Economical 360mpa grade carbon-manganese cryogenic steel with super low temperature toughness and method of manufacturing the same

Abstract

The application discloses a saving type 360MPa-grade carbon-manganese cryogenic steel with super-low-temperature toughness and a manufacturing method thereof, and belongs to the field of hot rolling of steel materials.The chemical components of the steel are as follows: C 0.09-0.11%, Si 0.15-0.25%, Mn 1.5-1.7%, no other micro-alloying components, and the balance of Fe and inevitable impurities; the preparation process is as follows: the temperature of a heating furnace is 1150-1250 DEG C, the holding time is greater than 150 min, and the furnace-out temperature is 1100-1200 DEG C; two-stage controlled rolling is adopted, the final rolling temperature is 750-810 DEG C, and relaxation waiting temperature treatment is carried out after rolling to ensure the phase transition proportion of the microstructure ferrite; then, super-fast cooling process is adopted, the cooling starting temperature is 700-740 DEG C, the final cooling temperature is 560-620 DEG C, and the average cooling speed is 30-80 DEG C. The application can produce 360MPa-grade ship plate steel, has good low-temperature toughness, and the thickness specification is 10-40 mm, and is suitable for LPG ship construction.

Description

Technical Field

[0001] This invention relates to the field of hot rolling of steel materials, and in particular to a cost-saving 360MPa grade carbon-manganese low-temperature steel with ultra-low temperature toughness and its manufacturing method. Background Technology

[0002] In the early 21st century, researchers in countries such as Japan and South Korea increasingly focused on LPG marine cryogenic steel, conducting in-depth research and applying the results to practical engineering projects. Japan's JFE Corporation has consistently held a leading position in the international market. With the continuous improvement of my country's economic level, the high-end ship plate market has flourished, and domestic steel manufacturing capabilities have significantly improved. Major ship plate manufacturers are attempting to research and produce LPG marine cryogenic steel to meet their own development needs. Currently, although companies like Nanjing Iron & Steel, Baosteel, and Ansteel are capable of related production, the lack of application experience means that cryogenic steel plates still need to be imported in practice, seriously impacting the improvement of my country's independent research and development capabilities for high-end shipbuilding and marine engineering equipment. Although mainstream classification societies have clear regulations on the product types and performance of marine cryogenic steel, the large variety and complex properties of marine cryogenic steel, along with the high similarity of the regulations, can hinder researchers from conducting subsequent steel grade development. Relevant personnel must pay close attention, referencing foreign research results to formulate specific research plans to ensure the stability of product performance.

[0003] Marine cryogenic steel refers to the main material used in the manufacture of cargo tanks and hull structures of liquefied gas carriers, produced in accordance with the construction specifications of classification societies. To improve the transport performance of LPG carriers, C-Mn series cryogenic steel is designed for temperatures above -55℃. Due to its inherent characteristics, it has attracted considerable attention. Relevant personnel should analyze its application standards to meet the needs of LPG carriers and promote the development of related industries.

[0004] Controlled rolling (TMCP) achieves a fine and uniform microstructure by rationally controlling the heating, deformation, and temperature regimes of the metal during hot rolling. In the finish rolling stage, austenite does not recrystallize; the rolling deformation flattens and elongates the austenite grains, forming strain-accumulated austenite in the form of dislocations, deformation bands, and cellular structures. This strain accumulation increases the nucleation sites for ferrite phase transformation, promoting ferrite nucleation and growth on defects within the deformed austenite grains, thereby refining the phase transformation microstructure. Controlled cooling controls the phase transformation behavior of high-temperature austenite during cooling by regulating the post-rolling cooling rate, the start temperature, and the end temperature, ultimately controlling the microstructure type, morphology, and distribution of the steel, and improving its microstructure and mechanical properties. TMCP offers advantages such as energy saving, simplified production processes, and improved comprehensive mechanical properties of steel; currently, TMCP is widely used in the production of high-strength ship hull structural steel plates both domestically and internationally.

[0005] CN201610815114 discloses "A High-Strength, High-Toughness, Low Yield-to-Temperature Steel for -70℃ and its Manufacturing Method." The steel plate's chemical composition by mass percentage is: C = 0.05–0.20, Si ≤ 0.30, Mn = 0.85–1.50, P ≤ 0.005, S ≤ 0.003, Al = 0.015–0.050, Ni = 0.35–0.70. It also contains one or more of Mo ≤ 0.10, Nb ≤ 0.05, Ti ≤ 0.025, and Ca ≤ 0.005, with the remainder being Fe and unavoidable impurities. This invention, due to the addition of expensive alloys such as Ni and Mo, results in high alloy costs. Furthermore, the steel plate requires normalizing and tempering heat treatment, and it does not employ economical composition control or TMCP process control, leading to high manufacturing costs and complex processes.

