A ship plate steel with high plasticity, impact toughness and excellent weldability and its production method.

Abstract

This invention discloses a ship plate steel with high plasticity, excellent impact toughness, and excellent weldability, and its production method. The composition of the ship plate steel includes C, Si, Mn, Als, P, S, Re, and the balance Fe and impurities. The preparation method includes: heating the billet at a temperature of 1150–1250℃, a heating rate of 8–12 min / cm, and a furnace exit temperature in the range of 1170±30℃; rolling: the first-stage rolling temperature is ≥1050℃, the intermediate billet thickness is 3 times the thickness H of the finished steel plate, and the second-stage rolling temperature is ≤960℃. The final rolling temperature is 830±30℃; the red-hot temperature after rolling is determined according to the thickness H of the finished steel plate. When H≤16mm, the red-hot temperature after rolling is 620±40℃; when 16<H≤25mm, the red-hot temperature after rolling is 640±30℃. The maximum thickness of the finished steel plate obtained by this invention is 25mm, the yield strength ReH≥320MPa, the tensile strength Rm≥420MPa, the elongation A≥30%, the longitudinal impact energy KV2≥200J at 0℃, and the steel plate microstructure is ferrite + pearlite. It is an economical high-plasticity ship plate steel.

Description

Technical Field

[0001] This invention relates to the technical field of ship plate steel manufacturing methods, and in particular to a ship plate steel with high plasticity, impact toughness and excellent weldability, and its production method. Background Technology

[0002] Ship plate steel used in shipbuilding not only needs high strength, but also good plasticity and weldability. Generally, to maintain high strength, ship plates require the addition of higher levels of carbon and other alloying elements. Although the strength is significantly increased, the plasticity and weldability of the steel plate are reduced, and the tensile reduction of area is not high.

[0003] Search results:

[0004] Chinese invention patent application CN103147005A discloses a TMCP type E36 ship plate with good low-temperature toughness and its manufacturing method. Its chemical composition by weight percentage is: C: 0.04–0.12%; Mn: 1.40–1.60%; Si: 0.10–0.50%; P≤0.018%; S≤0.010%; Alt: 0.020–0.070%; Ti: 0.010–0.030%; Nb: 0.010–0.040%; Ni: 0.10–0.30%; Cu: 0.08–0.30%; Mo: 0.005–0.020%; the balance being Fe and unavoidable trace impurities. Although the steel of this invention has a low C content, it contains a large amount of Nb, Cu and other precious alloys. The tensile test of the steel plate has a low reduction of area, less than 30%, so the cost is high and the plasticity is average.

[0005] Chinese invention patent application CN101876033A discloses a low-temperature high-toughness ship plate steel and its production method. Its composition by mass percentage is: C≤0.16%, Si: 0.10~0.50%, Mn: 0.90~1.60%, P≤0.025%, S≤0.025%, Ni≤0.80%, Cr≤0.20%, Cu≤0.35%, Mo≤0.08%, Nb≤0.05%, Ti≤0.02%, with the remainder being Fe and unavoidable impurities. The process involves electric furnace smelting, LF ladle refining, VD furnace vacuum treatment, casting, heating, controlled rolling, straightening, shot blasting, quenching, normalizing, and cutting to obtain the finished steel plate. The maximum thickness of the steel plate is 110mm. The steel plate has high strength (Re≥350MPa, Rm≥480MPa, A5 / %≥35), good low-temperature impact toughness (longitudinal impact energy ≥250J at -60℃). Although the steel invented has good plasticity, its actual production requires the addition of a large amount of precious alloys such as Nb, Ti, Cu, Cr, and Mo, and it also requires heat treatment by quenching and normalizing, resulting in high production costs.

[0006] Currently, there is no economical ship plate steel on the market that combines excellent plasticity with excellent weldability. Summary of the Invention

[0007] The purpose of this invention is to address the urgent need in the shipbuilding industry for ship plate steel that is low in cost, possesses high plasticity, and excellent weldability, while existing steel plates cannot simultaneously meet these requirements. This invention provides a ship plate steel with high plasticity, impact toughness, and excellent weldability, along with its production method. This invention effectively solves the problem of achieving good plasticity and weldability in high-strength steel plates.

[0008] This invention discloses a ship plate steel with high plasticity, excellent impact toughness and weldability, and a method for producing the same. The ship plate steel contains the following chemical composition by mass percentage: C: 0.10-0.12%, Si: 0.25-0.30%, Mn: 0.85-1.0%, P≤0.02%, S≤0.003%, Als: 0.025-0.04%, Re: 0.01-0.018%, Re / S≥5, Ceq=C+1 / 6Mn≤0.28%, with the remainder being Fe and unavoidable impurities.

