EP3366797B1

EP3366797B1 - Method for producing a hot press member

Info

Publication number: EP3366797B1
Application number: EP16857079.4A
Authority: EP
Inventors: Koichi Nakagawa; Shinjiro Kaneko; Takeshi Yokota; Kazuhiro Seto
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2015-10-19
Filing date: 2016-10-03
Publication date: 2019-12-18
Anticipated expiration: 2036-10-03
Also published as: EP3366797A4; JP2017078188A; MX2018004772A; US20190093191A1; JP6222198B2; WO2017068756A1; EP3366797A1; CN108138289A; KR20180063303A

Description

TECHNICAL FIELD

The present invention relates to a method of manufacturing a hot pressed member.

BACKGROUND

Recent years have seen strong demand to improve the fuel efficiency of vehicles, for global environment protection. This has led to intense need for lighter automotive bodies. To ensure safety even with thinner automotive members, steel sheets as blank sheets of such members needs to be strengthened. However, strengthening a steel sheet typically leads to lower formability. Hence, problems such as difficulty in forming and degraded shape fixability arise in the manufacture of members using high strength steel sheets as blank sheets.
In view of such problems, a technique of manufacturing a high strength automotive member by applying a hot press process to a steel sheet has been put into actual use. In the hot press process, after heating a steel sheet to an austenite region, the steel sheet is conveyed to a press machine. In the press machine, the steel sheet is formed into a member of a desired shape using a press tool, and simultaneously quenched. In this cooling process (quenching) in the press tool, the microstructure of the member undergoes phase transformation from austenite phase to martensite phase. A high strength member of a desired shape is thus obtained.
Demand to improve the anti-crash property of automotive members has also been growing recently, in order to ensure the safety of drivers and passengers. To meet this demand, increasing the uniform elongation of an automotive member is effective in enhancing the ability to absorb energy upon collision (collision energy absorbing performance). There has thus been strong demand for hot pressed members having excellent uniform elongation as well as high strength.
To meet this demand, JP 2013-79441 A (PTL 1) proposes a hot press formed part obtained by forming a thin steel sheet by a hot press forming method. The hot press formed part proposed in PTL 1 includes: a chemical composition containing, in mass%, C: 0.15 % to 0.35 %, Si: 0.5 % to 3 %, Mn: 0.5 % to 2 %, P: 0.05 % or less, S: 0.05 % or less, Al: 0.01 % to 0.1 %, Cr: 0.01 % to 1 %, B: 0.0002 % to 0.01 %, Ti: (N content) × 4 % to 0.1 %, and N: 0.001 % to 0.01 %, with a balance consisting of Fe and incidental impurities; and a microstructure including, in area ratio, martensite: 80 % to 97 %, retained austenite: 3 % to 20 %, and a balance: 5 % or less. PTL 1 states that, with the proposed technique, a metallic microstructure with an appropriate amount of retained austenite can be obtained, and a hot pressed member having higher ductility can be realized.
JP 2010-65293 A (PTL 2) proposes a hot pressed member having excellent ductility. The hot pressed member described in PTL 2 includes: a composition containing, in mass%, C: 0.20 % to 0.40 %, Si: 0.05 % to 3.0 %, Mn: 1.0 % to 4.0 %, P: 0.05 % or less, S: 0.05 % or less, Al: 0.005 % to 0.1 %, and N: 0.01 % or less, with a balance consisting of Fe and incidental impurities; and a microstructure in which the area ratio of ferrite phase is 5 % to 55 % and the area ratio of martensite phase is 45 % to 95 % with respect to the whole microstructure, and the mean grain size of ferrite phase and martensite phase is 7 µm or less. The hot pressed member has high strength of 1470 MPa to 1750 MPa in tensile strength TS, and high ductility of 8 % or more in total elongation El.
PTL 3 relates to a high-strength hot-pressed part obtained by performing a hot pressing process on a steel sheet, where the part has a specific chemical composition, microstructure and tensile properties. PTL 3 further relates to a method of manufacturing the hot-pressed part.
PTL 4 relates to a steel sheet used for hot stamping, which is characterized by a specific composition and martensitic transformation start temperature. PTL 4 further relates to a hot stamping process, in which the steel sheet is provided, heated, transferred to a die for stamping so as to obtain a formed component, and cooling the formed component.
PTL 5 relates to a hot-formed member having a specific chemical composition, microstructure and tensile strength, as well as a manufacturing method which provides the hot-formed member.
PTL 6 relates to a method for manufacturing a hot-pressed member including heating a specific coated steel sheet to 850°C to 950°C, and starting hot press forming when the temperature of the coated steel sheet which has been heated is 650°C to 800°C.

CITATION LIST

Patent Literatures

PTL 1: JP 2013-79441 A
PTL 2: JP 2010-65293 A
PTL 3: EP 3 181 715 A1 (prior art under Article 54(3) EPC)
PTL 4: CN 104 846 274 A ( EP 3 260 569 A1 )
PTL 5: WO 2015/102051 A1 ( EP 3 093 359 A1 )
PTL 6: WO 2015/001705 A1 ( EP 3 017 892 A1 )

SUMMARY

(Technical Problem)

With the techniques described in PTL 1 and PTL 2, high strength of 1500 MPa or more in tensile strength TS is achieved by strengthening martensite phase by C, but there is a problem of insufficient uniform elongation in terms of enhancing collision energy absorbing performance.
A hot pressed member is typically subjected to a baking finish after the production. Heat treatment in this baking finish increases yield stress YS. To enhance anti-crash property, not only high uniform elongation but also high YS is important. Accordingly, a hot pressed member that has excellent heat treatment hardenability so that YS increases as high as possible as a result of the heat treatment in the baking finish is desired. The techniques described in PTL 1 and PTL 2 are, however, not concerned with such heat treatment hardenability.
It could therefore be helpful to provide a hot pressed member having all of: high strength of 1500 MPa or more in tensile strength TS; high ductility of 6.0 % or more in uniform elongation uEl; and excellent heat treatment hardenability of increasing in yield stress YS by 150 MPa or more when subjected to heat treatment (baking finish), and an advantageous method of manufacturing the same. In this description, "excellent heat treatment hardenability" means a property that, when a hot pressed member is heat treated, the difference (hereafter denoted by "ΔYS") between the yield stress YS after the heat treatment and the yield stress YS before the heat treatment is 150 MPa or more.

(Solution to Problem)

As a result of conducting extensive study on various factors that influence yield stress YS and uniform elongation uEl in a hot pressed member having high strength of 1500 MPa or more in tensile strength TS, we discovered the following.