[0006] CN201611003652 discloses "A carbon-manganese low-temperature steel for liquefied gas ships and its manufacturing method." The chemical composition of the steel plate is: C = 0.03-0.08, Si = 0.10-0.30, Mn = 0.60-1.30, P ≤ 0.015, S ≤ 0.005, Nb = 0.010-0.080, Al = 0.010-0.060, with the remainder being Fe and unavoidable impurities. This invention employs an Nb micro-alloying process, which increases the alloy cost to some extent, and some examples do not meet the strength requirement of 360MPa carbon-manganese low-temperature steel.

[0007] The patent disclosed in CN201810099802, "A method for manufacturing a low yield strength ratio carbon-manganese low-temperature steel," describes a steel with the following chemical composition by weight percentage: C = 0.05–0.09, Si = 0.10–0.40, Mn = 1.30–1.50, P ≤ 0.010, S ≤ 0.003, Nb ≤ 0.010, Al = 0.030–0.060, Ti = 0.01–0.02, with the remainder being Fe and unavoidable impurities. This invention patent employs microalloying and proposes a controlled rolling and cooling process, but it does not propose the use of an online relaxation and heating process or ultra-rapid cooling technology. Furthermore, some examples do not meet the strength requirement of 360 MPa carbon-manganese low-temperature steel. Summary of the Invention

[0008] Purpose of the invention: The purpose of this invention is to propose a method for manufacturing a cost-effective carbon-manganese ship plate steel with ultra-low temperature toughness, specifically for the production process of 360MPa grade carbon-manganese low-temperature steel.

[0009] Technical solution: The present invention describes a low-temperature toughness, energy-saving 360MPa grade carbon-manganese low-temperature steel with the following chemical composition and mass percentage: C 0.09~0.11%, Si 0.15~0.25%, Mn 1.5~1.7%, without any other microalloying components, with the balance being Fe and unavoidable impurities.

[0010] Furthermore, its chemical composition and mass percentage are as follows: C 0.09%, Si 0.15%, Mn 1.5%, with no other microalloying components, and the balance being Fe and unavoidable impurities.

[0011] Furthermore, its chemical composition and mass percentage are as follows: C 0.11%, Si 0.25%, Mn 1.7%, with no other microalloying components, and the balance being Fe and unavoidable impurities.

[0012] Furthermore, its chemical composition and mass percentage are as follows: C 0.10%, Si 0.20%, Mn 1.6%, with no other microalloying components, and the balance being Fe and unavoidable impurities.

[0013] Furthermore, a method for manufacturing a cost-saving 360MPa grade carbon-manganese low-temperature steel with ultra-low temperature toughness includes heating, rolling, and ultra-rapid cooling to obtain the cost-saving 360MPa grade carbon-manganese low-temperature steel with ultra-low temperature toughness.

[0014] Furthermore, in the heating process, the furnace temperature is 1150–1250℃, the holding time is greater than 150 min, and the furnace exit temperature is 1100–1200℃.

[0015] Furthermore, in the rolling and ultra-rapid cooling process, a two-stage controlled rolling process is adopted, with a final rolling temperature of 750–810°C, followed by a relaxation and temperature-restoring treatment.

[0016] Subsequently, an ultra-fast cooling process was adopted, with a cooling start temperature of 700-740℃, a final cooling temperature of 560-620℃, and an average cooling rate of 30-80℃.

[0017] Furthermore, the prepared low-temperature toughness, economical 360MPa grade carbon-manganese low-temperature steel has a yield strength greater than 360MPa, a tensile strength of 490-620MPa, an elongation greater than 27%, a Charpy impact strength greater than 80J at -60℃, and a thickness of 10-40mm.

[0018] Furthermore, in the prepared low-temperature toughness, the S content of the energy-saving 360MPa grade carbon-manganese low-temperature steel is controlled below 0.002%, the P content is controlled below 0.012%, the Ds inclusion grade is controlled below 2.0, and the initial billet center segregation is controlled below C1.0.

