Hot-rolled steel sheet and method for producing the same

Abstract

The following elements, namely, 0.060% ≤ carbon ≤ 0.1%, 1.4% ≤ manganese ≤ 1.8%, 0.3% ≤ silicon ≤ 0.8%, 0.01% ≤ aluminum ≤ 0.1%, 0.01% ≤ niobium ≤ 0.09%, 0.01% ≤ titanium ≤ 0.09%, 0.002% ≤ phosphorus ≤ 0.02%, 0% ≤ sulfur ≤ 0.005%, 0% ≤ nitrogen ≤ 0.01%, where 0.045% ≤ Ti + Nb ≤ 0.060%, 0% ≤ chromium ≤ 0.5%, 0% ≤ molybdenum ≤ 0.4%, 0% ≤ vanadium ≤ 0.4%, 0% ≤ calcium ≤ 0.005%, 0% ≤ copper ≤ 1%, 0% ≤ nickel ≤ 1%, 0% ≤ boron ≤ 0.05%, 0% ≤ magnesium ≤ 0.05%, and the remaining composition has a composition composed of iron and inevitable impurities generated by processing. The microstructure of the steel sheet contains, by area fraction, 10% - 20% bainite and 80% - 92% ferrite, the aspect ratio of the particles of bainite and ferrite is less than 1.75, and the cumulative amount of retained austenite and martensite is 0% - 10%. A hot-rolled steel sheet.

Description

Technical Field

[0001] The present invention relates to a hot-rolled steel sheet suitable for use as an automotive steel sheet.

Background Art

[0002] Automotive parts are required to meet two conflicting needs, namely, ease of forming and strength. In recent years, a third requirement of improving fuel efficiency has been imposed on automobiles from the perspective of the global environment. Therefore, currently, automotive parts need to be made from materials with high formability in order to meet the standard of ease of adaptation to complex automotive assemblies. At the same time, it is necessary to improve the strength for vehicle crashworthiness and durability, and to reduce the weight and improve the fuel efficiency of the vehicle.

[0003] Therefore, in order to reduce the amount of materials used in automobiles by increasing the strength of the materials, active research and development are being carried out. On the contrary, when the strength of the steel sheet is increased, the formability decreases. Therefore, the development of materials having both high strength and high formability is required.

[0004] Previous research and development in the field of high-strength and high-formability steel sheets has led to several methods for manufacturing high-strength and high-formability steel sheets, some of which are listed herein for the ultimate understanding of the present invention.

[0005] CN113201691 discloses a hot-rolled steel plate for hydraulic bulging at the 590 MPa level and a manufacturing method thereof, belonging to the technical field of steel materials engineering. The 590 MPa grade hot-rolled steel plate for hydraulic bulging contains the following chemical components, namely, 0.02 - 0.04% C, 0.05% or less Si, 1.05 - 1.15% Mn, 0.040 - 0.050% Nb, 0.050 - 0.060% Ti, 0.30 - 0.60% Cr, 0.02% or less P, 0.003% or less S, 0.0040% or less N, 0.010 - 0.050% Als, and 0.010 - 0.050% Ca by weight percentage, with the balance being Fe and inevitable impurities. This invention adopts a micro / low C-Nb-Ti-Cr microalloy component system and obtains a microstructure of ultrafine granular acicular ferrite and nano-precipitation phases through controlled rolling and controlled cooling process technologies. The mechanical properties of the steel plate provided by this invention meet the following requirements. That is, the yield strength is 500 MPa or more, the tensile strength is 590 MPa or more, the elongation is 25% or more, the n value is 0.12 or more, d-a is recognized in a 180-degree cold bending test, and the grain size is 12 grades or more. Therefore, the problem of poor formability of existing high-strength steels can be effectively solved. However, CN113201691 does not show homogeneous properties in all three directions, namely the transverse direction, the longitudinal direction, and the diagonal direction, and thus its use in the automotive industry is restricted.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] The object of the present invention is to solve these problems by making available a hot-rolled steel plate having the following simultaneously. - A maximum tensile strength of 590 MPa or more in three directions, namely the transverse direction, the longitudinal direction, and the diagonal direction, preferably 600 MPa or more in the three directions of the transverse direction, the longitudinal direction, and the diagonal direction. - A yield strength of 500 MPa to 600 MPa in three directions, namely the transverse direction, the longitudinal direction, and the diagonal direction, preferably 510 MPa to 580 MPa in the three directions of the transverse direction, the longitudinal direction, and the diagonal direction. - An overall elongation of 20% or more in three directions, namely the transverse direction, the longitudinal direction, and the diagonal direction, preferably 22% or more in the three directions of the transverse direction, the longitudinal direction, and the diagonal direction. - A hole expansion ratio of 75% or more, preferably 77% or more.

