EP2987887B1

EP2987887B1 - High strength hot rolled steel sheet and method for producing same

Info

Publication number: EP2987887B1
Application number: EP14785555.5A
Authority: EP
Inventors: Kazuhiko Yamazaki; Katsumi Nakajima; Chikara Kami
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2013-04-15
Filing date: 2014-03-17
Publication date: 2019-09-11
Anticipated expiration: 2034-03-17
Also published as: EP2987887A4; US20160076124A1; EP2987887A1; CN105102662A; KR20160041850A; MX2015014437A; WO2014171063A1; MX2020003923A

Description

Technical Field

The present invention relates to a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more, which is suitable for a material for structural parts and frameworks of automobiles, frames of trucks, steel pipes, and the like.

Background Art

In recent years, automobile exhaust gas regulations have been tightened from the viewpoint of global environmental conservation. Under such circumstances, an improvement of fuel efficiency of automobiles, e.g., trucks, has been an important issue and enhancement of strength and reduction in thickness of the material employed have been further required. Along with this, in particular, high strength hot rolled steel sheets have been actively applied to materials for automobile parts.
Also, in accordance with a demand for further reduction in the construction cost of pipeline, reduction in the material cost of steel pipes have been required. Consequently, instead of UOE steel pipes formed from steel plate, high strength welded steel pipes produced from coil-shaped hot rolled steel sheets with low-price and high productivity, have been noted as transport pipes.
As described above, demands for high strength hot rolled steel sheets having predetermined strength as materials for automotive parts and materials for steel pipes have increased year after year. In particular, the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is highly expected to serve as a material capable of improving fuel efficiency of automobile by leaps and bounds or a material capable of reducing the construction cost of pipeline to a large extent.
However, as the strength of the steel sheet increases, the toughness is degraded in general. Therefore, in order to provide the toughness required of automotive parts and steel pipes to the high strength hot rolled steel sheet, various studies have been conducted on the improvement of toughness.

[Toughness]

For example, Patent Literature 1 proposes a hot rolled steel sheet with sheet thickness: 4.0 mm or more and 12 mm or less, having a composition containing, on a percent by mass basis, C: 0.04% to 0.12%, Si: 0.5% to 1.2%, Mn: 1.0% to 1.8%, P: 0.03% or less, S: 0.0030% or less, Al: 0.005% to 0.20%, N: 0.005% or less, Ti: 0.03% to 0.13%, and the balance being Fe and incidental impurities and a microstructure in which the area fraction of bainite phase is more than 95% and the average grain size of the bainite phase is 3 µm or less, wherein a difference between the Vickers hardness at the position at 50 µm from the surface layer and the Vickers hardness at the position one-quarter of the sheet thickness is specified to be 50 or less, and a difference between the Vickers hardness at the position one-quarter of the sheet thickness and the Vickers hardness at the position at one-half of the sheet thickness is specified to be 40 or less. It is mentioned that according to the technology proposed in Patent Literature 1, a high strength hot rolled steel sheet exhibiting excellent toughness and having tensile stress: 780 MPa or more is obtained by specifying the principal phase to be fine bainite and reducing the hardness distribution in the sheet thickness direction.
Patent Literature 2 proposes a method for manufacturing a steel sheet, including the steps of heating a steel material satisfying, on a percent by mass basis, C: 0.05% to 0.18%, Si: 0.10% to 0.60%, Mn: 0.90% to 2.0%, P: 0.025% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.001% to 0.1%, and N: 0.002% to 0.01%, and the balance being Fe and incidental impurities, to 950°C or higher and 1,250°C or lower, starting rolling, completing the rolling at 820°C or higher, performing cooling to 600°C to 700°C at a cooling rate of 20°C/s or more, performing holding at that temperature range for 10 to 200 seconds or performing slow cooling and, thereafter, performing cooling to 300°C or lower at a cooling rate of 5°C/s or more, wherein the metal microstructure is specified to be ferrite: 70% to 90%, martensite or a mixed phase of martensite and austenite: 3% to 15%, and the remainder: bainite (including the case of 0%) on an area fraction relative to the whole microstructure basis and, in addition, the average grain size of the above-described ferrite is specified to be 20 µm or less. It is mentioned that according to the technology proposed in Patent Literature 2, a high toughness steel sheet which has a tensile strength of 490 N/mm² or more and which exhibits a low yield ratio, where the yield ratio is 70% or less, is obtained by specifying the metal microstructure to be a microstructure including ferrite having fine crystal grains, martensite or a mixed phase of martensite and austenite, and the like.
Patent Literature 3 proposes a method for manufacturing a thick high strength hot rolled steel sheet, including the steps of subjecting a steel material containing, on a percent by mass basis, C: 0.02% to 0.25%, Si: 1.0% or less, Mn: 0.3% to 2.3%, P: 0.03% or less, S: 0.03% or less, Al: 0.1% or less, Nb: 0.03% to 0.25%, and Ti: 0.001% to 0.10%, where (Ti + Nb/2)/C < 4 is satisfied, to hot rolling, applying first cooling after finish rolling of the hot rolling is completed, where accelerated cooling is performed at an average cooling rate of hot-rolled sheet surface of 20°C/s or more and less than martensite formation critical cooling rate until the surface temperature reaches the Ar₃ transformation temperature or lower and the Ms temperature or lower, applying second cooling, where quenching is performed until the sheet thickness center temperature reaches 350°C or higher and lower than 600°C, performing coiling into the shape of a coil at a coiling temperature of 350°C or higher and lower than 600°C on a sheet thickness center temperature basis, and applying third cooling, where at least the position at one-quarter of the sheet thickness in the coil thickness direction to the position at three-quarters of the sheet thickness is held or retained at a temperature range of 350°C to 600°C for 30 minutes or more, sequentially. It is mentioned that according to the technology proposed in Patent Literature 3, a material for X65 grade or higher of high strength electric resistance welded steel pipe exhibiting excellent low-temperature toughness is obtained by specifying the microstructure of the hot rolled steel sheet to be a bainite phase or bainitic ferrite phase and, furthermore, adjusting the amount of grain boundary cementite to a specific value or less. A further high-strength hot rolled steel sheet having excellent toughness and a method for producing the same is disclosed in Patent Literature 4.

