Steel for ultra-high heat input welding and its manufacturing method

Abstract

The manufacturing method of ultra-large heat input welding steel and the manufactured welding steel adopt advantageous control of oxide-based inclusions in steel plate, adopt low-temperature large reduction rolling to produce, and control the rolling process to control the size and distribution of oxides in the steel plate, thereby achieving the manufacturing of ultra-large heat input welding steel. The method has low production costs, easy-to-control production process, simple operation, is suitable for mass production, and has excellent mechanical properties and welding heat effect properties of the base material.

Description

Technical Field

[0001] This application belongs to the technical field of steel rolling, and specifically relates to steel for super-large heat input welding and its manufacturing method.

[0002] This application claims the priority of a Chinese patent application with an application number of 202310238411.3 and an invention title of "Steel for Super-Large Heat Input Welding and Its Manufacturing Method", which was filed with the China Patent Office on March 13, 2023, and all of its contents are incorporated herein by reference.

Background Art

[0003] In the fields of large steel structures such as ships, buildings, and bridges, welding is a crucial process in manufacturing. By increasing the heat input, the welding efficiency can be improved, and the manufacturing cycle of the project can be shortened. Especially in the case of thick steel plates, conventional welding methods require multi-pass welding forming and complex auxiliary operations before welding and between layers. However, if a large heat input welding method is adopted, it can be formed in one pass, and the production efficiency can be improved several times or dozens of times. Therefore, in various construction fields, the manufacturing of large steel welded structures tends to adopt the large heat input welding method, thereby significantly reducing the manufacturing cost and improving the production efficiency.

[0004] The heat input that general steel for large heat input welding can withstand is 100 - 400 kJ / cm. However, with the development of industries such as construction and shipbuilding, the requirements for welding technology with a larger heat input are increasing. Moreover, in order to ensure the safety and reliability of the construction structure, it is required that the steel has high mechanical properties even at a super-large heat input of 500 - 1500 kJ / cm.

[0005] Conventional technologies offer several high-strength steels and manufacturing methods for ultra-high heat input welding. However, most require the addition of large amounts of expensive components such as nickel or calcium-magnesium core wires or zirconium-calcium core wires during the steel smelting process to improve the low-temperature toughness of welded joints under these high heat input welding conditions, thus significantly increasing production costs. Furthermore, these technologies all improve the performance of the steel from the perspective of alloy composition, and controlling the content of components through process operation is difficult, leading to longer smelting processes and reduced production efficiency.

[0006] If we can provide a method for producing high-strength steel for ultra-high heat input welding that does not require expensive components and shortens the smelting process, then its future potential is promising. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] Therefore, the technical problem that this application aims to solve is to provide ultra-high heat input welding steel and a method for manufacturing the same by solving the defects of conventional ultra-high heat input welding steel, such as high production costs, high control difficulty, and low production efficiency. [Means for solving the problem]

[0008] Therefore, this application provides the following technical solutions. This application is, The steelmaking steps involve converter smelting and LF furnace refining, followed by the casting of slabs. The slab is heated, and a two-stage rolling process is employed. In the first stage, the rolling temperature is 900-1000°C, and the reduction ratio per pass is greater than 20% in the recrystallized region. After the steel sheet has cooled to below 800°C, the second stage of rolling is performed in the non-recrystallized region, with a reduction ratio per pass greater than 20%. The total compression ratio of the rolling steps is controlled to be 5 or more, and the compression ratio of the second stage of rolling accounts for 65-75% of the total compression ratio. The present invention provides a method for manufacturing steel for ultra-high heat input welding, which includes a cooling step in which the steel is cooled after the completion of the rolling step.

[0009] Selectively, the final rolling temperature of the first stage of rolling is 900-950°C, and / or The final rolling temperature in the rolling step is controlled to be within a range of 20 to 50°C higher than the ferrite transformation onset temperature (Ar3).

[0010] Selectively, the reduction ratio per pass in the first stage of the rolling process is 21-23%, and / or The reduction ratio per pass during the second stage of the rolling process is 21-26%.

[0011] The slab heating temperature is selectable to be 1050-1150°C, and the slab heating time is 330 min or longer.

[0012] The slab heating time is selectable between 330 and 350 minutes.

