A low-compression-ratio s355nl super-thick steel plate with a thickness of 250-300 mm and a manufacturing method thereof

Abstract

The present application relates to a kind of thickness > 250 ~ 300mm low compression ratio S355NL extra-thick steel plate and its manufacturing method, the chemical composition of the steel plate: C:0.08 ~ 0.12%, Si:0.20 ~ 0.40%, Mn:1.45 ~ 1.65%, P:≤0.005%, S:≤0.001%, Al:0.02 ~ 0.05%, Nb:0.02 ~ 0.05%, V:0.03 ~ 0.06%, Ni:0.25 ~ 0.45%, Ti:0.01 ~ 0.03%, the balance is Fe and inevitable impurity element, carbon equivalent Ceq≤0.45%.Production using large thickness thick continuous casting slab, rolling compression ratio is less than 2.0.The internal quality and mechanical property of steel plate meet: yield strength ≥275Mpa, tensile strength ≥450Mpa, elongation after fracture ≥22%, the Charpy impact energy of transverse direction at-50 ℃ single value ≥100J, thickness direction tensile reduction of area ≥35%.Steel plate UT flaw detection can meet the requirement of NB / T47013.3 I level.The metallographic microstructure of steel plate is uniform fine ferrite + pearlite structure.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of steel plate manufacturing, and particularly relates to a low-compression-ratio S355NL super-thick steel plate and a manufacturing method thereof. BACKGROUND

[0002] With the increase of infrastructure construction projects in China, the market demand for super-thick steel plates is increasing, and is developing towards large thickness, large single weight and high quality performance. The S355NL super-thick steel plate is usually used for important components in the energy field, and the service safety thereof is highly required in design, so the product must have good internal quality, welding performance, excellent low-temperature impact performance and excellent resistance to lamellar tearing.

[0003] Under the background of the national green low-carbon high-quality development strategy, domestic steel enterprises have increased investment in the research and development of super-thick continuous casting slab manufacturing technology and low-compression-ratio rolling technology, and have achieved certain results. Publicly reported materials show that the maximum thickness of the steel plate produced by using continuous casting slabs has reached 250 mm, breaking the traditional manufacturing process of super-thick steel plates using ingots and breaking the traditional restriction that the rolling compression ratio must be greater than 3. However, due to the limitation of the thickness and metallurgical quality and stability of the continuous casting slab, the thickness of the super-thick steel plate produced by using the continuous casting slab directly rolling process is below 200 mm, and the performance qualification rate and the flaw detection qualification rate of the steel plate with a thickness of 100-200 mm are generally not high. The steel plate with a thickness of more than 200-250 mm is produced by forging + rolling, which can significantly improve the internal quality of the super-thick steel plate under the condition of low compression ratio, but the manufacturing cycle is long, the yield is low, and the production cost is high, which does not have popularization value. There is no related report on the process technology of producing high-performance steel plates with a thickness of more than 250-300 mm by using continuous casting slabs directly rolling.

[0004] Based on the above prior art, the person skilled in the art is committed to developing a low-compression-ratio high-performance S355NL super-thick steel plate with a larger thickness, which realizes good internal quality and excellent comprehensive mechanical properties under the condition of low-compression-ratio rolling, reduces the production cost, and is suitable for batch production. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a low-compression-ratio high-performance S355NL super-thick steel plate with a thickness of more than 250-300 mm, which can be applied to the manufacturing of key core components in the energy industry, has good internal quality, high low-temperature impact toughness and excellent resistance to lamellar tearing under the condition of a compression ratio of less than 2.0.

[0006] The technical scheme adopted by the present application to solve the above problems is as follows: a low-compression-ratio high-performance S355NL extra-thick steel plate with a thickness of 250-300 mm, the chemical components of the steel plate are as follows in percentage by weight: C: 0.08-0.12%, Si: 0.20-0.40%, Mn: 1.45-1.65%, P: ≤0.005%, S: ≤0.001%, Al: 0.02-0.05%, Nb: 0.02-0.05%, V: 0.03-0.06%, Ni: 0.25-0.45%, Ti: 0.01-0.03%, and the balance is Fe and inevitable impurities, the carbon equivalent Ceq is ≤0.45%, and the calculation formula is: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, and the compression ratio of the steel plate is less than 2.0.

