A tough steel plate and a method for manufacturing the same

Abstract

The present invention is used in the technical field of special steel materials, and discloses a tough steel plate, wherein the chemical composition of the steel plate includes, by mass percentage, C0.04-0.06%, Si0.14-0.22%, Mn1.50-1.65%, Nb0.02-0.05%, Ti0.01-0.02%, Cu0.20-0.32%, Cr0.14-0.18%, Ni0.40-0.80%, Al0.01-0.04%, P≦0.015%, S≦0.010% and the balance of Fe and impurities. The tough steel plate adopts an innovative thermomechanical rolling TMCP process, so that the manufactured extra-thick steel plate can fully meet the use requirements of earthquake-resistant bridge structures.

Description

Technical Field

[0001] The invention relates to the technical field of special steel materials, in particular to a toughness steel plate and a manufacturing method thereof. Background Art

[0002] In earthquake-prone areas, parts of bridge seismic structures, especially shock-absorbing diaphragms, need to withstand large plastic strains of 3%-5% for many times, and their working stress exceeds the yield strength of bridge steel many times. Under this working condition, the shock-absorbing diaphragms are required to have high resistance to large deformation and high low-cycle fatigue life under this working condition, so as to ensure the safety of the bridge. Studies have shown that the thickness of the steel plate required for making seismic isolation structures is 50-100mm, the tensile strength is 560-650MPa, the elongation is greater than 20%, the yield strength is less than 0.72, and the straightness of this type of extra-thick plate is required to be ≦5mm / 2m;

[0003] Conventional TMCP thick plate rolling production process: slab heating → descaling → controlled rough rolling → intermediate slab waiting temperature → controlled finishing rolling → pre-straightening → controlled cooling → straightening → flaw detection → finishing. Among them, rough rolling is generally carried out in the austenite recrystallization zone, that is, the rolling temperature is higher than the austenite non-recrystallization temperature Tnr. The intermediate slab can be water-cooled to shorten the slab waiting time and improve production efficiency. Finishing rolling can be carried out in the austenite non-recrystallization zone and the rolling temperature is lower than Tnr. It can also be completed in the austenite + ferrite two-phase zone. The rolling temperature at this time is mainly determined by Ar3;

[0004] In order to achieve high strength, the microstructure of steel usually contains non-equilibrium medium-temperature or low-temperature phase transformation structures, such as acicular ferrite or bainite / martensite, etc. These microscopic non-equilibrium phases are regulated by controlling the cooling process. The water-cooled steel plate undergoes a multi-roll straightening process, and its plate shape control is difficult, which often restricts the improvement of production efficiency and reduces the performance of the product. When the microstructure of the hot-rolled thick steel plate is a complex phase structure composed of soft phase ferrite F + hard phase acicular ferrite AF, its yield strength ratio is lower than that of the steel plate with acicular ferrite as a single microstructure. When producing complex phase steel plates, the steel plates after final rolling need to be cooled in sections, that is, When the steel plate enters the austenite + ferrite two-phase region and is air-cooled, a partial ferrite phase transformation occurs, and then it is rapidly cooled, that is, the entire temperature-time curve is tortuous, and the final cooling temperature is usually lower than 400°C. For example, the publication number CN112877601B (see Table 1 Patent No. 1) proposes a 60-100mm thick multiphase structure steel plate, and its yield strength ratio is about 0.8; the publication number CN114480809A (see Table 1 Patent No. 9) proposes a 60-100mm thick multiphase structure high-toughness steel plate, and its yield strength ratio is about 0.84. It can be seen that based on the above patents and other patents listed in Table 1 Patent report, although people can determine the use of continuous converter steelmaking BOF, refining outside the furnace LF, vacuum degassing (RH or VD), slab continuous casting (CC), controlled rolling and controlled cooling (TMCP) technical routes and the idea of ​​regulating the complex phase microstructure of the steel of the present invention, but these cannot meet the special requirements of high-end bridge seismic steel. Therefore, for the specific seismic steel involved in the present invention, people still need to conduct a lot of experiments to explore the chemical composition of the steel plate, especially the TMCP process parameters, and conduct a lot of work such as the required mechanical properties series test. It should be pointed out that: considering the pre-straightening and straightening of the thick plate rolling mill Due to the limitation of equipment capacity, the shape defects (such as warping) of extra-thick (thickness t≥50mm) steel plates often have to be corrected with the help of subsequent flattening equipment, which greatly reduces production efficiency and increases production costs. Furthermore, due to the complexity of the segmented cooling path, there is a temperature gradient along the thickness of the steel plate, and the phase change is not synchronized, resulting in a large residual stress inside the steel plate. The shape of the steel plate with large residual stress will deteriorate after being processed by the user. Therefore, downstream users often need to flatten the extra-thick steel plates offline, which reduces the performance of the steel plates. At present, there has been no report on an efficient solution to the shape problem of extra-thick TMCP steel plates. Summary of the invention

[0005] The object of the present invention is to provide a tough steel plate and a method for manufacturing the same, so as to solve the problem of immature TMCP process parameters of extra thick steel plates mentioned in the above background technology.