[0009] This invention discloses a method for producing ship plate steel with high plasticity, impact toughness, and excellent weldability, comprising: hot metal pretreatment → converter smelting → LF furnace refining → RH furnace vacuum treatment → slab casting → slow cooling of cast billets → heating of cast billets → rolling → laminar flow cooling → straightening → air cooling, wherein:

[0010] (1) Smelting process: Cast slabs with a thickness of 200-300 mm according to the required chemical composition weight percentage of steel plates, and feed rare earth (Re) during the casting process to control Re / S≥5;

[0011] (2) Heating of billet: The heating temperature is controlled at 1150~1250℃, the heating rate is 8~12min / cm, and the furnace exit temperature is in the range of 1170±30℃;

[0012] (3) Rolling process: The rolling mill adopts two-stage controlled rolling. The first stage opening rolling temperature is ≥1050℃, and the intermediate billet waiting thickness is 3 times the finished steel plate thickness H. The second stage opening rolling temperature and the final rolling temperature are determined according to the finished steel plate thickness H. When H≤16mm, the second stage opening rolling temperature is ≤960℃ and the final rolling temperature is 830±30℃. When 16<H≤25mm, the second stage opening rolling temperature is ≤950℃ and the final rolling temperature is 840±20℃.

[0013] (4) The temperature of the steel plate after rolling is determined according to the thickness H of the finished steel plate. When H≤16mm, the temperature of the steel plate after rolling is 620±40℃; when 16<H≤25mm, the temperature of the steel plate after rolling is 640±30℃.

[0014] The finished steel plate obtained by this invention has a maximum thickness of 25mm, a yield strength ReH≥320MPa, a tensile strength Rm≥420MPa, an elongation A≥30%, and a longitudinal impact energy KV2≥200J at 0℃. The microstructure of the steel plate is F (ferrite) + P (pearlite).

[0015] The reasons for limiting the main chemical components in the steel of this invention are as follows:

[0016] Carbon (C) is the most effective element for increasing the strength of steel. As the carbon content increases, the strength of the steel also increases, but it reduces the steel's plasticity, elongation, and impact toughness, and also affects the steel's weldability. To improve the plasticity of the steel, the steel of this invention adopts a low-carbon design, with the carbon content controlled at 0.10–0.12%.

[0017] Si can improve the hardness and strength of the solid solution in steel, but an increase in Si content is detrimental to the toughness of the heat-affected zone and makes it prone to cracking defects. The Si content of the steel in this invention is designed to be 0.25-0.30%.

[0018] Mn is a strengthening alloying element in steel. It has a strong affinity for carbon and is an effective element for expanding the austenite phase region, refining grains, and ensuring overall performance. It does not deteriorate the deformability of steel, but excessive content will cause grain coarsening at high temperatures. In this invention, the Mn content is controlled within the range of 0.85% to 1.0%. Simultaneously, to improve weldability, Ceq = C + 1 / 6Mn ≤ 0.28%.

[0019] P and S are impurity elements in steel. P has a strong solid solution strengthening effect in steel, improving its strength and resistance to atmospheric corrosion. However, P tends to segregate severely in localized areas, reducing the steel's plasticity and toughness, and is extremely detrimental to low-temperature toughness. S is prone to segregation and enrichment in steel. Sulfide inclusions are accumulation sites for hydrogen, causing the metal to form defective structures. Sulfur also acts as a promoter of hydrogen adsorption. In this invention, the steel contains P ≤ 0.02% and S ≤ 0.004%.

[0020] Al is the main deoxidizing element in steel. In steel, Al can combine with N to form AlN, which can hinder the growth of austenite at high temperatures, thus refining the grain size. However, when the Al content is too high, it easily leads to an increase in inclusions in the steel, which is detrimental to its toughness. The Al content of the steel in this invention is controlled at 0.025–0.04%.

[0021] Rare earth element Re has a strong chemical affinity for impurities such as O and S in molten steel, and can act as a strong deoxidizer and desulfurizer, thus purifying the steel. The oxides, sulfides, or oxysulfides formed by the reaction with O and S in molten steel have high melting points and can serve as heterogeneous nucleation centers during solidification, refining the solidification structure of the steel. Trace amounts of rare earth elements dissolved in steel can produce alloying effects, affecting the phase transformation process, altering the composition and structure of the phase transformation products, thereby improving the fatigue performance of the steel and enhancing the corrosion resistance of ship plates. The steel of this invention controls Re to 0.01-0.018% and Re / S ≥ 5.