Patent Literature

PTL 1: Japanese Unexamined Patent Application Publication No. 2012-062557
PTL 2: Japanese Unexamined Patent Application Publication No. 2007-056294
PTL 3: Japanese Unexamined Patent Application Publication No. 2010-174343
PTL 4: WO 2012/036307

Summary of Invention

Technical Problem

[Toughness]

In the technology proposed in Patent Literature 1, the high strength hot rolled steel sheet having tensile strength: 980 MPa or more is obtained. However, the control of the bainite microstructure is insufficient and, thereby, there is a problem that excellent low-temperature toughness cannot be obtained stably.
Also, in the technology proposed in Patent Literature 2, the metal microstructure of the steel is specified to be the structure including a ferrite phase as a primary phase, although in the case where the tensile strength is in the 980 MPa class, the toughness of the ferrite phase may be degraded significantly.
Also, in the technology proposed in Patent Literature 3, an improvement of the low-temperature toughness by controlling the amount of grain boundary cementite is intended, although the hot rolled steel sheet strength is insufficient and, as shown in the example thereof, tensile strength: about 800 MPa is the maximum. In this regard, in the case where a high strength hot rolled steel sheet having tensile strength: 980 MPa or more is obtained on the basis of the technology proposed in Patent Literature 3, it is necessary that the C content be increased. However, the control of the grain boundary cementite becomes difficult as the C content increases, so that excellent toughness cannot be obtained stably in some cases.
The present invention solves the above-described problems included in the technologies of the related art advantageously, and it is an object to provide a high strength hot rolled steel sheet having high strength of tensile strength: 980 MPa or more, further exhibiting good toughness and, in particular, having a sheet thickness of 4 mm or more and 15 mm or less and a method for manufacturing the same.

Solution to Problem

[Toughness]

In order to achieve the object, the present inventors conducted intensive research to improve the toughness of a hot rolled steel sheet while the high strength of tensile strength TS: 980 MPa or more had. Specifically, the bainite phase was noted, where it is known that the bainite phase has good strength-toughness balance in general, and various factors affecting the strength and the toughness of the hot rolled steel sheet, in which the primary phase of the microstructure was bainite, were studied. As a result, it was found that allowing laths of the bainite phase to become fine was very effective in enhancing strength and improving toughness of the hot rolled steel sheet. Then, further studies were conducted. As a result, it was found that the toughness was improved considerably while the high strength of tensile strength TS: 980 MPa or more was maintained by adding predetermined amounts of Ti and V, specifying the primary phase to be more than 85% on an area fraction basis of bainite phase, specifying the lath interval of the bainite phase to be 400 nm or less in average, and specifying the length of long axis of the lath to be 5.0 µm or less in average.
The present invention has been completed on the basis of the above-described findings and additional studies. That is, the gist configuration of the present invention is as described in product claim 1 and method claim 2.

[Toughness]

According to the present invention, a high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and exhibiting excellent toughness is obtained. Therefore, the car body weight can be reduced while the safety of the automobile is ensured and an environmental load can be reduced by applying the present invention to structural parts and frameworks of automobiles, frames of trucks, and the like. In the case where a welded steel pipe produced from the hot rolled steel sheet according to the present invention serving as a material instead of the UOE pipe produced from a steel plate serving as a material is applied to a transport pipe, the productivity is improved and the cost can be further reduced.
Also, the present invention can stably produce a hot rolled steel sheet exhibiting improved toughness while high strength of tensile strength: 980 MPa or more has and, therefore, is very useful for the industry.

Description of Embodiment

The embodiment will be specifically described below.
To begin with, reasons for the limitation of the chemical composition of the hot rolled steel sheet according to the present invention will be described. Hereafter the term "%" representing the chemical composition refers to "percent by mass" unless otherwise specified.

C: 0.10% or more and 0.16% or less

C enhances the strength of the steel and facilitates formation of bainite. Therefore, in the present invention, it is necessary that the C content be 0.10% or more. On the other hand, if the C content is more than 0.16%, formation control of bainite becomes difficult, formation of hard martensite increases, and the toughness of the hot rolled steel sheet is degraded. Consequently, the C content is specified to be more than 0.10% and 0.16% or less. In this regard, in the case where the amount of Mn is 2.5% or more and 3.5% or less, the amount of C is preferably 0.06% or more and 0.15% or less.

Si: 1.0% or less

Si is an element which suppresses coarse oxides and cementite to impair the toughness and which contributes to solute strengthening. If the content is more than 1.0%, the surface quality of the hot rolled steel sheet is degraded significantly and degradation in the chemical conversion treatability and the corrosion resistance is caused. Therefore, the Si content is specified to be 1.0% or less, and preferably 0.4% or more and 0.8% or less.

Mn: 1.8% or more and 3.5% or less

Mn is an element which contributes to enhancement of strength of the steel through solid solution and which facilitates formation of bainite through improvement of the hardenability. In order to obtain such effects, it is necessary that the Mn content be 1.8% or more. On the other hand, if the Mn content is more than 3.5%, center segregation becomes considerable, and the toughness of the hot rolled steel sheet is degraded. Therefore, the Mn content is specified to be 1.8% or more and 3.5% or less. In this regard, 1.8% or more and 2.5% or less is preferable.

