Cold-rolled precipitation strengthened steel and manufacturing method thereof

Abstract

The invention discloses steel, which comprises more than 90% of Fe and inevitable impurities, and further comprises the following chemical elements in percentage by weight: 0.06% < = C < = 0.075%, 0.3% < = Si < = 0.4%, 1.25% < = Mn < = 1.4%, 0.015% < = P, 0.0015% < = S, 0.02% < = Nb < = 0.03%, 0.075% < = Ti < = 0.095%, 0.025% < = Al < = 0.04%, 0.0006% < = B, 0.005% < = N, 0.035% < = Cu, 0.05% < = Cr, 0.01% < = Mo and 0.005% < = V. The invention further discloses a method for manufacturing the steel. The method comprises the following steps: smelting; casting is conducted; hot rolling; pickling and cold rolling; continuous annealing; and flattening. The steel and the manufacturing method thereof provided by the invention have the advantages of low production cost, energy conservation and low consumption in the manufacturing process, and excellent comprehensive performance (especially in terms of yield strength, tensile strength and elongation at break).

Description

Technical Field

[0001] The invention relates to the field of metallurgy, and in particular to a cold-rolled precipitation-strengthened steel and a manufacturing method thereof. Background Art

[0002] High-strength cold-rolled precipitation-hardened steel is widely used in the automotive industry due to its good mechanical properties, welding properties and cold forming properties. It is suitable for manufacturing automotive structural parts, reinforcements, support parts, etc. In particular, cold-rolled precipitation-hardened steel has been increasingly used in the manufacture of automotive seat slide products in recent years. Seat slides are important safety components on passenger cars, and the quality of the steel used has always been a concern in the market.

[0003] With the rapid development of the automobile industry at home and abroad, the requirements for the strength of automobile steel are increasing. In order to meet the needs of high-strength thinning, weight reduction and energy saving for passenger cars, the yield strength of automobile steel must be above 800MPa. Although many companies can produce precipitation-strengthened steel, the overall performance of the precipitation-strengthened steel produced is relatively low. The precipitation-strengthened steel produced by most companies still remains below 500MPa in yield strength, and there are few manufacturers that can produce products above 550MPa. This makes it difficult to adapt to the automotive industry's strong demand for higher and higher strength of automobile steel plates to meet the needs of thinning and weight reduction.

[0004] Generally speaking, the strength of automotive steel can be improved by adding alloying elements such as Nb, V, and Ti. However, the precipitation strengthening and fine grain strengthening effects of these alloying elements are limited. When the strength reaches a certain level, the improvement of strength by these alloying elements will no longer be obvious. In addition, adding too many alloying elements will also lead to a significant increase in manufacturing costs. Therefore, it is necessary to further improve the strength of precipitation-strengthened steel through other means.

[0005] In the prior art, a variety of high strength precipitation strengthened steel products have been disclosed.

[0006] Patent document CN102409252A discloses an ultra-high strength cold-rolled steel sheet and a manufacturing method thereof, which contains, by weight%, 0.05%≤C≤0.08%, 0.1%≤Si≤0.4%, 0.5%≤Mn≤1.0%, 0.008%≤P≤0.03%, S≤0.006%, 0.06%≤Ti≤0.13%, 0.015%≤Al<0.045%, 0.2%≤Cu≤0.55%, 0.08%≤Ni≤0.2%, 0.3%≤Cr≤0.6%. Although the yield strength of the cold-rolled steel sheet is above 700MPa, the tensile strength is above 800MPa, and the elongation is above 7%, its overall performance still has a large room for improvement, so as to achieve a yield strength of above 800MPa, a tensile strength of above 850MPa, and an elongation at break of above 8%. In addition, the elements Cu, Ni, and Cr contained in the cold-rolled steel sheet are common alloying elements of weathering steel. In other words, the finished cold-rolled steel sheet is mainly used for weathering steel products such as container plates, and is not suitable for use as automotive steel (such as steel for automotive seat rails).

