Hot-rolled steel sheet having high hole expandability and method for manufacturing the same

Abstract

The hot-rolled steel sheet has, by weight, a chemical composition comprising: 0.15% ≤ C ≤ 0.20%, 0.50% ≤ Mn ≤ 2.00%, 0.25% ≤ Si ≤ 1.25%, 0.10% ≤ Al ≤ 1.00% with 1.00% ≤ (Al + Si) ≤ 2.00%, 0.001% ≤ Cr ≤ 0.250%, P ≤ 0.02%, S ≤ 0.005%, N ≤ 0.008%, and optionally one or more of the following elements: 0.005% ≤ Mo ≤ 0.250%, 0.005% ≤ V ≤ 0.250%, 0.0001% ≤ Ca ≤ 0.003%, and 0.001% ≤ Ti ≤ 0.025%, the remainder being Fe and unavoidable impurities, wherein the microstructure comprises ferrite and bainite for a sum of surface fraction greater than 5% and strictly lower than 20%, the remainder being constituted of tempered martensite; and the hot-rolled steel sheet comprises a ferrite layer at the surface having a thickness less than 5% of the thickness of the hot-rolled steel sheet.

Description

[0001] This application is a divisional application of the invention patent application filed on September 9, 2019, with application number "201980058734.6" and invention title "Hot-rolled steel plate with high hole expansion ratio and manufacturing method thereof". Technical Field

[0002] The present invention relates to hot-rolled steel sheets having a yield strength of 780 MPa to 1000 MPa, a tensile strength of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa, and a hole expansion ratio of more than 45%, which can be used to manufacture structural components of vehicles. Background Technology

[0003] Reducing vehicle weight to decrease CO2 emissions is a major challenge for the automotive industry. This weight reduction must be combined with safety requirements. The steelmaking industry is constantly developing new high-strength steels to meet these requirements. With the increasing use of high-strength steels in automotive applications, the demand for steels that offer both increased strength and improved hole-opening properties is growing. Consequently, several families of steels offering various strength grades have been proposed.

[0004] Publication EP1138796 describes hot-rolled steel sheets with tensile strengths exceeding 1000 MPa, suitable for use in vehicle components. The manufacture of such hot-rolled steel sheets necessitates expensive alloying elements, such as molybdenum, which, due to its hardening effect, enables the acquisition of a fully bainitic structure and high mechanical properties; and vanadium, which allows for the acquisition of fine nitrides and carbides, as well as high levels of tensile mechanical properties.

[0005] In WO2018108653, hot-rolled flat steel sheets are produced with tensile strengths of 800 MPa to 1500 MPa, yield strengths greater than 700 MPa, elongation of 7% to 25%, and expansion values ​​greater than 20%. These martensitic hot-rolled steel sheets are produced via a so-called quenching and partitioning process, in which the sheet is first cooled within a range where the martensite transformation is incomplete. Then, the sheet is reheated within a temperature range where carbon is partitioned (i.e., diffuses from martensite and enriches austenite) to stabilize it. The sheet is then cooled to room temperature. Therefore, the final steel sheet contains partitioned martensite, fresh martensite, and retained austenite. However, implementing this method requires specific equipment and production lines.

[0006] Publication WO2012130434 describes a heat treatment that is variable in width over a coated plate having a duplex or martensitic microstructure, thereby obtaining a metal sheet with customized mechanical properties across the width of the metal strip. However, this method requires specific and dedicated production equipment. Furthermore, localized heat treatment can introduce residual stress and flatness issues. Summary of the Invention

[0007] One object of the present invention is to provide high-strength hot-rolled steel sheets without the need for the addition of large amounts of expensive elements.

[0008] Another objective of this invention is to manufacture hot-rolled steel sheets using conventional production lines without increasing manufacturing costs.

[0009] Therefore, the present invention aims to provide flat hot-rolled high-strength steel with a yield strength of 780 MPa to 1000 MPa, a tensile strength TS of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa, a total elongation of more than 8%, and a HER of more than 45%.

