High-formability and ultra-high-strength hot-rolled strip steel, and manufacturing method therefor

Abstract

Provided in the present disclosure is hot-rolled strip steel having a yield strength of greater than or equal to 900 MPa. In addition to Fe and inevitable impurities, the hot-rolled strip steel further comprises the following chemical elements in percentages by mass: 0.07-0.2% of C, 0.02-0.20% of Si, 1.2-2.5% of Mn, 0.3-1.0% of Cr, 0.13-0.2% of Ti and 0.01-0.06% of Al. Further provided is a manufacturing method for the hot-rolled strip steel, the method comprising: subjecting a plate blank to heating, rough rolling and finish rolling to obtain strip steel, then subjecting the strip steel to a staged online quenching treatment, and subjecting same to coiling, a water blowing treatment and a tempering heat treatment. The hot-rolled strip steel of the present disclosure has an alloy cost significantly lower than that of the existing hot-rolled strip steel having a yield strength grade of 900 MPa, and also has relatively high plasticity and good cold formability.

Description

A high-formability, ultra-high-strength hot-rolled strip steel and its manufacturing method Technical Field

[0001] This disclosure relates to a strip steel and a method for manufacturing the same, and more particularly to a high-formability, ultra-high-strength hot-rolled strip steel and a method for manufacturing the same. Background Technology

[0002] Against the backdrop of green, low-carbon, energy-saving, and weight-reducing initiatives, the demand for hot-rolled high-strength steel is booming in sectors such as commercial vehicle modification. As strength levels increase, steel consumption decreases, leading to a reduction in overall vehicle weight. This lowers material costs for modification vehicle manufacturers and reduces energy consumption during vehicle operation. Consequently, in recent years, the strength levels of steel used in modification vehicles have been steadily increasing, rising rapidly from a yield strength of 235-355 MPa to 700 MPa, with a further demand for 800-1000 MPa. To meet the requirements of modification vehicle manufacturers in terms of formability and vehicle safety, steel used in modification vehicles must not only possess excellent bending and impact resistance but also good sheet shape.

[0003] Currently, the three main processes for producing hot-rolled high-strength steel with a yield strength ≥900MPa in China include: conventional hot continuous rolling, quenching and tempering heat treatment, and online quenching. Chinese Patent Publication No. CN109023091A discloses a hot-rolled ultra-high-strength steel plate with a tensile strength ≥900MPa and its preparation method. The preparation method of this steel plate is based on a conventional controlled rolling and controlled cooling process. The microstructure of this steel plate consists of fine acicular ferrite and dispersed precipitated second phases, exhibiting low carbon equivalent and good formability. However, this steel plate incorporates expensive alloying elements, including 0.03–0.05% Nb and 0.15–0.25% Mo by mass, increasing material costs.

[0004] Chinese Patent Publication No. CN102618793A discloses a quenched and tempered steel with a yield strength of 960 MPa and its manufacturing method. The steel plate is prepared using an offline quenching-tempering heat treatment process, achieving a yield strength ≥960 MPa and a Charpy impact energy AKv (-40℃) ≥47 J. However, this steel plate also incorporates a large amount of expensive alloying elements, including 0.20-0.60% Mo, 0.015-0.055% Nb, and 0.020-0.060% V by mass, resulting in high material costs. Furthermore, the quenching and tempering heat treatment process further increases manufacturing costs.

[0005] Chinese Patent Publication No. CN104561827A discloses a high-strength steel with a yield strength of 900-1000 MPa and its production method. The steel plate is prepared using an online quenching-tempering process, resulting in a tempered martensite microstructure. The yield strength is 900-1080 MPa, the tensile strength is 950-1200 MPa, the elongation is ≥10%, and the impact energy at -40℃ is ≥27 J. However, this steel plate also contains a large amount of expensive alloying elements, including 0.10-0.45% Mo and 0.10-0.50% Ni by mass, leading to high alloying costs.

[0006] According to the production process, the patent applications disclosed above have the following characteristics when producing high-strength steel with a yield strength of 900MPa or higher: 1) Conventional hot rolling process: To improve strength, expensive alloying elements such as Nb, V and Mo are usually added, which leads to a significant increase in alloy cost; in addition, the increase in strength will lead to an increase in the internal stress of the steel coil, and the plate shape defects are more likely to occur after cutting, which cannot meet the requirements of users with high plate shape requirements; 2) Offline quenching and tempering process: It is usually used to produce high-strength steel with high requirements for comprehensive mechanical properties, such as engineering machinery steel with specific requirements for weldability, fatigue performance and low temperature impact performance. These high-strength steels typically contain a large number of precious alloying elements, and the quenching and tempering heat treatment further increases production costs, making it difficult to meet users' low-cost requirements; 3) Online quenching process: commonly used to produce high-strength steels such as wear-resistant steels, which has a significant low-cost advantage compared to traditional quenching and tempering processes. However, online quenching processes require quenching to be completed on a layered cooling roller table, which makes it easier for plate shape defects to occur compared to quenching and tempering processes; in addition, because the tempering temperature of these steels is not high, the plasticity of the material is relatively low, which cannot meet the requirements for high formability such as small-angle cold bending. Summary of the Invention

[0007] In view of the above-mentioned shortcomings in the prior art, especially the problem that hot-rolled strip steel with a yield strength of ≥900MPa has high alloy cost and cannot meet the high formability requirements such as small-angle cold bending, the inventors have optimized the alloy composition of the steel.