[0007] Patent document CN107164700A discloses an 800MPa grade high yield ratio cold-rolled steel strip and its manufacturing method, which contains, by weight: 0.05%≤C≤0.08%, Si≤0.15%, 1.6%≤Mn≤1.9%, P≤0.02%, S≤0.008%, 0.045%≤Nb≤0.065%, 0.09%≤Ti≤0.11%, 0.02%≤Als≤0.06%, 0.15%≤Mo≤0.2% and N≤0.005%, with the remainder being Fe and impurities. A high content of alloys such as Mn, Mo, and Nb is added to the steel strip, and the manufacturing cost is relatively high. In addition, the soaking temperature of the steel strip during the manufacturing process is above 760°C. Therefore, it is difficult for the steel strip and the manufacturing method provided by the patent document to achieve the goal of energy saving and consumption reduction.

[0008] Based on the above reasons, the inventors hope to develop a cold-rolled precipitation-strengthened steel with low cost (for example, lower content of alloying elements such as Mn, Mo, and Nb), low energy consumption in the manufacturing process (for example, lower annealing soaking temperature, etc.), and excellent comprehensive performance (for example, yield strength ≥≥800MPa, tensile strength ≥850MPa, and elongation at break ≥8%). This cold-rolled precipitation-strengthened steel has good application value and is suitable for use as automotive steel (for example, steel for automotive seat slide rails). Summary of the invention

[0009] Therefore, the main object of the present invention is to provide a steel material (especially cold-rolled precipitation-strengthened steel) and a method for manufacturing the same, wherein the steel material has low production cost, energy-saving and low-consumption manufacturing process, and excellent comprehensive performance (especially in terms of yield strength, tensile strength and elongation at break).

[0010] In one aspect, the present application provides a steel material (particularly a cold-rolled precipitation-strengthened steel), wherein the steel material contains, in addition to more than 90% of Fe and unavoidable impurities, the following chemical elements in wt%:

[0011] 0.06%≤C≤0.075%, 0.3%≤Si≤0.4%, 1.25%≤Mn≤1.4%, P≤0.015%, S≤0.0015%, 0.02%≤Nb≤0.03%, 0.075%≤Ti≤0.095%, 0.025%≤Al≤0.04%, B≤0.0006%, N≤0.005%, Cu≤0.035%, Cr<0.05%, Mo≤0.01% and V≤0.005%.

[0012] Preferably, the steel contains the following chemical elements in wt%:

[0013] 0.06%≤C≤0.075%, 0.3%≤Si≤0.4%, 1.25%≤Mn≤1.4%, P≤0.015%, S≤0.0015%, 0.02%≤Nb≤0.03%, 0.075%≤Ti≤0.095%, 0.025%≤Al≤0.04%, B≤0.0006%, N≤0.005%, Cu≤0.035%, Cr≤0.05%, Mo≤0.01% and V≤0.005%, the balance being Fe and unavoidable impurities.

[0014] Preferably, the microstructure of the steel material is ferrite+cementite+microalloy precipitation phase, and the microalloy precipitation phase includes NbC and TiN precipitates; preferably, the main components of the microalloy precipitation phase are NbC and TiN precipitates.

[0015] Preferably, the ferrite in the microstructure is 90-95 volume %.

[0016] Preferably, cementite in the microstructure is 4-8 volume %.

[0017] Preferably, the total amount of NbC and TiN precipitates in the microstructure is 1-2 volume %; preferably, the size (ie, maximum length) of the NbC and TiN precipitates is 10-20 nm, as measured by scanning electron microscopy.

[0018] Preferably, the grains of the finished steel are mainly in an unrecovered state and have a small grain size (see Figure 2 ).

[0019] Preferably, the grain size of the finished steel product is ≥ grade 10. Preferably, in this document, "grain size" is measured according to standard GB / T6394.

[0020] Preferably, the yield strength of the steel is ≥810 MPa, the tensile strength is ≥≥860 MPa, and the elongation at break is ≥11%.

[0021] Preferably, the steel material is a steel plate or a steel strip.

