A 35 kg grade cold-rolled low-temperature-resistant thin steel sheet and a manufacturing method thereof

Abstract

The application discloses a 35kg-grade cold-rolled low-temperature-resistant thin steel plate, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: C: 0.04-0.08%; Mn: 0.30-0.60%; Al: 0.02-0.06%; Nb: 0.015-0.030%; and Ti: 0.005-0.020%. In addition, the application also discloses a manufacturing method of the 35kg-grade cold-rolled low-temperature-resistant thin steel plate, which comprises the following steps: (1) smelting and casting; (2) hot rolling: in the heating stage, the slab heating temperature is controlled to be 1220-1280 DEG C, and the slab holding time is greater than or equal to 150 min; (3) pickling and cold rolling; (4) continuous annealing: the annealing soaking temperature is controlled to be 770-810 DEG C, and the soaking holding time is 100-180 s; and (5) flattening. The 35kg-grade cold-rolled low-temperature-resistant thin steel plate has excellent low-temperature resistance.

Description

Technical Field

[0001] This invention relates to an alloy steel material and its manufacturing method, and more particularly to a thin plate steel material and its manufacturing method. Background Technology

[0002] For steel materials, the material state typically transitions from a ductile to a brittle state as the temperature drops below a certain point; this temperature is known as the ductile-brittle transition temperature. Brittle fracture at room temperature and brittle fracture at low temperatures exhibit essentially the same characteristics: no significant plastic deformation occurs before fracture, the fracture surface is flush, and the fracture occurs suddenly. Therefore, when materials are in a brittle state, fracture failure is more likely to occur, leading to partial or complete failure of parts or systems, and this should be avoided as much as possible.

[0003] Various low-temperature resistant steels are already available in existing technologies:

[0004] For example, Chinese patent document CN107287515A, published on June 9, 2017, entitled "Low-Temperature Impact Resistant and High Elongation Q345E Hot-Rolled Strip Steel and Production Method Thereof," discloses a low-temperature impact resistant and high elongation Q345E hot-rolled strip steel and its production method. This technical solution, through the principles of controlled rolling and cooling and bainitic phase transformation strengthening, aims to improve strength and low-temperature impact toughness while reducing alloy costs. However, this technical solution is aimed at hot-rolled steel plates and does not involve cold-rolled continuously annealed steel.

[0005] For example, Chinese patent document CN107699792A, published on February 16, 2018, entitled "A Low-Temperature Resistant Microalloyed Steel and its Production Process," discloses a low-temperature resistant microalloyed steel. This technical solution effectively controls the growth of austenite grains during the rolling heating process using Nb+Ti microalloying technology to meet the technical requirements of various low-temperature indicators. However, this technical solution is aimed at hot-rolled steel plates or H-beams and does not involve cold-rolled continuously annealed steel.

[0006] For example, Chinese patent document CN111705270A, published on September 25, 2020, entitled "A Method for Preparing 800MPa Grade Low-Temperature Resistant High-Strength Steel," discloses a method for preparing 800MPa grade low-temperature resistant high-strength steel. This method involves heating, descaling, rough rolling, finish rolling, pre-straightening, online water cooling, air cooling, and tempering of a slab obtained from continuous casting to obtain the low-temperature resistant high-strength steel. However, this technical solution also pertains to hot-rolled products and does not involve the production of cold-rolled low-temperature resistant thin plates.

[0007] Based on this, it is desirable to obtain a technical solution for cold-rolled thin sheets that can have excellent low-temperature resistance. Summary of the Invention

[0008] One of the objectives of this invention is to provide a 35kg grade cold-rolled low-temperature resistant thin steel sheet with a simple alloy composition, a simple manufacturing process, strong adaptability, and excellent low-temperature performance.

[0009] Based on the above-mentioned objectives, this invention provides a 35kg grade cold-rolled low-temperature resistant thin steel sheet, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0010] C: 0.04–0.08%;

[0011] Mn: 0.30–0.60%;

[0012] Al: 0.02–0.06%;

[0013] Nb: 0.015–0.030%;

[0014] Ti: 0.005~0.020%.

