Steel sheet with excellent toughness, ductility, and strength, and method for manufacturing the same.

Abstract

To provide a cold rolled and heat treated steel sheet having a high combination of ductility and strength. The present invention provides a cold-rolled and heat-treated steel sheet having a composition including 0.1%≦C≦0.4%, 3.5%≦Mn≦8.0%, 0.1%≦Si≦1.5%, Al≦3%, Mo≦0.5%, Cr≦1%, Nb≦0.1%, Ti≦0.1%, V≦0.2%, B≦0.004%, 0.002%≦N≦0.013%, S≦0.003%, and P≦0.015%, whose surface structure is comprised of between 8 and 50% retained austenite with an average C content of at least 0.4%, up to 80% intercritical ferrite (the ferrite grains, if any, have an average size of up to 1.5 μm), up to 1% cementite (the cementite grains, if any, have an average size of less than 50 nm), martensite, and / or bainite.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing hot-rolled and annealed steel sheets that have high cold-rollability and toughness, and a combination of high ductility and strength, and is suitable for the production of cold-rolled and heat-treated steel sheets, and to hot-rolled and annealed steel sheets produced by this method.

[0002] The present invention relates to a method for manufacturing cold-rolled and heat-treated steel sheets having a high combination of ductility and strength, and to cold-rolled and heat-treated steel sheets obtained by this method. [Background technology]

[0003] In the automotive industry in particular, there is a continuous demand for vehicle weight reduction to improve fuel efficiency and enhanced safety through the use of steel with high tensile strength, from the perspective of protecting the global environment. Such steel can actually be used to manufacture parts with thinner thicknesses while guaranteeing the same or improved level of safety.

[0004] To this end, steels containing minute alloying elements have been proposed that achieve hardening simultaneously through precipitation and refinement of crystal grain size. Following the development of such steels, even stronger steels called advanced high-tensile steels have been developed, which maintain a good level of strength and good cold formability.

[0005] To achieve even higher tensile strength levels, steels exhibiting TRIP (transformation-induced plasticity) behavior with a highly favorable combination of properties (tensile strength / deformability) have been developed. These properties are related to the microstructure of such steels, which consists of a ferrite matrix containing bainite and retained austenite. The retained austenite is stabilized by the addition of silicon or aluminum, and these elements delay the precipitation of carbides in the austenite and bainite. The presence of retained austenite gives the undeformed sheet high ductility. Under the effect of subsequent deformation, for example, when stress is applied uniaxially, the retained austenite in parts made of TRIP steel gradually transforms into martensite, resulting in substantial hardening and delayed necking.

[0006] To achieve an improved combination of strength and ductility, the plate is made of austenite or Two-phase region The domain is annealed, cooled to a quenching temperature below the Ms transformation point, and then distribution The process of heating to a certain temperature and maintaining that temperature for a predetermined time is known as "quenching and distribution It was further proposed that the sheet be manufactured by the following method. The resulting steel sheet has a structure containing martensite and retained austenite, and optionally bainite and / or ferrite. The retained austenite is distribution Carbon from the martensite inside distribution It has a high carbon content due to this, and the martensite contains a low proportion of carbides.

[0007] All of these steel sheets exhibit a good balance of resistance and ductility.

[0008] However, new challenges arise regarding the manufacture of such sheets. In particular, the manufacturing method of such steel sheets generally involves casting a steel semi-finished product before heat treatment to impart the final properties to the steel, hot-rolling this semi-finished product to produce a hot-rolled steel sheet, and then winding this hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet is cold-rolled to a desired thickness and subjected to heat treatment selected according to the desired final structure and properties to obtain a cold-rolled and heat-treated steel sheet.

[0009] Due to the composition of these steels, a high level of resistance is achieved throughout the manufacturing process. In particular, hot-rolled steel sheets exhibit high hardness before cold rolling, which impairs their cold-rollability. As a result, the range of available sizes for cold-rolled sheets is narrowed.

[0010] To solve this problem, it has been proposed to batch anneal the hot-rolled steel sheets for several hours at a temperature generally between 500°C and 700°C before cold rolling.

[0011] Batch annealing actually reduces the hardness of hot-rolled steel sheets, and therefore improves their cold-rollability.

[0012] However, this solution is not entirely satisfactory.

[0013] In fact, batch annealing generally results in a decrease in the final properties of steel, particularly its ductility and strength.

[0014] Furthermore, hot-rolled steel sheets exhibit insufficient toughness after batch annealing, which can lead to band fracture during further processing. [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] Therefore, the present invention aims to provide a hot-rolled steel sheet and a method for manufacturing the same, which are suitable for the manufacture of cold-rolled and heat-treated steel sheets having high mechanical properties, particularly a combination of high ductility and strength, while also improving cold-rollability and toughness.

[0016] Furthermore, the present invention aims to provide a cold-rolled and heat-treated steel sheet having a combination of superior mechanical properties compared to a similar steel sheet manufactured by a method including batch annealing before cold rolling, and a method for manufacturing the same. [Means for solving the problem]

[0017] For this purpose, the present invention provides the following steps, namely - By weight percentage 0.1% ≦ C ≦ 0.4% 3.5% ≦ Mn ≦ 8.0% 0.1% ≦ Si ≦ 1.5% Al ≦ 3% Mo ≦ 0.5% Cr ≦ 1% Nb ≦ 0.1% Ti ≦ 0.1% V ≦ 0.2% B ≦ 0.004% 0.002% ≦ N ≦ 0.013% S ≦ 0.003% P ≦ 0.015% Casting a steel having a composition containing the above and the balance being iron and inevitable impurities resulting from steelmaking to obtain a steel semi-finished product - Reheating the steel semi-finished product to a temperature T included between 1150°C and 1300°C Reheating step - A hot rolling step of hot rolling the reheated semi-finished product at a temperature included between 800°C and 1250°C, wherein the final rolling temperature T FRT is 800°C or higher, thereby obtaining a hot rolled steel sheet - Cooling the hot rolled steel sheet at a cooling rate V included between 1°C / second and 150°C / second c1 to a coiling temperature T of 650°C or lower coil and coiling the hot rolled steel sheet at the coiling temperature T coil Thereafter - T ICAmin ~T ICAmax Continuous annealing temperature T included between ICA A step of continuously annealing the hot rolled steel sheet at the continuous annealing temperature T ICAmin = 650°C, T ICAmax is the temperature at which 30% austenite is generated during heating, and the hot rolled steel sheet is held at the continuous annealing temperature T ICA for a continuous annealing time t included between 3 seconds and 3600 seconds ICA Thereafter - A step of cooling the hot rolled steel sheet to room temperature, wherein the hot rolled steel sheet has an average cooling rate V of at least 1°C / second between 600 and 350°CICA A process to obtain a steel sheet that has been cooled, thereby hot-rolled and annealed. - A process to obtain cold-rolled steel sheets by cold-rolling steel sheets that have been hot-rolled and annealed at a cold-rolling reduction ratio between 30% and 70%. This relates to a method for manufacturing steel plates, including the method described above.