[0022] The reasons for setting the main production process parameters in this invention are as follows:

[0023] Heating slabs can improve the plasticity of steel, reduce deformation resistance, improve the internal structure and properties of the metal, and facilitate rolling. The maximum heating temperature should generally be 100-150℃ below the solidus. However, excessively high temperatures or prolonged heating times may cause defects such as severe oxidation, overheating, and burning in the steel, and may also cause excessive growth of austenite grains, deteriorating the mechanical properties of the steel. To ensure sufficient time for homogenization of the microstructure and dissolution of carbides at a suitable temperature, resulting in a metal microstructure with high plasticity, low deformation resistance, and good processing performance, the heating temperature of the steel in this invention is controlled at 1150-1250℃, the heating rate at 8-12 min / cm, and the furnace exit temperature within the range of 1170±30℃.

[0024] The first-stage rolling process primarily aims to break down the grains through high-temperature rapid rolling. To ensure that the first-stage rolling process remains within the austenite recrystallization region, this invention controls the initial rolling temperature to be ≥1050℃. The second-stage rolling is carried out in the non-recrystallized austenite region. To ensure the acquisition of fine and uniform ferrite grains, the total reduction rate in the non-recrystallization region needs to be greater than a certain value, and the final rolling temperature should be higher than Ar3. Calculations show that the Ar3 temperature for the steel of this invention is 798℃. Therefore, the intermediate billet thickness to be heated in this invention is three times the thickness H of the finished steel plate, and the final rolling temperature is determined based on the thickness H of the finished steel plate. When H≤16mm, the final rolling temperature is 830±30℃; when 16<H≤25mm, the final rolling temperature is 840±20℃.

[0025] Post-rolling cooling is a heat treatment process that utilizes the residual heat after rolling. As the cooling rate increases, the reheating temperature decreases, and fine pearlite forms in the steel plate, increasing its strength and improving its plasticity and toughness. However, if the cooling rate is too high and the reheating temperature is too low, a mixed structure containing bainite will form, affecting the steel plate's properties and reducing its plasticity and toughness. The steel of this invention requires a ferrite + pearlite structure and good plasticity and toughness. Therefore, the post-rolling reheating temperature is determined based on the thickness H of the finished steel plate. When H ≤ 16 mm, the post-rolling reheating temperature is 620 ± 40 °C; when 16 < H ≤ 25 mm, the post-rolling reheating temperature is 640 ± 30 °C.

[0026] The present invention has the following advantages over the prior art:

[0027] The steel plate of this invention has a maximum thickness of 25mm, a yield strength ReH≥320MPa, a tensile strength Rm≥420MPa, an elongation A≥30%, and a longitudinal impact energy KV2≥200J at 0℃. The microstructure of the steel plate is F (ferrite) + P (pearlite).

[0028] This invention relates to a type of ship plate steel with high plasticity, excellent impact toughness, and good weldability, which can be used in shipbuilding or other structural components requiring high plasticity and good weldability.

[0029] This invention has the advantages of simple manufacturing process and can be implemented in various metallurgical enterprises. Attached Figure Description

[0030] Figure 1 This is a metallographic diagram of the steel plate obtained in Example 1 of the present invention. Detailed Implementation

[0031] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way.

[0032] In practical applications, depending on the steel plate production specifications and batches, there are different component contents, specific process control conditions, and corresponding mechanical property indicators within the control range. In order to better illustrate and explain the present invention, Tables 1 to 3 list the components, process conditions, and mechanical properties of the embodiments of the present invention (the steel grades involved in the present invention) and the comparative examples (existing steel grades) for comparison.

[0033] Table 1 below lists the chemical composition (wt%) values ​​of the steel plates in various embodiments and comparative examples of the present invention;

[0034] Table 2 below lists the main production process parameters for the steel plates in various embodiments and comparative examples of the present invention.

[0035] Table 3 below lists the test results of the main mechanical properties of the steel plates in various embodiments and comparative examples of the present invention.

[0036] A method for producing ship plate steel with high plasticity, impact toughness, and excellent weldability according to various embodiments of the present invention includes: hot metal pretreatment → converter smelting → LF furnace refining → RH furnace vacuum treatment → slab casting → slow cooling of cast billet → heating of cast billet → rolling → laminar flow cooling → straightening → air cooling, wherein:

[0037] (1) Smelting process: Cast slabs with a thickness of 200-300 mm according to the required chemical composition weight percentage of steel plates, and feed rare earth (Re) during the casting process to control Re / S≥5;

[0038] (2) Heating of billet: The heating temperature is controlled at 1150~1250℃, the heating rate is 8~12min / cm, and the furnace exit temperature is in the range of 1170±30℃;

[0039] (3) Rolling process: The rolling mill adopts two-stage controlled rolling. The first stage opening rolling temperature is ≥1050℃, and the intermediate billet waiting thickness is 3 times the finished steel plate thickness H. The second stage opening rolling temperature and the final rolling temperature are determined according to the finished steel plate thickness H. When H≤16mm, the second stage opening rolling temperature is ≤960℃ and the final rolling temperature is 830±30℃. When 16<H≤25mm, the second stage opening rolling temperature is ≤950℃ and the final rolling temperature is 840±20℃.

