Low-cost cold-rolled low-alloy high-strength steel having marginal difference in strength between transverse and longitudinal directions, and manufacturing method therefor

Abstract

Disclosed in the present invention is a low-cost cold-rolled low-alloy high-strength steel having a marginal difference in strength between the transverse and longitudinal directions. The steel comprises Fe and inevitable impurities, and further comprises the following chemical elements in percentage by mass: C: 0.045-0.085%; Mn: 1.16-1.50%; Al: 0.02-0.05%; Ti: 0.10-0.16%; and 0<Si≤0.2%. The microstructure thereof comprises ferrite, granular bainite, and precipitated carbonitrides. Further disclosed in the present invention is a method for manufacturing low-alloy high-strength steel. The method comprises the steps of: smelting and casting; hot rolling; acid pickling and cold rolling, wherein the cold-rolling reduction rate is controlled to be 1-20%; and annealing, involving: heating strip steel to 530-680°C and maintaining the temperature, then cooling same to 440-520°C at a cooling rate of 5-20°C / s and maintaining the temperature, and then cooling same to room temperature at a cooling rate of 1-10°C / s.

Description

Low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference and manufacturing method thereof TECHNICAL FIELD

[0001] The present application relates to a steel sheet and a manufacturing method thereof, and in particular to a low-alloy ultra-high-strength steel and a manufacturing method thereof. BACKGROUND

[0002] With the development of automobile lightweight, more and more automobile structural parts are prepared from high-strength steel, and the required yield strength gradually reaches or exceeds 700 MPa.

[0003] In the prior art, cold-rolled high-strength steel with a yield strength of 700 MPa is either added with high alloy to improve the strength of the product or combined with carbide precipitation, cold rolling and incomplete annealing to improve the strength of the product.

[0004] For example, the Chinese patent document with the publication number CN105200332A and the publication date of December 30, 2015 and the title of "700 MPa grade thin gauge high strength steel strip and its production method" discloses a 700 MPa grade thin gauge high strength steel strip and its production method, and the sum of the addition amount of only Mn+Cr two alloys is as high as 1.8% or more, resulting in a high carbon equivalent of the steel material, which increases the manufacturing cost.

[0005] For another example, the Chinese patent document with the publication number CN109207843A and the publication date of January 15, 2019 and the title of "A high-strength cold-rolled steel strip and a manufacturing method thereof" discloses a high-strength cold-rolled steel strip, which has high strength, good plasticity, low carbon equivalent and good weldability, but the transverse and longitudinal strength difference is large, especially the yield strength, which is more than 50 MPa higher than the longitudinal yield strength. When the transverse and longitudinal strength difference is too large, it will cause abnormal springback of the steel material under complex stamping conditions and unqualified size precision.

[0006] For another example, the Chinese patent document with the publication number CN115109994A and the publication date of September 27, 2022 and the title of "A high-strength cold-rolled hot-dip galvanized micro-alloyed steel strip and a manufacturing method thereof" discloses a high-strength cold-rolled hot-dip galvanized micro-alloyed steel strip, which uses Nb instead of Ti to obtain a steel material with a transverse and longitudinal yield strength difference of less than 50 MPa. However, on the one hand, using Nb instead of Ti will result in high manufacturing cost, and on the other hand, the yield strength of the above-mentioned steel strip is also lower than 700 MPa. SUMMARY

[0007] One of the purposes of the present application is to provide a low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference, which has ultra-low alloy cost and low carbon equivalent while having small transverse and longitudinal yield strength difference.

[0008] To achieve the above object, the present application provides a low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0009] C: 0.045-0.085%;

[0010] Mn: 1.16-1.50%;

[0011] Al: 0.02-0.05%;

[0012] Ti: 0.10-0.16%;

[0013] 0

[0014] The microstructure thereof contains ferrite + granular bainite + precipitated carbonitride.

[0015] Further, in the low-alloy high-strength steel according to the present application, the mass percentages of the chemical elements are as follows:

[0016] C: 0.045-0.085%;

[0017] Mn: 1.16-1.50%;

[0018] Al: 0.02-0.05%;

[0019] Ti: 0.10-0.16%;

[0020] 0

[0021] the balance being Fe and inevitable impurities.

