JP2014205890A - High strength hot rolled steel sheet excellent in bore expanding workability and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet excellent in bore expanding workability with tensile strength of 980 MPa or more.SOLUTION: A steel raw material containing, by mass%, C:over 0.1% to 0.2%, Si:1.0% or less, Mn:1.5 to 2.5%, Al:0.10% or less, N:0.007% or less, Ti:0.07 to 0.2%, V:over 0.1% to 0.3%, is heated at 1200°C or more, applied to rough rolling and finish rolling at completion temperature of 850 to 950°C, first step cooling for cooling to a cooling stop temperature of 500 to 600°C at an average cooling rate of 20 to 80°C/s, then second step cooling for cooling to a cooling stop temperature of 330 to 470°C at the average cooling rate of 90°C/s or more, and winding up at winding temperature of 330 to 470°C. This leads a high strength hot rolled steel sheet having a structure of a bainite phase of 90% or more by area ratio as a main phase, dispersed cementite of 0.8 mass% or less and an average particle diameter of 150 nm or less, and excellent bore expanding workability with tensile strength of 980 MPa or more.

Description

  The present invention relates to a high-strength hot-rolled steel sheet having a tensile strength TS of 980 MPa or more, which is suitable as a material for automobile undercarriage parts, structural parts, frames, truck frames, and the like, and in particular, to expand holes. It relates to improvement of workability. Note that the “steel plate” here includes a steel strip.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of protecting the global environment, and further weight reduction of automobile bodies has been demanded. Therefore, a further increase in strength (thinning) is required for the steel sheet for automobiles, which is a material for automobile parts. However, generally, when the strength of a steel plate is increased, the formability (workability) is lowered, so that improvement of the formability of the high strength steel plate is desired. In particular, high strength steel sheets for automobile parts are strongly requested to improve the hole expansion workability (also referred to as stretch flangeability), and various studies have been made.

  For example, Patent Document 1 includes mass%, C: 0.05 to 0.15%, Si: 0.2 to 1.2%, Mn: 1.0 to 2.0%, P: 0.04% or less, S: 0.005% or less, Ti: 0.05 to 0.15. %, Al: 0.005 to 0.10%, N: 0.007% or less, and after heating the steel material having the composition of the balance iron and inevitable impurities to 1150 to 1350 ° C, preferably more than 1200 ° C and 1350 ° C or less , 850 to 950 ° C., preferably over 900 ° C. and finished at a finishing temperature of 950 ° C. or less, and after the hot rolling is finished, it is cooled to 530 ° C. at an average cooling rate of 30 ° C./s or more. A method for producing a high-strength hot-rolled steel sheet is described in which the coil is cooled to a winding temperature of 300 to 500 ° C. at an average cooling rate of 100 ° C./s or more and wound at the winding temperature. As a result, after having a structure composed of a single phase of bainite phase having an average particle size of 5 μm or less, preferably more than 3.0 to 5.0 μm, TS of 780 MPa or more is obtained by leaving 0.02% or more of solid solution Ti. It is said that stretch flangeability and fatigue resistance will be remarkably improved while maintaining. Instead of a structure consisting of a single phase of bainite phase, a bainite phase having an area ratio of 90% or more and a second phase other than the bainite phase, and a structure in which the average particle size of the second phase is 3 μm or less It ’s good.

Patent Document 2 includes mass%, C: 0.01 to 0.08%, Si: 0.30 to 1.50%, Mn: 0.50 to 2.50%, P: 0.03% or less, S: 0.005% or less, and Ti: 0.01 to 0.20%, Nb: contains 0.01 to 0.04% of one or, in slab and the balance of iron and unavoidable impurities, after the rolling end temperature was subjected to hot rolling as Ar ₃ transformation point to 950 ° C., High-strength hot-rolled steel sheet that is cooled to 650-800 ° C at a cooling rate of 20 ° C / s or higher, then air-cooled for 2-15s, and further cooled to 350-600 ° C at a cooling rate of 20 ° C / s or higher. The manufacturing method is described. As a result, a high-strength hot-rolled steel sheet having a ferrite-bainite dual-phase structure in which the proportion of ferrite with a grain size of 2 μm or more is 80% or more, TS: 690 MPa or more and excellent in hole expansibility and ductility is obtained Yes. Note that one or two of Ca and REM may be contained in an amount of 0.0005 to 0.01%.

