CROSS-REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2015/000729, filed Feb. 17, 2015 and claims priority to Japanese Patent Application No. 2014-036569, filed Feb. 27, 2014, the disclosures of each of these applications being incorporated herein by reference in their entireties for all purposes.

FIELD OF THE INVENTION

The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more which can preferably be used as a material for, for example, the structural members, skeleton members, and chassis such as suspensions of automobiles, the frame parts of trucks, and steel pipes and to a method for manufacturing the steel sheet.

BACKGROUND OF THE INVENTION

Nowadays, automobile exhaust gas regulations are being strengthened from the viewpoint of global environment conservation. In addition, increasing the fuel efficiency of automobiles is a major issue, and there is a growing demand for the strengthening and thickness reduction of materials used. In such a situation, there is a trend toward actively using high-strength hot-rolled steel sheets as materials for automobile parts. High-strength hot-rolled steel sheets are used not only for, for example, the structural members and skeleton members of automobiles but also for chassis of automobiles and the frame parts of trucks.

In addition, in response to, for example, a significant increase in the price of crude oil and a demand for the diversification of sources of energy supply, there is a growing trend toward drilling oil wells or natural gas wells and constructing pipelines in very cold areas such as Russia, Canada, and Alaska. In such a situation, since there is a growing demand for further decreasing the construction costs of pipelines, there is a demand for decreasing the material costs of steel pipes. Therefore, there is a trend toward using high-strength welded steel pipes, which are manufactured from hot-rolled steel sheets having a coil shape with increased productivity and decreased cost, as transporting pipes instead of UOE steel pipes, which are manufactured from thick steel plates.

As described above, the demand for high-strength hot-rolled steel sheets having a specified strength as materials for automobile parts and steel pipes is growing year by year. In particular, a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more is highly expected to be a material which can markedly increase the fuel efficiency of automobiles or significantly decrease the construction costs of pipelines.

Generally, however, an increase in the strength of a steel sheet may result in a decrease in toughness and punching workability. In particular, in the case of truck frame members having a large thickness where punching is performed many times, a steel sheet having excellent punching workability and toughness is required. Therefore, various investigations have been conducted in order to provide excellent punching workability and toughness to a high-strength hot-rolled steel sheet.

For example, Patent Literature 1 discloses a hot-rolled steel sheet having a thickness of 4.0 mm or more and 12 mm or less, the steel sheet having a chemical composition containing, by mass %, C: 0.04% to 0.12%, Si: 0.5% to 1.2%, Mn: 1.0% to 1.8%, P: 0.03% or less, S: 0.0030% or less, Al: 0.005% to 0.20%, N: 0.005% or less, Ti: 0.03% to 0.13%, and the balance being Fe and inevitable impurities, a microstructure including more than 95% of a bainite phase in terms of area ratio, in which the average grain diameter of the bainite phase is 3 μm or less, a difference in Vickers hardness of 50 or less between positions located at 50 μm from the surface layer and at ¼ of the thickness, and a difference in Vickers hardness of 40 or less between positions located at ¼ of the thickness and at ½ of the thickness. In addition, it is said that, according to the technique disclosed in Patent Literature 1, by forming a bainite phase having a small grain diameter as a main phase, and by decreasing a variation in hardness in the thickness direction, it is possible to obtain a high-strength hot-rolled steel sheet having a tensile strength of 780 MPa or more and excellent toughness.

