Ti microalloyed P-containing economical high-strength weathering steel with yield strength of 450MPa and production method of Ti microalloyed P-containing economical high-strength weathering steel

Abstract

The invention discloses Ti microalloyed P-containing economical high-strength weathering steel with yield strength of 450 MPa and a production method thereof. The weathering steel comprises the following chemical components: 0.015% < = C < = 0.035%, 0.10% < = Si < = 0.20%, 0.50% < = Mn < = 0.70%, 0.040% < = P < = 0.050%, S < = 0.003%, 0.010% < = Als < = 0.045%, 0.045% < = Ti < = 0.060%, 0.80% < = Cr < = 1.00%, 0.15% < = Ni < = 0.20%, 0.30% < = Cu < = 0.35%, N < = 0.0040%, O < = 0.0020% and the balance of Fe and inevitable impurities. The microstructure of the weathering steel is a three-phase structure of quasi-polygonal ferrite, acicular ferrite and nano precipitated phase. The production method of the weathering resistant steel sequentially comprises the working procedures of converter smelting, RH vacuum treatment, LF refining, slab continuous casting, heating, rough rolling, finish rolling, laminar cooling, coiling and warehousing. Through low-cost alloy design and an easy-to-produce process flow, the high-strength weathering steel with the yield strength of 450 MPa and good surface quality, mechanical property low-temperature toughness and excellent atmospheric corrosion resistance is obtained, and the obtained steel plate and steel belt are widely applied to the fields of railway vehicles, containers, iron towers, high-speed and bridge engineering and the like.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa and a production method thereof. Background Art

[0002] Weathering steel, also known as atmospheric corrosion resistant steel, is a low alloy steel between ordinary steel and stainless steel. It is widely used in railways, vehicles, bridges, towers, photovoltaics, high-speed engineering and other steel structures that are exposed to the atmosphere for a long time. With the advancement of iron and steel metallurgy and controlled rolling and controlled cooling technology, the optimization of chemical composition and production process to reduce production costs without reducing the performance of materials has become the focus of research for technicians in this industry.

[0003] The existing patent CN111235464B discloses a titanium microalloyed economical high-strength weathering steel and its production method. The Ti microalloying design is adopted, the Mn content is reduced, and the TMCP process is combined to effectively refine the grain size, promote the combination of Ti with C and N to precipitate fine carbonitrides, give full play to the crystallization strengthening and precipitation strengthening effects of the Ti element, improve the strength, and obtain the weathering steel performance: yield strength ≥ 450MPa, tensile strength 550MPa ~ 650MPa, elongation after fracture ≥ 24%, Kv2 ≥ 60J at -60℃. However, due to the high C content (0.08% ~ 0.14%), it is in the peritectic steel area, and the ingot is prone to corner cracks during the continuous casting production process; due to the use of a micro-Ni composition scheme, and the Cu content is 0.20% ~ 0.40%, the Ni / Cu ratio is much less than 1 / 2, and copper brittleness is very likely to occur during the production process, which leads to the birth of ingot cracks, so higher requirements are put forward for the stability control of the production process.

[0004] Patent CN114990441B discloses a weathering steel for -70℃ environment and its production method, which adopts low C-high Ni-high Cr composition system, adopts low temperature rolling in the finishing stage, and adopts two-stage laminar cooling to achieve medium temperature coiling (450℃~520℃), obtains uniform and fine ferrite single-phase structure, which is conducive to improving the impact energy at -70℃, and its performance is yield strength ≥355MPa, tensile strength ≥470MPa, elongation after fracture ≥22%, and KV2 at -70℃ ≥60J. However, Ni in this invention is a precious metal with a high content, resulting in a production cost significantly higher than that of the same product.

[0005] Patent CN117947354A discloses weather-resistant hot-rolled steel strip and its production method. The weather-resistant steel strip involves steel plates with a thickness of ≤6.0mm. Since the N and Ti contents are generally high, coarse TiN particles are easily formed in actual production, which leads to a significant decrease in low-temperature impact toughness, especially the large fluctuation of impact energy under conditions of ≤-40℃.