[0007] Further, the internal quality and mechanical properties of the low-compression-ratio high-performance S355NL extra-thick steel plate with a thickness of 250-300 mm meet the following requirements: the yield strength is ≥275 MPa, the tensile strength is ≥450 MPa, the elongation after fracture is ≥22%, the Charpy impact energy at -50 ℃ in the transverse direction is ≥100 J, and the thickness direction tensile reduction of area is ≥35%. The UT flaw detection of the steel plate can meet the requirements of NB / T47013.3 I level.

[0008] The present application belongs to iron-based alloy, and the main chemical elements in the steel and their effects are as follows:

[0009] C: can significantly improve the strength of the steel plate, but is not conducive to the toughness and plasticity, and the C content in the present application is set to be 0.08-0.12% by comprehensively considering the strength and toughness of the steel plate.

[0010] Mn: manganese is a good deoxidizer and desulfurizer in the steelmaking process, and can improve the strength and toughness of the steel plate, but excessive content will cause serious segregation of the continuous casting billet, and the manganese content in the present application is controlled to be 1.45-1.65%.

[0011] Si: is mainly used as a reducing agent and deoxidizer during steelmaking, and has a certain solid solution strengthening effect, and if the content is too high, the low-temperature toughness of the steel plate will be reduced, and therefore the Si content in the present application is set to be 0.20-0.40%.

[0012] S and P are harmful elements, which increase the brittleness of the steel, reduce the impact toughness and welding performance, and therefore the lower the content of S and P, the better, and the S content in the present application is set to be ≤0.001%, and the P content is set to be ≤0.005%.

[0013] Ni: nickel can significantly improve the low-temperature impact toughness, but if the content is too high, it will adversely affect the surface quality of the steel plate, and at the same time, nickel is a precious alloy and should be limited in use, and the Ni content in the present application is set to be 0.25-0.45% by comprehensively considering the performance and production cost of the steel plate.

[0014] Alt: Aluminum is mainly used for deoxidation and grain refinement, and the content of Alt in the patent is controlled at 0.02-0.05%.

[0015] Nb, V: Niobium and vanadium micro-alloying elements form carbonitride with C and N elements, which can delay austenite recrystallization, refine ferrite grains, and improve the strength and toughness of the steel plate. In the present invention, the content of Nb is controlled at 0.02-0.05%, and the content of V is controlled at 0.03-0.06%.

[0016] Ti: Carbonitride is formed with C and N elements, which can delay austenite recrystallization, refine ferrite grains, and improve the strength and toughness of the steel plate. In the present invention, the content of Ti is controlled at 0.010-0.03%.

[0017] The manufacturing method of the low-compression-ratio high-performance S355NL thick steel plate with a thickness of >250-300mm comprises the following contents:

[0018] The large-thickness continuous casting slab is directly rolled, and the production process flow is as follows: KR pretreatment→ converter smelting→ LF refining→ RH refining→ slab continuous casting→ slab slow cooling→ slab heating→ slab slow cooling after discharging→ slab heating→ steel plate rolling→ steel plate slow cooling→ flaw detection→ normalizing→ finishing→ inspection→ warehousing.

[0019] The specific operation of the main process is as follows:

[0020] 1) Smelting process

[0021] The smelting raw material is pretreated by KR molten iron, and after converter smelting, slagging treatment is performed, and S≤0.001%, P≤0.005%, H≤1.0ppm, O≤20ppmm, and N≤40ppm are strictly controlled; low superheat argon protection is used for full-process pouring in the continuous casting process, and through dynamic soft reduction technology, the segregation C class is controlled at 0.5 level, and the porosity is controlled at 0.5 level or better; the slab is covered and slowly cooled for more than 48 hours after being discharged, so as to ensure that the hydrogen in the steel is fully diffused.

[0022] 2) Heating and rolling process

[0023] The twice-heating and once-rolling process is adopted for production.

