Hot-rolled steel sheet and its production method

Abstract

FIELD: metallurgy.SUBSTANCE: hot-rolled steel sheet used as a material for the automotive industry. The sheet has a composition that includes the following elements, in wt.%: 0.060 ≤ carbon ≤ 0.1, 1.4 ≤ manganese ≤ 1.8, 0.3 ≤ silicon ≤ 0.8, 0.01 ≤ aluminum ≤ 0.1, 0.02 ≤ niobium ≤ 0.05, 0.01 ≤ titanium ≤ 0.04, 0.002 ≤ phosphorus ≤ 0.02, sulfur ≤ 0.005, nitrogen ≤ 0.01, 0.045 ≤ Ti+Nb ≤ 0.060, optionally at least one element selected from: chromium ≤ 0.5, molybdenum ≤ 0.4, vanadium ≤ 0.4, calcium ≤ 0.005, copper ≤ 1, nickel ≤ 1, boron ≤ 0.05, magnesium ≤ 0.05, cerium ≤ 0.1, zirconium ≤ 0.05, the rest is composed of iron and inevitable impurities caused by processing. The microstructure of the sheet comprises, in area fractions, 10-18% bainite, 82-90% ferrite, and 0-10% combined residual austenite and martensite, with the aspect ratio of the bainite and ferrite grains being less than 1.75, where the aspect ratio is the ratio of the maximum Feret diameter (Fmax) of the largest dimension intersecting the grain to the largest dimension intersecting the grain measured at an angle of 90° from the specified Fmax (Fmax90°). The steel sheet simultaneously has a tensile strength greater than or equal to 590 MPa in three directions, which are the transverse direction, longitudinal direction and diagonal direction, a yield strength of 500-600 MPa in three directions, which are the transverse direction, longitudinal direction and diagonal direction, a total elongation greater than or equal to 20% in three directions, which are the transverse direction, longitudinal direction and diagonal direction, measured in accordance with the JIS Z2241 standard.EFFECT: sheet has the required uniform mechanical properties in all directions.11 cl, 4 tbl

Description

[0001] The present invention relates to hot-rolled steel sheets suitable for use as steel sheets for automobiles.

[0002] Automotive parts must meet two incompatible requirements: ease of molding and strength. However, in recent years, a third requirement has been imposed on automobiles: improving fuel consumption due to global environmental issues. Therefore, automotive parts must now be made of materials with high moldability to meet the criteria of easy installation in complex automotive structures, and at the same time, they must have increased strength to ensure crash safety and vehicle durability, while reducing vehicle weight to improve fuel efficiency.

[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 their 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 the development of several methods for producing high-strength steel sheets with high formability, some of which are listed here for the final evaluation of the present invention:

[0005] CN 113201691 discloses a hot-rolled steel sheet with a strength of 590 MPa for hydraulic drawing and a manufacturing method thereof, and belongs to the technical field of steel material engineering. Hot rolled steel sheet grade 590 MPa for hydraulic drawing includes the following chemical components with a content in mass percent: 0.02 - 0.04% C, less than or equal to 0.05% Si, 1.05 - 1.15% Mn, 0.040 - 0.050% Nb, 0.050 - 0.060% Ti, 0.30 - 0.60% Cr, 0.02%, less than or equal to 0.02% P, less than or equal to 0.003% S, less than or equal to 0.0040% N, 0.010 - 0.050% Al, 0.0010 - 0.0050% Ca0, the rest is Fe and unavoidable impurities. The invention utilizes a low-C-Nb-Ti-Cr microalloying system and achieves a microstructure of ultrafine-grained acicular ferrite and nanophases through controlled rolling and a controlled cooling process.The mechanical properties of the steel sheet provided by the invention meet the following requirements: yield strength greater than or equal to 500 MPa, tensile strength greater than or equal to 590 MPa, elongation greater than or equal to 25 percent, n value greater than or equal to 0.12, da qualification in the 180-degree cold bending test, and grain size greater than or equal to grade 12, so that the problem of low formability characteristics of existing high-strength steel can be effectively solved. However, the sheet of document CN 113201691 does not present uniform properties in all three directions, namely, the transverse direction, longitudinal direction, and diagonal direction, which limits its use in the automotive industry.

