Strip steel, fine blanked steel material and manufacturing method therefor

Abstract

Disclosed in the present invention is a strip steel, which comprises the following chemical elements in mass percentages: C: 0.16-0.27 wt%, Si: 0.01-0.5 wt%, Mn: 0.5-1.5 wt%, Al: 0.01-0.1 wt%, Cr: 0.6-1.2 wt%, Ti: 0.01-0.05 wt%, B: 0.0001-0.005 wt%, N≤0.006 wt%, and the balance of iron and inevitable impurities, wherein the microstructure of the strip steel comprises granular pearlite, and the diameter of spherical cementite particles in the granular pearlite is 1.0-2.4 μm. Further disclosed in the present invention are a fine blanked steel material and a manufacturing method therefor. In addition, further disclosed in the present invention is a manufacturing method for the strip steel, which method comprises: smelting; continuous casting; and hot rolling: comprising the reduction of multiple passes of strip steel, wherein the reduction rate of the last pass is 10-20%, and the rolling pressure of the last pass is 150-200 tons; and after hot rolling, pickling and bell furnace annealing are directly performed without cold rolling, with the annealing temperature being 660-710°C. The strip steel disclosed in the present invention adopts a reasonable chemical composition and process design, which can not only satisfy the subsequent complex stamping requirements, but also simplify the process steps and reduce the production cost.

Description

A strip steel, a fine-stamped steel product and its manufacturing method Technical Field

[0001] This invention relates to the technical field of steel plates and their manufacturing methods, specifically to strip steel and fine-stamped steel and their manufacturing methods. Background Technology

[0002] In automobiles, precision adjustment mechanism components require easy formability and extremely high dimensional accuracy after forming, with smooth deformed surfaces and virtually no tear bands. The microstructure of such steel sheets is primarily ferrite and granular pearlite, lacking banded structures and decarburized layers, with granular pearlite accounting for more than 90% of the volume. This microstructure ensures easy machining into precision-formed parts with smooth shear surfaces, and also guarantees satisfactory strength and surface hardness after heat treatment. To achieve good stamping performance, high plasticity and relatively low strength with uniform properties are desired in the steel sheet. This property is ideally achieved by increasing the spheroidization rate (i.e., the volume proportion of granular pearlite). The main factors affecting spheroidization include the original microstructure of the hot-rolled steel, the cold rolling process, and the heat treatment process.

[0003] Patent document CN202211185612.3 discloses a cold-rolled precision-stamped steel strip and its preparation method, belonging to the field of high-carbon precision-stamped steel technology. The chemical composition of the cold-rolled precision-stamped steel strip includes: C, Si, Mn, P, S, Al, Cr, Nb, Fe, and impurities from the preparation of the cold-rolled precision-stamped steel strip; by mass fraction, the content of C element is 0.80% to 0.90%, and the content of Nb element is 0.02% to 0.12%. The cold-rolled precision-stamped steel strip has a yield strength of 400 to 500 MPa, a tensile strength of 600 to 700 MPa, an elongation after fracture (A50) of 20% to 30%, a hardness (HV5) of 165-180 HV, a spheroidization rate ≥95%, and an average diameter of spheroids of 0.8 to 1.5 μm.

[0004] Patent document CN201410231375.9 discloses a method for manufacturing fine-stamped steel and its adjusting mechanism fine-stamped parts. The chemical composition (wt%) of the fine-stamped steel is as follows: C: 0.12~0.24, Si: ≤0.40, Mn: 0.30~0.80, P≤0.015, S≤0.005%, Cr: ≤0.40, Ni≤0.40, Mo: ≤0.10, Ca: 0.0015~0.0040%, with the balance being Fe and unavoidable inclusions. The method includes the following process route: molten iron - smelting (ladle refining) - casting - billet heating - hot rolling - pickling - cold rolling - spheroidizing annealing - fine stamping - heat treatment. According to the literature, the non-metallic inclusion content of the above-mentioned fine-stamped parts is: various inclusions ≤ 1.5 grade, elongation after fracture: A ≥ 30%, fine-stamping difficulty: S3 grade, which can obtain fine-stamped parts with high dimensional accuracy, high strength and high toughness.

[0005] However, there are currently few publicly available patent documents regarding this type of steel sheet, and most of the existing patent documents describe the preparation of fine-stamped steel through hot rolling, pickling, cold rolling, and spheroidizing annealing to obtain cold-rolled fine-stamped strip (i.e., cold-rolled fine-stamped steel strip), followed by fine-stamping and heat treatment to obtain the finished fine-stamped steel. The reason for choosing cold-rolled strip is that hot-rolled steel has large pearlite lamellae, high strength, poor plasticity, and is generally difficult to form directly. Furthermore, hot-rolled steel has poor surface quality, poor thickness accuracy, and is prone to banded structures, making direct spheroidizing annealing difficult. Therefore, hot-rolled steel requires subsequent cold rolling and spheroidizing annealing before the complex fine-stamping process can proceed. However, the cold-rolled strip process is complex, resulting in high production and management costs. Therefore, providing a type of strip steel and its manufacturing method that can not only meet the complex subsequent stamping requirements but also simplify the process steps and reduce production costs has become an urgent technical problem to be solved. Summary of the Invention

[0006] The present invention provides a strip steel to solve the above-mentioned technical problems.

