Cold-rolled heat-treated steel sheet

Abstract

Provided is a cold-rolled heat-treated steel sheet made of steel having a composition containing, in weight percentages, 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%. The structure consists, in surface fraction, of: 8% to 50% of retained austenite; up to 80% sub-temperature ferrite, if there are ferrite grains, the ferrite grains have an average size of up to 1.5 [mu] m; and up to 1% cementite, the cementite particles having an average size of less than 50 nm if there are cementite particles; and a bainite, the bainite being a bainite containing less than 100 carbides per 100 mm2 of surface unit, in particular a carbide-free bainite. Thus, a steel sheet having excellent toughness, ductility, and strength is provided.

Description

[0001] This divisional patent application is a divisional application of the patent application with the filing date of December 18, 2018, application number 201880081933.4, and invention title "Steel Plate with Excellent Toughness, Ductility and Strength and Manufacturing Method Thereof". Technical Field

[0002] The present invention relates to a method for manufacturing a hot-rolled annealed steel plate for manufacturing a cold-rolled heat-treated steel plate having high cold-rollability and toughness and suitable for producing a highly combined cold-rolled heat-treated steel plate having ductility and strength, and to a hot-rolled annealed steel plate produced by this method.

[0003] The present invention also relates to a method for manufacturing a cold-rolled heat-treated steel plate having a highly combined ductility and strength, and to a cold-rolled heat-treated steel plate obtained by this method. Background Art

[0004] Especially in the automotive industry, there is a continuous need to lighten vehicles to improve fuel efficiency of vehicles in consideration of global environmental protection and a continuous need to increase safety by using steels with high tensile strength. Such steels can indeed be used to produce components with a lower thickness while ensuring the same or improved safety level.

[0005] For this purpose, steels with microalloying elements have been proposed, and the hardening of these steels is obtained simultaneously by precipitation and by refinement of the grain size. After the development of these steels, higher-strength steels called Advanced High Strength Steels have been developed, which maintain a good strength level and good cold formability.

[0006] For the purpose of obtaining an even higher tensile strength level, steels showing TRIP (Transformation Induced Plasticity) behavior and having a highly favorable combination of properties (tensile strength / deformability) have been developed. These properties are associated with 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 hinder carbide precipitation in austenite and bainite. The presence of retained austenite gives high ductility to the undeformed sheet. Under the action of subsequent deformation, for example when subjected to uniaxial stress, the retained austenite in the component made of TRIP steel gradually transforms into martensite, thereby causing a large degree of hardening and delaying the occurrence of necking.

[0007] In order to achieve an improved combination of strength and ductility, it is further proposed to produce sheets by means of the so-called "quenching and partitioning" process, in which the sheets are annealed in the austenite region or in the critical region, cooled to a quenching temperature below the Ms transformation point, and thereafter heated to the partitioning temperature and held at this temperature for a given time. The resulting steel sheets have a microstructure comprising martensite and retained austenite and optionally comprising bainite and / or ferrite. The retained austenite has a high C content due to the partitioning of carbon from the martensite during partitioning, and the martensite contains a low fraction of carbides.

[0008] All these steel sheets exhibit a good balance of resistance and ductility.

[0009] However, new challenges have emerged in the manufacture of such sheets. In particular, the manufacturing process of such steel sheets generally includes: casting a steel semi-finished product, hot rolling the semi-finished product to produce a hot-rolled steel sheet, and then coiling the hot-rolled steel sheet before the heat treatment gives its final properties to the steel. The hot-rolled steel sheet is then cold-rolled to the desired thickness and subjected to a heat treatment selected according to the desired final microstructure and properties to obtain a cold-rolled heat-treated steel sheet.

[0010] Due to the composition of these steels, a high level of resistance is achieved throughout the manufacturing process. In particular, the hot-rolled steel sheet exhibits a high hardness before cold rolling, which impairs its cold rollability. As a result, the range of available sizes of the cold-rolled sheet is reduced.

[0011] To solve this problem, it is proposed to subject the hot-rolled steel sheet to batch annealing at a temperature of approximately 500 °C to 700 °C for a number of hours before cold rolling.

[0012] Batch annealing does reduce the hardness of the hot-rolled steel sheet and thus improves its cold rollability.

[0013] However, this solution is not entirely satisfactory.

[0014] In fact, the batch annealing treatment generally results in a reduction of the final properties of the steel, in particular a reduction of the ductility and strength of the steel.

[0015] In addition, the hot-rolled steel sheet exhibits insufficient toughness after batch annealing, which may be the cause of strip breakage during further processing.

[0016] Therefore, the present invention aims to provide a hot-rolled steel sheet and a method for manufacturing the same, which has improved cold rollability and toughness, and is suitable for producing cold-rolled heat-treated steel sheets with high mechanical properties, in particular with a high combination of ductility and strength.

[0017] The present invention also aims to provide a cold-rolled heat-treated steel sheet and a method for manufacturing the same, which has a high combination of mechanical properties compared to similar steel sheets produced by a method including a batch annealing treatment before cold rolling. Summary of the Invention

[0018] To this end, the present invention relates to a method for manufacturing a steel sheet, the method comprising the following steps:

[0019] - Casting steel to obtain a steel semi-finished product, the steel having a composition comprising the following, by weight percentage:

[0020] 0.1% ≤ C ≤ 0.4%

[0021] 3.5% ≤ Mn ≤ 8.0%

[0022] 0.1% ≤ Si ≤ 1.5%

[0023] Al ≤ 3%

[0024] Mo ≤ 0.5%

[0025] Cr ≤ 1%

[0026] Nb ≤ 0.1%

[0027] Ti ≤ 0.1%

[0028] V ≤ 0.2%

[0029] B ≤ 0.004%

[0030] 0.002% ≤ N ≤ 0.013%

[0031] S ≤ 0.003%

[0032] P ≤ 0.015%,

[0033] the balance being iron and inevitable impurities resulting from smelting,

[0034] - Reheating the steel semi-finished product to a temperature T of 1150°C to 1300°C 再加热 ,

[0035] - Hot-rolling the reheated semi-finished product at a temperature of 800°C to 1250°C to obtain a hot-rolled steel sheet, wherein the final rolling temperature T FRT is higher than or equal to 800°C,

[0036] - Cooling the hot-rolled steel sheet at a cooling rate V of 1°C / s to 150°C / s c1 to a coiling temperature T of less than or equal to 650°C 卷取 , and coiling the hot-rolled steel sheet at the coiling temperature T 卷取 and then

[0037] - At T ICA最小 to T ICA最大 of the continuous annealing temperature T ICA carry out continuous annealing on the hot-rolled steel sheet, wherein, T ICA最小 = 650 °C, and T ICA最大 is the temperature at which 30% austenite is formed during heating. Keep the hot-rolled steel sheet at the continuous annealing temperature T ICA for a continuous annealing time t of 3 s to 3600 s ICA , then,

[0038] - Cool the hot-rolled steel sheet to room temperature. The hot-rolled steel sheet is cooled at an average cooling rate V of at least 1 °C / s between 600 °C and 350 °C ICA to obtain a hot-rolled annealed steel sheet,

[0039] - Cold-roll the hot-rolled annealed steel sheet at a cold rolling reduction rate of 30% to 70% to obtain a cold-rolled steel sheet.