[0006] The purpose of the present invention is to solve these problems by providing hot rolled steel sheets that simultaneously have:

[0007] - tensile strength greater than or equal to 590 MPa in three directions that are the transverse direction, the longitudinal direction and the diagonal direction, and preferably greater than or equal to 600 MPa in three directions that are the transverse direction, the longitudinal direction and the diagonal direction,

[0008] - yield strength of 500-600 MPa in three directions that are transverse direction, longitudinal direction and diagonal direction, and preferably yield strength of 510-580 MPa in three directions that are transverse direction, longitudinal direction and diagonal direction,

[0009] - a total elongation greater than or equal to 20% in three directions that are the transverse direction, the longitudinal direction and the diagonal direction, and preferably a total elongation greater than or equal to 22% in three directions that are the transverse direction, the longitudinal direction and the diagonal direction,

[0010] - opening expansion ratio greater than or equal to 75% and preferably greater than or equal to 77%

[0011] In a preferred embodiment, the steel sheets according to the invention may also have a yield strength to tensile strength ratio of 0.5 or more.

[0012] Preferably, this steel has formability, particularly rolling, with good weldability and coating ability.

[0013] Another object of the present invention is also to provide a method for producing these sheets that is compatible with conventional industrial methods while being robust to changes in production parameters.

[0014] The hot rolled steel sheet of the present invention may be optionally coated with zinc or zinc alloys to improve its corrosion resistance.

[0015] The carbon content in the steel is 0.060-0.1%. Carbon is an element necessary for imparting the mechanical properties of the steel of the present invention by promoting the formation of ferrite, and carbon also imparts strength to the steel through precipitation strengthening due to the formation of titanium carbide or niobium carbides, so carbon plays a key role in increasing the strength. However, a carbon content of less than 0.06% will not be able to impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.1%, the steel exhibits poor weldability, which limits its application to automotive parts. The preferred content for the present invention may be 0.065-0.1%, and more preferably 0.07-0.095%.

[0016] The manganese content in the steel of the present invention is 1.4-1.8%. This element is gammagenic and also affects the Bs and Ms temperatures, therefore it plays an important role in controlling the formation of bainite. The purpose of adding manganese is, in fact, to impart hardenability to steel. It was found that a manganese content of at least 1.4% by mass ensures the strength and hardness of the steel sheet. However, when the manganese content is more than 1.8%, it causes adverse effects such as slowing down the transformation of austenite during cooling after hot rolling. In addition, a manganese content above 1.8% promotes axial segregation, therefore reducing the formability and also deteriorating the weldability of the present steel. The preferred manganese content for the present invention may be 1.45-1.7%, and more preferably 1.45-1.65%.

[0017] The silicon content in the steel of the present invention is 0.3-0.8%. Silicon causes solid solution strengthening, especially for ferrite and bainite microstructures. In addition, a higher silicon content can slow down the precipitation of cementite. However, a disproportionate silicon content leads to problems such as surface defects such as tiger stripes, which adversely affect the coating ability of the steel of the present invention. Therefore, the concentration is controlled within the upper limit of 0.8%. The preferred silicon content for the present invention can be 0.35-0.75%, and more preferably 0.35-0.6%.

[0018] Aluminum is an element present in the steel of the present invention in an amount of 0.01-0.1%. Aluminum is an alphagenic element and imparts ductility to the steel of the present invention. Aluminum in steel tends to combine with nitrogen to form aluminum nitride, therefore, from the perspective of the present invention, the aluminum content should be kept as low as possible and preferably 0.02-0.06%.

[0019] Niobium is an essential element for the present invention. The niobium content in the steel of the present invention can be 0.01-0.09% and is added to the steel of the present invention to form carbides or carbonitrides to impart strength to the steel of the present invention by precipitation strengthening. The preferred content for the present invention can be 0.02-0.07%, and more preferably 0.02-0.05%. Titanium is also an essential element added to the steel of the present invention in an amount of 0.01-0.09%, preferably 0.01-0.06%. Like niobium, it participates in the formation of carbides and carbonitrides, thus playing a role in strengthening and ensuring the strength of the steel. However, it also contributes to the formation of TiN, which appears during the solidification of the cast product. The amount of Ti is limited to 0.09% to prevent the formation of coarse TiN, which impairs hole expansion. If the titanium content is below 0.01%, it has no effect on the steel of the present invention.