Patent Document 3 describes a high-strength thin steel sheet having excellent hole expandability and ductility. The high-strength thin steel sheet described in Patent Document 3 is in mass%, C: 0.01 to 0.20%, Si: 1.50% or less, Al: 1.5% or less, Mn: 0.5 to 3.5%, P: 0.2% or less, S : 0.0005 to 0.009%, N: 0.009% or less, Mg: 0.0006 to 0.01%, O: 0.005% or less, and Ti: 0.01 to 0.20%, Nb: 0.01 to 0.10%. Consists of iron and inevitable impurities.
[Mg%] ≧ ([O%] / 16 × 0.8) × 24 (1)
[S%] ≤ ([Mg%] / 24- [O%] / 16 × 0.8 + 0.00012) × 32 (2)
[S%] ≦ 0.0075 / [Mn%] (3)
It is a thin steel sheet that satisfies all of the above and whose structure is mainly a bainite phase. As a result, TS: 980MPa or higher high strength steel sheet with excellent hole expansibility and ductility. In the technique described in Patent Document 3, the balance of addition of O, Mg, Mn, and S is adjusted to a certain condition, and uniform refinement of (Nb, Ti) N is achieved by utilizing composite precipitation of MgO and MgS. It is said that fine uniform voids are generated in the cross-section of the punched hole to relieve stress concentration during the hole expanding process and improve the hole expandability.

JP 2012-12701 A JP 2002-180190 A JP 2005-120437 A

  In the technique described in Patent Document 1, the target strength is the tensile strength TS: 780 MPa or more, but if the C content is increased, the tensile strength TS: 980 MPa or more can be ensured. it can. However, if the C content is increased to further increase the strength, it becomes difficult to control the amount of precipitation of Ti carbide, and 0.02% or more of the solid solution Ti required to improve hole expansion workability can be stabilized. There was a problem that it was difficult to remain.

  In the technique described in Patent Document 2, the steel sheet structure is a mixed structure of ferrite and bainite in which the ratio of ferrite having a grain size of 2 μm or more is 80% or more, and the obtained steel sheet strength is up to about 976 MPa, Tensile strength TS: It is difficult to achieve higher strength of 980 MPa or higher, and even if tensile strength TS: 980 MPa or higher is obtained, the toughness of the ferrite phase is significantly reduced and excellent hole expansion workability is achieved. There was a problem that could not be secured.

  Moreover, in the technique described in Patent Document 3, uniform (Nb, Ti) N refinement is achieved, fine uniform voids are generated in the cross section of the punched hole, and stress concentration during hole expansion processing is reduced. Although the hole expandability (hole expandability) is being improved, the (Nb, Ti) N uniform refinement reduces the distance between (Nb, Ti) N and facilitates the connection of voids generated during local deformation. Therefore, there was a problem that the local elongation may decrease.

  The present invention provides a high-strength hot-rolled steel sheet having a further excellent hole expansion workability and a method for producing the same while solving such problems of the prior art and maintaining a high strength of tensile strength: 980 MPa or more. Objective. The high-strength hot-rolled steel sheet targeted by the present invention is a thin steel sheet having a thickness of 2 to 4 mm.

  In order to achieve the above-described object, the present inventors diligently studied various factors affecting the hole expansion workability while maintaining a high strength of tensile strength TS: 980 MPa or more. As a result, when the tensile strength TS: 980MPa or higher is maintained with the bainite phase as the main phase, cementite acts as a starting point for void formation during hole expansion or local deformation. It has been found that when the amount of is increased, the voids are easily connected, the local ductility is lowered, and the hole expanding workability is lowered. It has also been found that when the particle size of cementite increases, coarse voids are formed on the punching end face of the punching process, which is a pretreatment for the hole expanding process, and the hole expanding processability is lowered.

  For this reason, the present inventors conducted further research, and in order to improve the hole expansion workability and further the local ductility while maintaining a high strength of tensile strength TS: 980 MPa or more, C Adjust the content balance of Si, Ti and V, further optimize the production conditions, adjust the cementite to 0.8% by mass and the average particle size of cementite to 150nm or less, and widen the space between cementites. I found out that this is important.

The present invention has been completed on the basis of such findings with further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: more than 0.1% and 0.2% or less, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less, N: 0.007 %, Ti: 0.07 to 0.2%, V: more than 0.1% and 0.3% or less, the composition consisting of the balance Fe and inevitable impurities, and the main phase is a bainite phase with an area ratio of 90% or more , Cementite having a structure composed of one or more selected from the martensite phase, austenite phase, and ferrite phase, the balance other than the main phase being 10% or less in area ratio, and dispersed in the structure Is a high-strength hot-rolled steel sheet excellent in hole expansion workability, characterized by having a mass% of 0.8% or less, an average particle size of 150 nm or less, and a tensile strength TS of 980 MPa or more.

(2) In (1), in addition to the above composition, in terms of mass%, Nb: 0.005-0.1%, B: 0.0002-0.002%, Cu: 0.005-0.3%, Ni: 0.005-0.3%, Cr: 0.005 A high-strength hot-rolled steel sheet comprising one or more selected from ˜0.3% and Mo: 0.005 to 0.3%.
(3) In (1) or (2), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.0003 to 0.01% and REM: 0.0003 to 0.01% by mass%. A high-strength hot-rolled steel sheet characterized by that.