[0022] The present application obtains a microstructure of ferrite + granular bainite + precipitated carbonitride by matching the components and process design, thereby obtaining a low-cost low-carbon equivalent steel with high strength, high formability and low transverse and longitudinal strength difference. Specifically, in the low-alloy ultra-high-strength steel according to the present application, the design principles of the chemical elements are as follows:

[0023] C: In the low-alloy ultra-high-strength steel according to the present application, the C element affects both phase transition and carbonitride precipitation. When the mass percentage of C is too low, effective phase transition strengthening cannot be formed, and the amount of carbonitride formation is also very limited; when the mass percentage of C is too high, although phase transition is more likely to occur, it will also cause a substantial increase in carbon equivalent and the size of carbonitride, affecting the weldability and formability. Therefore, in the low-alloy ultra-high-strength steel according to the present application, the mass percentage of C can be controlled between 0.045-0.085%.

[0024] Mn: In the low-alloy ultra-high-strength steel described in the present application, Mn element is one of the core elements affecting strength and controlling phase transformation. When the mass percentage content of Mn element is too low, it will cause insufficient granular bainite transformation, resulting in insufficient strength; when the mass percentage content of Mn element is too high, although it is beneficial to improve the strength of the steel, on the one hand it will lead to the increase of carbon equivalent, affecting the weldability, on the other hand it will also increase the manufacturing cost. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Mn element can be controlled between 1.16-1.50%.

[0025] Al: In the low-alloy ultra-high-strength steel described in the present application, Al element can be used as a deoxidizing element and a carbonitride forming element to improve the quality of the steel and improve the strength of the steel. At the same time, Al element is also a ferrite forming element and a stabilizing element. When the mass percentage content of Al element is too low, the deoxidation of molten steel is insufficient, and the steel purity will be affected; when the mass percentage content of Al element is too high, it is not conducive to the formation of granular bainite. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Al element can be controlled between 0.02-0.05%.

[0026] Ti: In the low-alloy ultra-high-strength steel described in the present application, Ti element is the main carbonitride forming element, which is used to refine grains and improve strength. When the mass percentage content of Ti element is too low, the formed carbonitride precipitation is not enough, and the required strength cannot be achieved; when the mass percentage content of Ti element is too high, it is easy to form coarse TiN particles and coarse titanium carbonitride in the steel, which is not conducive to the forming performance of the steel. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Ti element can be controlled between 0.10-0.16.

[0027] Si: In the low-alloy ultra-high-strength steel described in the present application, Si element is a necessary element for deoxidization and solid solution strengthening, and is also a ferrite forming and stabilizing element. However, when the mass percentage content of Si element is too high, it will affect the formation of granular bainite, and also worsen the weldability and increase the alloy cost of steel manufacturing. Therefore, in the low-alloy ultra-high-strength steel described in the present application, the mass percentage content of Si element can be controlled to be 0

[0028] Further, in the low-alloy high-strength steel described in the present application, it further contains at least one of the following elements:

[0029] 0

[0030] 0

[0031] 0

[0032] 0 < V ≤ 0.1 wt. %;

[0033] 0 < Cr ≤ 0.15 wt. %.

[0034] Therefore, in some embodiments, the low-alloy high-strength steel according to the present application contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.045-0.085%; Mn: 1.16-1.50%; Al: 0.02-0.05%; Ti: 0.10-0.16%; 0 < Si ≤ 0.2%; Nb: ≤ 0.02%; Ce: ≤ 0.006%; La: ≤ 0.006%; V: ≤ 0.1%; Cr: ≤ 0.15%. In some embodiments, the low-alloy high-strength steel according to the present application contains the following chemical elements in the following mass percentages: C: 0.045-0.085%; Mn: 1.16-1.50%; Al: 0.02-0.05%; Ti: 0.10-0.16%; 0 < Si ≤ 0.2%; Nb: ≤ 0.02%; Ce: ≤ 0.006%; La: ≤ 0.006%; V: ≤ 0.1%; Cr: ≤ 0.15%; and the balance is Fe and inevitable impurities. In some embodiments, the low-alloy high-strength steel according to the present application contains at least three of Nb, Ce, La, V and Cr.

[0035] In the present application, Nb, Ce, La, V and Cr can be selectively added to the steel as optional elements. Among them, La and Ce elements as rare earth elements have the effect of improving steel quality, modifying inclusions, and refining grains; Nb and V elements as strong carbonitride forming elements can form a large amount of carbonitride precipitates, refine grains, and improve strength; Cr element can adjust the phase transformation of steel, and further improve the hardenability, which is beneficial to the formation of granular austenite.