[0018] Preferably, the hot-rolled and annealed steel sheet has a surface fraction of - Ferrite (ferrite grains have an average size of up to 3 μm.) - Up to 30% austenite, - Up to 8% fresh martensite, and - Cementite with an average Mn content lower than 25% It has an organization consisting of [this].

[0019] Generally, hot-rolled and annealed steel sheets have a Vickers hardness lower than 400 HV.

[0020] Preferably, the hot-rolled and annealed steel sheet has a humidity of at least 50 J / cm² at 20°C. 2 It has a Charpy energy of .

[0021] Preferably, the method further includes a step of pickling the hot-rolled steel sheet between winding and continuous annealing, and / or after continuous annealing.

[0022] Preferably, continuous annealing time t ICA It falls between 200 seconds and 3600 seconds.

[0023] Preferably, this method further includes cold rolling, - Cold-rolled steel sheet annealing temperature T between 650 and 1000°C anneal Heat until, and - Cold-rolled steel sheet annealed at temperature T anneal The annealing time t is included between 30 seconds and 10 minutes. anneal Hold for the duration This includes the following.

[0024] In the first embodiment, the annealing temperature T anneal is T ICAmin It falls within the range of ~Ae3.

[0025] In the second embodiment, the annealing temperature T anneal It falls within the range of Ae3 to 1000℃.

[0026] According to one embodiment, this method further involves annealing a cold-rolled steel sheet at an annealing temperature T anneal The cooling rate V is included in the range of 1°C / sec to 70°C / sec from room temperature. c2 The process includes a step of cooling to obtain a cold-rolled and heat-treated steel sheet.

[0027] In another embodiment, this method further involves annealing a cold-rolled steel sheet at an annealing temperature T anneal After holding in place, the following sequential steps are taken, namely, - Cold-rolled steel sheet annealed at temperature T anneal The holding temperature T is included in the range of 350°C to 550°C. H Cooling rate V is included in the range of 1°C / sec to 70°C / sec. c2 The cooling process, - Holding time t included between 10 seconds and 500 seconds H The cold-rolled steel sheet is held at a temperature T H The process of holding it, then - Holding temperature T of cold-rolled steel sheet H The cooling rate V is included in the range of 1°C / sec to 70°C / sec from room temperature. c3 A process to obtain a steel sheet that has been cooled, cold-rolled, and heat-treated. Includes.

[0028] Preferably, the method involves a tempering temperature T between 170 and 450°C. T The tempering time t is included between 10 seconds and 1200 seconds. T The process further includes a step of tempering the cold-rolled and heat-treated steel sheet.

[0029] Preferably, the method further includes a step of coating the cold-rolled and heat-treated steel sheet with Zn or a Zn alloy or Al or an Al alloy.

[0030] In another embodiment, the method further includes the following steps: - The heated cold-rolled steel sheet is annealed at an annealing temperature T anneal From Mf+20℃ to the quenching temperature QT, which falls between Mf+20℃ and Ms-20℃, a cooling rate V high enough to avoid the formation of ferrite and pearlite during cooling is required. c4 The process of hardening, - Cold-rolled steel sheets are quenched at a temperature between 350°C and 500°C (QT). distribution temperature T P Reheat until the cold-rolled steel sheet is ready. distribution temperature T P It falls within the range of 3 seconds to 1000 seconds. distribution Time t P A process to maintain during - A process of cooling cold-rolled steel sheet to room temperature to obtain a cold-rolled and heat-treated steel sheet. Includes.

[0031] In the first modified example of this embodiment, the annealing temperature T anneal During annealing, the cold-rolled steel sheet has a surface fraction of - Ferrite between 10% and 45% - Austenite, and - Up to 0.3% cementite (cementite grains, if present, have an average size smaller than 50 nm). It is an organization that consists of such members.

[0032] In a second modification of this embodiment, the annealing temperature T anneal It is higher than Ae3, and cold-rolled steel sheets, during annealing, - Austenite, and - Up to 0.3% cementite (cementite grains, if present, have an average size smaller than 50 nm). It has an organization consisting of [this].

[0033] This cold-rolled steel sheet distribution temperature T PAfter maintaining this temperature, the cold-rolled steel sheet can be immediately cooled to room temperature.

[0034] In the modified form, distribution temperature T P During the period from holding the cold-rolled steel sheet in the galvanizing bath to its cooling to room temperature, the cold-rolled steel sheet is hot-dip plated in the galvanizing bath.

[0035] Preferably, the Si content in this composition is a maximum of 1.4%.

[0036] Furthermore, the present invention, in weight percent, 0.1% ≤ C ≤ 0.4% 3.5% ≤ Mn ≤ 8.0% 0.1% ≤ Si ≤ 1.5% Al ≤ 3% Mo ≤ 0.5% Cr ≤ 1% Nb ≤ 0.1% Ti ≤ 0.1% V ≤ 0.2% B ≤ 0.004% 0.002% ≤ N ≤ 0.013% S ≤ 0.003% P ≤ 0.015% Cold-rolled steel sheets are made from steel having a composition that includes, with the remainder being iron and unavoidable impurities resulting from smelting, in terms of surface fraction, - Residual austenite between 8 and 50%, - Up to 80% Two-phase region Ferrite (ferrite grains, if present, have an average size of up to 1.5 μm), - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm), - Martensite and / or bainite The invention also relates to cold-rolled and heat-treated steel sheets having a structure consisting of the above.

[0037] In one embodiment, the structure has at least 10% of the surface fraction Two-phase region Contains ferrite.

[0038] In another embodiment, the structure is, by surface fraction, - Residual austenite between 8 and 50%, - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm), - Martensite and / or bainite It consists of.

[0039] According to one embodiment, the martensite consists of tempered martensite and / or fresh martensite.

[0040] In the first modified example of this embodiment, the structure is, by surface fraction, - Having an average carbon content of at least 0.4% and an average manganese content of at least 1.3*Mn%, where Mn% represents the average manganese content in the steel composition, including retained austenite. - Between 40% and 80% Two-phase region Ferrite, - Up to 15% martensite and / or bainite, - Up to 0.3% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0041] In a second modification of this embodiment, the structure is, by surface fraction, - Retained austenite having an average carbon content of at least 0.4% between 8% and 30%. - Martensite and / or bainite between 70% and 92%, and - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0042] In another embodiment, the structure is, by surface fraction, - Up to 45% Two-phase region Ferrite, - Residual austenite between 8% and 30%, - distribution Martensite, - Up to 8% fresh martensite, and - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0043] In the first modified example of this embodiment, the structure is, by surface fraction, - Between 10% and 45% Two-phase region Ferrite, - Residual austenite between 8% and 30%, - distribution Martensite, - Up to 8% fresh martensite, and - Up to 0.3% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0044] In a second modification of this embodiment, the structure is, by surface fraction, - Residual austenite between 8% and 30%, - distribution Martensite, - Up to 8% fresh martensite, and - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0045] Preferably, the Si content in the composition is a maximum of 1.4%.