[0006] To achieve the above-mentioned purpose, the present invention provides the following technical solution: a ductile steel plate, the chemical composition of which, in mass percentage, includes: C0.04-0.06%, Si0.14-0.22%, Mn1.50-1.65%, Nb0.02-0.05%, Ti0.01-0.02%, Cu0.20-0.32%, Cr0.14-0.18%, Ni0.40-0.80%, Al0.01-0.04%, P≦0.015%, S≦0.010% and the remainder of Fe and impurities.

[0007] Preferably, the thickness of the steel plate is 50-100 mm, the tensile strength of the steel plate is 560-650 MPa, and the yield strength ratio of the steel plate is lower than 0.72.

[0008] Preferably, the AKV2 of the steel plate at -40°C is greater than 300J, the fracture toughness characteristic CTOD value of the steel plate at -80°C is greater than 0.20mm, and the crack arrest toughness Kca of the steel plate at -10°C is greater than 9100N / mm 3 / 2 ,The unevenness of the steel plate is less than 4mm / 2m, and the unevenness of the whole plate is less than 6mm.

[0009] A method for manufacturing a ductile steel plate, the method comprising the following steps:

[0010] S1. Pre-desulfurization of molten iron;

[0011] S2. Converter steelmaking;

[0012] S3.LF ladle refining;

[0013] S4.RH vacuum degassing;

[0014] S5. Continuous casting of slabs with a thickness of 300 mm or more;

[0015] S6. Slab heating;

[0016] S7. Rough rolling;

[0017] S8. Finish rolling;

[0018] S9. Thermal straightening;

[0019] S10. Control cooling;

[0020] S11. Finishing

[0021] Preferably, the heating temperature of the slab in S6 is 1100-1150° C., and the slab is kept warm for 30-60 minutes after the heating of the slab in S6 is completed.

[0022] Preferably, the steel plate is subjected to two-stage rough rolling, wherein the final rolling temperature of the second stage rough rolling is ≥930°C.

[0023] Preferably, the steel plate is subjected to two-stage finish rolling, and the total reduction ratio of the steel plate after the two-stage finish rolling is ≥50%, and the final rolling temperature of the steel plate is 700-830°C.

[0024] Preferably, the hot straightening time of the steel plate is controlled within 90-120 seconds.

[0025] Preferably, the starting cooling temperature of the controlled cooling of the steel plate is 680-720°C, the cooling rate of the controlled cooling of the steel plate is controlled at 3-10°C / s, and the final cooling temperature of the controlled cooling of the steel plate is controlled at 250-450°C.

[0026] Preferably, after the controlled cooling of the steel plate is completed, it needs to be slowly and naturally cooled to room temperature.

[0027] Compared with the prior art, the beneficial effects of the present invention are: the ductile steel plate:

[0028] An innovative thermo-mechanical rolling TMCP process is adopted, namely: the reheating temperature of the continuous casting slab is 1100-1150℃, and the holding time after reaching the temperature is 30-60min; the steel plate is subjected to two-stage rough rolling, wherein the final rolling temperature of the second stage rough rolling is ≥930℃; the steel plate is subjected to two-stage finish rolling, with a total reduction rate ≥50% and a final rolling temperature of 700-830℃; the steel plate needs to be hot straightened within 90-120s; the steel plate is then controlled cooled, with the initial cooling temperature at 680-720℃, the cooling rate controlled at 3-10℃ / s, the final cooling temperature controlled at 250-450℃, and then slowly cooled to room temperature, so that the manufactured extra-thick steel plate can fully meet the use requirements of seismic bridge structures, especially the production and manufacturing requirements of extra-thick steel plates with high requirements for strength and low-temperature toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of a typical microstructure of the steel plate F1-1 of the present invention;

[0030] Figure 2 Schematic diagram of a typical microstructure of the steel plate F1-2 of the present invention;

[0031] Figure 3 Schematic diagram of a typical microstructure of the steel plate F3-2 of the present invention;

[0032] Figure 4 Schematic diagram of typical microstructure of steel plate F7-3 of the present invention;

