The present invention relates to a high-strength steel plate having excellent formability, toughness, and weldability, and a method for producing the same.
Background technology
[0002]
In recent years, automobiles have high strength in order to reduce the weight of the vehicle body, improve fuel efficiency, reduce carbon dioxide emissions, and absorb collision energy in the event of a collision to ensure the protection and safety of passengers. Steel plates are often used. However, in general, when the strength of a steel sheet is increased, the formability (ductility, hole expandability, etc.) decreases and it becomes difficult to process into a complicated shape. Therefore, the strength and formability (ductility, hole expandability, etc.) It is not easy to achieve both, and various technologies have been proposed so far.
[0003]
For example, in Patent Document 1, in a high-strength steel plate having a tensile strength of 780 MPa or more, the steel plate structure has a space ratio of ferrite: 5 to 50%, retained austenite: 3% or less, and balance: martensite (average aspect ratio). : 1.5 or more), a technique for improving the strength-elongation balance and the strength-elongation flange balance is disclosed.
[0004]
In Patent Document 2, a composite structure composed of ferrite having an average crystal grain size of 10 μm or less, martensite of 20% by volume or more, and other second phase is formed in a high-strength hot-dip galvanized steel sheet, and has corrosion resistance and resistance. Techniques for improving secondary process brittleness are disclosed.
[0005]
Patent Documents 3 and 8 disclose a technique for ensuring high elongation even at high strength by using the metal structure of a steel sheet as a composite structure of ferrite (soft structure) and bainite (hard structure).
[0006]
Patent Document 4 describes in high-strength steel sheets that the space factor is 5 to 30% for ferrite and 50 to 95% for martensite, the average grain size of ferrite is 3 μm or less in diameter equivalent to a circle, and the average grain of martensite. A technique for forming a composite structure having a diameter equivalent to a circle and having a diameter of 6 μm or less to improve elongation and stretch ferrite properties is disclosed.
[0007]
In Patent Document 5, precipitation-enhanced ferrite precipitated by controlling the precipitation distribution mainly by the precipitation phenomenon (interphase interfacial precipitation) caused by grain boundary diffusion at the phase interface during the transformation from austenite to ferrite is used as the main phase. , A technique for achieving both strength and elongation is disclosed.
[0008]
Patent Document 6 discloses a technique in which a steel sheet structure has a ferrite single-phase structure and ferrite is reinforced with fine carbides to achieve both strength and elongation.
[0009]
In Patent Document 7, in a high-strength thin steel plate, the austenite grains having a required C concentration at the interface between the ferrite phase, the bainite phase, and the martensite phase and the austenite grains are set to 50% or more to provide elongation and hole expansion. The technology to secure is disclosed.
[0010]
In recent years, high-strength steel with a tensile strength of 590 to 1470 MPa has been used for some parts in order to reduce the weight of automobiles. In order to achieve further weight reduction, it not only enhances the formability (ductility, hole expandability, etc.)-strength balance, but also balances formability with various properties (toughness, weldability, etc.). Also need to be increased at the same time.

The scope of the claims
[Claim 1]
Ingredient composition is mass%,
C: 0.05 to 0.30%,
Si: 2.50% or less,
Mn: 0.50 to 3.50%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.001 to 2.000%,
N: 0.0150% or less,
O: 0.0050% or less,
Remaining: In a steel sheet consisting of Fe and unavoidable impurities,
The microstructure in the region from 1/8 t (t: plate thickness) to 3/8 t (t: plate thickness) from the surface of the steel plate is by volume%.
Needle-shaped ferrite: 20% or more,
Martensite: 10% or more
Including
Bulk ferrite: 20% or less,
Residual austenite: 2.0% or less,
Structures other than the structure in which bainite and bainitic ferrite are further added to all the above structures: 5% or less
Limited to
Moreover, the martensite satisfies the following formula (A).
A high-strength steel plate having excellent formability, toughness, and weldability.
[Number 1]

