WO2024111526A1

WO2024111526A1 - High-strength hot-rolled steel sheet and method for producing same

Info

Publication number: WO2024111526A1
Application number: PCT/JP2023/041536
Authority: WO
Inventors: 寛長谷川; 広志松田; 隼佑飛田
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-11-22
Filing date: 2023-11-17
Publication date: 2024-05-30

Abstract

The purpose of the present invention is to provide a high-strength hot-rolled steel sheet having excellent toughness and punchability and also having excellent strength and toughness after post-heating; and a method for producing the high-strength hot-rolled steel sheet.　Provided is a high-strength hot-rolled steel sheet having a chemical composition comprising, in % by mass, 0.04 to 0.18% of C, 0.1 to 3.0% of Si, 0.5 to 3.5% of Mn, more than 0% and 0.050% or less of P, more than 0% and 0.010% or less of S, more than 0% and 1.5% or less of Al, more than 0% and 0.010% or less of N, more than 0% and 0.003% or less of O, 0.040 to 0.150% of Ti, and a remainder comprising Fe and unavoidable impurities, in which a steel structure contains bainite as a main phase, retained austenite is contained in an amount less than 3% by volume, a ((solid solution Ti amount)/(total Ti amount)) ratio is 0.30 or more and less than 0.80, and the amount of Ti that is present in the form of precipitates each having a grain diameter of 100 nm or more is 0.010 to 0.030% by mass.

Description

High strength hot rolled steel sheet and method for manufacturing same

The present invention relates to high-strength hot-rolled steel sheets and their manufacturing methods, and in particular to high-strength hot-rolled steel sheets suitable as materials for automotive parts and their manufacturing methods.

In order to improve the crashworthiness and fuel economy of automobiles, hot-rolled steel sheets used in automobile parts are required to have high strength. On the other hand, in hot-rolled steel sheets with high strength, cracks caused by insufficient workability become prominent during pressing, so improvements in pressing methods and workability of steel sheets are required. Regarding the method, studies are being conducted to improve workability by heating the steel sheet (original sheet). In this specification, the heat treatment applied when processing the steel sheet (original sheet) into parts is also called post-heating. On the other hand, development of steel sheets is being conducted taking into account the characteristics of parts after post-heating and processing (after post-heating processing). In addition, if end cracks occur in the trim process, which is the stage before post-heating, not only will they deteriorate press formability, but they will also lead to various defects such as a decrease in fatigue properties, so the stability of the trim end surface, i.e., excellent punchability, is also required. In response to these various issues, various methods and steel sheets are being developed.

Patent Document 1 discloses a technique for improving stretch flangeability by setting the processing temperature (post-heating temperature) at 400 to 1000°C. Patent Document 2 discloses a technique for high-strength hot-rolled steel sheet with a TS of 730 MPa or more. The hot-rolled steel sheet disclosed in Patent Document 2 has a structure in which bainite is the main phase and 80% or more of the total Ti is solid-solubilized Ti. This results in heat treatment hardening with an increase in YS (yield strength) and TS of 100 MPa or more after heat treatment in which the steel sheet is heated to a temperature range of 500°C to the Ac1 transformation point and held for 60 minutes. Patent Document 3 discloses a technique for a hot-rolled steel sheet with a structure in which martensite and/or tempered martensite are the main phases, and which has excellent toughness, punchability, delayed fracture resistance, etc.

JP 2002-113527 A JP 2015-57514 A JP 2018-188675 A

However, Patent Document 1 does not take into consideration the performance of the part, such as strength and toughness, after post-heating, and there is room for improvement. The steel plate disclosed in Patent Document 2 has issues with toughness after heat treatment due to a significant increase in strength and fine precipitation of carbides after heat treatment, and there is room for improvement as no consideration is given to the punchability of the original plate before heat treatment. Patent Document 3 focuses on the properties of the original plate, and there is no consideration at all to the strength and toughness of the steel plate after post-heating, and there is room for improvement.

The present invention was made in consideration of the above circumstances, and aims to provide a high-strength hot-rolled steel sheet that has excellent toughness and punchability, and also has excellent strength and toughness after post-heating, and a manufacturing method thereof.

The inventors focused on the precipitation behavior of Ti after post-heating of hot-rolled steel sheets, and came up with the idea of improving the properties of the steel sheets after post-heating by controlling the initial coarse Ti-containing precipitates and the amount of dissolved Ti before post-heating. As a result, they discovered that by adjusting the chemical composition to make bainite the main phase, making the amount of residual γ less than 3% by volume, making the ratio of the amount of dissolved Ti to the Ti content (amount of dissolved Ti/total Ti) 0.30 or more and less than 0.80, and making the amount of Ti present as precipitates with a grain size of 100 nm or more 0.010 to 0.030 mass%, the hot-rolled steel sheet (original sheet) can have high strength, excellent punchability and toughness, and still exhibit properties similar to those of the original sheet even after post-heating, resulting in excellent toughness and high strength, and thus completing the present invention.

In the present invention, high strength means that the tensile strength (TS) is 780 MPa or more and less than 1320 MPa.
In the present invention, "excellent toughness" means that in a Charpy impact test using a test piece taken from a hot-rolled steel sheet (original sheet or hot-rolled steel sheet after post-heating), the ductile fracture surface ratio at -40°C is 50% or more. The thickness of the test piece is 0.6 to 3.0 mm, and when the thickness of the hot-rolled steel sheet exceeds 3.0 mm, the test piece taken from the hot-rolled steel sheet is ground on both sides to a thickness of 3.0 mm, and then subjected to the Charpy impact test.
In the present invention, excellent punchability means that when punched with a clearance of 5 to 30%, the clearance range where no chipping or cracking occurs on the end face is 10% or more.
In the present invention, excellent strength after post-heating means that the decrease in strength of the hot-rolled steel sheet after post-heating is 40 or less in Vickers hardness compared to the strength of the hot-rolled steel sheet (original sheet) before post-heating.
In the present invention, post-heating means a heat treatment in which the hot-rolled steel sheet (original sheet) is heated to 400° C. or higher.

