CN114008231B

CN114008231B - High-strength hot-rolled steel sheet and method for producing same

Info

Publication number: CN114008231B
Application number: CN202080043029.1A
Authority: CN
Inventors: 山崎和彦; 廷胡延·多恩
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2019-06-14
Filing date: 2020-06-01
Publication date: 2022-06-07
Anticipated expiration: 2040-06-01
Also published as: JPWO2020250735A1; KR102635009B1; WO2020250735A1; JP6819840B1; CN114008231A; KR20220005094A

Abstract

The invention provides a high-strength hot-rolled steel sheet having excellent ductility, fatigue characteristics, and resistance to rough punching. The tensile strength is 1180MPa or more and Ra is 2.00 μm or less. The component composition contains C: 0.09-0.20%, Si: 0.2-2.0%, Mn: 1.0-3.0%, P: 0.100% or less, S: 0.0100% or less, Al: 0.01-2.00%, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, B: 0.0005 to 0.0200%, further comprising Cr: 0.10 to 1.50%, etc. The area ratio of the upper bainite phase as the main phase is 50% or moreAbove 90% and below, and the average particle diameter is 12.0 μm or below. The second phase is equal to retained austenite, has an area ratio of 10% or more and less than 50%, and has a circumference of 300000 μm/mm, the equivalent circle diameter of 0.5 μm or more²The above.

Description

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

Technical Field

The present invention relates to a high-strength hot-rolled steel sheet and a method for manufacturing the same.

Background

In recent years, from the viewpoint of global environmental conservation, exhaust gas regulations of automobiles are being strengthened, and improvement of fuel efficiency of automobiles is an important issue. Materials for automobiles are required to have further higher strength and thinner walls.

Therefore, high-strength hot-rolled steel sheets are actively used as materials for automobile members. High-strength hot-rolled steel sheets are used not only for structural members and frame members of automobiles but also for running members, frame members, and the like.

In particular, a high-strength hot-rolled steel sheet having a Tensile Strength (TS) of 1180MPa or more is expected as a material that can dramatically improve the fuel efficiency of automobiles.

However, as the strength of steel sheets increases, material properties such as ductility, fatigue properties, and resistance to punching asperity generally deteriorate. Particularly, steel sheets used as traveling members of automobiles are required to have a combination of excellent properties of these materials. That is, it is required to ensure these material properties and high strength in a well-balanced manner at a high level.

Various studies have been made to increase the strength of steel sheets without deteriorating the properties of these materials (see patent documents 1 to 4).

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open No. 2014-227583

Patent document 2: japanese patent laid-open publication No. 2016-211073

Patent document 3: japanese laid-open patent publication No. 2009-84637

Patent document 4: international publication No. 2014/188966

Disclosure of Invention

Problems to be solved by the invention

However, patent documents 1 to 4 do not disclose a high-strength hot-rolled steel sheet having a tensile strength of 1180MPa or more and excellent in ductility, fatigue characteristics, and punching-coarsening resistance.

Accordingly, an object of the present invention is to provide a high-strength hot-rolled steel sheet having a tensile strength of 1180MPa or more and excellent in ductility, fatigue characteristics, and resistance to punch roughening, and a method for producing the same.

Means for solving the problems

The present inventors have conducted extensive studies and, as a result, have found that the above object can be achieved by adopting the following constitution, thereby completing the present invention.

Namely, the present invention provides the following [1] to [7 ].

[1]A high-strength hot-rolled steel sheet having a tensile strength of 1180MPa or more and an arithmetic average surface roughness Ra of 2.00 [ mu ] m or less, comprising: contains, in mass%, C: 0.09% or more and 0.20% or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01% or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005% or more and 0.0200% or less, and further contains a metal selected from the group consisting of Cr: 0.10% or more and 1.50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% or less and V: 0.05% to 0.50% inclusive, and the balance of Fe and unavoidable impurities; and a microstructure including an upper bainite phase and a second phase, wherein the area ratio of the upper bainite phase is 50% or more and less than 90%, the average grain size of the upper bainite phase is 12.0 [ mu ] m or less, and the second phase is selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phaseAt least one of the group consisting of 10% or more and less than 50% in area ratio of the second phase, and 300000 μm/mm in circumferential length of the second phase having an equivalent circle diameter of 0.5 μm or more²The above.

[2] The high-strength hot-rolled steel sheet according to the above [1], wherein the above-mentioned composition further contains, in mass%, a component selected from the group consisting of Cu: 0.01% or more and 0.50% or less and Ni: 0.01% to 0.50% of at least one of the group.

[3] The high-strength hot-rolled steel sheet according to the above [1] or [2], wherein the above-mentioned composition further contains, in mass%: 0.0002% or more and 0.0300% or less.

[4] The high-strength hot-rolled steel sheet according to any one of the above [1] to [3], wherein the above-mentioned composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0002% or more and 0.0100% or less, Mg: 0.0002% or more and 0.0100% or less and REM: at least one selected from the group consisting of 0.0002% to 0.0100%.

[5] The high-strength hot-rolled steel sheet according to any one of the above [1] to [4], which has a plated layer on a surface thereof.

[6] A method for producing a high-strength hot-rolled steel sheet according to any one of the above items [1] to [4], wherein a steel material having a composition of any one of the above items [1] to [4] is heated to 1150 ℃ or higher, the heated steel material is rough-rolled to obtain a rough-rolled sheet, the rough-rolled sheet is subjected to high-pressure water descaling at a collision pressure of 2.5MPa or higher, the rough-rolled sheet subjected to high-pressure water descaling is finish-rolled at a finish-rolling temperature of (RC-100) DEG C or higher and (RC +100) DEG C or lower to obtain a finish-rolled sheet, RC is defined by the following formula (1), and the finish-rolled sheet is cooled at an average cooling rate of 20 ℃/s or higher to a cooling stop temperature of (Bs-150) DEG C or higher and Bs ℃ or lower, wherein Bs is defined by the following formula (2), and when the finish rolling finishing temperature is RC ℃ or higher, the time from the finish rolling finishing to the cooling start is 2.0s or less, the finish rolled sheet after the cooling is coiled at the cooling stop temperature, and the finish rolled sheet after the coiling is cooled to (Bs-300) ° c at an average cooling rate of 0.10 ℃/min or more.

(1)RC＝850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V

(2)Bs＝830-270×C-90×Mn-70×Cr-37×Ni-83×Mo

Here, each element symbol in the above formula represents the content of each element in the above composition in mass%. In the case of an element not contained in the above-mentioned composition, the symbol of the element in the above-mentioned formula is calculated as 0.

[7] The method for producing a high-strength hot-rolled steel sheet according to item [6], wherein the finish-rolled sheet cooled after the coiling is subjected to a plating treatment.

Effects of the invention

According to the present invention, a high-strength hot-rolled steel sheet having a tensile strength of 1180MPa or more and excellent in ductility, fatigue characteristics, and resistance to punch roughening, and a method for producing the same can be provided.

By using the high-strength hot-rolled steel sheet of the present invention for structural members, frame members, running members such as suspensions, frame members, and the like of automobiles, the weight of automobile bodies can be reduced while ensuring the safety of automobiles. Therefore, it can contribute to reduction of environmental load.

Drawings

Fig. 1 is a schematic view showing a test piece used in a plane bending fatigue test.