[0008] Therefore, in a first aspect, this disclosure provides a hot-rolled strip steel with a yield strength ≥900 MPa, wherein the strip steel, in addition to containing Fe and unavoidable impurities, contains the following chemical elements in the following mass percentages: C: 0.07–0.2%, Si: 0.02–0.2%, Mn: 1.2–2.5%, Cr: 0.3–1.0%, Ti: 0.13–0.2%, Al: 0.01–0.06%.

[0009] In one specific embodiment, the hot-rolled strip steel of this disclosure is composed of the following chemical elements in mass percentage: C: 0.07-0.2%, Si: 0.02-0.2%, Mn: 1.2-2.5%, Cr: 0.3-1.0%, Ti: 0.13-0.2%, Al: 0.01-0.06%; the balance being Fe and unavoidable impurities, or the balance being Fe, selectively added elements and unavoidable impurities.

[0010] By adopting the C-Mn-Ti-Cr alloy system of the present invention and rationally designing the content ratio of alloying elements, hot-rolled strip steel with a yield strength ≥900MPa with low alloy cost can be obtained without adding or with a small amount of other alloying elements as needed.

[0011] In one specific embodiment, the hot-rolled strip steel of this disclosure selectively incorporates Nb, V, Mo, and Ca, and the content of Nb, V, Mo, and Ca as a percentage by mass satisfies the following formula: Nb+V≤0.04%, Mo≤0.08%, Ca≤0.004%. By selectively incorporating the above alloying elements, the microstructure of the strip steel can be further refined, the strength-toughness matching of the material can be optimized, and the cold forming capability of the strip steel can be improved.

[0012] In one specific embodiment, the hot-rolled strip steel of this disclosure unavoidably contains impurities including P, S, O, and N, and the content of P, S, O, and N by mass percentage satisfies the following formula: P ≤ 0.02%, S ≤ 0.01%, O ≤ 0.008%, N ≤ 0.005%. By controlling the impurity elements within the above ranges, the influence of impurities on the plasticity and toughness of the steel can be reduced.

[0013] In one specific embodiment, the chemical composition of the hot-rolled strip steel of this disclosure also satisfies the following relationships 1 and 2: 3.5% ≤ 12×C + Mn + 2.5×(Cr + V) + 2×Mo ≤ 5.5% (Equation 1), Ti - 3.5×N ≥ 0.12% (Equation 2), where each element symbol represents the mass percentage content of the corresponding element. By ensuring that the contents of C, Mn, Cr, V, and Mo satisfy the above relationship 1, it is possible to ensure that the high-strength steel has sufficient hardenability and can achieve online quenching under the cooling conditions provided by conventional laminar flow cooling processes, while avoiding excessive martensite strength leading to deterioration of formability. By ensuring that the contents of Ti and N satisfy the above relationship 2, it is possible to ensure the presence of sufficient Ti element in the steel, thereby effectively pinning austenite grain boundaries and refining the austenite structure during hot rolling, while forming TiC nanoprecipitates during subsequent tempering, thus improving the strength of the strip steel after tempering.

[0014] In one specific embodiment, the microstructure of the hot-rolled strip steel of this disclosure comprises tempered martensite and ferrite, preferably composed of tempered martensite and ferrite. In one specific embodiment, the volume percentage of ferrite is ≤20%, for example, 3-20%. In one specific embodiment, both the tempered martensite and ferrite contain dispersed TiC nanoprecipitates. In one specific embodiment, the size of the TiC nanoprecipitates is ≤10 nm. By giving the hot-rolled strip steel the above-mentioned microstructure, it is possible to better ensure that the strip steel reaches the required strength level, while obtaining high plasticity and good cold bending performance.

[0015] In a second aspect, this disclosure also provides a method for manufacturing the aforementioned hot-rolled strip steel, comprising the following steps:

[0016] S1: Heat the slab to 1240-1300℃ and homogenize it for 30-90 minutes; perform rough rolling on the slab to obtain strip steel, with an exit temperature of 1040-1080℃.

[0017] S2: The strip steel is finished rolled at a final rolling temperature of 840-900℃;

[0018] S3: Perform staged online quenching treatment on the strip steel;

[0019] S4: The strip steel enters the coiler for winding;

[0020] S5: Perform water blowing treatment on the strip steel;

[0021] S6: The strip steel is subjected to tempering heat treatment at a temperature of 580-700℃.

[0022] Using the above scheme, heating the slab to a temperature of 1240-1300℃ can ensure sufficient solid solution of Ti; the roughing mill exit temperature of 1040-1080℃ can ensure that the strip steel obtains sufficient strength; the finishing mill final rolling temperature of 840-900℃ can increase the rolling deformation energy, promote the ferrite transformation of the strip steel during air cooling on the laminar cooling roller table, and promote the interphase precipitation of TiC during the ferrite phase transformation, significantly improving the precipitation strengthening effect of TiC.

