Low-yield-ratio hot rolled steel coil for welded gas cylinder and preparation method of low-yield-ratio hot rolled steel coil

Abstract

The invention relates to the technical field of metallurgical hot-rolled plate strip steel, in particular to a steel low-yield-ratio hot-rolled coil for a welded gas cylinder and a preparation method thereof.The hot-rolled coil comprises, by weight, 0.14%-0.16% of C, 0.06%-0.10% of Si, 0.85%-0.95% of Mn, smaller than or equal to 0.020% of P, smaller than or equal to 0.008% of S, 0.020%-0.045% of Als, smaller than or equal to 0.005% of N and the balance Fe and inevitable impurities; the strength and toughness of the welding cylinder steel are guaranteed by properly adjusting the content of Mn and Si, the structure morphology of the steel is controlled through the hot rolling TMCP controlled rolling and controlled cooling process, and the method has the beneficial effects that under the condition that the yield strength and the tensile strength are improved, a hot rolled coil with the low yield-tensile ratio is produced, and the strength and toughness of the welding cylinder steel are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of metallurgical hot-rolled plate and strip steel, in particular to a low-yield ratio hot-rolled steel coil for welding gas cylinders and a preparation method thereof. Background Art

[0002] Welding gas cylinders generally refer to steel cylinders containing flammable and explosive liquids or gases. They are special equipment used to store gases during the welding process. These cylinders usually have the ability to withstand high pressures and can safely store and transport the various gases required for welding. They are an indispensable and important part of the welding process.

[0003] The construction of welding gas cylinders must meet stringent safety standards. Generally speaking, welding gas cylinders consist of a cylinder body, valve, and accessories. The cylinder body is made of high-strength material to withstand the pressure of the gas inside. To ensure the performance of the steel used in welding gas cylinders, strict mechanical properties such as high elongation and yield-to-strength ratio are required.

[0004] The Mn and Si contents of traditional welded gas cylinders affect the strength and toughness of the welded cylinder steel. Too high Si content may increase the ductile-brittle transition temperature and deteriorate the toughness, making it impossible to control the steel's microstructure. The yield strength ratio of the welded cylinder steel is too large, and the toughness of the welded cylinder steel is low. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention provides a low-yield ratio hot-rolled steel coil for welding gas cylinders and a preparation method thereof. The Mn and Si contents are appropriately adjusted to ensure the strength and toughness of the welding cylinder steel. The microstructure of the steel is controlled by the hot-rolling TMCP controlled rolling and controlled cooling process to improve the yield strength and tensile strength, thereby producing a low-yield ratio hot-rolled coil with high strength and high toughness for welding gas cylinders.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A low-yield ratio hot-rolled steel coil for welding gas cylinders. The composition of the hot-rolled coil is as follows by weight percentage: C: 0.14% to 0.16%, Si: 0.06% to 0.10%, Mn: 0.85% to 0.95%, P: ≤0.020%, S: ≤0.008%, Als: 0.020% to 0.045%, N: ≤0.005%, and the remainder is Fe and unavoidable impurities.

[0008] Furthermore, the yield strength of the low-yield-to-tensile ratio hot-rolled coil of steel for welding gas cylinders is ≥360 MPa, the tensile strength is 460-560 MPa, the elongation after fracture is ≥26%, and the yield-to-tensile ratio is less than 0.77.

[0009] Furthermore, the method for preparing the low yield ratio hot-rolled coil of steel for welding gas cylinders includes the following contents:

[0010] Steelmaking process: Steel plates are rolled from continuous casting billets that are smelted in a converter, refined in an electric furnace, and poured. The converter uses slag tapping. During the casting process, the tundish uses a submerged nozzle to protect the casting throughout the entire process.

[0011] Hot rolling process: heating furnace temperature: 1180 ~ 1210 ℃; rough rolling final rolling temperature: 1000 ~ 1060 ℃; finishing rolling final rolling temperature: 840 ~ 880 ℃; coiling temperature: 630 ~ 670 ℃;

[0012] Cooling process: adopts rear-stage cooling, and the strip is air-cooled for 10 to 20 seconds after finishing rolling. The temperature drop change parameters are predicted by the cooling water laminar flow control model to track the strip position and calculate the manifold position for cooling water. After air cooling, the manifold water cooling starts. The rear-stage manifold is set according to the thickness of the production specifications and the water-cooling manifold position predicted and calculated by the laminar flow control model to control 1-3 groups of manifolds to discharge water densely. As the rolling speed increases, the water discharge method is first sparse and then dense. According to the feedback input from the pyrometer detection, the laminar flow calculation model is used to control the number of opening and closing groups of water-cooling manifolds, and the cooling rate is controlled to be not less than 25℃ / s.

[0013] Furthermore, the finishing rolling temperature is controlled to obtain pearlite and ferrite with uniform structure, with pearlite accounting for 20-30% and ferrite accounting for 70-80%.

