Thin-gauge low-yield-ratio container steel plate and production method thereof

Abstract

The invention provides a thin-gauge low-yield-ratio container steel plate and a production method thereof. The thin-gauge low-yield-ratio container steel plate comprises the following components: 0.11%-0.15% of C, 0.25%-0.40% of Si, 1.25%-1.35% of Mn, less than or equal to 0.015% of P, less than or equal to 0.005% of S, 0.025%-0.045% of Als, 0.010%-0.018% of Nb, 0.0010%-0.0015% of Ca, 0.0040%-0.0065% of N and the balance of Fe and inevitable trace elements. Compared with the prior art, the steel plate has the advantages that relatively economical components are adopted, the manganese content is properly controlled, a trace amount of Nb is added, and a heat treatment process is designed, so that the low strength ratio (less than or equal to 0.75) and good low-temperature (-40 DEG C) impact toughness of the thin SA516Gr.70 steel plate are ensured.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel plate production, and in particular relates to a thin-gauge low-yield-to-strength ratio container steel plate and a production method thereof. Background Art

[0002] Low-yield ratio steel for container and storage tanks refers to steel with a low yield strength to tensile strength ratio, typically requiring a yield ratio of 0.85 or less. This steel exhibits excellent plasticity, toughness, and fatigue resistance, making it widely used in the manufacture of storage tanks for the oil, natural gas, and chemical industries. This ensures that plastic deformation, rather than fracture, occurs preferentially under overload conditions, improving tank safety and extending fatigue life.

[0003] In the prior art, the patent with publication number CN114134414 A published on March 4, 2022 discloses "A low yield ratio and high toughness steel plate and its preparation method". The invention adopts (mass percentage) in the composition design: C: 0.09%~0.11%, Si: 0.10%~0.20%, Mn: 1.40%~1.50%, P≤0.015%, S≤0.003%, Als: 0.010%~0.040%, Cr: 0.15%~0.25%, Cu: 0.15%~0.20%, Ni: 0.20%~0.30%, Nb: 0.025%~0.040%, Ti: 0.010%~0.040%, and the rest is Fe and unavoidable impurities. The production method is as follows: (1) the raw materials are smelted, refined, alloyed, and calcium treated to obtain molten steel, which is finally continuously cast into ingots. The ingots are cut and then slowly cooled; (2) heating; (3) two-stage rolling is adopted, in which the first stage adopts a transverse-longitudinal rolling method. After the first stage rolling is completed, the intermediate ingot is accelerated to cool to 730°C; when the red-return temperature reaches 760°C, the second stage rolling is carried out. After the second stage rolling is completed, the temperature is relaxed to 720°C on the surface of the steel plate; (4) the steel plate is water-cooled after rolling; and (5) pile cooling. The thickness of the steel plate produced is 40-80mm, which does not meet the requirements of thin specifications.

[0004] Therefore, it is necessary to provide a thin-gauge low-yield-to-strength ratio container steel plate. Summary of the Invention

[0005] The present invention aims to provide a thin-gauge, low-yield ratio container steel plate and a production method thereof. By designing the composition, appropriately controlling the manganese content, adding a trace amount of Nb, and designing a heat treatment process, the thin-gauge (5mm) low-yield ratio container steel plate SA516 Gr.70 is achieved with excellent low-temperature impact properties and a low yield ratio.

[0006] The specific technical solutions of the present invention are as follows:

[0007] A thin gauge low yield ratio container steel plate comprising the following components in percentage by mass:

[0008] C 0.11~0.15%, Si 0.25~0.40%, Mn 1.25~1.35%, P≤0.015%, S≤0.005%, Als 0.025~0.045%, Nb 0.010~0.018%, Ca 0.0010-0.0015%, N

[0009] 0.0040-0.0065%, the rest are Fe and inevitable trace elements.

[0010] The nominal thickness of the thin-gauge low-yield-to-strength ratio container steel plate is 5 mm, and the allowable tolerance range in actual production is -0.3 to +0.8 mm.