[0015] Furthermore, the present invention also provides a 35kg grade cold-rolled low-temperature resistant thin steel sheet, the chemical element mass percentage ratio of which is:

[0016] C: 0.04–0.08%;

[0017] Mn: 0.30–0.60%;

[0018] Al: 0.02–0.06%;

[0019] Nb: 0.015–0.030%;

[0020] Ti: 0.005~0.020%;

[0021] The balance is Fe and unavoidable impurities.

[0022] Furthermore, in the unavoidable impurities of the 35kg grade cold-rolled low-temperature resistant thin steel sheet described in this invention: Si≤0.05%, P≤0.01%, S≤0.008%.

[0023] The design principles of each element of the 35kg-grade cold-rolled low-temperature resistant thin steel sheet of this invention are as follows:

[0024] C: Carbon is the main element affecting the strength of the finished product. It forms cementite with iron, which directly affects low-temperature toughness. In this technical solution, when the carbon content is higher than 0.08%, a peritectic reaction occurs during steelmaking and casting, easily causing transverse cracks in the slab. Therefore, in this invention, the carbon content is controlled within the range of 0.04% to 0.08%. This ensures a certain strength in the matrix while avoiding the peritectic reaction zone.

[0025] Mn: Manganese is a solid solution strengthening element that can expand the γ region of austenite. Increasing the Mn / C ratio can lower the ductile-brittle transition temperature of steel. Therefore, in this invention, the Mn content is related to the C content, and its range is 0.30% to 0.60%.

[0026] Al: The main purpose of adding aluminum to steel is deoxidation. Because molten steel contains a certain amount of free oxygen after the converter, adding aluminum removes this free oxygen, reducing internal inclusions in the steel. This has a significant positive effect on the low-temperature brittleness of the finished product. Simultaneously, maintaining a certain excess aluminum can also form AlN particles with nitrogen, thereby refining the grain size and lowering the ductile-brittle transition temperature. Based on this, the aluminum content in this invention is controlled at 0.02–0.06%.

[0027] Niobium (Nb) is a strong carbide-forming element. NbC compounds have high hardness and high melting point, and do not easily grow after precipitation at high temperatures. It can play a role in precipitation strengthening and grain refinement, thereby lowering the ductile-brittle transition temperature. Therefore, it is one of the core controlled elements in this invention. Based on this, the niobium content in this invention is controlled at 0.015–0.030%.

[0028] Ti: Titanium, like nitrogen and carbon, is a strong carbide-forming element, and its role is similar to that of Nb. In addition, the addition of Ti in this invention serves another important purpose: to reduce corner cracking in the slab and lower process costs. Based on this, the titanium content in this invention is controlled at 0.005–0.020%.

[0029] The unavoidable impurity elements in this invention are mainly Si, S, and P. Si is an unavoidable residual element introduced during the steelmaking process, while P and S, as harmful elements, easily cause non-metallic inclusions, adversely affecting low-temperature toughness; therefore, their content should be as low as possible. Furthermore, their content can be controlled as follows: Si ≤ 0.05%, P ≤ 0.01%, S ≤ 0.008%.

[0030] Furthermore, the microstructure of the 35kg grade cold-rolled low-temperature resistant thin steel sheet described herein is refined ferrite + second-phase precipitation strengthening particles.

[0031] The microstructure of steel, in descending order of its brittle-ductile transition temperature (Tk), is: pearlite—upper bainite—ferrite—lower bainite—tempered martensite. Furthermore, as the size of the second phase increases, the material's toughness decreases and the ductile-brittle transition temperature increases; however, spherical second phases exhibit relatively better toughness. Based on this, the present invention controls the microstructure to consist of refined ferrite + second-phase precipitated reinforcing particles, thereby enabling the matrix to possess certain strength and low-temperature toughness.

[0032] Furthermore, the ferrite grain size of the 35kg grade cold-rolled low-temperature resistant thin steel sheet of the present invention is above grade 9.

