EP4726069A1

EP4726069A1 - High-strength steel sheet and method for manufacturing same

Info

Publication number: EP4726069A1
Application number: EP24845397.9A
Authority: EP
Inventors: Kohei KAWAZOME; Hiroshi Hasegawa; Hiroshi Matsuda
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-07-26
Filing date: 2024-07-09
Publication date: 2026-04-15
Also published as: WO2025023016A1; JP7666761B1; JPWO2025023016A1; MX2026000769A; CN121532532A; KR20260025164A

Abstract

Objects are to provide a high strength steel sheet having a tensile strength of 980 MPa or greater and having excellent ductility, fatigue strength, and blankability and to provide a method for manufacturing the same.

A high strength steel sheet has a predetermined chemical composition. In the high strength steel sheet, a primary phase is a bainite phase that includes upper bainite and granular bainite and is present in an area fraction of 85% or greater, and a hard secondary phase includes fresh martensite and/or retained austenite phases and is present in an area fraction of less than 15%; the area fraction of the granular bainite with respect to the area fraction of the upper bainite in the bainite phase is 5% or greater and 40% or less; a difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 is 30% or less with respect to 0.3TS, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position; and the upper bainite phase has an average grain size of 7 µm or less.

Description

Technical Field

The present invention relates to a high strength steel sheet (in particular, hot rolled steel sheet) that is suitable for use in automotive members and which, in particular, has improved fatigue resistance and blankability in addition to improved strength and ductility. The present invention also relates to a method for manufacturing the high strength steel sheet.

Background Art

In recent years, there has been a need for a worldwide effort to reduce CO₂ emissions to achieve global environment conservation. In particular, there is a strong need to improve the fuel efficiency of automobiles, and, therefore, attempts are being made to reduce the weight of automobile bodies. An effective way to reduce the weight of automobile bodies without decreasing their strength is to increase the strength of a steel sheet that serves as a material of automotive members, to reduce the thickness of the steel sheet. In particular, steel sheets having a tensile strength of 980 MPa or greater are materials that are expected to dramatically improve the fuel efficiency of automobiles when they have a reduced thickness.
If the tensile strength of a steel sheet is increased, however, its ductility decreases, which degrades the press formability of the steel sheet. Automotive parts, in particular, chassis components, such as suspension components, need to be formed with a complex shape so as to have stiffness, and, accordingly, materials of automotive parts need to have high press formability and, in particular, excellent ductility.
In addition, since the durability of automotive parts decreases as their thickness decreases, it is necessary for steel sheets to have improved fatigue strength so as to ensure durability. Automotive parts, in particular, chassis components, such as suspension components, are subjected to repeated loads from tires, and, therefore, if the fatigue strength is low, the durability of the parts may fall below their expected design durability as the mileage increases. In general, however, increasing the strength of steel sheets does not necessarily increase their fatigue strength; in addition, the increase in the strength of the material increases the likelihood of the occurrence of defects including recesses and protrusions in edge surfaces resulting from blanking, and thus, fractures due to the defects, which act as origins, are formed, which results in degraded fatigue resistance.
Thus, regarding steel sheets having increased strength, specifically, a tensile strength of 980 MPa or greater, there is a need to have excellent fatigue strength and excellent blankability as well as excellent ductility. To date, various studies have been conducted to improve the fatigue strength and blankability of steel sheets while increasing their tensile strength (Patent Literature 1 to 3).
Patent Literature 1 discloses a technology regarding a high strength hot rolled steel sheet having excellent formability and fatigue resistance, which can be obtained by controlling manufacturing conditions for hot rolling to form a primary phase of ferrite and control a shape and dispersion state of inclusions.
Patent Literature 2 discloses a technology regarding a high strength hot rolled steel sheet having excellent blanking fatigue resistance and workability, which can be obtained by controlling a shape and a hardness of martensite in a thickness middle portion.
Patent Literature 3 discloses a technology regarding a high strength hot rolled steel sheet having excellent flangeability and blankability, which can be obtained by controlling manufacturing conditions for hot rolling to form a primary phase of bainite and control an amount of a precipitate as well as a shape and dispersion state of a hard secondary phase.

Citation List

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2014-31560
PTL 2: Japanese Unexamined Patent Application Publication No. 2015-214718
PTL 3: Domestic Re-publication of PCT International Publication for Patent Application No. 2017-017933

Summary of Invention

Technical Problem

Unfortunately, the technologies of the related art described in Patent Literature 1 to 3 present the following problems.
The technologies described in Patent Literature 1 and 2 do not provide a tensile strength of 980 MPa or greater.
The technology described in Patent Literature 3 provides a hot rolled steel sheet having a tensile strength of 980 MPa or greater, excellent fatigue strength, and excellent blankability. The technology described in Patent Literature 3 provides a hot rolled steel sheet having a tensile strength of 980 MPa or greater, excellent blankability, and excellent flangeability. However, fatigue strength, which is a matter of concern in the present application, is not referred to, which means that whether good fatigue strength is necessarily achieved is unknown.
As described, in the related art, technologies have not been established for manufacturing a high strength hot rolled steel sheet having a tensile strength of 980 MPa or greater and having excellent ductility, fatigue strength, and blankability.
Accordingly, an object of the present invention is to provide a high strength hot rolled steel sheet having a tensile strength of 980 MPa or greater and having excellent ductility, fatigue strength, and blankability and to provide a method for manufacturing the same.
In the present invention, "excellent ductility" refers to cases where a uniform elongation is 6.0% or greater. "Excellent fatigue strength" refers to cases where a ratio of plane bending fatigue strength (fatigue limit ratio) with respect to the tensile strength is 0.50 or greater in a plane bending test with 2×10⁶ cycles, performed at a stress ratio of -1. "Excellent blankability" refers to cases where, after blanking is performed with a 10 mmφ punch, with at least three clearances including 10% and 20% within a clearance range of 10 to 20%, an edge surface of each blanked hole does not have cracks, flaws, a brittle fracture surface, or a secondary shear plane.