The present invention has the following configuration.
[1] In mass%,
C: 0.04 to 0.18%,
Si: 0.1 to 3.0%,
Mn: 0.5 to 3.5%,
P: more than 0% and not more than 0.050%;
S: more than 0% and 0.010% or less;
Al: more than 0% and not more than 1.5%;
N: more than 0% and not more than 0.010%;
O: more than 0% and 0.003% or less; and Ti: 0.040 to 0.150%;
The balance is Fe and unavoidable impurities,
The steel structure has bainite as the main phase and retained austenite at a volume fraction of less than 3%;
the ratio of the amount of dissolved Ti to the Ti content (amount of dissolved Ti/total amount of Ti) is 0.30 or more and less than 0.80;
A high-strength hot-rolled steel sheet, in which the amount of Ti present as precipitates having a grain size of 100 nm or more is 0.010 to 0.030 mass %.
[2] The composition further comprises, in mass%,
Cr: 0.005 to 2.0%,
Cu: 0.005 to 0.5%,
Ni: 0.005 to 2.0%,
Mo: 0.005 to 1.0%,
V: 0.005 to 0.5%,
B: 0.0002 to 0.0050%,
Ca: 0.0001 to 0.0050%,
REM: 0.0001 to 0.0050%,
Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50%
The high strength hot rolled steel sheet according to [1], comprising one or more selected from the following:
[3] The high strength hot rolled steel sheet according to [1] or [2], wherein the amount of Fe present as precipitates having a grain size of 100 nm or more is more than 0 mass% and 0.100 mass% or less.
[4] A method for producing a high strength hot rolled steel sheet according to any one of [1] to [3],
A slab having the above-mentioned composition is heated to a temperature range of 1150 to 1300°C and held at that temperature range for 0.2 to 3.5 hours;
Next, when hot rolling is performed,
After rough rolling with a total reduction of 80 to 90% in a temperature range of 1080 ° C. or more, finish rolling is performed, and in the finish rolling, rolling is performed under conditions where the reduction per pass is 25% or less at T (° C.) or less calculated by the following formula, and after the completion of the finish rolling, the steel is allowed to cool for 1.0 s or more.
Next, the steel sheet is cooled at an average cooling rate of 50°C/s or more in a temperature range up to 550°C, and then coiled at a coiling temperature of the Ms point (°C) or higher and 550°C or lower.
T(℃)=800+1000[Ti]
Here, [Ti] is the Ti content (mass %).
[5] A method for producing a high strength hot rolled steel sheet according to [4], wherein cooling is stopped at a cooling stop temperature in a temperature range of 480 to 550°C during the period from cooling at an average cooling rate of 50°C/s or more in a temperature range up to 550°C to coiling, the cooling is held at the cooling stop temperature ±20°C for 0.5 to 4.0 s, and then coiled at the coiling temperature.

The present invention provides a high-strength hot-rolled steel sheet that is excellent in toughness and punchability, and also has excellent strength and toughness after post-heating, and a method for manufacturing the same.

According to the present invention, a high-strength hot-rolled steel sheet that exhibits excellent strength and toughness even after post-heating, and is suitable as a material for automobile parts, can be obtained.
By using the high-strength hot-rolled steel sheet of the present invention, products such as high-strength automobile parts that exhibit excellent strength and toughness can be obtained even after post-heating for improving workability and fatigue properties.

Below, the embodiments of the high-strength hot-rolled steel sheet and its manufacturing method of the present invention will be described in detail. Note that the present invention is not limited to the following embodiments.

<High-strength hot-rolled steel sheet>
The high-strength hot-rolled steel sheet of the present invention may be either a black skin as hot-rolled, or a hot-rolled steel sheet called a white skin which is further pickled after hot rolling. The high-strength hot-rolled steel sheet of the present invention preferably has a thickness of 0.6 mm or more. The high-strength hot-rolled steel sheet of the present invention preferably has a thickness of 10.0 mm or less. When the high-strength hot-rolled steel sheet of the present invention is used as a material for automobile parts, the thickness is more preferably 1.0 mm or more. When the high-strength hot-rolled steel sheet of the present invention is used as a material for automobile parts, the thickness is more preferably 6.0 mm or less. The width of the high-strength hot-rolled steel sheet of the present invention is preferably 500 mm or more, more preferably 700 mm or more. The width of the high-strength hot-rolled steel sheet of the present invention is preferably 1800 mm or less, more preferably 1400 mm or less.

The high-strength hot-rolled steel sheet of the present invention has a specific chemical composition and a specific steel structure. Here, the chemical composition and steel structure will be explained in that order.

First, we will explain the composition of the high-strength hot-rolled steel sheet of the present invention. Note that "%" representing the content of the composition of the components means "mass %."

The composition of the high-strength hot-rolled steel sheet of the present invention is, in mass%, C: 0.04-0.18%, Si: 0.1-3.0%, Mn: 0.5-3.5%, P: 0.050% or less (excluding 0%), S: 0.010% or less (excluding 0%), Al: 1.5% or less (excluding 0%), N: 0.010% or less (excluding 0%), O: 0.003% or less (excluding 0%), Ti: 0.040-0.150%, with the balance being Fe and unavoidable impurities.

C: 0.04 to 0.18%
C is an element that is effective in increasing TS by generating and strengthening bainite, and in suppressing a decrease in strength after post-heating by combining with Ti, N, etc. to generate precipitates. If the C content is less than 0.04%, such effects are not sufficiently obtained, and the TS of the steel plate (original plate) of 780 MPa or more or excellent strength after post-heating is not obtained. On the other hand, if the C content exceeds 0.18%, the toughness and punchability are significantly decreased, and the characteristics of the present invention are not obtained. Therefore, the C content is set to 0.04 to 0.18%. The C content is preferably set to 0.05% or more. The C content is preferably set to 0.16% or less, and more preferably set to 0.11% or less.

Si: 0.1 to 3.0%
Silicon is an element effective in strengthening the solid solution of steel, suppressing cementite in bainite, and suppressing the decrease in strength after post-heating. To obtain these effects, the content must be 0.1% or more. On the other hand, if the Si content exceeds 3.0%, polygonal ferrite is excessively formed and the steel structure of the present invention cannot be obtained. Therefore, the Si content is set to 0.1 to 3.0%. The Si content is preferably set to 0.2% or more. The Si content is preferably set to 2.0% or less, and more preferably set to 1.5% or less.