[0040] Preferably, the hot-rolled annealed steel sheet has a microstructure composed of the following in terms of surface fraction:

[0041] - Ferrite, the ferrite grains having an average size of at most 3 μm,

[0042] - At most 30% austenite,

[0043] - At most 8% fresh martensite, and

[0044] - Cementite, the cementite having an average Mn content of less than 25%.

[0045] Generally, the hot-rolled annealed steel sheet has a Vickers hardness of less than 400 HV.

[0046] Preferably, the hot-rolled annealed steel sheet has a Charpy energy of at least 50 J / cm at 20 °C 2 .

[0047] Preferably, between coiling and continuous annealing and / or after continuous annealing, the method further includes a step of pickling the hot-rolled steel sheet.

[0048] Preferably, the continuous annealing time t ICA is 200 s to 3600 s.

[0049] Preferably, after cold rolling, the method further includes:

[0050] - Heat the cold-rolled steel sheet to an annealing temperature T of 650 °C to 1000 °C 退火 , and

[0051] - Keep the cold-rolled steel sheet at the annealing temperature T退火 Maintain an annealing time t of 30 s to 10 min at 退火 .

[0052] In the first embodiment, the annealing temperature T 退火 is T ICA最小 to Ae3.

[0053] In the second embodiment, wherein the annealing temperature T 退火 is from Ae3 to 1000 °C.

[0054] In the embodiment, the method further comprises the step of: cooling the cold-rolled steel sheet at a cooling rate V of 1 °C / s to 70 °C / s c2 from the annealing temperature T 退火 to room temperature to obtain a cold-rolled heat-treated steel sheet.

[0055] In another embodiment, after maintaining the cold-rolled steel sheet at the annealing temperature T 退火 , the method further comprises the following sequential steps:

[0056] - cooling the cold-rolled steel sheet at a cooling rate V of 1 °C / s to 70 °C / s c2 from the annealing temperature T 退火 to a holding temperature T of 350 °C to 550 °C H ,

[0057] - holding the cold-rolled steel sheet at the holding temperature T H for a holding time t of 10 s to 500 s H , then,

[0058] - cooling the cold-rolled steel sheet at a cooling rate V of 1 °C / s to 70 °C / s c3 from the holding temperature T H to room temperature to obtain a cold-rolled heat-treated steel sheet.

[0059] Preferably, the method further comprises the step of: tempering the cold-rolled heat-treated steel sheet at a tempering temperature T of 170 °C to 450 °C T for a tempering time t of 10 s to 1200 s T .

[0060] Preferably, the method further comprises the step of coating the cold-rolled heat-treated steel sheet with Zn or a Zn alloy, or with Al or an Al alloy.

[0061] In another embodiment, the method further comprises the following steps:

[0062] - cooling the heated cold-rolled steel sheet at a cooling rate V high enough to avoid the formation of ferrite and pearlite upon cooling c4 from the annealing temperature T 退火The quenching temperature QT is quenched to Mf + 20°C to Ms - 20°C,

[0063] - The cold-rolled steel sheet is reheated from the quenching temperature QT to the partitioning temperature T of 350°C to 500°C P , and the cold-rolled steel sheet is at the partitioning temperature T P and held for a partitioning time t of 3 s to 1000 s P ,

[0064] - The cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled heat-treated steel sheet.

[0065] In the first variant of this embodiment, the annealing temperature T 退火 is such that the cold-rolled steel sheet has a structure consisting of the following by surface fraction when annealed:

[0066] - 10% to 45% ferrite,

[0067] - austenite, and

[0068] - at most 0.3% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0069] In the second variant of this embodiment, the annealing temperature T 退火 is above Ae3, and the cold-rolled steel sheet has a structure consisting of the following when annealed:

[0070] - austenite, and

[0071] - at most 0.3% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0072] After holding the cold-rolled steel sheet at the partitioning temperature T P , the cold-rolled steel sheet can be immediately cooled to room temperature.

[0073] In a variant, between holding the cold-rolled steel sheet at the partitioning temperature T P and cooling the cold-rolled steel sheet to room temperature, the cold-rolled steel sheet is hot dip coated in a bath.

[0074] Preferably, the Si content in the composition is at most 1.4%.

[0075] The present invention also relates to a cold-rolled heat-treated steel sheet made of steel having a composition containing the following by weight percentage:

[0076] 0.1% ≤ C ≤ 0.4%

[0077] 3.5% ≤ Mn ≤ 8.0%

[0078] 0.1% ≤ Si ≤ 1.5%

[0079] Al ≤ 3%

[0080] Mo ≤ 0.5%

[0081] Cr ≤ 1%

[0082] Nb ≤ 0.1%

[0083] Ti ≤ 0.1%

[0084] V ≤ 0.2%

[0085] B ≤ 0.004%

[0086] 0.002% ≤ N ≤ 0.013%

[0087] S ≤ 0.003%

[0088] P ≤ 0.015%,

[0089] The balance is iron and unavoidable impurities resulting from smelting, wherein the cold-rolled steel sheet has a structure composed of the following in terms of surface fraction:

[0090] - 8% to 50% retained austenite,

[0091] - up to 80% hypoeutectoid ferrite. If there are ferrite grains, the ferrite grains have an average size of at most 1.5 μm, and

[0092] - up to 1% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm,

[0093] - martensite and / or bainite. Optionally, the bainite can be bainite containing less than 100 carbides per 100 mm 2 of surface unit, especially carbide-free bainite.

[0094] In an embodiment, the structure contains at least 10% hypoeutectoid ferrite in terms of surface fraction.

[0095] In another embodiment, the structure is composed of the following in terms of surface fraction:

[0096] - 8% to 50% retained austenite,

[0097] - up to 1% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm,

[0098] - martensite and / or bainite. Optionally, the bainite can be bainite containing less than 100 carbides per 100 mm 2The surface unit contains bainite with less than 100 carbides, especially carbide-free bainite.

[0099] In an embodiment, the martensite consists of tempered martensite and / or fresh martensite.

