Steel for internal pressure pipe and method for producing the same

Abstract

The application discloses a steel preparation method for an internal pressure pipe, and the main chemical components and mass percentage of the steel are as follows: carbon 0.05-0.15%, silicon 0-0.1%, manganese 0.8-1.0%, sulfur ≤0.01%, phosphorus ≤0.03%, titanium 0.02-0.04%, niobium 0.01-0.03%, aluminum 0.02-0.06%, nitrogen ≤0.008%, and the rest is Fe and residual impurity elements in the smelting process. The production process flow is as follows: hot metal pretreatment, desulfurization, converter smelting, LF refining, continuous casting, slab slow cooling, slab heating, hot continuous rolling, coil slow cooling and (pickling). The steel plate produced by the method has a thickness of 1.4-8 mm, a yield strength of greater than or equal to 400 MPa and a tensile strength of greater than 500 MPa.

Description

Technical Field

[0001] This invention relates to the field of steel processing technology for internal pressure pipes, specifically to a type of steel for internal pressure pipes and its preparation method. Background Technology

[0002] With the intensification of global warming, energy conservation and emission reduction have become one of the important manufacturing requirements for the automotive industry, highlighting the importance of lightweighting in automobiles. Internal pressure forming technologies, such as hydroforming, are one of the important methods to achieve automotive lightweighting. Internal pressure forming can achieve higher strength and rigidity, reduce the number of molds and processes, reduce welding, improve precision, and has a higher forming limit, enabling the processing of complex integral structures.

[0003] As a raw material for hydroformed parts, internal pressure tube steel needs to possess good strength, plasticity, and formability. In existing technologies, traditional internal pressure tube steel often requires the addition of numerous expensive alloying elements to ensure strength, resulting in high manufacturing costs, and it is difficult to simultaneously achieve good weldability and low-temperature toughness. Therefore, developing a lower-cost, high-performance internal pressure tube steel and its preparation method is of great significance for promoting automotive lightweighting. Summary of the Invention

[0004] (a) Technical problems to be solved The technical problem to be solved by the present invention is to provide a steel for internal pressure pipes and a method for preparing the same. This steel has good plasticity and formability while ensuring high strength, and has low production cost.

[0005] (II) Technical Solution To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: a steel for internal pressure pipes, characterized in that the chemical composition of the steel, by mass percentage, includes: carbon 0.05%-0.15%, silicon 0-0.1%, manganese 0.8%-1.0%, sulfur ≤0.01%, phosphorus ≤0.03%, titanium 0.02%-0.04%, niobium 0.01%-0.03%, aluminum 0.02%-0.06%, nitrogen ≤0.008%, with the balance being iron and unavoidable impurity elements during the smelting process.

[0006] As an improvement, the metallographic structure of the steel is ferrite + pearlite.

[0007] As an improvement, the steel has a yield strength ≥400MPa and a tensile strength ≥500MPa.

[0008] A method for preparing steel for internal pressure pipes includes the following sequential steps: S1. Smelting and refining: The molten iron is desulfurized, smelted in a converter and refined in an LF furnace to obtain molten steel with the chemical composition described in claim 1. S2, Continuous casting: molten steel is continuously cast to obtain slabs; S3. Slab heating: The slab is heated and then hot rolled. S4. Controlled rolling and controlled cooling: The heated slab is rough rolled and finished rolled, then laminar flow cooling is performed, and then it is coiled to obtain hot-rolled coils of steel for internal pressure tubes.

[0009] As an improvement, in step S1, the sulfur content is controlled to be ≤0.01% during desulfurization treatment; the LF refining station outlet temperature is 1570-1595℃, and the refining time is ≥30 minutes.

[0010] As an improvement, in step S2, in step (2), the continuous casting speed is controlled at 0.8-1.5 m / min, and a light pressing mode is adopted; the slab after continuous casting is cooled to room temperature.

[0011] As an improvement, in step S3, the slab heating temperature is 1180-1250℃, the heating time is ≥180 minutes, and the solution treatment time is 20-35 minutes.

[0012] As an improvement, in step S4, the roughing rolling start temperature is 1145-1190℃, the roughing rolling finish temperature is 1050-1110℃; the finishing rolling start temperature is 980-1060℃, and the finishing rolling finish temperature is 860-900℃.

[0013] As an improvement, in step S4, the final cooling temperature of laminar flow cooling is 560-620℃, and the winding temperature is 560-620℃.

[0014] As an improvement, the thickness of the steel used for the internal pressure pipe is 1.4–8 mm.