[0006] In summary, the existing weathering steel production technology still has some different defects and cannot meet the requirements of high performance and low cost at the same time. Summary of the invention

[0007] In view of the problem that the current weathering steel production technology cannot take into account both the high performance and low cost of weathering steel, the present invention provides a Ti micro-alloyed P-containing economical high-strength weathering steel with a yield strength of 450MPa and a production method thereof.

[0008] The invention obtains 450MPa-grade high-strength weathering steel with good surface quality, excellent mechanical properties, low-temperature toughness and atmospheric corrosion resistance through low-cost alloy design and easy-to-produce process flow.

[0009] The Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa provided by the present invention contains the following chemical components in percentage by mass:

[0010] 0.015%≤C≤0.035%, 0.10%≤Si≤0.20%, 0.50%≤Mn≤0.70%, 0.040%≤P≤0.050%, S≤0.003%, 0.010%≤Als≤0.045%, 0.045%≤Ti≤0.060%, 0.80%≤Cr≤1.00%, 0.15%≤Ni≤0.20%, 0.30%≤Cu≤0.35%, N≤0.0040%, O≤0.0020%, and the balance is Fe and unavoidable impurities.

[0011] In the composition of weathering steel, C must have a certain content in order to promote the formation of carbide strengthening phase, thereby ensuring the strength level of the steel strip; when its content is too high, the precipitated carbide particles will increase, which is not conducive to the hole expansion performance of the steel plate; when its content is too low, the strength will not meet the design requirements. The present invention controls the C content between 0.015% and 0.035%, and forms a nano-TiC precipitation phase by adding Ti to fix free C and reduce Fe 3 C content, while achieving the purpose of strengthening ferrite.

[0012] Si has a solid solution strengthening effect, which can simultaneously improve the strength and elongation of the steel strip and improve the formability of the steel strip. However, when the Si content is high, Fe 2 SiO 4Red rust is not easy to remove during pickling. In order to improve the surface quality of the steel strip and reduce the difficulty of smelting in the steel plant itself and the difficulty of removing red rust iron scale for users, the Si content is designed to be 0.10% to 0.20% in the present invention.

[0013] Mn is a solid solution strengthening element. According to the strength requirements of this steel grade, when the content is too low, the strength will not meet the design requirements; when the content is too high, the proportion of pearlite will increase, and high Mn content will easily lead to Mn component segregation in the ingot, which is one of the main reasons for the appearance of banded structure in the steel strip, and the banded structure will cause the plastic toughness of the steel strip to drop sharply. At the same time, for carbon manganese steel, the interaction between C and Mn has a certain influence on the formation and distribution of pearlite structure. When the content of both is too high, coarse pearlite and banded structure are easily generated. Therefore, under the premise that the strength meets the design requirements, the C content and Mn content are designed to be as low as possible. In the present invention, the Mn content is 0.50% to 0.70%.

[0014] As the strongest element for strengthening ferrite, P has strong solid solution strengthening and cold work hardening effects. When used as an alloying element, it can improve its strength and atmospheric corrosion resistance of the steel strip. However, when the content is too high, the cold stamping performance of the steel strip will drop sharply. At the same time, there will be segregation of the components of the continuous casting slab, causing uneven organization, increasing temper brittleness, and significantly reducing the plasticity and toughness of the steel strip, so that the steel strip will appear "cold brittle" phenomenon and brittle cracking during cold working. However, with the advancement of metallurgical technology, the segregation of P-containing steel leads to a significant reduction in the risk of plasticity and toughness of the steel strip, creating conditions for the development of high-strength and high-weathering steel. The P content of the present invention is 0.040% to 0.050%.

[0015] S is an impurity element in steel and is easy to form MnS in steel. The amount and shape of sulfides in steel directly affect the hole expansion performance of the steel strip, especially strip-shaped sulfide inclusions are prone to cracks during deformation, so the designed S content is ≤0.003%.