[0010] Another object of the invention is to provide a steel plate with high resistance to crack initiation and propagation, thereby preventing any brittle fracture of components made from the steel plate. To this end, the invention aims to provide a steel plate with a strength exceeding 50 J / cm² at 20°C. 2 Charpy V fracture energy of flat hot-rolled steel sheet.

[0011] This invention relates to hot-rolled steel sheets having, by weight percent, the following chemical composition: 0.15% ≤ C ≤ 0.20%, 0.50% ≤ Mn ≤ 2.00%, 0.25% ≤ Si ≤ 1.25%, 0.10% ≤ Al ≤ 1.00%, wherein 1.00 ≤ (Al + Si)≤2.00, 0.001%≤Cr≤0.250%, P≤0.02%, S≤0.005%, N≤0.008%, and optionally one or more of the following elements: 0.005%≤Mo≤0.250%, 0.005%≤V≤0.250%, 0.0001%≤Ca≤0.0030% and 0.001%≤Ti≤0.025%, with the remainder being iron and unavoidable impurities, and wherein the microstructure comprises ferrite and bainite with a total surface fraction greater than 5% and strictly less than 20%, with the remainder consisting of tempered martensite.

[0012] In a preferred embodiment, the silicon content is 0.40% to 0.90%.

[0013] In another preferred embodiment, the aluminum content is 0.30% to 0.90%.

[0014] In another preferred embodiment, the sum of the aluminum content and the silicon content is 1.20% to 2.00%.

[0015] The hot-rolled steel sheet of the present invention has a yield strength YS of 780 MPa to 1000 MPa and a tensile strength TS of 950 MPa to 1150 MPa, preferably 980 MPa to 1150 MPa.

[0016] According to the present invention, the total elongation of the steel is greater than 8%.

[0017] According to the present invention, the hole expansion value of the steel is higher than 45%.

[0018] According to the present invention, the Charpy V energy of steel at 20°C is higher than 50 J / cm². 2 .

[0019] The thickness of the steel in this invention is 1.8 mm to 4.5 mm, preferably 1.8 mm to 3.5 mm.

[0020] According to the present invention, the hot-rolled steel sheet includes a ferrite layer at its surface having a thickness of less than 5% of the thickness of the hot-rolled steel sheet.

[0021] According to the present invention, the hot-rolled steel sheet is coated with zinc or a zinc-based alloy.

[0022] In a first embodiment, the zinc-based coating comprises 0.01 wt% to 8.0 wt% Al, optionally 0.2 wt% to 8.0 wt% Mg, with the remainder being Zn.

[0023] In the second embodiment, the zinc-based coating comprises 0.15% to 0.40% by weight of Al, with the remainder being Zn.

[0024] This invention provides a method for producing hot-rolled steel sheets, comprising the following steps in sequence:

[0025] - Provide a steel semi-finished product having the above composition, then

[0026] - The steel semi-finished product is hot-rolled at a final rolling temperature of 875°C to 950°C to obtain a steel plate, and then...

[0027] - Cool the steel plate at a rate V higher than 50°C / second. R1 Cooling, thus obtaining a cooled steel sheet, then

[0028] - Coiling is performed at a temperature T below 160°C and below Mf to obtain a coiled steel sheet, then...

[0029] - The wound plate is heat-treated to a heat treatment temperature θ. A Duration t A θ A and t A To make P A =θ A (22+log 10 t A) θ is between 15400 and 17500. A Let K represent tA Expressed in hours.

[0030] In a first embodiment of the invention, the heat treatment temperature θ is between 400°C and 475°C in an inert atmosphere or an HNX atmosphere. A The heat treatment step of the manufacturing method is carried out in batch processing, and the duration t of the heat treatment temperature is... A The time ranges from 10 to 25 hours.

[0031] In a second embodiment of the present invention, the heat treatment step is carried out on a continuous annealing line at a heat treatment temperature θ of 500°C to 600°C. A The duration t at the heat treatment temperature A The duration is 40 to 100 seconds, preferably 50 to 100 seconds.

[0032] In a preferred embodiment of the present invention, P A The parameter is in the range of 15,500 to 17,000.

[0033] The manufacturing method also includes an acid pickling step after the winding step and before the heat treatment.