P: 0.04% or less

P is an element which contributes to enhancement of strength of the steel through solid solution but is an element which segregates at grain boundaries, in particular prior-austenite grain boundaries, to cause degradation in low-temperature toughness and workability. Consequently, it is preferable that the P content be minimized, although the content up to 0.04% is allowable. Therefore, the P content is specified to be 0.04% or less. However, when the P content is excessively reduced, an effect corresponding to an increase in the smelting cost is not obtained, so that the P content is specified to be preferably 0.003% or more and 0.03% or less, and more preferably 0.005% or more and 0.02% or less.

S: 0.006% or less

S forms coarse sulfides by bonding to Ti and Mn and degrades the workability of the hot rolled steel sheet. Consequently, it is preferable that the S content be minimized, although the content up to 0.006% is allowable. Therefore, the S content is specified to be 0.006% or less. However, when the S content is excessively reduced, an effect corresponding to an increase in the smelting cost is not obtained, so that the S content is specified to be preferably 0.0003% or more and 0.004% or less, and more preferably 0.0005% or more and 0.002% or less.

Al: 0.10% or less

A1 is an element which functions as a deoxidizing agent and which is effective in improving cleanliness of the steel. On the other hand, excessive addition of Al causes increases in oxide inclusions, degrades the toughness of the hot rolled steel sheet and, in addition, causes an occurrence of flaw. Therefore, the Al content is specified to be 0.10% or less, preferably 0.005% or more and 0.08% or less, and further preferably 0.01% or more and 0.05% or less.

N: 0.008% or less

Ni precipitates as nitrides by bonding to nitride-forming elements and contributes to making crystal grains fine. However, N bonds to Ti at a high temperature to form coarse nitrides easily and degrades the toughness of the hot rolled steel sheet. Consequently, the N content is specified to be 0.008% or less, preferably 0.001% or more and 0.006% or less, and more preferably 0.002% or more and 0.005% or less.

Ti: 0.05% or more and 0.20% or less

Ti is one of the most important elements in the present invention. Ti contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, Ti forms many fine (Ti,V)C clusters at low temperatures of 300°C or higher and 450°C or lower, reduces the amount of cementite in the steel, and improve the toughness of the hot rolled steel sheet. In order to exert such effects, it is necessary that the Ti content be 0.05% or more. On the other hand, if the Ti content is excessive and is more than 0.20%, the above-described effects are saturated, an increase in coarse precipitates is caused, and degradation in the toughness of the hot rolled steel sheet is caused. Therefore, the Ti content is limited to within the range of 0.05% or more and 0.20% or less, and preferably 0.08% or more and 0.15% or less.

V: more than 0.1% and 0.3% or less

V is one of the most important elements in the present invention. V contributes to enhancement of strength of the steel through formation of carbonitrides to make crystal grains fine and through precipitation strengthening. Also, V improves the hardenability and contributes to formation and making fine of bainite phase. In addition, V forms many fine (Ti,V)C clusters at low temperatures of 300°C or higher and 450°C or lower, reduces the amount of cementite in the steel, and improves the toughness of the hot rolled steel sheet. In order to exert such effects, it is necessary that the V content be more than 0.1%. On the other hand, if the V content is excessive and is more than 0.3%, the above-described effects are saturated, so that the cost increases. Therefore, the V content is limited to within the range of more than 0.1% and 0.3% or less, and preferably 0.15% or more and 0.25% or less.
The basic components of the hot rolled steel sheet according to the present invention are as described above. The hot rolled steel sheet according to the present invention may further contain, as necessary, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, and Mo: 0.005% or more and 0.4% or less for the purpose of, for example, improvement of toughness and enhancement of strength.

Nb: 0.005% or more and 0.4% or less

Nb is an element which contributes to enhancement of strength of the steel through formation of carbonitrides. In order to exert such an effect, it is preferable that the Nb content be 0.005% or more. On the other hand, if the Nb content is more than 0.4%, deformation resistance increases, so that a rolling force of hot rolling increases in production of the hot rolled steel sheet, a load to a rolling mill becomes too large, and rolling operation in itself may become difficult. Meanwhile, if the Nb content is more than 0.4%, coarse precipitates are formed and the toughness of the hot rolled steel sheet tends to be degraded. Therefore, the Nb content is preferably specified to be 0.005% or more and 0.4% or less. In this regard, 0.01% or more and 0.3% or less is more preferable and 0.02% or more and 0.2% or less is further preferable.

B: 0.0002% or more and 0.0020% or less

B is an element which segregates at austenite grain boundaries and which suppresses formation and growth of ferrite. Also, B is an element which improves the hardenability and which contributes to formation and making fine of bainite phase. In order to exert these effects, it is preferable that the B content be 0.0002% or more. However, if the B content is more than 0.0020%, formation of martensite phase is facilitated, so that the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, in the case where B is contained, the content thereof is specified to be preferably 0.0002% or more and 0.0020% or less. In this regard, 0.0004% or more and 0.0012% or less is more preferable.

Cu: 0.005% or more and 0.2% or less

Cu is an element which contributes to enhancement of strength of the steel through solid solution. Also, Cu is an element which has a function of improving hardenability, which lowers, in particular, the bainite transformation temperature, and which contributes to making bainite phase fine. In order to obtain these effects, it is preferable that the Cu content be 0.005% or more, although if the content thereof is more than 0.2%, degradation in the surface quality of the hot rolled steel sheet is caused. Therefore, the Cu content is specified to be preferably 0.005% or more and 0.2% or less. In this regard, 0.01% or more and 0.15% or less is more preferable.