Detailed Description

[ high-Strength Hot-rolled Steel sheet ]

The high-strength hot-rolled steel sheet of the present invention is a high-strength hot-rolled steel sheet as follows: the tensile strength is 1180MPa or more, the arithmetic average roughness Ra of the surface is 2.00 mu m or less, and the alloy has the following components: contains, in mass%, C: 0.09% or more and 0.20% or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01% or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005 percentAbove and 0.0200% or less, further containing a metal selected from the group consisting of Cr: 0.10% or more and 1.50% or less, Mo: 0.05% to 0.45%, Nb: 0.005% or more and 0.060% or less and V: 0.05% to 0.50% inclusive, and the balance of Fe and unavoidable impurities; and a microstructure including an upper bainite phase and a second phase, wherein an area ratio of the upper bainite phase is 50% or more and less than 90%, an average grain diameter of the upper bainite phase is 12.0 [ mu ] m or less, the second phase is at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase, an area ratio of the second phase is 10% or more and less than 50%, and a circumferential length of the second phase having an equivalent circle diameter of 0.5 [ mu ] m or more is 300000 [ mu ] m/mm²The above.

The high-strength hot-rolled steel sheet of the present invention is excellent in ductility, fatigue characteristics and resistance to punch roughening.

High strength means a Tensile Strength (TS) of 1180MPa or more.

The term "excellent ductility" (excellent ductility) means that the value (TS. times. U-El) obtained by multiplying the Tensile Strength (TS) by the uniform elongation (U-El) is 6000MPa ·% or more, as described later.

The term "excellent fatigue properties" means that the value obtained by dividing the fatigue strength at 50 ten thousand cycles obtained by the plane bending fatigue test by the Tensile Strength (TS) is 0.50 or more, as described later.

The excellent punching harshness resistance (excellent punching harshness resistance) means that the maximum height roughness Rz of the punched end face after punching with a punch of 10mm phi at a gap of 12 + -1% is 35 μm or less on average and the standard deviation of Rz is 10 μm or less, as described later.

Generally, in order to obtain a tensile strength of 1180MPa or more, a lower bainite phase and/or a tempered martensite phase having high hardness are used as a main phase in a microstructure of a steel sheet. However, they have low ductility.

Therefore, in the present invention, the main phase is upper bainite having high ductility, and the second phase is at least one selected from the group consisting of lower bainite having high hardness and/or tempered martensite, fresh martensite, and retained austenite. This can provide excellent ductility while maintaining high strength (tensile strength of 1180MPa or more).

The main phase means 50% or more in terms of area ratio.

In general, the fatigue life of a steel sheet is determined by the time required for the generation and growth of fatigue cracks. By delaying these times, the fatigue characteristics are excellent.

In the present invention, the time required for the generation of fatigue cracks is delayed by controlling the arithmetic average roughness Ra of the steel sheet surface. The time required for the growth of fatigue cracks is delayed by controlling the perimeter of the second phase having an equivalent circle diameter of 0.5 μm or more. Thereby, excellent fatigue characteristics are obtained.

Further, in the automobile chassis and frame members which are subjected to a large number of punching processes, it is required in terms of appearance quality that the roughness of the end face after punching is not increased (excellent punching coarseness resistance). Thus, the average particle diameter and the composition of the components of the main phase are controlled. Thereby, excellent punching harshness resistance is obtained.

The high-strength hot-rolled steel sheet of the present invention is a so-called hot-rolled steel sheet, and has a composition and a microstructure described below. Hereinafter, the "high-strength hot-rolled steel sheet" or the "hot-rolled steel sheet" is also simply referred to as "steel sheet".

The thickness of the steel sheet is not particularly limited, and is, for example, 6.0mm or less. The lower limit is also not particularly limited, and is, for example, 1.0mm or more.

< composition of ingredients >

First, the reasons for limiting the composition of the steel sheet will be described. Hereinafter, "%" in the component composition means "% by mass" unless otherwise specified.

C: 0.09% or more and 0.20% or less

C promotes the formation of bainite by increasing the strength of the steel and improving hardenability, and increases the percentage of the second phase. When the upper bainite is transformed, C is distributed into the non-transformed austenite, and the non-transformed austenite is stabilized. Thereby, in the cooling after coiling, the non-transformed austenite becomes a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase). Therefore, the C content is 0.09% or more, preferably 0.10% or more, and more preferably 0.11% or more.

On the other hand, if the C content is too large, the second phase increases, and ductility becomes insufficient. Therefore, the C content is 0.20% or less, preferably 0.18% or less, and more preferably 0.16% or less.

Si: more than 0.2% and less than 2.0% >

Si contributes to solid solution strengthening and to improvement of the strength of steel. Si has an effect of suppressing the formation of Fe-based carbides, and suppresses the precipitation of cementite during upper bainite transformation. Thereby, C is distributed to the non-transformed austenite, and the non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) in cooling after coiling. In order to obtain these effects, the Si content is 0.2% or more, preferably 0.4% or more, and more preferably 0.5% or more.

On the other hand, Si forms a secondary scale on the surface of the steel sheet in hot rolling. When the Si content is too large, the secondary oxide scale becomes too thick, the arithmetic average roughness Ra of the surface of the descaled steel sheet becomes too large, and the fatigue characteristics become insufficient. Therefore, the Si content is 2.0% or less, preferably 1.8% or less, and more preferably 1.6% or less.

Mn: 1.0% or more and 3.0% or less

Mn is solid-dissolved to contribute to an increase in the strength of steel, and promotes the formation of a bainite phase and a martensite phase by improving hardenability. In order to obtain such an effect, the Mn content is 1.0% or more, preferably 1.3% or more, and more preferably 1.5% or more.

On the other hand, if the Mn content is too large, the second phase increases, and the ductility becomes insufficient. Therefore, the Mn content is 3.0% or less, preferably 2.6% or less, and more preferably 2.4% or less.

P: less than 0.100% (including 0%), (iv)

P is solid-dissolved to contribute to the increase in strength of the steel. However, P segregates to austenite grain boundaries during hot rolling, thereby causing cracks during hot rolling. Even if the generation of cracks can be avoided, the cracks segregate in the grain boundaries, and the low-temperature toughness is reduced, thereby reducing the workability. Therefore, the P content is preferably as low as possible, and it is acceptable to contain P up to 0.100%. Therefore, the P content is 0.100% or less, preferably 0.050% or less, and more preferably 0.020% or less.

S: less than 0.0100% (including 0%), "Zhi

S combines with Ti and Mn to form coarse sulfides, which reduces the punch roughening resistance. Therefore, the S content is preferably as low as possible, and it is acceptable to contain S up to 0.0100%. Therefore, the S content is 0.0100% or less, preferably 0.0050% or less, and more preferably 0.0030% or less.

Al: 0.01% or more and 2.00% or less

Al acts as a deoxidizer and is effective for improving the cleanliness of steel. When Al is too small, the effect is not necessarily sufficient. In addition, Al has an effect of suppressing the formation of Fe-based carbides, similarly to Si, and suppresses the precipitation of cementite at the time of upper bainite transformation. Thereby, C is distributed to the non-transformed austenite, and the non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) in cooling after coiling. Therefore, the Al content is 0.01% or more, preferably 0.015% or more, and more preferably 0.020% or more.

On the other hand, excessive addition of Al causes an increase in oxide-based inclusions, lowers the punch-roughening resistance, and causes occurrence of defects. Therefore, the Al content is 2.00% or less, preferably 1.80% or less, and more preferably 1.60% or less.