[0046] The present invention is described in detail below and illustrated by example with reference to the accompanying figures, without introducing any limitations. [Brief explanation of the drawing]

[0047] [Figure 1] These are micrographs showing the microstructure of hot-rolled and batch-annealed steel sheets for comparison. [Figure 2]This is a micrograph showing the microstructure of hot-rolled steel that has undergone continuous annealing according to the present invention. [Figure 3] This graph compares the mechanical properties of hot-rolled and batch-annealed steel sheets, or cold-rolled and heat-treated steel sheets manufactured from either hot-rolled and continuous steel sheets. [Modes for carrying out the invention]

[0048] According to the present invention, the carbon content is between 0.1% and 0.4%. Carbon is an element that stabilizes austenite. Below 0.1%, it is difficult to achieve a high level of tensile strength. If the carbon content exceeds 0.4%, cold rolling properties decrease and weldability deteriorates. Preferably, the carbon content is between 0.1% and 0.2%.

[0049] The manganese content falls between 3.5% and 8.0%. Manganese provides solid solution hardening and a refinement effect on the microstructure. Therefore, manganese contributes to an increase in tensile strength. At content above 3.5%, Mn is used to provide important stabilization of austenite in the microstructure throughout the entire manufacturing process and in the final structure. In particular, when the Mn content exceeds 3.5%, a final structure of cold-rolled and heat-treated steel sheets containing at least 8% retained austenite can be achieved. Furthermore, high ductility is obtained due to the stabilization of retained austenite by Mn. Above 8.0%, weldability deteriorates, and at the same time, segregation and inclusions degrade the damage properties.

[0050] Silicon is highly efficient at increasing strength and stabilizing austenite through solid solutions. Furthermore, silicon delays cementite formation during cooling by significantly slowing carbide precipitation. This is due to the fact that silicon has very low solubility in cementite, and Si increases the activity of carbon in austenite. Therefore, a process of displacing Si at the interface takes place prior to cementite formation. Thus, the enrichment of austenite by carbon leads to its stabilization at room temperature.

[0051] Therefore, the Si content is at least 0.1%. However, the Si content is limited to 1.5%. This is because exceeding this value results in excessively high rolling loads, making the hot rolling process difficult. Furthermore, cold rolling properties also decrease. In addition, excessively high Si content leads to the formation of silicon oxide on the surface, which impairs the coating properties of the steel.

[0052] The Si content is preferably a maximum of 1.4%. In fact, when the Si content is a maximum of 1.4%, the occurrence of red scale (also called tiger stripes) caused by the presence of iron olivine (Fe2SiO4) during hot rolling is reduced or suppressed.

[0053] Aluminum is a very effective element for deoxidizing steel in the liquid phase during refinement. Preferably, the Al content is 0.003% or higher in order to obtain sufficient deoxidation of the liquid steel.

[0054] Furthermore, similar to Si, Al stabilizes retained austenite and delays cementite formation during cooling. However, the Al content should be 3% or less to avoid inclusion formation, prevent oxidation problems, and ensure the material's hardening properties.

[0055] The steel of the present invention may contain at least one element selected from molybdenum and chromium.

[0056] Molybdenum increases hardening properties, stabilizes retained austenite, and reduces central segregation, which can result from manganese content and is detrimental to formability. Above 0.5%, excessive molybdenum can form carbides, which can be detrimental to ductility.

[0057] However, even without the addition of molybdenum (Mo), steel can contain at least 0.001% Mo as an impurity. When Mo is added, the Mo content is generally 0.05% or more.

[0058] Chromium enhances the hardenability of steel and contributes to achieving high tensile strength. A maximum of 1% chromium is acceptable. In fact, beyond 1%, a saturation effect is observed, and adding chromium becomes pointless and costly. When chromium is added, its content is generally at least 0.01%. If chromium is not added voluntarily, it may be present as an impurity at a low content of about 0.001%.

[0059] Microalloying elements such as titanium, niobium, and vanadium can be added in concentrations of up to 0.1% Ti, up to 0.1% Nb, and up to 0.2% V to obtain additional precipitation hardening. In particular, titanium and niobium are used to control particle size during solidification.

[0060] When adding Nb, its content is preferably at least 0.01%. If it exceeds 0.1%, a saturation effect is achieved, and adding more than 0.1% Nb is pointless and costly.

[0061] When adding Ti, its content is preferably at least 0.015%. When the Ti content is between 0.015% and 0.1%, precipitation occurs at very high temperatures in the form of TiN, and then at lower temperatures in the form of fine TiC, resulting in hardening. Furthermore, when titanium is added in addition to boron, titanium inhibits the bonding of boron with nitrogen, while nitrogen bonds with titanium. Therefore, when adding boron, a titanium content higher than 3.42N is preferable. However, in order to avoid the precipitation of coarse TiN precipitates that increase the hardness of hot-rolled and cold-rolled steel sheets during the manufacturing process, the Ti content should remain below 0.1%.

[0062] Optionally, the steel composition may include boron to enhance the hardenability of the steel. When boron is added, its content is higher than 0.0002%, preferably higher than 0.0005%, and at most 0.004%. In fact, beyond these limits, a saturation level in terms of hardenability can be expected.

[0063] Generally, sulfur, phosphorus, and nitrogen are present in the steel composition as impurities.

[0064] The nitrogen content is generally at least 0.002%. The nitrogen content should not exceed 0.013% to prevent the precipitation of coarse TiN and / or AlN precipitates from degrading ductility.

[0065] Regarding sulfur, at sulfur content exceeding 0.003%, the presence of excess sulfides such as MnS reduces ductility, and particularly in pore expansion tests, lower values ​​are observed in the presence of such sulfides.

[0066] Phosphorus hardens in solid solutions, but its tendency to segregate, particularly at grain boundaries or co-segregate with manganese, reduces spot weldability and hot ductility. For these reasons, its content must be limited to 0.015% to obtain good spot weldability.

[0067] The remainder consists of iron and unavoidable impurities. These impurities may include up to 0.03% Cu and up to 0.03% Ni.

[0068] The present invention aims to provide hot-rolled and annealed steel sheets suitable for the manufacture of cold-rolled and heat-treated steel sheets, which have high toughness and high cold-rollability, and a high combination of ductility and strength.

[0069] Furthermore, the method according to the present invention aims to manufacture such cold-rolled and heat-treated steel sheets.

[0070] The inventors investigated the problems of low toughness in hot-rolled and batch-annealed steel sheets, and the deterioration of the mechanical properties of cold-rolled and heat-treated steel sheets produced from such hot-rolled and batch-annealed steel sheets compared to steel sheets that would not have been annealed, and found that these problems stem from four main factors.

[0071] In particular, the inventors discovered that batch annealing forms a coarse cementite with a high concentration of manganese, which is therefore extremely stable in hot-rolled and batch-annealed steel sheets. The inventors further found that this stabilized cementite does not completely dissolve during the subsequent standard heat treatment of cold-rolled steel sheets. As a result, some of the Mn in the steel remains trapped in the cementite, and therefore its effect on the strength and ductility of the steel is suppressed.

[0072] The inventors further discovered that batch annealing coarses the microstructure of hot-rolled and batch-annealed steel sheets, resulting in a coarser final microstructure of cold-rolled and heat-treated steel sheets and a deterioration in their mechanical properties.

[0073] Furthermore, the inventors discovered that minute alloying elements, particularly Nb, that may be present in the steel composition precipitate as coarse precipitates that do not harden the steel at an early stage during batch annealing, and as a result, cannot be used for precipitation hardening during the subsequent heat treatment of cold-rolled steel sheets.