[0022] In another aspect, the present application provides a method for manufacturing the above-mentioned steel material, the method comprising the following steps:

[0023] 1) Smelting molten steel;

[0024] 2) Casting the smelted molten steel to obtain a casting billet;

[0025] 3) heating, rough rolling, finish rolling and coiling the ingot;

[0026] 4) pickling and cold rolling;

[0027] 5) Continuous annealing;

[0028] 6) Smooth and produce finished steel products.

[0029] Preferably, in step 1), the smelting includes converter smelting and refining (such as RH refining) of the molten steel, the stop-blowing temperature of the converter smelting is ≥1600°C, and the refining is carried out for ≥15 minutes at a vacuum degree of ≤200Pa.

[0030] Preferably, in step 2), the casting is performed by continuous casting, and the casting speed is ≤1.5 m / min. The theoretical liquid temperature of the molten steel is 1522°C, and the solidification temperature is 1480°C.

[0031] Preferably, in step 3), the heating temperature is 1250° C. to 1280° C. This temperature is conducive to the precipitation of NbN, NbC, TiC and TiN, and is conducive to grain refinement and strength improvement.

[0032] Preferably, in step 3), rough rolling, finish rolling and coiling are all carried out in a hot rolling line.

[0033] Preferably, in step 3), the rough rolling temperature is 1020° C. to 1080° C. When the rough rolling temperature is lower than 1020° C., the precipitated NbN, NbC, TiC, and TiN will be coarse, and the strengthening effect on the steel plate will be weakened, resulting in reduced strength. When the rough rolling temperature is higher than 1080° C., the strength will increase, but the elongation at break will decrease.

[0034] Preferably, in step 3), the finishing rolling temperature is 860°C to 900°C.

[0035] Preferably, in step 3), the coiling temperature is 490° C. to 550° C. The inventors have found through research that coiling at a lower temperature of 490° C. to 550° C. can further improve the strength of the steel, ensure uniform steel structure, and reduce the difference in structure in the length and width directions.

[0036] Preferably, in step 3), the microstructure of the steel after hot coiling is ferrite+bainite.

[0037] Preferably, in step 4), pickling and cold rolling are carried out in a pickling tandem rolling mill.

[0038] Preferably, in step 4), the pickling speed is 100 m / min to 200 m / min. The pickling speed of 100 m / min to 200 m / min can effectively remove the oxide scale on the surface of the steel.

[0039] Preferably, in step 4), the cold rolling reduction ratio is 40% to 60%. The inventors have found through research that a relatively low reduction ratio of 40% to 60% can reduce the rolling load of the rolling mill and reduce the annealing recrystallization dynamics of the hard rolled coil, making the strip structure after annealing more likely to be in an incompletely recovered state.

[0040] Preferably, in step 5), continuous annealing is performed in a non-oxidizing continuous annealing furnace, the soaking temperature is 700°C to 750°C, and the soaking time (i.e., the holding time of the soaking section) is 60 seconds to 120 seconds. The inventors have found through research that the use of such a lower annealing temperature is conducive to making the strip steel structure in an incompletely recovered state, which can not only ensure strength, but also save energy and reduce consumption.

[0041] Preferably, in step 6), the flattening rate is 0.01% to 0.1%, for example, 0.03% to 0.1%. The inventors have found through research that when the flattening rate is 0.01% to 0.1%, it is beneficial for the finished steel to obtain a good plate shape and uniform mechanical properties.

[0042] The flattening process is a slight rolling process after annealing to improve the uniformity of the shape and properties of steel (such as steel plate or steel strip). The flattening process is similar to the cold rolling process, but the degree of reduction is much lower than that of cold rolling. The degree of flattening is characterized by the flattening rate, which indicates the amount of reduction of the steel.

[0043] Preferably, the microstructure of the finished steel is ferrite+cementite+microalloyed precipitates, and the microalloyed precipitates include NbC and TiN precipitates; preferably, the main components of the microalloyed precipitates are NbC and TiN precipitates.