N: less than 0.010% (including 0%), (iv)

N is precipitated as a nitride by bonding with the element forming the nitride, and contributes to the refinement of crystal grains. However, N is easily bonded to Ti at high temperature to form coarse nitrides, and an excessive content thereof lowers the punch roughening resistance. Therefore, the N content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

Ti: more than 0.001% and less than 0.030%

Ti improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Ti forms nitrides in the high temperature range of the austenite phase (the range of high temperatures in the austenite phase and the range of higher temperatures than the austenite phase (casting stage)). This suppresses precipitation of BN, and B becomes a solid solution state. Thus, hardenability necessary for forming the upper bainite phase is obtained, which contributes to improvement of strength. Ti can be rolled in the austenite non-recrystallization region by increasing the recrystallization temperature of the austenite phase during hot rolling. This contributes to the refinement of the grain size of the upper bainite phase and the increase in the circumferential length of the second phase, and improves the resistance to punch-roughening and fatigue characteristics. In order to exhibit these effects, the Ti content is 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

On the other hand, if the Ti content is too high, coarse nitrides are formed, and the resistance to punch roughening becomes insufficient. Therefore, the Ti content is less than 0.030%, preferably 0.028% or less, and more preferably 0.025% or less.

B: 0.0005% or more and 0.0200% or less

B promotes the formation of an upper bainite phase by segregating at the prior austenite grain boundary and suppressing the formation of ferrite, thereby contributing to the improvement of the strength of the steel sheet. In order to exhibit these effects, the B content is 0.0005% or more, preferably 0.0006% or more, and more preferably 0.0007% or more.

On the other hand, if the B content is too large, the above effect is saturated. Therefore, the B content is 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less.

The steel sheet further contains at least one selected from the group consisting of Cr, Mo, Nb, and V in the following content.

[ Cr: more than 0.10% and less than 1.50% >

Cr improves the strength of the steel sheet by solid solution strengthening. Cr is an element forming carbide, and segregates at the interface between the upper bainite phase and the non-transformed austenite during transformation of the upper bainite after coiling, thereby reducing the transformation driving force of bainite and stopping transformation of the upper bainite in a state where the non-transformed austenite remains. The non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) by being cooled thereafter. In order to exhibit these effects, when Cr is contained, the Cr content is 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more.

On the other hand, like Si, Cr forms a secondary scale on the surface of the steel sheet in hot rolling. Therefore, when the Cr content is too large, the secondary oxide scale becomes too thick, the arithmetic average roughness Ra after descaling becomes too large, and the fatigue characteristics become insufficient. Therefore, when Cr is contained, the Cr content is 1.50% or less, preferably 1.40% or less, more preferably 1.30% or less, still more preferably 1.20% or less, and particularly preferably 1.00% or less.

Mo: more than 0.05% and less than 0.45% >

Mo promotes the formation of a bainite phase by improving hardenability, and contributes to the improvement of the strength of the steel sheet. In addition, Mo is an element forming carbide, like Cr, and segregates at the interface between the upper bainite phase and the non-transformed austenite during transformation of the upper bainite after coiling, thereby reducing the transformation driving force of bainite and stopping transformation of the upper bainite in a state where the non-transformed austenite remains. The non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) by being cooled thereafter. In order to obtain such effects, when Mo is contained, the Mo content is 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more.

On the other hand, if the Mo content is too large, the second phase increases, and the ductility becomes insufficient. Therefore, when Mo is contained, the Mo content is 0.45% or less, preferably 0.40% or less, and more preferably 0.30% or less.

Nb: more than 0.005% and less than 0.060%

Nb improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. Similarly to Ti, Nb can be rolled in an austenite unrecrystallized region by increasing the recrystallization temperature of the austenite phase during hot rolling. This contributes to the refinement of the grain size of the upper bainite phase and the increase in the circumferential length of the second phase, and improves the resistance to punch-roughening and fatigue characteristics. In addition, Nb is an element forming carbide, like Cr, and segregates at the interface between the upper bainite phase and the non-transformed austenite during transformation of the upper bainite after coiling, thereby reducing the transformation driving force of bainite and stopping transformation of the upper bainite in a state where the non-transformed austenite remains. The non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) by being cooled thereafter. In order to exhibit these effects, when Nb is contained, the Nb content is 0.005% or more, preferably 0.010% or more, and more preferably 0.015% or more.

On the other hand, if the Nb content is too large, the second phase increases, and the ductility becomes insufficient. Therefore, when Nb is contained, the Nb content is 0.060% or less, preferably 0.050% or less, and more preferably 0.040% or less.

V: more than 0.05% and less than 0.50% >

V improves the strength of the steel sheet by precipitation strengthening or solid solution strengthening. In addition, V can be rolled in the austenite non-recrystallization region by raising the recrystallization temperature of the austenite phase at the time of hot rolling, similarly to Ti. This contributes to the refinement of the grain size of the upper bainite phase and the increase in the circumferential length of the second phase, and improves the resistance to punch-roughening and fatigue characteristics. In addition, V is an element forming carbide, similarly to Cr, and segregates at the interface between the upper bainite phase and the non-transformed austenite during transformation of the upper bainite after coiling, thereby reducing the transformation driving force of bainite and stopping transformation of the upper bainite in a state where the non-transformed austenite remains. The non-transformed austenite is changed into a second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase) by being cooled thereafter. In order to exhibit these effects, when V is contained, the V content is 0.05% or more, preferably 0.10% or more, and more preferably 0.15% or more.

On the other hand, if the V content is too large, the second phase increases, and ductility becomes insufficient. Therefore, when V is contained, the V content is 0.50% or less, preferably 0.40% or less, and more preferably 0.30% or less.

By adding the above elements to the composition of the steel sheet, desired properties can be obtained.

The composition of the steel sheet may contain other elements described below as necessary for the purpose of further enhancing the strength of the steel sheet, or further improving the properties such as ductility, fatigue properties, and resistance to punching asperity, for example.

Other elements

For example, the steel sheet may further contain at least one selected from the group consisting of Cu and Ni in the following content.

(Cu: 0.01% or more and 0.50% or less)

Cu is solid-dissolved to contribute to the increase in strength of the steel. In addition, Cu promotes the formation of a bainite phase by increasing hardenability, and contributes to the improvement of strength. In order to obtain these effects, when Cu is contained, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more.

On the other hand, if the Cu content is too large, the surface properties of the steel sheet may be reduced, and the fatigue characteristics may be insufficient. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less, and more preferably 0.30% or less.

(Ni: 0.01% or more and 0.50% or less)

Ni is solid-dissolved to contribute to an increase in the strength of steel. In addition, Ni promotes the formation of a bainite phase by increasing hardenability, and contributes to the improvement of strength. In order to obtain these effects, when Ni is contained, the Ni content is preferably 0.01% or more, and more preferably 0.05% or more.

On the other hand, if the Ni content is too large, the second phase increases, and the ductility may become insufficient. Therefore, when Ni is contained, the Ni content is preferably 0.50% or less, and more preferably 0.30% or less.

For example, the steel sheet may contain Sb in the following content.

(Sb: 0.0002% or more and 0.0300% or less)

Sb suppresses nitriding of the surface of a steel material such as a billet at the stage of heating the steel material, and suppresses precipitation of BN in the surface layer portion of the steel material. Further, the presence of solid solution B makes it possible to obtain hardenability necessary for bainite to be formed in the surface layer portion of the steel sheet, thereby improving the strength of the steel sheet. In order to exhibit such an effect, when Sb is contained, the Sb content is preferably 0.0002% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.

On the other hand, if the Sb content is too large, the rolling load may be increased, and the productivity may be lowered. Therefore, when Sb is contained, the Sb content is preferably 0.0300% or less, more preferably 0.0250% or less, and further preferably 0.0200% or less.

For example, the steel sheet may further contain at least one selected from the group consisting of Ca, Mg, and REM in the following contents.