Mn: 0.5 to 3.5%
Mn is an element effective in suppressing ferrite and generating bainite. If the Mn content is less than 0.5%, this effect is not sufficiently obtained, and polygonal ferrite and the like are generated, making it impossible to obtain the microstructure of the present invention. On the other hand, if the Mn content exceeds 3.5%, martensite increases and the toughness decreases significantly, making it impossible to obtain the excellent toughness of the present invention. Therefore, the Mn content is set to 0.5 to 3.5%. The Mn content is preferably set to 1.0% or more. Moreover, the Mn content is preferably set to 2.7% or less.

P: more than 0% and 0.050% or less P reduces toughness, so it is desirable to reduce the amount as much as possible. In the present invention, a P content of up to 0.050% is acceptable. Therefore, the P content is set to 0.050% or less. The P content is preferably set to 0.030% or less. There is no particular lower limit, and the P content may be more than 0%, but if the P content is less than 0.001%, production efficiency decreases, so the P content is preferably 0.001% or more.

S: more than 0% and 0.010% or less S reduces toughness, so it is preferable to reduce the amount as much as possible, but in the present invention, an S content of up to 0.010% is acceptable. Therefore, the S content is set to 0.010% or less. The S content is preferably set to 0.0050% or less, and more preferably set to 0.0020% or less. There is no particular lower limit, and the S content may be more than 0%, but if the S content is less than 0.0002%, production efficiency decreases, so the S content is preferably 0.0002% or more.

Al: more than 0% and not more than 1.5% Al acts as a deoxidizer, and is preferably added in the deoxidization process. The Al content may be more than 0%, but from the viewpoint of using it as a deoxidizer, the Al content is preferably 0.01% or more. On the other hand, if a large amount of Al is contained, a large amount of polygonal ferrite is generated, and the steel structure of the present invention cannot be obtained. In the present invention, the Al content is allowed up to 1.5%. Therefore, the Al content is set to 1.5% or less. The Al content is preferably set to 0.50% or less, more preferably 0.10% or less, and even more preferably 0.05% or less.

N: more than 0% and 0.010% or less N generates TiN and inhibits the precipitation of TiC, so it is preferable to reduce the amount as much as possible. In the present invention, an N content of up to 0.010% is acceptable. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.007% or less. There is no particular lower limit, and the N content may be more than 0%, but if the N content is less than 0.0005%, the production efficiency decreases, so the N content is preferably 0.0005% or more.

O: more than 0% and 0.003% or less O reduces toughness, so it is preferable to reduce the amount as much as possible. In the present invention, an O content of up to 0.003% is acceptable. Therefore, the O content is set to 0.003% or less. The O content is preferably set to 0.002% or less. There is no particular lower limit, and the O content may be more than 0%, but if the O content is less than 0.0002%, production efficiency decreases, so the O content is preferably 0.0002% or more.

Ti: 0.040 to 0.150%
Ti is the most important element in the present invention, and is an element necessary for obtaining excellent strength and toughness after post-heating by generating appropriate precipitates such as TiC after post-heating. If the Ti content is less than 0.040%, such effects are not sufficiently obtained, and excellent strength after post-heating is not obtained. On the other hand, if it exceeds 0.150%, the precipitates after post-heating become excessive, and excellent toughness after post-heating cannot be obtained. Therefore, the Ti content is set to 0.040 to 0.150%. The Ti content is preferably set to 0.050% or more. Moreover, the Ti content is preferably set to 0.120% or less.

The above components are the basic components of the high-strength hot-rolled steel sheet of the present invention. The high-strength hot-rolled steel sheet of the present invention contains the above components, with the remainder being Fe and unavoidable impurities.

In addition to the above components, the high-strength hot-rolled steel sheet of the present invention can further contain one or more selected from the following: Cr: 0.005-2.0%, Cu: 0.005-0.5%, Ni: 0.005-2.0%, Mo: 0.005-1.0%, V: 0.005-0.5%, B: 0.0002-0.0050%, Ca: 0.0001-0.0050%, REM: 0.0001-0.0050%, Sb: 0.0010-0.10%, Sn: 0.0010-0.50%.

Cr: 0.005 to 2.0%
Cr is an element effective in suppressing ferrite and generating bainite. In order to obtain such an effect, when Cr is contained, the Cr content is preferably 0.005% or more. On the other hand, when the Cr content exceeds 2.0%, the corrosion resistance may be significantly decreased, so when Cr is contained, the Cr content is preferably 2.0% or less. The Cr content is more preferably 0.1% or more. Moreover, the Cr content is more preferably 1.0% or less, and further preferably 0.8% or less.

Cu: 0.005 to 0.5%
Cu is an element effective in stabilizing austenite and generating bainite. In order to obtain such an effect, when Cu is contained, the Cu content is preferably 0.005% or more. On the other hand, when the Cu content exceeds 0.5%, the generation of Cu precipitates becomes significant, which may lead to a decrease in toughness, so when Cu is contained, the Cu content is preferably 0.5% or less. The Cu content is more preferably 0.05% or more. Moreover, the Cu content is more preferably 0.3% or less.

Ni: 0.005 to 2.0%
Ni is an element effective in suppressing ferrite and generating bainite. In order to obtain such an effect, when Ni is contained, the Ni content is preferably 0.005% or more. On the other hand, when the Ni content exceeds 2.0%, a large amount of martensite and residual γ are formed, which may lead to a decrease in toughness, so when Ni is contained, the Ni content is preferably 2.0% or less. The Ni content is more preferably 0.05% or more. Moreover, the Ni content is more preferably 0.8% or less, and further preferably 0.5% or less.

Mo: 0.005 to 1.0%
Mo is an element effective in improving the hardenability of the steel plate and generating bainite. In order to obtain such effects, when Mo is contained, the Mo content is preferably 0.005% or more. On the other hand, when the Mo content exceeds 1.0%, the generation of Mo-based precipitates becomes significant, which may lead to a decrease in toughness, so when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.05% or more. Moreover, the Mo content is more preferably 0.50% or less.

