Cold-rolled and heat-treated steel sheet and method of its manufacturing

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to cold-rolled steel sheet and can be used as a steel sheet for vehicles. Cold rolled and heat treated steel sheet comprises the following elements, wt.%: 0.3 ≤ C ≤ 0.45; 0.4 ≤ Mn ≤ 1.4; 0.1 ≤ Si ≤ 0.9; 0.01 ≤ Al ≤ 0.1; 0.1 ≤ Cr ≤ 0.8; 0.01 ≤ Nb ≤ 0.1; 0.1 ≤ Ni ≤ 0.9; 0.01 ≤ Мо ≤ 0.9; 0.01 ≤ Ti ≤ 0.1; 0.0001 ≤ В ≤ 0.010; Р ≤ 0.02; S ≤ 0.03; N ≤ 0.09; the rest of the composition consists of iron and inevitable impurities arising during manufacturing, the microstructure of the specified steel includes, in percentage of area, 1-5% bainite, the rest is tempered martensite.EFFECT: high values of tensile strength, yield strength, total elongation and hole expansion coefficient.15 cl, 4 tbl, 2 ex

Description

[0001] The present invention relates to a cold-rolled steel sheet that is suitable for use as a steel sheet for vehicles.

[0002] Automotive parts must meet two incompatible requirements: ease of molding and strength. However, in recent years, global environmental issues have added another requirement: reducing fuel consumption. Therefore, automotive parts must now be made of materials with high formability to meet the criteria of easy installation in complex automotive assemblies. At the same time, they must have increased strength to ensure impact resistance and vehicle durability, while simultaneously reducing vehicle weight to improve fuel economy. Furthermore, the steel part must be weldable without liquid metal embrittlement.

[0003] Therefore, intensive research and development is being conducted to reduce the number of materials used in automotive manufacturing by increasing their strength. Conversely, increasing the strength of steel sheets reduces formability, necessitating the development of materials with both high strength and high formability.

[0004] Earlier research and development in the field of high-strength steel sheets with high formability have led to several methods for producing high-strength steel sheets suitable for formability, some of which are listed in the description for unambiguous evaluation of the present invention:

[0005] EP3486346 presents a steel sheet with a specific chemical composition and a method for manufacturing the steel sheet. The steel sheet has a microstructure comprising martensite and bainite. The total area proportion of martensite and bainite in the overall microstructure is 95% or more and 100% or less. The remainder of the microstructure is at least one phase of ferrite and retained austenite. The microstructure includes certain inclusion clusters, and the inclusion cluster content in the microstructure is 5 clusters / mm. 2 or less. The microstructure includes former austenite grains with an average size greater than 5 μm. The steel sheet has a tensile strength of 1320 MPa or more. However, the bendability of EP3486346 steel cannot reach a value of 2.5 xt or less.

[0006] The prior art related to the production of high-strength steel sheets with high formability has one or more gaps: therefore, there is a need for a cold-rolled steel sheet with a strength of more than 1500 MPa and a method for producing the same.

[0007] The purpose of the present invention is to solve these problems by providing cold rolled and heat treated steel sheets that simultaneously have:

[0008] - tensile strength of 1500-1750 MPa and preferably 1580-1720 MPa,

[0009] - yield strength more than 1250 - 1450 MPa and preferably 1280 - 1400 MPa,

[0010] -total elongation of 6% or more,

[0011] - The opening distribution ratio is 40% or more, and preferably more than 45%.

[0012] In a preferred embodiment, the cold-rolled and heat-treated steel sheet has a YS / TS ratio of more than 0.60.

[0013] Preferably, such steel can also be well suited for forming, particularly for rolling, with suitable weldability and coating acceptance ability.

[0014] In a preferred embodiment, the cold-rolled and heat-treated steel sheet has a bendability of less than 3.0 when measured at an angle of 90° for r / t. The bending test is conducted in accordance with ASTM E290-22. The ratio of the punch radius to the material thickness is calculated as the bendability of the material.

[0015] Another object of the present invention is also to provide a method for producing these sheets that is compatible with conventional industrial applications, but at the same time robust to changes in production parameters.