  (4) The steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, followed by cooling consisting of two stages of first stage cooling and second stage cooling, and then a wound hot rolled steel sheet. In the above, the steel material is mass%, C: more than 0.1%, 0.2% or less, Si: 1.0% or less, Mn: 1.5 to 2.5%, P: 0.05% or less, S: 0.005% or less, Al: 0.10% or less , N: 0.007% or less, Ti: 0.07 to 0.2%, V: more than 0.1% and 0.3% or less, and a steel material having a composition consisting of the remaining Fe and inevitable impurities, and the heating causes the steel material to be 1200 ° C It is a process to heat above, and the finish rolling is a finish rolling finish temperature: 850 to 950 ° C., and the first stage cooling starts cooling within 1.5 s after finishing the finish rolling. And cooling to the first stage cooling stop temperature of 500 to 600 ° C. at an average cooling rate of 20 to 80 ° C./s, wherein the second stage cooling is the first stage cooling. After the completion of the rejection, the cooling is performed at an average cooling rate of 90 ° C./s or more within 3 s to the second stage cooling stop temperature of 330 to 470 ° C. After the second stage cooling ends, the second stage cooling stop temperature A method for producing a high-strength hot-rolled steel sheet excellent in hole-expanding workability, characterized by winding at a winding temperature.

(5) In (4), in addition to the above composition, in terms of mass%, Nb: 0.005-0.1%, B: 0.0002-0.002%, Cu: 0.005-0.3%, Ni: 0.005-0.3%, Cr: 0.005 A method for producing a high-strength hot-rolled steel sheet, comprising one or more selected from ˜0.3% and Mo: 0.005 to 0.3%.
(6) In (4) or (5), in addition to the above composition, the composition further contains one or two selected from Ca: 0.0003 to 0.01% and REM: 0.0003 to 0.01% by mass%. A method for producing a high-strength hot-rolled steel sheet.

  According to the present invention, it is possible to stably manufacture a hot-rolled steel sheet having a significantly improved hole expansion workability while maintaining a high strength of tensile strength: 980 MPa or more, and has a remarkable industrial effect. The hot-rolled steel sheet of the present invention can be used as a material for automobile undercarriage parts, structural parts, skeletons, truck frames, etc., while ensuring the safety of the automobile while reducing the weight of the vehicle body and reducing the environmental load. There is also an effect that it becomes possible to do.

First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. “%” Indicating the content of each component element means “% by mass” unless otherwise specified.
C: Over 0.1% and below 0.2%
C is an element that has an action of promoting the formation of bainite, increasing the strength of steel, and promoting the formation of bainite, and is one of the important elements in the present invention. In order to obtain such an effect, the C content needs to exceed 0.1%. On the other hand, since C combines with Fe to form cementite, the excessive C content increases the number of cementite, narrows the gap between the cementites that are the origin of voids, reduces local ductility, Spreading processability decreases. Further, if C is contained in excess of 0.2%, weldability is lowered. For these reasons, C is limited to the range of more than 0.1% and 0.2% or less. In addition, Preferably, it is 0.12 to 0.17%.

Si: 1.0% or less
Si is an element having an effect of suppressing the formation of coarse cementite as well as contributing to an increase in strength of the steel by solid solution, and is an important element in the present invention. Si contributes to the improvement of local ductility and hole expansion workability by widening the interval of cementite, which is the origin of voids, particularly through the action of suppressing the formation of coarse cementite. In order to acquire such an effect, it is desirable to contain 0.1% or more. On the other hand, if the content exceeds 1.0%, the surface properties of the steel sheet are remarkably deteriorated, and chemical conversion treatment properties and corrosion resistance are reduced. For this reason, Si was limited to 1.0% or less. In addition, Preferably it is 0.5 to 0.9%.

Mn: 1.5-2.5%
Mn is an element that contributes to increasing the strength of the steel by solid solution and further promotes the formation of a bainite phase through improvement of hardenability. In order to obtain such an effect, the content of 1.5% or more is required. On the other hand, if the content exceeds 2.5%, central segregation becomes prominent, the punched end face properties of the steel sheet are lowered, and the hole expansion workability is lowered. For this reason, the amount of Mn was limited to the range of 1.5 to 2.5%. In addition, Preferably it is 1.7 to 2.2% of range.

P: 0.05% or less
P dissolves and contributes to increasing the strength of the steel, but segregates at the grain boundaries, particularly the prior austenite grain boundaries, and lowers the low temperature toughness and workability. For this reason, it is preferable to reduce P as much as possible, but inclusion up to 0.05% is acceptable. Therefore, P is limited to 0.05% or less. In addition, Preferably it is 0.03% or less, More preferably, it is 0.02% or less.