[0008] In a preferred embodiment, the steel sheet according to the present invention can also exhibit a yield strength to tensile strength ratio of 0.5 or more.

[0009] Preferably, such steel can also have good weldability and coatability, as well as good compatibility with forming, particularly rolling.

[0010] Another object of the present invention is also to make available a manufacturing method for these plates that is stable against shifts in manufacturing parameters while being suitable for conventional industrial applications.

[0011] The hot-rolled steel sheet of the present invention may be coated with zinc or a zinc alloy in order to improve corrosion resistance.

Means for Solving the Problems

[0012] Carbon exists in the steel at 0.060% - 0.1%. Carbon is an element necessary to promote the formation of ferrite and impart mechanical properties to the steel of the present invention. Carbon imparts strength to the steel by precipitation strengthening through the formation of titanium carbide or niobium carbide. Therefore, carbon plays an extremely important role in increasing strength. However, a carbon content of less than 0.06% cannot impart tensile strength to the steel of the present invention. On the other hand, when the carbon content exceeds 0.1%, the steel exhibits insufficient pot weldability, and its application to automotive parts is restricted. The preferred content for the present invention can be maintained at 0.065% - 0.1%, more preferably 0.07 - 0.095%.

[0013] The manganese content of the steel of the present invention is 1.4% - 1.8%. This element is gammagenous and also affects the Bs temperature and Ms temperature, and thus plays an important role in controlling bainite formation. The purpose of adding manganese is essentially to impart hardenability to the steel. It has been found that at least 1.4 wt% of manganese is required to impart strength and hardenability to the steel plate. However, when the manganese content exceeds 1.8%, it has adverse effects such as delaying the transformation of austenite during cooling after hot rolling. Also, when the manganese content exceeds 1.8%, central segregation is promoted, formability decreases, and the weldability of the steel of the present invention also decreases. The preferred content for the present invention can be maintained at 1.45% - 1.7%, more preferably 1.45% - 1.65%.

[0014] The silicon content of the steel of the present invention is 0.3% - 0.8%. Silicon is a solid solution strengthening agent, especially for the ferrite and bainite in the microstructure. Furthermore, when the silicon content is high, the precipitation of cementite can be delayed. However, if the silicon content is unbalanced, problems such as surface defects like tiger strip that adversely affect the coating property of the steel of the present invention will occur. Therefore, the upper limit of the concentration is controlled at 0.8%. The preferred content for the present invention can be maintained at 0.35% - 0.75%, more preferably 0.35% - 0.6%.

[0015] Aluminum is an element present in the steel of the present invention in an amount of 0.01% to 0.1%. Aluminum is an alphagenous element and imparts ductility to the steel of the present invention. Aluminum in the steel tends to combine with nitrogen to form aluminum nitride. Therefore, from the viewpoint of the present invention, the aluminum content should be as low as possible, preferably maintained at 0.02% to 0.06%.

[0016] Niobium is an essential element for the present invention. The niobium content may be present in the steel of the present invention in an amount of 0.01% to 0.09%, and is added to the steel of the present invention to form carbides or carbonitrides in order to impart strength to the steel of the present invention by precipitation strengthening. The preferred content for the present invention can be maintained at 0.02 to 0.07%, more preferably 0.02 to 0.05%.

[0017] Titanium is also an essential element added to the steel of the present invention in an amount of 0.01% to 0.09%, preferably 0.01% to 0.06%. Similar to niobium, titanium is involved in the formation of carbides and carbonitrides, and plays a role in hardening the steel and imparting strength. However, titanium is also involved in forming TiN that appears during the solidification of cast products. Therefore, the amount of Ti is limited to 0.09% in order to avoid coarse TiN that is harmful to hole expansion. When the titanium content is less than 0.01%, titanium has no effect on the steel of the present invention.

[0018] The phosphorus component of the steel of the present invention is 0.002% to 0.02%. Phosphorus reduces spot weldability and hot rolling properties, particularly due to its tendency to segregate at grain boundaries or co-segregate with manganese. For these reasons, its content is limited to 0.02%, preferably less than 0.015%.