[0024] The continuous casting slab is heated in the walking beam furnace, and in order not to affect the production rhythm, the same process system is adopted for the twice-heating process. After the first heating is completed, the slab is discharged, then quickly discharged to the slow cooling area for stacking and slow cooling, and after the slab is naturally cooled to room temperature, the continuous casting slab is loaded into the heating furnace for heating again.

[0025] Both heating adopts sectional heating mode: total heating time is 500-550 min, the first heating section temperature is 1000-1180 DEG C, the second heating section temperature is 1160-1260 DEG C, the soaking section temperature is 1150-1250 DEG C, the total heating time of the second heating section and the soaking section is greater than or equal to 280 min.

[0026] The rolling deformation is all completed on the single stand rough rolling mill, the full longitudinal rolling mode is adopted, the rolling process adopts high permeation rolling process, and the steel plate is naturally cooled in air after rolling. The rough rolling temperature is 1050-1100 DEG C, and the rolling is completed by 4-6 passes. High pressure water is used for descaling after each rolling pass, the high permeation rolling is realized by using the temperature difference between the surface and the core of the continuous casting billet, that is, the differential temperature rolling is realized, and the deformation coefficient of at least two passes is greater than or equal to 0.5. The steel plate is stacked and slowly cooled for more than or equal to 72 hours after being discharged, and the hydrogen is fully expanded. R-Work roll radius of the R mill (598 mm), Δh-Reduction, H-Entry thickness.

[0027] 3) Heat treatment process

[0028] The normalizing process is adopted, the normalizing temperature is preferably controlled to be 880 DEG C ± 10 DEG C, the holding time coefficient is controlled to be 2.2-2.5 min / mm, and the air cooling is adopted. The normalizing treatment is carried out in a trolley furnace, and the steel plate is hoisted to a cooling bed as soon as possible after being discharged from the furnace to be cooled to below 200 DEG C.

[0029] The principle of the application of the above process measures and process parameters is as follows:

[0030] The technical difficulties of the low compression ratio rolling high-performance thick steel plate of the continuous casting billet lie in: (1) how to improve the metallurgical quality of the continuous casting billet itself, that is, to minimize the center segregation and center porosity defects of the continuous casting billet, and to fully remove the non-metallic inclusions and the gas hydrogen content in the steel. (2) how to realize the rolling pressure penetration in the limited deformation range in the thickness direction to achieve the effect of fully crushing the as-cast structure and pressing the core metallurgical defects. The composition and process design of the application is carried out around solving the above two technical problems. The main measures for improving the metallurgical quality of the continuous casting billet are as follows: 1) on the composition design, the contents of segregation elements such as C, Mn, S and P are as low as possible under the premise of ensuring the strength of the steel plate; 2) on the production process, the dynamic soft reduction of the continuous casting billet and the two heating high temperature diffusion technical means are adopted to reduce the center segregation and porosity defects of the continuous casting billet; the main measures for realizing the rolling deformation penetration are as follows: 1) the single stand full longitudinal rolling mode is adopted for production, the high temperature holding time of the continuous casting billet is prolonged, and the high temperature rolling is realized to reduce the deformation resistance; 2) the single pass reduction is increased, and the differential temperature rolling technology is adopted to ensure that the deformation coefficient of at least two passes is greater than or equal to 0.5.

[0031] Compared with the prior art, the application has the advantages that:

[0032] The present application relates to a kind of low compression ratio high-performance S355NL thick steel plate with thickness > 250 ~ 300mm, it is directly rolled using continuous casting billet, with less than 2.0 compression ratio, with good internal quality, higher strength and excellent low temperature impact toughness and anti-laminated tear performance.Under the condition, the yield strength and tensile strength margin of the product of the present application is 30Mpa or more, the Charpy impact energy at-50 DEG C is ≥100J, and the thickness direction tensile section shrinkage is ≥35%.The UT flaw detection of steel plate can meet the requirements of NB / T47013.3 I level.Through the normalizing treatment, the microstructure of the steel plate is uniform fine ferrite + pearlite mixed structure, and its typical morphology is shown in the following figure. Figure 1 . BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 It is the typical microstructure diagram (100x) of the low compression ratio S355NL thick steel plate of the present application. DETAILED DESCRIPTION

[0034] The present application is further described in detail below in conjunction with examples, which are exemplary and intended to explain the present application, and cannot be understood as limiting the present application.