[0034] The manufacturing method also includes a pickling step following the heat treatment.

[0035] In an embodiment of the first cooling scheme of the present invention, water cooling is used at a cooling rate V higher than 75°C / second. R1 To cool it down.

[0036] In the second cooling scheme, the cooling rate V is... R1 Cool down until it reaches an intermediate temperature T of 500°C to 550°C. i Then from T i start,

[0037] - Further air cooling is performed for a duration t2 of 1 to 5 seconds, then

[0038] - Allow the plate to cool at a rate V greater than 40°C / second. R2 cool down.

[0039] In a preferred embodiment of the invention, the air cooling is performed for a duration t2 of 2 to 3 seconds.

[0040] The steel plate according to the present invention can be used to manufacture structural components of vehicles. Attached Figure Description

[0041] The invention will now be described in more detail with reference to the accompanying drawings, but without introducing limitations:

[0042] - Figure 1The following is shown as the heat treatment parameter P for the steel composition according to the present invention. A =θ A (22+log 10 t A The change in pore expansion ratio HER is a function of ).

[0043] - Figure 2 The parameter P is shown as a parameter for the steel composition according to the present invention. A The change in tensile strength as a function of .

[0044] - Figure 3 An example of the microstructure of a hot-rolled steel sheet according to the present invention is shown.

[0045] - Figure 4 Examples of microstructures of hot-rolled steel sheets that do not correspond to the present invention are shown.

[0046] - Figure 5 The microstructure of an embodiment according to the invention is shown, wherein the steel plate comprises a ferrite layer on its surface. Detailed Implementation

[0047] In the following description of the invention, the yield stress YS, tensile strength TS, and total elongation of the steel plate are referenced to standard JIS Z2241. The expansion ratio HER is referenced to standard ISO 16630:2009.

[0048] To achieve the desired microstructure and mechanical properties, chemical composition and processing parameters are crucial. The steel composition, expressed as a weight percentage, is as follows:

[0049] - 0.15%≤C≤0.20%: If the carbon content is less than 0.15%, a tensile strength of 950 MPa cannot be achieved. If the carbon content is higher than 0.20%, the yield strength and tensile strength can exceed 1000 MPa and 1150 MPa respectively, and the total elongation can be less than 8%.

[0050] - 0.50%≤Mn≤2.00%: When the manganese content is below 0.50%, the hardenability of the steel decreases, and the sum of the ferrite surface fraction and the bainite surface fraction cannot be strictly lower than 20%, thus the tensile strength can be lower than 950 MPa. If the manganese content is greater than 2.00%, the risk of center segregation increases, thereby impairing the yield strength, tensile strength, and porosity.

[0051] - 0.25% ≤ Si ≤ 1.25%: Silicon is an element used for deoxidation in the liquid stage and for achieving solution hardening. If the Si content is less than 0.25%, the hardenability of the steel decreases. However, if the Si content exceeds 1.25%, the kinetics of carbide formation decrease. Therefore, the ferrite content can be higher than 20%, and the tensile strength can be lower than 950 MPa. In a preferred embodiment, the silicon content is 0.40% to 0.90%.

[0052] - 0.10% ≤ Al ≤ 1.00%: The addition of aluminum facilitates effective deoxidation in the liquid stage and promotes ferrite stabilization. If the aluminum content is below 0.10%, the sum of the ferrite surface fraction and bainite surface fraction in the hot-rolled sheet can be below 5%, and therefore the total elongation of the sheet can be below 8%. Above 1.00%, excessive ferrite will form during cooling, thus failing to achieve the yield strength and tensile strength levels required in this invention. In a preferred embodiment, the aluminum content is 0.30% to 0.90%.

[0053] - 1.00≤Al+Si≤2.00: When the sum of silicon and aluminum content is 1.00% to 2.00%, this allows for a microstructure containing more than 5% and less than 20% ferrite and bainite, thus resulting in increased ductility and elongation. In a preferred embodiment, the sum of silicon and aluminum content is 1.20% to 2.00% to promote the formation of a ferrite layer on the main surface of the steel sheet. The ferrite layer allows for a bending radius less than 1 divided by the sheet thickness in the rolling direction and less than 1.5 divided by the sheet thickness in the transverse direction.