[0023] In one specific embodiment, in the manufacturing method of this disclosure, in step S3, the temperature is cooled to 620℃~700℃ at a cooling rate of not less than 0.5×exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s, then cooled in air for 3-7s, and then cooled to below Ms-250℃ at a cooling rate of not less than exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s, where Ms is the martensitic transformation start temperature, Ms=539-423×C-11.0×Si-30.4×Mn-12.1×Cr-7.0×Mo, and each element symbol represents the value before the percentage sign in the mass percentage content of the corresponding element.

[0024] By employing the aforementioned staged online quenching process, especially by controlling the cooling rate within the range described above during the staged online quenching process, followed by high-temperature tempering heat treatment, the resulting strip steel microstructure includes tempered martensite, ferrite, and dispersed TiC nano-precipitates; and the volume percentage of ferrite is ≤20%, and the size of the TiC nano-precipitates is ≤10nm, thereby enabling the strip steel to achieve the required strength while also achieving high plasticity and good cold bending performance.

[0025] In one specific embodiment, in the manufacturing method of this disclosure, during step S5, the strip is leveled during the water blowing process. By leveling the strip, the strip shape is optimized.

[0026] In one specific embodiment, in the manufacturing method of this disclosure, step S6 involves: uncoiling the strip steel and then subjecting it to continuous tempering heat treatment for a holding time of 80-240 seconds; or, cutting the strip steel into plates and then subjecting them to tempering heat treatment for a holding time of 10-30 minutes. Using the above methods, delivery in either coil or plate condition can be achieved by utilizing different tempering equipment, demonstrating good manufacturability.

[0027] The hot-rolled strip steel and its manufacturing method disclosed herein have the following beneficial technical effects:

[0028] The hot-rolled strip steel disclosed herein adopts a low-cost alloy system of C-Mn-Ti-Cr. By rationally designing the content ratio of alloying elements, the alloy cost of hot-rolled strip steel with a yield strength ≥900MPa is reduced.

[0029] Based on reducing alloy costs, the manufacturing method disclosed herein employs a rolling-online quenching-high-temperature tempering process to obtain a tempered martensite + ferrite microstructure, with TiC nano-precipitates of less than 10 nm dispersed inside. This fully leverages the strength and plasticity enhancement effects brought about by microstructure regulation and precipitation strengthening, achieving the required strength, high plasticity, and good cold bending performance at a lower alloy cost.

[0030] The hot-rolled strip steel disclosed herein has a yield strength of 900-1050 MPa, a tensile strength of 950-1200 MPa, and a fracture elongation of ≥12%. Under cold bending conditions of d=3a and 90°, the sample surface does not wrinkle or crack. Attached Figure Description

[0031] Figure 1 shows the microstructure of the hot-rolled strip steel in Example 1.

[0032] Figure 2 shows the microstructure of the hot-rolled strip steel in Example 2.

[0033] Figure 3 shows the microstructure of the hot-rolled strip steel in Example 5.

[0034] Figure 4 shows the microstructure of the hot-rolled strip steel in Example 6. Detailed Implementation

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0036] In this paper, the volume percentage and grain size of ferrite were determined according to GB / T15749-2008 "Quantitative Metallographic Determination Method".

[0037] In this paper, yield strength, tensile strength and elongation at break were determined according to GB / T 228.1-2021 "Metallic materials, tensile test at room temperature".

[0038] In this paper, the cold bending performance was determined according to GB / T 232-2024 "Metallic Materials - Test Method for Bending".

[0039] As is well known in the art, exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s is one of the commonly used empirical formulas for the critical cooling rate of martensite, reflecting the influence of different elements on the critical cooling rate. The larger the absolute value of the coefficient, the stronger the influence of the corresponding element.

[0040] The design principles of each chemical element in the hot-rolled strip steel disclosed herein are as follows:

[0041] Carbon (C): Carbon (C) has a significant impact on the phase transformation process and products of steel. Increasing the C content can lower the critical cooling rate for martensite formation, making it easier to form martensite under the same cooling conditions. C exists in martensite in solid solution form, significantly improving its strength. However, a higher C content leads to the formation of brittle twinned martensite, which is detrimental to the steel's low-temperature impact toughness. During subsequent tempering, a higher C content easily leads to the formation of coarse carbides, resulting in a decrease in plasticity and impact toughness. On the other hand, an excessively low C content easily leads to the formation of large amounts of ferrite and other low-strength structures, failing to meet the strength requirements of this disclosure. Therefore, in the hot-rolled strip steel of this disclosure, the C content is controlled within the range of 0.07% to 0.2%.

[0042] Silicon (Si): Si is a solid solution element in steel and has a good deoxidizing effect. However, high Si content can easily lead to the formation of red iron scale on the surface during hot rolling and deteriorate the toughness and weldability of martensite. Therefore, in the hot-rolled strip steel of this disclosure, the Si content is controlled within the range of 0.02-0.2%.