[0021] The phosphorus content in the steel of the present invention is 0.002-0.02%. Phosphorus reduces 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 limited to 0.02% and preferably below 0.015%.

[0022] Sulfur is not an essential element, but may be contained as an impurity in steel. From the perspective of the present invention, the sulfur content should preferably be as low as possible, but it is 0.005% or less from the perspective of production cost. Furthermore, if a larger amount of sulfur is present in steel, it combines to form sulfides, especially with manganese, and reduces its beneficial effect on the steel of the present invention. Therefore, a sulfur content of less than 0.003% is preferable.

[0023] Nitrogen is limited to 0.01% in order to avoid aging of the material. Nitrogen forms nitrides, which impart strength to the steel of the present invention by precipitation strengthening with vanadium and niobium, but whenever the presence of nitrogen exceeds 0.01%, it can form a large amount of aluminum nitrides, which are harmful to the present invention, so the preferred upper limit of nitrogen content is 0.005%.

[0024] Chromium is an optional element for the present invention. The chromium content in the steel of the present invention can be 0-0.5%. Chromium is an element that provides hardenability to steel, but higher chromium content, above 0.5%, leads to axial segregation, similar to manganese.

[0025] Molybdenum is an optional element, the content of which is 0-0.4% in the steel of the present invention; molybdenum improves the hardenability of the steel of the present invention and affects the transformation of austenite into ferrite and bainite during cooling after hot rolling. However, the addition of molybdenum excessively increases the cost of adding alloying elements, therefore, for economic reasons, its content is limited to 0.4%. The preferred limit of molybdenum content is 0-0.3%.

[0026] Vanadium is an optional element, with a content of 0-0.4% in the steel of this invention. Vanadium effectively increases the strength of steel by forming carbides, nitrides, or carbonitrides, and the upper limit is 0.4% for economic reasons. These carbides, nitrides, or carbonitrides are formed during the second and third cooling stages. The preferred vanadium content is 0-0.3%.

[0027] Nickel can be added as an optional element in amounts of 0-1% to increase the strength of steel and improve its toughness. A minimum of 0.01% is required to achieve these effects. However, when its content exceeds 1%, nickel causes a deterioration in ductility.

[0028] Copper can be added as an optional element in amounts of 0-1% to increase the strength of steel and improve its corrosion resistance. A minimum of 0.01% is required to achieve these effects. However, when its content exceeds 1%, copper causes deterioration of hot ductility during hot rolling.

[0029] Calcium is an optional element that can be added to the steel of the present invention at a concentration of up to 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 preventing the harmful effects of sulfur during its globularization.

[0030] Other elements such as cerium, boron, magnesium or zirconium may be added individually or together in the following proportions: Ce ≤ 0.1%, B ≤ 0.05%, Mg ≤ 0.05% and Zr ≤ 0.05%. Up to the specified maximum content levels, these elements allow grain refinement during solidification.

[0031] 0.045 ≤ Ti +Nb ≤ 0.060

[0032] The total presence of titanium and niobium is maintained at 0.045 - 0.060% in order to give the steel of the present invention strength and improve the hole expansion ratio, since both niobium and titanium form carbonitrides or carbides, therefore, this ratio in the present invention maintains the achievement of a balance between the tensile strength by ensuring the formation of precipitates and the hole expansion ratio, due to the adequate formation of ferrite during cooling after hot rolling.

[0033] The rest of the steel composition consists of iron and inevitable impurities formed as a result of processing.

[0034] The microstructure of steel sheet includes: Ferrite accounts for 80-92% of the microstructure in area fractions for the steel of the present invention. Ferrite constitutes the primary phase of the steel as a matrix phase. Ferrite is collectively composed of polygonal ferrite and acicular ferrite. Ferrite imparts elongation and formability to the steel of the present invention in all three directions. To ensure an elongation of 20% or more, it is necessary to have at least 80% ferrite. Ferrite is formed during cooling after hot rolling in the steel of the present invention. However, whenever the ferrite content exceeds 92% in the steel of the present invention, the tensile strength is not achieved. Preferably, the ferrite content is 82-90%, and more preferably 85-90%.