[0033] Figure 5 This is a schematic diagram of the load-notch displacement curve of a 50 mm thick sample of the present invention;

[0034] Figure 6 This is a schematic diagram of the fracture of a 50 mm thick sample of the present invention;

[0035] Figure 7 Schematic diagram of the relationship between the crack arrest toughness Kca and temperature of the present invention;

[0036] Figure 8 Schematic diagram of a typical fracture of a steel plate of the present invention;

[0037] Fig. 9 Schematic diagram of the typical microstructure of the steel plate D4-1 of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] See also Figure 1-9 The present invention provides a technical solution: a ductile steel plate, the chemical composition of the steel plate includes, by mass percentage: C0.04-0.06%, Si0.14-0.22%, Mn1.50-1.65%, Nb0.02-0.05%, Ti0.01-0.02%, Cu0.20-0.32%, Cr0.14-0.18%, Ni0.40-0.80%, Al0.01-0.04%, P≦0.015%, S≦0.010% and the balance of Fe and impurities;

[0040] The chemical components contained in the steel of the present invention are further described as follows: C: is a strong solid solution strengthening element and is also one of the elements forming the precipitation strengthening phase. Too high C content will reduce the toughness and welding performance of the steel. The C content of the steel of the present invention is 0.04-0.06%.

[0041] Si: Si can be used as a deoxidizer and a reducing agent in the steelmaking process and has a certain solid solution strengthening effect. However, a high content is not good for the low temperature toughness and weldability of the steel plate. The silicon content of the present invention is controlled at 0.14-0.22%.

[0042] Mn: Manganese plays an important role in improving the strength of steel, improving low-temperature toughness, and reducing the tough-to-brittle transition temperature of steel, and it is low in cost. However, too high manganese can easily cause segregation of the ingot, causing the steel plate to produce a banded structure that is difficult to eliminate after rolling, reducing the transverse performance and lamellar tearing resistance of the steel plate. The manganese content of the present invention is controlled at 1.5-1.65%.

[0043] Nb: Niobium is one of the key elements for grain refinement and strengthening. It refines grains in two ways. First, niobium has a significant delay effect on austenite recrystallization, increases the recrystallization temperature, and prevents the growth of recrystallized austenite grains. Second, as the rolling temperature decreases, niobium carbon and nitride can be dispersed and precipitated before austenite transforms to ferrite, becoming nucleation points for ferrite, so that ferrite is formed at a smaller undercooling degree and is not easy to grow, thereby refining ferrite grains. As a niobium element that expands the non-recrystallization temperature range, it increases the area and fraction of high-angle grain boundaries by refining grains, thereby improving the crack arrest toughness of the steel plate. The niobium content of the present invention is controlled at 0.02-0.05%.

[0044] Ti: trace titanium combines with C and N in steel to form fine and stable C and N compound particles, which can effectively prevent the coarsening of austenite grains during the heating process of the slab. Titanium nitrides can inhibit the grain coarsening of the welding heat affected zone during welding, thereby improving the low-temperature toughness of the base metal and the welding heat affected zone. The titanium content of the present invention is controlled at Ti0.01-0.02%.

[0045] Ni: Nickel can effectively improve the low temperature toughness and corrosion resistance of steel, but as the nickel content increases, the production cost will increase significantly. The nickel content of the present invention is controlled at 0.40-0.80%.

[0046] Cu: Copper is an austenite stabilizing element. A proper amount of copper can improve the strength and corrosion resistance of the steel plate. However, adding too much copper can easily cause hot brittleness of the steel and damage the surface quality of the steel plate. The copper content of the present invention is controlled at 0.20-0.32%.

[0047] Cr: Chromium can significantly increase the strength of steel, but at the same time reduce the plasticity and toughness of steel. Chromium can also improve the oxidation resistance and corrosion resistance of steel. The chromium content of the present invention is controlled at 0.14-0.18%.

[0048] Al: Aluminum is an important deoxidizing element. A trace amount of aluminum can effectively reduce the inclusion content in steel and refine the grains, but too much aluminum will increase the tendency of cracks on the surface of the ingot. The aluminum content of the present invention is controlled at 0.01-0.04%.

[0049] The thickness of the steel plate is 50-100 mm, the tensile strength of the steel plate is 560-650 MPa, and the yield strength ratio of the steel plate is lower than 0.72.

[0050] The AKV2 of the steel plate at -40℃ is greater than 300J, the fracture toughness characteristic CTOD value of the steel plate at -80℃ is greater than 0.20mm, and the crack arrest toughness Kca of the steel plate at -10℃ is greater than 9100N / mm 3 / 2 ,The unevenness of the steel plate is less than 4mm / 2m, and the unevenness of the whole plate is less than 6mm.