[0043] Manganese (Mn): Mn is an important alloying element in this disclosure. The addition of Mn can suppress the formation of proeutectoid ferrite and improve the hardenability of steel. In addition, the addition of Mn can refine the microstructure, giving it a good balance of strength and toughness. However, excessive addition of Mn can easily lead to significant center segregation and the formation of high-carbon twinned martensite at the corresponding positions, which is detrimental to the toughness of martensitic high-strength steel. Therefore, in the hot-rolled strip steel of this disclosure, the Mn content is controlled within the range of 1.2% to 2.5%.

[0044] Chromium (Cr): Cr is an important alloying element in this disclosure. The addition of Cr can significantly improve the hardenability of steel and is beneficial for the formation of a full martensitic structure during quenching. In addition, Cr will form carbide precipitates during tempering, which have a certain resistance to tempering softening. Excessive Cr content can easily lead to a decrease in the strength and toughness of the strip steel. Therefore, in the hot-rolled strip steel of this disclosure, the Cr content is controlled in the range of 0.3-1.0%, preferably 0.3-0.84%.

[0045] Titanium (Ti): Ti is an important microalloying element in this disclosure. Ti can form fine, dispersed phase particles with C and N, which can pin austenite grain boundaries and refine the austenite structure during hot rolling. In the subsequent tempering process, Ti can generate nanoscale TiC precipitates, improving the yield strength and tensile strength of tempered martensite and exhibiting a significant anti-temper softening effect. To compensate for the temper softening caused by not using precious elements such as Mo and Nb, the hot-rolled strip steel of this disclosure has a relatively high Ti content, controlled within the range of 0.13-0.2%.

[0046] Aluminum (Al): Al is a deoxidizer for steel. The addition of a small amount of Al can refine the grains and improve impact toughness. Excessive Al content easily leads to the formation of alumina inclusions and promotes the formation of proeutectoid ferrite. Therefore, in the hot-rolled strip steel of this disclosure, the Al content is controlled within the range of 0.01-0.06%.

[0047] Niobium (Nb) and Vanadium (V): Nb and V are selectively added alloying elements. Nb has a strong dragging effect on austenite grain boundaries; even trace amounts of Nb can significantly refine austenite grains, thereby improving the strength and toughness of the material. Simultaneously, Nb is a strong carbide-forming element, capable of generating nano-Nb(C,N) precipitates in ferrite, thus increasing the strip strength. V significantly improves the hardenability of austenite and refines austenite grains, thereby increasing the strip strength and toughness. V is also a strong carbide-forming element, capable of generating nano-V(C,N) precipitates in ferrite, thus increasing the strip strength. To avoid excessive strength caused by precipitation strengthening after tempering, the total content of strong carbide-forming elements must be limited. Therefore, in the hot-rolled strip steel of this disclosure, the combined content of Nb and V is controlled to not exceed 0.04%. Furthermore, Nb and V are valuable elements; controlling their addition amounts can control production costs. In addition, controlling the total amount of Nb and V added within the above range can avoid excessive strength caused by precipitation strengthening after tempering, and can also reduce the coarsening of nano-precipitates during high-temperature tempering, effectively reducing toughness loss and benefiting the cold forming performance of strip steel.

[0048] Molybdenum (Mo): Mo is a selectively added alloying element. Mo can significantly improve the tempering resistance of strip steel, while inhibiting the coarsening of martensite and nano-precipitates during high-temperature tempering, thus improving the strength and toughness of the strip steel. However, Mo is also a valuable element, and its addition must be controlled to manage production costs. Therefore, in the hot-rolled strip steel disclosed herein, the Mo content is controlled to be no more than 0.08%.

[0049] Calcium (Ca): Ca is a purifying agent in the steelmaking process, which can improve the morphology of sulfides and enhance the impact toughness of steel. Excessive Ca content can easily lead to the formation of larger Ca compounds, which in turn deteriorates toughness. Therefore, in the hot-rolled strip steel of this disclosure, the Ca content is controlled to not exceed 0.004%.

[0050] Phosphorus (P), sulfur (S), oxygen (O), and nitrogen (N): P, S, O, and N are impurity elements in steel, and excessive content can significantly affect the plasticity and toughness of steel. Therefore, in the hot-rolled strip steel of this disclosure, the contents of the above-mentioned impurity elements are controlled to be P≤0.02%, S≤0.01%, O≤0.008%, and N≤0.005%, respectively.

[0051] In one specific embodiment, the chemical composition of the hot-rolled strip steel of this disclosure also satisfies the following relationships 1 and 2: 3.5% ≤ 12×C + Mn + 2.5×(Cr + V) + 2×Mo ≤ 5.5% (Equation 1), Ti - 3.5×N ≥ 0.12% (Equation 2).

[0052] By ensuring that the contents of C, Mn, Cr, V, and Mo satisfy the above-mentioned relationship 1, high-strength steel can be guaranteed to have sufficient hardenability, enabling online quenching under the cooling conditions provided by conventional laminar flow cooling processes. This fully completes the phase transformation process from untransformed austenite to martensite, while avoiding the deterioration of impact toughness and cold bending performance caused by excessive martensite strength. When 12×C+Mn+2.5×(Cr+V)+2×Mo is lower than the lower limit of 3.5% in Equation 1, the hardenability of the steel is insufficient, and ferrite is easily generated during the cooling process, resulting in low strength. When 12×C+Mn+2.5×(Cr+V)+2×Mo is higher than the upper limit of 5.5% in Equation 1, the quenching strength of the steel is too high, and cold bending cracks are prone to occur.