[0100] In a first variant of this embodiment, the structure consists of the following by surface fraction:

[0101] - 8% to 50% retained austenite, the retained austenite having an average C content of at least 0.4% and an average Mn content of at least 1.3*Mn%, where Mn% represents the average Mn content in the steel composition,

[0102] - 40% to 80% hypoeutectoid ferrite,

[0103] - At most 15% martensite and / or bainite, optionally, the bainite can be bainite with less than 100 carbides per 100 mm 2 of the surface unit, especially carbide-free bainite, and

[0104] - At most 0.3% cementite, if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0105] In a second variant of this embodiment, the structure consists of the following by surface fraction:

[0106] - 8% to 30% retained austenite, the retained austenite having an average C content of at least 0.4%,

[0107] - 70% to 92% martensite and bainite, optionally, the bainite can be bainite with less than 100 carbides per 100 mm 2 of the surface unit, especially carbide-free bainite, and

[0108] - At most 1% cementite, if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0109] In another embodiment, the structure consists of the following by surface fraction:

[0110] - At most 45% hypoeutectoid ferrite,

[0111] - 8% to 30% retained austenite,

[0112] - Partitioned martensite,

[0113] - At most 8% fresh martensite, and

[0114] - Cementite of at most 1%, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0115] In a first variant of this embodiment, the microstructure consists, by surface fraction, of:

[0116] - Hypoeutectoid ferrite of 10% to 45%,

[0117] - Retained austenite of 8% to 30%,

[0118] - Partitioned martensite,

[0119] - Fresh martensite of at most 8%, and

[0120] - Cementite of at most 0.3%, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0121] In a second variant of this embodiment, the microstructure consists, by surface fraction, of:

[0122] - Retained austenite of 8% to 30%,

[0123] - Partitioned martensite,

[0124] - Fresh martensite of at most 8%, and

[0125] - Cementite of at most 1%, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0126] Preferably, the Si content in the composition is at most 1.4%. BRIEF DESCRIPTION OF THE DRAWINGS

[0127] The present invention will now be described and illustrated in detail by way of examples without limitation with reference to the accompanying drawings, in which:

[0128] - Figure 1 is a micrograph showing the microstructure of a comparative hot-rolled batch-annealed steel sheet,

[0129] - Figure 2 is a micrograph showing the microstructure of a continuously annealed hot-rolled steel according to the present invention,

[0130] - Figure 3 is a graph comparing the mechanical properties of cold-rolled heat-treated steel sheets produced from hot-rolled batch-annealed steel sheets or hot-rolled continuously annealed steel sheets. DETAILED DESCRIPTION

[0131] According to the present invention, the carbon content is 0.1% to 0.4%. Carbon is an austenite stabilizing element. Below 0.1%, it is difficult to achieve a high tensile strength level. If the carbon content is greater than 0.4%, the cold rollability is reduced and the weldability becomes poor. Preferably, the carbon content is 0.1% to 0.2%.

[0132] The manganese content is 3.5% to 8.0%. Manganese provides solid solution hardening and refinement to the microstructure. Therefore, manganese helps to increase the tensile strength. When the content is higher than 3.5%, Mn is used to provide important stability to the austenite in the microstructure during the entire manufacturing process and the austenite in the final microstructure. In particular, in the case where the Mn content is higher than 3.5%, a final microstructure of the cold-rolled heat-treated steel sheet containing at least 8% retained austenite can be achieved. In addition, since the retained austenite is stabilized by Mn, high ductility can be obtained. Above 8.0%, the weldability becomes poor, and at the same time, segregation and inclusions deteriorate the damage performance.

[0133] Silicon is very effective in increasing the strength through solid solution and stabilizing austenite. In addition, during cooling, silicon delays the formation of cementite by greatly hindering the precipitation of carbides. This is caused by the fact that the solubility of silicon in cementite is very low and silicon increases the activity of carbon in austenite. Therefore, before any cementite is formed, there will be a step where Si is excluded at the interface. Therefore, the enrichment of carbon in austenite makes austenite stable at room temperature.

[0134] For this reason, the Si content is at least 0.1%. However, the Si content is limited to 1.5% because exceeding this value increases the rolling load too much and makes the hot rolling process difficult. The cold rollability is also reduced. In addition, when the content is too high, silicon oxide is formed on the surface, which will damage the coatability of the steel.

[0135] Preferably, the Si content is at most 1.4%. In fact, a Si content of at most 1.4% reduces or even suppresses the appearance of red scale (also known as tiger stripe) due to the presence of fayalite (Fe2SiO4) during hot rolling.

[0136] Aluminum is a very effective element for deoxidizing liquid-phase steel during processing. Preferably, in order to obtain sufficient deoxidation of liquid steel, the Al content is not less than 0.003%.

[0137] In addition, like Si, Al stabilizes the retained austenite and delays the formation of cementite during cooling. However, the Al content is not higher than 3% to avoid the appearance of inclusions, avoid oxidation problems, and ensure the hardenability of the material.

[0138] The steel according to the present invention may contain at least one element selected from molybdenum and chromium.

[0139] Molybdenum increases hardenability, stabilizes retained austenite, and reduces center segregation that may be caused by the manganese content and is detrimental to formability. Above 0.5%, Mo may form excessive carbides, which may be detrimental to ductility.

[0140] However, when Mo is not added, the steel may contain at least 0.001% of Mo as an impurity. When Mo is added, the Mo content is generally higher than or equal to 0.05%.

[0141] Chromium improves the quenchability of the steel and helps to achieve high tensile strength. Up to 1% of chromium is allowed. In fact, above 1%, the saturation effect is noted, and adding Cr is both useless and expensive. When Cr is added, its content is generally at least 0.01%. If no active addition of Cr is made, the Cr content may be present as an impurity in an amount as low as 0.001%.

[0142] To obtain additional precipitation hardening, microalloying elements such as titanium, niobium, and vanadium can be added in the following amounts: up to 0.1% of Ti, up to 0.1% of Nb, and up to 0.2% of V. In particular, titanium and niobium are used to control the grain size during solidification.

[0143] When Nb is added, its content is preferably at least 0.01%. Above 0.1%, the saturation effect is obtained, and adding more than 0.1% of Nb is both useless and expensive.

[0144] When Ti is added, its content is preferably at least 0.015%. When the Ti content is from 0.015% to 0.1%, precipitation at very high temperatures occurs in the form of TiN, and then precipitation at lower temperatures occurs in the form of fine TiC, resulting in hardening. In addition, when titanium is added in addition to boron, titanium prevents boron from combining with nitrogen, which combines with titanium. Therefore, when boron is added, the titanium content is preferably higher than 3.42N. However, the Ti content should be kept lower than or equal to 0.1% to avoid the precipitation of coarse TiN precipitates during the manufacturing process, thereby increasing the hardness of hot-rolled steel sheets and cold-rolled steel sheets.

[0145] Optionally, the steel composition contains boron to improve the quenchability of the steel. When B is added, the B content is higher than 0.0002%, and preferably higher than or equal to 0.0005%, up to 0.004%. In fact, above this limit, a saturation level is expected in terms of hardenability.

[0146] Sulfur, phosphorus, and nitrogen are usually present as impurities in the steel composition.

[0147] The nitrogen content is generally at least 0.002%. The nitrogen content must be at most 0.013% to prevent the precipitation of coarse TiN and / or AlN precipitates from deteriorating ductility.

[0148] Regarding sulfur, with a content higher than 0.003%, the ductility is reduced due to the presence of excessive sulfides such as MnS. In particular, in the presence of such sulfides, the hole expansion test shows lower values.

[0149] Phosphorus is an element that hardens in solid solution, but this element reduces the spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, the phosphorus content must be limited to 0.015% to obtain good spot weldability.

[0150] The balance consists of iron and inevitable impurities. Such impurities may contain up to 0.03% of Cu and up to 0.03% of Ni.

[0151] The method according to the present invention aims to provide a hot-rolled annealed steel sheet having high cold rollability and high toughness, and this hot-rolled annealed steel sheet is suitable for producing a cold-rolled heat-treated steel sheet having a high combination of ductility and strength.

[0152] The method according to the present invention also aims to manufacture such a cold-rolled heat-treated steel sheet.