[0019] Furthermore, the microstructure of the prepared low-temperature toughness, energy-saving 360MPa grade carbon-manganese low-temperature steel consists of ferrite and pearlite, with a ferrite content greater than 75%, thus avoiding or reducing the occurrence of overcooled degraded pearlite.

[0020] The components and their contents are described below:

[0021] Carbon (C) is an indispensable element in steel, significantly improving its strength and hardness. It has a significant impact on the steel microstructure. C dissolves into the matrix to form interstitial solid solutions, playing a role in solid solution strengthening and significantly increasing the strength of the matrix. As the carbon content increases, the tensile strength and yield strength of steel will increase, while the elongation and notched impact toughness will decrease. When the C content in steel is high, it is easy to aggravate cold cracking. Therefore, this invention adopts an ultra-low carbon design, in which a small amount of C forms microalloying element carbides in the steel, playing a role in second-phase strengthening and grain refinement. The C percentage content in this invention is set at 0.09% to 0.11%.

[0022] Si: In steel, it mainly improves the strength of steel through strong solid solution strengthening. It is also an essential element for deoxidation in steelmaking and can improve atmospheric corrosion resistance. However, it significantly reduces the plasticity and toughness of steel and significantly reduces the surface coating performance of steel. Therefore, taking into account factors such as strength, toughness, and plasticity, the Si percentage content in this invention is set to 0.15% to 0.25%.

[0023] Mn: A major element in steel. Manganese can improve the strength of materials. Although increasing the C content or Cr can also improve the strength, too much carbon affects the formability and weldability. Cr is too expensive and has limited reserves, which is not conducive to reducing costs. Mn is also the main element in steel to prevent hot brittleness. Taking into account the role of Mn, the percentage of Mn in this invention is set at 1.5% to 1.7%.

[0024] Beneficial effects: Compared with the prior art, the present invention can produce 360MPa grade ship plate steel with good low temperature toughness and a thickness of 10-40mm, which is suitable for LPG ship construction. Attached Figure Description

[0025] Figure 1 This is a scanned image of the microstructure of carbon manganese steel in Embodiment 1 of the present invention. Detailed Implementation

[0026] The present invention will be further described below with reference to embodiments.

[0027] The present invention discloses a low-temperature toughness, energy-saving 360MPa grade carbon-manganese low-temperature steel with the following chemical composition and mass percentage: C 0.09-0.11%, Si 0.15-0.25%, Mn 1.5-1.7%, with no other microalloying components, and the balance being Fe and unavoidable impurities. The material is prepared by post-rolling relaxation and ultra-rapid cooling processes, ensuring the stability of performance fluctuations.

[0028] Furthermore, its chemical composition and mass percentage are as follows: C 0.09%, Si 0.15%, Mn 1.5%, with no other microalloying components, the balance being Fe and unavoidable impurities; the material is prepared using a post-rolling relaxation and ultra-rapid cooling process, ensuring the stability of performance fluctuations.

[0029] Furthermore, its chemical composition and mass percentage are as follows: C 0.11%, Si 0.25%, Mn 1.7%, with no other microalloying components, the balance being Fe and unavoidable impurities; the material is prepared using a post-rolling relaxation and ultra-rapid cooling process, ensuring the stability of performance fluctuations.

[0030] Furthermore, its chemical composition and mass percentage are as follows: C 0.10%, Si 0.20%, Mn 1.6%, with no other microalloying components, the balance being Fe and unavoidable impurities; the material is prepared using a post-rolling relaxation and ultra-rapid cooling process, ensuring the stability of performance fluctuations.

[0031] Furthermore, a method for manufacturing a cost-saving 360MPa grade carbon-manganese low-temperature steel with ultra-low temperature toughness is provided. The main preparation process includes heating, rolling and ultra-rapid cooling to obtain the cost-saving 360MPa grade carbon-manganese low-temperature steel with ultra-low temperature toughness.

[0032] Furthermore, in the heating process, the furnace temperature is 1150–1250°C, the holding time is greater than 150 min, and the furnace exit temperature is 1100–1200°C.

[0033] Furthermore, in the rolling and ultra-rapid cooling process, a two-stage controlled rolling process is adopted, with a final rolling temperature of 750-810℃, followed by a relaxation and warming treatment to ensure the phase transformation ratio of ferrite in the microstructure.

[0034] Subsequently, an ultra-fast cooling process was adopted, with a cooling start temperature of 700-740℃, a final cooling temperature of 560-620℃, and an average cooling rate of 30-80℃.