[0015] (III) Beneficial Effects

[0016] The advantages of this invention compared to existing technologies are as follows: Through reasonable chemical composition design and controlled rolling and cooling processes, the steel produced by this invention for internal pressure pipes has a yield strength ≥400MPa and a tensile strength ≥500MPa, exhibiting a good balance between strength and plasticity, which meets the requirements for use in automotive hydroformed parts; at the same time, this invention reduces the addition of expensive alloying elements, thereby lowering production costs; furthermore, the steel grade of this invention has good weldability and formability, making it suitable for manufacturing complex integral structural parts and exhibiting a significant lightweighting effect. Attached Figure Description

[0017] Figure 1 This is a surface, 1 / 4, and center microstructure diagram of a steel for internal pressure pipes and its preparation method according to the present invention.

[0018] Figure 2 This is a production process flow diagram of a steel for internal pressure pipes and its preparation method according to the present invention. Detailed Implementation

[0019] The invention will now be described in further detail with reference to specific embodiments, but this should not be construed as limiting the scope of the subject matter of the invention to the following embodiments.

[0020] Examples 1-3 illustrate the preparation of steel for internal pressure pipes according to the technical solution of the present invention, with the specific steps as follows: Molten iron undergoes KR desulfurization treatment to control the sulfur content to ≤0.01%. It is then smelted in a 210-ton converter, followed by argon blowing in a ladle and refining in an LF furnace before being continuously cast into 230mm thick slabs. The LF refining time is ≥30 minutes, and the outlet temperature is controlled within the range of 1570-1595℃. The continuous casting speed is controlled at 0.8-1.5 m / min, using a light-reduction mode, and the slabs are cooled to room temperature in a stack.

[0021] The slab is heated to 1180-1250℃ for ≥180 minutes and solution treated for 20-35 minutes. After heating, the slab is rough rolled in 5-7 passes, then finished rolled on a 7-stand 2250℃ continuous rolling mill. After rolling, it is cooled by a laminar flow cooling system and then coiled into hot-rolled coils.

[0022] The specific chemical composition of Examples 1-3 is shown in Table 1, the rolling process parameters are shown in Table 2, and the mechanical properties are shown in Table 3.

[0023] The chemical composition of the steel used in the internal pressure pipe example is shown in Table 1 below.

[0024] Table 1 Chemical Composition Example C Si Mn P S Ti Nb Al Control range 0.05-0.15 0-0.1 0.8-1.0 ≤0.03 ≤0.01 0.02-0.04 0.01-0.03 0.02-0.06 Example 0.065 0.045 0.871 0.021 0.007 0.028 0.017 0.024 element N Control range ≤0.008 Example 0.0066

[0025] The specific heating process used in the example is 1200-1260℃, with a heating time of 130-300 min in the furnace. The first stage is rough rolling, with an initial rolling temperature of 1160-1190℃; the second stage is finish rolling, with an initial rolling temperature of 980-1060℃. After rolling, the steel strip is cooled using ACC-controlled cooling, and the coiling temperature is 560-610℃.

[0026] The specific rolling process parameters for the examples and comparative examples are shown in Table 2 below.

[0027] Table 2 Rolling process parameters

[0028] serial number Thickness (mm) Furnace temperature (°C) Roughing and finishing rolling temperatures (°C) Finishing rolling start temperature (°C) Finishing rolling temperature (°C) Winding temperature (°C) 1 1.8 1244 1105 1056 893 622 2 2 1242 1100 1048 885 619 3 2.5 1236 1191 1041 882 611

[0029] The specific mechanical properties of the embodiments and comparative examples are shown in Table 3 below.

[0030] Table 3 Mechanical Properties serial number Thickness (mm) <![CDATA[R el (MPa)]]> <![CDATA[R m (MPa)]]> <![CDATA[A 80 (%)]]> Example 1 1.8 466 533 22 Example 2 2 464 524 24 Example 3 2.5 455 513 24.5

[0031] The design basis for each chemical component of this invention is as follows: Carbon: Carbon is an effective and economical element for improving the strength of steel. When the carbon content is in the range of 0.05% to 0.15%, it can ensure strength while avoiding the formation of excessive lath martensite, thus ensuring the weldability and low-temperature toughness of the steel. Too low a carbon content will result in insufficient strength, while too high a carbon content will reduce low-temperature toughness and deteriorate weldability.

[0032] Silicon: Silicon can reduce the solubility of carbon in austenite, decrease the stability of supercooled austenite, and accelerate the phase transformation process. Silicon can inhibit the precipitation of carbides, increase the ferrite content, and play a role in solid solution strengthening, while also improving the elongation of steel plates. However, excessive silicon content will impair the toughness of steel, so it is controlled at 0-0.1%.

[0033] Manganese: Manganese can improve hardenability and increase material strength, but it is a positive segregation element in crystallization, and excessive content can easily lead to banded structures. This invention achieves the goal of improving strength by adding microalloying reinforcing elements such as titanium and niobium, while controlling the manganese content within a reasonable range of 0.8%–1.0%.