Solution to Problem

To achieve the above-described object, the inventors diligently conducted studies to improve the ductility, fatigue strength, and blankability of a hot rolled steel sheet while ensuring a tensile strength of 980 MPa or greater of the steel sheet. As a result, the following discoveries were made. Bainite should be used as a primary phase, and area fractions of upper bainite and granular bainite, which is soft and forms at or near a Bs temperature, should be controlled to be within particular ranges. In addition, the area fraction of fresh martensite and retained austenite phases, which constitute a hard secondary phase, should be controlled to be less than 15%. Furthermore, a difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 should be 30% or less with respect to 0.3TS (MPa) in order to reduce segregation, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position. In addition, an average grain size of the upper bainite should be 7 µm or less. With these conditions, it is possible to obtain a steel sheet having excellent ductility, fatigue strength, and blankability, while ensuring a tensile strength of 980 MPa or greater.
As referred to herein, an "upper bainite phase" is an assembly of lath bainitic ferrite. Specifically, the upper bainite is a microstructure containing an Fe-based carbide and/or a retained austenite phase between laths of lath bainitic ferrite.
Lath bainitic ferrite has a lath shape with a relatively high dislocation density in its interior, unlike lamellar (layered) ferrite in pearlite or polygonal ferrite. Accordingly, lath bainitic ferrite can be distinguished from them by using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).
If the upper bainite includes a retained austenite phase between laths, the portion of lath bainitic ferrite is solely regarded as the upper bainite and is distinguished from the retained austenite phase. "Fresh martensite" is martensite that does not contain Fe-based carbides.
"Granular bainite" is a microstructure formed of granular bainitic ferrite and containing an Fe-based carbide and/or a retained austenite phase between grains of the granular bainitic ferrite. Furthermore, the granular bainite has an aspect ratio of 2 or less, and, regarding an <axial direction>-rotation angle relationship, the grains of the granular bainite have grain boundaries that do not have the following orientational relationships: <15 16 18>-60°, <6 15 15>-56°, <6 7 7>-50°, <1 0 1>-60°, and <1 2 2>-17°. The crystal orientation can be determined by electron backscatter diffraction patterns (EBSD). Fresh martensite and/or retained austenite phases have a low sharpness (IQ value). Since prior austenite grains have a high aspect ratio and are large, grains having an aspect ratio of 2 or greater and a size of 10 µm or greater are deemed to be prior austenite grains in the present study. Accordingly, grains having a high IQ value and having an aspect ratio of 2 or less and a size of 10 µm or less, among grains having a misorientation of 15° or greater, from which upper bainite grains are excluded, are deemed to constitute the granular bainite.
Fresh martensite and/or retained austenite phases exhibit a brighter contrast than the upper bainite phase, lower bainite and/or tempered martensite phases, and a polygonal ferrite phase, in SEM images. Accordingly, fresh martensite and/or retained austenite phases can be distinguished from those microstructure by using an SEM.
The fresh martensite phase and the retained austenite phase exhibit similar contrasts as observed under an SEM but can be distinguished from each other by using electron backscatter diffraction patterns.
The present invention was made based on the above-described discovery and with further studies that were conducted. A summary of the present invention is as follows.

[1] A high strength steel sheet having a chemical composition containing, in mass%, C: 0.04 to 0.18%, Si: 0.1 to 2.5%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.010 to 1.0%, and N: 0.01% or less, with the balance being Fe and unavoidable impurities, wherein the high strength steel sheet includes a microstructure including a primary phase and a hard secondary phase, the primary phase being a bainite phase that is formed of upper bainite and granular bainite and being present in an area fraction of 85% or greater, the hard secondary phase including fresh martensite and/or retained austenite phases and being present in an area fraction of less than 15%, in the bainite phase, the area fraction of the granular bainite with respect to the area fraction of the upper bainite is 5% or greater and 40% or less, a difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 is 30% or less with respect to 0.3TS, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position, and the upper bainite phase has an average grain size of 7 µm or less.
[2] The high strength steel sheet according to [1], wherein the chemical composition further includes, in mass%, at least one of groups a to e: group a: one or two or more selected from V: 0.005 to 0.5%, Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.1%, group b: one or two or more selected from Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, and Mo: 0.005 to 0.5%, group c: B: 0.0002 to 0.005%, group d: one or two selected from Sb: 0.001 to 0.1%, and Sn: 0.001 to 0.1%, and group e: one or two or more selected from Ca: 0.0001 to 0.005%, Mg: 0.0001 to 0.005%, and REM: 0.0001 to 0.005%.
[3] A method for manufacturing the high strength steel sheet according to [1] or [2] including: casting a molten steel material having the chemical composition, the casting including rotating the molten steel material with an inductive electromagnetic stirring device at a rotational speed of 10 cm/s or greater in a horizontal plane with respect to a casting mold; heating a resulting steel material to 1150°C or greater; subjecting the heated steel material to rough rolling to form a steel sheet; subjecting the steel sheet to finish rolling under conditions including a finish rolling delivery temperature of (RC - 50)°C or greater and (RC + 100)°C or less; cooling the finish-rolled steel sheet under conditions including a time between completion of the finish rolling and start of the cooling of 2.0 s or less, an average cooling rate before Bs is reached of 30 °C/s or greater, which is a cooling rate on a surface, a residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less of 3.0 s or more and 10.0 s or less, and a cooling stop temperature of (Bs - 250)°C or greater and (Bs - 100) °C or less; coiling the cooled steel sheet under conditions including a coiling temperature of (Bs - 250) °C or greater and (Bs - 100) °C or less; and cooling a resultant to a temperature of (Bs - 400) °C or less at an average cooling rate of 1°C/s or less, where RC and Bs are defined by equations (1) and (2) below, respectively: RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu where, in equations (1) and (2), each of the element symbols represents a content (mass%) of the element, and the element symbol of an element that is not included represents 0.

Advantageous Effects of Invention

The present invention can provide a high strength steel sheet having a tensile strength of 980 MPa or greater and having excellent ductility, fatigue strength, and blankability and can also provide a method for manufacturing the same.
In cases where the high strength steel sheet of the present invention is used in automotive chassis components, such as suspensions, or in structural parts, frame parts, or truck frame parts, safety can be ensured, weight reduction of the automobile bodies can be realized, and, therefore, industrially significant effects are produced.

Brief Description of Drawings

[Fig. 1] Fig. 1 is a schematic diagram illustrating a shape of a test specimen for a plane bending fatigue test in the present invention.

Description of Embodiments

Embodiments of the present invention will be described below. The description below illustrates examples of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.
A steel sheet has the following chemical composition. In the description below, the unit "%" used to indicate a content of an element in the chemical composition is "mass%" unless otherwise specified.

<C: 0.04 to 0.18%>

C promotes a formation of bainite by improving hardenability and is, therefore, an element effective for improving strength. Furthermore, C lowers a Bs temperature, thereby causing bainite transformation to occur at a lower temperature, and, consequently, inhibits a formation of granular bainite. When a C content is less than 0.04%, these effects cannot be sufficiently produced, which makes it impossible to achieve a tensile strength of 980 MPa or greater. Accordingly, the C content is to be 0.04% or greater and is preferably 0.05% or greater and more preferably 0.06% or greater. On the other hand, when the C content is greater than 0.18%, an excessive amount of a hard secondary phase including fresh martensite and/or retained austenite phases is formed, which makes it impossible to achieve sufficient fatigue strength. Accordingly, the C content is to be 0.18% or less and is preferably 0.17% or less and more preferably 0.15% or less.

<Si: 0.1 to 2.5%>

Si contributes to improving a strength of steel through solid solution strengthening of the steel. Accordingly, a Si content is to be 0.1% or greater and is preferably 0.3% or greater and more preferably 0.5% or greater. Si is also an element that promotes a formation of ferrite, and, therefore, when the Si content is greater than 2.5%, ferrite is formed, which decreases fatigue strength. Accordingly, the Si content is to be 2.5% or less and is preferably 2.3% or less and more preferably 2.0% or less.