[0036] However, the addition of the above alloying elements will increase the manufacturing cost of the steel on the one hand, and will also increase the carbon equivalent and deteriorate the weldability of the steel on the other hand. Therefore, in the low-alloy high-strength steel according to the present application, the mass percentage of Nb can be controlled to be Nb ≤ 0.02 wt. %, the mass percentage of Ce can be controlled to be Ce ≤ 0.006 wt. %, the mass percentage of La can be controlled to be La ≤ 0.006 wt. %, the mass percentage of V can be controlled to be V ≤ 0.1 wt. %, and the mass percentage of Cr can be controlled to be Cr ≤ 0.15 wt. %.

[0037] Further, in the low-alloy high-strength steel according to the present application, the mass percentages of the chemical elements satisfy at least one of the following:

[0038] Ti / (Al+Nb) > 3.0;

[0039] 1.1% < Mn+Cr-Ti-Nb-V < 1.6%.

[0040] In the chemical composition design of the present application, while controlling the content of a single element, Ti / (Al+Nb) > 3.0 can also be controlled to regulate the precipitation of carbonitride.

[0041] In the present application, a certain balance needs to be found among the carbonitride-forming abilities of the three elements Al, Ti, and Nb. Al, Ti, and Nb are all carbonitride-forming elements, but in terms of carbonitride-forming ability, Nb > Ti > Al. If the content of Al is too high and the contents of Ti and Nb are too low, less carbonitride is formed during manufacturing, and the strength of the steel is not enough; if the content of Nb is too high, although it is easier to form carbonitride during manufacturing, on the one hand, it will cause the size of the carbonitride to be coarse, and on the other hand, it will consume more C elements and cause insufficient or inability to form granular bainite; if the content of Ti is too low and the contents of Nb or Al are too high, the precipitation of carbonitride and the phase transition of granular bainite will be unbalanced, leading to deterioration of the strength and forming performance of the product. Therefore, in the low-alloy high-strength steel described in the present application, Ti / (Al+Nb) > 3.0 is also controlled. In some embodiments, Ti / (Al+Nb) is controlled to be in the range of 3.0-5.5.

[0042] In the chemical composition design of the present application, while controlling the content of a single element, the ratio of alloying elements can also be controlled, so that 1.1 < Mn+Cr-Ti-Nb-V < 1.6, to regulate the optimal ratio of the precipitation of carbonitride and the formation of granular bainite. Mn and Cr are beneficial to the formation of granular bainite, and Ti, Nb, and V are strong carbide-forming elements, so the higher the value of the above formula, the more beneficial to the formation of granular bainite, and the lower the value of the above formula, the more beneficial to the precipitation of carbonitride. In some embodiments, Mn+Cr-Ti-Nb-V is controlled to be in the range of 1.1-1.4.

[0043] Further, in the low-alloy high-strength steel described in the present application, the carbon equivalent Cev < 0.32, where Cev = C + Mn / 6 + (Cr+Mo+V) / 5 + (Ni+Cu+Si) / 15, where the mass percentage of each chemical element is substituted into the value before the percentage sign.

[0044] In the present application, by controlling the carbon equivalent Cev < 0.32, the steel can be ensured to have excellent weldability. In some embodiments, Cev is 0.30-0.32.

[0045] Further, in the low-alloy high-strength steel of the present application, the unavoidable impurities include P≤0.02wt%, S≤0.006wt%, and N≤0.006wt%.

[0046] In the present application, the unavoidable impurities mainly include phosphorus (P), sulfur (S), and nitrogen (N), and it is desirable that their contents are as low as possible under the technical conditions.

[0047] Further, in the low-alloy high-strength steel of the present application, the volume fraction of ferrite in the microstructure thereof is≥90%, the volume fraction of granular bainite is≥5%, and the content of carbonitride is≤5%. In some embodiments, the sum of the volume fractions of ferrite and granular bainite in the microstructure of the low-alloy high-strength steel of the present application is≥95%, and the remainder is carbonitride.

[0048] Further, in the low-alloy high-strength steel of the present application,≥60% of the ferrite grains in the ferrite have an aspect ratio≤4. In some embodiments, 60-90% of the ferrite grains in the ferrite have an aspect ratio≤4.

[0049] Further, in the low-alloy high-strength steel of the present application, the difference between the transverse and longitudinal yield strengths is≤50MPa. In some embodiments, the difference between the transverse and longitudinal yield strengths of the low-alloy high-strength steel of the present application is≤40MPa. In some embodiments, the difference between the transverse and longitudinal yield strengths of the low-alloy high-strength steel of the present application is≤30MPa.