[0153] Compared with the sheet that has not undergone annealing, the inventors have studied the problem of low toughness of the hot-rolled batch-annealed steel sheet and the problem of deterioration of the mechanical properties of the cold-rolled heat-treated steel sheet manufactured from such a hot-rolled batch-annealed steel sheet, and found that these problems are caused by four main factors.

[0154] In particular, the inventors found that batch annealing leads to the formation of coarse cementite highly enriched in manganese, and thus this cementite is strongly stable in the hot-rolled batch-annealed steel sheet. The inventors also found that the thus-stabilized cementite cannot be completely dissolved during the subsequent standard heat treatment of the cold-rolled steel sheet. Therefore, a part of Mn in the steel remains in the cementite, thereby suppressing the influence of Mn on the strength and ductility of the steel.

[0155] The inventors also found that batch annealing also causes coarsening of the structure of the hot-rolled batch-annealed steel sheet, which leads to coarsening of the final structure of the cold-rolled heat-treated steel sheet and deteriorates the mechanical properties.

[0156] In addition, the inventors found that microalloying elements that can be included in the steel composition, especially Nb, precipitate as coarse precipitates that do not harden the steel during the early stage of batch annealing, and thus are no longer available during the subsequent heat treatment of the cold-rolled steel sheet for providing precipitation hardening.

[0157] Finally, the inventors found that batch annealing is carried out at temperatures and times that cause temper embrittlement, resulting in low toughness of the hot-rolled batch-annealed steel sheet.

[0158] To solve these problems, the inventors conducted experiments by raising the batch annealing temperature above the Ae1 transformation point of the steel.

[0159] However, the inventors found that using a higher batch annealing temperature, although limiting the formation of Mn-rich cementite, caused coarsening of the microstructure, thereby impairing the final properties of the cold-rolled heat-treated steel sheet.

[0160] Based on these results, the inventors found that if the hot-rolled steel sheet is annealed to have a microstructure containing the following, the cold-rollability and toughness can be greatly improved while ensuring the final properties of the cold-rolled heat-treated steel sheet:

[0161] - Ferrite, the ferrite having an average ferrite grain size of at most 3 μm,

[0162] - At most 30% austenite,

[0163] - At most 8% fresh martensite, and

[0164] - Cementite, the cementite having an average Mn content of less than 25%.

[0165] A fresh martensite fraction of at most 8% enables high toughness of the hot-rolled annealed steel sheet.

[0166] In particular, the inventors conducted experiments by subjecting hot-rolled steel sheets made of several types of steel to various annealing conditions, which resulted in changes in the austenite fraction and fresh martensite fraction after cooling to room temperature, and measured the Charpy energy of the steel sheets thus obtained at 20 °C.

[0167] 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. In addition, the inventors found that if the hot-rolled annealed steel sheet has a fresh martensite fraction of at most 8%, a high Charpy energy of at least 50 J / cm 2 is achieved at 20 °C.

[0168] In addition, cementite with an average Mn content of less than 25% means that it contributes to the dissolution of cementite during the final heat treatment of the cold-rolled steel sheet, which improves ductility and strength during further processing steps. In contrast, cementite with an average Mn content greater than 25% will result in a reduction in the mechanical properties of the cold-rolled heat-treated steel sheet produced from the hot-rolled annealed steel sheet.

[0169] Furthermore, an average ferrite grain size of at most 3 μm allows the production of cold-rolled heat-treated steel sheets with a very fine microstructure and improves their mechanical properties.

[0170] The inventors have also found that the above microstructure allows for a hardness of the hot-rolled annealed steel sheet of less than 400 HV, thus ensuring satisfactory cold-rollability of the hot-rolled annealed steel sheet.

[0171] The inventors have found that such a microstructure and these properties of the hot-rolled annealed steel sheet can be achieved by the following method: continuous annealing of the hot-rolled steel sheet at a continuous annealing temperature T ICA最小 from a minimum continuous annealing temperature T ICA最大 to a maximum continuous annealing temperature T ICA for a time of 3 s to 3600 s, the minimum continuous annealing temperature T ICA最小 = 650 °C, the maximum continuous annealing temperature T ICA最大 being the temperature at which 30% austenite is formed during heating; and subsequently cooling the hot-rolled steel sheet under specific cooling conditions.

[0172] In particular, the inventors have found that due to the high continuous annealing temperature T ICA , an annealing time of up to 3600 s is sufficient to achieve sufficient tempering of the microstructure, thus improving the cold-rollability of the hot-rolled annealed steel sheet while avoiding coarsening of the microstructure.

[0173] Furthermore, annealing the sheet at a temperature above 650 °C allows the hot-rolled steel sheet to soften, limits the Mn enrichment in the cementite particles to less than 25% and, if there are microalloying elements, limits the precipitation of the microalloying elements, and prevents coarsening of such precipitates, thus retaining the influence of C, Mn and microalloying elements on the final mechanical properties. Annealing the sheet at a temperature above 650 °C also limits the segregation of brittle impurities such as P at the grain boundaries.

[0174] The manufacturing method will now be described in further detail.

[0175] The method for producing the steel according to the present invention includes casting a steel having the chemical composition of the present invention.

[0176] Reheat the cast steel to a temperature T 再加热 of 1150 °C to 1300 °C.

[0177] When the slab reheating temperature T 再加热 is lower than 1150 °C, the rolling load increases too much and the hot-rolling process becomes difficult.

[0178] Above 1300 °C, oxidation is very strong, which can lead to scale loss and surface deterioration.

[0179] Hot-roll the reheated slab at a temperature of 1250 °C to 800 °C, with the final hot-rolling pass being carried out at a final rolling temperature T FRT above or equal to 800 °C.

[0180] If the final rolling temperature T FRT is lower than 800 °C, the hot workability is reduced.

[0181] After hot rolling, the steel is cooled at a cooling rate V of 1 °C / s to 150 °C / s c1 to a coiling temperature T lower than or equal to 650 °C 卷取 . Below 1 °C / s, a too coarse microstructure is produced and the final mechanical properties deteriorate. Above 150 °C / s, the cooling process is difficult to control.

[0182] The coiling temperature T 卷取 must be lower than or equal to 650 °C. If the coiling temperature is higher than 650 °C, deep intergranular oxidation forms below the scale, resulting in deterioration of the surface properties.

[0183] After coiling, the hot-rolled steel sheet is preferably pickled.

[0184] Then, the hot-rolled steel sheet is continuously annealed, that is, the unrolled hot-rolled steel sheet undergoes heat treatment by continuously traveling through a furnace.

[0185] At a continuous annealing temperature T between a minimum continuous annealing temperature T ICA最小 and a maximum continuous annealing temperature T ICA最大 the hot-rolled steel sheet is continuously annealed for a time of 3 s to 3600 s, the minimum continuous annealing temperature T ICA = 650 °C, the maximum continuous annealing temperature T ICA最小 being the temperature at which 30% austenite is formed upon heating. ICA最大 is the temperature at which 30% austenite is formed upon heating.