[0016] The cold rolled heat treated steel sheet of the present invention may optionally be coated with zinc or zinc alloys, or aluminum or aluminum alloys to improve its corrosion resistance (in the description, electrochemical, JVD (jet vapor deposition), CVD (chemical vapor deposition), hot dip and other coating processes).

[0017] Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

[0018] Carbon is present in the steel in an amount of 0.3-0.45%. Carbon is an element necessary for increasing the strength of the steel sheet by delaying the formation of ferrite and bainite during cooling after annealing. A content of less than 0.3% will not allow the steel of the present invention to have sufficient tensile strength and ductility. On the other hand, when the carbon content exceeds 0.45%, the weld zone and the heat-affected zone are significantly hardened, and thus the mechanical properties of the weld zone are deteriorated. Preferably, the carbon content limit is 0.31-0.4%, and more preferably, the limit is 0.32-0.38%.

[0019] The manganese content in the steel of the present invention is 0.4-1.4%. Manganese is an element that imparts strength, and a manganese content of at least 0.4% is necessary to ensure the strength and hardenability of the steel sheet by delaying the formation of ferrite. Therefore, a manganese content of 0.5-1.3% is preferable, and 0.55-0.9% is more preferable. However, when the manganese content exceeds 1.4%, it causes adverse effects such as slowing down the transformation of austenite into martensite, resulting in decreased ductility in the final product. Moreover, a manganese content above 1.4% will cause central liquation and reduce the weldability of this steel. In addition, a high manganese content is detrimental to delayed hydrogen attack, which is an important criterion for steel manufacturers and the automobile industry.

[0020] The silicon content in the steel of the present invention is 0.1-0.9%. Silicon is an element that contributes to the increase in strength through solid solution strengthening. Silicon is a component that can inhibit the precipitation of carbides during cooling after annealing, so silicon promotes the formation of martensite. However, silicon is also a ferrite-forming element and also increases the Ac3 transformation point, which shifts the annealing temperature to higher temperature ranges. Therefore, the silicon content is maintained at a maximum of 0.9%. Silicon content above 0.9% can also cause embrittlement due to tempering, and silicon also deteriorates the ability to accept a coating. The preferred silicon content limit is 0.2-0.8%, and more preferably 0.3-0.7%.

[0021] The aluminum content in the steel of the present invention is 0.01-0.1%. Aluminum can be added during steel preparation to deoxidize the steel to absorb oxygen. A content above 0.1% will increase the Ac3 point, thereby reducing productivity. In addition, in this range, aluminum binds nitrogen in the steel to form aluminum nitride, reducing the grain size, and aluminum also delays the precipitation of cementite. However, when the aluminum content exceeds 0.1% in the present invention, the amount and size of aluminum nitrides worsen hole expansion and bendability, and shift Ac3 to higher temperature ranges, which is very expensive for the industry to achieve, and also cause grain coarsening during soaking during annealing. The preferred limit of the aluminum content is 0.01-0.08%, and more preferably 0.01-0.06%.

[0022] Chromium is an essential element in the steel of the present invention, present in an amount of 0.1-0.8%. Chromium provides strength and hardening to the steel, but when used in amounts exceeding 0.8%, it degrades the surface quality of the steel. The preferred chromium content is 0.2-0.7%, and more preferably 0.2-0.6%.

[0023] Niobium is an essential element and can be present in an amount of 0.01-0.1%, preferably 0.01-0.09%, and more preferably 0.01-0.07%. It is suitable for forming carbonitrides to impart strength to the steel according to the invention by precipitation strengthening in the annealing soak temperature range, resulting in product hardening. However, when the niobium content exceeds 0.1%, niobium binds carbon to form a large amount of carbonitrides, which is unfavorable for the present invention, since a large amount of carbonitrides tends to reduce the ductility of the steel.

[0024] Nickel is an essential element and is present in an amount of 0.1-0.9% to increase the strength of the steel of the present invention and improve its toughness. To achieve these effects, a minimum content of 0.01% is preferred. The preferred nickel content range is 0.2-0.7%, and more preferably 0.3-0.6%.