[0033] Refining grain size can enhance the toughness of materials and reduce the ductile-brittle transition temperature. Therefore, this invention reduces the ductile-brittle transition temperature by refining the grain size (to level 9 or above).

[0034] Furthermore, the thickness of the 35kg grade cold-rolled low-temperature resistant thin steel sheet of the present invention is 0.7 to 2.0 mm.

[0035] Furthermore, the 35kg grade cold-rolled low-temperature resistant thin steel sheet of the present invention has a static tensile yield strength ≥300MPa, tensile strength ≥350MPa, and elongation ≥30% at room temperature; a static tensile yield strength ≥320MPa, tensile strength ≥400MPa, and elongation ≥25% at -60℃; and a ductile-brittle transition temperature below -60℃.

[0036] Another objective of this invention is to provide a method for manufacturing a 35kg grade cold-rolled low-temperature resistant thin steel sheet.

[0037] To achieve the above objectives, the present invention provides a method for manufacturing a 35kg-grade cold-rolled low-temperature resistant thin steel sheet, comprising the following steps:

[0038] (1) Smelting and casting;

[0039] (2) Hot rolling: Control the slab heating temperature to 1220℃~1280℃, and the slab holding time ≥150min;

[0040] (3) Pickling and cold rolling;

[0041] (4) Continuous annealing: control the annealing temperature to be 770-810℃ and the holding time to be 100-180s;

[0042] (5) Flat.

[0043] In the manufacturing method described in this invention, the continuous annealing step is a key step. Its main purpose is to recrystallize the rolled hardened structure through heat treatment. The annealing homogenization temperature range and annealing holding time have a significant impact on the performance of the finished product. In this invention, the annealing homogenization temperature is controlled at 770–810°C, and the homogenization holding time is controlled at 100–180 seconds. This is to ensure that the rolled hardened structure can be fully recrystallized, and to prevent secondary grain growth.

[0044] In addition, the manufacturing method described in this invention uses a relatively high slab heating temperature: 1220-1280°C. This is to ensure that more NbC and TiC compounds are precipitated during hot rolling, while ensuring that C and N compounds are fully dissolved, thereby refining the grains and reducing the ductile-brittle transition temperature.

[0045] Therefore, it can be seen that the composition ratio and manufacturing process of this invention need to work together in a suitable manner to achieve the purpose of this invention. The core point of this invention is that it adopts a low carbon and low manganese design in terms of composition, while adding Nb and Ti to play a role in refining the grain size. At the same time, trace amounts of Ti can reduce the occurrence rate of corner cracks in the slab, thereby improving the yield and reducing production costs. Combined with a specific continuous annealing production process, and through a suitable recrystallization annealing process, an ultra-fine grain product is obtained, thereby possessing low temperature resistance.

[0046] Furthermore, in step (2) of the manufacturing method described in this invention, the final rolling temperature is controlled to be 850°C to 910°C, and the coiling temperature is controlled to be 520°C to 580°C.

[0047] Furthermore, in step (3) of the manufacturing method described in this invention, the cold rolling reduction rate is controlled to be 60-75%.

[0048] When the reduction rate is low, the hot-rolled grains are not fully elongated and broken up, resulting in insufficient accumulated deformation energy. This leads to relatively large recrystallized grain sizes during annealing, causing a decrease in the yield strength of the finished product. Increasing the deformation amount, on the other hand, causes greater roll wear and problems such as hardened coils. However, increasing the deformation amount is beneficial for lowering the annealing homogenization temperature and improving the strip's yield strength. Based on this, the present invention uses a cold rolling reduction rate of 60–75%.

[0049] Furthermore, in step (5) of the manufacturing method described in this invention, the flatness ratio is controlled to be 1.0 to 1.4%.