[0044] Preferably, the grains of the finished steel are mainly in an incompletely recovered state and have a small grain size (the grain growth during the recrystallization process is significantly suppressed) (see Figure 2 ).

[0045] Preferably, the finished steel product can be further sheared into single steel plates / steel strips by a cross-cutting unit.

[0046] Compared with the prior art, the present invention has the following beneficial effects:

[0047] The steel material (cold-rolled precipitation-strengthened steel) of the present invention shows excellent comprehensive performance (especially in terms of yield strength, tensile strength and elongation at break). In addition, the manufacturing process of the present invention is simple and does not require equipment modification, and high-strength, high-elongation steel products can be manufactured at low cost and low energy consumption. For example, compared with CN102409252A, the present invention further improves the yield strength, tensile strength and elongation of automotive steel. Compared with CN107164700A, the present invention appropriately increases the Si content and reduces the content of alloying elements such as Mn, Nb, and Mo, thereby ensuring the excellent strength and elongation of the steel at low cost; at the same time, the heat treatment temperature in the manufacturing process of the present invention is lower (for example, the soaking temperature is below 750°C), which can achieve the energy-saving and consumption-reduction goals of the automotive industry.

[0048] Therefore, the steel product and the manufacturing method thereof of the present invention can be widely used in the fields of automobile safety parts, and is expected to promote further thinning and energy saving in the automobile industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 The microstructure of the hot-rolled steel of Example 1 of the present invention is shown;

[0050] Figure 2 The microstructure of the finished steel material of Example 1 of the present invention is shown. DETAILED DESCRIPTION

[0051] Through extensive research, the inventors unexpectedly found that by designing the chemical composition of the steel as described above, a steel with low cost, low energy consumption and excellent comprehensive performance (especially in terms of yield strength, tensile strength and elongation at break), especially cold-rolled precipitation-strengthened steel, can be obtained. Specifically, the design principle of the above-mentioned chemical elements is as follows. In this article, unless otherwise explicitly stated, the element content is calculated in mass percentage (wt%).

[0052] C: Element C is a strengthening element, which can combine with Nb and Ti elements in steel to form NbC and TiC, playing a role of precipitation strengthening. When the C content is lower than 0.06%, sufficient strengthening effect cannot be obtained. When the C content is higher than 0.075%, it is easy to precipitate from the ferrite grain boundary during annealing, thereby reducing the plasticity of the steel plate and also having an adverse effect on the welding performance of the product. Therefore, the C content in the steel of the present invention is controlled to be 0.06% to 0.075%, for example, 0.065% to 0.07%.

[0053] Si: Elemental Si is a deoxidizing element in the steelmaking process, has a certain solid solution strengthening effect, and can replace part of Mn to strengthen the matrix. When the Si content is lower than 0.3%, the strengthening effect of elemental Si cannot be fully exerted. When the Si content is higher than 0.4%, defects such as red iron scale are easily formed on the surface of hot-rolled steel, which is difficult to remove in the subsequent process. Therefore, the Si content in the steel of the present invention is controlled to be 0.3% to 0.4%, for example, 0.31% to 0.39%.

[0054] Mn: Element Mn has a strong solid solution strengthening effect and is an important toughening element. When the Mn content is less than 1.25%, it cannot play a strengthening role. When the Mn content is higher than 1.4%, the hardenability of the steel plate will increase, thereby affecting the welding performance; too high a Mn content will also increase the cost. Therefore, the Mn content in the steel of the present invention is controlled to be 1.25% to 1.4%, for example, 1.28% to 1.38%.

[0055] P: Element P has a good strengthening effect and is inexpensive. However, the P element is easily segregated at the grain boundary, making the grain boundary embrittled, resulting in low elongation at break of the finished product. Therefore, the P content in the steel of the present invention is controlled to not exceed 0.015%, for example, not more than 0.014%.

[0056] S: Element S is a common element in steel, and is easily combined with Mn to form MnS, which will affect the cold bending performance and hole expansion performance of the finished product. Therefore, the S content in the steel of the present invention is controlled to be no more than 0.0015%, for example, no more than 0.0014%.