Ni: 0.005% or more and 0.2% or less

Ni is an element which contributes to enhancement of strength of the steel through solid solution. Also, Ni has a function of improving hardenability and facilitates formation of bainite phase. In order to obtain these effects, it is preferable that the Ni content be 0.005% or more. However, if the Ni content is more than 0.2%, a martensite phase is generated easily, and the toughness of the hot rolled steel sheet may be degraded significantly. Therefore, the Ni content is specified to be preferably 0.005% or more and 0.2% or less, and more preferably 0.01% or more and 0.15% or less.

Cr: 0.005% or more and 0.4% or less

Cr forms carbides and contributes to enhancement of strength of the hot rolled steel sheet. In order to exert this effect, it is preferable that the Cr content be 0.005% or more. On the other hand, if the Cr content is excessive and is more than 0.4%, it is feared that the corrosion resistance of the hot rolled steel sheet is degraded. Therefore, the Cr content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.

Mo: 0.005% or more and 0.4% or less

Mo facilitates formation of bainite phase through improvement of the hardenability and contributes to improvement of the toughness and enhancement of strength of the hot rolled steel sheet. In order to obtain such effects, it is preferable that the Mo content be 0.005% or more. However, if the Mo content is more than 0.4%, a martensite phase is generated easily, and the toughness of the hot rolled steel sheet may be degraded. Therefore, the Mo content is specified to be preferably 0.005% or more and 0.4% or less, and more preferably 0.01% or more and 0.2% or less.
Meanwhile, the hot rolled steel sheet according to the present invention may contain, as necessary, one or two selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.

Ca: 0.0002% or more and 0.01% or less

Ca is effective in controlling the shape of sulfide inclusions and improving bending workability and the toughness of the hot rolled steel sheet. In order to exert these effects, it is preferable that the Ca content be 0.0002% or more. However, if the Ca content is more than 0.01%, surface defects of the hot rolled steel sheet may be caused. Therefore, the Ca content is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.

REM: 0.0002% or more and 0.01% or less

As with Ca, REM controls the shape of sulfide inclusions and improves adverse influences of sulfide inclusions on the bending workability and the toughness of the hot rolled steel sheet. In order to exert these effects, it is preferable that the REM content be 0.0002% or more. However, if the REM content is excessive and is more than 0.01%, the cleanliness of the steel tends to be degraded and the toughness of the hot rolled steel sheet tends to be degraded. Therefore, in the case where REM is contained, the content thereof is specified to be preferably 0.0002% or more and 0.01% or less. In this regard, 0.0004% or more and 0.005% or less is more preferable.
In the present invention, the remainder other than those described above is composed of Fe and incidental impurities. Examples of incidental impurities include Sb, Sn, and Zn. As for contents of them, Sb: 0.01% or less, Sn: 0.1% or less, and Zn: 0.01% or less are allowable.
Next, reasons for the limitation of the microstructure of the hot rolled steel sheet according to the present invention will be described.
The hot rolled steel sheet according to the present invention has a microstructure in which a primary phase is more than 85% on an area fraction basis of bainite phase, a secondary phase is at least one of ferrite phase, martensite phase, and retained austenite phase, 0% or more and less than 15% in total on an area fraction basis of secondary phase is contained, the average lath interval of laths of the above-described bainite phase is 400 nm or less, and the average long axis length of the above-described laths is 5.0 µm or less.

Fraction of bainite phase: more than 85% on an area fraction basis

The primary phase of the hot rolled steel sheet according to the present invention is a bainite phase having excellent strength-toughness balance. If the fraction of the bainite phase is 85% or less on an area fraction basis, a hot rolled steel sheet provided with predetermined strength and toughness is not obtained. Therefore, the fraction of the bainite phase is specified to be more than 85% on an area fraction basis, preferably 87% or more, and more preferably 90% or more. It is still more preferable that the fraction of the bainite phase be 100% on an area fraction basis and the microstructure be a bainite single phase microstructure.
Fraction of at least one of ferrite phase, martensite phase, and retained austenite phase (secondary phase): 0% or more and less than 15% in total on an area fraction basis
The hot rolled steel sheet according to the present invention includes a secondary phase, which is composed of at least one of ferrite phase, martensite phase, and retained austenite phase, as a microstructure other than the bainite phase serving as the primary phase. The microstructure is specified to be preferably a bainite single phase microstructure to impart predetermined strength and toughness to the hot rolled steel sheet. However, even in the case where at least one of ferrite phase, martensite phase, and retained austenite phase is included as the secondary phase, the total fraction of them of less than 15% on an area fraction basis is allowable. Therefore, the fraction of the above-described secondary phase in total is specified to be 0% or more and less than 15% on an area fraction basis, preferably 13% or less, and more preferably 11% or less.