[0186] Under these conditions, the microstructure of the steel formed during continuous annealing and before cooling to room temperature comprises:

[0187] [[ID=4I]]- ferrite,

[0188] - less than 30% austenite,

[0189] - cementite, the cementite having an average Mn content of less than 25%.

[0190] If the continuous annealing temperature is lower than 650 °C, the softening due to microstructure recovery during the continuous annealing treatment is insufficient, making the hardness of the hot-rolled annealed steel sheet higher than 400 HV. A continuous annealing temperature below 650 °C also enhances the segregation of brittle elements such as P at grain boundaries and results in poor toughness values, which are crucial for further processing of the steel sheet.

[0191] If the continuous annealing temperature is higher than T ICA最大, an excessive austenite fraction will be generated during continuous annealing, which may lead to insufficient stability of austenite during cooling and the formation of more than 8% fresh martensite.

[0192] If the continuous annealing time is less than 3 s, the hardness of the hot-rolled annealed steel sheet will be too high, especially higher than 400 HV, making the cold rollability of the hot-rolled steel sheet unsatisfactory. The continuous annealing time is preferably at least 200 s.

[0193] If the continuous annealing time is greater than 3600 s, the microstructure will coarsen; in particular, the ferrite grains have an average size greater than 3 μm. Preferably, the continuous annealing time is at most 500 s.

[0194] The austenite that can be generated during annealing is rich in carbon and manganese. In particular, the austenite has an average Mn content of at least 1.3*Mn% and an average C content of at least 0.4%, where Mn% represents the Mn content of the steel.

[0195] Therefore, the austenite is strongly stable.

[0196] Then the hot-rolled steel sheet is cooled from the annealing temperature T ICA to room temperature, where the average cooling rate V between 600 °C and 350 °C ICA is at least 1 °C / s. Under these conditions, temper embrittlement is restricted.

[0197] If the cooling rate between 600 °C and 350 °C is lower than 1 °C / s, segregation occurs in the hot-rolled annealed steel sheet, enhancing temper embrittlement and making the cold rollability of the hot-rolled annealed steel sheet unsatisfactory.

[0198] The hot-rolled annealed steel sheet thus obtained has a microstructure composed of the following:

[0199] - Ferrite,

[0200] - Up to 30% austenite,

[0201] - Up to 8% fresh martensite,

[0202] - Cementite, the cementite having an average Mn content lower than 25%.

[0203] Due to the stabilization of austenite by Mn, a fresh martensite fraction of up to 8% is achieved, so that austenite does not transform into fresh martensite or only transforms into fresh martensite to a very small extent during cooling.

[0204] The retained austenite of the hot-rolled annealed steel sheet has an average Mn content of at least 1.3*Mn%, where Mn% represents the Mn content of the steel, and the retained austenite of the hot-rolled annealed steel sheet has an average C content of at least 0.4%.

[0205] Optionally, a tempering treatment is carried out to further limit the fraction of fresh martensite.

[0206] In addition, the ferrite grains have an average size of at most 3 μm. In fact, compared with batch annealing, continuous annealing carried out during a relatively short time period does not lead to coarsening of the microstructure and thus allows to obtain a hot-rolled annealed sheet having a very fine microstructure.

[0207] At this stage, the hot-rolled annealed sheet has improved cold rollability and toughness compared to the hot-rolled steel sheet before annealing. In addition, the hot-rolled annealed steel sheet is suitable for the production of cold-rolled heat-treated steel sheets having high mechanical properties, in particular high ductility and high strength.

[0208] In particular, the hot-rolled annealed sheet has a Vickers hardness of less than 400 HV and thus has very good cold rollability.

[0209] In addition, the hot-rolled annealed steel sheet has a Charpy energy of at least 50 J / cm at 20 °C 2 . Thus, compared with the hot-rolled steel sheet subjected to batch annealing, the hot-rolled annealed steel sheet has very good workability and the risk of strip breakage during further processing is greatly reduced. Furthermore, the inventors have found that not only the Charpy energy of the hot-rolled annealed steel sheet is higher than the Charpy energy of the hot-rolled batch-annealed steel sheet, but also the Charpy energy of the hot-rolled annealed steel sheet is generally higher than the Charpy energy of the hot-rolled steel sheet from which the hot-rolled annealed steel sheet is produced.

[0210] After cooling to room temperature, the hot-rolled annealed steel sheet is optionally pickled. However, this step can be omitted. In fact, due to the short duration of continuous annealing, no or almost no internal oxidation occurs during continuous annealing. Preferably, if pickling is not carried out between hot rolling and continuous annealing, the hot-rolled annealed steel sheet is pickled at this stage.

[0211] Then, the hot-rolled steel sheet is cold-rolled at a cold rolling reduction rate of 30% to 70% to obtain a cold-rolled steel sheet. Below 30%, it is not conducive to recrystallization during subsequent heat treatment, which may impair the ductility of the cold-rolled steel sheet after heat treatment. Above 70%, there is a risk of edge cracking during cold rolling.

[0212] Then, the cold-rolled steel sheet is heat-treated on a continuous annealing line to produce a cold-rolled heat-treated steel sheet.

[0213] The heat treatment carried out on the cold-rolled steel sheet is selected according to the final target mechanical properties.

[0214] In any case, the heat treatment includes the following steps: heating the cold-rolled steel sheet to an annealing temperature T of 650 °C to 1000 °C 退火, and the cold-rolled steel sheet is held at the annealing temperature T 退火 for an annealing time of 30 s to 10 min.

[0215] In addition, the annealing temperature T 退火 is such that the structure formed during annealing contains at least 8% austenite.

[0216] If the annealing temperature is lower than 650 °C, cementite will form in the structure during annealing, resulting in deterioration of the mechanical properties of the cold-rolled heat-treated steel sheet.

[0217] The annealing temperature T 退火 is at most 1000 °C to limit the coarsening of austenite grains.

[0218] The reheating rate Vr to reach the annealing temperature T 退火 is preferably 1 °C / s to 200 °C / s.

[0219] According to the first embodiment, the annealing is intercritical annealing, and the annealing temperature T 退火 is lower than Ae3, and is such that the structure formed during annealing contains at least 8% austenite.

[0220] According to the second embodiment, the annealing temperature T 退火 is higher than or equal to Ae3 so as to obtain a structure composed of austenite and at most 1% cementite during annealing.

[0221] In the first embodiment, at the end of the holding at the annealing temperature, the austenite has a C content of at least 0.4% and an average Mn content of at least 1.3*Mn%.

[0222] Then the cold-rolled annealed steel sheet is directly cooled to room temperature, that is, there is no holding, tempering or reheating step between the annealing temperature T 退火 and room temperature, or indirectly cooled to room temperature, that is, having a holding, tempering and / or reheating step, to obtain a cold-rolled heat-treated steel sheet.

[0223] In any case, the cold-rolled heat-treated steel sheet has a structure (hereinafter referred to as the final structure) containing the following:

[0224] - 8% to 50% retained austenite,

[0225] - martensite, which may contain fresh martensite and / or partitioned or tempered martensite, and optionally contains bainite,

[0226] - at most 80% subcritical ferrite, and

[0227] - at most 1% cementite.

[0228] The retained austenite generally has an average C content of at least 0.4% and an average Mn content of generally at least 1.3*Mn%.