[0011] The composition of the thin gauge low yield ratio container steel plate satisfies the carbon equivalent CEV (%) ≤ 0.43%;

[0012] The structure of the thin-gauge low-yield ratio container steel plate is as follows: the matrix structure is ferrite+pearlite, the volume fraction of pearlite is 22-30%, and the grain size at the position 1 / 4 from the surface is ≥11 levels.

[0013] The yield strength of the thin-gauge low-yield ratio container steel plate is ≥390Mpa, the tensile strength of the steel plate is ≥550Mpa, the elongation is ≥35%, the longitudinal -40°C impact energy is ≥100J, the yield ratio is ≤0.75, preferably ≤0.73.

[0014] The present invention provides a method for producing thin-gauge low-yield ratio container steel plates, which includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating furnace heating → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

[0015] The LF refining is as follows: after arriving at the refining station, argon is connected and the gas volume is adjusted to control the flow rate at 400-600NL / min; based on 100t / furnace, 200-250Kg of refining slag, 100-300Kg of lime, and 20-80Kg of fluorite are added. After the LF furnace is electrified and slagging for 6-10 minutes, deoxidizers such as Al particles are added. Nb iron is added according to the sampling results to fine-tune the composition. After the composition meets the standard, the calcium wire (100-150m) is fed, and the soft argon blowing time is ≥8min.

[0016] The slab continuous casting obtains a slab with a cross section of 180×1400 mm;

[0017] The slab heating furnace is specifically heated as follows: the temperature of the second heating section and the soaking section is controlled at 1240-1260°C, the billet thickness is 180mm, the heating time is (1.1-1.3)×H, H is the billet thickness in mm, and the heating time is in min, that is, the heating time is 198-234min.

[0018] The hot rolling is carried out using a 3800mm medium and thick plate rolling mill. The hot rolling process is divided into two stages: the starting rolling temperature of the rough rolling is ≥1100°C, the finishing rolling temperature is ≥980°C, and the intermediate thickness is 55-60mm; the starting rolling temperature of the finishing rolling is ≤980°C, the finishing rolling temperature is 840-870°C, and the cumulative deformation rate below 900°C is ≥50%.

[0019] The heat treatment adopts normalizing, and the normalizing process is: temperature 860±10°C, time: 3.5×H, time unit is min; H is thickness, unit is mm.

[0020] This invention utilizes the addition of microalloying Nb and an appropriate addition of alloying Mn. Niobium is used to form fine NbC, NbN, or Nb(C,N) precipitates in the high-temperature austenite region. These precipitates pin austenite grain boundaries, inhibiting grain growth. This, combined with grain boundary strengthening and grain refinement, significantly improves the material's strength and toughness, ensuring a low yield ratio. Furthermore, this thin-gauge (5mm) low-yield ratio container steel plate meets the low-temperature impact performance requirements of thin-gauge steel at -40°C.

[0021] Because the medium-thick steel plate is relatively thin (5mm), it must be rolled quickly at a relatively high temperature (finishing rolling temperature 840-870°C) to ensure a normal plate shape. After normalizing, a ferrite + pearlite matrix structure is obtained, minimizing the bainite structure generated by the rapid cooling of thin gauges during the rolling stage. This ensures high strength and toughness (low yield ratio and -40°C impact resistance) that meet standard requirements. The heat treatment process in the present invention is designed based on actual production conditions and the precipitation characteristics of Nb.

