Low-cost non-nb non-quenched 700mpa high-strength steel and manufacturing method thereof

Abstract

The application provides a low-cost Nb-free non-quenched 700MPa high-strength steel and a manufacturing method thereof, the high-strength steel comprises the following chemical components by weight percentage: C: 0.050-0.100%, Si: 0.20-0.60%, Mn: 1.60-1.85%, P: ≤0.020%, S: ≤0.010%, Alt: 0.010%-0.060%, B: 0.0008%-0.0025%, Ti: 0.008%-0.020%, Cr: ≤0.15%, Nb: ≤0.006%, Mo: ≤0.08%, Ni: ≤0.15%, Cu: ≤0.15%, N: ≤0.0060%, O: ≤0.0030%, H: ≤0.0002%, Pcm: ≤0.25%, and the balance is Fe and inevitable impurities. Under the premise of not adding micro-alloying elements such as Nb, the whole process fine-grain concept of moderate low-temperature heating, two-stage recrystallization rolling and post-rolling controlled cooling is adopted to produce the Nb-free non-quenched 700MPa high-strength steel, and the cost of per ton of steel can be reduced by more than 100 yuan. The steel plate manufactured by the manufacturing method has a yield strength of ≥600MPa, a tensile strength of ≥700MPa, a non-proportional elongation A 50mm ≥30%, a longitudinal impact energy at -20 DEG C of ≥200J, a metallographic structure mainly of lath bainite and granular bainite.

Description

Technical Field

[0001] This invention relates to the field of iron and steel metallurgy technology, specifically to a low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel and its manufacturing method. Background Technology

[0002] In China, mainstream steel mills often produce high-strength steel with a tensile strength of 700 MPa without quenching and tempering by adding precious metals such as Cr, Mo, Ni, Cu, and V, as well as a high content of the microalloying element Nb, resulting in high alloy costs. For example, patent CN101768698A discloses a low-cost high-strength steel plate with a yield strength of 700 MPa without quenching and tempering and its manufacturing method. Its composition design clearly includes Nb: 0.04%~0.08%, Ti: 0.09%~0.15%, and Cr: 0.3%~0.8%, which belongs to a typical "high Ti+Nb" composite microalloying route. Furthermore, several other patents in the industry for 700MPa-grade high-strength steel involve the addition of Nb. For example, patent CN114774787A discloses a hot-rolled steel strip for 700MPa-grade microalloyed high-strength drill pipe and its production method, in which Nb: 0.061~0.066%, Ti: 0.019~0.023%, Mo: 0.17~0.18%, and Cr: 0.19~0.22%. It is evident that Nb has become an indispensable microalloying element in 700MPa-grade non-quenched and tempered high-strength steel. However, the above-mentioned Nb-containing technical routes have the following prominent problems in actual production:

[0003] First, the alloy cost is high. In recent years, the price of niobium-iron alloy has fluctuated continuously and remained at a high level. As an expensive microalloying element, every 0.01% increase in the amount of Nb added will significantly increase the cost per ton of steel. At the same time, the above-mentioned route usually requires the addition of other expensive alloying elements such as Cr, Mo, Ni, and Cu, which further increases the overall alloy cost.

[0004] Secondly, the rolling rhythm is severely constrained. During controlled rolling of Nb-containing steel, due to the solute drag effect of Nb and the strong inhibitory effect of Nb(C,N) precipitation on recrystallization, a relatively long waiting time is usually required between the two rolling stages to allow the intermediate billet temperature to drop from the recrystallization zone to the non-recrystallization zone. On hot-roll mill lines, this waiting time typically relies on natural air cooling, with the waiting time for 30-50mm thick intermediate billets usually exceeding 1.5-2.5 minutes. This severely impacts the mill's hourly output and line efficiency, becoming a bottleneck restricting the production rhythm. The efficiency loss due to waiting time is even more significant in the production of thick steel plates.

[0005] Third, the high heating temperature requirement further increases energy consumption and production costs. The complete dissolution of Nb (C,N) in Nb-containing steel usually requires a high heating temperature (generally 1220~1280℃), which not only increases the fuel consumption of the heating furnace, but also aggravates the oxidation and burning loss on the surface of the billet, and shortens the service life of the heating furnace.