[0074] Finally, the inventors found that batch annealing, performed at a certain temperature and for a certain time, induces tempering embrittlement, resulting in low toughness of hot-rolled and batch-annealed steel sheets.

[0075] To solve these problems, the inventors conducted experiments by increasing the batch annealing temperature to exceed the Ae1 transformation point of steel.

[0076] However, the inventors found that using higher batch annealing temperatures, while limiting the formation of Mn-rich cementite, leads to coarsening of the microstructure, impairing the final properties of the cold-rolled and heat-treated steel sheets.

[0077] Based on these findings, the inventors of this invention believe that hot-rolled steel sheets are, - Ferrite with an average ferrite particle size of up to 3 μm, - Up to 30% austenite, - Up to 8% fresh martensite, and - Cementite with an average Mn content of less than 25% We have found that annealing to have a microstructure containing [specific microstructure] can greatly improve cold-rollability and toughness, while guaranteeing the final properties of the cold-rolled and heat-treated steel sheet.

[0078] A fresh martensite fraction of up to 8% makes it possible to achieve high toughness in hot-rolled and annealed steel sheets.

[0079] In particular, the inventors conducted experiments to determine the changes in austenite and fresh martensite fractions after subjecting hot-rolled steel sheets made from several steel compositions to various annealing conditions and cooling to room temperature, and measured the Charpy energy of the resulting steel sheets at 20°C.

[0080] Based on these experiments, the inventors found that the Charpy energy is an increasing function of the annealing temperature and a decreasing function of the fresh martensite fraction. Furthermore, the inventors found that when a hot-rolled and annealed steel sheet has a fresh martensite fraction of up to 8%, the Charpy energy is at least 50 J / cm² at 20°C. 2 We discovered that a high Charpy energy can be achieved.

[0081] Furthermore, cementite with an average Mn content of less than 25% allows for easier cementite melting during the final heat treatment of cold-rolled steel sheets, which means improved ductility and strength during further processing steps. In contrast, cementite with an average Mn content exceeding 25% will result in a decrease in the mechanical properties of cold-rolled and heat-treated steel sheets produced from the aforementioned hot-rolled and annealed steel sheets.

[0082] In addition, having an average ferrite grain size of up to 3 μm makes it possible to manufacture cold-rolled and heat-treated materials with a very fine microstructure, thereby increasing their mechanical properties.

[0083] The inventors have further found that the above microstructure makes it possible to achieve a hardness of hot-rolled and annealed steel sheets lower than 400 HV, which ensures satisfactory cold-rollability of the hot-rolled and annealed steel sheets.

[0084] The inventors have determined that the microstructure and properties of this hot-rolled and annealed steel sheet are such that, compared to a hot-rolled steel sheet, the minimum continuous annealing temperature T ICAmin = The maximum continuous annealing temperature T is the temperature at which 30% austenite is formed during heating from 650°C. ICAmax The continuous annealing temperature T included in the interval ICA We found that this can be achieved by performing continuous annealing for a time between 3 and 3600 seconds, followed by cooling the hot-rolled steel sheet under specific cooling conditions.

[0085] In particular, the inventors have found a high continuous annealing temperature T ICA Therefore, we found that an annealing time of up to 3600 seconds is sufficient to achieve adequate tempering of the microstructure, thereby improving the cold-rollability of hot-rolled and annealed steel sheets while avoiding microstructuring.

[0086] Furthermore, annealing the sheet at a temperature higher than 650°C allows for the softening of the hot-rolled steel sheet, limiting the Mn concentration of cementite particles to less than 25%, and limiting the precipitation of minute alloying elements, even if present. This prevents the coarsening of such precipitates, thereby preserving the influence of C, Mn, and minute alloying elements on the final mechanical properties. This also limits the segregation of brittle impurities such as P at grain boundaries.

[0087] The manufacturing method will be explained in more detail below.

[0088] A method for producing steel according to the present invention includes casting steel having the chemical composition of the present invention.

[0089] Cast steel is produced within a temperature range of 1150°C to 1300°C. reheat It will be reheated until it reaches the desired temperature.

[0090] Slab reheating temperature T reheat If the temperature is below 1150°C, the rolling load becomes too large, making hot rolling difficult.

[0091] Above 1300℃, oxidation becomes extremely severe, leading to scale loss and surface degradation.

[0092] The reheated slab is hot-rolled at a temperature between 1250°C and 800°C, with the final hot-rolling pass reaching a final rolling temperature of 800°C or higher. FRT It will be held at [location].

[0093] Final rolling temperature T FRT If the temperature is below 800°C, the hot workability decreases.

[0094] After hot rolling, the steel is cooled at a rate V between 1°C / sec and 150°C / sec. c1 Therefore, winding temperature T is 650℃ or less. coil It is cooled to a certain temperature. Below 1°C / second, an excessively coarse microstructure is created, degrading the final mechanical properties. Above 150°C / second, the cooling process becomes difficult to control.

[0095] Winding temperature T coil The temperature must be 650°C or lower. If the winding temperature exceeds 650°C, deep intergranular oxidation will form beneath the scale, leading to deterioration of surface properties.

[0096] It is preferable to pickle the hot-rolled steel sheet after winding.

[0097] Next, the hot-rolled steel sheet is continuously annealed. That is, the unrolled hot-rolled steel sheet undergoes heat treatment by continuously moving through the furnace.

[0098] Hot-rolled steel sheets have a minimum continuous annealing temperature T ICAmin = The maximum continuous annealing temperature T is the temperature at which 30% austenite is formed during heating from 650°C. ICAmax The continuous annealing temperature T included in the interval ICAThe material is then continuously annealed for a period of time between 3 seconds and 3600 seconds.

[0099] Under these conditions, the microstructure of the steel created during continuous annealing, before cooling to room temperature, - Ferrite, - Less than 30% austenite, - Cementite with an average Mn content of less than 25% It consists of.

[0100] If the continuous annealing temperature is lower than 650°C, the softening due to microstructural recovery during the continuous annealing process is insufficient, resulting in a hardness exceeding 400 HV for the hot-rolled and annealed steel sheet. Furthermore, continuous annealing temperatures below 650°C enhance the segregation of brittle elements such as P at grain boundaries, leading to insufficient toughness, which is critical for further processing of the steel sheet.

[0101] The continuous annealing temperature is T ICAmax If the concentration is higher, an excessively high austenite fraction may develop during continuous annealing, resulting in insufficient austenite stabilization and potentially leading to the formation of more than 8% fresh martensite upon cooling.

[0102] If the continuous annealing time is shorter than 3 seconds, the hardness of the hot-rolled and annealed steel sheet will be too high, especially above 400 HV, resulting in unsatisfactory cold-rollability. A continuous annealing time of at least 200 seconds is preferable.

[0103] If the continuous annealing time is longer than 3600 seconds, the microstructure becomes coarser, and in particular, the ferrite grains have an average size exceeding 3 μm. The continuous annealing time is preferably a maximum of 500 seconds.