[0022] Compared with the prior art, the present invention adopts more economical composition, properly controls the manganese content, adds a trace amount of Nb, and designs a heat treatment process, thereby ensuring the low strength ratio (≤0.75) and good low-temperature (-40°C) impact toughness of SA516 Gr.70 steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The matrix structure of Example 1;

[0024] Figure 2 This is the heart tissue of Example 1;

[0025] Figure 3 This is the matrix structure of Example 2;

[0026] Figure 4 This is the heart tissue of Example 2;

[0027] Figure 5 This is the matrix structure of Comparative Example 1;

[0028] Figure 6 This is the core tissue of Comparative Example 1;

[0029] Figure 7 This is the matrix structure of Comparative Example 2;

[0030] Figure 8 This is the core tissue of Comparative Example 2. DETAILED DESCRIPTION

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with 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. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0032] Example 1

[0033] A method for producing thin-gauge (5mm) low-yield ratio container steel plates includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating in a heating furnace → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

[0034] The main process parameters are:

[0035] LF refining is as follows: after arriving at the refining station, connect argon gas and adjust the gas volume to 485NL / min; according to 100t / furnace, add 230Kg of refined slag, 180Kg of lime and 50Kg of fluorite; after the LF furnace is electrified for 8.5 minutes, add deoxidizers such as Al particles, and add Nb iron to fine-tune the composition according to the sampling results. After the composition meets the standard, feed 126m of calcium wire and the soft argon blowing time is 11 minutes.

[0036] Slab heating furnace heating control: the temperature of the second heating section and the soaking section is controlled at 1245~1258℃, the billet thickness is 180mm, and the heating time is 212 minutes.

[0037] The hot rolling process consists of two stages: rough rolling starts at 1130°C and finishes at 1110°C, and finishing rolling starts at 978°C and finishes at 856°C. The cumulative deformation rate is approximately 60% below 900°C. After rolling, air cooling and rapid hot straightening are used. The finished steel plate has a thickness of 5.26mm.

[0038] The heat treatment normalizing temperature is 857-863℃, and the furnace time is 17.5min.

[0039] After inspection, the chemical composition of the 5mm thick SA516 Gr.70 steel produced is C 0.13%, Si 0.31%, Mn1.28%, P 0.011%, S 0.003%, Als 0.031%, Nb 0.014%, Ca 0.0014%, N 0.0042%, and the rest is Fe and inevitable trace elements. The matrix structure is ferrite + pearlite (see Figure 1 ), of which the total volume fraction of pearlite is about 26%, and the grain size is 11.5; the core structure is shown Figure 2 The steel plate has a yield strength of 406 MPa, a tensile strength of 558 MPa, a yield strength ratio of 0.73, an elongation of 38.5%, and a longitudinal impact energy of -40°C (sample size 10×5×55 mm) of 102 J, 89 J, and 113 J, respectively, with an average value of 101 J.

[0040] Example 2

[0041] A method for producing thin-gauge (5mm) low-yield ratio container steel plates includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating in a heating furnace → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

[0042] The main process parameters are:

[0043] LF refining is as follows: after arriving at the refining station, connect argon gas and adjust the gas volume to 506NL / min; according to 100t / furnace, add 233Kg of refined slag, 198Kg of lime, and 48Kg of fluorite. After the LF furnace is electrified for 9 minutes to slag, add deoxidizers such as Al particles, and add Nb iron to fine-tune the composition according to the sampling results. After the composition meets the standard, feed 133m of calcium wire and the soft argon blowing time is 10min.

[0044] Heating temperature control of slab heating furnace: the temperature of the second heating section and the soaking section is controlled at 1243~1252℃, the billet thickness is 180mm, and the heating time is 206 minutes.

[0045] The hot rolling process is divided into two stages: rough rolling starts at 1143°C and finishes at 1102°C, and finishing rolling starts at 975°C and finishes at 862°C. The cumulative deformation rate below 900°C is approximately 63%. After rolling, air cooling and rapid hot straightening are used. The finished steel plate thickness is 5.22mm.

[0046] The heat treatment normalizing temperature is 855-866℃, and the furnace time is 17.5min.

[0047] After inspection, the chemical composition of the 5mm thick SA516 Gr.70 steel produced is C 0.12%, Si 0.34%, Mn 1.27%, P 0.010%, S 0.002%, Als 0.037%, Nb 0.015%, Ca 0.0013%, N 0.0046%, and the rest is Fe and inevitable trace elements. The matrix structure is ferrite + pearlite (see Figure 3 ), of which the total volume fraction of pearlite is about 25%, the grain size is 11; the core structure is Figure 4 The steel plate has a yield strength of 412 MPa, a tensile strength of 571 MPa, a yield strength ratio of 0.72, an elongation of 36.5%, and a longitudinal impact energy of -40°C (sample size 10×5×55 mm) of 118 J, 122 J, and 96 J, respectively, with an average value of 112 J.