Here, di is the circle equivalent diameter [μm] of the i-th largest island-shaped martensite in the microstructure in the region of 1/8 t (t: plate thickness) to 3/8 t (t: plate thickness), and a. i is the aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of 1 / 8t (t: plate thickness) to 3 / 8t (t: plate thickness).
[Claim 2]
The component composition is, instead of a part of Fe, further in mass%.
Ti: 0.30% or less,
Nb: 0.10% or less,
V: 1.00% or less
Including one or more of
The high-strength steel sheet having excellent formability, toughness, and weldability according to claim 1.
[Claim 3]
The component composition is, instead of a part of Fe, further in mass%.
Cr: 2.00% or less,
Ni: 2.00% or less,
Cu: 2.00% or less,
Mo: 1.00% or less,
W: 1.00% or less,
B: 0.0100% or less,
Sn: 1.00% or less,
Sb: 0.20% or less
Including one or more of
The high-strength steel plate having excellent formability, toughness, and weldability according to claim 1 or 2, wherein the steel sheet has the above-mentioned features.
[Claim 4]
The component composition further contains, in mass%, one or more of Ca, Ce, Mg, Zr, La, Hf, and REM in a total of 0.0100% or less instead of a part of Fe. The high-strength steel plate having excellent formability, toughness, and weldability according to any one of claims 1 to 3.
[Claim 5]
Any of claims 1 to 4, wherein the martensite of the microstructure contains 30% or more of tempered martensite in which fine carbides having an average diameter of 1.0 μm or less are precipitated in volume% with respect to all martensite. A high-strength steel plate having excellent formability, toughness, and weldability according to item 1.
[Claim 6]
The formability, toughness, and weldability according to any one of claims 1 to 5, which are characterized by having a zinc plating layer or a zinc alloy plating layer on one side or both sides of the high-strength steel plate, are excellent. High-strength steel plate.
[Claim 7]
The high-strength steel plate having excellent formability, toughness, and weldability according to claim 6, wherein the zinc plating layer or the zinc alloy plating layer is an alloyed plating layer.
[Claim 8]
A manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to any one of claims 1 to 4.
A steel piece having the composition according to any one of claims 1 to 4 is subjected to hot rolling, and hot rolling is completed at 850 ° C to 1050 ° C to obtain a steel sheet after hot rolling.
The hot-rolled steel sheet was cooled from 850 ° C to 550 ° C at an average cooling rate of 30 ° C / sec or higher, and wound at a temperature equal to or lower than the bainite transformation start temperature Bs point defined by the following formula.
Cool from the Bs point to (Bs point -80) ° C under the conditions that satisfy the following formula (1) to make a hot-rolled steel sheet.
The hot-rolled steel sheet was cold-rolled with or without a rolling reduction of 10% or less to manufacture a heat-treated steel sheet.
The heat treatment steel plate is calculated by dividing the elapsed time in a temperature range from (Ac1 + 25) ° C. to Ac3 point temperature and 700 ° C. to the maximum heating temperature or (Ac3-20) ° C., whichever is lower, into 10 parts. Heat under the condition that satisfies the following formula (3), and keep it in the temperature range from the maximum heating temperature of -10 ° C to the maximum heating temperature for 150 seconds or less.
From the heat holding temperature, cool with an average cooling rate of 25 ° C / sec or more in the temperature range of 700 ° C to 550 ° C.
Cooling is limited to the range that satisfies the following equations (4) and (5), which are calculated by dividing the residence time in the temperature range up to 300 ° C from the lower of 550 ° C or Bs point into 10 parts. A method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability.
Bs point (° C) = 611-33, [Mn] -17, [Cr]
-17, [Ni] -21, [Mo] -11, [Si]
+30 ・ [Al] + (24 ・ [Cr] +15 ・ [Mo]
+5500 ・ [B] +240 ・ [Nb]) / (8 ・ [C])
[Element]: Mass% of element
[Number 2]