<Mn: 1.0 to 3.0%>

Mn is an element that stabilizes austenite and is, therefore, an element effective for improving strength by inhibiting the formation of ferrite. Furthermore, Mn lowers a Bs temperature, thereby causing bainite transformation to occur at a lower temperature, and, consequently, inhibits the formation of granular bainite. When a Mn content is less than 1.0%, these effects cannot be sufficiently produced, and, consequently, ferrite and granular bainite are formed, which makes it impossible to achieve a tensile strength of 980 MPa or greater and excellent fatigue strength. Accordingly, a Mn content is to be 1.0% or greater and is preferably 1.2% or greater and more preferably 1.5% or greater. On the other hand, when the Mn content is greater than 3.0%, the hard secondary phase including fresh martensite and/or retained austenite phases increases, which makes it impossible to achieve sufficient fatigue strength. Accordingly, the Mn content is to be 3.0% or less and is preferably 2.8% or less and more preferably 2.5% or less.

<P: 0.1% or less>

P degrades weldability, and, therefore, an amount of P is desirably reduced as much as possible. In the present invention, a P content of up to 0.1% is permissible. Accordingly, the P content is to be 0.1% or less. While the lower limit is not particularly limited, when the P content is less than 0.003%, the cost of refining increases, and, accordingly, the P content is preferably 0.003% or greater and more preferably 0.005% or greater.

<S: 0.01% or less>

S degrades weldability, and, therefore, an amount of S is preferably reduced as much as possible; however, in the present invention, a S content of up to 0.01% is permissible. Accordingly, the S content is to be 0.01% or less. While the lower limit is not particularly limited, when the S content is less than 0.0001%, production efficiency decreases, and, accordingly, the S content is preferably 0.0001% or greater and more preferably 0.0005% or greater.

<Al: 0.010 to 1.0%>

Al acts as a deoxidizing agent and is, therefore, an element effective for improving a cleanliness of steel. When an amount of Al is excessively small, the effect is not sufficiently produced. Accordingly, an Al content is to be 0.010% or greater and is preferably 0.015% or greater and more preferably 0.020% or greater. Al is also an element that promotes a formation of ferrite, and, therefore, when the Al content is greater than 1.0%, ferrite is formed, which decreases fatigue strength. Accordingly, the Al content is to be 1.0% or less and is preferably 0.8% or less and more preferably 0.5% or less.

<N: 0.01% or less>

N combines with a nitride-forming element and, thus, precipitates as a nitride, thereby contributing to refining grains. However, N tends to form a coarse nitride by combining with Ti at a high temperature, and, therefore, including an excessive amount of N decreases fatigue strength. Accordingly, a N content is to be 0.01% or less and is preferably 0.008% or less and more preferably 0.006% or less. While the lower limit is not particularly limited, when the N content is less than 0.001%, production efficiency decreases, and, accordingly, the N content is preferably 0.001% or greater.
The balance is Fe and unavoidable impurities.
The components described above constitute a basic chemical composition of the high strength steel sheet of the present invention. Furthermore, if necessary, any of the following elements may be included.
V, Ti, and Nb form carbides to accomplish precipitation strengthening and are, therefore, elements effective for improving strength. Accordingly, in cases where V, Ti, and/or Nb are included, it is preferable that a V content be 0.005 to 0.5%, a Ti content be 0.005 to 0.2%, and a Nb content be 0.005 to 0.1%. When the contents of V, Ti, and Nb are greater than the respective upper limits, carbides may become coarse, which may degrade blankability. Accordingly, the V content is preferably 0.05% or greater and more preferably 0.1% or greater and is more preferably 0.3% or less. The Ti content is more preferably 0.01 or greater and 0.1% or less. The Nb content is more preferably 0.01 or greater and 0.08% or less.
Cr, Ni, Cu, and Mo are elements that stabilize austenite and are, therefore, elements effective for improving strength by inhibiting the formation of ferrite. Furthermore, Cr, Ni, Cu, and Mo lower the Bs temperature, thereby causing bainite transformation to occur at a lower temperature, and, consequently, inhibit the formation of granular bainite. Accordingly, in cases where Cr, Ni, Cu, and/or Mo are included, it is preferable that a Cr content be 0.005 to 1.0%, a Ni content be 0.005 to 0.5%, a Cu content be 0.005 to 0.5%, and a Mo content be 0.005 to 0.5%. When the contents of Cr, Ni, Cu, and Mo are greater than the respective upper limits, an excessive amount of the hard secondary phase including fresh martensite and/or retained austenite phases is formed, which may make it impossible to obtain the steel microstructure of the present invention. The Cr content is preferably 0.01% or greater and more preferably 0.3% or greater and is more preferably 0.8% or less. The Ni content is preferably 0.01% or greater and more preferably 0.05% or greater and is more preferably 0.3% or less. The Cu content is preferably 0.01% or greater and more preferably 0.05% or greater and is more preferably 0.3% or less. The Mo content is preferably 0.01% or greater and more preferably 0.05% or greater and is more preferably 0.3% or less.
In cases where B is included, it is preferable that an amount of B be 0.0002 to 0.005%. B segregates at austenite grain boundaries and inhibits the formation of ferrite, thereby further promoting the formation of upper bainite and granular bainite, and is, therefore, an element effective for further improving the strength of the steel sheet. It is preferable that a B content be 0.0002% or greater so that the effects can be produced. Accordingly, the B content is to be 0.0002% or greater and is preferably 0.0005% or greater and more preferably 0.0007% or greater. When the B content is greater than 0.005%, however, the effects no longer increase. Accordingly, the B content is to be 0.005% or less and is preferably 0.004% or less and more preferably 0.003% or less.
Sb inhibits the removal of elements from a surface of a steel material during the heating of the steel material and is, therefore, an element effective for inhibiting a decrease in the strength of steel. Accordingly, in cases where Sb is included, it is preferable that a content of Sb be 0.001 to 0.1%. When the Sb content is greater than the upper limit, the steel sheet may become brittle. The Sb content is more preferably 0.005% or greater and 0.05% or less.
Sn inhibits the formation of pearlite and is, therefore, an element effective for inhibiting a decrease in the strength of steel. In cases where Sn is included, it is preferable that a content of Sn be 0.001 to 0.1% so that the effect can be produced. When the Sn content is greater than the upper limit, the steel sheet may become brittle. The Sn content is more preferably 0.005% or greater and 0.05% or less.
Ca, Mg, and REM control a shape of inclusions and are, therefore, elements effective for improving blankability. In cases where Ca, Mg, and/or REM are included, it is preferable that a Ca content be 0.0001 to 0.005%, a Mg content be 0.0001 to 0.005%, and a REM content be 0.0001 to 0.005% so that the effect can be produced. When the Ca content and the REM content are greater than the upper limits, an amount of inclusions may increase, which may degrade fatigue strength. The Ca content is more preferably 0.0005% or greater and 0.003% or less. The Mg content is more preferably 0.0005% or greater and 0.003% or less. The REM content is more preferably 0.0005% or greater and 0.003% or less. "REM (rare earth elements)" is a generic term that refers to Sc, Y, and 15 elements ranging from lanthanum (La, atomic number 57) to lutetium (Lu, atomic number 71), and the "REM content" as referred to herein is a total content of these elements. Regarding each of the components (V, Ti, Nb, Cr, Ni, Cu, Mo, B, Sb, Sn, Ca, Mg, and REM) of groups a to e, when its content is less than the lower limit mentioned above, the component is deemed to be present as an unavoidable impurity.
Now, a microstructure of the high strength steel sheet of the present invention will be described.
The high strength steel sheet of the present invention includes the following microstructure. A primary phase is a bainite phase that is formed of upper bainite and granular bainite and is present in an area fraction of 85% or greater. A hard secondary phase including fresh martensite and/or retained austenite phases is present in an area fraction of less than 15%. In the bainite phase, the area fraction of the granular bainite with respect to the area fraction of the upper bainite is 5% or greater and 40% or less, and the upper bainite phase has an average grain size of 7 µm or less. A difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 is 30% or less with respect to 0.3TS, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position.