Average lath interval of laths of bainite phase: 400 nm or less

Average long axis length of laths of bainite phase: 5.0 µm or less

It is very important for enhancement of strength and enhancement of toughness of the hot rolled steel sheet to make laths of bainite phase fine. The present inventors found that the sizes of laths of bainite phase, specifically, the lath interval and the long axis length of the lath, were factors which influenced greatly the strength and the toughness of the hot rolled steel sheet. Consequently, in the present invention, predetermined strength and toughness are added to the hot rolled steel sheet by specifying the lath interval and the long axis length of the lath of bainite phase.
In the case where the average lath interval of laths of the bainite phase is more than 400 nm or the average long axis length of laths of the bainite phase is more than 5.0 µm, a hot rolled steel sheet exhibiting predetermined strength and toughness according to the present invention in combination is not obtained. Therefore, the average lath interval of laths of the bainite phase is specified to be 400 nm or less, and preferably 350 nm or less. Also, the average long axis length of laths of the bainite phase is specified to be 5.0 µm or less, and preferably 4.0 µm or less. In this regard, lower limits of the average lath interval of laths of the bainite and the average long axis length of laths of the bainite phase are not particularly specified. The lath interval and the long axis length are determined on the basis of the bainite transformation temperature and, therefore, usually the average lath interval of laths of the bainite phase is 100 nm or more and the average long axis length of laths of the bainite phase is 1.0 µm or more.
A high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and having toughness required of a material for structural parts of automobiles and a material for steel pipes, e.g., line pipes, is obtained by specifying the composition and the microstructure, as described above. In this regard, the sheet thickness of the hot rolled steel sheet according to the present invention is not specifically limited, although the sheet thickness is specified to be preferably about 4 mm or more and 15 mm or less.
Next, a preferable method for manufacturing the hot rolled steel sheet according to the present invention will be described.
The present invention is characterized by heating a steel having the above-described composition to 1,200°C or higher, applying hot rolling composed of rough rolling and finish rolling in which the accumulated rolling reduction is 50% or more in a temperature range of 1,000°C or lower and the finishing temperature is 820°C or higher and 930°C or lower, starting cooling within 4.0 s of the hot rolling, performing cooling at an average cooling rate of 20°C/s or more, and performing coiling at a coiling temperature of 300°C or higher and 450°C or lower.
The method for manufacturing a steel is not necessarily particularly limited, and any common method can be applied, wherein a molten steel having the above-described composition is refined in a converter or the like, and a steel, e.g., a slab, is produced by a casting method, e.g., a continuous casting method. In this regard, an ingot-making and blooming method may be used.
Meanwhile, in the present invention, electro-magnetic stirrer (EMS), intentional bulging soft reduction casting (IBSR), and the like can be applied to reduce component segregation of the steel during continuous casting. Equiaxial crystals are formed in the sheet thickness center portion by applying an electro-magnetic stirrer treatment, so that segregation can be reduced. Also, in the case where the intentional bulging soft reduction casting is applied, segregation in the sheet thickness center portion can be reduced by preventing flowing of the molten steel in an unsolidified portion of the continuous casting slab. The toughness described below can be brought to a more excellent level by applying at least one of these segregation reduction treatments.

Heating temperature of steel: 1,200°C or higher

In steel material, e.g., a slab, most of carbonitride-forming elements, e.g., Ti, are present as coarse carbonitrides. The presence of these coarse nonuniform precipitates causes degradation in various characteristics (for example, strength, toughness, and hole expansion workability) of the hot rolled steel sheet. Consequently, the steel material before hot rolling is heated to allow coarse precipitates to form solid solutions. In order to allow these coarse precipitates to form solid solutions sufficiently, it is necessary that the heating temperature of the steel be 1, 200°C or higher. However, if the heating temperature of the steel is too high, an occurrence of slab flaw and reduction in yield due to scale-off are caused. Therefore, the heating temperature of the steel is specified to be preferably 1,350°C or lower, and more preferably 1,220°C or higher and 1,300°C or lower.
In this regard, the steel material is heated to the heating temperature of 1, 200°C or higher and is held for a predetermined time. If the holding time is more than 4,800 seconds, the amount of generation of scale increases and, as a result, scale biting and the like occurs easily in the following hot rolling step, and the surface quality of the hot rolled steel sheet tends to be degraded. Therefore, the holding time of the steel material in the temperature range of 1,200°C or higher is specified to be preferably 4,800 seconds or less, and more preferably 4,000 seconds or less.
Following the heating of the steel material, the steel material is subjected to hot rolling having rough rolling and finish rolling. The condition of the rough rolling is not specifically limited insofar as predetermined sheet bar dimensions are ensured. Following the rough rolling, the finish rolling is applied. In this regard, preferably, descaling is performed before the finish rolling or between stands during rolling. In the finish rolling, the accumulated rolling reduction is specified to be 50% or more in a temperature range of 1,000°C or lower and the finishing temperature is specified to be 820°C or higher and 930°C or lower.

Accumulated rolling reduction in temperature range of 1,000°C or lower: 50% or more

In order to make laths of the bainite phase fine, it is necessary that the rolling reduction in a relatively low temperature range be increased and crystal grains after rolling be allowed to become crystal grains elongated in the rolling direction (crystal grains having a high elongation rate). If the accumulated rolling reduction at 1,000°C or lower is less than 50%, it becomes difficult to make bainite having a predetermined lath structure (average lath interval: 400 nm or less, average long axis length: 5.0 µm or less), and the toughness of the hot rolled steel sheet is degraded. Therefore, the accumulated rolling reduction at 1,000°C or lower is specified to be 50% or more, and preferably 60% or more. However, if the accumulated rolling reduction in a temperature range of 1,000°C or lower is excessively high, crystal grains are excessively elongated in the rolling direction and ferrite is generated easily, so that it may also be difficult to make bainite having a predetermined lath structure. Consequently, the accumulated rolling reduction in a temperature range of 1,000°C or lower is specified to be preferably 80% or less.
Finishing temperature: 820°C or higher and 930°C or lower If the finishing temperature of the finishing rolling is lower than 820°C, rolling is performed at a temperature of two-phase region of ferrite + austenite, so that a deformation microstructure remains after rolling and the toughness of the hot rolled steel sheet is degraded. On the other hand, if the finishing temperature is higher than 930°C, austenite grains grow, and a bainite phase of the hot rolled steel sheet obtained after cooling is coarsened. As a result, it becomes difficult to make a predetermined microstructure, and the toughness of the hot rolled steel sheet is degraded. Therefore, the finishing temperature is specified to be 820°C or higher and 930°C or lower, and preferably 840°C or higher and 920°C or lower. Here, the finishing temperature refers to the surface temperature of a sheet.