[0013] Selectively, the cooling rate in the cooling step is 11°C / s or higher, the final cooling temperature is controlled to be above the bainite transformation start temperature Bs and within the range of 20-40°C, and then air-cooled to 350°C or lower.

[0014] However, Ar3 = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo-0.35(H-8), where the unit of Ar3 is °C, H is the target steel plate thickness, and the unit is mm.

[0015] In the converter smelting step, the ratio of molten iron to clean iron scrap is 7-8:1, and the temperature of the molten iron is 1350-1450°C.

[0016] In the casting step, the casting temperature is controlled to 1540-1560°C and the drawing speed is controlled to 1.1-1.3 m / min, respectively.

[0017] Selectively, the chemical composition of the slab includes C: 0.05-0.16 wt%, Si: 0.1-0.4 wt%, Mn: 0.9-1.6 wt%, P ≤ 0.01 wt%, S: 0.003-0.02 wt%, Cr: 0.05-0.20 wt%, Ni: 0.1-0.4 wt%, Ti: 0.02-0.04 wt%, Ca: 0.001-0.0025 wt%, with the remainder being Fe and unavoidable impurities.

[0018] The following provides a detailed explanation of the effects and selection of usage amounts of the components included in this invention. S:S is the main element that causes hot brittleness during the hot rolling process and should be controlled within a reasonable range. S combines with Mn in steel to form MnS inclusions, and especially when the Mn content in the steel is high, the formed MnS is not only numerous but also large in size. During the hot rolling process, MnS stretches along the rolling direction due to its plasticity, forming MnS inclusion bands along the rolling direction, which seriously impairs the low-temperature impact toughness, elongation, and Z-direction properties of the steel sheet. Cr:Cr can effectively improve the hardenability of steel sheets. When its content is greater than 0.1%, combined with an ultra-rapid quenching process, it forms a microstructure based on fine needle ferrite. However, if the content is too high, it negatively affects the weldability of the steel sheet. Ni: Ni is an effective element for improving strength and low-temperature toughness. However, if the Ni content is too high, scale on the steel sheet becomes difficult to remove, thereby affecting the surface quality of the steel sheet. Moreover, since Ni is expensive, adding too much increases manufacturing costs.

[0019] This application further provides ultra-high heat input welding steel manufactured by the above manufacturing method.

[0020] For selectable applications, the steel for ultra-high heat input welding has a base material with a yield strength of 460 MPa or higher, a tensile strength of 560-620 MPa, an elongation of 26% or higher, and an impact energy of 280 J or higher at -40°C.

[0021] Optionally, under the condition that the welding heat input is 600 kJ / cm, the tensile strength of the heat-affected zone of the weld is 580 Mpa or more, and the impact energy at -40 °C is 220 J or more.

Advantages of the Invention

[0022] The technical solution of the present application has the following advantages.

[0023] The present invention relates to a method for manufacturing ultra-high heat input welding steel, comprising: a steelmaking step of performing converter smelting and LF furnace refining to cast a slab; a rolling step of heating the slab and employing a two-stage rolling process, wherein the first stage of rolling is performed in the recrystallized region with a rolling temperature of 900 to 1000°C and a reduction ratio of more than 20% per pass, and the steel plate is allowed to cool down until the temperature drops to 800°C or below, at which point the second stage of rolling, which is rolling in the non-recrystallized region, is performed with a reduction ratio of more than 20% per pass, the total compression ratio of the rolling steps is controlled to be 5 or more, and the compression ratio of the second stage of rolling accounts for 65 to 75% of the total compression ratio of the rolling steps; and a cooling step after the completion of rolling. This invention is the first to rethink the conventional approach to oxide metallurgy and employ low-temperature, high-reduction rolling technology to produce ultra-high heat input welding steel. By controlling the reduction ratio per pass and total compression ratio in two-stage rolling, limiting the compression ratio in the second stage of rolling, and combining this with the TMCP process, the size and distribution of oxides in the steel sheet are controlled, solving the difficult problem of controlling the dense distribution of oxide inclusion particles in the steel sheet and realizing the production of ultra-high heat input welding steel. This method has the advantages of low production costs, easy control of the production process, simple operation, and suitability for mass production. Furthermore, it does not require the addition of expensive components to control the oxide content during the steelmaking stage, shortens the rolling cycle, improves production efficiency, and has high added value. It excels mainly in the mechanical properties of the base material and the performance of the weld heat-affected zone, and in particular, the low-temperature toughness of the weld heat-affected zone is stable under high heat input welding conditions. Therefore, it can be widely applied to various large and ultra-large steel structures that require high-efficiency welding. From the perspective of downstream welding users, this invention further improves welding efficiency, significantly reduces the workload of welding operations, saves users labor costs for welding, and drastically shortens the time it takes for users to manufacture steel components, thereby bringing enormous value to users.