[0104] The austenite produced during annealing is rich in carbon and manganese, and in particular has an average Mn content of at least 1.3*Mn% (where Mn% represents the Mn content of steel) and an average C content of at least 0.4%.

[0105] Therefore, austenite becomes very stable.

[0106] Next, the hot-rolled steel sheet is annealed at temperature T ICA It is cooled from 600°C to room temperature, where the average cooling rate V is between 600°C and 350°C. ICA The temperature is at least 1°C / second. Under these conditions, temper embrittlement is limited.

[0107] If the cooling rate between 600°C and 350°C is lower than 1°C / second, segregation occurs in the hot-rolled and annealed steel sheet, increasing tempering brittleness, and thus the cold-rollability cannot be satisfied.

[0108] The hot-rolled and annealed steel sheet obtained in this way is - Ferrite, - Up to 30% austenite, - Up to 8% fresh martensite, - Cementite with an average Mn content of less than 25% It has an organization consisting of [this].

[0109] To stabilize the austenite with Mn, a fresh martensite fraction of up to 8% is achieved, so that the austenite does not transform into fresh martensite at all, or only slightly, upon cooling.

[0110] The retained austenite in hot-rolled and annealed steel sheets has an average Mn content of at least 1.3*Mn% (where Mn% represents the Mn content of the steel) and an average C content of at least 0.4%.

[0111] To further limit the fresh martensite fraction, tempering is optionally performed.

[0112] Furthermore, the ferrite grains have an average size of up to 3 μm. In fact, continuous annealing, which is performed in a relatively short time compared to batch annealing, does not result in microcoarsening and therefore makes it possible to achieve hot-rolled and annealed sheets with a very fine microstructure.

[0113] At this stage, the hot-rolled and annealed sheet has improved cold-rollability and toughness compared to the hot-rolled steel sheet before annealing. Furthermore, the hot-rolled and annealed steel sheet is suitable for producing cold-rolled and heat-treated steel sheets with high mechanical properties, particularly high ductility and strength.

[0114] In particular, hot-rolled and annealed sheets have a Vickers hardness lower than 400 HV, and therefore exhibit very good cold-rollability.

[0115] Furthermore, hot-rolled and annealed steel sheets should have a humidity of at least 50 J / cm² at 20°C. 2 It has a Charpy energy of . Therefore, hot-rolled and annealed steel sheets have very good workability, and the risk of band breakage during further processing is significantly reduced compared to hot-rolled steel sheets that would have been batch-annealed. Furthermore, the inventors have found that not only is the Charpy energy of hot-rolled and annealed steel sheets higher than that of hot-rolled and batch-annealed steel sheets, but that the Charpy energy of hot-rolled and annealed steel sheets is generally higher than that of hot-rolled steel sheets produced from them.

[0116] After cooling to room temperature, the hot-rolled and annealed steel sheets are optionally pickled. However, this step may be omitted. In fact, because the duration of continuous annealing is short, internal oxidation does not occur at all or hardly at all during continuous annealing. If pickling is not performed between hot rolling and continuous annealing, it is preferable to pickle the hot-rolled and annealed steel sheets at this stage.

[0117] Next, this hot-rolled steel sheet is cold-rolled, with a cold-rolling reduction ratio of 30% to 70% to obtain cold-rolled steel sheet. If the reduction ratio is less than 30%, it is not favorable for recrystallization during subsequent heat treatment, and there is a risk of impairing the ductility of the cold-rolled steel sheet after heat treatment. If it exceeds 70%, there is a risk of edge cracking during cold rolling.

[0118] Subsequently, the cold-rolled steel sheets are heat-treated in a continuous annealing line to produce cold-rolled and heat-treated steel sheets.

[0119] The heat treatment applied to cold-rolled steel sheets is selected according to the desired final mechanical properties.

[0120] In either case, the heat treatment involves annealing the cold-rolled steel sheet at a temperature between 650 and 1000°C. anneal Heat to the annealing temperature T anneal The annealing time t is included between 30 seconds and 10 minutes. anneal This includes a step of holding the object for a certain period of time.

[0121] Furthermore, the annealing temperature T anneal This refers to a structure in which the annealed material contains at least 8% austenite.

[0122] If the annealing temperature is lower than 650°C, cementite is formed in the microstructure during annealing, leading to a deterioration in the mechanical properties of cold-rolled and heat-treated steel sheets.

[0123] To limit the coarsening of austenite grains, the annealing temperature T anneal The maximum temperature is 1000°C.

[0124] Annealing temperature T anneal The reheating rate Vr is preferably between 1°C / second and 200°C / second.

[0125] According to the first embodiment, annealing is Two-phase region The process is annealed, and the annealing temperature is T. anneal The hardness is lower than Ae3, and the microstructure created during annealing contains at least 8% austenite.

[0126] According to the second embodiment, in order to obtain a microstructure consisting of austenite and up to 1% cementite during annealing, the annealing temperature T anneal It is Ae3 or higher.

[0127] In the first embodiment, at the end of holding at the annealing temperature, the austenite has a carbon content of at least 0.4% and an average manganese content of at least 1.3*Mn%.

[0128] Next, the cold-rolled and annealed steel sheet is directly subjected to annealing at temperature T anneal Steel sheets are obtained that have been cooled to room temperature, cold-rolled, and heat-treated, either without or indirectly with holding, tempering, or reheating steps between the temperature and room temperature, i.e., with holding, tempering, and / or reheating steps.

[0129] In any case, cold-rolled and heat-treated steel sheets are - Residual austenite between 8% and 50%, - Martensite (fresh martensite and / or distribution Alternatively, it may include tempered martensite and optionally bainite. - Up to 80% Two-phase region Ferrite, and - Up to 1% cementite It has an organization that includes (hereinafter referred to as the "final organization").

[0130] Residual austenite generally has an average carbon content of at least 0.4% and an average manganese content of at least 1.3*Mn%.

[0131] Due to the manganese content of cementite, which can be up to 25% in the microstructure of hot-rolled and annealed steel sheets, cementite readily dissolves during annealing. Depending on the heat treatment performed, a small amount of cementite may remain in the final structure. However, the cementite fraction in the final structure remains less than 1% in all cases. Furthermore, cementite particles, if present, have an average size of less than 50 nm.

[0132] Martensite is fresh martensite and distribution Martensite or tempered martensite may be included.

[0133] As will be explained in more detail below, distributionMartensite has an average carbon content that is significantly lower than the nominal carbon content of steel. This lower carbon content is due to the transformation of martensite into austenite between 350°C and 500°C during quenching of steel below the Ms temperature. distribution temperature T P carbon during retention distribution It arises from.

[0134] In contrast, tempered martensite has an average carbon content equal to the nominal carbon content of the steel. Tempered martensite results from the tempering of martensite made by quenching the steel below its Ms temperature.

[0135] distribution Martensite can be distinguished from tempered martensite and fresh martensite on polished and etched sections with known reagents, such as Nital reagent, as observed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD).

[0136] This organization is bainite, especially 100mm 2 It may include carbide-free bainite containing fewer than 100 carbides per surface unit.

[0137] The ferrite fraction depends on the annealing temperature during heat treatment.

[0138] If ferrite is present in the final tissue, Two-phase region It is ferrite.