Start of forced cooling: within 4.0 s of completion of finish rolling

Forced cooling is started within 4.0 s of, preferably just after, completion of the finish rolling, cooling is stopped at the coiling temperature, and coiling into the shape of a coil is performed. If the time from completion of the finish rolling to start of the forced cooling is more than 4.0 s and is long, austenite grains become coarse, and a bainite phase is coarsened. Also, austenite grains become coarse, so that the hardenability of the steel sheet increases and a martensite phase is generated easily. In the case where the bainite phase is coarsened and the martensite phase is generated easily, predetermined excellent toughness cannot be obtained. Therefore, the forced cooling start time is limited to within 4.0 s of completion of the finish rolling.

Average cooling rate: 20°C/s or more

If the average cooling rate from the finishing temperature to the coiling temperature is less than 20°C/s, a bainite phase having a predetermined area fraction is not obtained. Therefore, the above-described average cooling rate is specified to be 20°C/s or more, and preferably 30°C/s or more. The upper limit of the average cooling rate is not particularly specified. However, if the average cooling rate is too large, the surface temperature becomes too low, and martensite is generated on the steel sheet surface easily. Therefore, the average cooling rate is specified to be preferably 60°C/s or less. In this regard, the above-described average cooling rate is specified to be an average cooling rate of the steel sheet surface.

Coiling temperature: 300°C or higher and 450°C or lower

If the coiling temperature is lower than 300°C, hard martensite phase and retained austenite phase are formed in the microstructure of the inside of the steel sheet. As a result, the hot rolled steel sheet cannot be made a predetermined microstructure and predetermined toughness cannot be obtained. On the other hand, if the coiling temperature is more than 450°C, ferrite and pearlite increase in the microstructure of the inside of the steel sheet. As a result, the lath interval of the bainite phase increases and, thereby, the toughness of the hot rolled steel sheet is degraded significantly. For the above-described reasons, the coiling temperature is specified to be within the range of 300°C or higher and 450°C or lower, and preferably 330°C or higher and 430°C or lower.
In this regard, after the coiling, the hot rolled steel sheet may be subjected to temper rolling following the common method or be subjected to pickling to remove scale formed on the surface. Alternatively, a galvanization process, e.g., hot dip galvanizing or electrogalvanizing, and a chemical conversion treatment may further be applied.

EXAMPLE 1

A molten steel having the composition shown in Table 1 was refined in a converter, and a slab (steel) was produced by a continuous casting method. In the continuous casting, those other than Hot rolled steel sheet No. 1' of Steel Al in Tables 1 to 3 described below were subjected to electro-magnetic stirrer (EMS) for the purpose of segregation reduction treatment of the components. Subsequently, these steel materials were heated under the conditions shown in Table 2, and were subjected to hot rolling having rough rolling and finish rolling under the conditions shown in Table 2. After the finish rolling was completed, cooling was performed under the conditions shown in Table 2, and coiling was performed at coiling temperatures shown in Table 2, so that hot rolled steel sheets having sheet thicknesses shown in Table 2 were produced.
Test pieces were taken from the resulting hot rolled steel sheets, and microstructure observation, a tensile test, and a Charpy impact test were performed. The microstructure observation method and various testing methods were as described below.

(i) Microstructure observation

Fraction of microstructure

A test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in three fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 3,000 times, and the area fraction of each phase was quantified on the basis of an image treatment.

Lath interval of laths of bainite phase

A test piece having size: 10 mm × 15 mm was taken from the hot rolled steel sheet, thin film samples for transmission electron microscope (TEM) observation of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) were produced, and photographs were taken in ten fields of view of each position with TEM at the magnification of 30,000 times. Five straight lines at intervals of 10 mm were drawn at right angles to long axes of at least three laths which were shown in each photograph having a size of 120 mm × 80 mm and which were successively arranged side by side. The length of each line segment between the intersection points of the straight line and the lath boundary was measured and the average value of the resulting lengths of the segments was specified to be the average lath interval.

Long axis length of lath of bainite phase

A test piece for a scanning electron microscope (SEM) was taken from the hot rolled steel sheet, a sheet thickness cross-section parallel to the rolling direction was polished and, thereafter, the microstructure was allowed to appear with a corrosive liquid (3% nital solution). Photographs were taken in five fields of view of each of the position at one-quarter of the sheet thickness and the position at one-half of the sheet thickness (center position of the sheet thickness) with a scanning electron microscope (SEM) at the magnification of 10,000 times. The lengths of long axes of at least 10 laths which were shown in each photograph, where at least three laths were successively arranged side by side, were measured and the average value of the resulting lath long axis lengths was specified to be the average lath long axis length.

(ii) Tensile test

JIS No. 5 test pieces (GL: 50 mm) were taken from the hot rolled steel sheet in such a way that the tensile direction and the rolling direction form a right angle. A tensile test was performed in conformity with JIS Z 2241 (2011) and the yield strength (yield point) YP, the tensile strength (TS), and the total elongation El were determined.