[0229] Since the Mn content in cementite in the microstructure of the hot-rolled annealed steel sheet is at most 25%, the cementite is liable to dissolve during annealing. Depending on the heat treatment carried out, a small fraction of the cementite may remain in the final microstructure. However, the cementite fraction in the final microstructure will in any case remain below 1%. Additionally, if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0230] The martensite can include fresh martensite and partitioned martensite or tempered martensite.

[0231] As described in further detail below, the partitioned martensite has an average C content strictly below the nominal C content of the steel. This low C content is due to the partitioning of carbon from the martensite formed upon quenching below the Ms temperature of the steel to austenite during the holding period at the partitioning temperature T P between 350 °C and 500 °C.

[0232] In contrast, the tempered martensite has an average C content equal to the nominal C content of the steel. The tempered martensite is produced by the tempering of the martensite formed upon quenching below the Ms temperature of the steel.

[0233] The partitioned martensite can be distinguished from the tempered martensite and the fresh martensite on portions polished and etched with reagents known per se, such as Nital reagent, as observed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD).

[0234] The microstructure can contain bainite, in particular carbide-free bainite, with fewer than 100 carbides per 100 mm 2 of surface unit.

[0235] The ferrite fraction depends on the annealing temperature during the heat treatment.

[0236] The ferrite is hypoeutectoid ferrite when present in the final microstructure.

[0237] Thus, when ferrite is present, the ferrite is inherited from the microstructure of the hot-rolled annealed steel sheet which is then cold-rolled and recrystallized. Thus, the ferrite has an average grain size of at most 1.5 μm.

[0238] The preferred heat treatment carried out on the cold-rolled steel sheet will now be described in further detail.

[0239] In a first preferred heat treatment, at an annealing temperature T below or above Ae3 退火After holding at the following temperature, the cold-rolled steel sheet is cooled to room temperature at a cooling rate Vc2 of 1 °C / s to 70 °C / s.

[0240] The cold-rolled steel sheet is cooled to room temperature at a cooling rate Vc2, or is cooled to a holding temperature T of 350 °C to 550 °C at a cooling rate Vc2 H and is held at the holding temperature T H for a time of 10 s to 500 s. It is shown that this heat treatment to promote, for example, Zn coating by a hot-dip process does not affect the final mechanical properties. After an optional holding at the holding temperature T H the cold-rolled steel sheet is cooled to room temperature at a cooling rate Vc3 of 1 °C / s to 70 °C / s.

[0241] Optionally, after cooling to room temperature, the cold-rolled heat-treated steel sheet is tempered at a temperature T of 170 °C to 450 °C t for a tempering time t of 10 s to 1200 s t .

[0242] This treatment can achieve the tempering of martensite that may form during cooling to room temperature after annealing. Therefore, the martensite hardness is reduced and the ductility is improved. Below 170 °C, the tempering treatment is not effective enough. Above 450 °C, the strength loss becomes high and the balance between strength and ductility is no longer improved.

[0243] The structure of the cold-rolled heat-treated steel sheet obtained by the first preferred heat treatment consists of the following, by surface fraction:

[0244] - 8% to 50% retained austenite having an average C content of at least 0.4%,

[0245] - at most 80% hypoeutectoid ferrite,

[0246] - at most 92% martensite and / or bainite,

[0247] - at most 1% cementite.

[0248] The martensite consists of tempered martensite and / or fresh martensite.

[0249] This structure may contain bainite, in particular carbide-free bainite, with less than 100 carbides per 100 mm 2 of the surface unit.

[0250] The average size of the cementite particles is less than 50 nm.

[0251] The ferrite fraction and austenite fraction depend on the annealing temperature during the heat treatment.

[0252] In the first variant of the first preferred heat treatment, the annealing temperature T退火 below Ae3, and preferably such that the structure formed upon annealing contains 40% to 80% ferrite.

[0253] In this first variant, the final structure preferably contains, by surface fraction:

[0254] - 8% to 50% retained austenite having an average C content of at least 0.4% and an average Mn content of at least 1.3*Mn%,

[0255] - 40% to 80% hypoeutectoid ferrite with ferrite grains having an average size of at most 1.5 μm,

[0256] - at most 15% martensite (consisting of tempered martensite and / or fresh martensite) and / or bainite,

[0257] - at most 0.3% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0258] In a second variant of the first preferred heat treatment, the annealing temperature is higher than or equal to Ae3.

[0259] In the second variant, the final structure contains:

[0260] - 8% to 30% retained austenite having an average C content of at least 0.4%,

[0261] - 70% to 92% martensite (consisting of tempered martensite and / or fresh martensite) and / or bainite,

[0262] - at most 1% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0263] In the second preferred heat treatment, the cold-rolled steel sheet is subjected to a quenching and partitioning process.

[0264] For this purpose, after holding at the annealing temperature T 退火 the cold-rolled steel sheet is quenched from the annealing temperature T at a cooling rate Vc4 high enough to avoid the formation of ferrite and pearlite upon cooling to a quenching temperature QT below the Ms transformation point of austenite. 退火 The cooling rate Vc4 to reach the quenching temperature QT is preferably at least 2 °C / s.

[0265] During this quenching step, austenite is partially transformed into martensite.

[0266] During this quenching step, austenite is partially transformed into martensite.

[0267] The quenching temperature is selected between Mf + 20 °C and Ms - 20 °C according to the desired final microstructure, in particular according to the desired fractions of partitioned martensite and retained austenite in the final microstructure. For each specific composition and each microstructure of the steel, a person skilled in the art knows how to determine the Ms starting transformation point and the Mf finishing transformation point of austenite by dilatometry.

[0268] If the quenching temperature QT is lower than Mf + 20 °C, the fraction of partitioned martensite in the final microstructure is too high. Furthermore, if the quenching temperature QT is higher than Ms - 20 °C, the fraction of partitioned martensite in the final microstructure is too low, such that a high ductility will not be achieved.

[0269] A person skilled in the art knows how to determine the quenching temperature suitable for obtaining the desired microstructure.

[0270] Optionally, the cold-rolled steel sheet is held at the quenching temperature QT for a holding time tQ of 2 s to 200 s, preferably 3 s to 7 s, to avoid the formation of ε-carbides in the martensite, which would lead to a reduction in the ductility of the steel.

[0271] Then the cold-rolled steel sheet is reheated to a partitioning temperature T of 350 °C to 500 °C P and held at the partitioning temperature T P for a partitioning time t of 3 s to 1000 s P . During this partitioning step, carbon diffuses from the martensite to the austenite, thereby achieving C enrichment of the austenite.

[0272] If the partitioning temperature T P is higher than 500 °C or lower than 350 °C, the elongation of the final product is not satisfactory.

[0273] Optionally, the cold-rolled steel sheet is hot-dip coated in a bath at a temperature, for example, below or equal to 480 °C. Any type of coating can be used and in particular zinc or zinc alloys (such as zinc-nickel alloy, zinc-magnesium alloy or zinc-magnesium-aluminum alloy), aluminum or aluminum alloys (such as aluminum-silicon alloy).