V: 0.005 to 0.5%
V is an element effective in improving the hardenability of a steel sheet and generating bainite. In order to obtain such effects, when V is contained, the V content is preferably 0.005% or more. On the other hand, when the V content exceeds 0.5%, the generation of V-based precipitates becomes significant, which may lead to a decrease in toughness, so when V is contained, the V content is preferably 0.5% or less. The V content is more preferably 0.01% or more. Moreover, the V content is more preferably 0.1% or less.

B: 0.0002 to 0.0050%
B is an element effective in improving the hardenability of a steel sheet and generating bainite. In order to obtain such effects, when B is contained, the B content is preferably 0.0002% or more. On the other hand, when the B content exceeds 0.0050%, B-based compounds increase, and toughness may decrease. Therefore, when B is contained, the B content is preferably 0.0050% or less. The B content is more preferably 0.0005% or more. Moreover, the B content is more preferably 0.0040% or less.

Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%
Ca and REM (rare earth elements) are each an element effective in improving workability by controlling the shape of inclusions. In order to obtain such an effect, when Ca and REM are contained, the respective contents are preferably 0.0001% or more. On the other hand, when the respective contents of Ca and REM exceed 0.0050%, the influence of the increase in the amount of inclusions becomes significant, and the toughness may decrease. Therefore, when Ca and REM are contained, the respective contents of Ca and REM are preferably 0.0050% or less. The Ca content is more preferably 0.0005% or more. Moreover, the Ca content is more preferably 0.0030% or less. The REM content is more preferably 0.0005% or more. Moreover, the REM content is more preferably 0.0030% or less. Note that REM is a collective term for Sc, Y, and 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here is the total content of these elements.

Sb: 0.0010 to 0.10%, Sn: 0.0010 to 0.50%
Sb and Sn are elements that suppress surface reactions such as oxidation, denitrification, and deboronization, improve the surface properties of the steel sheet, and are effective in improving toughness. In order to obtain such effects, when Sb and Sn are contained, the contents of Sb and Sn are preferably 0.0010% or more. On the other hand, when the Sb content exceeds 0.10% or the Sn content exceeds 0.50%, the steel sheet may be embrittled and the toughness may be significantly reduced. Therefore, when Sb is contained, the Sb content is preferably 0.10% or less, and when Sn is contained, the Sn content is preferably 0.50% or less. The Sb content is more preferably 0.0050% or more. Moreover, the Sb content is more preferably 0.030% or less. The Sn content is more preferably 0.0050% or more. Moreover, the Sn content is more preferably 0.050% or less.

In addition, even if the content of Cr, Cu, Ni, Mo, V, B, Ca, REM, Sb, and Sn is less than the above lower limit, the effect of the present invention is not impaired. Therefore, when the content of these components is less than the above lower limit, these elements are treated as being contained as unavoidable impurities.

In addition to the above-mentioned composition, the present invention may further contain, by mass%, one or more of Mg, As, W, Ta, Pb, Zr, Hf, Te, Bi, and Se in a total amount of 0.3% or less. It is preferable to limit the content of each of these elements to 0.03% or less.

Next, we will explain the steel structure of the high-strength hot-rolled steel sheet of the present invention.

The steel structure of the high-strength hot-rolled steel sheet of the present invention has bainite as the main phase, with the volume fraction of residual γ being less than 3%.

Main phase: bainite In the present invention, in order to obtain high strength and excellent toughness, the structure has bainite as the main phase. If ferrite, pearlite, residual γ, etc. become the main phase, it becomes difficult to achieve both high strength and excellent toughness and punchability. In addition, if martensite becomes the main phase, it is not preferable because the toughness and punchability decrease. Therefore, the steel structure has bainite as the main phase. Note that bainite may be any of upper bainite, lower bainite, tempered bainite, and bainitic ferrite. Note that, in the present invention, the main phase means a phase that occupies 50% or more in terms of area ratio. The area ratio of the main phase is preferably 55% or more, more preferably 65% or more. In addition, the area ratio of the main phase is preferably 95% or less.

Amount of retained austenite (residual γ): less than 3% Since retained austenite (residual γ) is a structure that reduces the toughness of the steel plate and significantly reduces the strength and toughness by transforming into pearlite after post-heating, it is preferable to reduce it as much as possible. In the present invention, the volume fraction of retained γ is allowed to be less than 3%. Therefore, the volume fraction of retained γ is set to less than 3%. The volume fraction of retained γ is preferably less than 2%, and more preferably less than 1%. There is no particular limit on the lower limit of the volume fraction of retained γ, and the volume fraction of retained γ may be 0%.

The phases other than bainite and residual gamma (other phases) include one or more of ferrite, pearlite, and martensite. The total area ratio of the other phases is preferably 40% or less. There is no particular lower limit to the area ratio of the other phases, but the total area ratio of the other phases is preferably 3% or more, more preferably 5% or more, and even more preferably 10% or more.

Solute Ti content/total Ti content: 0.30 or more and less than 0.80 If the ratio of the solute Ti content to the Ti content (Solute Ti content/total Ti content) is less than 0.30, the amount of solute Ti that becomes precipitates during post-heating to offset the strength reduction is insufficient, resulting in a reduction in strength after post-heating, or in the form of fine precipitates, resulting in a reduction in toughness. On the other hand, if it is 0.80% or more, the strength increase due to precipitation after post-heating becomes significant, and excellent toughness after post-heating cannot be obtained. Therefore, the solute Ti content/total Ti content is 0.30 or more and less than 0.80. It is preferably 0.35 or more. It is also preferably 0.70 or less. The solute Ti content/total Ti content is determined by the method described in the examples.

Amount of Ti present as precipitates with a grain size of 100 nm or more: 0.010 to 0.030 mass%
By containing a certain amount of Ti-containing precipitates having a grain size of 100 nm or more, the growth of the precipitates competes with the precipitation of new TiC during post-heating, thereby appropriately suppressing the precipitation of fine TiC, and excessive strength increase and toughness decrease can be suppressed. To obtain such an effect, the amount of Ti present as precipitates having a grain size of 100 nm or more must be 0.010 mass% or more. On the other hand, if the amount of Ti exceeds 0.030 mass%, the decrease in toughness due to coarse precipitates becomes significant, so the amount of Ti present as precipitates having a grain size of 100 nm or more must be 0.030 mass% or less. Therefore, the amount of Ti present as precipitates having a grain size of 100 nm or more is set to 0.010 to 0.030 mass%. It is preferably set to 0.013 mass% or more. It is also preferably set to 0.027 mass% or less. The amount of Ti present as precipitates having a grain size of 100 nm or more is determined by the method described in the examples.