The microstructure of the high strength steel sheet of the present invention includes, as a primary phase, a bainite phase formed of upper bainite and granular bainite. When the area fraction of the bainite phase is less than 85%, excellent blankability cannot be achieved. Accordingly, the area fraction of the bainite phase is to be 85% or greater and is preferably 90% or greater and more preferably 95% or greater.

The hard secondary phase is controlled to be formed of fresh martensite and/or retained austenite phases and is present in an area fraction of less than 15%. Because of this, macroscopic stress concentration does not generate at phase interfaces and is, thus, prevented from acting as fatigue fracture origins, during a fatigue test, and, consequently, excellent fatigue strength can be achieved. Accordingly, the area fraction of the hard secondary phase including fresh martensite and/or retained austenite phases is to be less than 15% and is preferably 10% or less and more preferably 5% or less.
The area fraction of the remaining microstructure, other than the upper bainite, granular bainite, fresh martensite phase, or retained austenite phase, may be 3% or less, which is permissible. Examples of the remaining microstructure include lower bainite, ferrite, tempered martensite, and pearlite.

Granular bainite has better ductility than upper bainite. When the area fraction of the granular bainite with respect to the area fraction of the upper bainite in the bainite phase is less than 5%, excellent ductility cannot be achieved. Accordingly, the area fraction of the granular bainite with respect to the area fraction of the upper bainite in the bainite phase is to be 5% or greater and is preferably 8% or greater and more preferably 10% or greater. On the other hand, when the area fraction of the granular bainite is greater than 40% with respect to the area fraction of the upper bainite, fatigue strength is significantly degraded. Accordingly, the area fraction of the granular bainite with respect to the area fraction of the upper bainite is to be 40% or less and is preferably 35% or less and more preferably 30% or less.

Grain boundaries have an effect of making it difficult for slip deformation to propagate to adjacent grains and, consequently, can improve fatigue strength. Accordingly, the average grain size of the upper bainite phase is to be 7 µm or less and is preferably 6 µm or less. While the lower limit is not particularly limited, when the average grain size is excessively small, ductility may decrease, and, accordingly, the average grain size is preferably 1 µm or greater and more preferably 2 µm or greater.

When segregation of a component such as Mn becomes excessive at the 1/2 thickness position, the hard secondary phase including fresh martensite and/or retained austenite phases is locally formed, which results in a difference in hardness from that of the base microstructure, and, consequently, the likelihood of fracturing and roughening during blanking increases. When the difference |Hv1 - Hv2| between the hardness Hv1 at the 1/2 thickness position and the hardness Hv2 at the 1/4 thickness position is greater than 30% with respect to 0.3TS, blankability significantly decreases. Accordingly, the difference |Hv1 - Hv2| between the hardness Hv1 at the 1/2 thickness position and the hardness Hv2 at the 1/4 thickness position is to be 30% or less and is preferably 25% or less and more preferably 20% or less, with respect to 0.3TS. The lower limit need not be particularly limited and may be 0%. "TS" refers to the tensile strength. The hardnesses Hv1 and Hv2 and the TS are measured in accordance with the methods described in the Examples section.
The high strength steel sheet of the present invention has a tensile strength of 980 MPa or greater and has excellent ductility, excellent fatigue strength, and excellent blankability collectively. Because of the high tensile strength, the high strength steel sheet of the present invention can ensure safety even if the steel sheet has a reduced thickness and, therefore, can be used in components of trucks and automobiles. Note that in the present invention, the values of the area fractions of the microstructure and mechanical properties are values measured by the methods described in the Examples section.
Next, a method for manufacturing the high strength steel sheet, according to an embodiment of the present invention, will be described. In the description below, the symbol "°C" regarding temperatures indicates a surface temperature of an object (steel material or steel sheet) unless otherwise specified.
The high strength steel sheet of the present invention can be manufactured by successively performing processes (1) to (6), described below. Each of the processes will be described below.

(1) Steelmaking and Casting
(2) Heating
(3) Hot Rolling
(4) Cooling (first cooling)
(5) Coiling
(6) Cooling (second cooling)

(1) Steelmaking and Casting

Steelmaking for preparing a molten steel material can be performed by any method. For example, molten steel having the chemical composition described above can be prepared by performing steelmaking with a known method, for example, in a converter, to provide the molten steel material. The raw materials to be used may include scrap metals. The obtained molten steel material is subjected to a continuous casting process to form a steel material. Inductive electromagnetic stirring has an effect of reducing segregation of components that are formed during solidification, thereby promoting the formation of granular bainite; this effect is produced because columnar crystals that are grown in a casting mold are caused to be split, and consequently equiaxed crystals are formed at a region extending from the 1/4 thickness position to the 1/2 thickness position. When a rotational speed for the electromagnetic stirring is less than 10 cm/s, columnar crystals cannot be sufficiently split, and, therefore, the effect is not produced. Accordingly, the rotational speed is to be 10 cm/s or greater and is preferably 15 cm/s or greater and more preferably 20 cm/s or greater, in a horizontal plane.
While the upper limit of the rotational speed for the electromagnetic stirring is not particularly limited, the rotational speed is preferably 50 cm/s or less, more preferably 45 cm/s or less, and more preferably 40 cm/s or less, for a reason associated with manufacture. After the steel material has been manufactured by a continuous casting process, the steel material may be directly subjected to the subsequent heating step; alternatively, the steel material may be cooled to form a warm slab or a cold slab, and this steel material may be subjected to the heating step. The chemical composition of the high strength steel sheet that is finally obtained is the same as the chemical composition of the steel material used.