(A) To achieve high uniform elongation uEl of 6.0 % or more, a microstructure having an appropriate amount of retained austenite is necessary. To obtain a microstructure having an appropriate amount of retained austenite with less than 0.30 mass% C, the Mn content needs to be 3.5 % or more. Mn contributes to increased strength, so that high strength can be ensured even with less than 0.30 % C.
(B) The dislocation density and ΔYS of the hot pressed member correlate with each other. To achieve ΔYS of 150 MPa or more, the dislocation density of the hot pressed member needs to be 1.0 × 10¹⁶/m² or more.
(C) An appropriate amount of retained austenite can be generated by, before hot pressing a steel sheet containing 3.5 % or more Mn as mentioned above, performing heat treatment of heating the steel sheet to a ferrite-austenite dual phase temperature range and retaining the steel sheet at a predetermined temperature in the temperature range for 1 hr or more and 48 hr or less beforehand to cause Mn to concentrate in austenite. Moreover, by subjecting the obtained steel sheet to a predetermined heating process and a hot press forming process, a hot pressed member having a dislocation density of 1.0 × 10¹⁶/m² or more can be yielded.

The present invention is based on these discoveries. The present invention is defined in the appended claims.

(Advantageous Effect)

The hot pressed member obtainable according to the present invention has all of: high strength of 1500 MPa or more in tensile strength TS; high ductility of 6.0 % or more in uniform elongation uEl; and excellent heat treatment hardenability of increasing in yield stress YS by 150 MPa or more when subjected to heat treatment (baking finish). Such a hot pressed member can be advantageously obtained by the method of manufacturing a hot pressed member according to the present invention.

DETAILED DESCRIPTION

(Chemical composition)

The chemical composition of a hot pressed member obtainable by the present invention is described below. In the following description, "mass%" is simply written as "%" unless otherwise noted.

C: 0.090 % or more and less than 0.30 %

C is an element that increases the strength of the steel. In addition, in heat treatment for the hot pressed member, yield stress is increased by dislocation locking of solute C. To achieve the effects and ensure a tensile strength TS of 1500 MPa or more, the C content is 0.090 % or more. If the C content is 0.30 % or more, the degree of solid solution strengthening by C increases, which makes it difficult to adjust the tensile strength TS of the hot pressed member to less than 2300 MPa.

Mn: 3.5 % or more and less than 11.0 %

Mn is an element that increases the strength of the steel and also concentrates in austenite to improve the stability of austenite, and is the most important element in the present invention. To achieve the effects and ensure a tensile strength TS of 1500 MPa or more and a uniform elongation uEl of 6.0 % or more, the Mn content is 3.5 % or more. If the Mn content is 11.0 % or more, the degree of solid solution strengthening by Mn increases, which makes it difficult to adjust the tensile strength TS of the hot pressed member to less than 2300 MPa.
If the C content and the Mn content are in the respective ranges mentioned above, a hot pressed member having tensile property of 6.0 % or more in uniform elongation can be yielded stably, with a tensile strength TS of 1500 MPa or more and preferably less than 2300 MPa. In more detail, to ensure a strength of 1500 MPa or more and less than 1700 MPa in tensile strength TS, it is preferable to set C: 0.090 % or more and less than 0.12 % and Mn: 4.5 % or more and less than 6.5 %, or C: 0.12 % or more and less than 0.18 % and Mn: 3.5 % or more and less than 5.5 %. To ensure a strength of 1700 MPa or more and less than 1900 MPa in tensile strength TS, it is preferable to set C: 0.090 % or more and less than 0.12 % and Mn: 6.5 % or more and less than 8.5 %, or C: 0.12 % or more and less than 0.18 % and Mn: 5.5 % or more and less than 7.5 %. To ensure a strength of 1800 MPa or more and less than 1980 MPa in tensile strength TS, it is preferable to set C: 0.18 % or more and less than 0.30 % and Mn: 3.5 % or more and less than 4.5 %. To ensure a strength of 2000 MPa or more and less than 2300 MPa in tensile strength TS, it is preferable to set C: 0.090 % or more and less than 0.12 % and Mn: 8.5 % or more and less than 11.0 %, C: 0.12 % or more and less than 0.18 % and Mn: 7.5 % or more and less than 11.0 %, or C: 0.18 % or more and less than 0.30 % and Mn: 4.5 % or more and less than 6.5 %.

Si: 0.01 % to 2.5 %

Si is an element that increases the strength of the steel by solid solution strengthening. To achieve the effect, the Si content is 0.01 % or more. If the Si content is more than 2.5 %, surface defects called red scale occur significantly in hot rolling, and also the rolling load increases. The Si content is therefore 0.01 % or more and 2.5 % or less. The Si content is preferably 0.02 % or more. The Si content is preferably 1.5 % or less.

P: 0.05 % or less

P is an element that exists in the steel as an incidental impurity, and segregates to crystal grain boundaries and like and causes adverse effects such as a decrease in the toughness of the member. The P content is therefore desirably as low as possible, but 0.05 % or less P is allowable. Accordingly, the P content is 0.05 % or less, and more preferably 0.02 % or less. Excessive dephosphorization leads to higher refining cost, and so the P content is desirably 0.0005 % or more.

S: 0.05 % or less

S is contained in the steel incidentally. S exists in the steel as a sulfide inclusion, and decreases the ductility, toughness, and the like of the hot pressed member. The S content is therefore desirably as low as possible, but 0.05 % or less S is allowable. Accordingly, the S content is 0.05 % or less, and more preferably 0.005 % or less. Excessive desulfurization leads to higher refining cost, and so the S content is desirably 0.0005 % or more.

Al: 0.005 % to 0.1 %

Al is an element that acts as a deoxidizer. To achieve the effect, the Al content is 0.005 % or more. If the Al content is more than 0.1 %, Al combines with nitrogen to form a large amount of nitride. This causes a decrease in the blanking workability and quench hardenability of the steel sheet as a blank sheet. The Al content is therefore 0.005 % or more and 0.1 % or less. The Al content is preferably 0.02 % or more. The Al content is preferably 0.05 % or less.

N: 0.01 % or less

N is typically contained in the steel incidentally. If the N content is more than 0.01 %, nitrides such as AlN form during heating in hot rolling or hot press. This causes a decrease in the blanking workability and quench hardenability of the steel sheet as a blank sheet. The N content is therefore 0.01 % or less. The N content is more preferably 0.0030 % or more. The N content is more preferably 0.0050 % or less. In the case where N is contained incidentally without adjustment, the N content is approximately less than 0.0025 %. To prevent an increase in refining cost, the N content is desirably 0.0025 % or more.
In addition to the basic components described above, the chemical composition may contain the following optional components.

A group: one or more selected from Ni: 0.01 % to 5.0 %, Cu: 0.01 % to 5.0 %, Cr: 0.01 % to 5.0 %, and Mo: 0.01 % to 3.0 %

Ni, Cu, Cr, and Mo are each an element that increases the strength of the steel and improves quench hardenability. One or more of them may be selected and added according to need. To achieve the effect, the content of each element is 0.01 % or more. To prevent an increase in material cost, the Ni, Cu, and Cr contents are each 5.0 % or less, and the Mo content is 3.0 % or less. The content of each element is preferably 0.01 % or more and 1.0 % or less.