Bs: Bs point (° C)
WM: Composition of each element (% by mass)
Δt (n): Elapsed time (seconds) from (Bs-10 × (n-1)) ° C to (Bs-10 × n) ° C during cooling from cooling after hot rolling to 400 ° C through winding. )
[Number 3]

Δt: 1/10 of the elapsed time (seconds)
WM: Composition of each element species (mass%)
Fγ (n): Average reverse transformation rate in the nth section
T (n): average temperature (° C.) in the nth section
[Number 4]

[Number 5]

Δt: 1/10 of the elapsed time (seconds)
Bs: Bs point (° C)
T (n): Average temperature (° C.) at each step
WM: Composition of each element species (mass%)
[Claim 9]
A manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to any one of claims 1 to 4.
A steel piece having the composition according to any one of claims 1 to 4 is subjected to hot rolling, and hot rolling is completed at 850 ° C to 1050 ° C to obtain a steel sheet after hot rolling.
The hot-rolled steel sheet was cooled from 850 ° C to 550 ° C at an average cooling rate of 30 ° C / sec or higher, and wound at a temperature equal to or lower than the bainite transformation start temperature Bs point defined by the following formula.
A hot-rolled steel sheet was manufactured by cooling from the Bs point to (Bs point-80) ° C under the conditions satisfying the following formula (1).
The hot-rolled steel sheet was subjected to the first cold rolling or not, and a steel sheet for intermediate heat treatment was manufactured.
The steel sheet for intermediate heat treatment is heated under the condition of satisfying the following formula (2), which is calculated by dividing the elapsed time in the temperature range from 700 ° C. to (Ac3-20) ° C. into 10 at a temperature of (Ac3-20) ° C. or higher. death,
Next, from the heating temperature, the average cooling rate in the temperature range of 700 ° C to 550 ° C is set to 30 ° C / sec or more, and the average cooling rate in the temperature range from the Bs point to (Bs-80) ° C is set to 20 ° C / sec or more. Then, the residence time from the (Bs-80) ° C. to the Ms point was set to 1000 seconds or less, and the average cooling rate from the Ms point to the (Ms-50) ° C. was limited to 100 ° C./sec or less and cooled to obtain an intermediate heat-treated steel plate. ,
The cooled intermediate heat-treated steel sheet was subjected to or without a second cold rolling with a rolling reduction of 10% or less to manufacture a heat-treated steel sheet.
The heat treatment steel plate is calculated by dividing the elapsed time in a temperature range from (Ac1 + 25) ° C. to Ac3 point temperature and 700 ° C. to the maximum heating temperature or (Ac3-20) ° C., whichever is lower, into 10 parts. Heat under the condition that satisfies the following formula (3), and keep it in the temperature range from the maximum heating temperature of -10 ° C to the maximum heating temperature for 150 seconds or less.
From the heat holding temperature, the average cooling rate in the temperature range of 700 ° C to 550 ° C is set to 25 ° C / sec or more, and the residence time in the temperature range from 550 ° C or Bs point, whichever is lower, to 300 ° C is set. A method for producing a high-strength steel plate having excellent formability, toughness, and weldability, which comprises cooling by limiting the temperature to a range satisfying the following formulas (4) and (5) calculated by dividing into 10 parts.
Bs point (° C) = 611-33, [Mn] -17, [Cr]
-17, [Ni] -21, [Mo] -11, [Si]
+30 ・ [Al] + (24 ・ [Cr] +15 ・ [Mo]
+5500 ・ [B] +240 ・ [Nb]) / (8 ・ [C])
[Element]: Mass% of element
[Number 6]

Bs: Bs point (° C)
WM: Composition of each element (% by mass)
Δt (n): Elapsed time (seconds) from (Bs-10 × (n-1)) ° C to (Bs-10 × n) ° C during cooling from cooling after hot rolling to 400 ° C through winding. )
Ms point (° C) = 561-474 [C] -33 ・ [Mn]
-17, [Cr] -17, [Ni] -21, [Mo]
-11 ・ [Si] +30 ・ [Al]
[Element]: Mass% of element
[Number 7]