[0007] This invention provides a steel strip containing the following chemical elements by mass percentage:

[0008] C: 0.16–0.27 wt%, Si: 0.01–0.5 wt%, Mn: 0.5–1.5 wt%, Al: 0.01–0.1 wt%, Cr: 0.6–1.2 wt%, Ti: 0.01–0.05 wt%, B: 0.0001–0.005 wt%, N ≤ 0.006 wt%, with the balance being iron and unavoidable impurities. The microstructure of the strip steel includes granular pearlite, wherein the diameter of the spherical cementite particles in the granular pearlite is 1.0–2.4 μm.

[0009] Using the above technical solution, the strip steel is easy to forge into plate form, thus meeting the complex stamping requirements, and is less prone to cracking during the process.

[0010] Optionally, B and Cr satisfy the following formula: 200 * B wt% + Cr wt% > 1.0 wt%, where B and Cr are represented by their mass percentage values ​​before the symbols. In some embodiments, B and Cr satisfy the following formula: 200 * B wt% + Cr wt% ≥ 1.4 wt%.

[0011] Optionally, the microstructure of the strip steel also includes ferrite, with a volume percentage of ferrite ≤10%, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or 9%, and a volume percentage of granular pearlite ≥90%, such as 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%. In some embodiments, the microstructure of the strip steel is ferrite + granular pearlite.

[0012] Optionally, Al: 0.01–0.08 wt%, N < 0.0045%.

[0013] Optionally, the strip steel also contains P and S, wherein, by mass percentage, P ≤ 0.015 wt% and S ≤ 0.01 wt%.

[0014] Optionally, the tensile strength of the strip is ≤460 MPa, for example 420–460 MPa, and the elongation is ≥25%, for example 25%–35%. In some embodiments, the tensile strength of the strip is <460 MPa, and the elongation is >25%.

[0015] The present invention also provides a fine-stamped steel product, which is obtained by fine-stamping and heat treatment of the strip steel obtained in the above embodiments.

[0016] Using the above technical solution, the tensile strength of the precision stamping steel can reach ≥1500MPa, for example, 1500~1850MPa.

[0017] Optionally, the fine-stamped steel has a tensile strength ≥1500MPa, for example 1500-1850MPa, and a yield strength ≥1000MPa, for example 1000-1250MPa. In some embodiments, the fine-stamped steel has a tensile strength >1500MPa and a yield strength >1000MPa.

[0018] Optionally, the microstructure of fine-stamped steel is lath martensite.

[0019] The present invention also provides a method for manufacturing strip steel in the above embodiments, comprising the steps of:

[0020] Smelting;

[0021] Continuous casting;

[0022] Hot rolling: This includes multiple passes of rolling down the strip, with the last pass having a reduction rate of 10%-20% and a rolling pressure of 150-200 tons.

[0023] Pickling and bell-type furnace annealing, with the annealing temperature in the bell-type furnace annealing being 660-710℃;

[0024] Cold rolling is not performed in the manufacturing process.

[0025] The above technical solution not only meets the complex stamping requirements, but also simplifies the process steps and reduces production costs.

[0026] Optionally, in the hot rolling step, the reduction rate of the last pass is 10-15%, and the rolling pressure of the last pass is 160-200 tons.

[0027] Optionally, in the hot rolling step, the heating temperature is controlled at 1230-1280℃, for example 1230-1260℃, and the finishing mill inlet temperature is ≥1200℃.

[0028] Optionally, the multiple passes are six passes, wherein the reduction rate of the first pass F1 to the third pass F3 is ≥50%, the pass temperature of the first pass F1 to the third pass F3 is ≥1000℃, the reduction rate of the fourth pass F4 is ≥40%, the reduction rate of the fifth pass F5 is ≥40%, and the final rolling temperature is 870-930℃.

[0029] Optionally, a laminar flow cooling step is included between the hot rolling step and the pickling and bell-type furnace annealing steps. The process conditions for the laminar flow cooling step are as follows: the water cooling rate is 70-120℃ / s, and after cooling to 560-620℃, the temperature is further cooled to 450-550℃ at a water cooling rate of ≤10℃ / s and then coiled.

[0030] Optionally, the annealing time in the pickling and bell-type furnace annealing steps is 10 to 30 hours.

[0031] The present invention also provides a method for manufacturing the fine-stamped steel described in the above embodiments. This method uses the strip steel manufacturing method described in the above embodiments to obtain strip steel, and then performs fine stamping and heat treatment on the strip steel to obtain fine-stamped steel. The heat treatment process can involve heating to 850-920℃ and holding for 20-30 minutes, followed by water quenching or oil quenching, with a cooling rate ≥40℃ / s.