[0054] - P≤0.02%: If the phosphorus content exceeds 0.02%, segregation can occur at the grain boundaries, which can reduce the elongation of the steel plate.

[0055] Furthermore, when the coiled steel sheet undergoes further heat treatment, phosphorus at such a high content can cause temper embrittlement. Preferably, the phosphorus content is higher than 0.0005% because obtaining lower levels of phosphorus in the steelmaking plant is expensive, without corresponding significant benefits to mechanical properties.

[0056] - S≤0.005%: The sulfur content is limited to 0.005% to reduce the formation of sulfides that are detrimental to the ductility of the sheet. Preferably, the sulfur content is higher than 0.0005% because achieving lower levels during steelmaking is very expensive, and there is no corresponding significant benefit in terms of mechanical properties.

[0057] - N ≤ 0.008%: If the nitrogen content exceeds 0.008%, some elements may precipitate in liquid or solid form as nitrides or carbonitrides. Since coarse precipitates reduce the ductility of hot-rolled steel sheets, they must be avoided. Preferably, the nitrogen content is above 0.001%. However, reducing the nitrogen content to less than 0.001% is expensive and does not result in a significant improvement in mechanical properties.

[0058] - 0.001%≤Cr≤0.250%: Chromium improves hardenability. If the Cr content is less than 0.001%, hardenability cannot be obtained. If the Cr content exceeds 0.250%, the risk of macrosegregation and microsegregation increases, and therefore the tensile strength may be lower than 950 MPa.

[0059] - 0.005%≤Mo≤0.250%: Molybdenum can be added as an optional element to increase hardenability, i.e., to make it easier to achieve martensite formation during cooling. Below 0.005%, this effective effect cannot be achieved. However, since molybdenum is an expensive element, its content is limited to 0.250%, making the manufacture of steel plates cost-effective.

[0060] - 0.005%≤V≤0.250%: Vanadium is an optional element that allows for the production of steel sheets with high toughness after batch heat treatment. However, adding more than 0.250% is not cost-effective.

[0061] - 0.0001%≤Ca≤0.0030%: Calcium can also be added as an optional element. The addition of Ca in the liquid stage allows for the formation of fine oxides or oxysulfides. These particles act as nucleating agents for the subsequent fine precipitation of titanium nitrides / carbonitrides. The reduction in carbonitride size enables improved porosity.

[0062] - 0.001%≤Ti≤0.025%: Titanium can also be added as an optional element. When titanium is above 0.025%, it tends to precipitate in the liquid phase as coarse titanium nitrides, which reduces the ductility of the plate. However, reducing titanium to levels below 0.001% is difficult in the industrial stage without providing any additional benefit to the mechanical properties.

[0063] The remaining components are iron and unavoidable impurities produced during smelting.

[0064] The microstructure of the hot-rolled steel sheet according to the present invention will now be described in detail.

[0065] According to the present invention, the sum of ferrite and bainite is greater than 5% and strictly less than 20%. If their sum is not strictly less than 20%, the yield strength and tensile strength decrease and cannot reach the minimum values ​​of 780 MPa and 950 MPa, respectively. Furthermore, the porosity will be low. When the ferrite and bainite content is less than 5%, the ductility of the steel plate decreases.

[0066] The remaining portion of the microstructure consists of tempered martensite. Within the framework of this invention, tempered martensite is defined as recycled martensite containing precipitated cementite that can coalesce at the highest tempering temperature. Its characteristics correspond to stage 3 of so-called martensitic tempering, described in the following publication: A. Constant, G. Henry, JC Charbonnier: “Principes de bases des traitements thermiques, thermomécaniqueset thermochimiques des aciers”, PYC edition, 1992, pp. 190-191.

[0067] According to the present invention, steel sheets are manufactured by a hot rolling process. This allows for the production of steel sheets having two parallel and opposing main surfaces, which also have an edge that can be designated as a second surface. According to one embodiment of the invention, the hot-rolled steel sheet includes a ferrite layer at its main surfaces, the ferrite layer having a thickness less than 5% of the thickness of the hot-rolled steel sheet.