Δt: 1/10 of the elapsed time (seconds)
F γ (n): Average reverse transformation rate in the nth section
T (n): average temperature (° C.) in the nth section
[Number 8]

Δt: 1/10 of the elapsed time (seconds)
WM: Composition of each element species (mass%)
Fγ (n): Average reverse transformation rate in the nth section
T (n): average temperature (° C.) in the nth section
[Number 9]

[Number 10]

Δt: 1/10 of the elapsed time (seconds)
Bs: Bs point (° C)
T (n): Average temperature (° C.) at each step
WM: Composition of each element species (mass%)
[Claim 10]
The method for manufacturing a steel sheet for heat treatment according to claim 9, wherein the first cold rolling is a rolling reduction of 80% or less.
[Claim 11]
The method for producing a steel sheet for heat treatment according to claim 9 or 10, wherein the first cold rolling is cold rolling with a rolling reduction of more than 10%.
[Claim 12]
The heat-treated steel sheet is limited to a range that satisfies the above equations (4) and (5), which is calculated by dividing the residence time in the temperature range up to 300 ° C. from the lower of 550 ° C. or Bs point into 10 parts. 2. A method for manufacturing high-strength steel sheets.
[Claim 13]
The method for producing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 12, wherein tempering rolling with a rolling reduction of 2.0% or less is performed prior to the tempering treatment.
[Claim 14]
The manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 6.
In the method for producing a high-strength steel sheet having excellent formability, toughness, and weldability according to any one of claims 8 to 13, a plating bath containing zinc as a main component while staying at 550 to 300 ° C. To form a zinc-plated layer or a zinc alloy-plated layer on one or both sides of a steel sheet.
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.
[Claim 15]
The manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 6.
In the method for producing a high-strength steel sheet having excellent formability, toughness, and weldability according to any one of claims 8 to 13, the steel sheet is allowed to stay at 550 ° C to 300 ° C, cooled to room temperature, and then the steel sheet. A zinc plating layer or a zinc alloy plating layer is formed by electroplating on one side or both sides.
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.
[Claim 16]
The manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 6.
In the method for producing a high-strength steel sheet having excellent formability, toughness, and weldability according to claim 12 or 13, the steel sheet is immersed in a plating bath containing zinc as a main component during the tempering process, and one side or both sides of the steel sheet is used. To form a zinc plating layer or a zinc alloy plating layer
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.
[Claim 17]
The manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 6.
In the method for producing a high-strength steel sheet having excellent formability, toughness, and weldability according to claim 12 or 13, after performing a tempering treatment and cooling to room temperature, one or both sides of the steel sheet are electroplated. , Forming a zinc-plated layer or a zinc alloy-plated layer
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.
[Claim 18]
The moldability and toughness according to claim 7. And, it is a manufacturing method for manufacturing a high-strength steel plate having excellent weldability.
In the method for producing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 17, a zinc-plated layer or a zinc alloy is formed while the temperature remains at 300 ° C. to 550 ° C. after being immersed in a plating bath. The plating layer is heated from 450 ° C. to 550 ° C., and the zinc plating layer or the zinc alloy plating layer is alloyed.
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.
[Claim 19]
The manufacturing method for manufacturing a high-strength steel plate having excellent formability, toughness, and weldability according to claim 7.
In the method for producing a high-strength steel plate having excellent formability, toughness, and weldability according to any one of claims 15, 16 and 18, the heating of the plating layer or the zinc alloy plating layer in the tempering treatment. The temperature is changed from 450 ° C to 550 ° C, and the zinc plating layer or the zinc alloy plating layer is alloyed.
A method for producing a high-strength steel plate having excellent formability, toughness, and weldability.