[0032] The above technical solution not only facilitates the subsequent plate forging of strip steel, but also ensures the strength after heat treatment. After heat treatment and tempering, the tensile strength of fine stamping steel can reach ≥1500MPa and the yield strength can reach ≥1000MPa. Attached Figure Description

[0033] Figure 1 shows the metallographic microstructure of the strip steel after pickling and annealing in Example 1 of the present invention;

[0034] Figure 2 shows the metallographic microstructure of the pickled and annealed strip steel of Comparative Example 1 of the present invention. Detailed Implementation

[0035] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0036] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and the accompanying drawings.

[0037] Steel strip is a raw material steel sheet used for precision stamping and heat treatment. It is often desirable for steel strip to have low tensile strength and high plasticity, so as to facilitate the subsequent fine stamping process. Since fine-stamped steel is needed for adjusting parts in precision mechanisms such as automobiles, it is often desirable for steel strip to have sufficient strength after fine stamping and heat treatment.

[0038] The microstructure of the strip steel mainly consists of ferrite and granular pearlite, lacking banded structure and a decarburized layer. Increasing the proportion of granular pearlite can improve plasticity, toughness, machinability, and reduce the tendency for deformation cracking during final heat treatment. Therefore, existing technologies often aim to obtain strip steel by increasing the proportion of granular pearlite (i.e., spheroidization rate) to over 95%. However, the inventors discovered through experiments that, within certain elemental ranges, if the diameter of the spheroidized cementite is within a suitable range, even if the proportion of granular pearlite (i.e., spheroidization rate) reaches 90%, strip steel meeting the aforementioned requirements of lower strength and higher plasticity can still be obtained.

[0039] Based on this, the present invention provides a strip steel containing the following chemical elements by mass percentage: C: 0.16–0.27 wt%, Si: 0.01–0.5 wt%, Mn: 0.5–1.5 wt%, Al: 0.01–0.1 wt%, Cr: 0.6–1.2 wt%, Ti: 0.01–0.05 wt%, B: 0.0001–0.005 wt%, N ≤ 0.006 wt%, with the balance being iron and unavoidable impurities. The microstructure of this strip steel includes granular pearlite, wherein the diameter of the spherical cementite particles in the granular pearlite is 1.0–2.4 μm. In the microstructure of the strip steel satisfying the above element ranges, and with the diameter of the spherical cementite between 1.0 and 2.4 μm, this not only facilitates subsequent plate forging of the strip steel but also ensures that the fine-stamped steel after quenching and tempering heat treatment has sufficient tensile strength and is less prone to cracking during the processing.

[0040] Furthermore, in the above embodiments, B and Cr satisfy the condition: 200*B wt% + Cr wt% > 1.0wt%. Both B and Cr can effectively improve hardenability. A certain amount of Cr has an effect on the morphology and distribution of the spheroidized structure after annealing. The inventors found through experiments that by simultaneously satisfying 200*B wt% + Cr wt% > 1.0wt% within their respective ranges, the tensile strength of the strip steel after appropriate heat treatment can be improved.

[0041] Furthermore, within the range of elements defined in the above embodiments, the microstructure of the strip steel also includes ferrite, with a volume percentage of ferrite ≤10% and a volume percentage of granular pearlite ≥90%. This ensures that the tensile strength of the strip steel sheet is within a suitable range, thus facilitating subsequent precision stamping. In some embodiments, the microstructure of the strip steel is ferrite + granular pearlite.

[0042] In the above embodiments, the Al content is further controlled to be 0.01-0.08 wt% and the N content to be <0.0045%. This allows for better control of the austenite grain diameter within the microstructure of the fine-stamped steel parts during subsequent quenching and tempering heat treatment, thereby ensuring the hardenability of the fine-stamped steel parts and preventing deformation of their geometry.

[0043] Furthermore, by controlling P≤0.015wt% and S≤0.01wt%, the obtained strip is easier to form in subsequent plate forging.

[0044] Furthermore, the tensile strength of the strip is ≤460MPa, and the elongation is ≥25%. The obtained strip can be forged into sheet metal using most precision stamping dies without cracking.

[0045] Furthermore, the present invention also provides a fine-stamped steel product, specifically obtained by fine-stamping and heat-treating the strip steel of the above embodiments. When fine-stamping and heat-treating the strip steel, any existing fine-stamping and heat-treating process can be used. For example, when performing quenching and tempering heat treatment, the process conditions can be: heating to above 860°C and holding for 20 minutes, followed by water quenching or oil quenching, with a cooling rate >40°C / s.

[0046] Furthermore, the tensile strength of the fine-stamped steel in the above embodiment can reach 1500MPa, the yield strength can reach 1000MPa, and the microstructure of the fine-stamped steel is lath martensite.