[0035] Example 1

[0036] The thickness of the S355NL steel plate of the present embodiment is 255mm, and its chemical composition is as follows in terms of weight percentage: C: 0.09%, Si: 0.35%, Mn: 1.58%, P: 0.004%, S: ≤0.0008%, Al: 0.035%, Nb: 0.030%, V: 0.032%, Ni: 0.41%, Ti: 0.016%, the balance is Fe and inevitable impurity elements, carbon equivalent Ceq ≤0.45%, and the calculation formula is as follows: Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0037] The manufacturing process of the steel plate is as follows:

[0038] Large-thickness continuous casting slab is directly rolled, the continuous casting slab is heated twice and rolled once, and the production process flow is as follows:

[0039] KR pretreatment → converter smelting → LF refining → RH refining → slab continuous casting → slab slow cooling → slab heating → slab slow cooling after heating → slab heating → steel plate rolling → steel plate slow cooling → flaw detection → normalizing → finishing → inspection → warehousing.

[0040] The specific operation of main processes is as follows:

[0041] 1) Smelting process

[0042] The smelting raw materials undergo KR hot metal pretreatment, followed by slag removal after converter smelting. Saturation is strictly controlled to ensure S ≤ 0.001%, P ≤ 0.005%, H ≤ 1.0 ppm, O ≤ 20 ppm, and N ≤ 40 ppm. The continuous casting process employs low superheat with full argon protection during pouring. Dynamic light reduction technology controls slab segregation to Class C 0.5 and porosity to 0.5 or better. After the slab is removed from the casting line, it is slowly cooled under a cover for at least 48 hours to ensure sufficient hydrogen diffusion in the steel. In this embodiment, the molten steel superheat in the continuous casting process is 20°C. Low-magnification inspection of the slab shows center segregation of Class C 0.5 and porosity of 0.5.

[0043] 2) Heating and rolling process

[0044] It is produced using a double heating and single rolling process.

[0045] The continuously cast slab is heated in a walking beam furnace. To avoid affecting the production rhythm, the same process is used for both heating processes. After the first heating is completed, the slab is removed from the furnace and then quickly moved to a slow cooling area for slow cooling. After the slab has cooled naturally to room temperature in the air, it is loaded back into the furnace for heating.

[0046] The heating process employs a segmented heating method: The first heating cycle has a total heating time of 510 min, with the first heating segment temperature ranging from 1000 to 1180℃, the second heating segment temperature from 1160 to 1260℃, and the soaking section temperature from 1150 to 1250℃. The total heating time for the second and soaking sections is 300 min. The second heating cycle has a total heating time of 530 min, with the first heating segment temperature also ranging from 1000 to 1180℃, the second heating segment temperature from 1160 to 1260℃, and the soaking section temperature from 1150 to 1250℃. The total heating time for the second and soaking sections is 320 min.

[0047] All rolling deformation is completed on a single stand of the roughing mill, using a full longitudinal rolling method without controlled rolling. The rolling process employs a high-penetration rolling process, and the steel plate is naturally cooled in air after rolling. The initial rolling temperature is 1080℃, and rolling is completed in 6 passes. High-pressure water descaling is performed after each rolling pass. High-penetration rolling, i.e., differential temperature rolling, is achieved by utilizing the temperature difference between the surface and core of the continuously cast billet. Three of the rolling passes have a deformation coefficient ≥0.5, specifically 0.52, 0.57, and 0.53. After finishing, the steel plates are stacked and slowly cooled for ≥72 hours to allow for sufficient hydrogen diffusion.

[0048] 3) Heat treatment process

[0049] The normalizing process is adopted, with a normalizing temperature of 880±10℃ and a holding time coefficient of 2.3min / mm. Air cooling is used. The normalizing treatment is carried out in a bogie hearth furnace. After the steel plate is taken out of the furnace, it is quickly hoisted to the cooling bed to cool to below 200℃.