[0050] Further, in the low-alloy high-strength steel of the present application, both the transverse and longitudinal yield strengths are≥700MPa or more, both the transverse and longitudinal elongations at break are≥10%, and the ratio of the 90° bending radius to the plate thickness is r / t≤0.3. In some embodiments, the transverse yield strength of the low-alloy high-strength steel of the present application is≥705MPa, and the longitudinal yield strength is≥720MPa. In some embodiments, the transverse yield strength of the low-alloy high-strength steel of the present application is 705-790MPa, and the longitudinal yield strength is 720-830MPa.

[0051] Further, in the low-alloy high-strength steel of the present application, both the transverse and longitudinal tensile strengths are≥740MPa. In some embodiments, the transverse tensile strength of the low-alloy high-strength steel of the present application is≥745MPa, and the longitudinal tensile strength is≥770MPa. In some embodiments, the transverse tensile strength of the low-alloy high-strength steel of the present application is 745-830MPa, and the longitudinal tensile strength is 770-870MPa.

[0052] Another object of the present application is to provide a manufacturing method of low-alloy high-strength steel, which is green and efficient, and the manufactured low-alloy ultra-high-strength steel has the advantages of green low carbon and low-alloy addition amount, and has low transverse and longitudinal strength difference.

[0053] To achieve the above object, the present application provides a manufacturing method of low-alloy high-strength steel, which comprises the steps of:

[0054] smelting and casting;

[0055] hot rolling;

[0056] pickling and cold rolling: the cold rolling reduction is controlled to be 1-20%;

[0057] annealing: the strip steel is heated to 530-680℃ and kept, then cooled to 440-520℃ at a first cooling rate of 5-20℃ / s and kept, and then cooled to room temperature at a second cooling rate of 1-10℃.

[0058] By using the manufacturing method of the present application, a small amount of granular bainite can be obtained in the pre-heat rolling structure, and by optimizing the cold rolling deformation and annealing process, the optimal balance point of phase transformation strengthening and precipitation strengthening is obtained, so that a cold rolled high-strength steel with low cost carbon equivalent and small transverse and longitudinal difference is obtained, which has high strength, high forming property and low transverse and longitudinal strength difference.

[0059] In the pickling and cold rolling step of the present application, by controlling the cold rolling reduction to be 1-20%, it can be ensured that the subsequent strip steel does not produce elongated ferrite grains after annealing, so as to avoid too large transverse and longitudinal strength difference.

[0060] In the annealing step of the present application, the lower heating and holding temperature can ensure that the granular bainite in the strip steel does not decompose, and the carbonitride also does not coarsen. The subsequent cooling, holding and final cooling process is to obtain submicron-sized carbonitride precipitates under the premise of further controlling the growth size of carbonitride.

[0061] Further, in the annealing step of the manufacturing method of the present application, the strip steel is heated to 530-680℃ and kept for 90-180s, then cooled to 440-520℃ at a first cooling rate of 5-20℃ / s and kept for 5-30s, and then cooled to room temperature at a second cooling rate of 1-10℃.

[0062] Further, in the annealing step of the manufacturing method of the present application, hot galvanizing of the surface of the strip steel is completed in the process segment of cooling to 440-520℃ at a first cooling rate of 5-20℃ / s and keeping.

[0063] Further, in the hot rolling step of the manufacturing method, the slab delivery temperature is controlled to be 1250-1300℃, and the finish rolling temperature is controlled to be 890-940℃.

[0064] Further, in the hot rolling step of the manufacturing method, the strip is cooled to 440-520℃ at a cooling rate of ≥80℃ / s (e.g. 80-160℃ / s) after rolling and then coiled.

[0065] In the hot rolling step of the manufacturing method, the higher slab delivery temperature can ensure that the carbonitride formed first in the slab can be fully redissolved; the higher finish rolling temperature and ultrafast cooling rate can avoid batch precipitation and coarsening of the carbonitride; and the low-temperature coiling at 440-520℃ can form a small amount of granular bainite and a trace amount of fine carbonitride in the strip.

[0066] The low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference and the manufacturing method thereof have the following advantages and beneficial effects:

[0067] The cold-rolled low-alloy high-strength steel has low-alloy addition and low carbon equivalent, and also has low transverse and longitudinal strength difference.