[0047] As mentioned above, most existing technologies involve hot rolling, pickling, cold rolling, and spheroidizing annealing to obtain cold-rolled fine-stamped strip steel, which is then fine-stamped and heat-treated to produce fine-stamped steel. However, the cold-rolling process is complex, resulting in high production and management costs. Through experimentation, the inventors discovered that the cold rolling step is not always necessary. Therefore, this invention also provides a manufacturing method for the aforementioned strip steel, comprising the following steps:

[0048] Smelting; After smelting blast furnace iron and an appropriate amount of scrap steel in a converter, the steel is then refined in a ladle furnace (LF) + RH (Rheinstahl-Heratus) furnace. Calcium-silicon wire is fed into the molten steel to reduce the residue of large-sized blocky inclusions generated during the converter smelting process.

[0049] Continuous casting: The production of thin slabs by continuous casting and rolling of molten steel. The thickness of the cast steel billet is 60-85mm. At the same time, the superheat needs to be controlled within 15℃ to reduce the banded structure in the steel billet and improve segregation.

[0050] Hot rolling: This includes multiple passes of rolling down the strip, with the last pass having a reduction rate of 10%-20% and a rolling pressure of 150-200 tons.

[0051] After hot rolling, cold rolling is not performed; instead, pickling and bell-type furnace annealing are carried out directly. The annealing temperature in the bell-type furnace is 660-710℃. It should be noted that the final first-pass reduction rate can be understood as the reduction rate of the last pass in conventional hot rolling, or as a final reduction performed after the conventional hot rolling process, with a reduction rate of 10%-20%. This invention, by controlling the final pass reduction rate to 10-20%, the final pass rolling pressure to 150-200 tons, and the bell-type furnace annealing temperature to 660-710℃, after hot rolling, ensures that the size of the spherical cementite is between 1.0 and 2.4 μm. This not only facilitates subsequent strip forging but also ensures sufficient tensile strength in the heat-treated fine-stamped steel and reduces the likelihood of cracking during processing, simplifying the process and reducing production costs.

[0052] Furthermore, the present invention sets the final reduction rate to 10-20%. This is because the strip contains more martensitic structure at this time, and its strength is relatively high. While increasing the tension, excessive deformation can easily lead to strip breakage, thus affecting production. On the other hand, a smaller reduction amount can more precisely control the hot-rolled plate shape and thickness accuracy, making its diameter specifications better than ordinary hot-rolled products, and thus easier to perform precision stamping on it in the future.

[0053] Furthermore, in the above embodiments, the hot rolling step includes roughing and finishing rolling. The finishing rolling process involves pressing the strip through multiple stands, for example, 6 to 7 stands, with the last stand having a reduction rate of 10%-20%. The rolling pressure is controlled at 150-200 tons. Rolling pressure refers to the force applied by the rolls to the workpiece to cause plastic deformation. Specifically, it refers to the force applied by the rolls to the workpiece (i.e., the strip) during the rolling process, thereby causing plastic deformation of the strip. This invention utilizes a relatively small reduction rate of 10%-20% in the final pass combined with a relatively large rolling pressure of 150-200 tons, while simultaneously controlling the annealing temperature in the bell-type furnace at 660-710℃. This allows the austenite in the steel coil to undergo deformation-induced phase transformation under stress, generating martensite. This avoids the production of ferrite and pearlite during the subsequent slow cooling process of coiling, thus preventing component segregation within the steel coil. This, in turn, facilitates the formation of uniformly dispersed spherical cementite during bell-type furnace annealing. This not only benefits the subsequent strip forging but also ensures sufficient tensile strength in the fine-stamped steel after heat treatment. It simplifies the process steps and reduces production costs. Specifically, the final pass reduction rate is 10%-15%, and the rolling pressure is 160-200 tons.

[0054] This invention improves upon the traditional hot rolling process and further optimizes its management, eliminating the cold rolling step, shortening the process flow, and improving production efficiency, while ensuring product performance and the spheroidization rate of the microstructure.

[0055] Furthermore, in the above embodiments, in the hot rolling roughing step, the heating temperature is controlled at 1230-1280℃, preferably 1230-1260℃, and the finishing rolling inlet temperature is ≥1200℃. This allows the strip steel to deform rapidly under relatively hot conditions, while avoiding the formation of difficult-to-remove oxides on the surface due to excessively high temperatures.

[0056] Furthermore, in the hot rolling steps of the above embodiments, the reduction conditions for multiple passes are as follows: the reduction rate of the first pass F1 to the third pass F3 is ≥50%, for example, 50%-70%; the pass temperature of the first pass F1 to the third pass F3 is ≥1000℃, for example, 1000-1230℃; the reduction rate of the fourth pass F4 is ≥40%, for example, 40%-50%; and the rolling temperature of the fourth pass is ≥970℃, for example, 970-980℃. The reduction rate of the fifth pass is ≥40%, for example, 40%-50%, the rolling temperature of the fifth pass is ≥950℃, for example, 950-965℃, the reduction rate of the last pass is 10%-20%, the rolling pressure of the last pass is 150-200 tons, and the rolling temperature of the last pass (i.e., the final rolling temperature) is 870-930℃. This allows for the acquisition of more martensitic structure, avoids component segregation within the steel plate, and facilitates subsequent plate forging.

