High crack arrest performance eh420 grade marine steel plate and production method thereof

Abstract

The application discloses a high crack arrest performance EH420 grade marine steel plate and a production method thereof. The chemical components of the steel plate comprise the following components in percentage by mass: C 0.05-0.08%, Si 0.15-0.30%, Mn 1.50-1.70%, P≤0.015%, S≤0.003%, Nb 0.040-0.060%, V 0.020-0.040%, Ti 0.008-0.018%, Mo 0.10-0.25%, Als 0.020-0.040%, rare earth Ce 0.0010-0.0030%, B 0.0005-0.0020%, and a micro-alloy composite factor Mf value is controlled in 0.10-0.16. The CTOD value of the steel plate welded joint at -35 DEG C is greater than or equal to 0.75 mm, and the steel plate has high strength, high toughness and excellent crack arrest performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical manufacturing technology of structural steel for ships and marine engineering, specifically relating to an EH420 grade high-strength marine steel plate with excellent weld joint crack arrest performance and its production method using thermomechanical rolling (TMCP) process. Background Technology

[0002] With the increasing size of ships and the expansion of marine resource development into deep waters and polar regions, ship structures face more severe low-temperature, impact, and complex stress environments. EH420 grade steel is a key load-bearing material for shipbuilding and marine engineering, requiring not only high strength and low-temperature toughness but also excellent crack propagation resistance in welded joints, i.e., crack arrest performance (often characterized by crack tip opening displacement (CTOD)). Traditional EH420 steel plates often struggle to achieve a good balance between high strength (≥420MPa yield strength) and high crack arrest performance (e.g., -35℃ CTOD ≥0.7mm).

[0003] In the prior art, patent application CN121362924A discloses a high-performance EH40 marine steel plate, which uses low carbon and niobium, vanadium, titanium, and rare earth cerium (Ce) microalloying to achieve extremely high low-temperature impact toughness (-40℃ KV2≥270J) through the TMCP process. However, this technical solution is aimed at a lower strength level (EH40, yield strength≥390MPa), and its composition system and process (especially the high final cooling temperature) are designed to maximize impact toughness. If this solution is directly used to produce higher strength level EH420 steel plates, it often faces the dilemma of insufficient strength or sacrificing crack arrest performance to increase strength. Specifically, simply increasing the alloy content or lowering the final cooling temperature to increase strength can easily lead to coarsening of the weld heat-affected zone microstructure and increased internal stress, thereby significantly deteriorating the CTOD value.

[0004] Therefore, developing a new composition and process synergy system that can ensure the high strength and good low-temperature toughness of EH420 grade steel plates while achieving stable and excellent crack arrest performance of welded joints has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of the prior art and provide a high-crack-arrest EH420 grade marine steel plate and its production method. Through a unique composition design, combined with a deeply matched controlled rolling and controlled cooling process, this steel plate successfully achieves a synergistic improvement in high strength, high toughness, and ultra-high crack-arrest performance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a high-crack-arresting EH420 grade marine steel plate, the chemical composition of which, by mass percentage, is: C: 0.05-0.08%, Si: 0.15-0.30%, Mn: 1.50-1.70%, P≤0.015%, S≤0.003%, Nb: 0.040-0.060%, V: 0.020-0.040%, Ti: 0.008-0.018%, Mo: 0.10-0.25%, Als: 0.020-0.040%, rare earth Ce: 0.0010-0.0030%, B: 0.0005-0.0020%, carbon equivalent CEV≤0.38%, weld crack sensitivity index Pcm≤0.18%, with the balance being Fe and unavoidable impurities; wherein, the microalloying composite factor Mf = (Nb% / 0.05+ The value of (V% / 0.03 + Mo% / 0.15 + Ti% / 0.015) × (Ce% / 0.002) is controlled between 0.10 and 0.16. In the formula for calculating Mf, the values ​​0.05, 0.03, 0.15, and 0.015 represent the conventional empirical values ​​for the optimal microalloying effect of Nb, V, Mo, and Ti elements in the composition system of this invention; 0.002 represents the economically effective content value for the optimal purification and modification effect of rare earth Ce in this invention. The Mf value comprehensively characterizes the synergistic strengthening effect of the aforementioned microalloying elements and rare earth elements. Its value range of 0.10 to 0.16 was determined through extensive experiments, ensuring that the steel plate achieves high strength (EH420 grade) while also possessing excellent low-temperature toughness and crack arrest performance.

[0008] Preferably, the thickness of the steel plate is 10-50 mm.