[0006] In response to the above problems, the field has begun to explore the "Nb-free" technical route in recent years. Although the attempt to "free" Nb has reduced the cost of alloys to some extent, it still faces the following technical problems: how to ensure the strength and toughness of 700MPa while completely eliminating Nb, and how to effectively solve the inherent contradiction of slow rolling pace and low production efficiency of Nb-containing steel; at the same time, it is also necessary to achieve a low-cost, non-quenched and tempered medium and heavy plate production process.

[0007] In summary, developing a technical solution that can ensure the strength and toughness of 700MPa grade non-quenched and tempered high-strength steel while shortening the rolling cycle, without adding Nb, through reasonable principal component design and process innovation, has significant industrial application value and urgency.

[0008] Based on this, the present invention is proposed. Summary of the Invention

[0009] The purpose of this invention is to provide a low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel and its manufacturing method. Our company, relying on its own technology, achieves this without adding microalloying elements such as Nb. This is achieved by adjusting the ratio of C, Mn, and B in the steel to perform micro-Ti treatment, fully utilizing the roles of C, Mn, B, and Ti. Furthermore, without adding microalloying elements such as Nb, a full-process fine-graining concept involving moderate low-temperature heating, two-stage recrystallization rolling, and controlled cooling after rolling is adopted to produce Nb-free, non-quenched and tempered 700MPa high-strength steel. This reduces costs by more than 100 yuan per ton of steel, enabling low-cost, high-efficiency, and continuous production of Nb-free steel. Simultaneously, by controlling the [N], [O], and [H] content in the molten steel, a low-cost, molybdenum-free, non-quenched and tempered 700MPa high-strength steel plate with excellent low-temperature toughness is obtained.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] A low-cost, Nb-free, unquenched and tempered 700MPa high-strength steel comprises the following chemical composition by weight percentage: C: 0.050~0.100%, Si: 0.20~0.60%, Mn: 1.60~1.85%, P≤0.020%, S≤0.010%, Alt: 0.010%~0.060%, B: 0.0008%~0.0025%, Ti: 0.008%~0.020%, Cr: ≤0.15%, Nb≤0.006%, Mo≤0.08%, Ni≤0.15%, Cu≤0.15%, N≤0.0060%, O≤0.0030%, H≤0.0002%, Pcm≤0.25%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, the low-cost Nb-free unquenched and tempered 700MPa high-strength steel comprises the following chemical composition by weight percentage: C: 0.050~0.100%, Si: 0.20~0.60%, Mn: 1.60~1.85%, P≤0.020%, S≤0.010%, Alt: 0.010%~0.060%, B: 0.0010%~0.0018%, Ti: 0.010%~0.020%, Cr: ≤0.15%, Nb≤0.006%, Mo≤0.08%, Ni≤0.15%, Cu≤0.15%, N≤0.0060%, O≤0.0030%, H≤0.0002%, Pcm≤0.25%, with the balance being Fe and unavoidable impurities.

[0013] As a preferred technical solution, the low-cost Nb-free unquenched and tempered 700MPa high-strength steel comprises the following chemical composition by weight percentage: C: 0.076%, Si: 0.24%, Mn: 1.723%, P: 0.01%, S: 0.002%, Alt: 0.035%, B: 0.00145%, Ti: 0.015%, Nb: 0.006%, Pcm=0.18%, with the balance being Fe and unavoidable impurities.

[0014] A low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel manufacturing method includes hot metal pretreatment, top and bottom blowing converter, LF refining, VD vacuum refining, wide slab continuous casting, and hot coil rolling; wherein,

[0015] The molten iron pretreatment: the molten iron contains S ≥ 0.040%, and desulfurization is carried out using a molten iron pretreatment process, with a target of below 0.010%;

[0016] The top and bottom combined blowing converter oxygen blowing process lasts for 14-16 minutes, and the molten steel tapped is guaranteed to have P ≤ 0.015%, S ≤ 0.020%, and O ≤ 0.06%.

[0017] The LF refining and VD vacuum refining processes ensure that the molten steel temperature and composition meet the requirements for continuous casting and the target composition, wherein the gas content is: N≤0.0060%, O≤0.0030%, H≤0.0002%.

[0018] The wide slab continuous casting adopts full-process protective casting, with casting speed fluctuation within ±0.05m / min, molten steel level fluctuation within ±3mm, molten steel superheat controlled at 10~25℃, and slab low-magnification segregation below B1.5 grade and porosity below 1.5 grade.

[0019] Furthermore, the furnace coil rolling process includes:

[0020] ①After slow cooling for 48 hours, the 150 mm thick continuous casting billet is heated in a heating furnace at a temperature of 1150~1220℃ for a heating time of ≥120 min.