[0068] The method for manufacturing hot-rolled steel plates will now be described.

[0069] The semi-finished product, which can be further hot-rolled, has the steel composition described above. This semi-finished product can be in the form of an ingot or slab obtained by continuous casting, typically with a thickness of about 200 mm. Alternatively, the semi-finished product can also be in the form of a thin slab with a thickness of several tens of millimeters, or it can be a plate obtained by direct casting between counter-rotating rolls. The semi-finished product is heated to a temperature above 1150°C, making it easy to hot-roll, with the final hot-rolling temperature being between 875°C and 950°C. Hot rolling at temperatures below 875°C promotes austenite formation, followed by excessive ferrite formation during the cooling process, which reduces formability. If the hot-rolling temperature exceeds 950°C, the tendency for scaling increases, resulting in poor surface quality of the product.

[0070] Then, the hot-rolled product is cooled at a rate V of at least 50°C / second. R1Cooling is necessary to prevent ferrite formation until the coiling temperature is below 160°C and also below Mf, which represents the end temperature of the austenite-to-martensite transformation. According to Malcom Blair and Thomas L. Stevens' publication, "Steel castings Handbook - 6th Edition," the martensite termination temperature Mf is 245°C lower than the martensite initiation temperature Ms. This can be calculated using a formula derived by Andrews and published in the Journal of the Iron and Steel Institute, 203, 721-727, 1965.

[0071]

[0072] In a preferred embodiment, the hot-rolled product is wound at a temperature below 160°C and below (Mf-10°C). In this way, high microstructure uniformity is achieved along all the steel strips.

[0073] In one implementation of the cooling scheme, by a cooling rate V higher than 75°C / second R1 The cooling step is performed by a single-step water cooling process to obtain a martensitic microstructure matrix containing ferrite and bainite, wherein the sum of ferrite and bainite in the surface region is greater than 5% and strictly less than 20%.

[0074] In another embodiment of the cooling scheme, the cooling steps are performed through multi-step cooling, wherein the first cooling step is at the cooling rate V. R1 To achieve an intermediate temperature T of 500℃ to 550℃ i Then, air cooling is immediately performed for 1 to 5 seconds, preferably 2 to 3 seconds, t2, followed by a final cooling step at a cooling rate of over 40°C / second. This multi-step cooling allows for partial ferrite or bainite transformation, thus yielding 5% to 20% ferrite plus bainite within a martensitic matrix.

[0075] Regardless of the cooling method used, after winding, the temperature θ is then maintained. A Heat treatment of hot-rolled steel for a duration of t A , t A Indicates at temperature θ A The duration under θ A and t A Make the heat treatment parameter: P A =θ A (22+log 10 t A The value is between 15400 and 17500. Therefore, P... AThe combined thermal effects of temperature and duration were taken into account.

[0076] Based on existing technology, it is known that at high parameters P A At values ​​of P, some mechanical properties, such as the cavity expansion value and total elongation, are improved. Conversely, when parameter P... A As the value increases, the yield strength and tensile strength decrease. For martensitic steel, WO2012130434 discloses that when P... A The mechanical properties are optimal when the value is between 13,000 and 15,000. Specifically, the hole enlargement value increases with P. A And it increases continuously. For example Figure 1 As shown, in a surprising way, the present invention demonstrates that the aperture expansion value is related to P. A It decreases significantly above a specific value (approximately 16,000). Therefore, as by Figure 1 and Figure 2 As shown, when P A When the value is between 15,400 and 17,500, and particularly between 15,500 and 17,000, the present invention can obtain the desired mechanical properties.

[0077] According to the present invention, the heat treatment steps can be performed in a discontinuous (intermittent) or continuous manner.

[0078] In a first embodiment of the present invention, the heat treatment temperature θ is between 400°C and 475°C. A The manufacturing method involves a heat treatment step in which hot-rolled sheets are batch-coiled in a furnace with an inert atmosphere or an HNX atmosphere, and the duration t at the heat treatment temperature is specified. A The tempering process takes 10 to 25 hours to obtain a tempered martensitic matrix that combines good formability and tensile properties.