B group: one or more selected from Ti: 0.005 % to 3.0 %, Nb: 0.005 % to 3.0 %, V: 0.005 % to 3.0 %, and W: 0.005 % to 3.0 %

Ti, Nb, V, and W are each an element that increases the strength of the steel by precipitation strengthening, and also improves toughness by crystal grain refinement. One or more of them may be selected and added according to need.
Ti has not only the effect of increasing strength and improving toughness, but also the effect of forming a nitride more preferentially than B and improving quench hardenability by solute B. To achieve the effects, the Ti content is 0.005 % or more. If the Ti content is more than 3.0 %, the rolling load increases extremely in hot rolling, and also the toughness of the hot pressed member decreases. Accordingly, in the case of containing Ti, the Ti content is 0.005 % or more and 3.0 % or less. The Ti content is preferably 0.01 % or more. The Ti content is preferably 1.0 % or less.
To achieve the above-mentioned effect by Nb, the Nb content is 0.005 % or more. If the Nb content is more than 3.0 %, the amount of carbonitride increases, and ductility and lagging destruction resistance decrease. Accordingly, in the case of containing Nb, the Nb content is 0.005 % or more and 3.0 % or less. The Nb content is preferably 0.01 % or more. The Nb content is preferably 0.05 %.
V has not only the effect of increasing strength and improving toughness, but also the effect of precipitating as a precipitate or a crystallized product and improving hydrogen embrittlement resistance as a hydrogen trap site. To achieve the effects, the V content is 0.005 % or more. If the V content is more than 3.0 %, the amount of carbonitride increases considerably, and ductility decreases. Accordingly, in the case of containing V, the V content is 0.005 % or more and 3.0 % or less. The V content is preferably 0.01 % or more. The V content is preferably 2.0 % or less.
W has not only the effect of increasing strength and improving toughness, but also the effect of improving hydrogen embrittlement resistance. To achieve the effects, the W content is 0.005 % or more. If the W content is more than 3.0 %, ductility decreases. Accordingly, in the case of containing W, the W content is 0.005 % or more and 3.0 % or less. The W content is preferably 0.01 % or more. The W content is preferably 2.0 % or less.

C group: one or more selected from REM: 0.0005 % to 0.01 %, Ca: 0.0005 % to 0.01 %, and Mg: 0.0005 % to 0.01 %

REM, Ca, and Mg are each an element that improves ductility and hydrogen embrittlement resistance by morphological control of an inclusion. One or more of them may be selected and added according to need. To achieve the effect, the content of each element is 0.0005 % or more. To prevent a decrease in hot workability, the REM content and the Ca content are each 0.01 % or less. To prevent a decrease in ductility caused by the formation of a coarse oxide or sulfide, the Mg content is 0.01 % or less. The content of each element is preferably 0.0006 % to 0.01 %.

D group: Sb: 0.002 % to 0.03 %

Sb inhibits the formation of a decarburized layer in the steel sheet surface layer when heating or cooling the steel sheet, and so may be added according to need. To achieve the effect, the Sb content is 0.002 % or more. If the Sb content is more than 0.03 %, the rolling load increases, and productivity decreases. Accordingly, in the case of containing Sb, the Sb content is 0.002 % or more and 0.03 % or less. The Sb content is preferably 0.002 % or more and 0.02 % or less.

E group: B: 0.0005 % to 0.05 %

B improves quench hardenability during hot press and toughness after hot press, and so may be added according to need. To achieve the effect, the B content is 0.0005 % or more. If the B content is more than 0.05 %, the rolling load in hot rolling increases. Besides, martensite phase or bainite phase may form after hot rolling, and cause cracking in the steel sheet. Accordingly, in the case of containing B, the B content is 0.0005 % or more and 0.05 % or less, and preferably 0.0005 % or more and 0.01 % or less.
The balance other than the components described above consists of Fe and incidental impurities. As the incidental impurities, O (oxygen): 0.0100 % or less is allowable.

(Microstructure)

The microstructure of the hot pressed member obtainable by the present invention is described below.

Martensite phase: 70.0 % or more in volume fraction

To ensure a tensile strength TS of 1500 MPa or more, martensite phase of 70.0 % or more in volume fraction needs to be the main phase. To contain the desired amount of retained austenite phase, martensite phase is preferably 97 % or less.

Retained austenite phase: 3.0 % to 30.0 % in volume fraction

Retained austenite phase enhances uniform elongation by a transformation induced plasticity (TRIP) effect upon deformation, and is the most important microstructure in the present invention. In this embodiment, the volume fraction of retained austenite phase is 3.0 % or more, to achieve a uniform elongation uEl of 6.0 % or more. If the volume fraction of retained austenite phase is more than 30.0 %, hard martensite phase transformed after the TRIP effect is developed increases excessively, and toughness decreases. The volume fraction of retained austenite phase is therefore 3.0 % or more and 30.0 % or less. The volume fraction of retained austenite phase is preferably 5.0 or more. The volume fraction of retained austenite phase is preferably 20.0 % or less.
For the formation of the above-mentioned appropriate amount of retained austenite phase, it is important to use a steel sheet containing an appropriate amount of Mn, subject the steel sheet to predetermined heat treatment before hot press to cause Mn to concentrate in austenite, and appropriately adjusting a heating process in hot press.
As the balance other than martensite phase and retained austenite phase, 10 % or less (including 0 %) bainite phase, ferrite phase, cementite, and pearlite in volume fraction in total is allowable.
In the present invention, the volume fraction of each phase is determined as follows.
The volume fraction of retained austenite is determined by the following method. An X-ray diffraction test piece is cut out of the hot pressed member, mechanically polished and chemically polished so that the measurement plane is at a position of 1/4 of the thickness, and then subjected to X-ray diffraction. Using CoKα radiation as an incident X-ray, the peak integrated intensity for the retained austenite (γ) planes of {200}, {220}, and {311} and the peak integrated intensity for the ferrite (α) planes of {200} and {211} are measured. For a total of six patterns of α{200} - γ{200}, α{200} - γ{220}, α{200} - γ{311}, α{211} - γ{200}, α{211} - γ{220}, and α{211} - γ(311}, the retained γ volume fraction obtained from each integrated intensity ratio is calculated. Their mean value is set as "the volume fraction of retained austenite phase".
The volume fraction of the balance is determined by the following method. A microstructure observation test piece is collected from the hot pressed member so that the observation plane is parallel to the rolling direction and perpendicular to the rolling plane. The observation plane is polished, and etched with a 3 vol% nital solution to expose the microstructure. The microstructure at a position of 1/4 of the sheet thickness is observed using a scanning electron microscope (at 1500 magnifications) and photographed. From the obtained micrograph, the microstructure is identified and the microstructure proportion is calculated by image analysis. A phase observed as black with a relatively smooth surface is identified as ferrite phase. A phase observed as white in film or lump form in crystal grain boundaries is identified as cementite. A phase in which ferrite phase and cementite form in layers is identified as pearlite. A phase in which a carbide forms between laths and a phase made of bainitic ferrite having no carbide in grains are identified as bainite phase. The occupancy area ratio of each phase in the micrograph is calculated, and the area ratio is set as the volume fraction on the assumption that the microstructure is homogeneous three-dimensionally.
The volume fraction of martensite phase is calculated by subtracting the volume fraction of the balance and the volume fraction of the retained austenite phase from 100 %.