[0036] Bainite accounts for 10-20% of the microstructure in area fractions in the steel of the present invention. Bainite collectively consists of upper bainite and lower bainite, granular bainite, lath bainite, and carbide-free bainite. To ensure a tensile strength of 590 MPa or more in all three directions, bainite content of 10% is required. Bainite begins to form at the cooling stop temperature and is formed before winding. The bainite content is preferably 10-18%, and more preferably 10-15%.

[0037] The combined amount of bainite and ferrite always exceeds 90% to ensure a balance of strength and formability. In a preferred embodiment, the combined amount of bainite and ferrite always exceeds 95% and more preferably 97%. The combined presence of bainite and ferrite provides a tensile strength of 590 MPa, since the presence of bainite and ferrite ensures formability. Furthermore, the bainite and ferrite grains of the present invention have an aspect ratio of less than 1.75 and preferably less than 1.70 μm, which in turn imparts strength to the steel in all three directions, while simultaneously ensuring a hole expansion ratio of 75% or more.

[0038] Martensite and retained austenite are optional components of the steel of the present invention and may be present in a combined area of ​​0-10% and are found in trace amounts. In the present invention, martensite includes both fresh martensite and tempered martensite. Martensite imparts strength to the steel of the present invention. When the martensite content exceeds 10%, it imparts excessive strength, and the yield strength exceeds the acceptable upper limit. In a preferred embodiment, the combined amount of martensite and retained austenite is 0-5%.

[0039] In addition to the above-mentioned microstructure, the microstructure of hot rolled steel sheet does not contain microstructural components such as pearlite and cementite, but they can be found in trace amounts.

[0040] The 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 in the form of 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.

[0041] For example, a slab with the above-described chemical composition is produced by continuous casting. The slab is optionally subjected to direct soft reduction during the continuous casting process to avoid axial segregation and ensure the local carbon to nominal carbon ratio is below 1.10. The slab obtained by the continuous casting process can be used directly at high temperature after continuous casting, or can be first cooled to room temperature and then reheated for hot rolling.

[0042] The temperature of the slab being hot rolled is preferably at least 1200°C and should be below 1300°C. If the slab temperature is below 1200°C, the rolling mill is subjected to excessive stress. Therefore, the slab temperature is preferably high enough to ensure hot rolling is completed in the 100% austenitic range. Reheating at temperatures above 1275°C should be avoided, as it leads to productivity loss and is also costly on an industrial scale. Therefore, the preferred reheating temperature is 1200-1275°C.

[0043] The finishing temperature of hot rolling in the present invention is 850 to 975°C, and preferably 880 to 930°C.

[0044] The hot rolled strip thus obtained is then cooled, wherein the cooling is started immediately after the hot rolling is completed, and the hot rolled strip is cooled from the hot rolling completion temperature to a cooling stop temperature range of 480 to 550°C at a cooling rate of more than 20°C / s and preferably at a cooling rate of 40 to 150°C / s and more preferably 40 to 120°C / s. The preferred cooling stop temperature range is 490 to 540°C and more preferably 500 to 540°C.

[0045] After this, the hot-rolled strip is coiled at a coiling temperature range of 480-550°C, preferably 490-540°C. Then, the coiled hot-rolled strip is cooled to room temperature to obtain a hot-rolled steel sheet.

[0046] Examples

[0047] The following tests, examples, illustrative illustrations and tables, which are presented in this document, are not limiting in nature and should be considered for illustrative purposes only and will demonstrate the advantageous features of the present invention.

[0048] Steel sheets made from steels with different compositions are shown in Table 1, where the content of each element is expressed as a mass percentage, and the remainder represents iron and other process impurities. The steel sheets were produced according to the process parameters specified in Table 2, respectively. Table 3 then presents the microstructures of the steel sheets obtained during testing, and Table 4 presents the results of evaluating the obtained properties.

[0049]

[0050] Table 2 presents the process parameters implemented on the steels in Table 1.

[0051] Table 2

[0052]

[0053] I = according to the invention; R = comparison; underlined values: not in accordance with the invention.

[0054] Table 3 shows the results of tests conducted in accordance with standards on various microscopes, such as a scanning electron microscope, to determine the microstructure of steels both according to the invention and the comparison steels. The aspect ratio is the ratio of the maximum Feret diameter (Fmax) of the largest dimension intersecting the grain to the largest dimension intersecting the grain, measured at an angle of 90° from the specified Fmax (Fmax90°).