[0016] The basic function of Al in the present invention is mainly deoxidation in the steelmaking process. The Als content in steel is generally less than 0.010%; at the same time, if the Als content exceeds 0.060%, its grain refinement effect is weakened. The present invention sets the Als content to 0.010% to 0.045%.

[0017] Cr can delay the pearlite transformation, significantly improve the hardenability of steel, facilitate the formation of bainite structure, and refine the structure to achieve a strengthening effect. Too high Cr content will deteriorate the processing and formability of the material. The principle of selecting the Cr content is to promote the formation of bainite and avoid the formation of martensite structure that has an adverse effect on elongation. Therefore, the Cr content in the present invention is 0.90% to 1.00%.

[0018] Ni can significantly improve the corrosion resistance of steel. At the same time, nickel and copper elements form a Ni-containing copper-rich phase and are retained in the outer oxide layer in a solid state, reducing the copper enrichment in the matrix and the chance of forming a liquid copper-rich phase, thereby avoiding the occurrence of hot brittle defects. Therefore, Ni / Cu in steel is generally controlled to be ≥1 / 2; however, too high nickel will increase the adhesion of the oxide scale, and pressing it into the steel will form hot rolling defects on the surface. Moreover, nickel is a precious metal, and too high a content thereof will significantly increase the alloy cost of the steel; therefore, the Ni content in the present invention is 0.15-0.20%.

[0019] Cu is conducive to forming a dense, well-adhesive amorphous oxide (hydrocarbon oxide) protective layer on the surface of steel, which has a significant anti-corrosion effect; copper and sulfur generate insoluble sulfides, thereby offsetting the harmful effects of S on the corrosion resistance of steel; according to the calculation formula of atmospheric corrosion resistance index I, too little or too much copper content will reduce the calculated value of I, and the peak value of Cu content is 0.35%. Therefore, the Cu content in the present invention is 0.30-0.35%.

[0020] Titanium combined with carbon and nitrogen can form tiny carbonitrides to pin grain boundaries, delay recrystallization, inhibit the growth of austenite grains, and produce fine grain strengthening and precipitation strengthening effects; increasing the titanium content can increase its carbonitride volume fraction, enhance the fine grain strengthening and precipitation strengthening effects; increasing the nitrogen content in steel is conducive to promoting the precipitation of titanium second phase; but too high a titanium or nitrogen content is likely to form large-sized TiN, resulting in reduced bending performance, impact performance, and forming performance, and too high a nitrogen content will increase the aging tendency and cold brittleness and hot brittleness of the steel, and damage the welding performance and cold bending performance of the steel; therefore, the Ti content in the present invention is 0.045% to 0.060%, and the N content is ≤0.0040%.

[0021] O is a harmful element in steel and an inevitable element in the steelmaking process. The present invention achieves ultra-low oxygen content control through RH vacuum treatment, and the O content is ≤0.0020%.

[0022] The microstructure of the weathering steel is a three-phase structure of quasi-polygonal ferrite (QF)+acicular ferrite structure (AF)+nano precipitate phase (MC).

[0023] Mechanical properties of weathering steel: ReL≥450MPa, Rm≥550MPa, A≥22%.

[0024] Bending properties of weathering steel: 180° bending test D=2a at room temperature without cracks.

[0025] Low-temperature impact toughness of weathering steel at -60℃: The impact energy of full-size V-notch specimen at -60℃ is KV2≥80J, the impact energy of 3 / 4-size specimen is KV2≥60J, and the impact energy of V-notch specimen is KV2≥40J.

[0026] The present invention also provides a production method for the above-mentioned 450MPa yield strength Ti microalloyed P-containing economical high-strength weathering steel, comprising the following steps: converter smelting → RH vacuum treatment → LF refining → slab continuous casting → heating → rough rolling → finishing rolling → laminar cooling → coiling → warehousing.

[0027] Before the converter smelting process, the molten iron must be desulfurized to a S content of ≤0.003%.

[0028] In the converter smelting process, high carbon ferromanganese is used for manganese and high carbon ferrochrome is used for chromium, and no decarburization is performed.