[0025] Molybdenum is an essential element and is present in the steel of the present invention in an amount of 0.01-0.9%. Molybdenum plays an effective role in improving hardening and strengthening, and delaying the formation of ferrite and bainite during cooling after annealing when it is added in an amount of at least 0.01%. Mo is also beneficial for the toughness of hot-rolled products, which facilitates production. However, the addition of molybdenum excessively increases the cost of adding alloying elements, so for economic reasons, its content is limited to 0.9%. The preferred limit of molybdenum content is 0.01-0.7%, and more preferably 0.01-0.6%.

[0026] Titanium is an essential element that is added to the steel of the present invention in an amount of 0.01 to 0.1%, preferably 0.01 to 0.09%. It is suitable for forming carbides, nitrides, and carbonitrides to impart strength to the steel of the invention by precipitation strengthening during annealing in the soaking temperature range, resulting in strengthening of the product. However, when the titanium content exceeds 0.1%, titanium binds carbon to form a large amount of precipitates, and this is unfavorable for the present invention because a large amount of precipitates tends to reduce the ductility of the steel. The preferred limit of the titanium content is 0.01 to 0.08%, and more preferably 0.01 to 0.06%.

[0027] Boron is an essential element added in amounts of 0.0001-0.010%, preferably 0.001-0.004%, to strengthen steel. Boron binds nitride to form boron nitride, which imparts strength to the steel of the present invention. Boron also imparts hardenability to the steel of the present invention. However, when boron is added in amounts exceeding 0.010%, the rollability of the steel sheet is significantly reduced. Further boron segregation may occur at grain boundaries, which adversely affects formability.

[0028] The phosphorus content in the steel of the present invention is limited to 0.02%. Phosphorus is an element that causes solid-solution strengthening. Therefore, a small amount of phosphorus, at least 0.002%, can be beneficial. However, phosphorus also has its own adverse effects, such as reduced spot weldability and hot ductility, especially due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is preferably limited to a maximum of 0.015%.

[0029] Sulfur is not an essential element, but may be present as an impurity in steel. The sulfur content should preferably be as low as possible, but can be 0.03% or less, and preferably no more than 0.005%, from a production cost perspective. Furthermore, if sulfur is present in excess in steel, it combines, especially with Mn and Ti, to form sulfides, which are detrimental to the bendability, hole expansion, and elongation of the steel of the present invention.

[0030] The nitrogen content is limited to 0.09% to avoid aging of the material and minimize the precipitation of nitrides during solidification, which are harmful to the mechanical properties of steel.

[0031] Vanadium is an optional element that can be added to the steel of the present invention in an amount of 0-0.1%, preferably 0.001-0.1%. Like niobium, it participates in the formation of carbonitrides, thus playing a role in strengthening. However, it also contributes to the formation of VN, which occurs during the solidification of the cast product. The amount of V is limited to 0.1% to avoid the formation of coarse VN, which is detrimental to hole expansion. If the vanadium content is less than 0.001%, it has no effect on the steel of the present invention.

[0032] Copper can be added as an optional element in an amount of 0-2% to increase the strength of the steel of the present invention and improve its corrosion resistance. To achieve these effects, a minimum content of 0.01% is preferred. However, when its content exceeds 2%, it may degrade the surface properties.

[0033] Calcium is an optional element that can be added to the steel of the present invention in an amount of 0-0.005%, preferably 0.001-0.005%. Calcium is added to the steel of the present invention as an optional element, especially during the treatment of inclusions. Calcium promotes the purification of the steel by retaining harmful sulfur content during its globularization.

[0034] Other elements such as cerium, magnesium or zirconium may be added individually or together in the following proportions: Ce ≤ 0.1%, Mg ≤ 0.05% and Zr ≤ 0.05%. Up to the maximum specified content levels, these elements allow the inclusion grain to be refined during solidification.

[0035] The rest of the steel's composition consists of iron and inevitable impurities formed during manufacturing.

[0036] The microstructure of the steel sheet, according to the invention, includes 1 - 5% bainite, the rest is tempered martensite in fractions of the area.