(2) Heating

First, the steel material is heated to a heating temperature of 1150°C or greater. In the steel material after it has been cooled to a low temperature, substantially all precipitate-forming elements exist non-uniformly as coarse precipitates. In the case where additive elements exist as coarse, non-uniform precipitates, an effect of the additive elements cannot be sufficiently produced, which makes it impossible to obtain desired steel microstructure. Accordingly, it is necessary to heat the steel material before hot rolling to dissolve the coarse precipitates. Accordingly, the heating temperature for the steel material is to be 1150°C or greater and is preferably 1180°C or greater and more preferably 1200°C or greater. Meanwhile, when the heating temperature for the steel material is excessively high, slab defects are formed, and a decrease in yield due to scaling-off occurs. Accordingly, the heating temperature for the steel material is preferably 1350°C or less, more preferably 1300°C or less, and even more preferably 1280°C or less. Regarding the heating, it is preferable, from the standpoint of achieving a uniform temperature of the steel material, that the steel material be held at the heating temperature after the steel material has been heated to the heating temperature. While the duration of the holding at the heating temperature (holding time) is not particularly limited, it is preferable, from the standpoint of enhancing the uniformity of the temperature of the steel material, that the holding time be 1800 seconds or more. Meanwhile, when the holding time is more than 10000 seconds, an amount of scale formed increases. As a result, during the subsequent hot rolling, scale defects and the like are likely to be formed, and, consequently, a decrease in yield due to a poor surface quality occurs. Accordingly, the holding time is preferably 10000 seconds or less and more preferably 8000 seconds or less. This heating step may also be a step in which the steel material before hot rolling is directly subjected to hot rolling (hot direct rolling), with a high temperature of the steel material after casting being maintained (i.e., with a temperature within the above-mentioned range of the heating temperature being maintained).

(3) Hot Rolling

Subsequently, the steel material that has been heated (or remaining hot after casting) is subjected to hot rolling, which includes rough rolling and finish rolling. The conditions for the rough rolling are not particularly limited as long as desired transfer bar dimensions can be ensured.
First, the steel material is subjected to rough rolling to form a rough-rolled sheet. Before the formed rough-rolled sheet is subjected to finish rolling, descaling involving the spraying of high-pressure water (high-pressure water descaling) may be performed at an entry side of a finishing rolling mill.
In the present invention, then regarding the finish rolling, a finish rolling delivery temperature is to be (RC - 50)°C or greater and (RC + 100)°C or less, where RC is a temperature defined by equation (1) below. RC is a lower-limit temperature for austenite recrystallization, which is estimated from the chemical composition. When the finish rolling delivery temperature is less than (RC - 50)°C, strain relaxation due to recrystallization is unlikely to occur, therefore, the process proceeds to cooling in a state in which strain has accumulated in austenite, and consequently, the formation of granular bainite is excessively promoted. Accordingly, the finish rolling delivery temperature is to be (RC - 50)°C or greater and is preferably (RC - 30)°C or greater and more preferably RC °C or greater.
On the other hand, when the finish rolling delivery temperature is greater than (RC + 100)°C, austenite grains become coarse, and, consequently, the average grain size of the upper bainite is increased, which makes it impossible to achieve sufficient fatigue strength. Accordingly, the finish rolling delivery temperature is to be (RC + 100)°C or less and is preferably (RC + 80)°C or less and more preferably (RC + 50)°C or less. RC is defined by equation (1) below. RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V
In equation (1), each of the element symbols represents a content (mass%) of the element, and the element symbol of an element that is not included represents 0.

(4) Cooling (first cooling)

Subsequently, the resulting hot rolled steel sheet is cooled (first cooling). In this process, the time between the completion of the hot rolling and the start of the cooling (cooling start time) is to be 2.0 s or less after the completion of the finish rolling. When the cooling start time is greater than 2.0 s, austenite grains become coarse, and, consequently, the average grain size of the upper bainite is increased, which makes it impossible to achieve sufficient tensile strength and fatigue strength. Accordingly, the cooling start time is to be 2.0 s or less and is preferably 1.5 s or less and more preferably 1.0 s or less.
Furthermore, an average cooling rate over a range of the finish rolling delivery temperature to Bs is to be 30°C/s or greater, which is a cooling rate on a surface. When the average cooling rate over a range of the finish rolling delivery temperature to Bs is excessively low, an excessive amount of ferrite is formed, which makes it impossible to achieve sufficient tensile strength and fatigue strength. Accordingly, the average cooling rate is to be 30°C/s or greater and is preferably 40°C/s or greater and more preferably 50°C/s or greater. While the upper limit is not particularly limited, when the average cooling rate is excessively high, it becomes difficult to manage a cooling stop temperature. Accordingly, the average cooling rate is preferably 500°C/s or less and is more preferably 300°C/s or less, and even more preferably 150°C/s or less.
Furthermore, regarding the cooling, a residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less is to be 3.0 s or more and 10.0 s or less. When the residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less is less than 3.0 s, a sufficient amount of granular bainite cannot be obtained. Accordingly, the residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less is to be 3.0 s or more and is preferably 4.0 s or more and more preferably 5.0 s or more. When the residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less is greater than 10.0 s, an excessive amount of granular bainite is formed, which makes it impossible to achieve sufficient fatigue strength. Accordingly, the residence time for a temperature of (Bs - 100) °C or greater and Bs °C or less is to be 10.0 s or less and is preferably 9.0 s or less and more preferably 8.0 s or less.
Furthermore, regarding the cooling, forced cooling may be performed such that the average cooling rate can be achieved, and methods for the cooling are not particularly limited. The cooling stop temperature is to be (Bs - 250)°C or greater and (Bs - 100)°C or less. When the cooling stop temperature is less than (Bs - 250)°C, a microstructure of lower bainite is formed. All types of lower bainite are microstructure having high strength but have poor ductility. Accordingly, the cooling stop temperature is to be (Bs - 250)°C or greater and is preferably (Bs - 220)°C or greater and more preferably (Bs - 200)°C or greater. On the other hand, when the cooling stop temperature is greater than (Bs - 100)°C, an excessive amount of granular bainite is formed, which makes it impossible to achieve excellent fatigue strength. Accordingly, the cooling stop temperature is to be (Bs - 100)°C or less and is preferably (Bs - 120)°C or less and more preferably (Bs - 150)°C or less. Bs is defined by equation (2) below. Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu
In equation (2), each of the element symbols represents a content (mass%) of the element, and the element symbol of an element that is not included represents 0.

(5) Coiling

Subsequently, the cooled hot rolled steel sheet is coiled under conditions including a coiling temperature of (Bs - 250)°C or greater and (Bs - 100)°C or less. When the coiling temperature is less than (Bs - 250)°C, lower bainite and/or tempered martensite are formed, which results in insufficient ductility. Accordingly, the coiling temperature is to be (Bs - 250)°C or greater and is preferably (Bs - 230)°C or greater and more preferably (Bs - 200)°C or greater. On the other hand, when the coiling temperature is greater than (Bs - 100)°C, an excessive amount of granular bainite is formed, which makes it impossible to achieve excellent fatigue strength. Accordingly, the coiling temperature is to be (Bs - 100)°C or less and is preferably (Bs - 120)°C or less and more preferably (Bs - 150)°C or less.