[0009] Preferably, the mechanical properties of the steel plate meet the following requirements: yield strength ReH ≥ 440 MPa, tensile strength Rm: 550~680 MPa, elongation after fracture A ≥ 22%, longitudinal Charpy V-notch impact energy KV2 ≥ 140 J at -40℃, and crack tip opening displacement CTOD characteristic value δm ≥ 0.75 mm at -35℃ for welded joints.

[0010] Secondly, the present invention provides a method for producing the above-mentioned high crack arrest performance EH420 grade marine steel plate, comprising the following steps:

[0011] S1. Smelting and refining: After converter smelting, LF ladle refining and RH vacuum circulation degassing, clean molten steel that meets the above composition requirements is obtained, wherein the molten steel after RH treatment has [H]≤1.2ppm, [O]≤20ppm, and [N]≤40ppm;

[0012] S2. Continuous casting: Molten steel is poured into continuously cast billets, and a crystallizer and end electromagnetic stirring combined with dynamic light reduction technology are used to control the center segregation of the billet;

[0013] S3. Slab heating: Heat the continuously cast slab to 1200~1230℃, and soak it for ≥60min to ensure that the alloying elements are fully dissolved;

[0014] S4. Controlled rolling: Two-stage controlled rolling is adopted. The first stage is high-temperature large deformation rolling in the austenite recrystallization zone, with an initial rolling temperature of above 1120℃; the second stage is rolling in the non-recrystallization zone of austenite, with the initial rolling temperature strictly controlled to ≤860℃ and the final rolling temperature controlled between 760 and 800℃.

[0015] S5. Controlled cooling: Laminar flow cooling is performed immediately after rolling, and the cooling process is divided into two stages:

[0016] The first stage (rapid cooling stage): the steel plate is rapidly cooled from 750-780℃ to 580-620℃ at a cooling rate of 10-18℃ / s.

[0017] The second stage (slow cooling stage): The steel plate is slowly cooled from the final temperature of the first stage to the final cooling temperature of 460-500℃ at a cooling rate of 4-8℃ / s.

[0018] S6. Stacking and slow cooling: After the steel plates are cooled, they are stacked and the slow cooling time is ≥24 hours.

[0019] Preferably, in the S5 controlled cooling step, a head, tail and side shielding device is used, with a head shielding length of 0-2.0m, a tail shielding length of 0-2.5m and a side shielding length of 0-2.0m, so that the overall temperature difference after the steel plate turns red is ≤40℃.

[0020] Preferably, in the S1 smelting and refining step, the sulfur content [S] of the molten steel at the end of the LF refining process is ≤0.002%.

[0021] Preferably, in the S2 continuous casting step, electromagnetic stirring in the crystallizer and electromagnetic stirring at the end are used, and the center segregation of the billet is no greater than Class C 2.0.

[0022] Preferably, the method does not involve any form of offline heat treatment.

[0023] The beneficial effects of this invention are as follows:

[0024] 1. Innovative Composition Design: Molybdenum (Mo) and boron (B) were introduced and synergistically optimized with niobium (Nb), vanadium (V), titanium (Ti), and rare earth cerium (Ce). Mo significantly improved hardenability, ensuring a fine bainitic microstructure in the core of the thick plate; B and Ce synergistically purified and strengthened grain boundaries. An innovatively defined microalloying composite factor, Mf (0.10–0.16), quantitatively characterized the synergistic ratio of key microalloying elements with rare earth elements. This ratio is crucial for achieving the optimal balance between high strength and high crack arrest performance.

[0025] 2. Collaborative Process Innovation: A TMCP process was developed that is highly compatible with the aforementioned high hardenability composition system. Lower rolling temperatures in the non-recrystallization zone (final rolling 760–800℃) accumulate higher deformation energy. The unique "two-stage" cooling regime, especially the extremely low final cooling temperature (460–500℃), ensures a microstructure dominated by fine bainite and promotes the full precipitation of Nb, V, and Ti carbonitrides. This achieves high strength while significantly reducing weld sensitivity and crack propagation driving force.

[0026] 3. Superior Overall Performance: While meeting and far exceeding the basic requirements for EH420 steel in GB / T 712-2022, the steel plate of this invention achieves outstanding crack arrest performance with a stable CTOD value ≥0.75mm at -35℃ weld joints, while maintaining a high toughness level of over 140J in impact energy at -40℃, with uniform performance throughout the thickness direction. This solves the industry bottleneck of further improving the crack arrest performance of high-strength ship plate steel.

[0027] 4. Production economy: The entire process is based on the conventional TMCP process, which does not require expensive offline heat treatment (such as normalizing and tempering) or ultra-fast cooling equipment. The cost of alloys is controllable and it is suitable for large-scale industrial production. Detailed Implementation

[0028] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0029] Examples 1-6

[0030] EH420 high-strength steel plates with thicknesses of 10 mm, 20 mm, 24 mm, 30 mm, 40 mm, and 50 mm were produced according to the chemical composition and manufacturing method described in this invention, and were respectively labeled as Examples 1 to 6.