[0057] Furthermore, a laminar flow cooling step is included between the hot rolling step and the pickling and bell-type furnace annealing steps in the above embodiments. The process conditions for the laminar flow cooling step are as follows: first, the strip is cooled to 560-620°C at a water cooling rate of 70-120°C / s, and then cooled to 450-550°C at a water cooling rate of ≤10°C / s (e.g., 5-10°C / s) before coiling. During the initial cooling process, a higher cooling rate is required, mainly to avoid the formation of pearlite structure and to prevent the pearlite structure from segregating. When the temperature drops below 620°C, the water cooling rate needs to be appropriately reduced to prevent overcooling. Coiling can be carried out when the temperature drops to 450-550°C, mainly because if the temperature drops below 450°C, the strip strength will be too high, which will cause strip breakage and thus affect the production schedule.

[0058] Furthermore, in the bell-type furnace annealing step of the above embodiments, the annealing temperature is 660-710℃ and the annealing time is 10h-30h, which helps to spheroidize and anneal the hot-rolled mixed structure of martensite, bainite, etc. into granular pearlite structure with a diameter between 1.0 and 2.4μm.

[0059] This invention also provides a method for manufacturing fine-stamped steel. This method uses the strip steel manufacturing method described in the above embodiments to obtain strip steel, which is then fine-stamped and heat-treated to obtain fine-stamped steel. After the above rolling and heat treatment, the microstructure within the strip steel can form ≥90% spheroidized structure, with the diameter of the spheroidal cementite between 1.0 and 2.4 μm. This not only facilitates subsequent plate forging of the strip steel but also ensures the strength after quenching and tempering heat treatment. After quenching and tempering heat treatment, the tensile strength of the fine-stamped steel can reach over 1500 MPa, and the yield strength can reach over 1000 MPa.

[0060] The design principles of each chemical element in the strip steel provided by this invention are described below:

[0061] Carbon: Carbon is an important strengthening element that can significantly improve the strength of steel through solid solution strengthening. However, an excessively high carbon mass percentage will not only significantly increase the carbon equivalent of the strip steel and reduce its weldability, but also reduce its plasticity and increase its cold brittleness and aging sensitivity. Therefore, the carbon mass percentage cannot be too high. Thus, the carbon mass percentage in the steel plate of this invention is controlled at 0.16 to 0.27 wt%.

[0062] Silicon: Silicon is a ferrite solid solution strengthening element that can improve strength. However, adding silicon will increase the carbon equivalent, reduce weldability, and is also detrimental to phosphating properties. Therefore, the mass percentage of silicon in the steel plate of this invention is controlled at 0.01 to 0.5 wt%.

[0063] Manganese: Manganese has a strong solid solution strengthening effect and can effectively improve the strength of steel plates. Therefore, in order to meet the high strength requirements after heat treatment, a certain amount of Mn is added to the cold-rolled steel plate of the present invention. However, since an excessively high mass percentage of Mn will significantly increase the carbon equivalent of the steel plate, reduce weldability, and aggravate segregation, it will be detrimental to the forming properties such as plasticity, hole expansion, and bending performance. Therefore, the mass percentage of Mn in the steel plate of the present invention is controlled at 0.5 to 1.5 wt%.

[0064] Aluminum: Al acts as a deoxidizer, combining with N to form AlN, which helps prevent austenite grain coarsening. However, when the Al content exceeds 0.1 wt%, the purity of the steel sheet decreases. Therefore, the steel sheet of the present invention controls the mass percentage of Al to be 0.01 to 0.1 wt%, and more preferably, the mass percentage of aluminum is 0.01 to 0.08 wt%.

[0065] Chromium: In steel, chromium is mainly used to delay the incubation time of austenite transformation, improve hardenability, and postpone the transformation of ferrite and pearlite. It inhibits the formation of these structures during cooling, allowing the steel to directly enter the martensitic transformation region at a low cooling rate. Therefore, the mass percentage of chromium in the steel plate of this invention is controlled at 0.6–1.2 wt%. Simultaneously, an appropriate amount of chromium can also optimize the size and distribution of spheroidized cementite during spheroidizing annealing, resulting in a more ideal microstructure.

[0066] Boron: Boron is an element that strongly segregates austenite grain boundaries in steel. It can reduce the grain boundary energy of austenite, inhibit the formation of proeutectoid ferrite nuclei, and has a strong ability to improve hardenability. Only a very small amount of boron is needed to save a large amount of expensive alloying elements. However, there is an optimal boron content for improving hardenability. To effectively improve hardenability, the boron content in this invention must be above 0.0001 wt%. However, when its content exceeds 0.005 wt%, the effect of improving hardenability reaches saturation. Therefore, the boron content in this invention is controlled between 0.0001 and 0.005 wt%.