REM (Rare earth Metal) is a generic name of 2 elements of Sc (scandium) and Y (yttrium) and 15 elements (lanthanoid) from La (lanthanum) to Lu (lutetium) in total of 17 elements.

(Ca: 0.0002% or more and 0.0100% or less)

Ca controls the shape of oxide and sulfide inclusions to improve the resistance to punch roughening. In order to exhibit these effects, when Ca is contained, the Ca content is preferably 0.0002% or more, and more preferably 0.0004% or more.

On the other hand, if the Ca content is too high, surface defects of the steel sheet may be caused, and fatigue characteristics may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0100% or less, and more preferably 0.0050% or less.

(Mg: 0.0002% or more and 0.0100% or less)

Mg controls the shape of oxide and sulfide inclusions in the same manner as Ca, and improves the punch roughening resistance. In order to exhibit these effects, when Mg is contained, the Mg content is preferably 0.0002% or more, and more preferably 0.0004% or more.

On the other hand, if the Mg content is too large, the cleanliness of the steel may be deteriorated, and the punch-roughening resistance may be insufficient. Therefore, when Mg is contained, the Mg content is preferably 0.0100% or less, and more preferably 0.0050% or less.

(REM: 0.0002% or more and 0.0100% or less)

REM controls the shape of oxide and sulfide inclusions in the same manner as Ca, and improves the resistance to punch roughening. In order to exhibit these effects, when REM is contained, the REM content is preferably 0.0002% or more, and more preferably 0.0004% or more.

On the other hand, if the REM content is too high, the cleanliness of the steel may be deteriorated, and the resistance to punch-roughening may be insufficient. Therefore, when REM is contained, the REM content is preferably 0.0100% or less, and more preferably 0.0050% or less.

(residual)

In the composition of the steel sheet, the balance other than the above components (elements) is made up of Fe and unavoidable impurities. Examples of the inevitable impurities include Zr, Co, Sn, Zn, Pb, and the like, and the total content thereof may be 0.5% or less.

< microstructure >

Next, the reason for limiting the microstructure of the steel sheet will be described.

Main photo: the area ratio of the upper bainite phase is 50% or more and less than 90%, and the average grain diameter of the upper bainite phase is 12.0 μm or less

The upper bainite phase is used as a main phase. Thereby being excellent in ductility. By setting the area ratio of the upper bainite phase to 50% or more and setting the average grain size of the upper bainite phase to 12.0 μm or less, both excellent ductility and excellent resistance to punch coarsening can be achieved.

For the reason that the above-described effects are more excellent, the area ratio of the upper bainite phase is preferably 60% or more, more preferably 70% or more, and still more preferably 80% or more.

For the same reason, the average grain size of the upper bainite phase is preferably 11.0 μm or less, more preferably 10.0 μm or less, and still more preferably 9.0 μm or less. The lower limit is not particularly limited, and is, for example, preferably 1.0 μm or more, and more preferably 2.0 μm or more.

On the other hand, when the area ratio of the upper bainite phase is too large, the tensile strength is less than 1180 MPa. Therefore, the area ratio of the upper bainite phase is less than 90%.

Second phase: at least one (second phase) selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase has an area ratio of 10% or more and less than 50%, and a second phase having an equivalent circle diameter of 0.5 μm or more has a circumference of 300000 μm/mm²The above

At least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase is used as the second phase.

In order to obtain a tensile strength of 1180MPa or more, the area ratio of the second phase is 10% or more, preferably 11% or more, more preferably 13% or more, and still more preferably 15% or more.

On the other hand, when the area ratio of the second phase is too large, ductility becomes insufficient. Therefore, the area ratio of the second phase is less than 50%, preferably 40% or less, more preferably 35% or less, and still more preferably 30% or less.

The second phase having an equivalent circle diameter of 0.5 μm or more has a perimeter of 300000 μm/mm²The above. Thus, the growth of fatigue cracks is inhibited by the second phase in the plane bending fatigue test, and the fatigue properties are excellent. The fatigue characteristics are improved as the circumference length of the second phase having an equivalent circle diameter of 0.5 μm or more is longer, and therefore, 350000 μm/mm is preferable²Above, more preferably 400000 μm/mm²Above, 450000 μm/mm is more preferable²The above. The upper limit of the circumference is not particularly limited, but is preferably 900000 μm/mm²Hereinafter, more preferably 800000 μm/mm²The following.

The balance other than the main phase and the second phase is, for example, at least one selected from the group consisting of a pearlite phase and a polygonal ferrite phase. These margins may not be provided. From the viewpoint of obtaining the effect of the present invention, the area ratio of the remainder is preferably 0% or more and less than 3% in total.

The upper bainite phase is an aggregate of lathy ferrites having an orientation difference of less than 15 °, and means a structure having Fe-based carbides and/or a residual austenite phase between the lathy ferrites (however, the upper bainite phase also includes a case where no Fe-based carbides and/or residual austenite phase are present between the lathy ferrites).

Lathy ferrite is different from lamellar (lamellar) ferrite and polygonal ferrite in the pearlite phase, has a lathy shape, and has a high dislocation density in the inside. Therefore, lath-shaped ferrite, lamellar (lamellar) ferrite in the pearlite phase, and polygonal ferrite can be distinguished from each other by using SEM (scanning electron microscope) and TEM (transmission electron microscope).

The dislocation density of lamellar ferrite is lower than that of lath-shaped ferrite. Therefore, the pearlite phase and the upper bainite phase can be easily distinguished from each other using SEM, TEM, or the like.

In the case where a retained austenite phase is present between the laths, only the lath-like ferrite portion is regarded as an upper bainite phase and is distinguished from the retained austenite phase.

The lower bainite phase and/or tempered martensite phase is an aggregate of lathy ferrites having an orientation difference of less than 15 °, and means a structure having Fe-based carbides in the lathy ferrites (however, the structure also includes a case where Fe-based carbides are present between lathy ferrites).

The lower bainite and tempered martensite can be distinguished from each other by observing the orientation and crystal structure of Fe-based carbides in the lath-shaped ferrite using a TEM (transmission electron microscope). However, in the present invention, since they have substantially the same characteristics, they are not distinguished from each other.

The lower bainite phase and/or tempered martensite phase have Fe-based carbides in the lath ferrite, and thus can be distinguished from the upper bainite phase using SEM or TEM.

The dislocation density of lamellar ferrite is lower compared to lath ferrite. Therefore, the pearlite phase and the lower bainite phase and/or the tempered martensite phase can be easily distinguished from each other using SEM, TEM, or the like.

The fresh martensite phase and the retained austenite phase have no Fe-based carbides, as compared to the lower bainite phase and/or the tempered martensite phase. In addition, the contrast of SEM images of fresh martensite phase and retained austenite phase is bright compared to upper bainite phase, lower bainite phase and/or tempered martensite phase and polygonal ferrite. Thus, the fresh martensite phase and the retained austenite phase can be distinguished from these tissues using SEM.

The fresh martensite phase and the retained austenite phase, although having the same shape and contrast under SEM, can be distinguished from each other by using Electron back scattering Diffraction Patterns (EBSD) method.

The area ratios of the upper bainite phase, the lower bainite phase, and/or the tempered martensite phase (second phase), the retained austenite phase (second phase), the pearlite phase, and the polygonal ferrite phase can be measured by the methods described in examples described later.

The average grain size of the upper bainite phase can be measured by the method described in the examples described later.

The circumferential length of the second phase having an equivalent circle diameter of 0.5 μm or more can be measured by the method described in the examples described later.