[0029] Decarburization is carried out in the RH vacuum treatment process to achieve ultra-low oxygen content control; then quantitative and precise carbon addition is carried out in the LF refining process.

[0030] Heating process: To ensure that alloy elements such as Ti, Mn, Cr, and P are fully dissolved, the heating temperature of the continuous casting slab is 1250±10℃, and the heating time is 200-280min, of which the holding time is ≥35min.

[0031] Rough rolling process: The thickness of the intermediate billet at the rough rolling outlet is 40mm to 60mm.

[0032] Hot coil box: When the thickness of the intermediate billet is ≤45mm, the intermediate billet is coiled and scaled.

[0033] The process parameters of the finishing rolling process are controlled as follows:

[0034] ① When the finished product thickness is less than 6.0mm, the finishing rolling entrance temperature is 1030-1080℃, and the final rolling temperature is 840-870℃;

[0035] ② When the finished product thickness is 6.0mm-10.0mm, the finishing rolling entrance temperature is 980-1020℃, and the final rolling temperature is 830-860℃;

[0036] ③ When the finished product thickness is 10.0mm~12.0mm, the finishing rolling entrance temperature is ≤970℃, and the final rolling temperature is 820~850℃.

[0037] Laminar cooling process: After finishing rolling, the steel strip adopts the front-stage rapid cooling method, the laminar cooling rate is ≥70℃ / s, and the midpoint temperature is 610℃~650℃.

[0038] Coiling process: In order to ensure that there is enough nano-level TiC, under the action of the strong hardenability element Cr, the coiling temperature is 580-620°C, and then it is coiled and slowly cooled to room temperature in a slow cooling pit.

[0039] Compared with the prior art, the present invention is beneficial in that:

[0040] (1) The present invention adopts a composition design of micro-low C and Ti composite micro-alloying, and obtains a 450MPa grade high-strength weathering steel with good surface quality, excellent mechanical properties, low-temperature toughness and atmospheric corrosion resistance through precise heating, rapid cooling in the front stage, and medium-high temperature coiling. The mechanical properties are: ReL≥450MPa, Rm≥550MPa, A≥18%. The impact energy of the full-size V-notch specimen at -60°C is KV2≥80J, the impact energy of the 3 / 4-size specimen is KV2≥60J, and the impact energy of the half-size specimen is KV2≥40J.

[0041] (2) The obtained steel plates and steel strips are widely used in the fields of railway vehicles, containers, roads and bridges, etc., and have the characteristics of simple production process, low alloy composition and process cost, and excellent low-temperature resistance and toughness of the products.

[0042] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 These are microscopic morphology images of the weathering steel produced in Example 2-1 at different magnifications. DETAILED DESCRIPTION

[0044] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0045] With respect to the Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa of the present invention, three embodiments with different chemical compositions are provided. The chemical compositions of the weathering steels of Embodiments 1-3 are shown in Table 1, and the remainder is Fe and unavoidable impurities.

[0046] Table 1 Chemical composition content of weathering steel of Examples 1-3

[0047]

[0048] In Table 1, the weather resistance index I=26.01(wt.%Cu)+3.88(wt.%Ni)+1.20(wt.%Cr)+1.49(wt.%Si)+17. 28(wt.%P)-7.29(wt.%Cu)×(wt.%Ni)-9.10(wt.%Ni)×(wt.%P)-33.39(wt.%Cu) 2 .

[0049] Carbon equivalent CE=(wt.%C)+0.584{(wt.%Si) / 24+(wt.%Mn) / 16+(wt.%Cu) / 15+(wt.%Ni) / 20+[(wt.%Cr)+(wt.%Mo)+(wt.%Nb)+(wt.%V)] / 5+5(wt.%B).

[0050] Cold crack sensitivity index Pcm = (wt.% C) + (wt.% Si) / 30 + [(wt.% Mn) + (wt.% Cu) + (wt.% Cr)] / 20 + (wt.% Ni) / 60 + (wt.% Mo) / 15 + (wt.% V) / 10 + 5 (wt.% B).