[0067] Nitrogen: In Ti-containing steel, an appropriate amount of N readily forms TiN with Ti at high temperatures, which is beneficial for strengthening the matrix and improving the weldability of the steel plate. However, if the mass percentage of N is too high, it can easily coarsen TiN or cause excessive N to dissolve, reducing the plasticity and expansion / flanging performance of the steel. Furthermore, excessive N content can lead to the formation of AlN nitrides during hot rolling, resulting in reduced punching performance and hardenability of the base steel plate. Therefore, in the technical solution of this invention, the mass percentage of N is controlled at N ≤ 0.006 wt%, and more preferably, the mass percentage of N is N ≤ 0.0045 wt%.

[0068] Titanium: In this invention, Ti is a strong carbide and nitride forming element, possessing strong grain refinement and precipitation strengthening effects. It can react with N at high temperatures to form TiN, which is beneficial for strengthening the matrix. The mass percentage of Ti in this invention is controlled at 0.01–0.05 wt% because too low a percentage would result in insufficient grain refinement and precipitation strengthening, while too high a percentage would lead to excessively coarse TiN, potentially causing cracking during subsequent fine blanking and heat treatment.

[0069] The unavoidable impurities in this invention are mainly S and P. Among them, phosphorus increases the cold brittleness of the strip, reduces its plasticity, and also has an adverse effect on welding performance. Therefore, the lower the content of impurity P, the better. However, considering the smelting cost, the mass percentage of P can be controlled at P≤0.015wt%.

[0070] Sulfur readily combines with Mn in steel to form MnS, which deteriorates mechanical properties, hole expansion properties, and other forming properties. Therefore, the lower the S content, the better. Thus, in the above embodiments, the mass percentage of S is controlled to be S≤0.01wt%.

[0071] The technical effects of the present invention will be further illustrated below with reference to the embodiments and accompanying drawings.

[0072] Examples 1-6 and Comparative Examples 1-4

[0073] Table 1 lists the mass percentage of each chemical element in the strip steel of Examples 1-6 and Comparative Examples 1-4.

[0074] Table 1 (wt%, balance is Fe and other unavoidable impurities)

[0075] The strip steel in Examples 1-6 was obtained using the following steps, and Comparative Examples 1-4 also used the following method, the difference being that the parameter ranges in some steps are not within the ranges described below (the process parameters for specific steps can be found in Tables 2 and 3):

[0076] Smelting: After blast furnace molten iron and an appropriate amount of scrap steel are smelted in a converter, they are refined in an LF+RH furnace, and a silicon-calcium wire is fed into the molten steel.

[0077] Continuous casting: The production of thin slabs by continuous casting and rolling of molten steel, with the thickness of the cast steel billet being 60-85mm, while the superheat is controlled within 15℃.

[0078] Hot rolling: The billet is heated in a soaking furnace, and the heating temperature and finishing rolling temperature are controlled. The heating temperature is 1230-1280℃, and the finishing rolling inlet temperature is ≥1200℃. Rolling is carried out in a 6-stand finishing mill. The reduction rate of each pass from the first pass F1 to the third pass F3 is ≥50%, and the temperature of each pass from the first pass F1 to the third pass F3 is ≥1000℃. The reduction rate of the fourth pass F4 is ≥40%, and the temperature is ≥970℃. The reduction rate of the fifth pass F5 is ≥40%, and the temperature is ≥950℃. The reduction rate of the sixth pass is 10%-20%, and the rolling temperature of the sixth pass is 870-930℃. The rolling pressure of the sixth pass is 150-200 tons. Constant speed rolling is used according to the thickness specification, and the rolling speed is controlled at 3.0-10.0 m / s. The finishing rolling temperature is controlled, and the rolling thickness is 1.2-6.0 mm.

[0079] (4) Laminar flow cooling: The water cooling rate is 70-120℃ / s. After cooling to 560-620℃, the temperature is then cooled to 450-550℃ at a water cooling rate of ≤10℃ / s before winding.

[0080] (5) Pickling and bell-type furnace annealing: After pickling, bell-type furnace annealing is carried out. The annealing temperature is 660-710℃ and the annealing time is 10h-30h.

[0081] Tables 2 and 3 show the specific process parameters for the strip manufacturing methods of Examples 1-6 and Comparative Examples 1-4 of the present invention.

[0082] Table 2

[0083] Table 3

[0084] The steel strips from Examples 1-6 and Comparative Examples 1-4 were used to test their tensile strength, elongation, and spheroidization rate. The diameter of the spheroidal cementite within the granular pearlite was also tested. The tensile strength and elongation tests followed GB / T288.1-2021. The spheroidal cementite diameter was measured under 1000x magnification using an optical microscope, with the average diameter length calculated for at least five images. The microstructure and spheroidization rate were tested using GB / T38770-2020, where spheroidization rate is the volume percentage of granular pearlite in the microstructure relative to the overall microstructure. The test results are recorded in Table 4.