[0035] Furthermore, the prepared low-temperature toughness, economical 360MPa grade carbon-manganese low-temperature steel has a yield strength greater than 360MPa, a tensile strength of 490-620MPa, an elongation greater than 27%, a Charpy impact strength greater than 80J at -60℃, and a thickness of 10-40mm.

[0036] Furthermore, in the prepared low-temperature toughness, the S content of the energy-saving 360MPa grade carbon-manganese low-temperature steel is controlled below 0.002%, the P content is controlled below 0.012%, the Ds inclusion grade is controlled below 2.0, and the initial billet center segregation is controlled below C1.0.

[0037] Furthermore, the microstructure of the prepared low-temperature toughness, energy-saving 360MPa grade carbon-manganese low-temperature steel consists of ferrite and pearlite, with a ferrite content greater than 75%, thus avoiding or reducing the occurrence of overcooled degraded pearlite.

[0038] Example

[0039] The composition of the energy-saving carbon-manganese ship plate steel in Examples 1 to 8 is shown in Table 1; Table 2 shows the main process parameters of Examples 1 to 8; and the mechanical property test results of Examples 1 to 8 are shown in Table 3.

[0040] Table 1 Chemical composition of the products in the examples (wt.%, Fe balance)

[0041] Example C Mn P S Si 1 0.10247 1.53813 0.00971 0.00141 0.21245 2 0.09252 1.55892 0.00768 0.00102 0.2068 3 0.10491 1.54938 0.01213 0.00108 0.20699 4 0.09333 1.57451 0.00522 0.00112 0.26546 5 0.09944 1.57792 0.00974 0.00108 0.24148 6 0.09973 1.5437 0.01022 0.00157 0.22264 7 0.09819 1.56298 0.00823 0.00096 0.20876 8 0.09443 1.56071 0.00932 0.00093 0.26623

[0042] Table 2 Process parameters and conditions for the examples

[0043]

[0044] Table 3 Mechanical property results of the examples

[0045]

[0046] Figure 1 The microstructure of the carbon-manganese low-temperature steel produced by the TMCP process in Example 1 is shown. This microstructure has the typical characteristics of the low-temperature steel microstructure of the present invention. It can be seen that the microstructure of the carbon-manganese low-temperature steel is composed of ferrite and pearlite, with ferrite as the main matrix and a ferrite content of more than 75%. Therefore, this microstructure has good low-temperature impact toughness. At the same time, the presence of pearlite ensures the strength grade of the carbon-manganese steel, and increasing the proportion of pearlite is beneficial to improving the tensile strength of the low-temperature steel. Avoiding or reducing the occurrence of supercooled degraded pearlite in the microstructure can reduce the fluctuation of impact toughness and ensure the stability of performance.

Claims

1. A saving 360 MPa grade carbon-manganese cryogenic steel having super low temperature toughness, characterized in that, The chemical composition and mass percentage are as follows: C 0.09-0.11%, Si 0.15-0.25%, Mn 1.5-1.7%, no other micro-alloying component, and the balance of Fe and inevitable impurities; The preparation process comprises heating, rolling and ultra-fast cooling, and the saving 360MPa-grade carbon manganese low-temperature steel with super-low-temperature toughness is prepared; In the heating process, the heating furnace temperature is 1150-1250℃, the holding time is greater than 150min, and the discharge temperature is 1100-1200℃; In the rolling and ultra-fast cooling process, two-stage controlled rolling is adopted, the finish rolling temperature is 750-810℃, and relaxation waiting treatment is carried out after rolling; Then, the ultra-fast cooling process is adopted, the cooling starting temperature is 700-740℃, the final cooling temperature is 560-620℃, and the average cooling speed is 30-80℃; The yield strength of the prepared saving 360MPa-grade carbon manganese low-temperature steel with super-low-temperature toughness is greater than 360MPa, the tensile strength is 490-620MPa, the elongation is greater than 27%, the Charpy impact at-60℃ is greater than 80J, and the thickness specification is 10-40mm; In the prepared saving 360MPa-grade carbon manganese low-temperature steel with super-low-temperature toughness, S is controlled to be less than or equal to 0.002%, P is controlled to be less than or equal to 0.012%, the Ds inclusion grade is controlled to be less than or equal to 2.0, and the initial slab center segregation is controlled to be less than or equal to C1.0; The microstructure of the prepared saving 360MPa-grade carbon manganese low-temperature steel with super-low-temperature toughness is composed of ferrite and pearlite, and the ferrite content is greater than 75%.

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