(Dislocation density)

Dislocation density: 1.0 × 10¹⁶/m² or more

The dislocation density of the hot pressed member influences ΔYS, and is the most important index in the present invention. It is considered that, when the hot pressed member is subjected to heat treatment (baking finish), solute C locks to mobile dislocations, as a result of which yield stress YS increases. To achieve ΔYS of 150 MPa or more, the dislocation density of the hot pressed member needs to be 1.0 × 10¹⁶/m² or more. The upper limit of the dislocation density is substantially 5.0 × 10¹⁶/m². The dislocation density of the hot pressed member is preferably 1.2 × 10¹⁶/m² or more. The dislocation density of the hot pressed member is preferably 4.5 × 10¹⁶/m² or less.
In the present invention, the dislocation density is determined by the following method. An X-ray diffraction test piece is cut out of the hot pressed member, mechanically polished and chemically polished so that the measurement plane is at a position of 1/4 of the thickness, and then subjected to X-ray diffraction. Using CoKα₁ radiation as an incident X-ray, the peak half-value widths of α{110}, α{211}, and α{220} are measured. The measured peak half-value widths of α{110}, α{211}, and α{220} are corrected to true half-value widths using a strain-free standard test piece (Si), and then strain (ε) is calculated based on the Willaimson-Hall method. The dislocation density (ρ) is calculated using the strain (ε) and the Burgers vector (b = 0.286 nm), according to the following expression: $ρ = 14.4 \times ε^{2} / b^{2} .$

(Properties)

The hot pressed member in this embodiment has the following properties: high strength of 1500 MPa or more and preferably less than 2300 MPa in tensile strength TS; high ductility of 6.0 % or more and substantially 20 % or less in uniform elongation uEl; and ΔYS of 150 MPa or more and substantially 300 MPa or less.

(Coated layer)

The hot pressed member in this embodiment preferably has a coated layer.
In the case where the steel sheet used as a blank sheet of the hot pressed member is a coated steel sheet, a coated layer remains in the surface layer of the yielded hot pressed member. In such a case, scaling is suppressed during heating in hot press. The hot pressed member can thus be put to use without descaling the surface, which contributes to improved productivity.
The coated layer is preferably a zinc or zinc alloy coated layer or an aluminum or aluminum alloy coated layer. In the case where corrosion resistance is required, a zinc or zinc alloy coated layer is better than an aluminum or aluminum alloy coated layer, because the corrosion rate of the steel substrate can be reduced by the sacrificial protection effect of zinc. Moreover, in the case of hot pressing the coated steel sheet, a zinc oxide film forms in the initial stage of heating in the hot press process, so that evaporation of Zn can be prevented in the subsequent treatment of the hot pressed member.
Examples of the zinc or zinc alloy coating include typical hot-dip galvanizing (GI), galvannealing (GA), and Zn-Ni-based coating. Zn-Ni-based coating is particularly preferable. A Zn-Ni-based coated layer can remarkably suppress scaling during hot press heating, and also prevent liquid metal embrittlement cracking. To achieve the effects, the Zn-Ni-based coated layer preferably contains 10 mass% to 25 mass% Ni. If more than 25 % Ni is contained, the effects are saturated.
Examples of the aluminum or aluminum alloy coated layer include Al-10 mass% Si coating.

(Manufacturing method)

The method of manufacturing a hot pressed member according to the invention is described below. First, a slab having the above-mentioned chemical composition is heated, and hot rolled to obtain a hot rolled steel sheet. The hot rolled steel sheet is then subjected to predetermined heat treatment (Mn concentration heat treatment) (described later), to obtain a first blank steel sheet. After this, the first blank steel sheet is cold rolled to obtain a cold rolled steel sheet. The cold rolled steel sheet is then subjected to predetermined annealing, to obtain a second blank steel sheet.
The second blank steel sheet obtained in this way is subjected to a predetermined heating process and a hot press forming process, to obtain a hot pressed member. Each process is described in detail below.

The obtainment of the hot rolled steel sheet is not limited, and may be performed according to a usual method. It is preferable to obtain molten steel having the above-mentioned chemical composition by steelmaking in a converter or the like, and process the molten steel into a slab by a continuous casting method in order to prevent macrosegregation. An ingot casting method or a thin slab continuous casting method may be used instead of the continuous casting method.
The obtained slab is cooled to the room temperature, and then charged into a heating furnace for reheating. Alternatively, an energy saving process such as a process of charging the slab into the heating furnace as a warm slab without cooling the slab to the room temperature or a process of heat-retaining the slab for a short time and then immediately hot rolling the slab may be used.
The obtained slab is heated to a predetermined heating temperature, and then hot rolled to obtain a hot rolled steel sheet. The heating temperature is, for example, 1000 °C to 1300 °C. The heated slab is typically hot rolled at a finisher entry temperature of 1100 °C or less and a finisher delivery temperature of 800 °C to 950 °C, cooled at an average cooling rate of 5 °C/s or more, and coiled at a coiling temperature of 300 °C to 750 °C, to obtain a hot rolled steel sheet.

Following this, the hot rolled steel sheet is heated to a first temperature that is Ac1 point or more and Ac3 point or less, retained at the first temperature for 1 hr or more and 48 hr or less, and then cooled to obtain the first blank steel sheet. This process causes Mn to concentrate in austenite, and is the most important process for manufacturing a hot pressed member that has the appropriate amount of retained austenite to achieve a uniform elongation uEl of 6.0 % or more and has a dislocation density of 1.0 × 10¹⁶/m² or more to achieve ΔYS of 150 MPa or more.

Heating temperature: Ac1 point or more and Ac3 point or less

The hot rolled steel sheet is heated to a ferrite-austenite dual phase temperature range, to cause Mn to concentrate in austenite. In Mn-concentrated austenite, the martensite transformation end temperature is the room temperature or less, and the formation of retained austenite is facilitated. If the heating temperature is less than Ac1 point, austenite does not form, and Mn cannot be concentrated in austenite. If the heating temperature is more than Ac3 point, the temperature is in an austenite single phase temperature range, and Mn does not concentrate in austenite. In both of the case where the heating temperature is less than Ac1 point and the case where the heating temperature is more than Ac3 point, a hot pressed member having a dislocation density of 1.0 × 10¹⁶/m² or more cannot be obtained. The heating temperature is therefore Ac1 point or more and Ac3 point or less. The heating temperature is preferably (Ac1 point + 20 °C) or more. The heating temperature is preferably (Ac3 point - 20 °C) or less.
Ac1 point (°C) and Ac3 point (°C) are calculated according to the following expressions: $Ac 1 point (° C) = 751 - 16 C + 11 Si - 28 Mn - 5.5 Cu - 16 Ni + 13 Cr + 3.4 Mo$
$Ac 3 point (° C) = 910 - 203 C^{1 / 2} + 44.7 Si - 4 Mn + 11 Cr$
where C, Si, Mn, Ni, Cu, Cr, and Mo are each the content (mass%) of the corresponding element. In the case where the element is not contained, the content of the element is assumed to be 0.