[0050] The 255 mm thick continuous casting billet produced by the above manufacturing process produces a low compression ratio high performance S355NL thick steel plate, the UT flaw detection meets the I level requirements of NB / T47013.3 standard, and has a good comprehensive mechanical property matching, as shown in Table 1.

[0051] Example 2

[0052] The thickness of the S355NL steel plate of this example is 285 mm, and the chemical composition in terms of weight percentage is: C: 0.11%, Si: 0.32%, Mn: 1.55%, P: 0.004%, S: ≤0.0006%, Al: 0.032%, Nb: 0.028%, V: 0.035%, Ni: 0.42%, Ti: 0.018%, the balance being Fe and inevitable impurity elements, carbon equivalent Ceq≤0.45%, the calculation formula is: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0053] The manufacturing process of the steel plate is as follows:

[0054] Large thickness continuous casting slab is directly rolled, the continuous casting slab is heated twice and rolled once, and the production process flow is:

[0055] KR pretreatment→converter smelting→LF refining→RH refining→slab continuous casting→slab slow cooling→slab heating→slab furnace slow cooling→slab heating→steel plate rolling→steel plate slow cooling→flaw detection→normalizing→finishing→inspection→warehouse.

[0056] The specific operation of the main process is as follows:

[0057] 1) Smelting process

[0058] The smelting raw material is pretreated by KR molten iron, and after converter smelting, slagging treatment is carried out, S≤0.001%, P≤0.005%, H≤1.0ppm, O≤20ppmm, N≤40ppm are strictly controlled; low superheat argon protection pouring is adopted in the continuous casting process, through dynamic soft reduction technology, the segregation C class is controlled to 0.5 level, and the porosity is 0.5 level or better; the slab is covered and slowly cooled for more than 48 hours after unloading, to ensure that the hydrogen in the steel is fully diffused. The superheat of the molten steel in the continuous casting process of this example is 18℃, the slab is examined by macroscopic examination, the center segregation is C class 0.5 level, and the porosity is 0.5 level.

[0059] 2) Heating and rolling process

[0060] Two heating and one rolling process is adopted for production.

[0061] The continuous casting slab is heated in the walking beam furnace, and the same process system is used for the two heating processes in order not to affect the production rhythm. After the first heating is finished, the slab is discharged from the furnace, and then quickly lowered to the slow cooling area for stacking and slow cooling. After the slab is naturally cooled to room temperature in the air, the continuous casting slab is loaded into the heating furnace for heating again.

[0062] Both the two heating processes use the sectional heating method. The total heating time of the first heating is 520 min, the temperature of the first heating section is 1000-1180 °C, the temperature of the second heating section is 1160-1260 °C, the temperature of the soaking section is 1150-1250 °C, and the total heating time of the second heating section and the soaking section is 315 min. The total heating time of the second heating is 550 min, the temperature of the first heating section is 1000-1180 °C, the temperature of the second heating section is 1160-1260 °C, the temperature of the soaking section is 1150-1250 °C, and the total heating time of the second heating section and the soaking section is 330 min.

[0063] The rolling deformation is all completed on the single stand of the rough rolling mill, and the full longitudinal rolling method is used for production without controlled rolling. The high permeability rolling process is used in the rolling process, and the steel plate is naturally cooled in the air after rolling. The open rolling temperature is 1080 °C, and the rolling is completed in 6 passes. High pressure water is used for phosphorus removal after the end of each rolling pass. The high permeability rolling is realized by using the temperature difference between the surface and the core of the continuous casting slab, i.e. differential temperature rolling. There are 2 passes with a deformation coefficient ≥0.5 in the rolling passes, which are 0.52 and 0.61 respectively. The steel plate is stacked and slow cooled for ≥72 hours after being lowered, so as to fully expand the hydrogen.

[0064] 3) Heat treatment process

[0065] The normalizing process is used, the normalizing temperature is 880±10 °C, the holding time coefficient is 2.4 min / mm, and the air cooling is used. The normalizing treatment is carried out in the bogie furnace, and the steel plate is lifted to the cooling bed as soon as possible after being discharged from the furnace to be cooled to below 200 °C.