[0031] 1. Production process flow and key parameter control

[0032] Smelting and Continuous Casting: After deep desulfurization pretreatment, molten iron is smelted in a converter. LF refining involves deep desulfurization and deoxidation, with sequential addition of alloys to ensure that the final molten steel has [S] ≤ 0.002%. RH vacuum treatment ensures [H] ≤ 1.2ppm, [O] ≤ 20ppm, and [N] ≤ 40ppm. Continuous casting employs an optimized secondary cooling mode, electromagnetic stirring, and light reduction to obtain a 250mm thick billet with center segregation ≤ Class C 2.0.

[0033] Heating: The slab is heated to 1210-1220℃ in a walking beam furnace and homogenized for 65-70 minutes.

[0034] Rolling and Cooling: A two-stage rolling process is strictly implemented. The initial rolling temperature in the first stage is above 1120℃, the initial rolling temperature in the finishing stage is controlled at 840-860℃, and the final rolling temperature is controlled at 775-795℃. After rolling, the product immediately enters a laminar flow cooling system for two-stage cooling. Specific rolling and cooling parameters for each embodiment are shown in Table 2.

[0035] Slow cooling: Stack the steel plates and allow them to cool slowly for 24-36 hours.

[0036] 2. Chemical composition of the examples

[0037] The specific chemical composition (mass percentage) and calculated Mf values ​​of Examples 1-6 are shown in Table 1.

[0038] Table 1: Chemical composition (wt%) and Mf value of embodiments of the present invention

[0039]

[0040] 3. Rolling and Cooling Process Parameters for the Example

[0041] The specific rolling and cooling process parameters are shown in Table 2.

[0042] Table 2: Rolling process parameters of the embodiments of the present invention

[0043]

[0044] 4. Mechanical Properties of Examples

[0045] A comprehensive mechanical property test was conducted on the steel plates of the embodiments, and the results are shown in Table 3. All the steel plates of the embodiments exhibited excellent strength, plasticity, and low-temperature impact resistance. In particular, the CTOD value of the welded joints at -35℃ was consistently above 0.75mm, achieving the core objective of this invention.

[0046] Table 3: Mechanical properties of the steel plates in the embodiments of the present invention

[0047]

[0048] Comparative Example

[0049] Comparative Example 1

[0050] Composition design: The composition of Example 3 of CN121362924A (C:0.08, Si:0.26, Mn:1.57, P:0.012, S:0.004, Nb:0.046, V:0.040, Ti:0.014, Als:0.020, Ce:0.0010) was adopted, without Mo and B.

[0051] Process: The TMCP process, similar to that in Example 5 of this application, is adopted. The finishing rolling temperature is 795°C, and single-stage cooling is used with a final cooling temperature of 650°C.

[0052] Performance results: Yield strength 415 MPa (only reaching the lower limit of EH420), tensile strength 540 MPa, impact energy at -40℃ 221 J, but CTOD value at -35℃ is only 0.32 mm. The microstructure is mainly polygonal ferrite and pearlite. This proves that the existing high-toughness scheme for EH40 cannot meet the dual requirements of high strength and high CTOD performance for EH420 level.

[0053] Comparative Example 2

[0054] Composition design: Same as in Example 2 of this invention, but without adding Mo.

[0055] Process: Same as Example 2.

[0056] Performance results: Yield strength 438 MPa, impact energy at -40℃ 170 J, CTOD value at -35℃ decreased to 0.58 mm, and the impact energy difference between the core and edge of the 40 mm thick plate reached 35 J. This proves that Mo is crucial for ensuring CTOD performance under high strength and the uniformity of microstructure in thick plates.

[0057] Comparative Example 3

[0058] Ingredient design: Same as in Example 2 of this invention.

[0059] Process: The rolling process is the same as in Example 2, but the two-stage cooling is eliminated, and the temperature is directly and rapidly cooled from 780°C to 300°C (cooling rate 20°C / s).

[0060] Performance results: The yield strength is as high as 500 MPa, but the impact energy at -40℃ drops sharply to 75 J, and the CTOD value at -35℃ is only 0.25 mm, resulting in poor plate shape. This proves that simply pursuing high strength while using an inappropriate rapid cooling process will seriously damage toughness and crack arrest performance. The second stage of "controlled-rate slow cooling" in this invention is indispensable for obtaining a high CTOD value.