Amount of Fe present as precipitates with a grain size of 100 nm or more: more than 0 mass% and 0.100 mass% or less (preferred condition)
In the present invention, in addition to the above, the amount of Fe present as precipitates having a particle size of 100 nm or more can be set to more than 0 mass% and 0.100 mass% or less to further improve punchability. Fe precipitates having a particle size of 100 nm or more serve as a path for cracks when forming a fracture surface during punching, and are effective in smoothing the fracture surface. On the other hand, if there are too many such precipitates, punchability may be impaired. Therefore, it is preferable that the amount of Fe present as precipitates having a particle size of 100 nm or more is more than 0 mass% and 0.100 mass% or less. It is more preferable that it is 0.001 mass% or more, and even more preferable that it is 0.004 mass% or more. The amount of Fe present as precipitates having a particle size of 100 nm or more can be determined by the method described in the examples.

<Method of manufacturing high-strength hot-rolled steel sheet>
The high-strength hot-rolled steel sheet of the present invention is produced by heating a slab having the above-mentioned composition to a temperature range of 1150 to 1300°C, holding the slab in the temperature range for 0.2 to 3.5 hours, and then performing hot rolling, which involves performing rough rolling at a total reduction of 80 to 90% in a temperature range of 1080°C or higher, followed by finish rolling, and rolling under conditions in which the reduction per pass at or below T (°C) calculated by the following formula is 25% or less, allowing the slab to cool for 1.0 s or more after the completion of finish rolling, and then cooling the slab at a temperature range up to 550°C at an average cooling rate of 50°C/s or more, and then coiling the slab at a coiling temperature of the Ms point (°C) or higher and 550°C or lower.
T(℃)＝800+1000[Ti]
Here, [Ti] is the Ti content (mass %).
The total reduction in the temperature range of 1080° C. or higher is determined from the ratio of the thickness of the slab before hot rolling to the thickness at 1080° C. The reduction per pass below T (° C.) is determined from the ratio of the thickness before and after each pass of rolling below T (° C.).

A detailed explanation is provided below. The above temperatures are the surface temperatures at the center of the width of the steel plate, and the above average cooling rates and cooling speeds are the average cooling rate and cooling speed at the surface at the center of the width of the steel plate, respectively. Furthermore, unless otherwise specified, the average cooling rate is [(cooling start temperature - cooling stop temperature) / cooling time from cooling start temperature to cooling stop temperature].

Slab heating temperature: 1150-1300°C
If the heating temperature of the slab is less than 1150°C, the dissolution of Ti-containing precipitates becomes insufficient, and the value of the amount of dissolved Ti/total Ti of 0.30 or more and less than 0.80, or the value of the amount of Ti present as precipitates with a particle size of 100 nm or more of 0.010 to 0.030 mass% cannot be obtained. On the other hand, if the heating temperature of the slab exceeds 1300°C, the dissolution of Ti-containing precipitates becomes excessive, and the value of the amount of dissolved Ti/total Ti of 0.30 or more and less than 0.80, or the value of the amount of Ti present as precipitates with a particle size of 100 nm or more of 0.010 to 0.030 mass% cannot be obtained. Therefore, the heating temperature of the slab is set to 1150 to 1300°C. The heating temperature is preferably 1170°C or more, more preferably 1185°C or more. The heating temperature is preferably 1280°C or less, more preferably 1265°C or less.

Holding time in the temperature range of 1150 to 1300°C: 0.2 to 3.5 hours If the holding time in the temperature range of 1150 to 1300°C is less than 0.2 hours, the dissolution of Ti-containing precipitates is insufficient. As a result, the value of the amount of dissolved Ti/total Ti of 0.30 to less than 0.80, or the value of the amount of Ti present as precipitates with a particle size of 100 nm or more of 0.010 to 0.030 mass% cannot be obtained. On the other hand, if the holding time in the above temperature range exceeds 3.5 hours, decarburization in the vicinity of the surface layer becomes significant, ferrite and residual γ are likely to be generated from the vicinity of the surface layer, and the structure of the present invention cannot be obtained. Therefore, the holding time of the slab in the above temperature range is 0.2 to 3.5 hours. The holding time is preferably 0.4 hours or more. The holding time is preferably 2.5 hours or less.

Total reduction rate at temperatures above 1080°C: 80-90%
After the above holding, hot rolling is performed. When hot rolling is performed, rough rolling with a total reduction of 80 to 90% is performed in a temperature range of 1080°C or more, and then finish rolling is performed. By performing rolling with a total reduction of 80 to 90% in a temperature range of 1080°C or more, the generation and growth of coarse Ti-containing precipitates with a particle size of 100 nm or more can be promoted, and the amount of Ti present as precipitates with a particle size of 100 nm or more can be set to 0.010 to 0.030 mass%. If the total reduction is less than 80%, the generation of precipitates with a particle size of 100 nm or more is insufficient, and the amount of Ti present as precipitates with a particle size of 100 nm or more is less than 0.010 mass%. On the other hand, if the total reduction exceeds 90%, the generation of precipitates with a particle size of 100 nm or more becomes excessive, and the amount of Ti present as precipitates with a particle size of 100 nm or more exceeds 0.030 mass%. Therefore, the total rolling reduction in the temperature range of 1080° C. or higher is set to 80 to 90%. The total rolling reduction is preferably set to 81% or higher. The total rolling reduction is preferably set to 88% or lower.