Heating retention time: 1 hr or more and 48 hr or less

The concentration of Mn in austenite progresses with the passage of the heating retention time. If the heating retention time is less than 1 hr, the concentration of Mn in austenite is insufficient, and the desired uniform elongation cannot be obtained. Besides, if the heating retention time is less than 1 hr, the concentration of Mn is insufficient, and Ms point does not decrease in the hot press process, so that a hot pressed member having a dislocation density of 1.0 × 10¹⁶/m² or more cannot be obtained. If the heating retention time is more than 48 hr, pearlite forms, making it impossible to achieve the desired uniform elongation. Moreover, a hot pressed member having a dislocation density of 1.0 × 10¹⁶/m² or more cannot be obtained. The heating retention time is therefore 1 hr or more and 48 hr or less. The heating retention time is preferably 1.5 hr or more. The heating retention time is preferably 24 hr or less.
Ms point (°C) is calculated according to the following expression: $Ms point (° C) = 539 - 423 C - 30.4 Mn - 17.7 Ni - 12.1 Cr - 7.5 Mo$
where C, Mn, Ni, Cr, and Mo are each the content (mass%) of the corresponding element. In the case where the element is not contained, the content of the element is assumed to be 0.
The cooling after the heating retention is not limited. It is preferable to appropriately perform the cooling by natural cooling (gradual cooling) or controlled cooling depending on the heating furnace used and the like.
The Mn concentration heat treatment is preferably performed in a batch annealing furnace or a continuous annealing furnace. The treatment conditions in the batch annealing furnace other than the above-mentioned conditions are not limited. For example, it is preferable to set the heating rate to 40 °C/hr or more and the cooling rate after the heating retention to 40 °C/hr or more, in terms of Mn concentration. The treatment conditions in the continuous annealing furnace other than the above-mentioned conditions are not limited. For example, it is preferable to, after performing the above-mentioned heating retention, cool the hot rolled steel sheet at an average cooling rate of 10 °C/s or more to a cooling stop temperature in a temperature range of 350 °C to 600 °C, cause the hot rolled steel sheet to stay in the temperature range for 10 sec to 300 sec, and then cool and coil the steel sheet, in terms of manufacturability.
The first blank steel sheet produced in this way can be used as a steel sheet for hot press. The microstructure of the first blank steel sheet has a feature that Mns/Mnα is 1.2 or more, where Mns is the Mn concentration in lath secondary phase and Mnα is the Mn concentration in lath ferrite. Here, "secondary phase" denotes the balance (austenite, martensite, pearlite, bainite) other than ferrite. If Mns/Mnα is less than 1.2, the concentration of Mn in austenite is insufficient, making it impossible to achieve sufficient uniform elongation and dislocation density after the hot press.

After this, the first blank steel sheet is cold rolled to obtain a cold rolled steel sheet, instead of performing the below-mentioned heating process and hot press forming process on the first blank steel sheet. To prevent abnormal grain growth in the subsequent annealing or the heating process immediately before the hot press, the reduction ratio in the cold rolling is preferably 30 % or more, and more preferably 50 % or more. To prevent an increase in rolling load and a decrease in productivity, the reduction ratio is preferably 85 % or less.

After this, the cold rolled steel sheet is subjected to annealing of heating the cold rolled steel sheet to Ac1 point or more and Ac3 point or less, retaining it at the temperature, and then cooling it, to obtain the second blank steel sheet. The annealing temperature is a predetermined temperature that is Ac1 point or more and Ac3 point or less. With this annealing temperature, the concentration of Mn in austenite is further facilitated in the annealing. The retention time at the predetermined temperature is 30 sec or more and 300 sec or less. If the retention time is 30 sec or more, the effect of the concentration of Mn is sufficient. If the retention time is 300 sec or less, productivity is maintained.
Pickling and/or temper rolling may be performed as appropriate between the processes.
The second blank steel sheet produced in this way can be used as a steel sheet for hot press. The microstructure of the second blank steel sheet has a feature that the mean grain size of ferrite is 10 µm or less, the mean grain size of secondary phase is 10 µm or less, and Mns/Mnα is 1.5 or more, where Mns is the Mn concentration in secondary phase and Mnα is the Mn concentration in ferrite. The mean grain size of ferrite and the mean grain size of secondary phase are determined by the following method. A microstructure observation test piece is collected from the second blank steel sheet so that the observation plane is parallel to the rolling direction and perpendicular to the rolling plane. The observation plane is polished, and etched with a 3 vol% nital solution to expose the microstructure. The microstructure at a position of 1/4 of the sheet thickness is observed using a scanning electron microscope (at 1500 magnifications) and photographed. From the obtained micrograph, the microstructure is identified based on the above-mentioned criteria. The mean grain size of each of ferrite and secondary phase is calculated according to linear analysis described in JIS G 0551 (2005).
Mns/Mnα is determined by the following method. A microstructure observation test piece is collected. Its observation plane is then polished, and etched with a 3 vol% nital solution to expose the microstructure. The microstructure at a position of 1/4 of the sheet thickness is observed using an electron probe microanalyzer (EPMA), and quantitative analysis of Mn is performed on 30 particles for each of ferrite and secondary phase. Regarding the Mn quantitative analysis results, the mean value of ferrite is set as Mnα, the mean value of secondary phase is set as Mns, and the value obtained by dividing the mean value Mns of secondary phase by the mean value Mnα of ferrite is set as Mns/Mnα.

In the case where no coated layer is formed on the surface of the second blank steel sheet, descaling treatment such as shot blasting needs to be performed on the hot pressed member after the hot press. In the case where a coated layer is formed on the surface of the second blank steel sheet, on the other hand, scaling is suppressed during heating in the hot press, so that descaling treatment after the hot press is unnecessary. This improves productivity.
The coating weight of the coated layer is preferably 10 g/m² to 90 g/m² per side, and more preferably 30 g/m² to 70 g/m² per side. If the coating weight is 10 g/m² or more, the effect of suppressing scaling during heating is sufficient. If the coating weight is 90 g/m² or less, productivity is not hampered. The components of the coated layer are as described above.

Following this, a heating process of heating the second blank steel sheet to a second temperature that is Ac3 point or more and 1000 °C or less and retaining it at the second temperature for 900 sec or less is performed.

Heating temperature: Ac3 point or more and 1000 °C or less

If the heating temperature is less than Ac3 point which is in an austenite single phase region, austenitization is insufficient. As a result, the desired amount of martensite in the hot pressed member cannot be ensured, and the desired tensile strength cannot be achieved. Besides, the hot pressed member cannot have a dislocation density of 1.0 × 10¹⁶/m² or more, making it impossible to achieve ΔYS of 150 MPa or more. If the heating temperature is more than 1000 °C, Mn concentrated in austenite is made uniform. Consequently, the desired amount of retained austenite cannot be ensured, and the desired uniform elongation cannot be achieved. Moreover, uniform Mn makes it impossible to decrease Ms point, so that the hot pressed member cannot have a dislocation density of 1.0 × 10¹⁶/m² or more and ΔYS of 150 MPa or more cannot be achieved. The heating temperature is therefore Ac3 point or more and 1000 °C or less. The heating temperature is preferably (Ac3 point + 30) °C or more. The heating temperature is preferably 950 °C or less.
The heating rate to the heating temperature (second temperature) is not limited, but is preferably 1 °C/s to 400 °C/s, and more preferably 10 °C/s to 150 °C/s. If the heating rate is 1 °C/s or more, productivity is not hampered. If the heating rate is 400 °C/s or less, stable temperature control is ensured.