[0051] According to the chemical composition content of Examples 1-3, two sets of specific production process methods are respectively adopted. For example, for the chemical composition of Example 1, two different production process methods of Example 1-1 and Example 1-2 are adopted, and the process parameters are different. The process parameters of the processes such as heating of the ingot, rough rolling, finishing rolling, laminar cooling, and coiling are detailed in Table 2. The mechanical properties, bending properties, low temperature toughness, and microstructure type of the produced weathering steel are shown in Table 3. The microscopic morphology of the weathering steel produced in Example 2-1 at different magnifications is shown in Table 3. Figure 1 .

[0052] Table 2. Production process parameters

[0053]

[0054]

[0055] Table 3. Performance test results

[0056]

[0057] It can be seen that the weathering steel microstructure produced in Examples 1-3 of the present invention is a three-phase structure of quasi-polygonal ferrite (QF) + acicular ferrite structure (AF) + nano-precipitated phase (MC). The mechanical properties are: ReL≥450MPa, Rm≥550MPa, A≥22%. The bending performance is shown as no cracks in the 180° bending test D=2a at room temperature. The impact energy of the full-size specimen of the V-notch at -60°C is KV2≥80J, the impact energy of the 3 / 4-size specimen is KV2≥60J, and the impact energy of the half-size specimen is KV2≥40J, which reflects the excellent low-temperature impact toughness.

[0058] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450MPa, characterized in that: Contains the following chemical components by mass percentage: 0.015%≤C≤0.035%, 0.10%≤Si≤0.20%, 0.50%≤Mn≤0.70%, 0.040%≤P≤0.050%, S≤0.003%, 0.010%≤Als≤0.045%, 0.045%≤Ti≤0.060%, 0.80%≤Cr≤1.00%, 0.15%≤Ni≤0.20%, 0.30%≤Cu≤0.35%, N≤0.0040%, O≤0.0020%, and the balance is Fe.

2. The Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 1, characterized in that: The microstructure of weathering steel is a three-phase structure of quasi-polygonal ferrite + acicular ferrite structure + nano-precipitation phase.

3. A method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 1 or 2, characterized in that: It includes the following processes: converter smelting → RH vacuum treatment → LF refining → slab continuous casting → heating → rough rolling → finishing rolling → laminar cooling → coiling → warehousing.

4. The method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: The process of desulfurizing the molten iron is preceded by the converter smelting process, in which the molten iron is desulfurized to a sulfur content of ≤0.003%.

5. The method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: The converter smelting process uses high carbon ferromanganese for manganese and high carbon ferrochrome for chromium without decarburization; decarburization is carried out in the RH vacuum treatment process, and then quantitative and precise carbon addition is carried out in the LF refining process.

6. The method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: In the heating process, the heating temperature of the continuous casting slab is 1250±10°C, and the heating time is 200-280min, wherein the holding time is ≥35min.

7. The method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: The thickness of the intermediate billet at the rough rolling outlet of the rough rolling process is 40 mm to 60 mm.

8. The method for producing a Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: The process parameters of the finishing rolling process are controlled as follows: ① When the finished product thickness is less than 6.0mm, the finishing rolling entrance temperature is 1030-1080℃, and the final rolling temperature is 840-870℃; ② When the finished product thickness is 6.0mm-10.0mm, the finishing rolling entrance temperature is 980-1020℃, and the final rolling temperature is 830-860℃; ③ When the finished product thickness is 10.0mm~12.0mm, the finishing rolling entrance temperature is ≤970℃, and the final rolling temperature is 820~850℃.

9. The method for producing Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: Laminar cooling process: After finishing rolling, the steel strip adopts the front-stage rapid cooling method, the laminar cooling rate is ≥70℃ / s, and the midpoint temperature is 610℃~650℃.

10. The method for producing Ti microalloyed P-containing economical high-strength weathering steel with a yield strength of 450 MPa as claimed in claim 3, characterized in that: The temperature of the coiling process is 580-620°C, and then it is coiled into a coil and slowly cooled to room temperature in a slow cooling pit.

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