[0014] Furthermore, the rear cooling is precisely controlled to obtain a uniform structure with a grain size of more than 10 levels.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1) Reasonable chemical composition ensures that the hot-rolled coil has high strength and toughness indicators, while preventing excessive Si content from increasing the ductile-brittle transition temperature and deteriorating toughness, thereby improving the high strength and toughness of the hot-rolled coil and the high-pressure bearing capacity of the container after the hot-rolled coil is prepared into welded gas cylinders.

[0017] 2) The high precision of final rolling temperature control eliminates the adverse effects of too low final rolling temperature on the yield ratio. By effectively controlling the amount of pearlite above 850°C, the yield ratio can be reduced while ensuring strength, thereby improving the toughness of the hot-rolled coil and the structural performance of the steel used for welding gas cylinders, ensuring the safety and convenience of welding gas cylinders during use.

[0018] 3) The rear cooling method can prevent the residual deformation defects in the austenite from being restored, ensure the deformation-induced phase transformation effect, prevent the coarsening of ferrite grains, and improve the strength of the hot-rolled coil of steel for welding gas cylinders. The rear cooling method accurately controls the coiling temperature, ensures a low yield ratio, and ensures the quality and efficiency of steel for welding gas cylinders.

[0019] 4) The yield strength of the low yield ratio hot rolled coil of steel for welding gas cylinders shall be ≥360MPa, the tensile strength shall be 460-560MPa, the elongation after fracture shall be ≥26%, and the yield ratio shall be less than 0.77. DETAILED DESCRIPTION

[0020] The specific embodiments of the present invention are further described below:

[0021] The following details the mechanism of action of the chemical components of the present invention.

[0022] C often forms carbides with other alloying elements in steel, which has a strengthening effect and helps improve the plasticity and toughness of steel. From the perspective of ensuring the strength of the steel plate, it is hoped that the C content will be maintained at a high level. However, a high C content will cause segregation in the steel and affect the toughness of the steel plate. Therefore, under the premise of ensuring the strength of the steel plate, the C content is controlled at 0.14% to 0.16%.

[0023] Si has a deoxidizing and desulfurizing effect in steel and can improve the strength of steel plates through solid solution strengthening. Adding an appropriate amount of Si to steel can increase the strength and hardness of the ferrite in the steel, making the ferrite structure more uniform, and can increase the elastic limit, yield strength and yield ratio, as well as fatigue strength and fatigue ratio of the steel. It can also increase strength and improve localized corrosion resistance to a certain extent. However, if the Si content is too high, it will have a negative impact on the toughness of the steel. Therefore, the Si content should be limited to Si: 0.06% to 0.10%.

[0024] Mn has a desulfurizing effect in steel, and Mn is easy to form MnS inclusions with S. The high strength and low toughness produced by the segregation of Mn in steel will increase the tendency of post-weld cracking and have an adverse effect on the toughness of the steel. Therefore, the Mn content should not be too high. Taking into account the strength, toughness and welding performance of the steel plate, the Mn content is set in the range of 0.85% to 0.95% in the composition design.

[0025] P is a harmful impurity element that increases the brittleness of steel and reduces its plasticity, toughness, and weldability. The lower its content, the more beneficial it is to steel performance. Considering on-site production processes, the P content is controlled at ≤ 0.020%.

[0026] Sulfur is a harmful element. Sulfides are generally distributed in a network pattern at grain boundaries, significantly reducing the toughness of steel and easily causing cracking during processing. Sulfur also causes thermal cracking during welding. Generally, the sulfur content in steel is strictly controlled. The S content is controlled to ≤ 0.008%.

[0027] Als is a deoxidizing element in steel. It can effectively refine grains by forming AlN in the steel. In order to obtain a uniform structure with a grain size of more than 10 levels after cooling, the Als content is limited to the range of 0.020 to 0.045%.

[0028] Nitrogen can cause strain aging and increase notch sensitivity in steel, and its effect is far greater than that of phosphorus. It increases the brittleness of steel, reduces impact toughness, and affects the performance of finished steel products. Therefore, the N content is controlled at ≤0.005%.

[0029] Preparation method of an embodiment of the present invention: Steelmaking process: Steel plates are rolled from continuous casting billets that are smelted in a converter, refined in an electric furnace, and cast; slag tapping is used in the converter; during the casting machine production process, an immersion nozzle is used in the tundish to protect the casting throughout the entire process. The purpose of the steelmaking process is to obtain casting billets with pure steel quality and uniform composition.

[0030] Hot rolling process: heating furnace temperature: 1180~1210℃; rough rolling final rolling temperature: 1000~1060℃; finishing rolling final rolling temperature: 840~880℃; coiling temperature: 630~670℃. The function of hot rolling process is to obtain uniform pearlite and ferrite by controlling the austenite deformation temperature and phase transformation temperature, and the finishing rolling final rolling temperature. Pearlite accounts for 20~30% and ferrite accounts for 70~80%. It has good plasticity and toughness, as well as applicable strength, creating good process conditions for subsequent surface quality control and precise control of organization.