< tensile Strength: 1180MPa or more

The high-strength hot-rolled steel sheet according to the present invention has a Tensile Strength (TS) of 1180MPa or more.

The upper limit is not particularly limited, and the tensile strength is preferably 1470MPa or less.

The Tensile Strength (TS) can be measured by the method described in the examples described later.

< arithmetic average roughness Ra: 2.00 μm or less

When the arithmetic average roughness Ra of the surface of the high-strength hot-rolled steel sheet of the present invention is too large, local stress concentration may occur at the bending apex portion in the plane bending fatigue test, and fatigue cracking may occur early.

Therefore, the arithmetic average roughness Ra of the surface of the high-strength hot-rolled steel sheet of the present invention is 2.00 μm or less in order to obtain excellent fatigue characteristics, and is preferably 1.90 μm or less, more preferably 1.80 μm or less, and further preferably 1.60 μm or less for the reason of further excellent fatigue characteristics. The lower limit is not particularly limited, but is, for example, preferably 0.30 μm or more, and more preferably 0.45 μm or more.

The arithmetic mean roughness Ra is the arithmetic mean roughness Ra of the surface of the plating layer when the plating layer described later is formed, and is the arithmetic mean roughness Ra of the surface of the steel sheet itself when the plating layer described later is not formed.

The arithmetic average roughness Ra can be measured by the method described in the examples described later.

< coating >

The high-strength hot-rolled steel sheet of the present invention may have a plating layer on the surface thereof for the purpose of improving corrosion resistance and the like.

Examples of the plating layer include: hot-dip coating, electroplated coating, etc.

Examples of the hot-dip coating include a zinc coating layer, and specific examples thereof include a hot-dip coating layer, an alloyed hot-dip coating layer, and the like.

Examples of the plating layer include a zinc plating layer.

The thickness of the plating layer (plating layer deposition amount) is not particularly limited, and conventionally known values can be used.

[ method for producing high-strength Hot-rolled Steel sheet ]

Next, a method for manufacturing a high-strength hot-rolled steel sheet according to the present invention will be described.

The method for producing a high-strength hot-rolled steel sheet according to the present invention (hereinafter, also simply referred to as "the production method of the present invention") is a method for producing the high-strength hot-rolled steel sheet according to the present invention, wherein a steel material having the above-described composition is heated to 1150 ℃ or higher, the heated steel material is subjected to rough rolling to obtain a rough-rolled sheet, the rough-rolled sheet is subjected to high-pressure descaling at a collision pressure of 2.5MPa or higher, the rough-rolled sheet subjected to the high-pressure descaling is subjected to finish rolling at a finish rolling temperature of (RC-100) ° c or higher and (RC +100) ° c or lower to obtain a finish-rolled sheet, RC is defined by the following formula (1), and the finish-rolled sheet is cooled at an average cooling rate of 20 ℃/s or higher to a cooling stop temperature of (Bs-150 ℃) or lower, bs is defined by the following formula (2), and when the finish rolling finishing temperature is RC ℃ or higher, the time from the finish of the finish rolling to the start of the cooling is 2.0s or less, the finish rolled sheet after the cooling is coiled at the cooling stop temperature, and the finish rolled sheet after the coiling is cooled to (Bs-300) ° c at an average cooling rate of 0.10 ℃/min or higher.

(1)RC＝850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V

(2)Bs＝830-270×C-90×Mn-70×Cr-37×Ni-83×Mo

In the following description, the temperature indicates the temperature of the surface of a steel material, a rough rolled plate, a finish rolled plate, and the like, which will be described later. For example, the average cooling rate of forced cooling described later is based on the average cooling rate of the surface of the finish rolled sheet.

First, a steel material such as a billet having the above-described composition is prepared. The method for producing a steel material such as a billet is not particularly limited, and any conventional method can be employed. For example, the following methods can be mentioned: the molten steel having the above-described composition is melted in a converter or the like by a known method, and a billet is produced by a casting method such as a continuous casting method. A known casting method such as an ingot-cogging rolling method may be used. Scrap steel may also be used as the raw material.

In order to reduce the segregation of steel components during continuous casting, a segregation reducing treatment such as electromagnetic stirring (EMS) or soft reduction casting (IBSR) may be applied. By the electromagnetic stirring, equiaxed crystals are formed in the center of the plate thickness, and segregation can be reduced. By the soft reduction casting, the flow of molten steel in the non-solidified portion of the continuously cast slab is prevented, and the segregation in the center portion of the plate thickness can be reduced. By applying at least one of these segregation reducing treatments, press formability, low-temperature toughness, and the like can be improved.

< heating temperature of raw steel: 1150 ℃ or higher

In a steel material such as a billet cooled to a low temperature, most of elements forming carbonitrides such as Ti are unevenly precipitated as coarse carbonitrides. The presence of such coarse and uneven precipitates causes deterioration of various characteristics (for example, strength, resistance to punch-roughening, and the like).

Therefore, the steel material before hot rolling is heated to form a solid solution of coarse precipitates. In order to sufficiently dissolve the coarse precipitates before hot rolling, the heating temperature of the steel material is 1150 ℃ or higher, preferably 1180 ℃ or higher, and more preferably 1200 ℃ or higher.

On the other hand, when the heating temperature of the steel material is too high, billet flaws may occur, and the yield may be lowered due to descaling. Therefore, the heating temperature of the steel material is preferably 1350 ℃ or less, more preferably 1300 ℃ or less, and still more preferably 1280 ℃ or less.

The steel material is heated to a heating temperature of 1150 ℃ or higher and held for a predetermined time. In this case, if the holding time is too long, the amount of scale formed may increase. In this case, scale biting and the like are likely to occur in the subsequent hot rolling, and the surface roughness and fatigue characteristics of the obtained steel sheet tend to deteriorate.

Therefore, the holding time of the steel material in the temperature range of 1150 ℃ or higher is preferably 10000 seconds or less, more preferably 8000 seconds or less, from the viewpoint of further improving the fatigue characteristics. The lower limit is not particularly limited, and is preferably 1800 seconds or more from the viewpoint of uniformity of heating of the steel material.

The steel material before hot rolling may be directly subjected to hot rolling (direct rolling) in a high temperature state after casting (i.e., in a state in which the temperature is maintained within the above-described heating temperature range).

Next, hot rolling including rough rolling and finish rolling is performed on the heated (or in a high-temperature state after casting) steel material. The conditions for rough rolling are not particularly limited as long as a desired sheet bar size can be secured.

And carrying out rough rolling on the steel raw material to obtain a rough rolled plate. The obtained rough rolled sheet was subjected to descaling by spraying high-pressure water (high-pressure water descaling) on the inlet side of the finishing mill before finish rolling.

< descaled collision pressure: 2.5MPa or more

In order to remove primary scale generated before finish rolling, high-pressure water descaling was performed on the rough rolled sheet.

The collision pressure for high-pressure water descaling (also simply referred to as "descaling collision pressure") is 2.5MPa or more, preferably 3.0MPa or more, and more preferably 3.5MPa or more. The collision pressure is a force per unit area by which high-pressure water collides against the surface of the roughly rolled plate. Thus, the arithmetic average roughness Ra of the surface of the obtained high-strength hot-rolled steel sheet can be controlled to 2.00 μm or less.

The upper limit of the descaling collision pressure is not particularly limited, but is preferably 15.0MPa or less, more preferably 14.5MPa or less, and still more preferably 12.0MP or less.

The descaling with high pressure water may be performed during the rolling between the stands of the finish rolling. Further, the rough rolled sheet may be cooled between stands for finish rolling as necessary.