[0021] ② Reversible repeated rolling is carried out using a 3500mm hot roll mill, and the rolling process adopts two-stage recrystallization controlled rolling.

[0022] ③ First stage controlled rolling, rolling start temperature 1000~1100℃, rolling reduction rate in recrystallization zone ≥15%, cumulative reduction rate in recrystallization zone ≥40%;

[0023] ④ After the first stage of rolling is completed, the intermediate billet is cooled by laminar flow to 900-980℃ and then the second stage of controlled rolling is carried out.

[0024] ⑤ The intermediate billet enters the second stage of controlled rolling, with a rolling start temperature of 880~970℃ and a rolling end temperature of 830~880℃. The cumulative reduction rate in the second stage is ≥50%.

[0025] ⑥ Accelerated cooling is adopted after rolling, with a cooling rate of 15~30℃ / s and a final cooling temperature of 350~450℃;

[0026] ⑦ The steel plate enters the hot straightening machine for straightening.

[0027] Furthermore, in the rolling process ④, the intermediate billet cooling rate is 5-10℃ / s.

[0028] Furthermore, the thickness of the high-strength steel manufactured by the manufacturing method is 20~25mm.

[0029] Furthermore, the high-strength steel manufactured using the above method has a yield strength ≥600MPa, a tensile strength ≥700MPa, and a non-proportional elongation A. 50mm ≥30%, longitudinal impact energy at -20℃ ≥200J, metallographic structure is mainly lath bainite and granular bainite.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] (1) The manufacturing method of the present invention can produce 700MPa grade Nb-free high-strength steel by using a reasonable ratio of C-Mn-B and micro-alloying Ti without adding Nb, and by adjusting the rolling process. The steel has low alloy cost, uniform structure in the longitudinal, transverse and thickness directions, and good toughness and plasticity.

[0032] (2) The manufacturing method of the present invention controls the microstructure of the intermediate billet by forced cooling under Nb-free conditions, thereby replacing the inhibitory effect of Nb on recrystallization and eliminating the waiting time for heating. Therefore, the present invention can greatly shorten the waiting time of the intermediate billet, effectively improve the rolling rhythm, and realize continuous production of controlled rolling process.

[0033] (3) The industry generally adopts “adding Nb” or “adding Ti” to achieve fine grain strengthening, but the technical solution of the present invention does the opposite – remove Nb, reduce Ti and add B, and use the hardenability of B to replace the fine grain effect of Nb. This technical concept is non-obvious. Attached Figure Description

[0034] Figure 1 The metallographic diagrams are of the high-strength steel manufactured in Example 1 of the present invention, wherein (a) is the surface metallographic diagram, (b) is the metallographic diagram of the 1 / 4 thickness direction, and (c) is the metallographic diagram of the center thickness direction.

[0035] Figure 2 The metallographic diagrams are of the high-strength steel manufactured in Comparative Example 1 of the present invention, wherein (a) is the surface metallographic diagram, (b) is the metallographic diagram of the 1 / 4 thickness direction, and (c) is the metallographic diagram of the center thickness direction. Detailed Implementation

[0036] The technical solution and effects of the present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0037] Example 1

[0038] The following example illustrates the process of obtaining low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel by adjusting the ratio of the main hardenability elements C-Mn-B in a hot-rolled coil production line.

[0039] The production process in this embodiment includes hot metal desulfurization, top and bottom blowing converter, LF refining, VD degassing, wide slab continuous casting, and hot metal rolling. The hot metal contains ≥0.040% S, and desulfurization is achieved through a hot metal pretreatment process, with a target of below 0.010%. The top and bottom blowing converter undergoes oxygen blowing for 15 minutes, ensuring the molten steel has P ≤0.015%, S ≤0.020%, and O ≤0.06%. Through LF refining and VD vacuum refining, the temperature and composition of the molten steel meet the requirements for continuous casting and the target composition, with the following gas content: N: 0.0042%, O: 0.0016%, H: 0.00012%. Wide slab continuous casting employs full-process protective casting, with casting speed fluctuations within ±0.05 m / min, molten steel level fluctuations within ±3 mm, and molten steel superheat controlled between 10 and 25°C. The slab exhibits low-magnification segregation below grade B1.5 and porosity below grade 1.5.