[0079] In a second embodiment of the present invention, the heat treatment step is carried out on a continuous annealing line at a heat treatment temperature θ of 500°C to 600°C. A The duration t at the heat treatment temperature A The time is 40 to 100 seconds, preferably 50 to 100 seconds, to obtain a tempered martensitic matrix that combines good formability and tensile properties.

[0080] A first pickling step can be added after winding, followed by a second pickling step after heat treatment, to remove surface oxides.

[0081] According to existing technology, martensitic steels that are subsequently tempered and slowly cooled may exhibit low toughness. In this invention, the steel composition and heat treatment conditions have been defined to obtain a toughness of at least 50 J / cm² at 20°C on the final hot-rolled steel sheet. 2The Charpy V energy is high. Therefore, the resulting steel plate does not exhibit temper embrittlement.

[0082] The thickness of hot-rolled steel sheets is typically 1.8 mm to 4.5 mm, preferably 1.8 mm to 3.5 mm.

[0083] The present invention will now be illustrated by the following embodiments, which are in no way limiting.

[0084] Example 1

[0085] Semi-finished products in casting form with thicknesses ranging from 28 mm to 40 mm have the compositions detailed in Table 1. For different compositions, the calcium content is 0.002% by weight, with the remainder being iron and impurities from smelting. The martensitic end temperature is calculated from the value of the martensitic initiation temperature according to: Mf = Ms - 245℃. These semi-finished products are heated at temperatures above 1150℃ and further hot-rolled to thicknesses ranging from 1.8 mm to 4.5 mm. Table 2 details the manufacturing conditions applied. Tests 1 to 15 correspond to implementation schemes of the first cooling scheme described above, and tests 16 to 18 correspond to the conditions of the second cooling scheme described above. Pickling steps are performed after coiling and after heat treatment. In tests 4 and 9, the hot-rolled steel sheets are galvanized (GI).

[0086]

[0087] Table 1. Steel composition (wt%) and martensitic transformation temperature.

[0088] Underlined value: does not correspond to this invention.

[0089]

[0090] Table 2. Underlined values: not corresponding to this invention.

[0091] The microstructure of heat-treated steel sheets was determined on polished specimens etched with Nital and observed using optical and scanning electron microscopy. The surface fraction of different components in the microstructure was measured by image analysis combined with quantification. Furthermore, the final presence of a ferrite layer on the main surface of the steel sheet was evaluated. The component proportions and the final ferrite layer thickness are recorded in Table 3. Table 4 summarizes the mechanical properties of the final heat-treated steel sheets. The yield stress YS, ultimate tensile strength TS, and total elongation have been determined according to standard JIS Z2241. The porosity has been determined according to ISO 16630:2009.

[0092] Charpy V energy has been measured on samples with subthickness dimensions at 20°C. The measured fracture energy is divided by the ligament area under the V notch of the test sample.

[0093] The hole enlargement method involves measuring the initial hole diameter Di (nominal: 10 mm) before stamping and then measuring the final hole diameter Df after stamping to determine when a through crack is observed along the thickness direction of the plate at the edge of the hole. The hole enlargement capacity Ac% is determined according to the following formula: Ac = 100 * (Df - Di) / Di. Therefore, Ac is used to quantify the plate's ability to withstand stamping at the level of the cut hole.

[0094]

[0095] Table 3: Microstructure characteristics of the heat-treated steel plate

[0096] Underlined value: does not correspond to this invention.

[0097] na: Not evaluated

[0098]

[0099] Table 4: Mechanical properties of the heat-treated steel plates.

[0100] Underlined values: TS, YS, total elongation or HER value is insufficient.

[0101] In experiments 1 to 7 and 16 to 17, the composition and manufacturing conditions corresponded to those of the present invention. Therefore, the desired microstructure was obtained. Figure 3 The microstructure obtained in Experiment 7 is shown: it contains 89% tempered martensite and 11% ferrite and bainite. As a result, high tensile properties and high porosity were obtained. Due to the Charpy energy at 20°C being much higher than 50 J / cm², the plate exhibits high toughness.