[0066] The 255 mm thick continuous casting slab produced by the above manufacturing process produces a low compression ratio high performance S355NL super thick steel plate. The UT flaw detection meets the I level requirements of NB / T47013.3 standard, and has a good matching comprehensive mechanical property. See Table 1 for details.

[0067] Example 3

[0068] The thickness of the S355NL steel plate of the present embodiment is 300 mm, and the chemical composition in terms of percentage by weight is: C: 0.10%, Si: 0.36%, Mn: 1.62%, P: 0.005%, S: ≤0.0005%, Al: 0.032%, Nb: 0.032%, V: 0.035%, Ni: 0.42%, Ti: 0.015%, the balance being Fe and inevitable impurity elements, and carbon equivalent Ceq≤0.45%, the calculation formula being: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0069] The manufacturing process of the steel plate is as follows:

[0070] The large-thickness continuous casting slab is directly rolled, the continuous casting slab is heated twice and rolled once, and the production process flow is as follows:

[0071] KR pretreatment→converter smelting→LF refining→RH refining→slab continuous casting→slab slow cooling→slab heating→slab slow cooling after slab discharge→slab heating→steel plate rolling→steel plate slow cooling→inspection→normalizing→finishing→inspection→storage.

[0072] The specific operation of the main process is as follows:

[0073] 1) Smelting process

[0074] The smelting raw material is pretreated by KR molten iron, and after converter smelting, slagging treatment is performed, and S≤0.001%, P≤0.005%, H≤1.0ppm, O≤20ppmm, and N≤40ppm are strictly controlled; the continuous casting process adopts low superheat full-range argon protection pouring, and the dynamic soft reduction technology is used to control the segregation C class 0.5 level and the porosity 0.5 level or better; the slab is slow-cooled for more than 48 hours after being discharged, so as to ensure that the hydrogen in the steel is fully diffused. The superheat of the molten steel in the continuous casting process of the present embodiment is 17°C, the slab is examined by macro examination, the center segregation is C class 0.5 level, and the porosity is 0.5 level.

[0075] 2) Heating and rolling process

[0076] The two-heating-one-rolling process is adopted for production.

[0077] The continuous casting slab is heated in the walking beam furnace, in order not to affect the production rhythm, the same process system is adopted for the two-heating process. After the first heating is completed, the slab is discharged, then quickly discharged to the slow cooling area for stacking and slow cooling, and after the slab is naturally cooled to room temperature, the continuous casting slab is loaded into the heating furnace for heating again.

[0078] Both heating adopts the way of subsection heating: the first heating total heating time is 540 min, the first heating section temperature is 1000-1180℃, the second heating section temperature is 1160-1260℃, the soaking section temperature is 1150-1250℃, the second heating section and soaking section total heating time is 325 min. The second heating total heating time is 545 min, the first heating section temperature is 1000-1180℃, the second heating section temperature is 1160-1260℃, the soaking section temperature is 1150-1250℃, the second heating section and soaking section total heating time is 345 min.

[0079] The rolling deformation is completed on the single stand rough rolling mill, and the steel plate is produced by full longitudinal rolling without controlled rolling. The high permeability rolling process is adopted in the rolling process, and the steel plate is naturally cooled in air after rolling. The rough rolling temperature is 1080℃, and the rolling is completed in 6 passes. High pressure water is used for phosphorus removal after each rolling pass. The high permeability rolling is realized by using the temperature difference between the surface and the core of the continuous casting billet, i.e. differential temperature rolling. The deformation coefficient of 2 passes is greater than or equal to 0.5, which is 0.51 and 0.56 respectively. The steel plate is stacked and slowly cooled for more than 72 hours after being discharged, so as to fully expand the hydrogen.

[0080] 3) Heat treatment process

[0081] The normalizing process is adopted, the normalizing temperature is 880±10℃, the holding time coefficient is 2.5 min / mm, and the air cooling is adopted. The normalizing treatment is carried out in the bogie furnace, and the steel plate is lifted to the cooling bed as soon as possible after being discharged from the furnace to cool to below 200℃.