[0040] In this embodiment, a 150 mm thick continuously cast billet is slowly cooled for 48 hours before being heated in a furnace. It is then reversibly rolled using a 3500 mm hot roll mill, employing a two-stage recrystallization controlled rolling process. The specific process is as follows:

[0041] The first stage rolling start temperature is 1061℃, the reduction rate of each rolling pass in the recrystallization zone is ≥15%, the cumulative reduction rate in the recrystallization zone is 40%, and the first stage finishing rolling temperature is 1035℃.

[0042] After the first stage of rolling, the intermediate billet is cooled using laminar flow at a rate of 6.9℃ / s to 970℃.

[0043] After cooling, the second stage of controlled rolling is carried out directly. The rolling start temperature is 969℃ and the rolling end temperature is 865℃. The cumulative reduction rate in the second stage is 72.77%.

[0044] After rolling, accelerated cooling is adopted, with a cooling rate of 25.4℃ / s and a final cooling temperature of 435℃. Then, it enters a hot straightener for straightening.

[0045] The chemical composition of this embodiment 1 is shown in Table 1, the rolling process is shown in Tables 2-4, the performance indicators are shown in Table 5, the hardness indicators are shown in Table 6, and the metallographic structure is shown in Table 7. Figure 1 .

[0046] Table 1. Chemical composition (wt%) of 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0047]

[0048] Table 2 Heating regime for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0049]

[0050] Table 3. Rolling process for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0051]

[0052] Table 4 Cooling process for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0053]

[0054] Table 5 Performance indicators of the 25mm thick low-cost Nb-free unheated 700MPa high-strength steel manufactured in this embodiment

[0055]

[0056] Table 6. HV10 Hardness Index in the Thickness Direction of Steel Plates

[0057]

[0058] As can be seen from Tables 5-6, the 25mm low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel produced in this embodiment by controlling the C-Mn-B ratio of the main hardenability elements and using a reasonable controlled rolling and cooling process can meet the requirements of product strength, toughness, and plasticity, and has a certain margin. It also has uniform hardness and microstructure in the thickness direction and excellent performance.

[0059] according to Figure 1 It can be seen that the metallographic structure of the 25mm thick low-cost Nb-free unquenched and tempered 700MPa high-strength steel manufactured in this embodiment is mainly lath bainite with +5%~10% granular bainite.

[0060] Example 2

[0061] In this embodiment, a 150 mm thick continuously cast billet is slowly cooled for 48 hours before being heated in a furnace. It is then reversibly rolled using a 3500 mm hot roll mill, employing a two-stage recrystallization controlled rolling process. The specific process is as follows:

[0062] The first stage rolling start temperature is 1050℃, the reduction rate of each rolling pass in the recrystallization zone is ≥15%, the cumulative reduction rate in the recrystallization zone is 42%, and the first stage finishing rolling temperature is 1039℃.

[0063] After the first stage of rolling, the intermediate billet is cooled using laminar flow at a rate of 7.8℃ / s to 953℃.

[0064] After cooling, the second stage of controlled rolling is carried out directly. The rolling start temperature is 950℃, the rolling end temperature is 840℃, and the cumulative reduction rate of the second stage is 73.05%.

[0065] After rolling, accelerated cooling is adopted, with a cooling rate of 26.1℃ / s and a final cooling temperature of 410℃. Then, it enters a hot straightener for straightening.

[0066] The chemical composition of this Example 2 is shown in Table 7, the rolling process is shown in Tables 8-10, the performance indicators are shown in Table 11, and the hardness indicators are shown in Table 12.

[0067] Table 7 Chemical composition (wt%) of 20mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0068]

[0069] Table 8 Heating regime for manufacturing 20mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0070]

[0071] Table 9. Rolling process for manufacturing 20mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0072]

[0073] Table 10 Cooling process for manufacturing 20mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0074]

[0075] Table 11 Performance indicators of the 20mm thick low-cost Nb-free unheated 700MPa high-strength steel manufactured in this embodiment

[0076]

[0077] Table 12 HV10 Hardness Index in Thickness Direction of Steel Plate

[0078]

[0079] As can be seen from Tables 11-12, the 20mm low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel produced in this embodiment by adjusting the C-Mn-B ratio of the main hardenability elements and using a reasonable controlled rolling and cooling process can meet the requirements of product strength, toughness, and plasticity, and has a certain margin. It also has uniform hardness and microstructure in the thickness direction and excellent performance.