[0053] By ensuring that the contents of Ti and N satisfy the above relationship 2, it is possible to guarantee that there is a sufficient amount of Ti element in the steel, which can effectively pin the austenite grain boundaries and refine the austenite structure during hot rolling. At the same time, it ensures the formation of a sufficient amount of nano-scale TiC precipitates during the subsequent tempering process, which can compensate for the insufficient strength of ferrite, improve the yield strength of tempered martensite and ensure that it reaches more than 900 MPa.

[0054] In one specific embodiment, the microstructure of the hot-rolled strip includes tempered martensite and ferrite, and both the tempered martensite and ferrite contain dispersed TiC nano-precipitates; the volume percentage of ferrite is ≤20%, and the size of the TiC nano-precipitates is ≤10nm.

[0055] This disclosure employs a unique preparation process to obtain a microstructure dominated by tempered martensite and ferrite, in which TiC nanoprecipitates with a size of less than 10 nm are dispersed, which can fully exert the effects of microstructure strengthening and precipitation strengthening, achieving high yield and tensile strength. Compared with a single tempered martensite microstructure, the introduction of ferrite gives it higher plasticity and good cold bending performance.

[0056] This disclosure also provides a method for manufacturing hot-rolled strip steel. The process involves obtaining a slab through a steelmaking continuous casting process, heating it in a heating furnace, and then rolling it into a hot-rolled strip steel through rough rolling, finish rolling, and online quenching in a layered cooling process, followed by tempering treatment.

[0057] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0058] S0: Steelmaking and refining are carried out using a converter or electric furnace, and slabs are obtained through continuous casting.

[0059] S1: The slab is heated, and after heating, it leaves the heating furnace and is then cut to a fixed width before entering the roughing mill to obtain strip steel.

[0060] To ensure sufficient solid solution of Ti, the slab is heated to 1240-1300℃ for 30-90 minutes. The soaking time is calculated after the core of the slab reaches the target temperature.

[0061] Sufficiently high descaling pressure should be maintained during the rough rolling stage to achieve good descaling results. The rough rolling exit temperature is a crucial indicator. Excessively high exit temperatures hinder austenite grain refinement, leading to a deterioration in the impact toughness of the strip. Conversely, excessively low exit temperatures result in the formation of numerous strain-induced TiC precipitates within the austenite. Due to the high precipitation temperature, these TiC precipitates are larger, exhibiting weak precipitation strengthening and reducing the effective Ti content during subsequent tempering, ultimately decreasing the strip's strength. Considering both factors, the rough rolling exit temperature should be controlled between 1040-1080℃.

[0062] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0063] S2: After rough rolling, the strip enters the finishing mill for finishing rolling.

[0064] A multi-stand continuous rolling process is employed, with the appropriate final rolling temperature selected based on the thickness specifications of the rolled strip. Compared to traditional rolling processes, the method disclosed herein requires a lower final rolling temperature. This is to increase the rolling deformation energy of the steel, promote a small amount of ferrite transformation during the segmented cooling of the strip in the laminar cooling roller table, and facilitate the interphase precipitation of TiC during the ferrite phase transformation, significantly improving the precipitation strengthening effect of TiC and compensating for the reduction in yield strength caused by the formation of a small amount of ferrite. On the other hand, to suppress strain-induced precipitation of the TiC second phase in austenite, the final rolling temperature should not be too low. Considering both factors, the final rolling temperature is controlled between 840-900℃.

[0065] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0066] S3: The finished strip enters the laminar flow cooling unit for staged online quenching treatment.

[0067] In one specific embodiment, in step S3, the strip steel is cooled to a temperature of 620℃~700℃ at a cooling rate of not less than 0.5×exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s (first stage cooling rate), and then cooled in air for 3-7s; then cooled to a temperature below Ms-250℃ (final cooling temperature) at a cooling rate of not less than exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s (second stage cooling rate), where Ms is the martensitic transformation start temperature, Ms=539-423×C-11.0×Si-30.4×Mn-12.1×Cr-7.0×Mo, where each element symbol represents the value before the percentage sign in the mass percentage content of the corresponding element.

[0068] The purpose of staged online quenching is to achieve precise control over the microstructure and precipitation of the strip steel. Cooling to 620℃~700℃ at a rate of not less than 0.5×exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s maintains a relatively fine austenite state during cooling, preventing microstructure coarsening. Cooling in air for 3-7s allows partial ferrite phase transformation to occur during air cooling, generating ferrite with a volume fraction not exceeding 20%. Then, the strip steel is cooled to below Ms-250℃ at a rate of not less than exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s to rapidly reduce the strip steel temperature below the final cooling temperature, achieving online quenching and transforming the untransformed austenite into martensite. Insufficient cooling rate or excessive final cooling temperature can easily generate other phase structures such as bainite and excessive ferrite, resulting in low strip strength and failure to reach the specified strength level after tempering.