< finish rolling finish temperature: (RC-100) DEG C or higher and (RC +100) DEG C or lower

The rough rolled sheet subjected to high-pressure water descaling is finish rolled at a predetermined finish rolling finishing temperature to obtain a finish rolled sheet.

When the finish rolling temperature is too low, rolling may be performed at a ferrite + austenite dual-phase region temperature. Therefore, the desired area ratio cannot be sufficiently obtained for the main phase and the second phase, and the tensile strength of 1180MPa or more cannot be ensured.

Therefore, the finish rolling finishing temperature is (RC-100) DEG C or more, preferably (RC-80) DEG C or more, and more preferably (RC-50) DEG C or more.

On the other hand, when the finish rolling finishing temperature is too high, the grain growth of austenite grains occurs remarkably, the austenite grains are coarsened, the average grain size of the upper bainite phase becomes large, and the punch coarsening resistance becomes insufficient.

Therefore, the finish rolling finishing temperature is (RC +100) ° C or less, preferably (RC +80) ° C or less, and more preferably (RC +50) ° C or less.

RC is defined by the following formula (1).

(1)RC＝850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V

Here, the symbol of each element in the formula (1) represents the content [ mass%) of each element in the above-described composition. In the case of an element not contained in the above-mentioned composition, the symbol of the element in the formula (1) is calculated as 0.

Next, the finish rolled sheet obtained by the finish rolling is cooled from the finish rolling temperature (hereinafter, also referred to as "forced cooling") to a cooling stop temperature (hereinafter, also referred to as "forced cooling") at an average cooling rate (hereinafter, also referred to as "average cooling rate").

< Cooling Start time: 2.0s or less after finish rolling

In a predetermined case, the time (cooling start time) from the end of finish rolling to the start of forced cooling is controlled. Specifically, when the finish rolling temperature is not less than RC ℃, if the cooling start time is too long, austenite grains grow, the average grain size of the upper bainite phase increases, and the resistance to punch-roughening becomes insufficient.

Therefore, when the finish rolling temperature is RC ℃ or higher, the cooling start time is 2.0s or less, preferably 1.5s or less, and more preferably 1.0s or less.

When the finish rolling temperature is lower than RC ℃, the cooling start time is not particularly limited, but is preferably 2.0s or less, more preferably 1.5s or less, and even more preferably 1.0s or less, from the viewpoint of ensuring the tensile strength by not restoring the strain introduced into the austenite grains.

< average cooling rate from finish rolling finish temperature to cooling stop temperature: 20 ℃/s or above

In the forced cooling, when the average cooling rate from the finish rolling temperature to the cooling stop temperature (hereinafter, also referred to as "average cooling rate of forced cooling") is too slow, the iron matrix is transformed before the upper bainite transformation, and the upper bainite phase having a desired area fraction cannot be obtained.

Therefore, the average cooling rate of forced cooling is 20 ℃/s or more, preferably 25 ℃/s or more, and more preferably 30 ℃/s or more.

On the other hand, the upper limit of the average cooling rate of the forced cooling is not particularly limited, but if it is too high, it is difficult to control the cooling stop temperature and it is difficult to obtain a desired microstructure, and therefore, it is preferably 500 ℃/s or less, more preferably 300 ℃/s or less, still more preferably 150 ℃/s or less, and particularly preferably 80 ℃/s or less.

< cooling stop temperature: (Bs-150) DEG C or higher and Bs DEG C or lower

In the case where the cooling stop temperature is too low, upper bainite transformation is promoted and the circumference of the second phase is less than 300000 μm/mm²The fatigue characteristics become insufficient.

Therefore, the cooling stop temperature is (Bs-150) DEG C or higher, preferably (Bs-140) DEG C or higher, and more preferably (Bs-130) DEG C or higher.

On the other hand, when the cooling stop temperature is too high, the upper bainite phase is not formed, the upper bainite phase cannot be obtained at an area ratio of 50% or more, the area ratio of the second phase becomes large, and the ductility becomes insufficient. In addition, no fine second phase is obtained, the perimeter of the second phase being less than 300000 μm/mm²The fatigue characteristics become insufficient.

Therefore, the cooling stop temperature is Bs ℃ or lower, preferably (Bs-20) DEG C or lower, and more preferably (Bs-50) DEG C or lower.

Bs is defined by the following formula (2).

(2)Bs＝830-270×C-90×Mn-70×Cr-37×Ni-83×Mo

Here, the symbol of each element in the formula (2) represents the content [ mass%) of each element in the above-described composition. In the case of an element not contained in the above-mentioned composition, the symbol of the element in the formula (2) is calculated as 0.

The finish rolled sheet forcibly cooled to the cooling stop temperature is wound at the cooling stop temperature, for example, into a coil shape. Therefore, the cooling stop temperature is also the coiling temperature.

< average cooling rate until (Bs-300) DEG C after coiling: 0.10 ℃/min or more

Then, the rolled sheet was cooled to (Bs-300) DEG C.

The average cooling rate after coiling affects the transformation behavior of the non-transformed austenite phase. When the average cooling rate up to (Bs-300) ° c after coiling is too slow, the non-transformed austenite phase decomposes and changes to the upper bainite phase or pearlite phase, and the area ratio of the desired second phase (at least one selected from the group consisting of the lower bainite phase and/or tempered martensite phase, fresh martensite phase, and retained austenite phase) cannot be secured.

Therefore, in order to obtain a desired area fraction of the second phase, the average cooling rate up to (Bs-300) DEG C after winding is 0.10 ℃/min or more, preferably 0.12 ℃/min or more, more preferably 0.15 ℃/min or more, and still more preferably 0.20 ℃/min or more.

On the other hand, if the average cooling rate up to (Bs-300) ° c after coiling is too high, the bainite transformation stagnation phenomenon does not occur, and it may be difficult to obtain the area ratio of a desired second phase (at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase).

Therefore, the average cooling rate to (Bs-300). degree.C.after coiling is preferably 1800 ℃ C/min or less, more preferably 1800 ℃ C/min or less, still more preferably 600 ℃ C/min or less, and particularly preferably 60 ℃ C/min or less.

The cooling method after coiling may be any cooling method as long as a desired average cooling rate can be obtained. Examples of the cooling method include natural air cooling, forced air cooling, gas cooling, spray cooling, water cooling, and oil cooling.

In the cooling after the coiling, the cooling stop temperature may be lower than (Bs-300). degree.C. Typically, cooling is to room temperature of about 10 ℃ to about 30 ℃. Then, temper rolling (skin pass rolling) may be performed in accordance with a conventional method. Alternatively, the scale may be removed by pickling.

The finish rolled sheet cooled after coiling (and optionally subjected to temper rolling and/or pickling) without performing plating treatment described later becomes the high-strength hot-rolled steel sheet of the present invention.

< plating treatment >

The finish rolled sheet cooled after coiling (and optionally subjected to temper rolling and/or pickling) may be subjected to plating treatment using a conventional plating line. Thereby, a plating layer is formed on the surface of the finish rolled sheet. When the plating treatment is performed, the finish rolled sheet after the plating treatment becomes the high-strength hot-rolled steel sheet of the present invention.

The plating treatment is not particularly limited, and examples thereof include: conventionally known hot dipping treatment, alloying hot dipping treatment, plating treatment, and the like.

The hot dip coating treatment includes, for example, hot dip coating treatment for forming a hot dip coating layer. Further, as the alloying hot-dip treatment, for example, an alloying hot-dip treatment (a treatment of forming an alloying hot-dip galvanized layer by performing an alloying treatment after a hot-dip galvanizing treatment) can be cited.