[0139] Therefore, ferrite, if present, is inherited from the microstructure of the hot-rolled and annealed steel sheet, which is then cold-rolled and recrystallized. As a result, the ferrite has an average grain size of up to 1.5 μm.

[0140] Here, we will explain in more detail the preferred heat treatments performed on cold-rolled steel sheets.

[0141] In the first preferred heat treatment, the annealing temperature T is lower than or higher than Ae3. anneal After being held, the cold-rolled steel sheet is cooled at a rate V between 1°C / sec and 70°C / sec. c2 It is then cooled to room temperature.

[0142] Cold-rolled steel sheets are cooled at a cooling rate V c2 It is cooled to room temperature, or the cooling rate V c2 Therefore, the holding temperature T is within the range of 350-550°C. H It is cooled down to a certain temperature, and held for 10 to 500 seconds at temperature T. H It is held at a holding temperature T. For example, such heat treatments that facilitate Zn coating by melting have been shown not to affect the final mechanical properties. H After holding the selected option, the cooling rate V falls within the range of 1°C / sec to 70°C / sec. c3 The cold-rolled steel sheet is then cooled to room temperature.

[0143] Optionally, after cooling to room temperature, the temperature T falls within the range of 170-450°C. t The tempering time included is between 10 and 1200 seconds. t During this time, the cold-rolled and heat-treated steel sheets are tempered.

[0144] This process allows for the tempering of martensite, which is created during the cooling process to room temperature after annealing. This reduces the hardness of the martensite and improves its ductility. Below 170°C, the tempering process is not sufficiently efficient. Above 450°C, the strength loss increases, and the balance between strength and ductility does not improve further.

[0145] The microstructure of the cold-rolled and heat-treated steel sheet obtained by the first preferred heat treatment is, in terms of surface fraction, - Retained austenite having an average carbon content of at least 0.4% between 8% and 50%, - Up to 80% Two-phase region Ferrite, - Up to 92% martensite and / or bainite, - Up to 1% cementite It consists of.

[0146] The martensite consists of tempered martensite and / or fresh martensite.

[0147] This organization is bainite, especially 100mm 2 It may include carbide-free bainite containing fewer than 100 carbides per surface unit.

[0148] The average size of cementite grains is less than 50 nm.

[0149] The fractions of ferrite and austenite depend on the annealing temperature during heat treatment.

[0150] In a first modification of the first preferred heat treatment, the annealing temperature T anneal The ferrite content is lower than Ae3, and preferably such that the microstructure produced during annealing contains 40% to 80% ferrite.

[0151] In this first modification, the final structure is preferably, in terms of surface fraction, - Retained austenite having an average carbon content of at least 0.4% and an average manganese content of at least 1.3*Mn% between 8 and 50%. - 40-80% Two-phase region Ferrite (ferrite grains have an average size of up to 1.5 μm), - Up to 15% martensite (consisting of tempered martensite and / or fresh martensite) and / or bainite, - Up to 0.3% cementite (cementite grains, if present, have an average size of less than 50 nm.) Includes.

[0152] In the second modification of the first preferred heat treatment, the annealing temperature is Ae3 or higher.

[0153] In this second variation, the final organization is: - Retained austenite with an average carbon content of at least 0.4%, ranging from 8 to 30%. - 70% to 92% martensite (consisting of tempered martensite and / or fresh martensite) and / or bainite, - Up to 1% cementite (cementite grains, if present, have an average size of less than 50 nm). It consists of.

[0154] In a second preferred heat treatment, the cold-rolled steel sheet is quenched and distribution It will be subjected to processing.

[0155] Therefore, the annealing temperature T anneal After being held in place, the cold-rolled steel sheet is annealed at annealing temperature T anneal From the quenching temperature QT, which is lower than the austenite Ms transformation point, to a cooling rate V high enough to avoid the formation of ferrite and pearlite during cooling. c4 It is then hardened.

[0156] Cooling rate V to quenching temperature QT c4 The temperature is preferably at least 2°C / second.

[0157] During this quenching process, austenite partially transforms into martensite.

[0158] The quenching temperature is between Mf+20°C and Ms-20°C, and the desired final microstructure is achieved, particularly the desired final microstructure. distribution The selection depends on the fractions of martensite and retained austenite. For each specific composition and microstructure of steel, those skilled in the art know how to determine the Ms and Mf transformation start and end points of austenite by measuring the coefficient of thermal expansion.

[0159] If the quenching temperature QT is lower than Mf + 20°C, the final microstructure will distribution The martensite fraction is too high. Also, if the quenching temperature QT is higher than Ms-20℃, the final microstructure... distribution The martensite fraction is too low, so it does not achieve high ductility.

[0160] A person skilled in the art knows how to determine the quenching temperature adapted to obtain the desired structure.

[0161] The cold-rolled steel sheet is optionally held at the quenching temperature QT for a holding time tQ included between 2 seconds and 200 seconds, preferably between 3 seconds and 7 seconds, in order to avoid the formation of epsilon carbides in the martensite which would lead to a decrease in the ductility of the steel.

[0162] Then, the cold-rolled steel sheet is reheated to a distribution temperature T P included between 350 and 500 °C distribution temperature T P and maintained for a time t distribution included between 3 seconds and 1000 seconds at P . During this distribution step, carbon diffuses from the martensite to the austenite, thereby achieving an enrichment of C in the austenite.

[0163] distribution temperature t P If it is higher than 500 °C or lower than 350 °C, the elongation of the final product is not satisfactory.

[0164] Optionally, the cold-rolled steel sheet is hot-dip galvanized in a bath at a temperature of, for example, 480 °C or lower. Any type of coating can be used, in particular zinc or a zinc alloy, such as zinc-nickel, zinc-magnesium or zinc-magnesium-aluminum alloy, aluminum or an aluminum alloy, such as aluminum-silicon.

[0165] distribution Immediately after the step, or if a hot-dip galvanizing step is carried out, after the hot-dip galvanizing step, the cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled and heat-treated steel sheet. The cooling rate to room temperature is preferably higher than 1 °C / second, for example included between 2 °C / second and 20 °C / second.

[0166] The final microstructure of the cold-rolled and heat-treated steel sheet obtained by the second preferred heat treatment is mainly determined by the annealing temperature T anneal It also depends on the quenching temperature (QT).

[0167] However, the microstructure of the cold-rolled and heat-treated steel sheet obtained in this way generally has a surface fraction of, - Residual austenite between 8% and 30%, - Up to 45% Two-phase region Ferrite, - distribution Martensite, - Up to 8% fresh martensite, - Up to 1% cementite It consists of.

[0168] Retained austenite is rich in carbon, and in particular has an average carbon content of at least 0.4%.

[0169] Ferrite, if available, Two-phase region It is a ferrite with an average particle size of up to 1.5 μm.

[0170] The proportion of fresh martensite in the tissue is 8% or less. In fact, a fresh martensite proportion higher than 8% will impair the hole expansion rate (HER).

[0171] In this second preferred heat treatment, during cooling from the annealing temperature and distribution Occasionally, small amounts of cementite may be produced. However, the fraction of cementite in the final structure remains below 1% in all cases, and the average size of cementite particles in the final structure remains below 50 nm.