[0061] Comparative Example 4

[0062] Composition design: The composition is as follows (C: 0.07%, Si: 0.20%, Mn: 1.60%, P: 0.010%, S: 0.002%, Nb: 0.0025%, V: 0.0025%, Ti: 0.0008%, Mo: 0.018%, Als: 0.030%, Ce: 0.0005%, B: 0.0010%; CEV: ~0.35%, Pcm: ~0.17%, balance: Fe and impurities).

[0063] Process: Same as Example 2.

[0064] Performance results: Yield strength 430 MPa, impact energy at -40℃ 190 J, CTOD value at -35℃ only 0.48 mm. This proves that microalloying elements and rare earth elements must be proportioned according to the Mf factor (0.10-0.16) defined in this invention to achieve optimal performance synergy.

[0065] The comparison between the examples and comparative examples shows that the "low carbon + Mo-Nb-V-Ti composite microalloying + Ce-B synergy" composition system provided by the present invention, combined with the TMCP process of "low temperature non-recrystallization rolling + low final cooling temperature two-stage cooling", successfully solves the technical problem that EH420 grade high-strength ship plate steel is difficult to have excellent crack arrest performance (CTOD≥0.75mm) at the same time.

[0066] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-crack-arrest EH420 grade marine steel plate, characterized in that, Its chemical composition by mass percentage is as follows: C: 0.05-0.08%, Si: 0.15-0.30%, Mn: 1.50-1.70%, P≤0.015%, S≤0.003%, Nb: 0.040-0.060%, V: 0.020-0.040%, Ti: 0.008-0.018%, Mo: 0.10-0.25%, Als: 0.020-0.040%, rare earth Ce: 0.0010-0.0030%, B: 0.0005-0.0020%, carbon equivalent CEV≤0.38%, weld crack sensitivity index Pcm≤0.18%, with the balance being Fe and unavoidable impurities; The value of the microalloying composite factor Mf = (Nb% / 0.05 + V% / 0.03 + Mo% / 0.15 + Ti% / 0.015) ×(Ce% / 0.002) is 0.10 to 0.

16.

2. The high crack arrest performance EH420 grade marine steel plate according to claim 1, characterized in that, The thickness of the steel plate is 10-50 mm.

3. The high crack arrest performance EH420 grade marine steel plate according to claim 1, characterized in that, The mechanical properties of the steel plate meet the following requirements: yield strength ReH ≥ 440 MPa, tensile strength Rm: 550~680 MPa, elongation after fracture A ≥ 22%, longitudinal Charpy V-notch impact energy KV2 ≥ 140 J at -40℃, and crack tip opening displacement CTOD characteristic value δm ≥ 0.75 mm at -35℃.

4. A method for preparing EH420 grade marine steel plates with high crack arrest performance as described in any one of claims 1-3, characterized in that, Includes the following steps: S1. Smelting and refining: After converter smelting, LF refining and RH vacuum treatment, molten steel that meets the composition requirements of claim 1 is obtained, wherein the molten steel after RH treatment has [H] ≤ 1.2 ppm, [O] ≤ 20 ppm, and [N] ≤ 40 ppm; S2. Continuous casting: Molten steel is poured into billets using electromagnetic stirring and dynamic light reduction technology; S3. Heating: Heat the slab to 1200~1230℃, and the soaking time is ≥60min; S4. Controlled rolling: Two-stage rolling is adopted. The first stage is rolling in the recrystallization zone, and the second stage is rolling in the non-recrystallization zone. The initial rolling temperature of the second stage is ≤860℃, and the final rolling temperature is controlled between 760 and 800℃. S5. Controlled cooling: Laminar flow cooling is performed immediately after rolling, and the cooling process is divided into two stages: - First stage: Cool the steel plate rapidly from 750-780℃ to 580-620℃ at a cooling rate of 10-18℃ / s; - Second stage: Continue to cool the steel plate slowly at a cooling rate of 4-8℃ / s until the final cooling temperature of 460-500℃; S6. Stacking and slow cooling: After cooling, the steel plates are stacked and slow-cooled for more than 24 hours.

5. The method according to claim 4, characterized in that, In the S5 controlled cooling step, head, tail and side shielding devices are used to ensure that the overall temperature difference of the steel plate after it turns red is ≤40℃.

6. The method according to claim 4, characterized in that, In the S1 smelting and refining step, the sulfur content [S] of the molten steel is ≤0.002% at the end of the LF refining process.

7. The method according to claim 4, characterized in that, In the S2 continuous casting step, electromagnetic stirring in the crystallizer and electromagnetic stirring at the end are used, and the center segregation of the billet is no greater than Class C 2.

0.

8. The method according to claim 4, characterized in that, The method does not involve any form of offline heat treatment.

Images