Examples

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the embodiments described below.

Production of Hot rolled Steel sheet

Molten steel having a composition shown in table 1 below (the balance being Fe and unavoidable impurities) was melted in a converter, and billets were produced by a continuous casting method.

The produced slabs were heated at slab heating temperatures [ ° c ] shown in table 2 below and at slab heating times [ s ] of 1150 ℃ or higher.

And carrying out rough rolling on the heated steel billet to obtain a rough rolling plate.

The surface of the obtained rough rolled sheet was subjected to high-pressure water descaling at a collision pressure [ MPa ] shown in table 2 below.

The rough rolled sheet subjected to high-pressure water descaling was subjected to finish rolling at a finish rolling temperature [ ° c ] shown in table 2 below, thereby obtaining a finish rolled sheet.

After finishing the finish rolling, the obtained finish rolled sheet is forcibly cooled. In table 2 below, as conditions for forced cooling, a cooling start time (time from the end of finish rolling to the start of forced cooling) [ s ], an average cooling rate (average cooling rate from the end of finish rolling to a cooling stop temperature) [ c/s ], and a cooling stop temperature [ c ] are described.

The finish rolled sheet after forced cooling was wound at a cooling stop temperature [ ° c ] shown in table 2 below.

The rolled sheet was cooled to (Bs-300) ℃ at an average cooling rate [. degree.C./min ] shown in Table 2 below.

RC [. degree.C ] and Bs [. degree.C ] shown in the following Table 2 are as described above.

Thus, hot-rolled steel sheets having a sheet thickness [ mm ] shown in Table 2 below were obtained. The hot-rolled steel sheet thus obtained was subjected to temper rolling, and then acid pickling (hydrochloric acid concentration: 10 mass%, temperature 85 ℃) was performed to remove oxide scale. Further, a plating treatment is performed on a part of the hot-rolled steel sheet to form a plated layer. More specifically, a hot dip galvanizing treatment is performed, and then an alloying treatment is performed. Thereby, an alloyed hot dip zinc coating layer is formed. In this case, the column entitled "presence or absence of plating treatment" in table 2 below is marked with "o".

< evaluation of Hot rolled Steel sheet >

Test pieces were cut out from the hot-rolled steel sheet obtained, and tests and evaluations described below were performed. The hot-rolled steel sheet having a plated layer is subjected to tests and evaluations described below after the plating treatment. The results are shown in table 3 below.

(i) Observation of the microstructure

Test pieces were cut out from the hot-rolled steel sheets obtained. The cut test piece was polished to expose a cross section (a cross section parallel to the rolling direction) at a position 1/4 where the plate thickness was removed from the plating layer. The exposed cross section was etched with an etching solution (3 mass% nital solution), and then observed with a Scanning Electron Microscope (SEM) at a magnification of 5000 times. 10 visual fields were photographed, and the area ratios [% ] of the upper bainite phase, the lower bainite phase, and/or the tempered martensite phase, the pearlite phase, and the polygonal ferrite phase were quantitatively determined by image processing.

The fresh martensite phase is difficult to distinguish from the retained austenite phase using SEM. Thus, an Electron Back Scattering Diffraction (EBSD) method is used. More specifically, for each crystal grain in which the fresh martensite phase and the retained austenite phase cannot be distinguished by SEM, a phase identified as the austenite phase at less than 50% by area ratio within the crystal grain is regarded as the fresh martensite phase, and a phase identified as the austenite phase at 50% or more by area ratio within the crystal grain is regarded as the retained austenite phase by the EBSD method.

The area ratios [% ] were determined for the fresh martensite phase and the retained austenite phase thus separated.

The circumference of the second phase having an equivalent circle diameter of 0.5 μm or more is determined as follows.

For each crystal grain of the second phase identified by the SEM or EBSD method, first, the area a is obtained by image processing_secondary[μm²]Then, the equivalent circle diameter d is obtained by using the following formula_secondary[μm]。

d_secondary＝2√(A_secondary/π)

The crystal grains of the second phase having an equivalent circle diameter of 0.5 μm or more were identified, and the perimeter thereof was measured by image processing. The total of the circumferences of the second phases having equivalent circle diameters of 0.5 μm or more in the measurement field is divided by the area of the measurement field. From this, the perimeter [ μm/mm ] of the second phase having an equivalent circle diameter of 0.5 μm or more was determined²]。

The average particle size of the upper bainite phase was measured in the following manner.

First, a test piece was cut out of a hot-rolled steel sheet and polished. More specifically, the test piece was polished with a colloidal silica solution so that the surface parallel to the rolling direction (the surface at the position of 1/4 mm) was the observation surface. Then, the area of 100. mu. m.times.100. mu.m on the observation surface of the test piece at 10 spots was measured by the EBSD method (acceleration voltage of electron beam: 20keV, measurement interval: 0.1. mu.m step). The grain size [ μm ] of the Area average (Area average) of the upper bainite phase was calculated by defining the threshold value of the large tilt angle grain boundary, which is generally recognized as a grain boundary, as 15 °, and visualizing the grain boundary having a crystal misorientation of 15 ° or more. In the calculation, OIM Analysis software manufactured by TSL corporation was used. In this case, as the definition of the crystal grains, the area average Grain size was determined by setting the Grain Tolerance Angle (Grain Tolerance Angle) to 15 °. The area average grain size of the upper bainite phase thus determined was defined as the average grain size [ μm ] of the upper bainite phase.

(ii) Measurement of arithmetic average roughness Ra

Test pieces (size: t (plate thickness) × 50mm (width) × 50mm (length)) were cut out from the hot-rolled steel sheets obtained. The cut test pieces were measured according to JIS B0601: 2013, the arithmetic average roughness Ra is measured. The arithmetic average roughness Ra was measured three times in each direction perpendicular to the rolling direction, and the average value of the measurements was determined as the arithmetic average roughness Ra of the hot-rolled steel sheet.

For the hot-rolled steel sheet having a plated layer, the arithmetic average roughness Ra of the surface of the plated layer was determined.

(iii) Tensile test

From the obtained hot-rolled steel sheet, test pieces (GL: 50mm) according to JIS5 were cut out in a direction perpendicular to the rolling direction, and the mechanical properties were determined.

Specifically, the cut test pieces were measured according to JIS Z2241: 2011A tensile test was conducted to determine the yield strength (yield point, YP) [ MPa ], Tensile Strength (TS) [ MPa ], total elongation (El) [% ], and uniform elongation (U-El) [% ]. For each hot-rolled steel sheet, tensile tests were carried out twice, and the average of the two tests was taken as the mechanical property value of the hot-rolled steel sheet.

In the present invention, when the value of TS × U-El [ MPa.% is 6000 MPa.% or more, it is evaluated that the ductility is excellent.

(iv) Plane bending fatigue test

Test pieces having the dimensions and shapes shown in FIG. 1 were cut out from the hot-rolled steel sheet obtained. The numerical values shown in fig. 1 are in units of "mm". The longitudinal direction of the test piece is a direction perpendicular to the rolling direction of the hot-rolled steel sheet. Using the cut test piece, a plane bending fatigue test was carried out in accordance with the provisions of JIS Z2275-1978. In the stress load mode, the stress ratio R is set to-1, and the frequency is set to 25 Hz. The load stress amplitude was varied in 6 steps, the stress cycle until fracture was measured, the S-N curve was obtained, and the fatigue strength (stress amplitude value) at 50 ten thousand cycles was obtained.

In the present invention, when the value of the fatigue strength divided by the Tensile Strength (TS) at 50 ten thousand cycles is 0.50 or more, the fatigue characteristics are evaluated to be excellent.