(iii) Charpy impact test

A subsize test piece (V-notch) having a thickness of 5 mm was taken from the hot rolled steel sheet in such a way that the longitudinal direction of the test piece and the rolling direction form a right angle. A Charpy impact test was performed in conformity with JIS Z 2242, the Charpy impact value (vE-₅₀) at a temperature of -50°C was measured, and the toughness was evaluated. Here, the hot rolled steel sheet having a sheet thickness of more than 5 mm was subjected to double-side polishing to produce a test piece having a sheet thickness of 5 mm. As for the hot rolled steel sheet having a sheet thickness of 5 mm or less, a test piece having the original sheet thickness was produced. Then, the test pieces were subjected to the charpy impact test. In the case where the measured vE_-50 value was 40 J or more, the toughness was evaluated as good.

The obtained results are shown in Table 3 and Table 4.

[Table 1]

Steel	Chemical composition (percent by mass) Remainder: Fe and incidental impurities										Remarks
Steel	C	Si	Mn	P	S	Al	N	Ti	V	Others	Remarks
A1	0,06	0,3	3,3	0,017	0,0017	0,073	0,0038	0,15	0,20	-	Comparative steel
B1	0,15	0,7	2,0	0,037	0,0032	0,036	0,0026	0,12	0,20	-	Invention steel
C1	0,21	0,8	2,2	0,026	0,0006	0,042	0,0033	0,10	0,15	-	Comparative steel
D1	0,14	1,3	2,0	0,005	0,0009	0,043	0,0042	0,11	0,30	-	Comparative steel
E1	0,15	0,2	2,3	0,026	0,0012	0,022	0,0031	0,12	0,15	-	Invention steel
F1	0,14	0,5	3,8	0,018	0,0006	0,038	0,0039	0,09	0,25	-	Comparative steel
G1	0,04	0,7	2,0	0,021	0,0028	0,031	0,0025	0,03	0,20	-	Comparative steel
H1	0,04	0,9	1,6	0,022	0,0015	0,028	0,0037	0,23	0,11	-	Comparative steel
I1	0,13	0,6	1,6	0,016	0,0010	0,010	0,0028	0,17	0,05	-	Comparative steel
J1	0,15	0,8	1,8	0,025	0,0010	0,047	0,0029	0,06	0,25	-	Invention steel
K1	0,15	0,3	3,0	0,027	0,0008	0,067	0,0074	0,08	0,20	-	Invention steel
L1	0,12	0,8	1,3	0,027	0,0021	0,019	0,0039	0,18	0,11	-	Comparative steel
M1	0,18	0,7	1,7	0,016	0,0009	0,055	0,0047	0,09	0,20	-	Comparative steel
P1	0,14	0,3	1,8	0,030	0,0008	0,037	0,0070	0,12	0,20	Ni:0.1, Cr:0.1	Invention steel
Q1	0,16	0,7	2,1	0,020	0,0011	0,047	0,0041	0,15	0,15	Mo:0.15	Invention steel
R1	0,15	0,5	1,9	0,035	0,0055	0,005	0,0034	0,11	0,22	B:0.0005	Invention steel
S1	0,16	0,9	2,4	0,029	0,0016	0,093	0,0033	0,11	0,30	Ca:0.005	Invention steel
T1	0,15	0,7	2,3	0,017	0,0045	0,031	0,0039	0,11	0,15	REM:0.005	Invention steel
U1	0,13	0,6	2,2	0,022	0,0035	0,027	0,0037	0,13	0,18	Cu:0.1	Invention steel

[Table 2]

Hot rolled steel sheet No.	Steel	Hot rolling condition						Sheet thickness (mm)	Remarks
Hot rolled steel sheet No.	Steel	Slab heating temperature (°C)	Finish rolling accumulated rolling reduction at 1000°Cor lower (%)	Finishing temperature (°C)	Cooling start time (s)*	Average cooling rate (°C/s)	Coiling temperature (°C)	Sheet thickness (mm)	Remarks
2	A1	1220	80	910	1	40	470	4	Comparative example
3	A1	1220	80	800	1	40	430	4	Comparative example
4	B1	1240	75	910	1,5	35	410	6	Invention example
5	B1	1220	75	850	1,5	35	380	6	Invention example
6	C1	1220	55	900	3	25	390	12	Comparative example
7	D1	1240	75	880	1,5	35	350	6	Comparative example
8	E1	1220	55	950	2,5	25	370	10	Comparative example
9	E1	1220	55	920	2,5	25	350	10	Invention example
10	F1	1220	55	840	3	25	380	12	Comparative example
11	G1	1220	60	870	2	30	320	8	Comparative example
12	H1	1240	50	910	3,5	20	430	14	Comparative example
13	I1	1220	75	880	1,5	35	380	6	Comparative example
14	J1	1220	45	910	3,5	20	400	14	Comparative example
15	J1	1220	50	880	3,5	20	310	14	Invention example
16	K1	1240	75	900	1,5	10	370	6	Comparative example
17	K1	1220	75	920	1,5	30	350	6	Invention example
19	L1	1170	80	850	1	40	430	4	Comparative example
20	M1	1260	60	900	2	30	280	8	Comparative example
23	P1	1200	60	840	2	30	410	8	Invention example
24	Q1	1280	80	910	1	40	380	4	Invention example
25	R1	1220	80	880	1	35	350	4	Invention example
26	S1	1220	75	840	1,5	35	360	6	Invention example
27	T1	1220	75	910	1,5	35	390	6	Invention example
28	U1	1220	75	870	1,5	45	370	6	Invention example
*time from completion of finish rolling to start of forced cooling

[Table 3]