[0037] The area fractions of phases in the microstructure are determined using the following method: a sample is cut from a steel sheet, polished, and etched with a known reagent to reveal the microstructure. The section is then examined using a scanning electron microscope, for example, at a magnification greater than 500x, in secondary electron mode.

[0038] The ferrite content is determined using SEM after etching with Nital or Picral / Nital reagent.

[0039] Tempered martensite accounts for at least 95% of the microstructure in area fractions. Tempered martensite is formed from martensite that forms during cooling after annealing, particularly at temperatures below Ms, and particularly below Ms-10°C. This martensite is then tempered during holding at the tempering temperature. The tempered martensite of the present invention imparts ductility and strength to such steel. Preferably, the tempered martensite content is 96-99%.

[0040] Bainite is present in an amount of 1-5%. For the purposes of this invention, bainite may include carbide-free bainite and / or lath bainite and granular bainite. When present, lath bainite has the form of laths with a thickness of 1-5 μm. When present, carbide-free bainite is bainite with a very low carbide density, less than 100 carbides per unit area of ​​100 μm. 2 , and possibly containing austenite islands. When present, granular bainite takes the form of a grain with carbides present within the grains. Bainite provides improved elongation. The preferred bainite content is 1-4%.

[0041] In addition to the above-mentioned microstructure, the microstructure of cold rolled and heat treated steel sheet does not contain microstructural components such as ferrite, fresh martensite, residual austenite, pearlite and cementite, without deteriorating the mechanical properties of steel sheets.

[0042] The cold-rolled steel sheet according to the invention can be produced by any suitable method. A preferred method involves preparing a semi-finished steel casting with the chemical composition according to the invention. Casting can be performed either as ingots or continuously as thin slabs or thin strips, i.e., with a thickness ranging from approximately 220 mm for slabs to several tens of millimeters for thin strip.

[0043] For example, a slab will be considered a semi-finished product. A slab with the above-described chemical composition is produced by continuous casting, preferably subjected to direct soft reduction during casting to ensure the elimination of central segregation and the reduction of porosity. A slab produced by continuous casting can be used directly at high temperatures after continuous casting, or can be first cooled to room temperature and then reheated for hot rolling.

[0044] The temperature of the slab undergoing hot rolling is preferably at least 1000°C, preferably above 1150°C, and should be below 1300°C. If the slab temperature is below 1150°C, the rolling mill is subjected to excessive stress, and the steel temperature may also drop to the ferrite transformation temperature during finish rolling, resulting in the steel being rolled in a state in which the structure contains transformed ferrite. Furthermore, the temperature should not exceed 1300°C, as this is industrially expensive.

[0045] The slab temperature is preferably high enough to allow hot rolling to be completed entirely in the austenitic range, while the finishing hot rolling temperature remains above 850°C. It is necessary that the final rolling be performed above 850°C, since below this temperature, the steel sheet exhibits a significant drop in rollability.

[0046] The sheet thus obtained is then cooled at a cooling rate of at least 5°C / s to a temperature lower than or equal to 580°C. Preferably, the cooling rate is lower than or equal to 100°C / s and higher than 10°C / s. Thereafter, the hot-rolled steel sheet is coiled at a coiling temperature of 580 to 500°C, preferably 500 to 570°C, and more preferably 510 to 550°C. Thereafter, the coiled hot-rolled steel sheet is allowed to cool, preferably to room temperature. Then, the hot-rolled sheet may be subjected to an optional descaling process, such as pickling, to remove scale generated during hot rolling and ensure that there is no scale on the surface of the hot-rolled steel sheet, before subjecting it to optional hot-zone annealing.

[0047] The hot-rolled sheet may optionally be subjected to hot-zone annealing at a temperature of 350 to 750°C for 1 to 96 hours. The temperature and time of this hot-zone annealing are selected to soften the hot-rolled sheet to facilitate cold rolling of the hot-rolled steel sheet. The hot-rolled sheet may then be subjected to an optional descaling process, such as pickling, to remove scale formed during the hot-zone annealing.