[0057] Nb: Element Nb is an important strengthening element in precipitation-strengthened steel. Nb forms interstitial intermediate phases such as NbN and NbC with elements N and C in the steel, which plays a role in pinning dislocations during the recrystallization process, thereby increasing the recrystallization time and refining the grains. When the Nb content is lower than 0.02%, the effect of refining the grains cannot be fully exerted. When the Nb content is higher than 0.03%, the effect of refining the grains will no longer be obvious, and the cost will be significantly increased. Therefore, the Nb content in the steel of the present invention is controlled to be 0.02% to 0.03%, for example, 0.021% to 0.029%.

[0058] Ti: Element Ti is an important strengthening element in precipitation-strengthened steel. Ti can combine with C and N elements in steel to form interstitial intermediate phases such as TiC and TiN, which play a role in refining grains. Compared with Nb, Ti is cheap. When the Ti content is lower than 0.075%, the strengthening effect of grain refinement is not obvious. When the Ti content is higher than 0.095%, the slab is prone to cracking. Therefore, the Ti content in the steel of the present invention is controlled to be 0.075% to 0.095%, for example, 0.077% to 0.095%.

[0059] Al: Element Al is an important deoxidizing element and can also form fine AlN, which plays a certain role in grain refinement. When the Al content is less than 0.025%, the deoxidation and grain refinement effects of Al are not obvious. When the Al content is higher than 0.04%, the castability of the slab is reduced. Therefore, the Al content in the steel of the present invention is controlled to be 0.025% to 0.04%, for example, 0.026% to 0.04%.

[0060] B: Element B has a certain strengthening effect. However, too high a B content will affect the addition of Si element. Therefore, in the present invention, the B content is controlled to be no more than 0.0006%, for example, no more than 0.0005%.

[0061] N: Element N can generally increase yield strength and tensile strength. However, too high a N content will lead to a decrease in r-value, easily cause aging, and also cause cracks on the slab surface. Therefore, the N content in the steel of the present invention is controlled to not exceed 0.005%, for example, not more than 0.0046%.

[0062] Cu: Adding element Cu can improve the weather resistance of steel. However, too high a Cu content will lead to increased costs. Therefore, the Cu content in the steel of the present invention is controlled to not exceed 0.035%, for example, not more than 0.033%.

[0063] Cr: Element Cr can improve the hardenability of steel and is generally used as an additive element for phase transformation strengthening steel. However, too high a Cr content will increase the cost. Therefore, the Cr content in the steel of the present invention is controlled to not exceed 0.05%, for example, not exceed 0.048%.

[0064] Mo: The addition of element Mo improves the hardenability of steel and is generally used as an additive element for phase transformation strengthening steel. However, too high a Mo content will increase the cost. Therefore, the Mo content in the steel of the present invention is controlled to not exceed 0.01%, for example, not exceed 0.008%.

[0065] V: Element V has a certain strengthening effect, and its strengthening mechanism is similar to that of Nb and Ti. However, too high a V content will lead to increased costs. Therefore, the V content in the steel of the present invention is controlled to not exceed 0.005%, for example, not exceed 0.0048%.

[0066] The steel material and the method for manufacturing the same according to the present invention will be further explained and illustrated in conjunction with specific embodiments. However, the following description is an illustrative description for explaining the present invention and is not intended to limit the technical scope of the present invention to the description scope.

[0067] Embodiment 1-21

[0068] The steel materials of Examples 1-21 of the present invention are prepared by the following steps:

[0069] 1) The molten steel is subjected to converter smelting and RH refining, wherein the stop-blowing temperature of the converter smelting is above 1600° C., the vacuum degree of the RH refining is below 200 Pa, and the RH refining time is at least 15 minutes.

[0070] 2) Casting the smelted molten steel to obtain a casting billet, wherein the casting speed is less than 1.5 m / min.