[0019] Sulfur is not an essential element and may be contained as an impurity in the steel. From the perspective of the present invention, it is preferable that the sulfur content be as low as possible, but from the perspective of manufacturing cost, it is 0.005% or less. Furthermore, when more sulfur is present in the steel, sulfur particularly combines with manganese to form sulfides, reducing its beneficial effect on the steel of the present invention. Therefore, less than 0.003% is preferable.

[0020] Nitrogen is limited to 0.01% to avoid material aging. Nitrogen forms nitrides that impart strength to the steel of the present invention through precipitation strengthening together with vanadium and niobium. However, whenever the nitrogen content exceeds 0.01%, there is a possibility of forming a large amount of aluminum nitride, which is harmful to the present invention. Therefore, the preferable upper limit of nitrogen is 0.005%.

[0021] Chromium is an optional element for the present invention. The chromium content that can be present in the steel of the present invention is 0% to 0.5%. Chromium is an element that provides hardenability to the steel, but a chromium content higher than 0.5% causes center segregation similar to manganese.

[0022] Molybdenum is an optional element that constitutes 0% to 0.4% of the steel of the present invention. Molybdenum enhances the hardenability of the steel of the present invention and affects the transformation from austenite to 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 preferable limit of molybdenum is 0% to 0.3%.

[0023] Vanadium is an optional element that constitutes 0% to 0.4% of the steel of the present invention. Vanadium is effective in increasing the strength of the 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 steps of cooling. The preferable limit of vanadium is 0% to 0.3%.

[0024] By adding nickel as an optional element in an amount of 0% to 1%, the strength of the steel can be increased and its toughness can be improved. A minimum of 0.01% is required to produce such an effect. However, when its content exceeds 1%, nickel causes a decrease in ductility.

[0025] Copper can be added as an optional element in an amount of 0% to 1% in order to increase the strength of the steel and improve its corrosion resistance. A minimum of 0.01% is required to produce such an effect. However, when its content exceeds 1%, copper causes a decrease in hot ductility during hot rolling.

[0026] Calcium is an optional element that can be added to the steel of the present invention up to 0.005%, preferably up to 0.001% to 0.005%. Calcium is added as an optional element to the steel of the present invention, particularly during inclusion treatment. Calcium contributes to the refinement of the steel by preventing harmful sulfur contents in the spheroidization of the steel.

[0027] Other elements such as cerium, boron, magnesium or zirconium can be added individually or in combination at the following ratios, i.e., Ce≤0.1%, B≤0.05%, Mg≤0.05% and Zr≤0.05%. Up to the maximum content levels shown, these elements enable the refinement of the crystal grains during solidification.

[0028] 0.045≤Ti + Nb≤0.060

[0029] The cumulative presence of titanium and niobium is maintained at 0.045% to 0.060% in order to impart strength and hole expansion ratio to the steel of the present invention because both niobium and titanium form carbonitrides or carbides. Therefore, this equation supports the present invention to successfully balance the tensile strength by ensuring the formation of precipitates, and imparts a hole expansion ratio by ensuring appropriate ferrite formation during cooling after hot rolling.

[0030] The remainder of the steel composition consists of iron and inevitable impurities resulting from processing.

[0031] The microstructure of the steel sheet includes the following.

[0032] Ferrite constitutes 80% - 92% of the microstructure in terms of area fraction for the steel of the present invention. Ferrite forms the main phase of the steel as the matrix phase. Ferrite cumulatively contains polygonal ferrite and acicular ferrite. Ferrite imparts elongation and formability to the steel of the present invention in all three directions. In order to ensure an elongation of 20% or more, it is necessary to have at least 80% ferrite. In the steel of the present invention, ferrite is formed during cooling after hot rolling. However, whenever the ferrite content exceeds 92% in the steel of the present invention, the tensile strength cannot be achieved. Preferably, the content of ferrite is 82% - 90%, more preferably 85% - 90%.

[0033] Bainite constitutes 10% - 20% of the microstructure in terms of area fraction for the steel of the present invention. Bainite is cumulatively composed of upper bainite and lower bainite, granular bainite, lath bainite, and bainite without carbide. In order to ensure a tensile strength of 590 MPa or more in all three directions, it is necessary to have 10% bainite. Bainite starts to form from the cooling stop temperature and occurs until coiling. The content of bainite is preferably 10 - 18%, more preferably 10 - 15%.