[0069] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0070] S4: After the laminar flow cooling process is completed, the strip enters the coiler for coiling.

[0071] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0072] S5: After coiling, the strip steel is subjected to water blowing treatment. Because the temperature of the steel coil is low after online quenching, the residual cooling water between the coil layers cannot evaporate in time, easily leading to rust on the strip steel surface. Therefore, the steel coil should be subjected to water blowing treatment on the leveling line promptly after coiling to prevent rust on the strip steel surface.

[0073] In one specific implementation, in order to optimize the strip shape, a certain leveling force is applied to level the strip during the water blowing process.

[0074] In one specific embodiment, the manufacturing method of this disclosure includes the following steps:

[0075] S6: After the steel coil has been blown, it will undergo tempering heat treatment at a temperature of 580-700℃.

[0076] Tempering within the aforementioned temperature range promotes martensitic recovery and increases the elongation of the strip. Simultaneously, ensuring sufficient nanoscale TiC precipitates during tempering guarantees adequate strip strength. Furthermore, higher tempering temperatures reduce internal stress, thereby improving the strip's shape.

[0077] In one specific implementation, the strip steel is uncoiled and then subjected to continuous tempering heat treatment with a holding time of 80-240 seconds. Continuous heat treatment allows for rapid heating and short holding times, maintaining a refined state while TiC is being deposited, resulting in a better balance of strength and toughness. Furthermore, the shorter tempering time significantly reduces carbon emissions and accelerates production.

[0078] In one specific implementation, a single-plate heat treatment process is used for tempering heat treatment, with a holding time of 10-30 minutes.

[0079] The manufacturing method disclosed herein employs a rolling-in-line quenching-high-temperature tempering process, resulting in a microstructure of tempered martensite + ferrite. The tempered martensite retains a relatively fine lath martensite structure, while the ferrite is abundant and has a small grain size, exhibiting a quasi-polygonal or long-axis polygonal morphology. Both phases contain dispersed TiC nanoprecipitates smaller than 10 nm, ensuring high strength. The high-temperature tempered martensite possesses relatively high plasticity, which, combined with the increased plasticity from ferrite, ensures high plasticity and good cold bending performance under conditions of relatively low alloy cost and required strength. Furthermore, high-temperature tempering effectively eliminates the internal stresses generated during martensitic transformation and smoothing, reducing strip shape distortion and further improving strip profile.

[0080] After the above process, the yield strength of the strip steel reaches 900-1050MPa, the tensile strength reaches 950-1200MPa, and the elongation at break is ≥12%. Under the cold bending conditions of d=3a and 90°, the sample surface does not wrinkle or crack.

[0081] The non-oriented electrical steel sheet and its manufacturing method disclosed herein will be further explained and described below with reference to specific embodiments and accompanying drawings. However, such explanation and description do not constitute an undue limitation on the technical solution of the present invention.

[0082] Examples 1-14

[0083] The hot-rolled strip steel of Examples 1-14 was obtained by the following steps:

[0084] S0: Slabs are obtained by using converter smelting, refining and continuous casting processes;

[0085] S1: Heat the slab in a heating furnace to 1240-1300℃ (slab exit temperature), and heat for 30-90 minutes; perform rough rolling on the slab to obtain strip steel, with an exit temperature of 1040-1080℃.

[0086] S2: The strip steel is finished rolled at a final rolling temperature of 840-900℃;

[0087] S3: Perform layer cooling and online quenching treatment on the strip steel;

[0088] S4: The strip steel enters the coiler for winding;

[0089] S5: Water blowing treatment of strip steel; and

[0090] S6: The strip steel is subjected to tempering heat treatment at a temperature of 580-700℃; among them, Examples 1-6 adopt uncoiling continuous tempering heat treatment (the strip steel is first uncoiled and then subjected to continuous tempering heat treatment), and Examples 7-14 adopt plate cutting tempering heat treatment (the strip steel is first cut and then subjected to tempering heat treatment).

[0091] Comparative Examples 1-4

[0092] The hot-rolled strip steel of Comparative Examples 1-4 was obtained by the following steps:

[0093] S0: Slabs are obtained by using converter smelting, refining and continuous casting processes;

[0094] S1: Heat the slab in a heating furnace to 1240-1300℃ (slab exit temperature), and heat for 30-90 minutes; perform rough rolling on the slab to obtain strip steel, with an exit temperature of 1040-1080℃.

[0095] S2: The strip steel is finished rolled at a final rolling temperature of 840-900℃;

[0096] S3: Cooling the strip steel;

[0097] S4: The strip steel enters the coiler for winding;

[0098] S5: Water blowing treatment of strip steel; and

[0099] S6 (optional): Tempering heat treatment of the strip at a temperature of 580-700℃.

[0100] Comparative Example 1 did not undergo tempering heat treatment (step S6). Comparative Example 2 used a one-stage online quenching process. Comparative Examples 2-4 used uncoiling continuous tempering heat treatment.