(6) Cooling (second cooling)

Subsequently, a resultant is cooled to a temperature of (Bs - 400)°C or less at an average cooling rate of 1°C/s or less (second cooling). When the average cooling rate over the range of the coiling temperature to (Bs - 400)°C or less is greater than 1°C/s, lower bainite and/or tempered martensite are formed, which results in insufficient ductility. Accordingly, the average cooling rate over the range of the coiling temperature to (Bs - 400)°C or less is to be 1°C/s or less and is preferably 0.8°C/s or less and more preferably 0.5°C/s or less. There are no reasons for particularly limiting the lower limit of the average cooling rate, and it is preferable, from the standpoint of manufacturing efficiency and the coarsening of carbides, that the average cooling rate be 0.001°C/s or greater. The temperature to be achieved by the cooling may be any temperature less than or equal to (Bs - 400)°C and is preferably 50°C. The cooling may be performed in any manner, and, for example, the coiled coil may be cooled in that state.
With the procedure described above, the high strength steel sheet of the present invention can be manufactured. Processes after the coiling and the subsequent cooling may be performed in accordance with a common procedure. For example, skin pass rolling may be performed, and pickling may be performed to remove scales formed on the surface.

EXAMPLES

Molten steels having the composition shown in Table 1 were prepared by steelmaking in a converter and then cast with a continuous casting process that used the rotational speed shown in Table 2. In this manner, steel slabs as steel materials were manufactured. The resulting steel materials were heated to the heating temperature shown in Table 2, and subsequently, the heated steel materials were subjected to hot rolling, which included rough rolling and finish rolling, to form hot rolled steel sheets. The finish rolling delivery temperature for the hot rolling was as shown in Table 2. Subsequently, the resulting hot rolled steel sheets were cooled under conditions including the average cooling rate and the cooling stop temperature shown in Table 2 (first cooling). The cooled hot rolled steel sheets were coiled at the coiling temperature shown in Table 2, and the coiled steel sheets were cooled at the average cooling rate shown in Table 2 (second cooling). In this manner, high strength steel sheets were obtained. After the cooling, skin pass rolling was performed, and pickling was performed. The pickling was performed at a temperature of 85°C with an aqueous hydrochloric acid solution having a concentration of 10 mass%.

[Table 1]

Steel Grade	Chemical Composition (mass%) *								RC (°C)	Bs (°C)	RC+100 (°C)	RC-50 (°C)	Bs-100 (°C)	Bs-250 (°C)	Bs-400 (°C)	Notes
Steel Grade	C	Si	Mn	P	S	Al	N	Others	RC (°C)	Bs (°C)	RC+100 (°C)	RC-50 (°C)	Bs-100 (°C)	Bs-250 (°C)	Bs-400 (°C)	Notes
A	0.043	1.82	1.75	0.033	0.0010	0.042	0.0077	-	773	661	873	723	561	411	261	Conforming Steel
B	0.177	0.87	2.48	0.045	0.0008	0.586	0.0041	-	796	559	896	746	459	309	159	Conforming Steel
C	0.108	2.44	1.92	0.022	0.0062	0.061	0.0091	-	783	628	883	733	528	378	228	Conforming Steel
D	0.137	1.09	2.93	0.010	0.0018	0.089	0.0045	Ti: 0.12, Mo: 0.21, B: 0.0005, Sn: 0.07	839	512	939	789	412	262	112	Conforming Steel
E	0.065	0.14	1.52	0.015	0.0013	0.012	0.0035	Cr: 0.71, Nb: 0.02, Ti: 0.01, Mg: 0.002	798	626	898	748	526	376	226	Conforming Steel
F	0.076	1.20	1.04	0.015	0.0010	0.045	0.0045	Ni: 0.24, Cr: 0.91, Nb: 0.06, Cu: 0.11	824	641	924	774	541	391	241	Conforming Steel
G	0.162	0.33	2.32	0.092	0.0034	0.121	0.0035	Mo: 0.33, V: 0.41, Cu: 0.22, Ca: 0.003	871	546	971	821	446	296	146	Conforming Steel
H	0.098	1.22	1.89	0.022	0.0089	0.134	0.0055	Cr: 0.41, Nb: 0.03, Ti: 0.08, B: 0.0041	848	605	948	798	505	355	205	Conforming Steel
I	0.106	0.92	2.62	0.011	0.0021	0.867	0.0040	Mo: 0.11, Ni: 0.41, V: 0.09, REM: 0.002	808	541	908	758	441	291	141	Conforming Steel
J	0.064	0.73	1.83	0.031	0.0006	0.050	0.0029	Ti: 0.08, Cr: 0.52, B: :0.0020, Sb: 0.018	811	612	911	761	512	362	212	Conforming Steel
a	0.033	0.87	1.03	0.029	0.0012	0.031	0.0044	-	765	727	865	715	627	477	327	Comparative Steel
b	0.187	1.14	2.13	0.012	0.0039	0.028	0.0035	Cr: 0.57, Ti: 0.07, B: 0.0031	833	548	933	783	448	298	148	Comparative Steel
c	0.094	2.62	1.64	0.025	0.0031	0.045	0.0055	Ni: 0.05, Ti: 0.017, Mo: 0.15, REM: 0.0004	790	643	890	740	543	393	243	Comparative Steel
d	0.083	0.99	3.12	0.013	0.0012	0.052	0.0037	Nb: 0.08, B: 0.0011, Ca: 0.0011	857	527	957	807	427	277	127	Comparative Steel
e	0.062	1.41	0.88	0.020	0.0015	0.065	0.0042	Mo: 0.29, B: 0.0024	793	710	893	743	610	460	310	Comparative Steel
f	0.071	0.67	2.88	0.009	0.0015	1.140	0.0062	Mo: 0.18, V: 0.27, Cu: 0.08, Mg: 0.0007	837	535	937	787	435	285	135	Comparative Steel

The underline means "outside the scope of the invention".
* The balance is Fe and unavoidable impurities.

[Table 2]