[0024] The manufacturing method of the steel for extra-large heat input welding according to the present application invents an advantageous control technology for oxide-based inclusions in the steel plate. By increasing the reduction ratio of rolling and the reduction amount per pass, the size of the metal oxide inclusions in the steel plate is controlled to be lower than the size of the effective grain size, and fine inclusions are sufficiently dispersed and distributed to promote the formation of needle ferrite in the grains. Further, the present application can also control the surface density of oxides, the content and size of needle ferrite, etc. in the steel plate by limiting parameters, and further realizes the control of the performance of the steel plate.

[0025] The steel for extra-large heat input welding according to the present application is manufactured by the specific method of the present application. In the manufactured steel plate, the content of needle ferrite is 75% or more, the size is 17 μm or less, the base material has a yield strength of 460 Mpa or more, a tensile strength of 560 - 620 Mpa, an elongation rate of 26% or more, and an impact energy at -40°C of 280 J or more. Under the condition that the heat input for welding is 600 kJ / cm, the tensile strength of the heat-affected zone of the weld is 580 Mpa or more, and the impact energy at -40°C is 220 J or more.

Embodiments for Carrying out the Invention

[0026] The following examples are provided for better understanding of the present application. They are not limited to the optimal embodiments, and do not limit the content and protection scope of the present application. Any product identical or similar to the present application obtained by someone's inspiration from the present application or by combining the features of the present application with other prior arts is within the protection scope of the present application.

[0027] In the examples, when specific experimental steps or conditions are not specified, the operations or conditions of the conventional experimental steps described in the literature in this field may be followed. When the manufacturers of the reagents or equipment used are not specified, all are ordinary reagent products that can be obtained as commercially available products. Examples and Comparative Examples

[0028] The manufacturing method of the steel for extra-large heat input welding includes the following steps.

[0029] 1. Component Design and Steelmaking Process The chemical composition of steel sheets includes C: 0.05-0.16 wt%, Si: 0.1-0.4 wt%, Mn: 0.9-1.6 wt%, P ≤ 0.01 wt%, S: 0.003-0.02 wt%, Cr: 0.05-0.20 wt%, Ni: 0.1-0.4 wt%, Ti: 0.02-0.04 wt%, Ca: 0.001-0.0025 wt%, with the remainder being Fe and unavoidable impurities. Steelmaking process: In the converter steelmaking step, the ratio of molten iron to clean iron scrap is 7-8:1, and the temperature of the molten iron is 1350-1450°C. Ferromanganese, ferrosilicon, and lime are added sequentially during the melting process. In LF refining, the main focus is on controlling inclusions and adjusting the alloy composition. Metallic manganese, ferrosilicon, etc., are added to adjust the molten steel composition. After energizing, the temperature is measured and sampled. Then, diffusion deoxidation and temperature adjustment are continued, and finally, sample is taken, the oxygen content is measured, and the steel is tapped. The casting temperature was controlled to 1540-1560°C, and the drawing speed was controlled to 1.1-1.3 m / min to produce continuously cast slabs.

[0030] 2. Rolling process 1) Slab heating The heating temperature was controlled to 1050°C to 1150°C, and the heating time was controlled to 330 to 350 minutes. 2) Slab rolling A two-stage controlled rolling process was employed. In the first stage, the rolling temperature was controlled to be above the critical temperature for recrystallization, within the range of 900-1000°C, and the reduction ratio per pass was greater than 20% in the recrystallized region. After that, the steel sheet was allowed to cool down, and once the temperature dropped below 800°C, the second stage of rolling, which is in the non-recrystallized region, was performed, with a reduction ratio per pass greater than 20% and the final rolling temperature controlled to be within a range of 20-50°C higher than that of Ar3.