[0068] In some embodiments, the cold-rolled high-strength steel has a transverse and longitudinal yield strength of ≥700MPa, a transverse and longitudinal elongation of ≥10%, a 90° bending radius to plate thickness ratio r / t of ≤0.3, and a transverse and longitudinal yield strength difference of ≤50MPa, and can be widely used in the preparation of various automobile structural parts. BRIEF DESCRIPTION OF DRAWINGS

[0069] Figure 1 shows the microstructure morphology of Example 1. DETAILED DESCRIPTION

[0070] The low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference and the manufacturing method thereof will be further explained and described below in conjunction with specific examples, but the explanation and description do not constitute an improper limitation on the technical solutions of the present application.

[0071] Examples 1-6 and Comparative Examples 1-3

[0072] Table 1-1 and Table 1-2 list the mass percentages of each chemical element in the low-cost cold-rolled low-alloy high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0073] Table 1-1. (The balance is Fe and other unavoidable impurities other than P, S, and N)

[0074] Table 1-2. (The balance is Fe and other unavoidable impurities other than P, S, and N)

[0075] The low-cost cold-rolled low-alloy high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 are prepared by the following steps:

[0076] (1) smelting and continuous casting;

[0077] (2) hot rolling: the slab discharge temperature is controlled to be 1250-1300℃, and the finish rolling temperature is 890-940℃; after rolling, the ultra-fast cooling and low-temperature coiling method is used to cool the steel strip to 440-520℃ at a cooling rate of ≥80℃ / s;

[0078] (3) pickling and cold rolling: the cold rolling reduction is controlled to be 1-20%;

[0079] (4) annealing: the strip steel is heated to 530-680℃ and kept for 90-180s, then cooled to 440-520℃ at a first cooling rate of 5-20℃ / s and kept for 5-30s, and then cooled to room temperature at a second cooling rate of 1-10℃.

[0080] In addition, in some embodiments, the hot-dip galvanizing of the surface of the strip steel can be completed in the process segment of cooling to 440-520℃ at a first cooling rate of 5-20℃ / s and keeping in step (4), so as to obtain a hot-dip galvanized product.

[0081] It should be noted that the composition and process of the embodiments 1-6 of the present application all meet the requirements of the present application, while the composition of the comparative example 1 is the same as that of the embodiment 6, and the process parameters do not meet the present application, and the composition and process of the comparative examples 2-3 do not meet the present application.

[0082] The specific process parameters of the low-cost cold-rolled low-alloy high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 are listed in Tables 2-1 and 2-2.

[0083] Table 2-1.

[0084] Table 2-2. Annealing process

[0085] In order to verify the implementation effect of the present application, the low-cost cold-rolled low-alloy high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3 are sampled, and the metallographic sample after polishing is eroded by 4% nitric acid alcohol solution for about 10s, washed and dried, and then microstructure observation is carried out by using an optical microscope or a scanning electron microscope, and the observation results are listed in Table 3.

[0086] Table 3 lists the microstructure observation results of the low-cost cold-rolled low-alloy high-strength steel of the embodiments 1-6 and the comparative steels of the comparative examples 1-3.

[0087] Table 3.

[0088] As can be seen from the above Table 3, the microstructure of the low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference of the embodiments 1-6 of the present application contains ferrite + granular bainite + precipitated carbonitride, the volume ratio of ferrite is greater than or equal to 90%, the ratio of granular bainite is greater than or equal to 5%, and the ratio of carbonitride is less than or equal to 5%.

[0089] In addition, Figure 1 shows the microstructure morphology of Example 1.

[0090] As shown in Figure 1, the microstructure of Example 1 is ferrite + granular bainite + carbonitride, wherein the ratio of carbonitride is 5.5%, the ratio of granular bainite is 4%, and the rest is ferrite.

[0091] The low-cost cold-rolled low-alloy high-strength steel of Examples 1-6 and the comparative steel of Comparative Examples 1-3 are resampled, and various performance tests are performed, and the test results are listed in Table 4. Among them, various performance tests include:

[0092] Tensile property test: GB / T 228.1-2021 "Metallic materials - Tensile test - Part 1: Method of test at room temperature" is used to test the low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0093] Bending property test: GB / T 232-2010 "Metallic materials - Bending test methods" is used to test the low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference of Examples 1-6 and the comparative steel of Comparative Examples 1-3.

[0094] Table 4.

[0095] As can be seen from the above Table 4, the transverse and longitudinal yield strength difference of the low-cost cold-rolled low-alloy high-strength steel with low transverse and longitudinal strength difference of the embodiments 1-6 of the present application is less than 50 MPa, the transverse and longitudinal yield strength is greater than 700 MPa, the transverse and longitudinal elongation is greater than 10%, and the ratio of 90° bending radius r (unit: mm) to plate thickness t (unit: mm) r / t is less than or equal to 0.3.