[0071] 3) The ingot is heated, roughly rolled, finished rolled and coiled, wherein the heating temperature is 1250°C to 1280°C, the rough rolling temperature is 1020°C to 1080°C, the finishing rolling temperature is 860°C to 900°C, and the coiling temperature is 490°C to 550°C.

[0072] Depend on Figure 1 It can be seen that the structure of the steel after hot coiling is ferrite + bainite.

[0073] 4) The hot-rolled steel is pickled and cold-rolled in a pickling continuous rolling mill, wherein the pickling speed is 100 m / min to 200 m / min and the cold rolling reduction ratio is 40% to 60%.

[0074] 5) The cold-rolled steel is continuously annealed in a non-oxidizing continuous annealing furnace. During the continuous annealing process, the soaking temperature is 700° C. to 750° C. and the soaking time is 60 seconds to 120 seconds.

[0075] 6) Flattening to obtain finished steel products, wherein the flattening rate is 0.01% to 0.1%.

[0076] Depend on Figure 2 It can be seen that the microstructure of the finished steel is ferrite+cementite+microalloy precipitation phase, and the microalloy precipitation phase is mainly composed of NbC and TiN precipitates.

[0077] In the microstructure of the finished steel of Examples 1 to 21, ferrite accounts for 90-95% by volume, cementite accounts for 4-8% by volume, and NbC and TiN precipitates account for 1-2% by volume in total. The size of the NbC and TiN precipitates is 10-20 nm.

[0078] Comparative Examples 1-3

[0079] Comparative Example 1 adopts substantially the same steps as Examples 1-21 to prepare steel. However, the chemical element composition and process parameters of the steel of Examples 1-21 meet the scope of protection required by the present invention, while the continuous annealing soaking temperature in Comparative Example 1, the rough rolling temperature in Comparative Example 2, and the Ti element content in Comparative Example 3 do not meet the scope of protection required by the present invention.

[0080] The chemical compositions of the steel materials of Examples 1-21 and Comparative Examples 1-3 are listed in Table 1. The process parameters of the manufacturing process of the steel materials of Examples 1-21 and Comparative Examples 1-3 are listed in Table 2.

[0081]

[0082]

[0083] According to the P6 specimen in the national standard GB / T 228.1, the tensile properties of Examples 1-21 and Comparative Examples 1-3 were tested. Table 3 shows the performance test results of Examples 1-21 and Comparative Examples 1-3.

[0084] Table 3

[0085]

[0086] As can be seen from Tables 1 to 3, Examples 1 to 21, whose steel chemical composition and manufacturing process parameters fall within the scope of protection claimed, have the specific microstructure (ferrite + cementite + microalloy precipitation phase) claimed for protection, the yield strength is 810 to 937 MPa, the tensile strength is 864 to 971 MPa, and the elongation at break is 11% to 15.1%. In contrast, at least one of the steel chemical composition and manufacturing process parameters of Comparative Examples 1, 2 and 3 does not fall within the scope of protection claimed, and although the elongation at break of the finished steel is 14.1 to 14.6%, the yield strength is only 659 to 790 MPa, and the tensile strength is only 753 to 856 MPa, which is significantly lower than the embodiments of the present invention.

[0087] In summary, the present invention obtains cold-rolled precipitation-strengthened steel with excellent comprehensive performance (especially in terms of yield strength, tensile strength and elongation at break) by designing the composition of the steel and cooperating with the specific manufacturing process parameters of the present invention. The present invention reduces the content of alloying elements such as Mn, Nb, and Mo by appropriately increasing the element Si content, thereby ensuring the excellent strength and elongation of the steel at low cost. The heat treatment temperature in the manufacturing process of the present invention is lower, which can achieve the energy-saving and consumption-reducing goals of the automotive industry. In addition, the manufacturing process of the present invention is simple and does not require equipment modification, and high-strength and high-elongation steel products can be manufactured at low cost and low energy consumption. Therefore, the steel products of the present invention and their manufacturing methods can be widely used in the fields of automotive safety parts, and are expected to promote further thinning and energy saving in the automotive industry.