Reduction rate per pass at T (°C) or less: 25% or less After the rough rolling, finish rolling is performed. In the finish rolling, if a reduction of more than 25% per pass is performed at T (°C) or less calculated by the following formula, Ti-containing precipitates are generated, and a value of 0.30 or more and less than 0.80 of the amount of dissolved Ti/total Ti cannot be obtained. Therefore, the reduction rate per pass at T (°C) or less is 25% or less. The reduction rate is preferably 20% or less, more preferably 18% or less. The lower limit of the reduction rate is not particularly limited, but since coarse grains may be generated when the reduction rate is 5% or less, it is preferable that the reduction rate is more than 5%. The reduction rate is more preferably 7% or more. In the present invention, it is not necessary to perform rolling at T (°C) or less (no pass at T (°C) or less). In the finish rolling, the reduction rate per pass above T (°C) is not particularly limited.
Incidentally, T (°C) can be calculated by the following formula.
T(℃)=800+1000[Ti]
Here, [Ti] is the Ti content (mass %).

Cooling for 1.0 s or more By cooling after the above-mentioned finish rolling, it is possible to release some strain and suppress the generation of Ti-containing precipitates during the subsequent cooling. In order to obtain such an effect, it is necessary to set the cooling time after the completion of the finish rolling to 1.0 s or more. The cooling time is preferably 1.5 s or more, more preferably 2.0 s or more, and further preferably 2.2 s or more. There is no particular limit to the upper limit of the cooling time, but if the cooling time is 5.0 s or less, it becomes easier to control the subsequent hot rolling, so the cooling time is preferably 5.0 s or less. Note that cooling means exposure to the atmosphere (air cooling) without active cooling (accelerated cooling) by pouring water or the like.

Cooling at an average cooling rate of 50°C/s or more in the temperature range up to 550°C After the above cooling, cooling at an average cooling rate of 50°C/s or more in the temperature range up to 550°C. If the average cooling rate up to 550°C is less than 50°C/s, ferrite and Ti-containing precipitates are excessively generated, and the phase structure and precipitates of the present invention cannot be obtained. Therefore, the average cooling rate in the temperature range from the cooling start temperature to 550°C after the above cooling is 50°C/s or more. The average cooling rate is preferably 70°C/s or more. There is no particular limit to the upper limit of the average cooling rate, but if the average cooling rate is 500°C/s or more, deterioration of the steel sheet shape may occur, so the average cooling rate is preferably less than 500°C/s, and more preferably 300°C/s or less.

Coiling temperature: Ms point (°C) or higher and 550°C or lower If the coiling temperature exceeds 550°C, ferrite and Ti-containing precipitates are excessively formed, and the phase structure and precipitates of the present invention cannot be obtained. On the other hand, if the coiling temperature is lower than the Ms point, martensite is excessively formed and the structure of the present invention cannot be obtained. Therefore, the coiling temperature is set to be Ms point (°C) or higher and 550°C or lower. It is preferably set to be (Ms point + 20)°C or higher. It is also preferably set to be 530°C or lower. The Ms point (°C) is the martensite transformation start temperature, and is determined by a processing formaster or the like. When the Ms point (°C) is determined by a processing formaster, for example, a sample is heated to 1250°C, held at that temperature for 300s, and then cooled at a cooling rate of 100°C/s, and the temperature at which the size of the sample changes from contraction to expansion can be determined as the Ms point (°C).

Holding time at cooling stop temperature ±20°C in the temperature range of 480 to 550°C: 0.5 to 4.0 s (optimal conditions)
In the present invention, it is preferable to stop cooling at a cooling stop temperature in a temperature range of 480 to 550 ° C. between cooling at an average cooling rate of 50 ° C./s or more in the temperature range up to 550 ° C. and winding, and to hold the cooling stop temperature ± 20 ° C. for 0.5 to 4.0 s. This can further improve punchability. Holding for 0.5 s or more can generate Fe-containing precipitates, and the punchability can be further improved. On the other hand, by setting the holding time to 4.0 s or less, the amount of Fe present as precipitates with a particle size of 100 nm or more can be easily set to 0.100 mass % or less, and it is easy to suppress the disappearance of the punchability improvement effect. Therefore, it is preferable that the holding time at the cooling stop temperature ± 20 ° C. is 0.5 to 4.0 s. More preferably, it is 0.5 to 2.0 s.

There are no particular limitations on the manufacturing method conditions other than those described above, but it is preferable to manufacture by adjusting the conditions as appropriate as follows. For example, it is preferable to perform finish rolling in four passes or more in order to reduce coarse grains that cause a decrease in workability.

The high-strength hot-rolled steel sheet of the present invention has excellent strength and toughness after post-heating. Here, the heating temperature of the post-heating is 400°C or higher. The upper limit of the heating temperature of the post-heating is not particularly limited, but an example of the heating temperature of the post-heating is 1150°C or lower. The heating time of the post-heating (holding time at the heating temperature) is not particularly limited, but an example of the heating time is more than 0 seconds. The heating time of the post-heating is 3600 seconds or less, for example.

　Steel having the composition shown in Table 1 was melted in a converter and made into a slab, which was then heated and hot rolled under the conditions shown in Table 2 to produce hot-rolled steel sheets (original sheets). The hot-rolled steel sheets obtained were used to perform the following test methods: structural observation, analysis of solute Ti and Ti-containing precipitates, analysis of Fe-containing precipitates, and evaluation of tensile properties, hardness, punchability, and toughness. The hot-rolled steel sheets were then post-heated as shown in Table 2, and the hardness, toughness, and delayed fracture resistance properties were evaluated using the hot-rolled steel sheets after post-heating according to the following test methods. The post-heating temperature was set to 400°C or higher, at which an improvement in stretch flangeability is observed, and the post-heating time was set to 3600 s or less from the viewpoint of productivity. Note that "-" in Table 2 indicates that the treatment was not performed. The Ms point (°C) in Table 2 was determined from tests using a processing formaster.