[0048] The hot rolled steel sheet is then cooled to room temperature, after which the hot rolled sheet is cold rolled with a reduction ratio of 35-90% to obtain cold rolled steel sheet.

[0049] Then the cold rolled steel sheet is annealed to give the steel of the present invention the target microstructure and mechanical properties.

[0050] During annealing, the cold-rolled steel sheet is subjected to heating, in which the cold-rolled steel sheet is heated from room temperature until it reaches the soaking temperature TA, which is from Ac3+10°C to Ac3+150°C at a heating rate HR1 of 1 to 30°C / s. It is preferable to have the HR1 rate of 1 to 20°C / s, and more preferably 1 to 10°C / s. Preferably, the TA temperature is 800-900°C.

[0051] Then, the cold-rolled steel sheet is held at the annealing soaking temperature TA for 100 to 1000 seconds to ensure adequate transformation to form at least 90%, and preferably 100%, of austenite at the end of the soaking. Then, the cold-rolled steel sheet is cooled at an average cooling rate CR1 of 5 to 200°C / s, preferably 8 to 100°C / s, and more preferably 10 to 70°C / s to a cooling stop temperature range CS1 of 220 to 320°C, and preferably 230 to 310°C, and more preferably 250 to 300°C. At this cooling stage, martensite is formed. If the CS1 temperature exceeds 320°C, the steel of the present invention has too much austenite, which is detrimental to the overall elongation.

[0052] Then the cold-rolled steel sheet is brought to a tempering temperature TT, which is 210–390°C, and held at the TT temperature for 1–500 seconds. Preferably, the TT tempering temperature is 220–370°C, and more preferably 250–350°C. During this tempering stage, the martensite formed during the cooling stage is transformed into tempered martensite. The tempering duration is selected so that no or a maximum of 2% of residual austenite remains in the cold-rolled steel sheet at the end of tempering.

[0053] Then the cold rolled steel sheet is cooled to room temperature at a cooling rate of at least 1°C / s to obtain a cold rolled and heat treated steel sheet.

[0054] The resulting cold-rolled, heat-treated steel sheet can then optionally be coated using any known method. The coating can be zinc or a zinc-based alloy, or aluminum or an aluminum-based alloy.

[0055] After coating, the product may optionally be annealed in a chamber furnace, preferably at 170–210°C for 12–30 hours, to ensure degassing of the coated parts. The sheets are then cooled to room temperature to produce a coated cold-rolled steel sheet.

[0056] Examples

[0057] The following tests and examples presented in this document are not limiting in nature and are to be considered for illustrative purposes only and will demonstrate the advantageous features of the present invention, as well as clarify the meaning of the parameters selected by the inventors after extensive experimentation, and further establish the properties that can be achieved by steel in accordance with the requirements of the present invention.

[0058] Samples of steel sheets according to the invention and some comparative grades were prepared with the compositions shown in Table 1 and the processing parameters shown in Table 2. The corresponding microstructures of these steel sheets are presented in Table 3, and the properties in Table 4.

[0059] Table 1 presents the steel compositions expressed in mass percent, and also shows the Ac3 and Ms for each steel and the temperatures. Ac3 and Ms are measured according to ASTM A1033-18 standards. The remainder of the steel composition is iron and unavoidable impurities.

[0060] Table 1: Sample composition

[0061]

[0062] Table 2 presents the annealing process parameters implemented on the steels in Table 1.

[0063] Next, before annealing the steels of the invention and the comparison steel, the samples are heated to 1150–1300°C and hot rolled. All samples are then cold rolled with a cold rolling reduction of 56%.

[0064]

[0065] Table 3 shows the results of tests carried out according to standards on various microscopes such as scanning electron microscope to determine the microstructural composition of the samples.

[0066]

[0067] From the table above, it can be seen that all samples, according to the invention, have the target microstructures.

[0068] Table 4 shows the mechanical and surface properties of steel.

[0069] Table 4: Mechanical properties of samples

[0070] Yield strength (YS), ultimate tensile strength (TS), and total elongation (TE) are measured in accordance with ASTM E8, published in July 2022. Hole expansion ratio (HER) is measured in accordance with ISO-16630. Bend test is performed in accordance with ASTM E290-22.