Hot rolled steel sheet No.	Steel	Microstructure of hot rolled steel sheet **								Remarks
		B area fraction (%)		F+M+γarea fraction (%)		B average lath interval (nm)		B lath average long axis length (µm)
		1/4 of sheet thickness	1/2 of sheet thickness	1/4 of sheet thickness	1/2 of sheet thickness	1/4 of sheet thickness	1/2 of sheet thickness	1/4 of sheet thickness	1/2 of sheet thickness
2	A1	83	84	17	16	290	320	2,8	3,4	Comparative example
3	A1	82	83	18	17	280	310	2,2	2,5	Comparative example
4	B1	86	88	14	12	340	370	3,2	3,9	Invention example
5	B1	87	89	13	11	330	350	2,8	3,1	Invention example
6	C1	87	90	13	10	380	400	4,6	4,9	Comparative example
7	D1	88	91	12	9	350	370	3,4	3,8	Comparative example
8	E1	88	90	12	10	330	360	5,2	5,8	Comparative example
9	E1	88	90	12	10	320	350	4,5	4.9	Invention example
10	F1	87	91	13	9	360	380	4,2	4,6	Comparative example
11	G1	89	92	11	8	350	380	5,3	6.1	Comparative example
12	H1	86	88	14	12	520	560	4,2	4.7	Comparative example
13	11	87	91	13	9	370	390	3,9	4,8	Comparative example
14	J1	87	90	13	10	420	440	5,7	6,5	Comparative example
15	J1	90	92	10	8	380	390	4,6	4,9	Invention example
16	K1	81	84	19	16	410	440	3,3	3,8	Comparative example
17	K1	88	92	12	8	280	330	3,5	3,9	Invention example
19	L1	86	88	14	12	370	390	2,6	3,1	Comparative example
20	M1	82	85	18	15	310	350	4,1	4,8	Comparative example
23	P1	86	89	14	11	310	350	3,4	3,9	Invention example
24	Q1	87	88	13	12	300	340	3,0	3,5	Invention example
25	R1	88	90	12	10	290	330	2,8	3,2	Invention example
26	S1	88	91	12	9	290	320	2,5	2,9	Invention example
27	T1	87	90	13	10	340	380	3,4	3,6	Invention example
28	U1	91	93	9	7	320	350	2,9	3,3	Invention example
** B: bainite phase, F: ferrite phase, M: martensite phase, γ: retained austenite phase

[Table 4]

Hot rolled steel sheet No.	Steel	Mechanical characteristics of hot rolled steel sheet				Remarks
Hot rolled steel sheet No.	Steel	Yield stress YP (Mpa)	Tensile strength TS (Mpa)	Total elongation EI (%)	vE_-50 (J)	Remarks
2	A1	809	982	12,9	28	Comparative example
3	A1	820	1012	11,7	31	Comparative example
4	B1	829	983	15,0	46	Invention example
5	B1	877	1028	14,8	53	Invention example
6	C1	842	990	17,3	31	Comparative example
7	D1	999	1157	13,5	36	Comparative example
8	E1	846	988	17,1	28	Comparative example
9	E1	879	1018	16,9	47	Invention example
10	F1	915	1072	16,0	27	Comparative example
11	G1	847	970	17,6	29	Comparative example
12	H1	894	1069	15,7	15	Comparative example
13	I1	862	1011	15,0	30	Comparative example
14	J1	786	928	18,9	19	Comparative example
15	J1	932	1063	17,8	47	Invention example
16	K1	804	993	13,2	18	Comparative example
17	K1	883	1023	15,3	68	Invention example
19	L1	819	978	13,6	34	Comparative example
20	M1	920	1122	12,5	23	Comparative example
23	P1	836	992	15,7	43	Invention example
24	Q1	942	1104	12,6	52	Invention example
25	R1	942	1091	13,3	72	Invention example
26	S1	967	1124	13,7	69	Invention example
27	T1	844	993	15,2	44	Invention example
28	U1	923	1078	14,2	50	Invention example

The hot rolled steel sheets of Invention examples are hot rolled steel sheets having predetermined strength (TS: 980 MPa or more) and excellent toughness (vE_-50 value: 40 J or more) in combination. Also, the hot rolled steel sheets of Invention examples have predetermined strength and excellent toughness at each of the position at 1/4 of sheet thickness and the position at 1/2 of sheet thickness (sheet thickness center position) and, therefore, are hot rolled steel sheets having good characteristics in the entire region in the sheet thickness direction. On the other hand, the hot rolled steel sheets of Comparative examples out of the scope of the present invention are unable to obtained predetermined strength or are unable to obtained sufficient toughness.

Claims

A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, comprising a composition and a microstructure,
the composition consisting of, on a percent by mass basis,
C: more than 0.10% and 0.16% or less, Si: 1.0% or less,
Mn: 1.8% or more and 3.5% or less, P: 0.04% or less,
S: 0.006% or less, Al: 0.10% or less,
N: 0.008% or less, Ti: 0.05% or more and 0.20% or less,
V: more than 0.1% and 0.3% or less, optionally at least one selected from
Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.4% or less, Mo: 0.005% or more and 0.4% or less, Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less, and
the balance being Fe and incidental impurities including Sb: 0.01 % or less, Sn: 0.1 % or less and Zn: 0.01 % or less, and
the microstructure comprising a primary phase and a secondary phase,

the primary phase being a bainite phase having an area fraction of more than 85%,

the secondary phase being at least one of ferrite phase, martensite phase, and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total,

the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 µm or less.
A method for manufacturing a high strength hot rolled steel sheet, comprising:
heating a steel material having the composition according to Claim 1 to 1,200°C or higher,

applying hot rolling having rough rolling and finish rolling, the finish rolling having an accumulated rolling reduction of 50% or more in a temperature range of 1,000°C or lower and a finishing temperature of 820°C or higher and 930°C or lower,

starting cooling within 4.0 s after the hot rolling,

performing cooling at an average cooling rate of 20°C/s or more, and

performing coiling at a coiling temperature of 300°C or higher and 450°C or lower.