Structure Observation The area ratio of bainite is the ratio of the area of bainite to the observed area. The area ratio of bainite was measured by cutting out a sample from the obtained hot-rolled steel sheet, polishing the plate thickness cross section parallel to the rolling direction, corroding it with 3% nital, and photographing the plate thickness 1/4 position with a SEM (scanning electron microscope) at a magnification of 1500 times in three fields of view. The area ratio of each structure was calculated from the image data of the obtained secondary electron image using Image-Pro manufactured by Media Cybernetics, and the average area ratio of the three fields of view was taken as the area ratio of each structure. The structure may be determined by a general classification, but can be determined, for example, as follows. In the image data, bainite is distinguished as black or dark gray containing carbides or martensite with a linear interface, and lower bainite is distinguished as black or dark gray, gray, or light gray containing oriented carbides. Martensite is a black to light gray structure containing regular carbides with multiple orientations. Alternatively, it is observed as white or light gray without carbides. Residual γ is observed as white or light gray without carbides. Since it may be difficult to distinguish between a part of martensite and residual γ, the residual γ was obtained by the method described below, and the area ratio of martensite was obtained by subtracting it from the total area ratio of martensite and residual γ obtained from the SEM image. In the present invention, the martensite may be fresh martensite, autotempered martensite, tempered martensite, etc. depending on the degree of tempering, and any of these martensites may be used. In addition, the bainite may be any of upper bainite, lower bainite, tempered bainite, etc., but upper bainite or tempered bainite is more preferable. The more the degree of tempering is strong, the black contrast of the base material becomes an image, so the color of the base material is a guideline, and in the present invention, the amount of carbide, the structure form, etc. are comprehensively judged, and the structure, including the structure described below, is classified into one of the structures with similar characteristics. The carbides are white dots or lines. In addition, as for the structures other than the above, ferrite is a structure that is black or dark gray and does not have a substructure such as carbides or laths inside, and pearlite can be distinguished as a black and white layered or partially interrupted layered structure. The amount of residual γ is determined as follows. After grinding the hot-rolled steel sheet to 1/4 + 0.1 mm of the sheet thickness, the surface polished by chemical polishing for another 0.1 mm is used as the measurement surface. For the measurement surface, the integrated reflection intensity of the (200), (220), and (311) surfaces of fcc iron (γ) and the (200), (211), and (220) surfaces of bcc iron (ferrite) is measured using Mo Kα1 rays in an X-ray diffraction device. Then, the volume fraction is calculated from the intensity ratio of the integrated reflection intensity from each surface of fcc iron to the integrated reflection intensity from each surface of bcc iron, and this is the amount of residual γ.

Table 3 shows the structures constituting the main phase and other structures that account for 50% or more of the area ratio of each structure obtained. Note that in Table 3, B means bainite, γ means retained austenite, and O means other phases. Other phases include one or more of ferrite, pearlite, and martensite.

Analysis of dissolved Ti and Ti-containing precipitates Test pieces with a width of 30 mm and a length of 30 mm were taken from the obtained hot-rolled steel sheet, and constant current electrolysis was performed in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 Vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol). The current density was 20 mA/cm ² , and the amount of electrolysis was about 0.2 g. The electrolyte after electrolysis was used as an analysis solution, and the concentrations (mass%) of Ti and Fe as a comparative element were measured using ICP mass spectrometry. Based on the obtained concentration, the concentration ratio of Ti to Fe was calculated, and the amount of dissolved Ti (mass%) was obtained by multiplying the amount of dissolved Ti by the amount of dissolved Ti (mass%) in the test piece. The amount of dissolved Ti (mass%) in the test piece was calculated by subtracting the total amount of components other than Fe (mass%) from 100 mass%. The amount of dissolved Ti (mass%) obtained was used to calculate the ratio to the amount of contained Ti (mass%). On the other hand, the test piece with the deposits on the surface after electrolysis was taken out of the electrolytic solution and immersed in an aqueous solution of sodium hexametaphosphate (500 mg/L) (hereinafter referred to as SHMP aqueous solution). Then, ultrasonic vibration was applied to peel off the deposits from the test piece and extract them into the SHMP aqueous solution. Next, the SHMP aqueous solution containing the deposits was filtered using a filter with a pore size of 100 nm, and then the deposits collected on the 100 nm filter were acid-decomposed, and the decomposition solution was analyzed using an ICP emission spectrometer to measure the absolute value of Ti in the decomposition solution. The obtained absolute value of Ti was divided by the amount of electrolyte to obtain the amount of Ti contained in the deposits with a particle size of 100 nm or more (mass %) when the total composition of the test piece was taken as 100 mass %). Next, the obtained amount of Ti (mass %) was divided by the amount of Ti contained in the test piece (mass %) to obtain the amount of Ti (mass %) present as deposits containing Ti with a particle size of 100 nm or more. The amount of electrolyte was determined by measuring the mass of the test piece after the deposit was peeled off and subtracting it from the mass of the test piece before electrolysis.

Analysis of Fe-containing precipitates A test piece with a width of 30 mm and a length of 30 mm was taken from the obtained hot-rolled steel sheet, and constant current electrolysis was performed in a non-aqueous solvent-based electrolyte (10% AA-based electrolyte: 10 Vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol). The current density was 20 mA/cm ² , and the amount of electrolysis was about 0.2 g. After electrolysis, the test piece with the precipitate attached to the surface was taken out of the electrolyte, immersed in an aqueous SHMP solution, and ultrasonic vibration was applied to peel off the precipitate from the test piece and extract it into the aqueous SHMP solution. Next, the aqueous SHMP solution containing the precipitate was filtered using a filter with a pore size of 100 nm, and then the precipitate collected on the 100 nm filter was decomposed with acid, and the decomposition solution was analyzed using an ICP emission spectrometer, and the absolute value of Fe in the decomposition solution was measured. The absolute value of Fe obtained was divided by the amount of electrolyte to obtain the amount of Fe contained in precipitates having a particle size of 100 nm or more (mass % when the total composition of the test piece was taken as 100 mass %). Next, the obtained amount of Fe (mass %) was divided by the amount of Fe contained in the test piece (mass %) to obtain the amount of Fe (mass %) present as precipitates containing Fe with a particle size of 100 nm or more. The amount of electrolyte was determined by measuring the mass of the test piece after the precipitates were peeled off and subtracting it from the mass of the test piece before electrolysis.

Tensile Test JIS No. 5 tensile test pieces (JIS Z 2241:2011) were taken from the obtained hot-rolled steel sheets in the direction parallel to the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241:2011 at a strain rate of 10 ⁻³ /s to determine TS. In the present invention, a TS of 780 MPa or more and less than 1320 MPa was considered to be acceptable.