In addition, Patent Literature 2 discloses a method for manufacturing a steel sheet, the method including heating a steel material having a chemical composition containing, by mass %, C: 0.05% to 0.18%, Si: 0.10% to 0.60%, Mn: 0.90% to 2.0%, P: 0.025% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.001% to 0.1%, N: 0.002% to 0.01%, and the balance being Fe and inevitable impurities to a temperature of 950° C. or higher and 1250° C. or lower, then starting rolling, then finishing rolling at a temperature of 820° C. or higher, then performing cooling at a cooling rate of 20° C./s or more to a temperature in the range of 600° C. to 700° C., then holding and/or slowly cooling the cooled steel sheet in the temperature range for 10 seconds to 200 seconds, then cooling the steel sheet to a temperature of 300° C. or lower at a cooling rate of 5° C./s or more, in order to form a metallographic structure including, by an occupancy ratio with respect to the whole microstructure, 70% to 90% of ferrite, 3% to 15% of martensite or a dual phase of martensite and austenite, and the balance being bainite (including 0%), in which the average grain diameter of the ferrite is 20 μm or less. In addition, it is said that, according to the technique disclosed in Patent Literature 2, by forming a metallographic structure including ferrite having a small grain diameter, martensite or a dual phase of martensite and austenite, and so forth, it is possible to obtain a high-toughness steel material having a tensile strength of 490 N/mm2or more and a low yield ratio of 70% or less.

In addition, Patent Literature 3 discloses a thick high-strength steel plate having excellent low-temperature toughness, the steel plate having a chemical composition containing, by mass %, C: 0.02% to 0.25%, Si: 1.0% or less, Mn: 0.3% to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03% to 0.25%, Ti: 0.001% to 0.10%, and the balance being Fe and inevitable impurities, in which the relationship (Ti+Nb/2)/C<4 is satisfied, a microstructure at a position located at 1 mm in the thickness direction from the surface being a single phase of bainite or bainitic ferrite, in which the ratio of the length of grain boundary cementite to the total length of the grain boundaries is 10% or less, and a thickness of the steel plate is 8.7 mm to 35.4 mm.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 2012-062557PTL 2: Japanese Unexamined Patent Application Publication No. 2007-056294PTL 3: Japanese Unexamined Patent Application Publication No. 2013-014844

SUMMARY OF THE INVENTION

In the case of the technique disclosed in Patent Literature 1, since no consideration is given to the state in which Fe-based carbides are precipitated in a bainite structure, there is a case where it is not possible to provide sufficient punching workability to a steel sheet. In addition, when a hot-rolled steel sheet is manufactured, in order to form a desired microstructure it is necessary to cool a finish-rolled steel sheet by performing two-step cooling in which the cooling rates of the two steps are different from each other, and therefore there is a problem in that it is difficult to manufacture the steel sheet.

In the case of the technique disclosed in Patent Literature 2, since the steel material has a metallographic structure including mainly a ferrite phase while having a tensile strength of 980 MPa grade, there may be a significant decrease in the toughness of a ferrite phase.

In the case of the technique disclosed in Patent Literature 3, although the amount of grain boundary cementite is controlled in order to increase low-temperature toughness, the strength of the hot-rolled steel sheet is insufficient. Since the tensile strength is about 800 MPa at most as indicated in the examples in the DESCRIPTION, and since it is difficult to stably achieve a high strength corresponding to a tensile strength of 980 MPa or more, there is a problem in that the strength of a steel sheet is insufficient.

An object of aspects of the present invention is, by solving the problems described above, to provide a high-strength hot-rolled steel sheet excellent in terms of punching workability and toughness and a method for manufacturing the steel sheet.

The present inventors, in order to achieve the object described above, diligently conducted investigations to increase the punching workability and toughness of a hot-rolled steel sheet while maintaining a high tensile strength of 980 MPa or more. Specifically, the present inventors, focusing on a bainite phase, which is generally known to have a good strength-toughness balance, diligently conducted investigations regarding a method for achieving good punching workability while maintaining a strength-toughness balance, and, as a result, found that it is very effective for increasing the strength, toughness, and punching workability of a hot-rolled steel sheet to decrease the bainite grain diameter and control the spacing between Fe-based carbides precipitated in bainite grains. In addition, from the results of additional investigations, it was found that, by adding C, Mn, Cr, and B in a well-balanced manner, by forming a microstructure including a bainite phase as a main phase in an amount of more than 90% in terms of area ratio, by controlling the average grain diameter of the bainite phase to be 2.5 μm or less, and by controlling the spacing between Fe-based carbides precipitated in bainite grains to be 600 nm or less, there is a significant increase in punching workability and toughness while maintaining a high strength corresponding to a tensile strength TS of 980 MPa or more.