[0101] Table 1 lists the mass percentage of each chemical element in the hot-rolled strip steel of Examples 1-14 and Comparative Examples 1-4. Table 2 lists the specific process parameters of the hot-rolled strip steel of Examples 1-14 and Comparative Examples 1-4.

[0102] As can be seen from Table 1, Comparative Examples 1-2 have the same chemical composition as Examples 1-2; Comparative Example 3 has a lower Ti content, which is not within the scope of this application; Comparative Example 4 has lower Mn and Cr contents, which are not within the scope of this application.

[0103] Performance testing:

[0104] The strip steel obtained in Examples 1-14 and Comparative Examples 1-4 was subjected to performance testing.

[0105] (1) Tensile properties at room temperature

[0106] At room temperature, plate-shaped tensile specimens taken from steel strips were used to test the steel strip samples of each embodiment and comparative example according to the Chinese national standard GB / T 228.1-2021 "Metallic materials - tensile testing at room temperature". The yield strength, tensile strength and elongation at break of the steel strips of each embodiment and comparative example were measured.

[0107] (2) Cold bending performance

[0108] According to the Chinese national standard GB / T 232-2024 "Metallic Materials - Bending Test Method", the strip steel samples of each embodiment and comparative example were tested to obtain the cold bending performance of the strip steel under the condition of cold bending punch diameter d = 3a and bending angle of 90°.

[0109] (3) Microstructure

[0110] The volume percentage of ferrite in the microstructure of the strip steel samples of each embodiment and comparative example was detected in accordance with the Chinese national standard GB / T15749-2008 "Quantitative Metallographic Determination Method".

[0111] The performance test results of Examples 1-14 and Comparative Examples 1-4 are shown in Table 3.

[0112] Table 3.

[0113] Note: In the table above, "qualified" means that the surface of the strip does not wrinkle or crack; "unqualified" means that the surface of the strip wrinkles or cracks.

[0114] As shown in Table 3, the strip steel of Examples 1-14 has a yield strength ≥900MPa, a tensile strength ≥950MPa, and an elongation at break ≥12%. Under cold bending conditions of d=3a and 90°, no wrinkling or cracking occurs on the sample surface. Therefore, the hot-rolled strip steel has high plasticity and good cold bending performance. In particular, Examples 1, 5, and 14 achieved high strength, high plasticity, and good cold bending performance without the addition of any precious metal elements.

[0115] The chemical compositions of Examples 7 and 8 are identical. The main difference in the preparation process lies in the cooling rate: in Example 8, the first-stage cooling rate did not reach 0.5 × exp(5.8 - 2.53 × C - 0.16 × Si - 0.82 × Mn - 0.95 × Cr - 1.87 × Mo) °C / s, and the second-stage cooling rate did not reach exp(5.8 - 2.53 × C - 0.16 × Si - 0.82 × Mn - 0.95 × Cr - 1.87 × Mo) °C / s. Other process parameters are essentially the same. Performance testing results show that the microstructure of Example 8 has a higher ferrite content, exceeding 20%, and the yield strength and tensile strength of Example 8 are significantly lower than those of Example 7, with the yield strength only slightly exceeding the specified value.

[0116] The above results show that controlling the cooling rate in the first stage to be ≥0.5×exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s and the cooling rate in the second stage to be ≥exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s can significantly improve the strength level and control the occurrence of partial ferrite phase transformation during air cooling (avoiding the formation of excessive ferrite).

[0117] Examples 10 and 9 have the same chemical composition. The main difference in the preparation process is that the final cooling temperature of Example 10 is higher than (Ms-250)℃; other process parameters are basically the same. Performance testing results show that the yield strength and tensile strength of Example 10 are significantly lower than those of Example 9, and the tensile strength is only slightly higher than the specified value.

[0118] The above results indicate that controlling the final cooling temperature below (Ms-250)℃ can significantly improve the strength level.

[0119] In the chemical composition of Example 12, Ti-3.5N was 0.12%, the lowest value among all examples, and the yield strength and tensile strength of the steel plate were 918 MPa and 958 MPa, respectively, which were also the lowest values ​​among all examples.

[0120] The above results indicate that a low effective Ti content will affect the strength of the strip steel. When Ti-3.5N ≥ 0.12%, sufficient effective Ti can be ensured in the steel to improve the yield strength of the tempered martensite and ensure that it reaches above 900 MPa.

[0121] In the chemical composition of Example 13, 12C+Mn+2.5(Cr+V)+2Mo is 5.5%, which is the highest value among all examples. The yield strength and tensile strength of the steel plate are 1024MPa and 1095MPa, respectively, which are also the highest values ​​among all examples.

[0122] The above results show that increasing the value of 12C+Mn+2.5(Cr+V)+2Mo can significantly improve the strength level of the strip steel; however, excessively high strength can easily lead to cold bending cracks. Therefore, in order to prevent this problem, the value needs to be controlled below 5.5%.

[0123] The microstructures of the hot-rolled strip steels in Examples 1, 2, 5, and 6 are shown in Figures 1 to 4, respectively. As can be seen from Figures 1 to 4, the microstructure of the hot-rolled strip steel of the present invention includes tempered martensite + a small amount of ferrite, wherein TiC nanoprecipitates with a size of less than 10 nm are dispersedly distributed, and the tempered martensite has a fine lath martensite structure; the ferrite grains are relatively small, exhibiting quasi-polygonal or long-axis polygonal morphology.