[0051] A method for manufacturing a ductile steel plate, the method comprising the following steps:

[0052] S1. Pre-desulfurization of molten iron;

[0053] S2. Converter steelmaking;

[0054] S3.LF ladle refining;

[0055] S4.RH vacuum degassing;

[0056] S5. Continuous casting of slabs with a thickness of 300 mm or more;

[0057] S6. Slab heating;

[0058] S7. Rough rolling;

[0059] S8. Finish rolling;

[0060] S9. Thermal straightening;

[0061] S10. Control cooling;

[0062] S11.Finishing.

[0063] The heating temperature of the slab heating in S6 is 1100-1150° C., and the slab heating in S6 is maintained for 30-60 minutes after completion.

[0064] The steel plate is subjected to two-stage rough rolling, wherein the final rolling temperature of the second stage rough rolling is ≥930°C.

[0065] The steel plate is subjected to two-stage finish rolling, and the total reduction rate of the steel plate after the two-stage finish rolling is ≥50%, and the final rolling temperature of the steel plate is 700-830°C.

[0066] The hot straightening time of steel plate is controlled within 90-120s.

[0067] The starting cooling temperature of the steel plate controlled cooling is 680-720°C, the cooling rate of the steel plate controlled cooling is controlled at 3-10°C / s, and the final cooling temperature of the steel plate controlled cooling is controlled at 250-450°C.

[0068] After the controlled cooling of the steel plate is completed, it needs to be slowly cooled naturally to room temperature.

[0069] Table 1 Comparison of this paper

[0070]

[0071] Table 1 (Continued) Comparison of this article

[0072]

[0073] Steelmaking and continuous casting: converter BOF, ladle refining LF, vacuum degassing RH or VD, slab continuous casting CC, ingot casting IC.

[0074] Example

[0075] The thickness of the high-strength and tough steel plate involved in this embodiment is in the range of 50-100 mm. The invention steel is subjected to molten iron pretreatment → converter steelmaking → ladle refining LF → vacuum degassing RH → slab continuous casting, and the components and their weight percentages thereof are: C0.04-0.06%, Si0.20-0.25%, Mn1.50-1.65%, P≤0.009%, S≤0.003%, Nb0.035-0.045%, Ti0.013-0.015%, Cu0.16-0.32%, Ni0.40-0.80%, Cr0.14-0.18%, Al0.035-0.040% and the balance Fe, and unavoidable impurity elements. The specific chemical composition of the steel used in each embodiment is listed in Table 2.

[0076] Table 2 Chemical composition of the invention steel (wt%)

[0077]

[0078] The slab with the above chemical composition is heated to 1100-1150°C and kept warm for 30-60 minutes after reaching the target temperature; it is hot rolled on a production line equipped with a four-roll reversible rolling mill and an accelerated cooling system. The production process parameters are shown in Table 3.

[0079] Table 3 Key points of steel rolling production process used in the examples

[0080]

[0081] Tables 4 to 6 give the measured data of rolling force parameters of typical embodiments. Table 7 gives the test results of mechanical properties of the steel plates of the embodiments.

[0082] Table 4-80-mm thick invention steel plate rolling parameters

[0083]

[0084] Table 5-65-mm thick invention steel plate rolling parameters

[0085]

[0086] Table 6-100-mm thick invention steel plate rolling parameters

[0087]

[0088] Table 7 Mechanical properties of the invention steel plate

[0089]

[0090] Table 7 (Continued) Mechanical properties of the invention steel plate

[0091]

[0092] The steel of the present invention has excellent brittle fracture resistance. The crack tip opening displacement CTOD characteristic value test results of the F7-3 steel plate of the embodiment are measured by a three-point bending test specimen as shown in Table 8. The load-notch displacement curve and the fracture of the 50 mm thick test specimen are shown in Table 8. Figure 5 and Figure 6 shown.

[0093] Table 8 Test results of crack tip opening displacement CTOD of invention steel

[0094]

[0095] In this paper, a gradient temperature wide plate double tensile test was carried out according to the Japanese Welding Association standard WES2815. The full thickness specimen of the steel plate with a maximum thickness of 100mm is 800mm long (longitudinal) and 500mm wide (transverse). Under the longitudinal principal tensile stress and transverse temperature gradient, the brittle crack is first induced at the low temperature end, and the moving crack propagates in the direction of the temperature increase. The crack arrest toughness is calculated according to the position of the end of the crack propagation. This paper completed 4 effective crack arrest toughness tests, thereby determining the -10℃ crack arrest toughness Kca (see Table 9). The relationship between the crack arrest toughness Kca and temperature is shown in the figure. Figure 7 The typical specimen fracture is shown in Figure 8 As shown (arrows point to the crack arrest position). As the thickness of the steel plate decreases, the crack arrest toughness gradually increases. It can be seen that the crack arrest toughness Kca of 50-100mm thick steel at -10℃ in this paper is higher than 9100N / mm 3 / 2 .