Retention time: 900 sec or less (including 0 sec)

With the passage of the retention time at the heating temperature (second temperature), concentrated Mn diffuses around and is made uniform. Accordingly, if the retention time is more than 900 sec, the desired amount of retained austenite cannot be ensured, and the desired uniform elongation cannot be achieved. Besides, uniform Mn makes it impossible to decrease Ms point, so that the hot pressed member cannot have a dislocation density of 1.0 × 10¹⁶/m² or more and ΔYS of 150 MPa or more cannot be achieved. The retention time is therefore 900 sec or less. The retention time may be 0 sec, that is, the heating may be stopped immediately after the second temperature is reached.
The heating method is not limited, and may be any typical heating method such as an electric furnace, a gas furnace, infrared heating, high frequency heating, or direct current heating. The atmosphere is not limited, and may be any of an air atmosphere and an inert gas atmosphere.

In the hot press forming process, the second blank steel sheet which has undergone the heating process is simultaneously press formed and quenched using a press tool for forming, to obtain a hot pressed member of a predetermined shape. Hot press forming is a process of press forming a heated thin steel sheet using a press tool and simultaneously quenching it, and is also referred to as "hot forming", "hot stamping", "die quenching", etc.
The forming start temperature in the press machine is not limited, but is preferably Ms point or more. If the forming start temperature is less than Ms point, the load of press forming increases, and the load on the press machine increases. The conveyance of the blank steel sheet before the forming start is typically performed with air cooling. Accordingly, the upper limit of the forming start temperature is the heating temperature in the immediately previous heating process in the manufacturing process. In the case where the blank steel sheet is conveyed in an environment where the cooling rate is accelerated by a refrigerant such as gas or liquid, the cooling rate is preferably decreased by a heat insulation jig such as a heat retention box.
The cooling rate in the press tool is not limited. In terms of productivity, the average cooling rate to 200 °C is preferably 20 °C/s or more, and more preferably 40 °C/s or more.
The removal time from the press tool and the cooling rate after the removal are not limited. As the cooling method, for example, a punch press tool is held at the bottom dead center for 1 sec to 60 sec, and the hot pressed member is cooled using a die press tool and the punch press tool. After this, the hot pressed member is removed from the press tool, and cooled. The cooling in the press tool and the cooling after the removal from the press tool may be performed in combination with a cooling method using a refrigerant such as gas or liquid. This improves productivity.

EXAMPLES

Molten steel having the chemical composition (the balance consisting of Fe and incidental impurities) listed in Tables 1 and 4 was obtained by steelmaking in a small vacuum melting furnace, to yield a slab. The slab was heated to 1250 °C, and further subjected to hot rolling including rough rolling and finish rolling, to obtain a hot rolled steel sheet. The finisher entry temperature was 1100 °C, and the finisher delivery temperature was 850 °C. The cooling rate after the hot rolling end was 15 °C/s on average from 800 °C to 600 °C, and the coiling temperature was 650 °C.
The obtained hot rolled steel sheet was heated to the heating temperature T1 (first temperature) listed in Tables 2 and 5, retained at the temperature for the time listed in Tables 2 and 5, and then cooled to obtain a first blank steel sheet. In some test examples, the first blank steel sheet was pickled, and cold rolled at a reduction ratio of 54 %, to obtain a cold rolled steel sheet (sheet thickness: 1.6 mm). The cold rolled steel sheet was further heated to the heating temperature T2 listed in Tables 2 and 5, and retained for the time listed in Tables 2 and 5. The cold rolled steel sheet was then cooled at a cooling rate of 15 °C/s. The cooling was stopped at 500 °C, and the cold rolled steel sheet was retained at the temperature for 150 sec, to obtain a second blank steel sheet.
In the test examples not involving cold rolling, the first blank steel sheet was subjected to microstructure observation, and Mns/Mnα was calculated by the above-mentioned method. The results are listed in Tables 2 and 5. In the other test examples, the second blank steel sheet was subjected to microstructure observation, and the mean grain size of ferrite, the mean grain size of secondary phase, and Mns/Mnα were calculated by the above-mentioned methods. The results are listed in Tables 2 and 5.
As listed in Tables 2 and 5, in some test examples, the second blank steel sheet was subjected to coating treatment. In Tables 2 and 5, "GI" denotes a hot-dip galvanized layer, "GA" denotes a galvannealed layer, "Zn-Ni" denotes a Zn-12 mass% Ni coated layer, and "Al-Si" denotes a Al-10 mass% Si coated layer. The coating weight of each coated layer was 60 g/m² per side.
The hot rolled steel sheet (first blank steel sheet) or the cold rolled steel sheet (second blank steel sheet) obtained in this way was subjected to a heating process under the conditions listed in Tables 3 and 6 and a hot press forming process, to obtain a hat-shaped hot pressed member. The hot press was performed using a punch press tool having a width of 70 mm and a shoulder radius R of 6 mm and a die press tool having a shoulder radius R of 7.6 mm, with a forming depth of 30 mm.
Regarding the heating process before the hot press forming process, in the case of performing the heating process using an electric heating furnace in the air atmosphere, the heating rate from the room temperature to 750 °C was 7.5 °C/s on average. The heating rate from 750 °C to the heating temperature was 2.0 °C/s on average. After reaching the heating temperature, the steel sheet was retained at the heating temperature in the case of keeping temperature. In the case of performing the heating process using a direct current heater in the air atmosphere, the heating rate from the room temperature to the heating temperature was 100 °C/s on average. The hot press starts at 750 °C. The steel sheet was cooled to 150 °C or less by a combination of: clamping the steel sheet using the die press tool and the punch press tool with the punch press tool being held at the bottom dead center for 15 sec; and air cooling on the die after release from the clamping. The average cooling rate from the hot pressing start temperature to 200 °C was 100 °C/s.
The obtained hot pressed member was heat treated (low temperature heat treatment) at 170 °C for 20 min. This corresponds to the baking finish condition in a typical automotive member manufacturing process. Before and after the low temperature heat treatment, a JIS No. 5 tensile test piece (parallel portion width: 25 mm, parallel portion length: 60 mm, GL = 50 mm) was collected from a hat top portion, and a tensile test was conducted according to JIS Z 2241 to determine the yield stress YS, the tensile strength TS, the total elongation tEl, and the uniform elongation uEl. The results are listed in Tables 3 and 6.