[0124] In comparison, Comparative Example 1 had a low fracture elongation and failed cold bending performance; Comparative Example 2 had no ferrite formation in its microstructure and a low fracture elongation, also failing cold bending performance; Comparative Example 3 had significantly low yield strength and tensile strength; and Comparative Example 4 had a 12C+Mn+2.5(Cr+V)+2Mo content of less than 3.5%, indicating low hardenability and insufficient martensite strength, resulting in low yield and tensile strength.

[0125] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0126] While this disclosure has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the disclosure in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of this disclosure to these descriptions. Various changes in form and detail can be made by those skilled in the art, including some simple deductions or substitutions, without departing from the spirit and scope of this disclosure.

Claims

1. A hot-rolled strip steel, wherein, The strip steel has a yield strength ≥900MPa. In addition to Fe and unavoidable impurities, the strip steel also contains the following chemical elements in the following mass percentages: C: 0.07~0.2%, Si: 0.02~0.2%, Mn: 1.2~2.5%, Cr: 0.3~1.0%, Ti: 0.13~0.2%, Al: 0.01~0.06%.

2. The hot-rolled strip steel as described in claim 1, wherein, The strip steel is composed of the following chemical elements by mass percentage: C: 0.07-0.2%, Si: 0.02-0.2%, Mn: 1.2-2.5%, Cr: 0.3-1.0%, Ti: 0.13-0.2%, Al: 0.01-0.06%; the balance being Fe and unavoidable impurities, or the balance being Fe, selectively added elements and unavoidable impurities.

3. The hot-rolled strip steel as described in claim 2, wherein, The selectively added elements include Nb, V, Mo, and Ca, and the contents of Nb, V, Mo, and Ca as a percentage by mass satisfy the following formula: Nb + V ≤ 0.04%, Mo ≤ 0.08%, Ca ≤ 0.004%.

4. The hot-rolled strip steel according to any one of claims 1 to 3, wherein, The unavoidable impurities include P, S, O, and N, and the content of P, S, O, and N as a percentage by mass satisfies the following formula: P ≤ 0.02%, S ≤ 0.01%, O ≤ 0.008%, N ≤ 0.005%.

5. The hot-rolled strip steel according to any one of claims 1 to 4, wherein, The chemical composition of the strip steel also satisfies the following relationship: 3.5% ≤ 12×C + Mn + 2.5×(Cr + V) + 2×Mo ≤ 5.5%, Ti - 3.5×N ≥ 0.12%, where each element symbol represents the mass percentage content of the corresponding element.

6. The hot-rolled strip steel according to any one of claims 1 to 5, wherein, The microstructure of the strip steel includes tempered martensite and ferrite, preferably composed of tempered martensite and ferrite, both of which contain dispersed TiC nano-precipitates; the volume percentage of the ferrite is ≤27%, preferably ≤20%, for example, the volume percentage of the ferrite is 3-20%; the size of the TiC nano-precipitates is ≤10nm.

7. The hot-rolled strip steel according to any one of claims 1 to 6, wherein, The strip has a yield strength of 900-1050 MPa, a tensile strength of 950-1200 MPa, an elongation at break of ≥12%, and / or, under cold bending conditions of d=3a and 90°, the surface of the strip does not wrinkle or crack.

8. A method for manufacturing hot-rolled strip steel according to any one of claims 1 to 7, wherein, The method includes the following steps: S1: Heat the slab to 1240-1300℃ and heat it for 30-90 minutes; perform rough rolling on the slab to obtain strip steel, with the exit temperature of the rough rolling being 1040-1080℃; S2: The strip steel is finished rolled at a final rolling temperature of 840-900℃; S3: Perform staged online quenching treatment on the strip steel; S4: The strip steel enters the coiler for coiling; S5: Perform water blowing treatment on the strip steel; and S6: The strip steel is subjected to tempering heat treatment at a temperature of 580-700℃.

9. The method of claim 8, wherein, In step S3, the strip steel is cooled to a temperature of 620℃~700℃ at a cooling rate of not less than 0.5×exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s, and then cooled in air for 3-7 seconds; then cooled to a temperature below Ms-250℃ at a cooling rate of not less than exp(5.8-2.53×C-0.16×Si-0.82×Mn-0.95×Cr-1.87×Mo)℃ / s. Where Ms is the martensitic transformation onset temperature, Ms = 539 - 423 × C - 11.0 × Si - 30.4 × Mn - 12.1 × Cr - 7.0 × Mo. The symbols for each element represent the numerical value before the percentage sign in the mass percentage content of the corresponding element.

10. The method of claim 8 or 9, wherein, In step S5, the strip steel is leveled during the water blowing process.

11. The method according to any one of claims 8 to 10, wherein, In step S6: The strip steel is uncoiled and then subjected to continuous tempering heat treatment for 80-240 seconds; or The strip steel is cut into plates, and then subjected to tempering heat treatment for 10-30 minutes.

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