[0274] Immediately after the partitioning step, or if a hot-dip coating step is carried out, immediately after the hot-dip coating step, the cold-rolled steel sheet is cooled to room temperature to obtain a cold-rolled heat-treated steel sheet. The cooling rate to reach room temperature is preferably higher than 1 °C / s, for example 2 °C / s to 20 °C / s.

[0275] The final microstructure of the cold-rolled heat-treated steel sheet obtained by the second preferred heat treatment mainly depends on the annealing temperature T 退火 and the quenching temperature QT.

[0276] However, the microstructure of the cold-rolled heat-treated steel sheet thus obtained generally consists, by surface fraction, of the following:

[0277] - 8% to 30% retained austenite,

[0278] - up to 45% hypoeutectoid ferrite,

[0279] - partitioned martensite,

[0280] - up to 8% fresh martensite,

[0281] - up to 1% cementite.

[0282] The retained austenite is rich in carbon and in particular has an average C content of at least 0.4%.

[0283] If there is ferrite, the ferrite is hypoeutectoid ferrite and has an average grain size of at most 1.5 μm.

[0284] The fraction of fresh martensite in the structure is less than or equal to 8%. In fact, a fraction of fresh martensite higher than 8% will damage the hole expansion rate HER.

[0285] In this second preferred heat treatment, a small fraction of cementite may form during cooling from the annealing temperature and during partitioning. However, the fraction of cementite in the final structure will in any case remain below 1%, and the average size of the cementite particles in the final structure remains below 50 nm.

[0286] In a first variant of the second preferred embodiment, the annealing temperature T 退火 is such that the cold-rolled steel sheet has a structure consisting of the following by surface fraction upon annealing:

[0287] - 10% to 45% ferrite,

[0288] - austenite, and

[0289] - up to 0.3% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0290] In this first variant, the final structure preferably contains by surface fraction:

[0291] - 10% to 45% hypoeutectoid ferrite having an average grain size of at most 1.5 μm,

[0292] - 8% to 30% retained austenite,

[0293] - partitioned martensite,

[0294] - up to 8% fresh martensite, and

[0295] - Cementite of at most 0.3%, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0296] The retained austenite is rich in Mn and C. In particular, the average C content in the retained austenite is at least 0.4%, and the average Mn content in the retained austenite is at least 1.3 * Mn%.

[0297] In a second variant of the second preferred embodiment, the annealing temperature T 退火 is higher than or equal to Ae3, such that the cold-rolled steel sheet has a structure composed of austenite and at most 0.3% cementite upon annealing.

[0298] In this second variant, the quenching temperature QT is preferably selected to obtain a structure composed of at most 8% to 30% austenite, at most 92% martensite, and at most 1% cementite immediately after quenching.

[0299] In this second variant, the final structure consists of, by surface fraction:

[0300] - 8% to 30% retained austenite,

[0301] - Partitioned martensite

[0302] - At most 8% fresh martensite, and

[0303] - At most 1% cementite, and if there are cementite particles, the cementite particles have an average size of less than 50 nm.

[0304] The retained austenite is rich in C, and the average C content in the retained austenite is at least 0.4%.

[0305] The above microstructural features are determined, for example, by observing the microstructure using a scanning electron microscope with a field emission gun ("FEG-SEM") coupled to an electron backscatter diffraction ("EBSD") device and transmission electron microscopy ("TEM") at a magnification greater than 5000x.

[0306] Examples:

[0307] As examples and for comparison, sheets made of steels composed according to Table 1, with the contents expressed in weight percentages, have been manufactured.

[0308] Table 1

[0309]

[0310]

[0311] In a first experiment, steels I1, I2, I3, I6, and I7 were cast to obtain ingots. At a temperature T of 1250 °C再加热 Next, the ingot is reheated, descaled, and hot-rolled at a temperature above Ar3 to obtain hot-rolled steel.

[0312] Then, the hot-rolled steel is cooled to the coiling temperature T at a cooling rate Vc1 of 1 °C / s to 150 °C / s. 卷取 and coiled at this temperature T. 卷取

[0313] Then, some of the hot-rolled steel is continuously annealed or batch-annealed at the annealing temperature T for an annealing time t. A A and then cooled to room temperature at an average cooling rate V between 600 °C and 350 °C. ICA

[0314] The manufacturing conditions of the hot-rolled and annealed steel sheets, as well as the austenite fraction generated during annealing, are reported in Table 2 below.

[0315] Table 2

[0316]

[0317]

[0318]

[0319] In Table 2, the underlined values are not values according to the present invention, and "n.d." means "not determined".

[0320] The inventors have studied the microstructure of the thus obtained hot-rolled and optionally annealed steel sheets by means of a scanning electron microscope with a field emission gun ("FEG-SEM") having a magnification greater than 5000x coupled to an electron backscatter diffraction ("EBSD") device and a transmission electron microscopy ("TEM").

[0321] In particular, the inventors have 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).

[0322] The inventors have also measured the Charpy energy and the Vickers hardness of the hot-rolled steel sheets at 20 °C. The characteristics of the microstructure and the mechanical properties are reported in Table 3 below.

[0323] Table 3

[0324]

[0325]

[0326]

[0327] In this table, n.d. means "not determined". The underlined values are not in accordance with the values of the present invention.

[0328] These experiments show that the target microstructure and target mechanical properties of the hot-rolled annealed steel sheet can be achieved only when the hot-rolled steel sheet is annealed under the conditions of the present invention.

[0329] In contrast, Examples I1A, I2A, I3A, I6A and I7A were not subjected to any annealing.

[0330] Therefore, the hardness of I1A, I2A, I3A, I6A and I7A is higher than 400 HV, resulting in insufficient cold rollability of these hot-rolled steel sheets.

[0331] Examples I1B, I2B and I3B were batch annealed for 25200 s at a temperature of 500 °C. Compared with Examples I1A, I2A and I3A which were not subjected to any annealing respectively, batch annealing reduced the hardness. However, batch annealing led to a reduction in the Charpy energy, resulting in insufficient workability of Examples I1B, I2B and I3B. In addition, batch annealing led to the formation of cementite highly enriched in Mn.

[0332] Examples I1C, I2C, I3C, I6C and I7C were also batch annealed for 25200 s at a temperature of 600 °C. Compared with Examples I1A, I2A, I3A, I6A and I7A respectively, the hardness of these examples was reduced due to batch annealing, and compared with Examples I1B, I2B and I3B, the hardness of these examples was further reduced. However, the Charpy energy remained below 50 J / cm 2 , and batch annealing led to the formation of cementite highly enriched in Mn.

[0333] Then the inventors conducted experiments by raising the batch annealing temperature to 650 °C which is higher than the Ae1 transformation point (Examples I1D, I2D, I3D, I6D and I7D). Compared with Examples I1C, I2C, I3C, I6C and I7C respectively, this higher batch annealing temperature increased the Charpy energy of the sheet and reduced the average Mn content in the cementite.

[0334] However, batch annealing at a temperature higher than Ae1 led to coarsening of the microstructure and a ferrite grain size greater than 3 μm.