[0031] 3. Cooling process Immediately after the rolling process is complete, the steel sheet is transported to the accelerated cooling control device (ACC) at the maximum conveying speed of the roller table, the cooling rate is set to 11°C / s or higher, and the final cooling temperature is set to B s The temperature was controlled to be within a range of 20-40°C higher than the specified temperature, and then the steel plate was naturally air-cooled to below 350°C.

[0032] The components and parameters in each example and comparative example are specifically controlled as follows.

[0033] [Table 1] JPEG0007860262000002.jpg106155

[0034] [Table 2]

[0035] [Table 3]

[0036] [Table 4] JPEG0007860262000006.jpg132155 Note: Ar3 = 910 - 310C - 80Mn - 20Cu - 15Cr - 55Ni - 80Mo - 0.35(H-8), where H is the target steel plate thickness, and the steel plate thickness is 50mm. Bs = 630 - 45Mn - 40V - 35Si - 30Cr - 25Mo - 20Ni - 15W.

[0037] [Table 5] JPEG0007860262000008.jpg160156

[0038] [Table 6]

[0039] Clearly, the above embodiments are merely illustrative examples for clarity and do not limit the embodiments. Those skilled in the art can make various other forms of variations or modifications based on the above description. It is not necessary, nor is it possible, to list all embodiments here. Any obvious variations or modifications derived therefrom still fall within the scope of the invention.

Claims

1. A method for manufacturing steel for ultra-high heat input welding, The steelmaking steps involve converter smelting and LF furnace refining, followed by the casting of slabs. The slab is heated, and a two-stage rolling process is employed. In the first stage, the rolling temperature is 900-1000°C, and the reduction ratio per pass is greater than 20% in the recrystallized region. After the steel sheet has cooled to below 800°C, the second stage of rolling is performed in the non-recrystallized region, with a reduction ratio per pass greater than 20%. The total compression ratio of the rolling steps is controlled to be 5 or more, and the compression ratio of the second stage of rolling accounts for 65-75% of the total compression ratio. The process includes a cooling step, which involves cooling the area after the rolling step is completed. The method is characterized in that the chemical composition of the slab includes C: 0.05 to 0.16 wt%, Si: 0.1 to 0.4 wt%, Mn: 0.9 to 1.6 wt%, P ≤ 0.01 wt%, S: 0.003 to 0.02 wt%, Cr: 0.05 to 0.20 wt%, Ni: 0.1 to 0.4 wt%, Ti: 0.02 to 0.04 wt%, Ca: 0.001 to 0.0025 wt%, with the remainder being Fe and unavoidable impurities.

2. The final rolling temperature in the first stage of rolling is 900 to 950°C. and / or the final rolling temperature of the rolling step is set to the ferrite transformation start temperature Ar ３ Control the temperature to be within a range of 20 to 50 degrees higher than Ar ３ = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo-0.35(H-8), and Ar ３ The method for manufacturing steel for ultra-high heat input welding according to claim 1, characterized in that the unit of is °C, H is the target steel plate thickness, and the unit is mm.

3. The reduction ratio per pass in the first stage of the rolling process is 21-23%. The method for manufacturing ultra-high heat input welding steel according to claim 1, characterized in that and / or the reduction ratio per pass in the second stage rolling process is 21 to 26%.

4. The method for producing steel for ultra-high heat input welding according to claim 1, characterized in that the slab heating temperature is 1050 to 1150°C and the slab heating time is 330 min or more.

5. The method for manufacturing ultra-high heat input welding steel according to claim 4, characterized in that the slab heating time is 330 to 350 mins.

6. The cooling rate in the aforementioned cooling step is 11°C / s or higher. A method for producing ultra-high heat input welding steel according to claim 1, characterized in that the final cooling temperature is controlled to be within a range of 20 to 40°C higher than the bainite transformation onset temperature Bs, and then air-cooled to 350°C or lower, and Bs = 630 - 45Mn - 40V - 35Si - 30Cr - 25Mo - 20Ni - 15W, where the unit of Bs is °C.

7. The method for manufacturing ultra-high heat input welding steel according to Claim 1, characterized in that in the steelmaking step, the ratio of molten iron to clean iron scrap is 7 to 8:1, and the temperature of the molten iron is 1350 to 1450°C.

8. The method for manufacturing ultra-high heat input welding steel according to Claim 1, characterized in that in the steelmaking step, the casting temperature is controlled to 1540 to 1560°C and the drawing speed is controlled to 1.1 to 1.3 m / min.

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