[0088] The above embodiments are only used to illustrate the technical solution of the present invention and are not intended to limit it. Although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the specific embodiments of the present invention can be modified and replaced by equivalents without departing from the spirit of the technical solution of the present invention, and these variations should be included in the scope of the present invention.

Claims

1. A steel material, characterized in that: The steel material contains, in addition to more than 90% of Fe and inevitable impurities, the following chemical elements in wt%: 0.06%≤C≤0.075%, 0.3%≤Si≤0.4%, 1.25%≤Mn≤1.4%, P≤0.015%, S≤0.0015%, 0.02%≤Nb≤0.03%, 0.075%≤Ti≤0.095%, 0.025%≤Al≤0.04%, B≤0.0006%, N≤0.005%, Cu≤0.035%, Cr≤0.05%, Mo≤0.01% and V≤0.005%.

2. The steel material according to claim 1, characterized in that: The steel material contains the following chemical elements in wt%: 0.06%≤C≤0.075%, 0.3%≤Si≤0.4%, 1.25%≤Mn≤1.4%, P≤0.015%, S≤0.0015%, 0.02%≤Nb≤0.03%, 0.075%≤Ti≤0.095%, 0.025%≤Al≤0.04%, B≤0.0006%, N≤0.005%, Cu≤0.035%, Cr≤0.05%, Mo≤0.01% and V≤0.005%, the balance being Fe and unavoidable impurities.

3. The steel material according to claim 1 or 2, characterized in that: The microstructure of the steel is ferrite+cementite+microalloyed precipitates, and the microalloyed precipitates include NbC and TiN precipitates; preferably, the ferrite is 90-95 volume %; preferably, the cementite is 4-8 volume %; preferably, the NbC and TiN precipitates are 1-2 volume % in total; preferably, the size of the NbC and TiN precipitates is 10-20 nm.

4. The steel material according to claim 1 or 2, characterized in that: The grain size of the steel material is ≥ grade 10.

5. The steel material according to claim 1 or 2, characterized in that: The yield strength of the steel is ≥810MPa, the tensile strength is ≥860MPa, and the elongation at break is ≥11%.

6. A method for manufacturing the steel material according to any one of claims 1 to 5, the method comprising the following steps: 1) Smelting molten steel; 2) Casting the smelted molten steel to obtain a casting billet; 3) heating, rough rolling, finish rolling and coiling the ingot; 4) pickling and cold rolling; 5) Continuous annealing; 6) Smooth and produce finished steel products.

7. The method according to claim 6, characterized in that In step 1), the smelting includes converter smelting and refining of the molten steel, the stop-blowing temperature of the converter smelting is ≥1600°C, and the refining is carried out at a vacuum degree of ≤200Pa for ≥15 minutes.

8. The method according to claim 6 or 7, characterized in that: In step 2), the casting is performed by continuous casting, and the casting speed is ≤1.5 m / min.

9. The method according to claim 6 or 7, characterized in that: In step 3), the heating temperature is 1250°C to 1280°C; the rough rolling temperature is 1020°C to 1080°C; the finishing rolling temperature is 860°C to 900°C; the coiling temperature is 490°C to 550°C; preferably, the structure of the steel after coiling is ferrite + bainite.

10. The method according to claim 6 or 7, characterized in that: In step 4), pickling and cold rolling are carried out in a pickling tandem rolling mill, the pickling speed is 100 m / min to 200 m / min, and the cold rolling reduction ratio is 40% to 60%.

11. The method according to claim 6 or 7, characterized in that: In step 5), continuous annealing is performed in a non-oxidizing continuous annealing furnace, with a soaking temperature of 700° C. to 750° C. and a soaking time of 60 seconds to 120 seconds.

12. The method according to claim 6 or 7, characterized in that: In step 6), the flattening rate is 0.01% to 0.1%; preferably, the microstructure of the finished steel is ferrite+cementite+microalloy precipitation phase, and the microalloy precipitation phase includes NbC and TiN precipitates.

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