Moreover, the volume fraction of martensite phase, the volume fraction of retained austenite phase, the volume fraction of the balance, and the dislocation density in the obtained hot pressed member were measured by the above-mentioned methods. The results are listed in Tables 3 and 6.

Table 1

Steel No.	C (mass%)	Mn (mass%)	Si (mass%)	P (mass%)	S (mass%)	Al (mass%)	N (mass%)	Ac1 point (°C)	Ac3 point (°C)	Ms point (°C)	Category
A	0.210	4.22	0.26	0.012	0.0015	0.031	0.0025	632	812	322	Conforming steel
B	0.160	3.80	0.25	0.013	0.0014	0.030	0.0022	645	825	356	Conforming steel
C	0.220	8.20	0.02	0.011	0.0150	0.035	0.0030	518	783	197	Conforming steel
D	0.220	10.40	0.03	0.022	0.0220	0.037	0.0040	457	775	130	Conforming steel
E	0.292	4.50	0.03	0.015	0.0220	0.042	0.0041	621	784	279	Conforming steel
F	0.350	4.15	0.25	0.015	0.0300	0.035	0.0023	632	784	265	Comparative steel
G	0.083	4.20	0.25	0.010	0.0400	0.035	0.0025	634	831	356	Comparative steel
H	0.221	12.50	0.26	0.015	0.0350	0.030	0.0020	400	776	66	Comparative steel
I	0.165	2.50	0.26	0.010	0.0350	0.031	0.0032	653	825	363	Comparative steel

Table 2

Blank steel sheet No.	Steel No.	Heat treatment of hot rolled steel sheet		Microstructure after heat treatment	Heat treatment of cold rolled steel sheet (annealing)		Microstructure after annealing			Type of blank steel sheet	Surface treatment of blank steel sheet	Category
Blank steel sheet No.	Steel No.	Heating temperature T1 (°C)	Retention time (hr)	Mns /Mnα	Heating temperature T2 (°C)	Retention time (s)	Mean grain size of ferrite (µm)	Mean grain size of secondary phase (µm)	Mns /Mnα	Type of blank steel sheet	Surface treatment of blank steel sheet	Category
A1	A	675	2.0	-	675	32	2.3	2.7	2.0	Cold rolled steel sheet	Zn-Ni	Example
A2	A	640	2.0	-	675	30	2.5	2.8	1.9	Cold rolled steel sheet	Zn-Ni	Example
A3	A	715	2.0	-	675	30	2.3	2.6	2.0	Cold rolled steel sheet	Zn-Ni	Example
A4	A	750	2.0	-	675	31	26	2.8	1.8	Cold rolled steel sheet	Zn-Ni	Example
A5	A	675	2.0	1.4	-	-	-	-	-	Hot rolled steel sheet	None	Example *
A6	A	835	2.0	-	675	30	2.5	8.9	0.8	Cold rolled steel sheet	None	Comparative Example
A7	A	600	2.5	-	675	32	3.5	5.2	0.6	Cold rolled steel sheet	GA	Comparative Example
A8	A	670	0.2	-	672	30	2.4	5.4	0.9	Cold rolled steel sheet	GI	Comparative Example
A9	A	675	50.0	-	674	30	29	59	0.8	Cold rolled steel sheet	None	Comparative Example
B1	B	675	2.5	-	680	35	3.2	3.6	1.6	Cold rolled steel sheet	None	Example
B2	B	675	2.5	1.3	-	-	-	-	-	Hot rolled steel sheet	None	Example *
C1	C	620	5.5	-	625	100	3.5	3.9	2.5	Cold rolled steel sheet	None	Example
C2	C	620	5.3	-	625	100	3.5	3.8	2.5	Cold rolled steel sheet	None	Comparative Example
C3	C	620	5.5	-	624	100	3.4	3.9	2.5	Cold rolled steel sheet	None	Comparative Example
D1	D	600	10.5	-	615	105	3.2	3.6	2.4	Cold rolled steel sheet	None	Example
E1	E	650	10.5	-	665	102	4.5	5.0	2.3	Cold rolled steel sheet	Zn-Ni	Example
E2	E	650	10.5	-	665	102	4.5	5.0	2.3	Cold rolled steel sheet	Zn-Ni	Example
E3	E	650	10.5	-	665	102	4.5	5.0	2.3	Cold rolled steel sheet	Zn-Ni	Example
E4	E	650	10.5	-	665	102	4.5	5.0	2.3	Cold rolled steel sheet	Al-Si	Example
F1	F	673	2.0	-	675	32	2.5	2.9	2.1	Cold rolled steel sheet	Zn-Ni	Comparative Example
G1	G	673	2.0	-	675	30	2.2	2.6	2.2	Cold rolled steel sheet	Zn-Ni	Comparative Example
H1	H	602	10.5	-	620	105	3.1	3.5	2.5	Cold rolled steel sheet	None	Comparative Example
I1	I	675	2.5	-	685	35	3.1	3.5	1.7	Cold rolled steel sheet	None	Comparative Example
A10	A	675	20	-	675	32	2.3	2.7	2.0	Cold rolled steel sheet	Zn-Ni	Example
A11	A	675	2.0	-	675	32	2.3	2.7	2.0	Cold rolled steel sheet	Zn-Ni	Example
A12	A	675	2.0	-	675	32	2.3	2.7	2.0	Cold rolled steel sheet	Zn-Ni	Example
A13	A	675	2.0	-	675	32	2.3	2.7	2.0	Cold rolled steel sheet	Zn-Ni	Comparative Example

*: Reference Example, not in accordance with invention

Table 4

Steel No.	C (mass%)	Mn (mass%)	Si (mass%)	P (mass%)	S (mass%)	Al (mass%)	N (mass%)	Ac1 point (°C)	Ac3 point (°C)	Ms point (°C)	Others (mass%)	Category
J	0.205	6.23	0.03	0.015	0.004	0.031	0.003	573	795	263	Ni: 0.02	Conforming steel
K	0.210	6.25	0.02	0.015	0.003	0.031	0.004	573	793	260	Cu: 0.02	Conforming steel
L	0.223	6.35	0.02	0.010	0.004	0.032	0.003	574	793	248	Cr: 0.30	Conforming steel
M	0.221	6.15	0.03	0.011	0.004	0.035	0.003	576	791	257	Mo: 0.25	Conforming steel
N	0.225	4.20	0.02	0.032	0.004	0.035	0.005	630	798	316	Ti: 0.03	Conforming steel
O	0.215	4.31	0.03	0.034	0.003	0.035	0.004	627	800	317	Nb: 0.03	Conforming steel
P	0.205	4.15	0.04	0.035	0.005	0.035	0.003	632	803	326	V: 0.03	Conforming steel
Q	0.217	4.36	0.06	0.035	0.002	0.034	0.005	626	801	315	W: 0.03	Conforming steel
R	0.218	4.25	0.22	0.015	0.004	0.036	0.005	631	808	318	Ti: 0.02, B: 0.002	Conforming steel
S	0.213	6.25	0.27	0.016	0.003	0.036	0.005	576	803	259	Ti: 0.02, B: 0.002	Conforming steel
T	0.156	6.23	0.15	0.016	0.002	0.038	0.004	576	812	284	REM: 0.001	Conforming steel
U	0.224	6.20	0.16	0.025	0.002	0.050	0.003	576	796	256	B: 0.003	Conforming steel
V	0.152	6.22	0.22	0.025	0.005	0.065	0.004	577	816	286	Ca: 0.003	Conforming steel
W	0.148	6.22	0.16	0.035	0.004	0.058	0.003	576	814	287	Mg: 0.003	Conforming steel
X	0.203	4.31	0.13	0.035	0.003	0.065	0.006	629	807	322	Sb: 0.008	Conforming steel