[0080] Example 3

[0081] In this embodiment, a 150 mm thick continuously cast billet is slowly cooled for 48 hours before being heated in a furnace. It is then reversibly rolled using a 3500 mm hot roll mill, employing a two-stage recrystallization controlled rolling process. The specific process is as follows:

[0082] The chemical composition of this Example 3 is shown in Table 13, the rolling process is shown in Tables 14-16, and the performance indicators are shown in Table 17.

[0083] Table 13 Chemical composition (wt%) of 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0084]

[0085] Table 14 Heating regime for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0086]

[0087] Table 15 Rolling process for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0088]

[0089] Table 16 Cooling process for manufacturing 25mm thick, low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel in this embodiment.

[0090]

[0091] Table 17 Performance indicators of the 25mm thick low-cost Nb-free unheated 700MPa high-strength steel manufactured in this embodiment

[0092]

[0093] As can be seen from Table 17, when the B content is ≤0.010%, the 25mm low-cost Nb-free non-quenched and tempered 700MPa high-strength steel produced in this embodiment by adjusting the ratio of the main hardenable elements C-Mn-B and a reasonable controlled rolling and cooling process can achieve an impact toughness of over 200J, but it cannot meet the requirements for product strength, toughness, plasticity and other requirements.

[0094] Comparative Example 1

[0095] The following example, using a hot-rolled coil mill production line to produce 700MPa high-strength steel containing Nb and without tempering, will be used to further illustrate the present invention.

[0096] The comparative production process route includes hot metal desulfurization treatment, top and bottom blowing converter, LF refining, VD degassing, wide slab continuous casting, and hot roll mill rolling.

[0097] This comparative example also uses a 150 mm thick continuously cast billet, which is slowly cooled for 48 hours before being heated in a furnace at 1260℃ for 240 minutes. Reversible repeated rolling is performed using a 3500 mm hot roll mill, employing a two-stage recrystallization controlled rolling process. The specific process is as follows:

[0098] The first stage rolling start temperature is 1089℃, the reduction rate of each rolling pass in the recrystallization zone is ≥15%, the cumulative reduction rate in the recrystallization zone is 43.3%, and the first stage finishing rolling temperature is 1012℃.

[0099] After the first stage of rolling is completed, the intermediate billet is not cooled by laminar flow and enters the waiting temperature stage;

[0100] Then, the second stage of controlled rolling is carried out, with a rolling start temperature of 930℃ and a rolling end temperature of 775℃. The cumulative reduction rate in the second stage is 61.53%.

[0101] After rolling, accelerated cooling is adopted at a rate of 18.4℃ / s, with a final cooling temperature of 453℃, and then it enters a hot straightener for straightening.

[0102] Comparative Example 1: Chemical composition is shown in Table 18; rolling process and cooling regime are shown in Tables 19-20; performance indicators are shown in Table 21; rolling rhythm comparison is shown in Table 22; microstructure is plate-shaped scutellarin + granular scutellarin + needle iron; metallographic structure is shown in Table 18. Figure 2 .

[0103] Table 18 Chemical composition (wt%) of 25mm thick Nb-containing unquenched and tempered 700MPa high-strength steel in this comparative example.

[0104]

[0105] Table 19 Rolling process for manufacturing 25mm Nb-containing unquenched and tempered 700MPa high-strength steel in this comparative example

[0106]

[0107] Table 20 Cooling process parameters for manufacturing 25mm Nb-containing unquenched and tempered 700MPa high-strength steel in this comparative example.

[0108]

[0109] Table 21 Performance indicators of 25mm Nb-containing unquenched and tempered 700MPa high-strength steel manufactured in this comparative example.

[0110]

[0111] Table 22 Comparison of rolling time between Example 1 and Comparative Example 1

[0112]

[0113] Example 1 achieved a level comparable to the comparative example, with the rolling process time reduced by 180 seconds, a reduction of nearly 50-60%. Example 1, without the addition of Nb, achieved low-cost operation of non-quenched and tempered 700MPa high-strength steel while simultaneously increasing the rolling speed by 20-30%.

[0114] This comparative example also directly proves that if the technical route of this invention (B-type Nb + intermediate cooling) is not adopted, but a conventional Nb-containing route is used, problems such as slow rolling pace and high heating temperature will be faced on the same production line.