[0082] The 300mm thick continuous casting billet produced by the above manufacturing process produces low compression ratio high performance S355NL ultra thick steel plate, UT flaw detection meets the I level requirement of NB / T47013.3 standard, and has good comprehensive mechanical properties. See Table 1 for details.

[0083] Table 1 Mechanical properties of steel plates produced in each embodiment

[0084]

[0085] Note: The average value is in ().

Claims

1. A method for manufacturing a low-compression-ratio S355NL super-thick steel plate having a thickness of > 250-300 mm, characterized in that: The chemical composition of the steel plate, in terms of weight percentage, is C: 0.08-0.12%, Si: 0.20-0.40%, Mn: 1.45-1.65%, P: ≤0.005%, S: ≤0.001%, Al: 0.02-0.05%, Nb: 0.02-0.05%, V: 0.03-0.06%, Ni: 0.25-0.45%, Ti: 0.01-0.03%, the balance being Fe and inevitable impurity elements, carbon equivalent Ceq ≤0.45%, Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, wherein the element symbols are the percentage contents of the corresponding elements; the manufacturing method comprises the following steps: 1) Steel smelting The smelting raw materials are subjected to KR molten iron pretreatment, converter smelting, and slagging treatment, so as to control the molten steel S ≤0.001%, P ≤0.005%, H ≤1.0 ppm, O ≤20 ppm, and N ≤40 ppm; the continuous casting process adopts low superheat full-range argon protection pouring, and the dynamic soft reduction technology is used to control the segregation C class 0.5 level and the porosity 0.5 level or better; after the slab is discharged, the cover is added for slow cooling for more than 48 hours, so as to ensure that the hydrogen in the steel is fully diffused; 2) Heating and rolling The two-time heating and one-time rolling process is adopted for production, and the sectional heating mode is adopted for both times of heating: the total heating time is 500-550 min, the first heating section temperature is 1000-1180℃, the second heating section temperature is 1160-1260℃, the soaking section temperature is 1150-1250℃, and the total heating time of the second heating section and the soaking section is ≥280 min; after the first heating is completed, the furnace is discharged, and then the continuous casting slab is quickly discharged to the slow cooling area for stacking and slow cooling; after the slab is naturally cooled to room temperature in the air, the continuous casting slab is loaded into the heating furnace for the second time of heating; The total rolling deformation is completed on a single stand roughing mill, using full longitudinal rolling, without controlled rolling, with a rolling reduction ratio of less than 2.0, using high permeability rolling technology, with the steel plate being naturally cooled in air after rolling, with a start rolling temperature of 1050-1100°C, with 4-6 passes to complete rolling, with high pressure water descaling after each rolling pass, with high permeability rolling being achieved using the temperature difference between the surface and the core of the continuous casting billet, i.e. differential temperature rolling, with at least 2 passes having a deformation coefficient of ≥0.5, ; R - roll radius of the rolling mill, Δ h - reduction, H - entry thickness; the steel plate is stack cooled for ≥72 hours after being discharged from the rolling mill, with the hydrogen being fully expanded; 3) Heat treatment process The normalizing process is adopted, the normalizing treatment is performed in a trolley furnace, the normalizing temperature is 880±10℃, the holding time coefficient is 2.2-2.5 min / mm, and the air cooling is performed; the steel plate is quickly lifted to the cooling bed after being discharged from the normalizing heating furnace and is cooled to below 200℃.

2. Process for the manufacture of a heavy steel plate of thickness > 250-300 mm with low reduction ratio S355NL according to claim 1, characterized in that: The internal quality and mechanical properties of the steel plate meet the requirements of: yield strength ≥275 MPa, tensile strength ≥450 MPa, elongation after fracture ≥22%, -50℃ transverse Charpy impact energy ≥100 J, and thickness direction tensile section shrinkage rate ≥35%; the UT flaw detection of the steel plate can meet the requirements of NB / T47013.3 I level; and the steel plate has a uniform and fine ferrite + pearlite microstructure.

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