresponding to the present invention is primarily composed of three components: first, relatively coarse polygonal ferrite (characteristic of proeutectoid ferrite); second, relatively fine ferrite (precipitated at lower temperatures (580°C-620°C)), primarily polygonal in shape; and third, pearlite (shown in black in the figure). The coarse polygonal ferrite helps reduce yield strength, while the fine ferrite and pearlite ensure the product's tensile strength. This multi-dimensional structure effectively reduces the yield-to-tensile ratio of the HP295 product.

[0073] Depend on Figure 6-7 As can be seen, the microstructure of the HP295 product obtained using the comparative process is primarily composed of fine ferrite and pearlite. Furthermore, the high cooling rate results in relatively fine ferrite grains, and the ferrite morphology is mostly elongated or acicular, with polygonal forms being rare. The presence of this ferrite not only increases tensile strength but also significantly enhances yield strength, thereby improving the yield-to-strength ratio of the HP295 product.

[0074] In summary, the key to the present invention lies in the cooling mode of the steel coil after rolling. A two-stage cooling mode is adopted: the cooling temperature of the first stage is set at 730-770°C, and the cooling temperature of the second stage is set at 580-620°C. The temperatures of the two-stage cooling mode are combined with the austenite deformation to obtain proeutectoid ferrite, pearlite with fine interlamellar spacing, and hardened ferrite structure; through the regulation of the structure, the yield strength is reduced and the tensile strength is increased, thereby reducing the yield strength ratio of the steel coil.

[0075] The parts of the embodiment herein that are not exhaustive of the midpoint values ​​of the technical scope claimed for protection by the present invention and the new technical solutions formed by equivalent replacement of single or multiple technical features in the technical solutions of the embodiments are also within the scope claimed for protection by the present invention; at the same time, in all the embodiments listed or not listed in the solutions of the present invention, each parameter in the same embodiment merely represents an example of its technical solution (i.e., a feasible solution), and there is no strict coordination and limitation relationship between the parameters, wherein the parameters can be replaced with each other without violating the axioms and the claims of the present invention, unless otherwise stated.

[0076] The technical means disclosed in the solutions of the present invention are not limited to the technical means disclosed in the above technical means, but also include technical solutions composed of any combination of the above technical features. The above is a specific embodiment of the present invention. It should be noted that for those skilled in the art, various improvements and modifications can be made without departing from the principles of the present invention, and such improvements and modifications are also considered to be within the scope of protection of the present invention.

[0077] The specific embodiments described herein are merely illustrative of the spirit of the present invention. Persons skilled in the art may make various modifications, additions, or substitutions to the described specific embodiments without departing from the spirit of the present invention or exceeding the scope of the appended claims.

Claims

1. A low yield ratio HP295 steel coil, characterized in that: The steel coil has a thickness of 2.0 mm to 3.5 mm, a lower yield strength of ≥295 MPa, a tensile strength of 440 MPa to 560 MPa, an elongation after fracture of ≥26%, and a yield strength ratio of ≤0.

75.

2. The low yield ratio HP295 steel coil according to claim 1, characterized in that: The steel coil composition and its mass percentage content are: 0.14-0.18% C, 0.05-0.10% Si, 0.82-0.90% Mn, P≤0.015%, S≤0.009%, 0.025-0.040% Als, and the balance is Fe and unavoidable impurities.

3. A method for preparing the low yield ratio HP295 steel coil according to claim 1, characterized in that: The method comprises the following steps: S1, heating the ingot and then descaling it with high-pressure water; S2, then rolling in a roughing mill, and then rolling in a finishing mill; S3. After rolling, laminar cooling is performed and coiling is carried out, and finally slow cooling treatment is performed to obtain HP295 steel coil.

4. The method for preparing a low yield ratio HP295 steel coil according to claim 3, characterized in that: The temperature of the ingot out of the heating furnace is 1180-1220℃, and the heating treatment time is 160-180min.

5. The method for preparing a low yield ratio HP295 steel coil according to claim 3, characterized in that: The pressure of high-pressure water is 22-30Mpa.

6. The method for preparing a low yield ratio HP295 steel coil according to claim 3, characterized in that: The final rolling temperature of the finishing mill is 840℃-880℃.

7. The method for preparing a low yield ratio HP295 steel coil according to claim 3, characterized in that: Laminar cooling adopts a two-stage cooling mode, with the first stage cooling temperature being 730-770°C and the second stage cooling temperature being 600°C-625°C.

8. The method for preparing a low yield ratio HP295 steel coil according to claim 7, characterized in that: The cooling rate of the first stage is 1-5℃ / s, and the cooling rate of the second stage is 10-25℃ / s.

9. The method for preparing a low yield ratio HP295 steel coil according to claim 3, characterized in that: The coiling temperature is 580℃-620℃.

10. Use of the low yield ratio HP295 steel coil according to claim 1 in welding gas cylinder steel.

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