[0172] In the first modification of the second preferred embodiment, the annealing temperature T anneal This is because, when cold-rolled steel sheets are annealed, the surface fraction is, - Ferrite between 10% and 45% - Austenite, and - Up to 0.3% cementite (cementite grains, if present, have an average size of less than 50 nm). It is an organization that consists of such members.

[0173] In this first modified example, the final structure is preferably, in terms of surface fraction, - 10-45% have an average particle size of up to 1.5 μm. Two-phase region Ferrite, - Residual austenite between 8% and 30%, - distribution Martensite, - Up to 8% fresh martensite, and - Up to 0.3% cementite (cementite grains, if present, have an average size of less than 50 nm.) Includes.

[0174] Retained austenite is rich in Mn and C. In particular, the average C content in retained austenite is at least 0.4%, and the average Mn content in retained austenite is at least 1.3*Mn%.

[0175] In a second modified example of the second preferred embodiment, the annealing temperature T anneal The elemental strength is Ae3 or higher, and as a result, the cold-rolled steel sheet has a structure consisting of austenite and up to 0.3% cementite after annealing.

[0176] In this second modification, the quenching temperature QT is preferably selected to obtain a structure immediately after quenching consisting of a maximum of 8% to 30% austenite, a maximum of 92% martensite, and a maximum of 1% cementite.

[0177] In this second modification, the final structure is determined by surface fraction. - Residual austenite between 8% and 30%, - distribution Martensite, - Up to 8% fresh martensite, and - Cementite up to 1% at most (cementite grains, if any, have an average size of less than 50 nm). It consists of

[0178] The retained austenite is rich in C, and the average C content in the retained austenite is at least 0.4%.

[0179] The characteristics of the above microstructure are determined, for example, by observing the microstructure using a scanning electron microscope equipped with a field emission gun ("FEG-SEM") at a magnification exceeding 5000× and combined with an electron backscatter diffraction ("EBSD") apparatus and a transmission electron microscope (TEM).

Examples

[0180] As examples and for comparison, plates made from the steel compositions according to Table I were manufactured, and their contents are expressed in weight percentages.

[0181]

Table 1

[0182] In the first experiment, steels I1, I2, I3, I6, and I7 were cast to obtain ingots. These ingots were reheated at a temperature T of 1250 °C reheat to remove the scale and hot-rolled at a temperature higher than Ar3 to obtain hot-rolled steel.

[0183] Next, the hot-rolled steel was coiled at a cooling rate V included between 1 °C / second and 150 °C c1 to a coiling temperature T coil and coiled at this temperature T coil .

[0184] Next, a part of the hot-rolled steel was continuously annealed or batch-annealed at an annealing temperature T A for an annealing time t A and then cooled to room temperature at an average cooling rate V between 600 °C and 350 °C ICA .

[0185] Report the manufacturing conditions of the hot-rolled and annealed steel plates in Table 2 below, and also report the austenite fraction generated during annealing.

[0186] [Table 2] TIFF0007880835000003.tif81170

[0187] In Table 2, the underlined values are not according to the present invention, and "n.d." means "not determined".

[0188] The inventors examined the microstructure of the thus obtained hot-rolled and optionally annealed steel plates by means of a scanning electron microscope equipped with a field emission gun ("FEG-SEM") at a magnification of 5000× and combined with an electron backscatter diffraction ("EBSD") device and a transmission electron microscope (TEM).

[0189] In particular, the inventors measured the ferrite grain size, the surface fraction of fresh martensite (FM), the surface fraction of austenite (RA), and the average Mn content in cementite (Mn% in cementite).

[0190] The inventors further measured the Charpy energy and Vickers hardness of the hot-rolled steel plate at 20°C. Report the characteristics of the microstructure and mechanical properties in Table 3 below.

[0191] <000091​​​​​​​​​​​​​These experiments demonstrate that the desired microstructure and mechanical properties of the hot-rolled and annealed steel sheet are achieved only when the hot-rolled steel sheet is annealed under the conditions of the present invention.

[0194] In contrast, examples I1A, I2A, I3A, I6A, and I7A underwent no annealing.

[0195] As a result, their hardness is higher than 400HV, and therefore the cold-rollability of these hot-rolled steel sheets is insufficient.

[0196] Examples I1B, I2B, and I3B were batch annealed at 500°C for 25,200 seconds. Batch annealing resulted in a decrease in hardness compared to examples I1A, I2A, and I3A, which were not annealed. However, batch annealing also resulted in a decrease in Charpy energy, so the workability of examples I1B, I2B, and I3B is insufficient. Furthermore, batch annealing led to the formation of cementite highly rich in manganese.

[0197] Examples I1C, I2C, I3C, I6C, and 7C were also subjected to batch annealing at a temperature of 600°C for 25,200 seconds. As a result of batch annealing, the hardness of these examples decreased compared to examples I1A, I2A, I3A, I6A, and I7A, and further decreased compared to examples I1B, I2B, and I3B. However, the Charpy energy was 50 J / cm². 2 The levels remained lower, and batch annealing resulted in the formation of Mn-rich cementite.

[0198] Next, the inventors conducted experiments by increasing the batch annealing temperature to 650°C, above the Ae1 transformation point (e.g., I1D, I2D, I3D, I6D, and I7D). This higher batch annealing temperature resulted in an increase in the Charpy energy of the plates and a decrease in the average Mn content in the cementite, compared to examples I1C, I2C, I3C, I6C, and I7C, respectively.

[0199] Nevertheless, batch annealing at temperatures above Ae1 resulted in microcoarsening, with ferrite grain sizes larger than 3 μm.

[0200] The inventors further increased the batch annealing temperature to 680°C (e.g., I1E and I3E). This increase in batch annealing temperature resulted in a further increase in Charpy energy and a further decrease in the average Mn content in cementite. However, this increase in batch annealing temperature also resulted in a further undesirable increase in ferrite grain size.

[0201] Thus, these examples demonstrate that even though batch annealing reduces the hardness of hot-rolled steel sheets, the Charpy energy of hot-rolled and batch-annealed steel sheets is generally insufficient to ensure high workability. Furthermore, batch annealing undesirably produces cementite that is very rich in manganese. These examples further show that increasing the batch annealing temperature can increase the Charpy energy and decrease the average manganese content in the cementite, but the Charpy energy is often below the target value of 50 J / cm². 2 The temperature remains lower, indicating that increasing the batch annealing temperature leads to undesirable coarsening of the microstructure.

[0202] Example I3L underwent continuous annealing, but the continuous annealing temperature was lower than 650°C. As a result, softening due to microstructural recovery was insufficient, and the hardness of Example I3L was higher than 400 HV, resulting in an insufficient Charpy energy.

[0203] Examples I1G and I3Q were continuously annealed at annealing temperatures that produced more than 30% austenite during annealing. As a result, the fresh martensite fraction in the hot-rolled and annealed steel sheets was higher than 8%, so the hardness of these examples was higher than 400 HV, and their Charpy energies were 50 J / cm². 2 Lower.