[0040] (4) The temperature of the steel plate after rolling is determined according to the thickness H of the finished steel plate. When H≤16mm, the temperature of the steel plate after rolling is 620±40℃; when 16<H≤25mm, the temperature of the steel plate after rolling is 640±30℃.

[0041] Table 1. List of chemical composition (wt%) values ​​of steel plates in various embodiments and comparative examples of the present invention.

[0042]

[0043] Table 2. List of main production process parameters for steel plates in various embodiments and comparative examples of the present invention.

[0044]

[0045] Table 3. List of test results of mechanical properties of steel plates in various embodiments and comparative examples of the present invention.

[0046]

[0047] As shown in Table 3, the ship plate steel produced in Examples 1-3 of this invention has a yield strength ReH of over 320 MPa, a tensile strength Rm of over 420 MPa, an elongation A of over 30%, and a longitudinal impact energy KV2 at 0℃ of over 200 J. The microstructure of the steel plate is F (ferrite) + P (pearlite), with F grain size of grade 8 or higher. See [reference needed]. Figure 1 , Figure 1 This is a metallographic diagram of the steel plate obtained in Example 1 of the present invention. As can be seen from the diagram, the steel plate has an F+P structure, which is uniform and has both high strength and good plasticity and impact toughness.

[0048] Meanwhile, the steel of this invention has a Ceq≤0.28%, which gives it good weldability, and the addition of rare earth elements gives it good corrosion resistance.

[0049] In contrast, the steel plate of Comparative Example 1 exhibits low plasticity, with an elongation below 25%, a longitudinal impact energy (KV2) below 170 J at 0°C, and a grain size (F) of grade 6. The steel plate of Comparative Example 2 also shows low plasticity, with an elongation below 25%, a yield strength (ReH) below 300 MPa, a longitudinal impact energy (KV2) below 170 J at 0°C, and a grain size (F) of grade 7. The overall mechanical properties of Comparative Examples 1 and 2 are significantly lower than those of Examples 1-3 of this invention.

[0050] The above embodiments are merely specific examples exemplified to explain the present invention and do not limit the present invention in any way. Any non-substantial changes made by any person based on the above content and form that do not depart from the scope of protection of the claims of the present invention should be considered to fall within the scope of protection of the claims of the present invention.

Claims

1. A type of ship plate steel with high plasticity, excellent impact toughness, and good weldability, characterized in that: The ship plate steel contains the following chemical composition by mass percentage: C: 0.10-0.12%, Si: 0.25-0.30%, Mn: 0.85-1.0%, P≤0.02%, S≤0.003%, Als: 0.025-0.04%, Re: 0.01-0.018%, Re / S≥5, Ceq=C+1 / 6Mn≤0.28%, with the remainder being Fe and unavoidable impurities; the maximum thickness of the finished steel plate is 25mm, yield strength ReH≥320MPa, tensile strength Rm≥420MPa, elongation A≥30%, longitudinal impact energy KV2≥200J at 0℃, and the steel plate microstructure is ferrite + pearlite with F grain size ≥8.

5. The method for producing ship plate steel with high plasticity, impact toughness, and excellent weldability includes: hot metal pretreatment → converter smelting → LF furnace refining → RH furnace vacuum treatment → slab casting → slow cooling of cast billets → heating of cast billets → rolling → laminar flow cooling → straightening → air cooling, wherein: (1) Smelting process: Cast slabs with a thickness of 200-300 mm according to the required chemical composition weight percentage of steel plates, and feed rare earth Re into the slabs during the casting process, controlling Re / S≥5; (2) Heating of billet: The heating temperature is controlled at 1150~1250℃, the heating rate is 8~12min / cm, and the furnace exit temperature is in the range of 1170±30℃; (3) Rolling process: The rolling mill adopts two-stage controlled rolling. The first stage opening rolling temperature is ≥1050℃, and the intermediate billet waiting thickness is 3 times the finished steel plate thickness H. The second stage opening rolling temperature and the final rolling temperature are determined according to the finished steel plate thickness H. When H≤16mm, the second stage opening rolling temperature is ≤960℃ and the final rolling temperature is 830±30℃. When 16<H≤25mm, the second stage opening rolling temperature is ≤950℃ and the final rolling temperature is 840±20℃. (4) The temperature of the steel plate after rolling is determined according to the thickness H of the finished steel plate. When H≤16mm, the temperature of the steel plate after rolling is 620±40℃; when 16<H≤25mm, the temperature of the steel plate after rolling is 640±30℃.

Images