[0085] Table 4

[0086] The strip steel plates from Examples 1-6 and Comparative Examples 1-4 were fine-stamped and heat-treated to obtain application examples 1-6 and comparative application examples 1-4. The heat treatment process involved heating to 860°C and holding for 20 minutes, followed by oil quenching at a cooling rate of 70°C / s. The tensile strength and yield strength of the fine-stamped steel in application examples 1-6 and comparative application examples 1-4 were then measured (test method: GB / T288.1-2021), and the results are recorded in Table 5. The microstructure of the fine-stamped steel in application examples 1-6 was observed using an optical microscope at 500x magnification. The results showed that the microstructure of the fine-stamped steel in application examples 1-6 consisted entirely of lath martensite.

[0087] Table 5

[0088] As shown in Tables 1-5 above, the microstructure of the strip steel obtained in Examples 1-6 is granular pearlite + ferrite. Granular pearlite has good cold deformation performance, machinability, and quenching process performance. Meanwhile, the tensile strength of the strip steel is <460MPa, elongation >25%, spheroidization rate >90%, and the diameter of the spheroidal cementite particles is in the range of 1.0-2.4μm. After fine blanking and heat treatment, its tensile strength can reach over 1500MPa and its yield strength can reach over 1000MPa, which is not significantly different from the strength of fine-blanked steel obtained by fine blanking and heat treatment of cold-rolled strip steel. Therefore, this invention reduces the cold rolling process, and the obtained strip steel is not only conducive to subsequent plate forging, but also ensures that the fine-blanked steel after heat treatment has sufficient tensile strength and yield strength, and is not prone to cracking during the treatment process, fully meeting the requirements of fine-blanked steel, simplifying the process steps, and reducing production costs.

[0089] Furthermore, Examples 1-4 and 6 improve the tensile strength of the strip steel after appropriate heat treatment by controlling the content ratio of B and Cr elements to satisfy 200*B wt% + Cr wt% > 1.0wt%, and utilizing the content ratio of the two elements to simultaneously satisfy 200*B wt% + Cr wt% > 1.0wt% within their respective ranges. Finally, the tensile strength of Application Examples 1-4 and Application Example 6 is better than that of Example 5.

[0090] Furthermore, in Examples 4 and 6, the Al content was further controlled to be 0.01–0.08 wt% and the N content to be <0.0045%. This allowed for better control of the austenite grain diameter within the microstructure of the fine-stamped steel parts during subsequent quenching and tempering heat treatment, thereby ensuring the hardenability of the fine-stamped steel parts and preventing deformation of their geometry.

[0091] Furthermore, in Example 1, the Cr content was controlled at 1-1.2 wt%, and the effect of Cr on the morphology and distribution of the spheroidized structure after annealing was utilized to obtain the ideal microstructure shown in Figure 1.

[0092] Furthermore, in Examples 2 and 4, the reduction rate of the last stand in hot rolling is controlled within the range of 10-15%, and the rolling pressure is further controlled within 160-200 tons. By setting a smaller reduction rate of 10-15% to match a larger rolling pressure of 160-200 tons, the spheroidization rate of the strip reaches 99%, which not only facilitates plate forging but also prevents cracking during subsequent fine blanking and heat treatment. In addition, the tensile strength of the obtained fine blanking steel is superior to that of other application examples.

[0093] In Comparative Example 1, because the carbon content was not in the range of 0.16-0.27 wt%, and the rolling pressure in the final pass of hot rolling was not in the range of 150-200 tons, and the annealing temperature was not in the range of 660-710℃, the diameter of the spherical cementite in the strip microstructure was not in the range of 1.0-2.4 μm. Consequently, the tensile strength and yield strength of Comparative Example 1 were lower than those of the aforementioned embodiments. Furthermore, the microstructure of Comparative Example 1 contained a certain amount of lamellar pearlite. Since lamellar pearlite is prone to cracking in subsequent processing, the tensile strength of the fine-stamped steel obtained from Comparative Example 1 after fine blanking and heat treatment was not only lower than that of Examples 1-6, but it was also prone to cracking during fine blanking and heat treatment. Further, comparing the metallographic structure of the strip steel of Comparative Example 1 (see Figure 2) with that of Example 1 (see Figure 1), it can be seen that the microstructure of the strip steel obtained from Comparative Example 1 is significantly different from that of Example 1.

[0094] In Comparative Example 2, the Cr content was outside the range of 0.6-1.2 wt%, the Ti content was outside the range of 0.01-0.05 wt%, the final reduction rate was outside the range of 10-20%, and the annealing temperature was outside the range of 660-710℃. Consequently, the diameter of the spherical cementite in the final microstructure was outside the 1.0-2.4 μm range disclosed in this invention. Therefore, the tensile strength and yield strength of Comparative Example 1 were also lower than those of the aforementioned embodiments. Furthermore, the microstructure of Comparative Example 2 contained a certain amount of lamellar pearlite. Since lamellar pearlite is prone to cracking in subsequent processing, the tensile strength of the fine-stamped steel obtained from Comparative Example 2 after fine blanking and heat treatment was not only lower than that of Examples 1-6, but it was also prone to cracking during fine blanking and heat treatment.