[0102] In tests 1 to 3, 6 to 7, and 16 to 17, a ferrite layer was present on the main surface of the steel plate, thus enabling the achievement of higher bending characteristics. In particular, for test 7, the bending radius divided by the plate thickness was less than 1 in the rolling direction and less than 1.5 in the transverse direction, indicating excellent bending characteristics.

[0103] Figure 5 a) and b) show the ferrite layers present on two opposing main surfaces of the steel plate in the manufactured plate during test 7.

[0104] Tests 8 to 11 and 18 did not match the manufacturing conditions specified in the invention. As a result, the heat-treated steel sheets did not meet the required mechanical properties.

[0105] In fact, in Experiment 8, the coiling temperature was above 160°C, exceeding the martensite end-transformation temperature. This resulted in an excessive amount of ferrite, reducing the tensile strength and porosity.

[0106] In experiments 9 and 10, parameter P A The temperature exceeds 17,500, and the batch heat treatment temperature exceeds 475°C. The final microstructure contains 80% tempered martensite, therefore the tensile strength does not match 950 MPa.

[0107] In Experiment 11, the final hot rolling temperature was below 875°C. Therefore, during cooling, austenite formation was promoted and excessive ferrite was produced. Figure 4 The microstructure obtained in Experiment 11 is shown, which contains 60% tempered martensite and 40% ferrite and bainite. Therefore, the yield strength, tensile strength, and porosity are insufficient.

[0108] In Experiment 18, the intermediate duration t2 in the cooling scheme was greater than 5 seconds. This resulted in the formation of excessive ferrite and bainite, reducing yield strength, tensile strength, and porosity.

[0109] In experiments 12 to 15, the steel composition was outside the scope of this invention. Therefore, the final steel sheet did not match the mechanical and microstructural characteristics.

[0110] In Experiment 12, the carbon, manganese, and silicon content of the steel exceeded the values ​​defined by this invention. Therefore, insufficient amounts of ferrite and bainite were present, and the porosity was inadequate.

[0111] Conversely, in Experiment 13, the carbon content was less than 0.15%, resulting in insufficient tensile strength and porosity values.

[0112] In Experiment 14, the carbon, silicon, aluminum, and chromium content of the steel were not according to the invention. In particular, due to the low carbon content, an excessive amount of ferrite and bainite was produced, which made it impossible to obtain sufficient tensile stress and porosity.

[0113] Ultimately, in Experiment 15, the manganese content was higher than 2%. As a result, insufficient amounts of ferrite and bainite were obtained, and the porosity did not reach 45%.

[0114] Therefore, the steel sheet according to the invention can be advantageously used to manufacture structural components of vehicles.

Claims

1. A hot-rolled steel sheet, by weight, said hot-rolled steel sheet having the following chemical composition: 0.15％≤C≤0.20％ 0.50% ≤ Mn ≤ 2.00% 0.25% ≤ Si ≤ 1.25% 0.10％≤Al≤1.00％， Where 1.00% ≤ (Al + Si) ≤ 2.00%, 0.001％≤Cr≤0.250％ P≤0.02％ S≤0.005％ N≤0.008％ And optionally one or more of the following elements: 0.005% ≤ Mo ≤ 0.250% 0.005％≤V≤0.250％ 0.0001% ≤ Ca ≤ 0.003% and 0.001% ≤ Ti ≤ 0.025%, The remainder consists of Fe and unavoidable impurities, wherein the microstructure comprises ferrite and bainite with a total surface fraction greater than 5% and strictly less than 20%, and the remainder consists of tempered martensite; and The hot-rolled steel sheet includes a ferrite layer at its surface with a thickness less than 5% of the thickness of the hot-rolled steel sheet.

2. The hot-rolled steel sheet according to claim 1, wherein the Si content is 0.40% to 0.90%.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein Al is included in the form of 0.30% to 0.90%.