[0034] The cumulative amount of bainite and ferrite always exceeds 90% in order to ensure the balance between strength and formability. In a preferred embodiment, the cumulative amount of bainite and ferrite is always greater than 95%, more preferably greater than 97%. The cumulative presence of bainite and ferrite imparts a tensile strength of 590 MPa due to the presence of bainite and ferrite, and the formability is guaranteed. Furthermore, the bainite and ferrite particles of the present invention have an aspect ratio of less than 1.75, preferably less than 1.70 microns, which in turn has a hole expansion rate of 75% or more and imparts strength to the steel in all three directions.

[0035] Martensite and retained austenite are optional constituents of the steel of the present invention and may be present in a cumulative amount of 0% to 10% by area fraction and are found in trace amounts. For the purposes of the present invention, martensite includes both fresh martensite and tempered martensite. Martensite imparts strength to the steel of the present invention. If the martensite exceeds 10%, it imparts excessive strength and the yield strength exceeds the acceptable upper limit. In a preferred embodiment, the cumulative amount of martensite and retained austenite is 0 to 5%.

[0036] In addition to the above microstructure, the microstructure of the hot-rolled steel sheet does not contain microstructure components such as pearlite and cementite, although they may be found in trace amounts.

[0037] The steel sheet according to the present invention can be produced by any suitable method. A preferred method involves providing a semi-finished casting of steel having the chemical composition according to the present invention. The casting can be made into an ingot or carried out continuously in the form of a thin slab or thin strip. That is, it has a thickness ranging from about 220 mm for a slab to several tens of millimeters for a thin strip.

[0038] For example, a slab having the above-described chemical composition is produced by continuous casting, and the slab is optionally subjected to direct soft reduction during the continuous casting process to avoid center segregation and keep the ratio of local carbon to nominal carbon below 1.10. The slab provided by the continuous casting method can be used directly at a high temperature after continuous casting or can first be cooled to room temperature and then reheated for hot rolling.

[0039] The temperature of the slab to be hot-rolled should preferably be at least 1200°C and must be less than 1300°C. If the temperature of the slab is less than 1200°C, an excessive load will be applied to the rolling mill. Therefore, the temperature of the slab is preferably high enough to complete hot rolling within the 100% austenite range. Reheating at a temperature exceeding 1275°C causes a loss of productivity and is also industrially expensive, so it must be avoided. Therefore, the preferred reheating temperature is 1200°C to 1275°C.

[0040] The finish hot-rolling temperature of the present invention is 850°C to 975°C, preferably 880°C to 930°C.

[0041] Next, the hot-rolled strip thus obtained is cooled. Here, the cooling starts immediately after the end of hot rolling, and the hot-rolled strip is cooled at a cooling rate exceeding 20°C / second, preferably 40°C / second to 150°C / second, more preferably 40°C / second to 120°C / second, to a cooling stop temperature range of 480°C to 550°C from the end of hot rolling. The preferred cooling stop range is 490°C to 540°C, more preferably 500°C to 540°C.

[0042] Thereafter, the hot-rolled strip is wound up in a winding temperature range of 480°C to 550°C, preferably 490°C to 540°C. The wound hot-rolled strip is cooled to room temperature to obtain a hot-rolled steel sheet.

Examples

[0043] The following tests, examples, illustrative examples, and tables presented in this specification are essentially non-limiting and are to be considered for illustrative purposes only and show the advantageous features of the present invention.

[0044] Steel sheets made of steels with different compositions are summarized in Table 1, and the steel sheets are manufactured according to the process parameters specified in Table 2 respectively. Thereafter, the microstructures of the steel sheets obtained during the tests are summarized in Table 3, and the results of evaluating the obtained properties are summarized in Table 4.

[0045]

Table 1

[0046] Table 2 Table 2 summarizes the process parameters carried out with the steel of Table 1.

[0047]

Table 2

[0048] Table 3 Table 3 illustrates the results of tests performed according to standards on different microscopes such as a scanning electron microscope to determine the microstructure of both the steel of the present invention and the reference steel. The aspect ratio is the ratio of the longest section particle dimension maximum Feret diameter (Fmax) to the longest section particle dimension measured at 90° of said Fmax (Fmax90°).