[0050] The 35kg-grade cold-rolled low-temperature resistant thin steel sheet of this invention, through the combination of composition and process, can achieve a static tensile yield strength ≥300MPa, tensile strength ≥350MPa, and elongation ≥30% at room temperature; a static tensile yield strength ≥320MPa, tensile strength ≥400MPa, and elongation ≥25% at -60℃; and a ductile-brittle transition temperature below -60℃. This material can be used in low-temperature applications such as structural components in cold regions and ceiling components for LPG tanks, with an operating temperature range of -70 to -50℃. Attached Figure Description

[0051] Figure 1 This is a metallographic image of Embodiment 1 of the present invention. Detailed Implementation

[0052] The following will further explain and illustrate the 35kg grade cold-rolled low-temperature resistant thin steel sheet and its manufacturing method according to the present invention with reference to specific embodiments and accompanying drawings. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0053] Examples 1-6 and Comparative Examples 1-3:

[0054] Table 1 lists the mass percentage of each chemical element in Examples 1-6 and Comparative Examples 1-3 of the present invention.

[0055] Table 1. (Balance represents Fe and other unavoidable impurities besides S, P, and Si, wt%)

[0056]

[0057]

[0058] As can be seen from Table 1, Comparative Examples 1 to 3 did not contain any Nb, and the Ti alloy (its extremely low content is unavoidable during smelting) had a carbon content 0.04% lower than the lower limit of the present invention, while the carbon content of Comparative Example 3 was 0.08% higher than the upper limit of the present invention.

[0059] Based on the above chemical composition design, the 35kg grade cold-rolled low-temperature resistant thin steel sheets of Examples 1-6 of the present invention are prepared by the following steps:

[0060] (1) Blast furnace hot metal → converter steelmaking → RH refining: In the converter process, the hot metal is smelted by top and bottom combined oxygen blowing to remove non-metallic inclusions and various impurities in the hot metal. In the RH refining process, the ladle steel is smelted twice by vacuum circulation degassing equipment. The main purpose is to degas, deoxidize, and add alloys to adjust the chemical element composition of the steel to the values ​​shown in Table 1.

[0061] (2) Continuous casting: After secondary refining, the molten steel is poured into the tundish. The superheat of the molten steel can be 10 to 20°C. It is rapidly cooled and solidified in the continuous casting equipment and cut into steel billets of a certain size by flame. The continuous casting speed can be 1.0 to 1.5 m / min, and the thickness of the steel billet can be 230 to 250 mm.

[0062] (3) Hot rolling: The slab heating temperature is controlled at 1220-1280℃, the slab holding time is more than 150min, the slab enters the 7-stand finishing mill after 5 passes of rough rolling, and the final rolling temperature of the finishing mill stand is 850-910℃; after laminar flow cooling, the strip coiling temperature is 520-580℃, and the thickness of the hot rolled strip is 2.3-6.0mm.

[0063] (4) Pickling is used to remove iron oxide scale and dirt from the surface of hot-rolled coils and achieve a certain degree of surface cleanliness. The thickness of the strip does not change during this process.

[0064] (5) Cold rolling: Control the cold rolling reduction rate to 60-75%;

[0065] (6) Continuous annealing: The annealing temperature is 770-810℃ and the holding time is 100-180s.

[0066] (7) Flattening to improve the plate shape, eliminate yield plateau, and reduce the risk of tensile strain marks during use. In this step, the flatness ratio is controlled to be 1.0 to 1.4%.

[0067] It should be noted that Comparative Examples 1-3 also adopted basically the same steps and processes, but their process parameters were different from those of the embodiments of the present invention.

[0068] Tables 2-1 and 2-2 list the specific process parameters for Examples 1-6 and Comparative Examples 1-3.

[0069] Table 2-1.

[0070]

[0071] Table 2-2.

[0072]

[0073] Table 3 lists the mechanical property test results of the 35kg grade cold-rolled low-temperature resistant thin steel sheets prepared in Examples 1-6 and Comparative Examples 1-3 of this invention. The room temperature tensile test method is GB / T 228.1-2010 Metallic materials, tensile testing—Part 1: Tensile testing at room temperature; the -60℃ low temperature tensile test method is GB / T 228.3-2019 Metallic materials, tensile testing—Part 3: Low temperature testing. The secondary brittle drop hammer test adopts GB / T 24173-2016 Test method for embrittlement of steel plates after secondary processing.