[0335] The inventors further increased the batch annealing temperature to 680 °C (Examples I1E and I3E). This increase in the batch annealing temperature further increased the Charpy energy and further reduced the average Mn content in the cementite. However, this increase in the batch annealing temperature also led to an undesired further increase in the ferrite grain size.

[0336] Thus, these examples show that even though batch annealing reduces the hardness of the hot-rolled steel sheet, the Charpy energy of the hot-rolled batch-annealed steel sheet is generally not sufficient to ensure high workability of the steel sheet. Additionally, batch annealing results in the undesired formation of cementite highly enriched in Mn. These examples further show that although an increase in the batch annealing temperature can increase the Charpy energy and reduce the average Mn content in the cementite, the Charpy energy still remains below 50 J / cm 2 of the target value in most cases, and an increase in the batch annealing temperature leads to undesired coarsening of the microstructure.

[0337] Example I3L was subjected to continuous annealing, however, at a continuous annealing temperature below 650 °C. Thus, due to insufficient softening by microstructure recovery, the hardness of Example I3L was higher than 400 HV and the Charpy energy was insufficient.

[0338] Examples I1G and I3Q were continuously annealed at an annealing temperature such that more than 30% austenite was formed during annealing. Thus, the fresh martensite fraction in the hot-rolled annealed steel sheet was higher than 8%, making the hardness of these examples higher than 400 HV and the Charpy energy of these examples lower than 50 J / cm 2 .

[0339] Examples I1F, I2H, I2J, I2K, I3H, I3M, I3, I3O, I3P, I3J, I6K, and I7K were subjected to continuous annealing under the conditions of the present invention. Thus, the hot-rolled annealed steel sheet has a Charpy energy of at least 50 J / cm at 20 °C 2 and a hardness less than or equal to 400 HV. Thus, these hot-rolled annealed steel sheets have satisfactory cold rollability and workability. Additionally, the microstructure of these examples results in an average ferrite grain size less than 3 μm and an average Mn content in the cementite lower than 25%. Thus, these hot-rolled steel sheets are suitable for the production of cold-rolled heat-treated steel sheets with high mechanical properties.

[0340] Observe the microstructure of the thus obtained hot-rolled annealed steel sheet.

[0341] The microstructures of Examples I1E and I1F are shown in Figure 1 and Figure 2 respectively.

[0342] 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 the microstructure of steel I1E produced by batch annealing above Ae1.

[0343] These experiments demonstrate that, unlike batch annealing, continuous annealing according to the present invention results in a very fine microstructure.

[0344] The inventors also conducted the following experiments: The experiments were used to evaluate the final properties of cold-rolled heat-treated steel produced by batch annealing at a temperature below Ae1 or above Ae1 or cold-rolled heat-treated steel that had undergone continuous annealing according to the present invention before cold rolling.

[0345] Specifically, steels I1, I2, I4, I5, I6, and I7 were cast to obtain ingots. The ingots were reheated at a temperature of 1250 °C, descaled, and hot-rolled at a temperature above Ar3 to obtain hot-rolled steel. 再加热 The hot-rolled steel plates were then coiled at temperature T.

[0346] Then the hot-rolled steel plates were batch annealed or continuously annealed. 卷取 The hot-rolled steel plates were then coiled at temperature T.

[0347] Then the hot-rolled steel plates were batch annealed or continuously annealed.

[0348] Then the hot-rolled annealed steel plates were cold-rolled at a cold rolling reduction rate of 50%, and the hot-rolled annealed steel plates were subjected to various heat treatments, including annealing and then cooling to room temperature at a cooling rate Vc1.

[0349] Then the yield strength, tensile strength, uniform elongation, and hole expansion ratio of the cold-rolled heat-treated steel plates thus obtained were measured.

[0350] The manufacturing conditions and the measured properties are reported in Tables 4 and 5.

[0351] In these tables, T 卷取 represents the coiling temperature, T A and t A are the batch or continuous annealing temperature and annealing time, HBA refers to batch annealing, ICA refers to continuous annealing according to the present invention, T 退火 is the annealing temperature, t 退火 is the annealing time, and VC1 is the cooling rate (or cooling condition).

[0352] The measured properties reported in Tables 4 and 5 are the yield strength YS, tensile strength TS, uniform elongation UE, and hole expansion ratio HER.

[0353] In these tables, "n.d." means "not determined". The underlined values are not values according to the present invention.

[0354] Table 4

[0355]

[0356]

[0357] Table 5

[0358]

[0359] The properties of the examples made of steel I4 are reported in Figure 3 (UTS represents the ultimate tensile strength, and UEI represents the uniform elongation).

[0360] In this figure, each curve corresponds to the annealing conditions after hot rolling (black squares: batch annealing at 600 °C for 300 min; white squares: continuous annealing at 700 °C for 2 min), and each point on each curve reports the ultimate tensile strength and uniform elongation obtained at a specific annealing temperature. It should be understood that the higher the annealing temperature, the higher the ultimate tensile strength.

[0361] Figure 3 And the results reported in Table 4 demonstrate that compared with batch annealing, performing continuous annealing of the present invention allows for an improved combination of ultimate tensile strength and elongation.

[0362] Therefore, the steel sheet manufactured according to the present invention can be advantageously used for manufacturing structural components or safety components of vehicles.

Claims

1. A cold-rolled heat-treated steel sheet, which is made of steel having a composition containing the following components 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％， The balance is iron and inevitable impurities generated during smelting, wherein the cold-rolled steel sheet has a structure composed of the following components by surface fraction: - 8% to 50% retained austenite, - up to 80% hypoeutectoid ferrite. If there are ferrite grains, the ferrite grains have an average size of at most 1.5 μm, and - up to 1% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm, - Martensite and carbide-free bainite, with less than 100 carbides per 100 mm 2 of the surface unit.

2. The cold-rolled heat-treated steel sheet according to claim 1, wherein the structure contains at least 10% hypoeutectoid ferrite by surface fraction.

3. The cold-rolled heat-treated steel sheet according to claim 1, wherein the structure is composed of the following components by surface fraction: - 8% to 50% retained austenite, - up to 1% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm, - Martensite and carbide-free bainite, with less than 100 carbides per 100 mm 2 of the surface unit.

4. The cold-rolled heat-treated steel sheet according to any one of claims 1 or 2, wherein the martensite is composed of tempered martensite and / or fresh martensite.

5. The cold-rolled heat-treated steel sheet according to claim 4, wherein the structure is composed of the following components by surface fraction: - 8% to 50% retained austenite, the retained austenite having an average C content of at least 0.4% and an average Mn content of at least 1.3*Mn%, where Mn% represents the average Mn content in the steel composition, - 40% to 80% hypoeutectoid ferrite, - Up to 15% martensite and carbide-free bainite, with less than 100 carbides per 100 mm 2 of surface unit, and - up to 0.3% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm.

6. The cold-rolled heat-treated steel sheet according to claim 4, wherein the structure is composed of the following components by surface fraction: - 8% to 30% retained austenite, the retained austenite having an average C content of at least 0.4%, - 70% to 92% martensite and carbide-free bainite, with less than 100 carbides per 100 mm 2 of surface unit, and - up to 1% cementite. If there are cementite particles, the cementite particles have an average size of less than 50 nm.

Images