[0204] Examples I1F, I2H, I2J, I2K, I3H, I3M, I3O, I3P, I3J, I6K and I7K were subjected to continuous annealing under the conditions of the present invention. As a result, the hot-rolled and annealed steel sheets have a Charpy energy at 20 °C of at least 50 J / cm 2 and a hardness of 400 HV or less. These hot-rolled and annealed steel sheets thus have sufficient cold rolling properties and workability. Further, the microstructure of these examples is such that the average ferrite grain size is smaller than 3 μm and the average Mn content in cementite is lower than 25%. Therefore, these hot-rolled steel sheets are suitable for the production of cold-rolled and heat-treated steel sheets having high mechanical properties.

[0205] The microstructure of the hot-rolled and annealed steel sheets thus obtained was observed.

[0206] (Ex)amples I1E and I1F are shown in FIGS. 1 and 2, respectively.

[0207] As can be seen in these figures, the microstructure of steel I1F produced by continuous annealing according to the present invention is much finer than that of steel I1E produced by batch annealing above Ae1.

[0208] These experiments demonstrate that, unlike batch annealing, continuous annealing according to the present invention results in a very fine microstructure.

[0209] The inventors further conducted experiments to evaluate the final properties of cold-rolled and heat-treated steels produced by batch annealing at a temperature below Ae1 or above Ae1, or subjected to continuous annealing according to the present invention before cold rolling.

[0210] In particular, steels I1, I2, I4, I5, I6 and I7 were cast to obtain ingots. These ingots were reheated at a temperature T of 1250 °C reheat scaled off, and hot-rolled at a temperature higher than Ar3 to obtain hot-rolled steel. <目标语言文本内容>

[0211] Next, the hot-rolled steel sheet is heated to temperature T coil I wound it up.

[0212] Next, the hot-rolled steel sheets were batch-annealed or continuously annealed.

[0213] Next, the hot-rolled and annealed steel sheet is cold-rolled at a reduction ratio of 50%, then annealed, and then cooled at V c1 Various heat treatments were performed, including cooling to room temperature.

[0214] Next, the yield strength, tensile strength, uniform elongation, and hole expansion ratio of the cold-rolled and heat-treated steel sheets obtained in this manner were measured.

[0215] The manufacturing conditions and measured characteristics are reported in Tables 4 and 5.

[0216] In these tables, T coil This indicates the winding temperature, T A and t A is the batch or continuous annealing temperature and time, where HBA indicates batch annealing, ICA indicates continuous annealing according to the present invention, and T anneal is the annealing temperature, and t anneal V is the annealing time, C1 This represents the cooling rate (or cooling conditions).

[0217] The measurement characteristics reported in Tables 4 and 5 are yield strength YS, tensile strength TS, uniform elongation UE, and hole expansion rate HER.

[0218] In these tables, "nd" means "not determined." The underlined values ​​are not according to the present invention.

[0219] [Table 4]

[0220] [Table 5]

[0221] The properties of an example fabricated from steel I4 are reported in Figure 3 (UTS representing tensile strength and UEl representing uniform elongation).

[0222] In this figure, each curve corresponds to the annealing conditions after hot rolling (black squares: batch annealing at 600°C for 300 minutes; white squares: continuous annealing at 700°C for 2 minutes). Each point on each curve reports the tensile strength and uniform elongation obtained at a specific annealing temperature, and it can be seen that the higher the annealing temperature, the higher the tensile strength.

[0223] The results reported in Figure 3 and Table 4 demonstrate that by performing the continuous annealing of the present invention, it is possible to achieve an improved combination of tensile strength and elongation compared to batch annealing.

[0224] Therefore, steel plates manufactured according to the present invention can be usefully used for the manufacture of structural or safety components of vehicles.

Claims

1. Cold-rolled and heat-treated steel sheet, in weight percent, 0.1% ≤ C ≤ 0.4% 3.5% ≤ Mn ≤ 8.0% 0.1% ≤ Si ≤ 1.5% Al ≤ 3% Mo ≤ 0.5% Cr ≤ 1% Nb ≤ 0.1% Ti ≤ 0.1% V ≤ 0.2% B ≤ 0.004% 0.002% ≤ N ≤ 0.013% S ≤ 0.003% P ≤ 0.015% Made from steel having a composition that includes, with the remainder being iron and unavoidable impurities resulting from smelting, the cold-rolled steel sheet has a surface fraction of, - Retained austenite having an average carbon content of at least 0.4% between 8% and 50%, - A ferrite with a maximum of 80% two-phase region, and if ferrite grains are present, ferrite with a maximum average size of 1.5 μm. - A maximum of 1% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. - Martensite and / or bainite Cold-rolled and heat-treated steel sheet having a structure consisting of [the following].

2. The aforementioned structure contains at least 10% two-phase ferrite in surface fraction, and if ferrite grains are present, ferrite having an average size of up to 1.5 μm. The cold-rolled and heat-treated steel sheet according to claim 1.

3. The aforementioned structure, in terms of surface fraction, - Residual austenite between 8% and 50%, - A maximum of 1% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. - Martensite and / or bainite A cold-rolled and heat-treated steel sheet according to claim 1, comprising the above.

4. The cold-rolled and heat-treated steel sheet according to any one of claims 1 to 3, wherein the martensite comprises tempered martensite and / or fresh martensite.

5. The aforementioned structure, in terms of surface fraction, - Having an average carbon content of at least 0.4% and an average manganese content of at least 1.3*Mn%, where Mn% represents the average manganese content in the steel composition, including retained austenite. - A two-phase ferrite between 40% and 80%, where ferrite grains are present, with a maximum average size of 1.5 μm. - Up to 15% martensite and / or bainite, where the martensite consists of tempered martensite and / or fresh martensite, and - A maximum of 0.3% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. A cold-rolled and heat-treated steel sheet according to claim 1, comprising the above.

6. The aforementioned structure, in terms of surface fraction, - Retained austenite having an average carbon content of at least 0.4% between 8% and 30%, - 70% to 92% martensite and / or bainite, where the martensite consists of tempered martensite and / or fresh martensite, and - A maximum of 1% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. A cold-rolled and heat-treated steel sheet according to claim 1, comprising the above.

7. The aforementioned structure, in terms of surface fraction, - A ferrite with a maximum of 45% two-phase region, and if ferrite grains are present, ferrite with a maximum average size of 1.5 μm. - Residual austenite between 8% and 30%, - Distribution martensite, - Up to 8% fresh martensite, and - A maximum of 1% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. A cold-rolled and heat-treated steel sheet according to claim 1 or 2, comprising the above.

8. The aforementioned structure, in terms of surface fraction, - A two-phase ferrite between 10% and 45%, where ferrite grains are present, with a maximum average size of 1.5 μm. - Residual austenite between 8% and 30%, - Distribution martensite, - Up to 8% fresh martensite, and - A maximum of 0.3% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. A cold-rolled and heat-treated steel sheet according to claim 1, comprising the above.

9. The aforementioned structure, in terms of surface fraction, - Residual austenite between 8% and 30%, - Distribution martensite, - Up to 8% fresh martensite, and - A maximum of 1% cementite, and if cementite grains are present, cementite with an average size of less than 50 nm. A cold-rolled and heat-treated steel sheet according to claim 1, comprising the above.

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