Aspects of the present invention have been completed on the basis of the knowledge described above, and the subject matter of aspects of the present invention is as follows.

[1] A high-strength hot-rolled steel sheet having a chemical composition containing, by mass %, C: 0.060% or more and 0.140% or less, Si: 1.00% or less, Mn: 1.30% or more and 2.50% or less, P: 0.030% or less, S: 0.0050% or less, Al: 0.070% or less, N: 0.010% or less, Ti: 0.060% or more and 0.140% or less, Cr: 0.10% or more and 0.50% or less, B: 0.0002% or more and 0.0020% or less, and the balance being Fe and inevitable impurities, in which C, Mn, Cr, and B satisfy relational expression (1) below, and a microstructure including a bainite phase as a main phase in an amount of more than 90% in terms of area ratio and, optionally, one, two, or all of a ferrite phase, a martensite phase, and a retained austenite phase as second phases in an amount of less than 10% in total in terms of area ratio, in which the average grain diameter of the bainite phase is 2.5 μm or less, in which the spacing between Fe-based carbides precipitated in bainitic ferrite grains in the bainite phase is 600 nm or less, and in which a tensile strength TS is 980 MPa or more.
5.0≤18C+Mn+1.3Cr+1500B≤6.0 (C, Mn, Cr, and B respectively denote the contents (mass %) of C, Mn, Cr, and B)   (1)

[2] The high-strength hot-rolled steel sheet according to item [1] above, the steel sheet having the chemical composition further containing, by mass %, one, two, or more selected from among V: 0.05% or more and 0.50% or less, Nb: 0.005% or more and 0.100% or less, Cu: 0.01% or more and 0.40% or less, Ni: 0.01% or more and 0.40% or less, and Mo: 0.01% or more and 0.40% or less.

[3] The high-strength hot-rolled steel sheet according to item [1] or [2] above, the steel sheet having the chemical composition further containing, by mass %, one or both selected from Ca: 0.0002% or more and 0.0055% or less and REM: 0.0002% or more and 0.0100% or less.

[4] A method for manufacturing the high-strength hot-rolled steel sheet according to any one of items [1] to [3] above, the method including heating a steel material to a temperature of 1150° C. or higher, performing rough rolling, performing finish rolling with a finish rolling start temperature of 950° C. or higher and 1130° C. or lower, a cumulative rolling reduction in the last three finish rolling passes of 40% or more, and a finishing delivery temperature of 830° C. or higher and 950° C. or lower, starting cooling within 3.0 seconds after finish rolling has been performed, performing cooling at an average cooling rate of 20° C./s or more, and performing coiling at a coiling temperature of 350° C. or higher and 500° C. or lower.

Here, in the present description, % used when describing the chemical composition of steel always refers to mass %.

In addition, in the present invention, “high-strength hot-rolled steel sheet” refers to a hot-rolled steel sheet having a tensile strength (TS) of 980 MPa or more.

According to aspects of the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having a tensile strength of 980 MPa or more and excellent in terms of punching workability and toughness. In addition, it is possible to stably manufacture such a high-strength hot-rolled steel sheet.

Therefore, by using the high-strength hot-rolled steel sheet according to aspects of the present invention for, for example, the structural members and skeleton members of automobiles or the frame members of trucks, it is possible to decrease the weight of the automobile bodies while maintaining the safety of the automobiles, and thus it is possible to decrease environmental loads.

In addition, by using, instead of UOE steel pipes which are manufactured from a thick steel plate, welded steel pipes which are manufactured from the high-strength hot-rolled steel sheet according to aspects of the present invention as transporting pipes, there is an increase in productivity, which results in a further decrease in cost. As described above, aspects of the present invention have a marked effect on the industry.