[0096] Table 9 Crack arrest toughness test results of the invention steel

[0097] Steel plate number Steel plate thickness / mm Test temperature range / ℃ <![CDATA[Kca fitting result at -10°C, N / mm 3 / 2 > F7-3 100 -5－-22 9140

[0098] The present invention has obvious characteristics. For example, thick (50-100 mm) steel plates combine low yield ratio (≤0.72), high strength (tensile strength of 560-650 MPa), high fracture toughness (CTOD) and crack arrest toughness. Because it is produced using the TMCP process of controlled rolling-hot straightening-controlled cooling, the straightness of the steel plates is guaranteed to meet 4 mm / 2 m. The steel plates listed in Table 2 do not need to be flattened offline, and the production efficiency is improved.

[0099] A large number of production tests have shown that when a chemical composition different from that of the invention steel is used, or a TMCP process flow and parameters different from those of the present invention are used, the mechanical properties of the produced steel plate cannot meet the requirements of the present invention. Table 10 shows the comparative test TMCP process used in this article, and Table 11 shows the chemical composition of the comparative test steel. Among them, the comparative steel is produced by molten iron pretreatment → converter steelmaking → ladle refining LF → vacuum degassing RH → slab continuous casting. The mechanical properties of the comparative test steel plate are listed in Table 12, and the typical microstructure (steel plate D4-1) is as follows Fig. 9 shown.

[0100] Table 10 Key points of steel rolling production process used in comparative test

[0101]

[0102] Table 11 Chemical composition of steel used in comparative tests (wt%)

[0103]

[0104] Table 12 Mechanical properties of comparative test steel plates

[0105]

[0106] Table 13 Flatness of Invention Steel Plate

[0107]

[0108] Table 13 (Continued) Flatness of Invention Steel Plate

[0109]

[0110] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A tough steel plate, characterized in that: The chemical composition of the steel plate includes, by mass percentage, C0.04-0.06%, Si0.14-0.22%, Mn1.50-1.65%, Nb0.02-0.05%, Ti0.01-0.02%, Cu0.20-0.32%, Cr0.14-0.18%, Ni0.40-0.80%, Al0.01-0.04%, P≦0.015%, S≦0.010% and the balance of Fe and impurities, and the delivery state is TMCP; The rolling process is divided into two stages: rough rolling and finishing rolling. Controlled rolling of rough rolling: the heating temperature of the slab is 1100-1150℃, and the slab is kept warm for 30-60min after heating. The steel plate is rough rolled in two stages, and the final rolling temperature of the second stage is ≥930℃; Controlled rolling of finishing rolling: the steel plate is subjected to two-stage finishing rolling, and the total reduction rate of the steel plate after the two-stage finishing rolling is ≥50%, and the final rolling temperature of the steel plate is 700-830℃, the hot straightening time of the steel plate is controlled within 90-120s, the starting cooling temperature of the steel plate controlled cooling is 680-720℃, and the cooling rate of the steel plate controlled cooling is controlled at 3-10℃ / s, and the final cooling temperature of the steel plate controlled cooling is controlled at 250-450℃, and the steel plate needs to be slowly cooled naturally to room temperature after the controlled cooling is completed; The thickness of the steel plate is 50-100mm, the tensile strength of the steel plate is 560-650MPa, the yield strength ratio of the steel plate is lower than 0.72, the AKV2 of the steel plate at -40°C is greater than 300J, the fracture toughness characteristic CTOD value of the steel plate at -80°C is greater than 0.20mm, and the crack arrest toughness Kca of the steel plate at -10°C is greater than 9100N / mm 3 / 2 ,The unevenness of the steel plate is less than 4mm / 2m, and the unevenness of the whole plate is less than 6mm.

2. The method for manufacturing a ductile steel plate according to claim 1, characterized in that: The manufacturing method comprises the following steps: S1. Pre-desulfurization of molten iron; S2. Converter steelmaking; S3.LF ladle refining; S4.RH vacuum degassing; S5. Continuous casting of slabs with a thickness of 300 mm or more; S6. Slab heating; S7. Rough rolling; S8. Finish rolling; S9. Thermal straightening; S10. Control cooling; S11.Finishing.

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