[0096] It should be noted that the combination of the technical features in the present case is not limited to the combination mode described in the claims of the present case or the combination mode described in the specific embodiments. All technical features described in the present case can be freely combined or combined in any way, unless contradictory to each other.

[0097] It should also be noted that the above-mentioned embodiments are only specific embodiments of the present application. Obviously, the present application is not limited to the above-mentioned embodiments, and similar changes or modifications made in accordance with the disclosure of the present application are directly derived or easily thought of by those skilled in the art, and should all belong to the protection scope of the present application.

Claims

1. A low-cost cold-rolled low-alloy high-strength steel with low difference in transverse and longitudinal strength, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following mass percentages: C：0.045～0.085％； Mn: 1.16–1.50%; Al：0.02～0.05％； Ti: 0.10–0.16%; 0 < Si ≤ 0.2%; Its microstructure contains ferrite, granular bainite, and precipitated carbonitrides.

2. The low-alloy high-strength steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C：0.045～0.085％； Mn: 1.16–1.50%; Al：0.02～0.05％； Ti: 0.10–0.16%; 0 < Si ≤ 0.2%; The balance is Fe and unavoidable impurities.

3. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, It also contains at least one of the following elements: 0 < Nb ≤ 0.02 wt%; 0 < Ce ≤ 0.006 wt%; 0 < La ≤ 0.006 wt%; 0 < V ≤ 0.1 wt%; 0 < Cr ≤ 0.15 wt%.

4. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Its mass percentage content of each chemical element satisfies at least one of the following conditions: Ti / (Al+Nb)≥3.0, 3.0≤Ti / (Al+Nb)≤5.5 are preferred; 1.1% ≤ Mn+Cr-Ti-Nb-V ≤ 1.6%, preferably 1.1% ≤ Mn+Cr-Ti-Nb-V ≤ 1.4%.

5. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Its carbon equivalent Cev ≤ 0.32, preferably 0.30~0.32; where Cev=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu+Si) / 15, where each chemical element is replaced with the value before the percentage sign of its mass content.

6. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Of its unavoidable impurities, P ≤ 0.02 wt%, S ≤ 0.006 wt%, and N ≤ 0.006 wt%.

7. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Its microstructure has a ferrite volume ratio of ≥90%, a granular bainite volume ratio of ≥5%, and a carbonitride content of ≤5%.

8. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, The ferrite contains ≥60%, preferably 60-90%, of ferrite grains with an aspect ratio ≤4.

9. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Its yield strength difference between the transverse and longitudinal directions is ≤50MPa.

10. The low-alloy high-strength steel as described in claim 1 or 2, characterized in that, Its transverse and longitudinal yield strengths are both greater than 700 MPa, its transverse and longitudinal fracture elongation are both greater than 10%, and its ratio of 90° bending radius to plate thickness r / t is less than 0.

3.

11. The method for manufacturing low-alloy high-strength steel according to any one of claims 1-10, characterized in that, It includes the following steps: Smelting and casting; Hot-rolled; Pickling and cold rolling: control the cold rolling reduction rate to 1-20%; Annealing: Heat the strip steel to 530-680℃ and hold it at that temperature, then cool it to 440-520℃ at a first cooling rate of 5-20℃ / s and hold it at that temperature, then cool it to room temperature at a second cooling rate of 1-10℃.

12. The manufacturing method as described in claim 11, characterized in that, In the annealing step, the strip is heated to 530-680℃ and held for 90-180s, then cooled to 440-520℃ at a first cooling rate of 5-20℃ / s and held for 5-30s, and then cooled to room temperature at a second cooling rate of 1-10℃.

13. The manufacturing method as described in claim 11, characterized in that, In the annealing step, hot-dip galvanizing of the strip surface is completed in a process section where the strip is cooled to 440-520°C at a first cooling rate of 5-20°C / s and held at that temperature.

14. The manufacturing method as described in claim 11, characterized in that, In the hot rolling process, the slab exit temperature is controlled at 1250-1300℃, and the finishing rolling temperature is 890-940℃.

15. The manufacturing method according to any one of claims 11-14, characterized in that, In the hot rolling process, the strip is cooled to 440-520°C at a cooling rate of ≥80°C / s after rolling and then coiled.

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