No.	Steel Grade	Manufacturing Conditions										Notes
		Casting	Heating	Hot Rolling	First Cooling				Coiling	Second Cooling
		Rotational Speed (cm/s)	Heating Temperature (°C)	Finish Rolling Delivery Temperature (°C)	Time Between Completion of Hot Rolling and Start of Cooling (s)	Average Cooling Rate (°C/s)	Residence Time for Temperature of (Bs-100)°C or Greater and Bs °C or Less (s)	Cooling Stop Temperature (°C)	Coiling Temperature (°C)	Average Cooling Rate (°C/s)	Cooling Stop Temperature (°C)
1	A	15	1250	830	0.2	35	4.5	515	515	<0.5	80	Inventive example
2	A	15	1250	900	0.2	35	4.5	515	515	<0.5	80	Comparative Example
3	A	15	1250	700	0.2	35	4.5	515	515	<0.5	80	Comparative Example
4	B	20	1200	860	0.4	60	5.5	430	430	<0.5	85	Inventive example
5	B	20	1200	860	2.2	60	5.5	430	430	<0.5	85	Comparative Example
6	B	20	1200	860	0.4	25	5.5	430	430	<0.5	85	Comparative Example
7	C	17	1250	800	0.7	40	4.0	465	465	<0.5	120	Inventive example
8	C	17	1250	800	0.7	40	1.5	465	465	<0.5	120	Comparative Example
9	C	17	1250	800	0.7	40	11.0	465	465	<0.5	120	Comparative Example
10	D	22	1180	850	0.8	80	6.0	450	450	<0.5	55	Inventive example
11	D	22	1180	850	0.8	80	6.0	420	420	<0.5	55	Comparative Example
12	D	22	1180	850	0.8	80	6.0	250	250	<0.5	55	Comparative Example
13	E	13	1230	810	0.1	75	5.0	420	420	<0.5	40	Inventive example
14	E	8	1230	810	0.1	75	5.0	420	420	<0.5	40	Comparative Example
15	F	15	1220	785	1.0	70	3.5	450	450	<0.5	80	Inventive example
16	F	15	1220	785	1.0	70	3.5	450	450	2	80	Comparative Example
17	G	11	1220	930	0.6	85	4.0	400	400	<0.5	55	Inventive example
18	H	17	1190	890	1.4	35	7.5	460	460	<0.5	40	Inventive example
19	I	23	1220	820	0.5	50	6.0	390	390	<0.5	65	Inventive example
20	J	18	1215	815	0.8	55	8.0	430	430	<0.5	50	Inventive example
21	a	19	1230	820	1.6	70	9.5	510	510	<0.5	85	Comparative Example
22	b	28	1200	880	1.8	60	4.5	380	380	<0.5	75	Comparative Example
23	c	26	1250	835	1.0	55	5.5	415	415	<0.5	70	Comparative Example
24	d	16	1250	905	0.6	65	4.0	375	375	<0.5	30	Comparative Example
25	e	13	1230	840	0.8	40	6.5	555	555	<0.5	55	Comparative Example
26	f	27	1260	870	1.4	45	7.0	360	360	<0.5	70	Comparative Example

The underline means "outside the scope of the invention".

Test specimens were cut from the obtained high strength steel sheets, and their microstructure and mechanical properties were evaluated with the procedures described below.

Test specimens for microstructure observation were cut from the obtained high strength steel sheets such that a thickness-wise cross section parallel to a rolling direction could serve as an observation surface. A surface of the prepared test specimens was polished, and the surface was then etched with an etchant (3% nital solution) to reveal microstructure. Subsequently, regions of 10 fields of view at a 1/4 thickness position were taken by using a scanning electron microscope (SEM) at a magnification of 5000×, and thus, SEM images of microstructure were obtained. The obtained SEM images were analyzed by image processing to quantify the area fractions of upper bainite (UB), polygonal ferrite (F), and lower bainite and/or tempered martensite (LB+TM). Regarding upper bainite (UB), granular bainite (GB), fresh martensite (FM), and a retained austenite phase (γ), since they are difficult to distinguish from one another with an SEM, they were identified by using electron backscatter diffraction patterns, and the area fraction of each of them and the average grain size were determined. The measured area fractions of the respective microstructure and the average grain size of the upper bainite are shown in Table 3. Table 3 also shows a total area fraction (B) of upper bainite and granular bainite and a total area fraction (FM+γ) of fresh martensite and a retained austenite phase.

[Table 3]

No.	Steel Grade	Microstructure												Tensile Strength (MPa)	Uniform Elongation (%)	Plane Bending Fatigue Strength for 2×10⁶ Cycles σw (MPa)	Fatigue Limit Ratio ow/TS (-)	Blankability	Notes
		Area Fraction (%)								Area Fraction of Granular Bainite with Respect to Area Fraction of Upper Bainite (%)	Average Grain Size of Upper Bainite Phase (µm)	\|Hv1-Hv2\| (Hv)	Ratio of Difference in Hardness with Respect to [0.3×Tensile Strength (MPa)] (%)
		UB	GB	B	FM	γ	FM+γ	F	LB+TM		Average Grain Size of Upper Bainite Phase (µm)	\|Hv1-Hv2\| (Hv)
1	A	81	9	90	8	2	10	0	0	10	5.4	31	9	1137	7.6	591	0.52	○	Inventive example
2	A	79	9	88	11	1	12	0	0	10	9.5	31	10	991	7.2	446	0.45	○	Comparative Example
3	A	40	49	89	10	1	11	0	0	55	1.2	36	12	1022	6.7	419	0.41	○	Comparative Example
4	B	79	14	93	5	2	7	0	0	15	4.9	12	3	1188	7.1	701	0.59	○	Inventive example
5	B	71	12	83	13	4	17	0	0	15	7.6	31	10	1005	8.0	392	0.39	○	Comparative Example
6	B	50	9	59	8	1	9	32	0	15	6.3	26	9	967	8.3	454	0.47	×	Comparative Example
7	C	75	19	94	5	1	6	0	0	20	1.8	23	8	1021	7.1	572	0.56	○	Inventive example
8	C	87	2	89	9	2	11	0	0	2	4.5	47	13	1182	5.7	650	0.55	○	Comparative Example
9	C	49	40	89	6	5	11	0	0	45	4.8	35	12	1013	8.1	557	0.48	○	Comparative Example
10	D	92	7	99	1	0	1	0	0	7	5.3	51	17	1006	7.8	486	0.58	○	Inventive example
11	D	43	52	95	2	3	5	0	0	55	5.6	49	15	1102	6.7	583	0.37	○	Comparative Example
12	D	42	3	45	0	0	0	0	55	7	5.2	12	3	1256	4.8	408	0.57	×	Comparative Example
13	E	81	11	92	3	5	8	0	0	12	6.3	54	18	991	8.7	716	0.60	○	Inventive example
14	E	86	1	87	8	5	13	0	0	1	5.2	91	31	987	5,2	582	0.59	×	Comparative Example
15	F	78	12	90	2	8	10	0	0	13	5.1	27	9	999	7.1	716	0.61	○	Inventive example
16	F	44	7	51	6	5	11	0	38	13	4.6	40	12	1068	4.9	619	0.58	×	Comparative Example
17	G	65	25	90	7	3	10	0	0	28	2.9	52	17	1023	7.7	609	0.51	○	Inventive example
18	H	60	36'	96	1	3	4	0	0	38	3.8	42	12	1163	8.6	651	0.56	○	Inventive example
19	I	81	16'	97	2	1	3	0	0	17	5.4	21	7	1056	8.7	522	0.62	○	Inventive example
20	J	83	7	90	6	4	10	0	0	8	5.9	51	15	1171	8.5	679	0.58	○	Inventive example
21	a	33	4	37	0	0	0	63	0	11	5.6	43	15	948	8.1	655	0.41	×	Comparative Example
22	b	40	32.	72	16	12	28	0	0	45	5.3	32	10	1075	8.4	484	0.45	×	Comparative Example
23	C	42	12	54	11	3	14	32	0	23	5.1	25	9	912	6.8	447	0.49	×	Comparative Example
24	d	43	19	62	21	17	38	0	0	31	4.9	15	5	1022	8.1	399	0.39	×	Comparative Example
25	e	32	32	63	5	3	8	29	0	50	5.8	10	4	948	7.8	379	0.40	×	Comparative Example
26	f	60	12.	72	4	5	9	19	0	17	4.7	19	7	955	7.9	439	0.46	×	Comparative Example

The underline means "outside the scope of the invention".
UB: upper bainite GB: granular bainite FM: fresh martensite γ: retained austenite F: polygonal ferrite TM: tempered martensite LB: lower bainite

JIS No. 5 tensile test specimens (JIS Z 2201) were cut from the obtained hot rolled steel sheets such that a tensile direction was perpendicular to the rolling direction. A tensile test in accordance with the specifications of JIS Z 2241 was conducted with a strain rate of 10^-3/s, to determine the tensile strength and the uniform elongation. In the present invention, tensile strengths of 980 MPa or greater were rated as "pass". Furthermore, in cases where the uniform elongation was 6.0% or greater, it was determined that the ductility was excellent.