[0074] Table 3.

[0075]

[0076]

[0077] As shown in Table 3, the static tensile yield strength of Examples 1-6 of the present invention at room temperature is 302 MPa to 347 MPa, the tensile strength is 364 MPa to 426 MPa, and the elongation is ≥31%. At -60℃, their static tensile yield strength is 330 MPa to 357 MPa, the tensile strength is 429 MPa to 465 MPa, and the elongation is ≥30%. Their ductile-brittle transition temperature is ≤-60℃. In contrast, the yield strength of Comparative Examples 1-3 at room temperature is all below 300 MPa, and the tensile strength of Comparative Example 2 is below 350 MPa. Furthermore, the tensile strength of Comparative Examples 1 and 2 at -60℃ is also below 400 MPa. In addition, the brittle transition temperature of Comparative Examples 1-3 is higher than that of the Examples.

[0078] Figure 1 A metallographic photograph of Embodiment 1 of the present invention is shown. From Figure 1As can be seen from the data, the microstructure of the 35kg-grade cold-rolled low-temperature resistant thin steel sheet of the present invention is ferrite, and its grain size is greater than 9.

[0079] Therefore, the 35kg grade cold-rolled low-temperature resistant thin steel sheet of the present invention has excellent low-temperature resistance and can be used in low-temperature environments between -50℃ and -70℃ without brittle cracking.

[0080] It should be noted that the combination of the various technical features in this invention is not limited to the combination methods described in the claims of this invention or the combination methods described in the specific embodiments. All technical features described in this invention can be freely combined or combined in any way, unless they contradict each other.

[0081] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A 35kg grade cold-rolled low-temperature resistant thin steel sheet, characterized in that, Its chemical element mass percentage ratio is as follows: C：0.04～0.08%； Mn: 0.30~0.60%; Al：0.02～0.06%； Nb: 0.015~0.030%; Ti: 0.005~0.020%; The balance consists of Fe and unavoidable impurities; Its static tensile yield strength at room temperature is ≥300MPa, tensile strength is ≥350MPa, and elongation is ≥30%; its static tensile yield strength at -60℃ is ≥320MPa, tensile strength is ≥400MPa, and elongation is ≥25%; its ductile-brittle transition temperature is below -60℃.

2. The 35kg grade cold-rolled low-temperature resistant thin steel sheet as described in claim 1, characterized in that, In unavoidable impurities: Si≤0.05%, P≤0.01%, S≤0.008%.

3. The 35kg grade cold-rolled low-temperature resistant thin steel sheet as described in claim 1, characterized in that, Its microstructure consists of refined ferrite and second-phase precipitation strengthening particles.

4. The 35kg grade cold-rolled low-temperature resistant thin steel sheet as described in claim 1, characterized in that, The grain size of ferrite is above grade 9.

5. The 35kg grade cold-rolled low-temperature resistant thin steel sheet as described in claim 1, characterized in that, Its thickness is 0.7~2.0mm.

6. A method for manufacturing a 35kg grade cold-rolled low-temperature resistant thin steel sheet as described in any one of claims 1-5, characterized in that, Including the following steps: (1) Smelting and casting; (2) Hot rolling: During the heating stage, the slab heating temperature is controlled at 1220℃~1280℃, and the slab holding time is ≥150min; (3) Pickling and cold rolling; (4) Continuous annealing: control the annealing temperature to be 770-810℃ and the holding time to be 100-180s; (5) Flat.

7. The manufacturing method as described in claim 6, characterized in that, In step (2), the final rolling temperature is controlled at 850℃~910℃ and the coiling temperature is controlled at 520~580℃.

8. The manufacturing method as described in claim 6, characterized in that, In step (3), the cold rolling reduction rate is controlled to be 60-75%.

9. The manufacturing method as described in claim 6, characterized in that, In step (5), the flatness ratio is controlled to be 1.0~1.4%.

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