[0071]

Claims

1. Cold rolled and heat treated steel sheet, including the following elements, wt.%: 0.3 ≤ C ≤ 0.45; 0.4 ≤ Mn ≤ 1.4; 0.1 ≤ Si ≤ 0.9; 0.01 ≤ Al ≤ 0.1; 0.1 ≤ Cr ≤ 0.8; 0.01 ≤ Nb ≤ 0.1; 0.1 ≤ Ni ≤ 0.9; 0.01 ≤ Mo ≤ 0.9; 0.01 ≤ Ti ≤ 0.1; 0.0001 ≤ B ≤ 0.010; P ≤ 0.02; S ≤ 0.03; N ≤ 0.09; The rest of the composition consists of iron and inevitable impurities that arise during manufacturing; the microstructure of the specified steel includes, in area fractions, 1-5% bainite, the rest is tempered martensite.

2. Cold-rolled and heat-treated steel sheet according to item 1, the composition of which includes 0.31-0.4% carbon.

3. Cold rolled and heat treated steel sheet according to item 1 or 2, the composition of which includes 0.5-1.3% manganese.

4. Cold-rolled and heat-treated steel sheet according to any of paragraphs 1-3, the composition of which includes 0.01-0.08% aluminum.

5. Cold rolled and heat treated steel sheet according to any one of paragraphs 1-4, the composition of which includes 0.2-0.8% silicon.

6. Cold rolled and heat treated steel sheet according to any one of paragraphs 1-5, in which bainite is 1-4%.

7. Cold rolled and heat treated steel sheet according to any one of paragraphs 1-6, in which the tempered martensite is 96-99%.

8. Cold rolled and heat treated steel sheet according to any one of paragraphs 1-7, wherein said sheet has a tensile strength of 1500-1750 MPa and a yield strength of 1250-1450 MPa.

9. A method for producing cold-rolled and heat-treated steel sheet, comprising the following sequential stages: preparation of a semi-finished product from steel having a composition according to any of paragraphs 1-5; reheating the specified semi-finished product to a temperature of 1000-1300°C; rolling the said semi-finished product in the austenitic range, where the hot rolling temperature is above 850°C, to obtain hot-rolled steel sheet; cooling the sheet at a cooling rate of at least 5°C / s to a winding temperature that is less than or equal to 580°C; and winding said hot-rolled sheet in a temperature range of 500-580°C; cooling the said hot rolled sheet to room temperature; it is not necessary to perform the descaling process on the said hot rolled steel sheet; annealing of hot rolled steel sheet is optional; it is not necessary to perform the descaling process on the said hot rolled steel sheet; cold rolling the said hot rolled steel sheet with a reduction ratio of 35-90% to obtain a cold rolled steel sheet; then heating the said cold-rolled steel sheet from room temperature to a temperature TA from Ac3+10°C to Ac3+150°C, with a heating rate HR1 from 1 to 30°C / s, and holding at this temperature for 100-1000 s; then cooling said cold-rolled steel sheet from TA to a temperature CS1 in the range of 220-320°C, with a cooling rate CR1 of 5-200°C / s; then the said cold-rolled steel sheet is brought to the tempering temperature TT, which is 210-390°C, and maintained at this temperature for 1-500 s; then cooling to room temperature at a cooling rate of at least 1 °C / s to obtain a cold-rolled and heat-treated steel sheet.

10. The method according to claim 9, wherein said winding temperature is 500-570°C.

11. The method according to claim 9 or 10, wherein CS1 is 230-310°C.

12. The method according to any one of claims 9-11, wherein HR1 is 1-20°C / s.

13. The method according to any one of claims 9-12, wherein TA is 800-900°C.

14. The use of a steel sheet obtained according to any of paragraphs 1-8 for the manufacture of a structural part of a vehicle.

15. The use of a steel sheet manufactured by the method according to any of paragraphs 9-13 for the manufacture of a structural part of a vehicle.

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