4. The hot-rolled steel sheet according to claim 1 or 2, wherein the Al+Si content is 1.20% to 2.00%.

5. The hot-rolled steel sheet according to claim 1 or 2, wherein the yield strength YS is from 780 MPa to 1000 MPa, and the tensile strength TS is from 950 MPa to 1150 MPa.

6. The hot-rolled steel sheet according to claim 1 or 2, wherein the total elongation is greater than 8%.

7. The hot-rolled steel sheet according to claim 1 or 2, wherein the hole expansion property (HER) is greater than 45%.

8. The hot-rolled steel sheet according to claim 1 or 2, wherein the Charpy V energy at 20°C is higher than 50 J / cm². 2 .

9. The hot-rolled steel sheet according to claim 1 or 2, wherein the thickness is from 1.8 mm to 4.5 mm.

10. The hot-rolled steel sheet according to claim 1 or 2, wherein the hot-rolled steel sheet is coated with zinc or a zinc-based alloy.

11. The hot-rolled steel sheet of claim 10, wherein the zinc-based coating comprises 0.01 wt% to 8.0 wt% Al, optionally 0.2 wt% to 8.0 wt% Mg, with the remainder being Zn.

12. The hot-rolled steel sheet of claim 10, wherein the zinc-based coating comprises 0.15% to 0.40% by weight of Al, with the remainder being Zn.

13. A method for manufacturing hot-rolled steel sheet, the method comprising the following steps in sequence: - Provide a steel semi-finished product having the composition according to any one of claims 1 to 4, then - The steel semi-finished product is hot-rolled at a final rolling temperature of 875°C to 950°C to obtain a steel plate, and then... - Cool the steel plate at a rate of at least 50°C / second. R1 Cooling, thus obtaining a cooled steel sheet, then -At temperatures below 160°C and below Mf, T 卷取 The steel sheet is then wound up to obtain a coiled steel plate. - The wound plate is heat-treated to a heat treatment temperature θ. A Duration t A θ A and t A To make P A =θ A (22+log 10 t A ) is between 15400 and 17500, θ A Let K represent the number of t. A Expressed in hours; This results in the microstructure of the hot-rolled steel sheet containing a total surface fraction of ferrite and bainite greater than 5% and strictly less than 20%, and the surface of the hot-rolled steel sheet having a ferrite layer with a thickness less than 5% of the thickness of the hot-rolled steel sheet.

14. The method for manufacturing hot-rolled steel sheet according to claim 13, wherein the heat treatment step is performed at a heat treatment temperature θ of 400°C to 475°C in an inert atmosphere or an HNX atmosphere. A The process is carried out in batches, with the duration t at the annealing temperature. A The time ranges from 10 to 25 hours.

15. The method for manufacturing hot-rolled steel sheet according to claim 13, wherein the heat treatment step is carried out on a continuous annealing line at a heat treatment temperature θ of 500°C to 600°C. A The duration t at the heat treatment temperature A The duration is 40 to 100 seconds.

16. The method for manufacturing hot-rolled steel sheet according to any one of claims 13 to 15, wherein P A The range is 15,500 to 17,000.

17. The method for manufacturing hot-rolled steel sheet according to any one of claims 13 to 15, further comprising a pickling step after the coiling step and before the heat treatment step.

18. The method for manufacturing according to any one of claims 13 to 15, further comprising a pickling step following the heat treatment step.

19. The method for manufacturing according to any one of claims 13 to 15, wherein the cooling is performed by water cooling, and wherein V R1 Above 75℃ / second.

20. The method for manufacturing according to any one of claims 13 to 15, wherein the cooling rate V is... R1 The cooling steps described below are to achieve an intermediate temperature T of 500°C to 550°C. i ,Then, - Perform another air cooling step for a duration of 1 to 5 seconds, t2, then -The plate is cooled at a rate V greater than 40°C / second. R2 cool down.

21. The manufacturing method according to claim 20, wherein the air cooling step is performed for a duration t2 of 2 to 3 seconds.

22. Use of the hot-rolled steel sheet according to any one of claims 1 to 12 or the hot-rolled steel sheet manufactured according to any one of claims 13 to 21 for manufacturing structural components of a vehicle.

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