[0034] Sulfur: Sulfur is a harmful element in high-strength steel. It easily produces MnS segregation and sulfide inclusions, which reduce the plasticity and toughness of the steel. Therefore, the sulfur content needs to be controlled to ≤0.01% and reduced as much as possible.

[0035] Phosphorus: Phosphorus is also a harmful element in high-strength steel. Although it can improve strength and hardness, it segregates severely, increases temper brittleness, significantly reduces the plasticity and toughness of steel, and has an adverse effect on weldability. Therefore, the phosphorus content should be controlled to ≤0.03%.

[0036] Titanium: Titanium combines with nitrogen to form TiN, which can reduce the adverse effects of nitrogen on steel. The presence of TiN can inhibit grain coarsening in the weld heat-affected zone and improve weldability. The titanium content should be controlled between 0.02% and 0.04%.

[0037] Niobium: Niobium is a carbide-forming element that can increase the recrystallization temperature of austenite. Through strain-induced precipitation of niobium carbonitrides, it pins austenite grains, refines the grain size, and improves strength and low-temperature toughness. The niobium content is controlled between 0.01% and 0.03%.

[0038] Aluminum: Aluminum combined with a small amount of other elements is beneficial to promoting the formation of acicular ferrite, but too much aluminum will impair the toughness of steel. Therefore, the aluminum content should be controlled between 0.02% and 0.06%.

[0039] Nitrogen: Nitrogen can dissolve in iron to form interstitial solid solutions, and can also form stable nitrides with elements such as aluminum and titanium to achieve precipitation strengthening. However, excessive nitrogen content will reduce the toughness of steel, increase brittleness, and lead to increased notch sensitivity. Therefore, the nitrogen content should be controlled to ≤0.008%.

[0040] The preparation method described in this invention is based on the following: by controlling rolling and cooling techniques, and using metal deformation and phase transformation as a foundation, solid solution strengthening, precipitation strengthening, dislocation strengthening, and grain refinement strengthening are completed under specified deformation and temperature conditions to obtain steel plates with excellent comprehensive properties. Appropriate grain sizes are obtained through high-temperature austenite region deformation recrystallization, low-temperature non-recrystallized austenite region deformation, and accelerated cooling after rolling, reducing the use of precious alloys and lowering manufacturing costs.

[0041] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents. In short, if those skilled in the art are inspired by these claims and design similar structural methods and embodiments without departing from the inventive spirit of the present invention, they should all fall within the protection scope of the present invention.

Claims

1. A type of steel for internal pressure pipes, characterized in that, The chemical composition of the steel, by mass percentage, includes: carbon 0.05%–0.15%, silicon 0–0.1%, manganese 0.8%–1.0%, sulfur ≤0.01%, phosphorus ≤0.03%, titanium 0.02%–0.04%, niobium 0.01%–0.03%, aluminum 0.02%–0.06%, nitrogen ≤0.008%, with the balance being iron and unavoidable impurities during the smelting process.

2. The steel for internal pressure pipes according to claim 1, characterized in that, The metallographic structure of the steel is ferrite + pearlite.

3. The steel for internal pressure pipes according to claim 1, characterized in that, The steel has a yield strength ≥400MPa and a tensile strength ≥500MPa.

4. The method for preparing steel for internal pressure pipes according to claim 1, characterized in that, Includes the following sequential steps: S1. Smelting and refining: The molten iron is desulfurized, smelted in a converter and refined in an LF furnace to obtain molten steel with the chemical composition described in claim 1. S2, Continuous casting: molten steel is continuously cast to obtain slabs; S3. Slab heating: The slab is heated and then hot rolled. S4. Controlled rolling and controlled cooling: The heated slab is rough rolled and finished rolled, then laminar flow cooled, and finally coiled to obtain hot-rolled coils of steel for internal pressure tubes.

5. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, In step S1, the sulfur content is controlled to be ≤0.01% during desulfurization treatment; the LF refining station outlet temperature is 1570-1595℃, and the refining time is ≥30 minutes.

6. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, In step S2, in step (2), the continuous casting speed is controlled at 0.8-1.5 m / min, and a light pressing mode is adopted; the slab after continuous casting is cooled to room temperature.

7. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, In step S3, the slab heating temperature is 1180-1250℃, the heating time is ≥180 minutes, and the solution treatment time is 20-35 minutes.

8. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, In step S4, the roughing rolling start temperature is 1145-1190℃, the roughing rolling finish temperature is 1050-1110℃; the finishing rolling start temperature is 980-1060℃, and the finishing rolling finish temperature is 860-900℃.

9. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, In step S4, the final cooling temperature of laminar flow cooling is 560-620℃, and the winding temperature is 560-620℃.

10. The method for preparing steel for internal pressure pipes according to claim 4, characterized in that, The thickness of the steel used for the internal pressure pipe is 1.4-8 mm.

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