[0055] Aspect ratio = (Fmax) / (Fmax90°).

[0056] Table 3

[0057]

[0058] I = according to the invention; R = comparison; underlined values: not in accordance with the invention.

[0059] Table 4 illustrates the mechanical properties of both the steel according to the invention and the comparison steels. Tensile tests were conducted in accordance with JIS Z2241 to determine tensile strength, yield strength, and total elongation.

[0060] The results of various mechanical tests carried out in accordance with standards are presented.

[0061]

Claims

1. Hot rolled steel sheet having a composition including the following elements, with the content expressed in mass percent: 0.060% ≤ carbon ≤ 0.1% 1.4% ≤ manganese ≤ 1.8% 0.3% ≤ silicon ≤ 0.8% 0.01% ≤ aluminum ≤ 0.1% 0.02% ≤ niobium ≤ 0.05% 0.01% ≤ titanium ≤ 0.04% 0.002% ≤ phosphorus ≤ 0.02% sulfur ≤ 0.005% nitrogen ≤ 0.01% 0.045% ≤ Ti+Nb ≤ 0.060% and may contain one or more of the following optional elements chromium ≤ 0.5% molybdenum ≤ 0.4% vanadium ≤ 0.4% calcium ≤ 0.005% copper ≤ 1% nickel ≤ 1% boron ≤ 0.05% magnesium ≤ 0.05% cerium ≤ 0.1% zirconium ≤ 0.05% the remainder consists of iron and inevitable impurities caused by processing, the microstructure of said steel sheet includes, in area fractions, 10% - 18% bainite, 82% - 90% ferrite, where the aspect ratio of bainite and ferrite grains is less than 1.75, where the aspect ratio is the ratio of the maximum Feret diameter (Fmax) of the largest dimension intersecting the grain to the largest dimension intersecting the grain measured at an angle of 90° from said Fmax (Fmax90°), and the combined amount of retained austenite and martensite is 0 - 10% and the steel sheet simultaneously has: tensile strength greater than or equal to 590 MPa in three directions, which are the transverse direction, the longitudinal direction and the diagonal direction, yield strength of 500 - 600 MPa in three directions, which are transverse direction, longitudinal direction and diagonal direction, total elongation greater than or equal to 20% in three directions, which are the transverse direction, the longitudinal direction and the diagonal direction, measured in accordance with JIS Z2241.

2. Hot rolled steel sheet according to item 1, the composition of which includes 0.35% - 0.75% silicon.

3. Hot rolled steel sheet according to item 1 or 2, the composition of which includes 0.065% - 0.1% carbon.

4. Hot rolled steel sheet according to any one of paragraphs 1-3, the composition of which includes 1.45% - 1.7% manganese.

5. Hot rolled steel sheet according to any one of paragraphs 1-4, the composition of which includes 0.02 - 0.06% aluminum.

6. Hot rolled steel sheet according to any one of paragraphs 1-5, in which the total amount of residual austenite and martensite is 0 - 5%.

7. A hot rolled steel sheet according to any one of paragraphs 1 to 6, which has a hole expansion ratio of 75% or more.

8. A method for producing hot-rolled steel sheet comprising the following sequential stages: - preparation of a semi-finished product from steel of the composition specified in any of paragraphs 1-5; - reheating the specified semi-finished product to a temperature of 1200 - 1275°C; - rolling of the specified semi-finished product in the austenitic range, where the temperature at the end of hot rolling is 850 - 975°C to obtain a hot-rolled steel strip; - then cooling the said hot rolled strip immediately after the end of hot rolling from the end temperature of hot rolling to the cooling stop temperature range, which is 480 - 550°C with a cooling rate of more than 20°C / s; - then winding of hot-rolled strip in the winding temperature range of 480 - 550°C; - then cooling the wound hot rolled strip to room temperature to obtain hot rolled steel sheet.

9. The method according to claim 8, wherein the hot rolling temperature is 880 - 930°C.

10. The method according to claim 8 or 9, wherein the cooling rate is 40 - 120°C / s.

11. The use of hot-rolled steel sheet according to any of paragraphs 1-7 for the manufacture of structural or protective parts of a vehicle.

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