[0031] Cooling process: A back-end cooling system is employed. After finishing, the strip is air-cooled for 10-20 seconds. A laminar flow model tracks the strip's position and predicts temperature drop parameters, calculating the manifold position for water cooling. After air cooling, water cooling begins in the manifolds. The model's initial pre-set settings control the dense water flow from one to three manifolds, depending on the production thickness. As the rolling speed increases, the water flow pattern shifts from sparse to dense. Feedback from pyrometer measurements after laminar flow is fed into the laminar flow calculation model to determine the number of manifold openings and closings, maintaining a cooling rate of no less than 25°C / s. This back-end cooling method ensures deformation-induced phase transformation, prevents ferrite grain coarsening, and achieves a uniform microstructure with a grain size of 10 or greater, improving the strength of hot-rolled coils used for welding gas cylinders. The back-end cooling system precisely controls the coiling temperature, ensuring a low yield ratio and ensuring the quality and efficiency of the steel used for welding gas cylinders.

[0032] Table 1 is the basic parameters of the hot rolled coils in the embodiment (%)

[0033]

[0034] Table 2 is the hot rolled coil rolling process parameters (℃)

[0035] Example Steel tapping temperature Actual value of rough rolling temperature Actual value of final rolling temperature Actual value of coiling temperature Example 1 1209 1060 880 652 Example 2 1182 1023 876 651 Example 3 1203 1050 875 650 Example 4 1195 1045 873 651 Example 5 1193 1051 874 653

[0036] Table 3 is the test results of the hot rolled coil performance of the embodiment

[0037] Example Yield / MPa Tensile strength / MPa Elongation Yield-to-strength ratio Example 1 368 478 36％ 0.77 Example 2 387 514 28％ 0.75 Example 3 396 511 27％ 0.76 Example 4 369 477 28％ 0.77 Example 5 388 521 30％ 0.74

[0038] From the above embodiments, it can be concluded that the low yield ratio hot rolled coil of steel for welding gas cylinders prepared by the present invention has a yield strength ≥360 MPa, a tensile strength of 460-560 MPa, an elongation after fracture ≥26%, and a yield ratio less than 0.77.

[0039] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solutions and concepts of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A low yield ratio hot rolled steel coil for welding gas cylinders, characterized in that: The composition of the hot-rolled steel coil is as follows by weight percentage: C: 0.14% to 0.16%, Si: 0.06% to 0.10%, Mn: 0.85% to 0.95%, P: ≤0.020%, S: ≤0.008%, Als: 0.020 to 0.045%, N: ≤0.005%, and the rest is Fe and unavoidable impurities.

2. The low yield ratio hot rolled coil of steel for welding gas cylinders according to claim 1, characterized in that: The low-yield-to-tensile ratio hot-rolled coil of steel for welding gas cylinders has a yield strength of ≥360 MPa, a tensile strength of 460-560 MPa, an elongation after fracture of ≥26%, and a yield ratio of less than 0.

77.

3. A method for preparing a low yield ratio hot rolled coil of steel for welding gas cylinders according to claim 1, characterized in that: The method for preparing the low-yield ratio hot-rolled coil of steel for welding gas cylinders comprises the following contents: Steelmaking process: Steel plates are rolled from continuous casting billets that are smelted in a converter, refined in an electric furnace, and poured. The converter uses slag tapping. During the casting process, the tundish uses a submerged nozzle to protect the casting throughout the entire process. Hot rolling process: heating furnace temperature: 1180 ~ 1210 ℃; rough rolling final rolling temperature: 1000 ~ 1060 ℃; finishing rolling final rolling temperature: 840 ~ 880 ℃; coiling temperature: 630 ~ 670 ℃; Cooling process: adopts rear-stage cooling, and the strip is air-cooled for 10 to 20 seconds after finishing rolling. The temperature drop change parameters are predicted by the cooling water laminar flow control model to track the strip position and calculate the manifold position for cooling water. After air cooling, the manifold water cooling starts. The rear-stage manifold is set according to the thickness of the production specifications and the water-cooling manifold position predicted and calculated by the laminar flow control model to control 1-3 groups of manifolds to discharge water densely. As the rolling speed increases, the water discharge method is first sparse and then dense. According to the feedback input from the pyrometer detection, the laminar flow calculation model is used to control the number of opening and closing groups of water-cooling manifolds, and the cooling rate is controlled to be not less than 25℃ / s.

4. The method for preparing a low yield ratio hot rolled coil of steel for welding gas cylinders according to claim 3, characterized in that: The finishing rolling temperature is controlled to obtain pearlite and ferrite with uniform structure, with the pearlite accounting for 20-30% and the ferrite accounting for 70-80%.

5. The method for preparing a low yield ratio hot rolled coil of steel for welding gas cylinders according to claim 3, characterized in that: The precise control of the rear cooling section can obtain a uniform structure with a grain size of more than 10 levels.

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