[0115] This invention simultaneously achieves three technical effects: omitting Nb reduces costs, omitting the preheating stage improves rolling speed, and ensures 700MPa-level performance. These three effects are inherently contradictory: omitting Nb reduces grain refinement, affecting strength and toughness; omitting the preheating stage affects recrystallization control, potentially leading to microstructure coarsening; maintaining 700MPa-level performance requires a sufficient strengthening mechanism. However, this embodiment resolves these contradictions through the synergistic effect of B replacing Nb to provide hardenability, trace amounts of Ti fixing N to protect B, intermediate cooling replacing the preheating stage, and accelerated cooling to obtain a bainitic microstructure. This multiple synergistic effect is unpredictable.

[0116] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel, characterized in that, The chemical composition includes the following weight percentages: C: 0.050~0.100%, Si: 0.20~0.60%, Mn: 1.60~1.85%, P≤0.020%, S≤0.010%, Alt: 0.010%~0.060%, B: 0.0008%~0.0025%, Ti: 0.008%~0.020%, Cr: ≤0.15%, Nb≤0.006%, Mo≤0.08%, Ni≤0.15%, Cu≤0.15%, N≤0.0060%, O≤0.0030%, H≤0.0002%, Pcm≤0.25%, with the balance being Fe and unavoidable impurities.

2. The low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel according to claim 1, characterized in that, The chemical composition includes the following weight percentages: C: 0.050~0.100%, Si: 0.20~0.60%, Mn: 1.60~1.85%, P≤0.020%, S≤0.010%, Alt: 0.010%~0.060%, B: 0.0010%~0.0018%, Ti: 0.010%~0.020%, Cr: ≤0.15%, Nb≤0.006%, Mo≤0.08%, Ni≤0.15%, Cu≤0.15%, N≤0.0060%, O≤0.0030%, H≤0.0002%, Pcm≤0.25%, with the balance being Fe and unavoidable impurities.

3. A method for manufacturing low-cost, Nb-free, non-quenched and tempered 700MPa high-strength steel, characterized in that, This includes hot metal pretreatment, top and bottom blowing converter, LF refining, VD vacuum refining, wide slab continuous casting, and hot coil rolling; among which, The molten iron pretreatment: the molten iron contains S ≥ 0.040%, and desulfurization is carried out using a molten iron pretreatment process, with a target of below 0.010%; The top and bottom combined blowing converter oxygen blowing process lasts for 14-16 minutes, and the molten steel tapped is guaranteed to have P ≤ 0.015%, S ≤ 0.020%, and O ≤ 0.06%. The LF refining and VD vacuum refining processes ensure that the molten steel temperature and composition meet the requirements for continuous casting and the target composition, wherein the gas content is: N≤0.0060%, O≤0.0030%, H≤0.0002%. The wide slab continuous casting adopts full-process protective casting, with casting speed fluctuation within ±0.05m / min, molten steel level fluctuation within ±3mm, molten steel superheat controlled at 10~25℃, and slab low-magnification segregation below B1.5 grade and porosity below 1.5 grade.

4. The manufacturing method according to claim 3, characterized in that, The hot coil rolling process includes: ①After slow cooling for 48 hours, the 150 mm thick continuous casting billet is heated in a heating furnace at a temperature of 1150~1220℃ for a heating time of ≥120 min. ② Reversible repeated rolling is carried out using a 3500mm hot roll mill, and the rolling process adopts two-stage recrystallization controlled rolling. ③ First stage controlled rolling, rolling start temperature 1000~1100℃, rolling reduction rate in recrystallization zone ≥15%, cumulative reduction rate in recrystallization zone ≥40%; ④ After the first stage of rolling is completed, the intermediate billet is cooled by laminar flow to 900-980℃ and then the second stage of controlled rolling is carried out. ⑤ The intermediate billet enters the second stage of controlled rolling, with a rolling start temperature of 880~970℃ and a rolling end temperature of 830~880℃. The cumulative reduction rate in the second stage is ≥50%. ⑥ Accelerated cooling is adopted after rolling, with a cooling rate of 15~30℃ / s and a final cooling temperature of 350~450℃; ⑦ The steel plate enters the hot straightening machine for straightening.

5. The manufacturing method according to claim 4, characterized in that, In the rolling process described above, the intermediate billet cooling rate is 5-10℃ / s.

6. The manufacturing method according to claim 3, characterized in that, The high-strength steel produced by the manufacturing method has a thickness of 20~25mm.

7. The manufacturing method according to claim 3, characterized in that, The high-strength steel produced by the method has a yield strength ≥600MPa, a tensile strength ≥700MPa, and a non-proportional elongation A. 50mm ≥30%, longitudinal impact energy at -20℃ ≥200J, metallographic structure is mainly lath bainite and granular bainite.

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