[0048] Comparative Example 1

[0049] A method for producing thin-gauge (5mm) low-yield ratio container steel plates includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating in a heating furnace → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

[0050] The main process parameters are:

[0051] LF refining is as follows: after arriving at the refining station, connect argon gas and adjust the gas volume to 475NL / min; according to 100t / furnace, add 220Kg of refined slag, 189Kg of lime and 53Kg of fluorite; after the LF furnace is electrified for 9 minutes to slag, add deoxidizers such as Al particles, and add Nb iron to fine-tune the composition according to the sampling results. After the composition meets the standard, feed 115m of calcium wire and the soft argon blowing time is 10.5min.

[0052] Heating temperature control of slab heating furnace: the temperature of the second heating section and the soaking section is controlled at 1236~1247℃, the billet thickness is 180mm, and the heating time is 223 minutes.

[0053] The hot rolling process consists of two stages: rough rolling starts at 1120°C and finishes at 1081°C, and finishing rolling starts at 983°C and finishes at 852°C. The cumulative deformation rate below 900°C is approximately 58%. After rolling, air cooling and rapid hot straightening are used. The finished steel plate has a thickness of 5.35mm.

[0054] The heat treatment normalizing temperature is 859-867℃, and the furnace time is 17.5min.

[0055] After inspection, the chemical composition of the 5mm thick SA516 Gr.70 steel produced is C 0.15%, Si 0.25%, Mn1.19%, P 0.014%, S 0.003%, Als 0.018%, Nb 0.013%, Ca 0.0013%, N 0.0058%, and the rest is Fe and inevitable trace elements; the matrix structure is ferrite + pearlite (see Figure 5 ), the total volume fraction of pearlite is about 14%, the grain size is 10.5; the core structure is Figure 6 The steel plate has a yield strength of 440 MPa, a tensile strength of 541 MPa, a yield strength ratio of 0.81, an elongation of 29%, and longitudinal impact energy at -40°C (specimen size 10×5×55 mm) of 44 J, 35 J, and 42 J, respectively, with an average of 40 J. The main reasons for these performance deviations are the high start-of-rolling temperature for finishing, low Mn content, a low proportion of pearlite, low tensile strength, a high yield strength ratio after cold rolling, and low impact energy.

[0056] Comparative Example 2

[0057] A method for producing thin-gauge (5mm) low-yield ratio container steel plates includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating in a heating furnace → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

[0058] The main process parameters are:

[0059] The LF refining process is as follows: after arriving at the refining station, connect argon gas and adjust the gas volume to 489NL / min; based on 100t / furnace, add 218Kg of refined slag, 186Kg of lime, and 47Kg of fluorite. After the LF furnace is electrified for 9.5 minutes, add deoxidizers such as Al particles, and add Nb iron to fine-tune the composition based on the sampling results. After the composition meets the standard, feed 112m of calcium wire and the soft argon blowing time is 9 minutes.

[0060] Heating temperature control of slab heating furnace: the temperature of the second heating section and the soaking section is controlled at 1240-1255°C, the billet thickness is 180mm, and the heating time is 202 minutes.

[0061] The hot rolling process is divided into two stages: the starting temperature of rough rolling is 1113°C, the finishing temperature is 1060°C, the starting temperature of finishing rolling is 981°C, and the finishing temperature is 858°C. The cumulative deformation rate below 900°C is about 61%. After rolling, air cooling and rapid hot straightening are adopted. The thickness of the steel plate after rolling is 5.19mm.

[0062] The heat treatment normalizing temperature is 851-862°C, and the furnace time is 17.5 minutes.