Vickers hardness test Samples were cut out from the obtained hot-rolled steel sheet and the hot-rolled steel sheet after post-heating, and the cross section of the sheet thickness parallel to the rolling direction was polished. Then, a Vickers hardness test was performed at a 1/4 position of the sheet thickness with a load of 5 kg and five measurement points, and the average (arithmetic mean) was taken as the Vickers hardness of the steel sheet. A difference in hardness (ΔHV) of 40 or less before and after post-heating was judged to be excellent in strength after post-heating and was rated as passing.

Punching test: Test pieces with a width of 50 mm and a length of 50 mm were taken from the obtained hot-rolled steel sheets, and punching was performed three times for each clearance range of 5 to 30% using a punching punch with a diameter of 10 mm, and the clearance range without chipping or cracking on the end surface was determined for all three times. A clearance range of 10% or more was considered to be acceptable.

Charpy impact test Test pieces with a width of 10 mm and a length of 55 mm were taken from the obtained hot-rolled steel sheet and the post-heated hot-rolled steel sheet, respectively, and Charpy impact test pieces with a V-notch having a tip angle of 45°, a tip radius of 0.25 mm, and a depth of 2 mm were prepared. Then, in accordance with JIS Z 2242:2018, the Charpy impact test was performed five times at -40 ° C. to evaluate the ductile fracture rate. The average value of the five ductile fracture rates of 50% or more was judged to be excellent in toughness and was passed. The plate thickness was 2.9 mm, and the notch direction was parallel to the rolling direction.

All of the inventive examples have a TS of 780 MPa or more and less than 1320 MPa, excellent punchability and toughness, and further excellent strength and toughness after post-heating. On the other hand, the comparative examples outside the scope of the present invention do not have one or more of the desired strength, punchability, and toughness, or do not achieve one or more of the desired strength and toughness after post-heating.

According to the present invention, it is possible to obtain a high-strength hot-rolled steel sheet having a TS of 780 MPa or more and less than 1,320 MPa, excellent punchability and toughness, and excellent strength and toughness after post-heating. When the high-strength steel sheet of the present invention is used for automobile parts, it can greatly contribute to improving the collision safety and fuel efficiency of automobiles.

Claims

In mass percent,
C: 0.04 to 0.18%,
Si: 0.1 to 3.0%,
Mn: 0.5 to 3.5%,
P: more than 0% and not more than 0.050%;
S: more than 0% and 0.010% or less;
Al: more than 0% and not more than 1.5%;
N: more than 0% and not more than 0.010%;
O: more than 0% and 0.003% or less; and Ti: 0.040 to 0.150%;
The balance is Fe and unavoidable impurities,
The steel structure has bainite as the main phase and retained austenite at a volume fraction of less than 3%;
the ratio of the amount of dissolved Ti to the Ti content (amount of dissolved Ti/total amount of Ti) is 0.30 or more and less than 0.80;
A high-strength hot-rolled steel sheet, in which the amount of Ti present as precipitates having a grain size of 100 nm or more is 0.010 to 0.030 mass %.
The composition further comprises, in mass%,
Cr: 0.005 to 2.0%,
Cu: 0.005 to 0.5%,
Ni: 0.005 to 2.0%,
Mo: 0.005 to 1.0%,
V: 0.005 to 0.5%,
B: 0.0002 to 0.0050%,
Ca: 0.0001 to 0.0050%,
REM: 0.0001 to 0.0050%,
Sb: 0.0010 to 0.10%, and Sn: 0.0010 to 0.50%
The high strength hot rolled steel sheet according to claim 1, comprising one or more selected from the following:
The high-strength hot-rolled steel sheet according to claim 1 or 2, in which the amount of Fe present as precipitates with a grain size of 100 nm or more is greater than 0 mass% and less than or equal to 0.100 mass%.
A method for producing a high strength hot rolled steel sheet according to claim 1 or 2,
A slab having the above-mentioned composition is heated to a temperature range of 1150 to 1300°C and held at that temperature range for 0.2 to 3.5 hours;
Next, when hot rolling is performed,
After rough rolling with a total reduction of 80 to 90% in a temperature range of 1080 ° C. or more, finish rolling is performed, and in the finish rolling, rolling is performed under conditions where the reduction per pass is 25% or less at T (° C.) or less calculated by the following formula, and after the completion of the finish rolling, the steel is allowed to cool for 1.0 s or more.
Next, the steel sheet is cooled at an average cooling rate of 50°C/s or more in a temperature range up to 550°C, and then coiled at a coiling temperature of the Ms point (°C) or higher and 550°C or lower.
T(℃)=800+1000[Ti]
Here, [Ti] is the Ti content (mass %).
A method for producing a high strength hot rolled steel sheet according to claim 3,
A slab having the above-mentioned composition is heated to a temperature range of 1150 to 1300°C and held at that temperature range for 0.2 to 3.5 hours;
Next, when hot rolling is performed,
After rough rolling with a total reduction of 80 to 90% in a temperature range of 1080 ° C. or more, finish rolling is performed, and in the finish rolling, rolling is performed under conditions where the reduction per pass is 25% or less at T (° C.) or less calculated by the following formula, and after the completion of the finish rolling, the steel is allowed to cool for 1.0 s or more.
Next, the steel sheet is cooled at an average cooling rate of 50°C/s or more in a temperature range up to 550°C, and then coiled at a coiling temperature of the Ms point (°C) or higher and 550°C or lower.
T(℃)=800+1000[Ti]
Here, [Ti] is the Ti content (mass %).
The method for manufacturing high-strength hot-rolled steel sheet according to claim 4, in which cooling is stopped at a cooling stop temperature in the temperature range of 480 to 550°C between the time of cooling at an average cooling rate of 50°C/s or more through the temperature range up to 550°C and the time of coiling, and the sheet is held at the cooling stop temperature ±20°C for 0.5 to 4.0 s, and then coiled at the coiling temperature.
The method for producing a high strength hot rolled steel sheet according to claim 5, wherein cooling is stopped at a cooling stop temperature in a temperature range of 480 to 550°C during the period from cooling at an average cooling rate of 50°C/s or more in a temperature range up to 550°C to coiling, and the steel sheet is held at the cooling stop temperature ±20°C for 0.5 to 4.0 s, and then coiled at the coiling temperature.