Table 5

Blank steel sheet No.	Steel No.	Heat treatment of hot rolled steel sheet		Microstructure after heat treatment	Heat treatment of cold rolled steel sheet (annealing)		Microstructure after annealing			Type of blank steel sheet	Surface treatment of blank steel sheet	Category
Blank steel sheet No.	Steel No.	Heating temperature T1 (°C)	Retention time (hr)	Mns /Mnα	Heating temperature T2 (°C)	Retention time (s)	Mean grain size of ferrite (µm)	Mean grain size of secondary phase (µm)	Mns /Mnα	Type of blank steel sheet	Surface treatment of blank steel sheet	Category
J1	J	625	2.0	1.4	-	-	-	-	-	Hot rolled steel sheet	None	Example *
K1	K	630	2.0	-	680	45	3.5	5.6	2.3	Cold rolled steel sheet	Zn-Ni	Example
L1	L	631	2.1	-	685	48	2.5	7.8	2.5	Cold rolled steel sheet	Zn-Ni	Example
M1	M	635	2.5	-	675	45	2.4	8.5	2.2	Cold rolled steel sheet	None	Example
N1	N	675	2.5	1.4	-	-	-	-	-	Hot rolled steel sheet	None	Example*
O1	O	675	3.0	-	675	40	3.2	6.2	2.4	Cold rolled steel sheet	Zn-Ni	Example
P1	P	670	3.5	-	672	150	4.5	7.5	2.5	Cold rolled steel sheet	Zn-Ni	Example
Q1	Q	675	3.5	-	680	44	5.2	8.0	3.2	Cold rolled steel sheet	Zn-Ni	Example
R1	R	675	40.0	-	680	46	3.5	5.9	3.5	Cold rolled steel sheet	Zn-Ni	Example
S1	S	630	35.0	-	645	52	2.6	8.2	3.4	Cold rolled steel sheet	Zn-Ni	Example
T1	T	625	2.5	-	630	55	4.2	6.5	3.0	Cold rolled steel sheet	Zn-Ni	Example
U1	U	630	1.7	-	645	56	5.0	6.3	2.5	Cold rolled steel sheet	Zn-Ni	Example
V1	V	615	1.7	-	630	52	5.2	7.2	2.2	Cold rolled steel sheet	Zn-Ni	Example
W1	W	620	3.0	-	635	105	5.1	6.4	2.6	Cold rolled steel sheet	None	Example
X1	X	680	35.0	-	690	115	5.0	4.3	2.2	Cold rolled steel sheet	None	Example

*: Reference Example, not in accordance with invention

All Examples achieved a tensile strength TS of 1500 MPa or more, a uniform elongation uEl of 6.0 % or more, and ΔYS of 150 MPa or more. Comparative Examples, on the other hand, failed to satisfy at least one of the properties.

INDUSTRIAL APPLICABILITY

The hot pressed member obtainable by the present invention is suitable as a structural member required to have high collision energy absorbing performance, such as an impact beam, a center pillar, or a bumper of a vehicle.

Claims

A method of manufacturing a hot pressed member, the method comprising:
heating a slab and hot rolling the slab to obtain a hot rolled steel sheet, the slab having a chemical composition consisting of, in mass%,
C: 0.090 % or more and less than 0.30 %,

Mn: 3.5 % or more and less than 11.0 %,

Si: 0.01 % to 2.5 %,

P: 0.05 % or less,

S: 0.05 % or less,

Al: 0.005 % to 0.1 %,

N: 0.01 % or less,

optionally one or more groups selected from
A group: one or more selected from Ni: 0.01 % to 5.0 %, Cu: 0.01 % to 5.0 %, Cr: 0.01 % to 5.0 %, and Mo: 0.01 % to 3.0 %,

B group: one or more selected from Ti: 0.005 % to 3.0 %, Nb: 0.005 % to 3.0 %, V: 0.005 % to 3.0 %, and W: 0.005 % to 3.0 %,

C group: one or more selected from REM: 0.0005 % to 0.01 %, Ca: 0.0005 % to 0.01 %, and Mg: 0.0005 % to 0.01 %,

D group: Sb: 0.002 % to 0.03 %, and

E group: B: 0.0005 % to 0.05 %,

with a balance being Fe and incidental impurities including O, oxygen, 0.0100 % or less;

heating the hot rolled steel sheet to a first temperature that is an Ac1 point or more and an Ac3 point or less, retaining the hot rolled steel sheet at the first temperature for 1 hr or more and 48 hr or less, and then cooling the hot rolled steel sheet to obtain a first blank steel sheet;

cold rolling the first blank steel sheet to obtain a cold rolled steel sheet and annealing the cold rolled steel sheet to obtain a second blank steel sheet, the annealing including heating the cold rolled steel sheet to a temperature that is the Ac1 point or more and the Ac3 point or less, retaining the cold rolled steel sheet at the temperature for 30 seconds or more and 300 seconds or less, and then cooling the cold rolled steel sheet,

performing a heating process of heating the second blank steel sheet to a second temperature that is the Ac3 point or more and 1000 °C or less and retaining the second blank steel sheet at the second temperature for 900 sec or less; and

thereafter performing a hot press forming process of simultaneously press forming and quenching the second blank steel sheet using a press tool for forming, to obtain a hot pressed member,

where Ac1 point, °C, and Ac3 point, °C, are calculated according to the following expressions: $Ac 1 point, ° C = 751 - 16 C + 11 Si - 28 Mn - 5.5 Cu - 16 Ni + 13 Cr + 3.4 Mo$
$Ac 3 point, ° C = 910 - 203 C^{1 / 2} + 44.7 Si - 4 Mn + 11 Cr$

where C, Si, Mn, Ni, Cu, Cr, and Mo are each the content in mass% of the corresponding element, in the case where the element is not contained, the content of the element is assumed to be 0.
The method of manufacturing a hot pressed member according to claim 1, further comprising
forming a coated layer on a surface of the second blank steel sheet, before the heating process.
The method of manufacturing a hot pressed member according to claim 2,
wherein the coated layer is any of a zinc or zinc alloy coated layer and an aluminum or aluminum alloy coated layer.
The method of manufacturing a hot pressed member according to claim 3,
wherein the zinc or zinc alloy coated layer contains Ni: 10 mass% to 25 mass%.
The method of manufacturing a hot pressed member according to any one of claims 2 to 4,
wherein a coating weight of the coated layer is 10 g/m² to 90 g/m² per side.