[0095] In Comparative Example 3, the final reduction rate was not within the range of 10-20%, and the annealing temperature was not within the range of 660-710℃. This resulted in the diameter of the spherical cementite in the final microstructure being outside the 1.0-2.4 μm range disclosed in this invention. Consequently, the tensile strength and yield strength of Comparative Example 3 were lower than those of the aforementioned embodiments. Furthermore, the microstructure of Comparative Example 1 contained a certain amount of lamellar pearlite. Since lamellar pearlite is prone to cracking during subsequent processing, the tensile strength of the fine-stamped steel obtained from Comparative Example 3 after fine blanking and heat treatment was not only lower than that of Examples 1-6, but it was also prone to cracking during fine blanking and heat treatment.

[0096] In Comparative Example 4, the Ti element content was not within the range of 0.01-0.05 wt%, the final rolling pressure was not within the range of 150-200 tons, and the annealing temperature was not within the range of 660-710℃. Consequently, the diameter of the resulting spherical cementite was not within the 1.0-2.4 μm range disclosed in this invention. Therefore, the tensile strength and yield strength of Comparative Example 4 were also lower than those of the aforementioned embodiments. Furthermore, the microstructure of Comparative Example 4 contained a certain amount of lamellar pearlite. Since lamellar pearlite is prone to cracking during subsequent processing, the tensile strength of the fine-stamped steel obtained from Comparative Example 4 after fine blanking and heat treatment was not only lower than that of Examples 1-6, but it was also prone to cracking during fine blanking and heat treatment.

[0097] While the present invention 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 invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A strip steel, characterized in that, It contains the following mass percentages of chemical elements: C: 0.16-0.27wt%, Si: 0.01-0.5wt%, Mn: 0.5-1.5wt%, Al: 0.01-0.1wt%, Cr: 0.6-1.2wt%, Ti: 0.01-0.05wt%, B: 0.0001-0.005wt%, N≤0.006wt%, the balance being iron and inevitable impurities, wherein the microstructure of the strip steel comprises granular pearlite, wherein the diameter of the spheroidal cementite particles in the granular pearlite is 1.0-2.4μm.

2. The strip steel of claim 1 wherein, Wherein B and Cr satisfy: 200*B wt%+Cr wt%>1.0wt%.

3. The strip steel of claim 1 wherein, The microstructure of the strip steel further comprises ferrite, the volume percentage of the ferrite being≤10%, the volume percentage of the granular pearlite being≥90%.

4. The strip steel of claim 1 wherein, Wherein, Al: 0.01-0.08wt%, N<0.0045%.

5. The strip steel of claim 1 wherein, The strip steel further contains P and S, wherein P≤0.015wt% and S≤0.01wt% by mass percentage.

6. The strip steel of claim 1 wherein, Wherein, The tensile strength of the strip steel is≤460MPa and the elongation is≥25%.

7. A fine-blanking steel material, characterized by, The fine blanking steel material is obtained by fine blanking and heat treatment of the strip steel according to any one of claims 1-6.

8. The fine-blanking steel material according to claim 7, wherein The fine blanking steel material has a tensile strength≥1500MPa and a yield strength≥1000MPa; and / or the microstructure of the fine blanking steel material is lath-shaped martensite structure.

9. A manufacturing method for manufacturing the strip steel according to any one of claims 1 to 6, characterized by, Comprise: Smelting; Continuous casting; Hot rolling: comprising multiple passes of reduction of the strip steel, wherein the reduction rate of the last pass is 10%-20% and the rolling pressure of the last pass is 150-200 tons; Pickling and batch annealing, wherein the annealing temperature in the batch annealing is 660-710℃; Wherein, the manufacturing method does not perform cold rolling.

10. The production method according to claim 9, wherein In the hot rolling step, the reduction rate of the last pass is 10-15% and the rolling pressure of the last pass is 160-200 tons.

11. The production method according to claim 9, wherein In the hot rolling step, the heating temperature is controlled to be 1230-1280℃ and the finish rolling inlet temperature is≥1200℃.

12. The production method according to claim 9, wherein The multiple passes are six passes, wherein the reduction rate of the first pass F1 to the third pass F3 is≥50%, the pass temperature of the first pass F1 to the third pass F3 is≥1000℃, the reduction rate of the fourth pass F4 is≥40%, the reduction rate of the fifth pass F5 is≥40%, and the finish rolling temperature is 870-930℃.

13. The production method according to claim 9, wherein The manufacturing method further comprises a laminar cooling step between the hot rolling step and the pickling and batch annealing step, wherein the process conditions of the laminar cooling step are: the water cooling speed is 70-120℃ / s, after being cooled to 560-620℃, the strip steel is cooled to 450-550℃ at a water cooling speed≤10℃ / s and is coiled.

14. The production method according to claim 9, wherein In the pickling and batch annealing step, the annealing time is 10h-30h.

15. A manufacturing method for manufacturing a fine-blanking steel material, characterized by, The strip steel is obtained by the manufacturing method according to any one of claims 9-14, and then the strip steel is subjected to fine blanking and heat treatment to obtain the fine blanking steel material.

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