Test specimens having dimensions and a shape as illustrated in Fig. 1 were cut from the obtained hot rolled steel sheets such that a longitudinal direction of the test specimens was perpendicular to the rolling direction, and a plane bending fatigue test was conducted in accordance with the regulations of JIS Z 2275. Stress loading modes were a stress ratio R of -1 and a frequency f of 25 Hz. A load stress amplitude was varied in six steps, and stress cycles before failure were measured to determine an S-N curve. Thus, the fatigue strength (fatigue limit) for 2×10⁶ cycles was determined. In the present invention, when a value obtained by dividing the fatigue limit by the tensile strength determined in the tensile test was 0.50 or greater, it was determined that the fatigue properties were excellent.

Test specimens (size: t (thickness) × 30 mm (width) × 30 mm (length)) were cut from the obtained hot rolled steel sheets. Blanking was performed on a central portion of each of the cut test specimens with a 10 mmφ cylindrical punch, with at least three clearances including 10% and 20% within a clearance range of 10 to 20%, to form a blanked hole. The clearance is a ratio [%] with respect to the thickness of the test specimen. In cases where an edge surface of the blanked hole did not have cracks, flaws, a brittle fracture surface, or a secondary shear plane as visually observed, a rating of "o" was given, which meant that the blankability was excellent. In cases where any of those were observed, a rating of "×" was given.

Samples for the measurement of hardness were cut from the obtained high strength steel sheets such that a thickness-wise cross section parallel to the rolling direction could serve as a hardness measurement cross section, and the hardness (Hv1) at a 1/2 thickness position and the hardness (Hv2) at a 1/4 thickness position were measured. Conditions for the measurement of a Vickers hardness included a load of 100 g and a holding time of 10 s. The measurement was performed at five points in a region extending ±1/20 thickness from each of the positions, with a step size of 250 µm or greater, and the results were averaged.
Subsequently, a percentage of the difference (|Hv1 - Hv2|) between the determined Hv1 and Hv2, with respect to the value of 0.3 times the tensile strength determined by the tensile test, was calculated, and evaluated.
In all of the Inventive Examples, the high strength steel sheets had a tensile strength of 980 MPa or greater and had excellent ductility, fatigue strength, and blankability. In contrast, in Comparative Examples, which fall outside the scope of the present invention, the tensile strength was not 980 MPa or greater, or excellent ductility, fatigue strength, or blankability was not achieved.

Claims

A high strength steel sheet comprising a chemical composition comprising, in mass%,
C: 0.04 to 0.18%,

Si: 0.1 to 2.5%,

Mn: 1.0 to 3.0%,

P: 0.1% or less,

S: 0.01% or less,

Al: 0.010 to 1.0%, and

N: 0.01% or less, with a balance of Fe and unavoidable impurities, wherein

the high strength steel sheet comprises microstructure comprising a primary phase and a hard secondary phase, the primary phase being a bainite phase that comprises upper bainite and granular bainite and being present in an area fraction of 85% or greater, the hard secondary phase comprising fresh martensite and/or retained austenite phases and being present in an area fraction of less than 15%,

in the bainite phase, the area fraction of the granular bainite with respect to the area fraction of the upper bainite is 5% or greater and 40% or less,

a difference |Hv1 - Hv2| between a hardness Hv1 and a hardness Hv2 is 30% or less with respect to 0.3TS, where Hv1 is a hardness at a 1/2 thickness position, and Hv2 is a hardness at a 1/4 thickness position, and

the upper bainite phase has an average grain size of 7 µm or less.
The high strength steel sheet according to claim 1, wherein the chemical composition further comprises, in mass%, at least one of groups a to e:
group a:
one or two or more selected from V: 0.005 to 0.5%, Ti: 0.005 to 0.2%, and Nb: 0.005 to 0.1%,

group b:
one or two or more selected from Cu: 0.005 to 0.5%, Ni: 0.005 to 0.5%, Cr: 0.005 to 1.0%, and Mo: 0.005 to 0.5%,

group c:
B: 0.0002 to 0.005%,

group d:
one or two selected from Sb: 0.001 to 0.1%, and Sn: 0.001 to 0.1%, and

group e:
one or two or more selected from Ca: 0.0001 to 0.005%, Mg: 0.0001 to 0.005%, and REM: 0.0001 to 0.005%.
A method for manufacturing the high strength steel sheet according to claim 1 or 2 comprising:
casting a molten steel material having the chemical composition, the casting comprising rotating the molten steel material with an inductive electromagnetic stirring device at a rotational speed of 10 cm/s or greater in a horizontal plane with respect to a casting mold;

heating a resulting steel material to 1150°C or greater;

subjecting the heated steel material to rough rolling to form a steel sheet;

subjecting the steel sheet to finish rolling under conditions including a finish rolling delivery temperature of (RC - 50)°C or greater and (RC + 100)°C or less;

cooling the finish-rolled steel sheet under conditions including a time between completion of the finish rolling and start of the cooling of 2.0 s or less, an average cooling rate before Bs is reached of 30 °C/s or greater, which is a cooling rate on a surface, a residence time for a temperature of (Bs - 100)°C or greater and Bs °C or less of 3.0 s or more and 10.0 s or less, and a cooling stop temperature of (Bs - 250)°C or greater and (Bs - 100)°C or less;

coiling the cooled steel sheet under conditions including a coiling temperature of (Bs - 250)°C or greater and (Bs - 100)°C or less; and

cooling a resultant to a temperature of (Bs - 400)°C or less at an average cooling rate of 1°C/s or less, where RC and Bs are defined by equations (1) and (2) below, respectively: RC (°C) = 750 + 120 × C + 100 × N + 10 × Mn + 250 × Ti + 5000 × B + 10 × Cr + 50 × Mo + 750 × Nb + 150 × V Bs (°C) = 830 - 270 × C - 90 × Mn - 70 × Cr - 37 × Ni - 83 × Mo - 20 × Cu

where, in equations (1) and (2), each of the element symbols represents a content (mass%) of the element, and the element symbol of an element that is not included represents 0.