[0049] Aspect ratio = (Fmax) / (Fmax90°)

[0050] The results are specified in this specification.

[0051]

Table 3

[0052] Table 4 Table 4 illustrates the mechanical properties of both the steel of the present invention and the reference steel. To determine the tensile strength, yield strength and total elongation, a tensile test is carried out according to JIS Z2241 standard.

[0053] The results of various mechanical tests carried out according to the standard are summarized.

[0054]

Table 4

Claims

1. A hot-rolled steel sheet comprising the following elements expressed in weight percent, namely, 0.060% ≤ carbon ≤ 0.1% 1.4% ≤ manganese ≤ 1.8% 0.3% ≤ silicon ≤ 0.8% 0.01% ≤ aluminum ≤ 0.1% 0.01% ≤ niobium ≤ 0.09% 0.01% ≤ titanium ≤ 0.09% 0.002% ≤ phosphorus ≤ 0.02% 0% ≤ sulfur ≤ 0.005% 0% ≤ nitrogen ≤ 0.01% wherein 0.045% ≤ Ti + Nb ≤ 0.060%, and may contain one or more of the following optional elements, 0% ≤ chromium ≤ 0.5% 0% ≤ molybdenum ≤ 0.4% 0% ≤ vanadium ≤ 0.4% 0% ≤ calcium ≤ 0.005% 0% ≤ copper ≤ 1% 0% ≤ nickel ≤ 1% 0% ≤ boron ≤ 0.05% 0% ≤ magnesium ≤ 0.05% the remaining composition has a composition composed of iron and inevitable impurities generated during processing, and the microstructure of the steel sheet contains, in area fraction, 10% - 20% bainite and 80% - 92% ferrite, the aspect ratio of the bainite and ferrite particles is less than 1.75, and the cumulative amount of retained austenite and martensite is 0% - 10%. A hot-rolled steel sheet.

2. The hot-rolled steel sheet according to claim 1, wherein the composition contains 0.35% - 0.75% of silicon.

3. The hot-rolled steel sheet according to claim 1 or 2, wherein the composition contains 0.065% - 0.1% of carbon.

4. The hot-rolled steel sheet according to claim 3, wherein the composition contains 0.02% - 0.07% of niobium.

5. The hot-rolled steel sheet according to any one of claims 1 - 4, wherein the composition contains 1.45% - 1.7% of manganese.

6. The hot-rolled steel sheet according to any one of claims 1 - 5, wherein the composition contains 0.01% - 0.06% of titanium.

7. The hot-rolled steel sheet according to any one of claims 1 - 6, wherein the composition contains 0.02% - 0.06% of aluminum.

8. The hot-rolled steel sheet according to any one of claims 1 - 7, wherein the cumulative amount of retained austenite and martensite is 0% - 5%.

9. The hot-rolled steel sheet according to any one of claims 1 - 8, wherein the steel sheet has a hole expansion rate of 75% or more.

10. A method for manufacturing a hot-rolled steel sheet, comprising the following continuous steps, namely, providing a steel composition according to any one of claims 1 - 7, reheating the semi-finished product to a temperature of 1200°C - 1300°C, In the austenite range where the finish rolling temperature of hot rolling is 850°C to 975°C, a step of rolling the semi-finished product to obtain a hot-rolled steel strip; Next, a step of cooling the hot-rolled strip, where the cooling starts immediately after the end of hot rolling; Next, a step of cooling the hot-rolled strip at a cooling rate exceeding 20°C / second to a cooling stop temperature range of 480°C to 550°C from the end of hot rolling; Thereafter, a step of winding the hot-rolled strip in a winding temperature range of 480°C to 550°C; Next, a step of cooling the wound hot-rolled strip to room temperature to obtain a hot-rolled steel sheet A method comprising the above steps.

11. The method according to claim 10, wherein the reheating temperature of the semi-finished product is 1200°C to 1275°C.

12. The method according to claim 10 or 11, wherein the finish rolling temperature of hot rolling is 880°C to 930°C.

13. The method according to any one of claims 10 to 12, wherein the cooling rate in step 1 of the cooling step is 40°C / second to 120°C / second.

14. Use of the steel sheet according to any one of claims 1 to 9 or the steel sheet manufactured according to the method according to claims 10 to 13 for the manufacture of structural parts or safety parts of a vehicle.

15. A vehicle comprising a part obtained according to claim 14.

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