(v) Evaluation of resistance to Blanking roughness

Test pieces (size: t (plate thickness) × 30mm (width) × 30mm (length)) were cut out from the hot-rolled steel sheets obtained. A punched hole was formed at the center of the cut test piece with a 10mm diameter cylindrical punch with a clearance of 12. + -. 1%. The gap is a ratio [% ] to the thickness of the test piece. Test pieces were quartered along the diagonal line so that the rolling direction end face and the rolling direction end face of the punched hole could be evaluated, respectively, to produce quartered test pieces. The punched end faces of the quartered test piece were measured according to JIS B0601: 2013, and the maximum height roughness Rz [ μm ] is measured.

More specifically, the measurement is performed in the following manner. First, a position a and a position B are set in the plate thickness direction on the punched end faces of the quartered test piece. The position A is a position 100 μm in the plate thickness direction from the outermost surface of the burr generation side. The position B is a position 100 μm in the fracture surface direction from the shear surface/fracture surface boundary of the punched end surface. The positions A and B were equally divided into 10 positions, and a roughness curve having a length of 1mm was measured in the arc direction (circumferential direction) at the total of 10 positions. The maximum height roughness Rz was calculated from the 10 roughness curves obtained. The calculated average value of Rz is defined as the Rz of the quartered test piece. The Rz is measured for all the four test pieces, and the average value of the obtained Rz is defined as Rz [ μm ] of the punched edge face of the hot-rolled steel sheet.

In addition, the standard deviation of Rz at 40 points in total obtained from all the quartered test pieces was calculated and taken as the standard deviation [ μm ] of Rz at the punched end face of the hot-rolled steel sheet. Since the punched end face is a curved surface, Rz is calculated in accordance with JIS B0601: 2013. No correction based on the cut-offs λ s and λ c is performed.

In the present invention, it is evaluated that the punch-roughening resistance is excellent when Rz of the punch end face is 35 μm or less and the standard deviation of Rz of the punch end face is 10 μm or less.

[ Table 1]

[ Table 2]

[ Table 3]

< summary of evaluation results >

In tables 1 to 3, underlined portions indicate that the ranges of the present invention are out of the ranges or appropriate ranges.

The hot rolled steel sheets of Nos. 1 to 3, 5 to 6, 11 and 13 to 20 have a Tensile Strength (TS) of 1180MPa or more, a high strength, and excellent ductility, fatigue characteristics and resistance to rough blanking.

On the other hand, in case of No.4 (the cooling stop temperature of forced cooling is low), the area ratio of the upper bainite phase is large, the tensile strength is less than 1180MPa, the circumferential length of the second phase is short, and the fatigue characteristics are insufficient.

In the case of No.7 (the average cooling rate after coiling was slow to (Bs-300). degree.C.), the area ratio of the second phase was small and the tensile strength was less than 1180 MPa.

In the case of No.8 (high finish rolling temperature), the average grain size of the upper bainite phase was large, and the resistance to punch-induced roughness was insufficient.

In the case of No.9 (low finish rolling temperature), the area ratio of the upper bainite phase was small and the tensile strength was less than 1180 MPa.

In the case of No.10 (low descaling collision pressure), the arithmetic average roughness Ra was large, and the fatigue characteristics were insufficient.

In case of No.12 (high cooling stop temperature of forced cooling), the area ratio of the second phase was large, and the ductility was insufficient. In addition, the second phase has a short circumferential length and insufficient fatigue properties.

In the case of No.21 (steel N containing much Ti), the resistance to punch roughening was insufficient.

In the case of No.22 (steel O containing no Cr, Mo, Nb, or V), the area ratio of the second phase was small, and the tensile strength was less than 1180 MPa.

In the case of No.23 (steel P containing much Cr), the arithmetic average roughness Ra was large, and the fatigue characteristics were insufficient.

Claims

1. A high-strength hot-rolled steel sheet having a tensile strength of 1180MPa or more and an arithmetic average surface roughness Ra of 2.00 μm or less,

comprising:

contains, in mass%, C: 0.09% or more and 0.20% or less, Si: 0.2% or more and 2.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.100% or less, S: 0.0100% or less, Al: 0.01% or more and 2.00% or less, N: 0.010% or less, Ti: 0.001% or more and less than 0.030%, and B: 0.0005% or more and 0.0200% or less, and further contains a compound selected from the group consisting of Cr: 0.10% or more and 1.50% or less, Mo: 0.05% or more and 0.45% or less, Nb: 0.005% or more and 0.060% or less and V: 0.05% to 0.50% inclusive, and the balance of Fe and unavoidable impurities; and

a microstructure comprising an upper bainite phase and a second phase,

the area ratio of the upper bainite phase is 50% or more and less than 90%,

the average grain diameter of the upper bainite phase is less than 12.0 μm,

the second phase is at least one selected from the group consisting of a lower bainite phase and/or a tempered martensite phase, a fresh martensite phase, and a retained austenite phase,

the area ratio of the second phase is 10% or more and less than 50%,

the second phase having an equivalent circle diameter of 0.5 μm or more has a perimeter of 300000 μm/mm²As described above.

2. The high-strength hot-rolled steel sheet according to claim 1, wherein the composition further contains, in mass%, a component selected from the group consisting of Cu: 0.01% or more and 0.50% or less and Ni: 0.01% or more and 0.50% or less.

3. The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the composition further contains, in mass%, Sb: 0.0002% or more and 0.0300% or less.

4. The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, wherein the composition further contains, in mass%, a component selected from the group consisting of Ca: 0.0002% or more and 0.0100% or less, Mg: 0.0002% or more and 0.0100% or less and REM: at least one selected from the group consisting of 0.0002% to 0.0100%.

5. The high-strength hot-rolled steel sheet according to any one of claims 1 to 4, wherein the surface has a plated layer.

6. A method for producing a high-strength hot-rolled steel sheet according to any one of claims 1 to 4, wherein,

heating a steel material having the composition according to any one of claims 1 to 4 to 1150 ℃ or higher,

rough rolling the heated steel stock to thereby obtain a rough rolled plate,

performing high-pressure water descaling on the rough rolled plate at a collision pressure of 2.5MPa or more,

finish rolling the rough-rolled sheet subjected to the high-pressure water descaling at a finish rolling finishing temperature of (RC-100) DEG C or higher and (RC +100) DEG C or lower to obtain a finish-rolled sheet, wherein RC is defined by the following formula (1),

cooling the finish-rolled sheet at an average cooling rate of 20 ℃/s or more to a cooling stop temperature of (Bs-150) ° C or more and Bs ℃ or less, wherein Bs is defined by the following formula (2), and when the finish-rolling end temperature is RC ℃ or more, the time from the finish of the finish rolling to the start of the cooling is 2.0s or less,

coiling the finish rolled sheet after the cooling at the cooling stop temperature,

cooling the rolled sheet after coiling to (Bs-300) DEG C at an average cooling rate of 0.10 ℃/min or more,

(1)RC＝850+100×C+100×N+10×Mn+700×Ti+5000×B+10×Cr+50×Mo+2000×Nb+150×V

(2)Bs＝830-270×C-90×Mn-70×Cr-37×Ni-83×Mo

here, each element symbol in the formula represents a content of each element in the component composition in mass%, and in the case of an element not contained in the component composition, the element symbol in the formula is calculated as 0.

7. The method for manufacturing a high-strength hot-rolled steel sheet according to claim 6, wherein the finish-rolled sheet subjected to the cooling after the coiling is subjected to plating treatment.