[0063] After inspection, the chemical composition of the 5mm thick SA516 Gr.70 steel produced is C 0.10%, Si 0.26%, Mn1.43%, P 0.016%, S 0.004%, Als 0.022%, Nb 0.029%, Ca 0.0013%, N 0.0062%, and the rest is Fe and inevitable trace elements; the matrix structure (see Figure 7 ), the total volume fraction of pearlite is about 13%, the grain size is 10; the core structure is Figure 8 The steel plate has a yield strength of 471 MPa, a tensile strength of 563 MPa, a yield strength ratio of 0.84, and an elongation of 25.5%. The longitudinal impact energy at -40°C (sample size 10×5×55mm) is 43 J, 49 J, and 32 J, respectively, with an average of 41.3 J. The low impact energy is primarily due to the high finishing rolling temperature, high P content, and a composition deviation from the required values.

[0064] The above embodiments are described to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A thin gauge low yield ratio container steel plate, characterized in that: The thin gauge low yield ratio container steel plate has a nominal thickness of 5 mm and includes the following components in percentage by mass: C 0.11~0.15%, Si 0.25~0.40%, Mn 1.25~1.35%, P≤0.015%, S≤0.005%, Als0.025~0.045%, Nb 0.010~0.018%, Ca 0.0010-0.0015%, N 0.0040-0.0065%, the rest are Fe and inevitable trace elements.

2. The thin gauge low yield ratio container steel plate according to claim 1, characterized in that: The structure of the thin-gauge low-yield ratio container steel plate is as follows: the matrix structure is ferrite+pearlite, the volume fraction of pearlite is 22-30%, and the grain size at the position 1 / 4 from the surface is ≥11 levels.

3. The thin gauge low yield ratio container steel plate according to claim 1 or 2, characterized in that: The yield strength of the thin-gauge low-yield ratio container steel plate is ≥390 MPa, the tensile strength of the steel plate is ≥550 MPa, the elongation is ≥35%, the longitudinal impact energy at -40°C is ≥100 J, and the yield ratio is ≤0.

75.

4. A method for producing thin gauge low yield ratio container steel plate according to any one of claims 1 to 3, characterized in that: The production method includes the following process flow: blast furnace smelting → molten iron pretreatment → converter smelting → deoxidation and alloying → LF refining → slab continuous casting → natural cooling → slab heating furnace heating → hot rolling → steel plate hot straightening → air cooling → finishing shearing → heat treatment → finished product.

5. The production method according to claim 4, characterized in that The LF refining is as follows: after arriving at the refining station, argon is connected and the gas volume is adjusted to control the flow rate at 400-600NL / min; based on 100t / furnace, 200-250Kg of refining slag, 100-300Kg of lime, and 20-80Kg of fluorite are added. After the LF furnace is electrified and slagging for 6-10 minutes, Al particles are added as a deoxidizer. Nb iron is added according to the sampling results to fine-tune the composition. After the composition meets the standard, 100-150m of calcium wire is fed, and the soft argon blowing time is ≥8min.

6. The production method according to claim 4, characterized in that The slab continuous casting obtains a slab with a cross-section of 180×1400 mm.

7. The production method according to claim 4, characterized in that The slab heating furnace is specifically heated as follows: the temperature of the second heating section and the soaking section is controlled at 1240-1260°C, the billet thickness is 180 mm, and the heating time is (1.1-1.3) × H, where H is the billet thickness in mm and the heating time is in min.

8. The production method according to claim 4, characterized in that The hot rolling process is divided into two stages: the starting temperature of rough rolling is ≥1100℃, the finishing temperature is ≥980℃, and the intermediate thickness is 55-60mm; the starting temperature of finishing rolling is ≤980℃, the finishing temperature is 840-870℃, and the cumulative deformation rate below 900℃ is ≥50%.

9. The production method according to claim 4, characterized in that The heat treatment adopts normalizing, and the normalizing process is: temperature 860±10°C, time: 3.5×H, time unit is min; H is thickness, unit is mm.

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