S: 0.005% or less
S combines with Ti and Mn to form coarse sulfides, thereby reducing workability. For this reason, it is preferable to reduce S as much as possible, but the content up to 0.005% is acceptable. For these reasons, S is limited to 0.005% or less. In addition, Preferably it is 0.003% or less, More preferably, it is 0.001% or less.

Al: 0.10% or less
Al is an element that acts as a deoxidizer and contributes effectively to improving the cleanliness of steel. In order to acquire such an effect, it is desirable to contain 0.005% or more. On the other hand, an excessive content exceeding 0.10% causes an increase in oxide inclusions, which causes generation of flaws and decreases the workability of the steel sheet. For this reason, Al was limited to 0.10% or less. In addition, Preferably it is 0.01 to 0.05%.

N: 0.007% or less
N is an element that combines with a nitride-forming element and precipitates as a nitride, contributing to refinement of crystal grains. However, N bonds with Ti at a high temperature and tends to be coarse nitrides, and tends to be the starting point of voids during hole expansion processing. For this reason, although it is preferable to reduce as much as possible in this invention, 0.007% is permissible. For these reasons, N is limited to 0.007% or less. In addition, Preferably it is 0.006% or less, More preferably, it is 0.005% or less.

Ti: 0.07 to 0.2%
Ti forms carbonitrides, refines the crystal grains, and contributes to increasing the strength of the steel by precipitation strengthening. In addition, Ti has the effect of reducing the amount of cementite in steel by forming many fine (Ti, V) C clusters in the temperature range of about 300 to 500 ° C (coiling temperature). One of the important elements. In order to exhibit such an effect, the content of 0.07% or more is required. On the other hand, an excessive content exceeding 0.2% saturates the above-described effects and causes an increase in coarse precipitates, resulting in a decrease in hole expansion workability. Further, since Ti promotes the formation of a ferrite phase, a desired structure cannot be secured, and the hole expansion workability is lowered. For this reason, Ti was limited to the range of 0.07 to 0.2%. In addition, Preferably it is 0.1 to 0.15%.

V: More than 0.1% and less than 0.3%
V is an element that forms carbonitrides to refine crystal grains, contributes to increase the strength of steel by precipitation strengthening, and contributes to the formation and refinement of bainite phase through improvement of hardenability. In addition, V has a function of forming a large number of fine (Ti, V) C clusters in the temperature range of about 300 to 500 ° C. (winding temperature) and reducing the amount of cementite in the steel. One of the important elements. In order to exhibit such an effect, the content of more than 0.1% is required. On the other hand, an excessive content exceeding 0.3% lowers the ductility and causes an increase in production cost. For this reason, V is limited to a range of more than 0.1% and 0.3% or less. In addition, Preferably it is 0.13-0.27%, More preferably, it is 0.15-0.25%.

  The above-mentioned components are basic components. In the present invention, Nb: 0.005 to 0.1%, B: 0.0002 to 0.002%, Cu: 0.005 to 0.3, in addition to the basic composition, if necessary, as necessary. %, Ni: 0.005-0.3%, Cr: 0.005-0.3%, Mo: One or more selected from 0.005-0.3%, and / or Ca: 0.0003-0.01%, REM: 0.0003- One or two selected from 0.01% can be contained.

One selected from Nb: 0.005-0.1%, B: 0.0002-0.002%, Cu: 0.005-0.3%, Ni: 0.005-0.3%, Cr: 0.005-0.3%, Mo: 0.005-0.3% 2 or more types
Nb, B, Cu, Ni, Cr, and Mo are all elements that contribute to an increase in the strength of the steel sheet, and can be selected as necessary to contain one or more.
Nb is an element that contributes to increasing the strength of steel through the formation of carbonitrides. In order to exhibit such an effect, it is preferable to contain 0.005% or more. On the other hand, if the content exceeds 0.1%, the deformation resistance increases, the rolling load of hot rolling increases, the burden on the rolling mill becomes too large, making the rolling operation itself difficult, and forming coarse precipitates. In addition, workability is reduced. For this reason, when it contains, it is preferable to limit Nb to 0.005 to 0.1% of range. In addition, More preferably, it is 0.01 to 0.05%, More preferably, it is 0.02 to 0.04%.

  B segregates at the austenite grain boundaries, suppresses the formation and growth of ferrite, improves hardenability, contributes to the formation and refinement of the bainite phase, and increases the strength of the steel. In order to exhibit such an effect, it is preferable to contain 0.0002% or more, but if it exceeds 0.002%, the workability is remarkably lowered. For this reason, when it contains, it is preferable to limit B to 0.0002 to 0.002% of range. In addition, More preferably, it is 0.0005 to 0.0015%.

  Cu is an element that has the effect of increasing the strength of steel by solid solution and improving the hardenability. Cu particularly lowers the bainite transformation temperature and contributes to refinement of the bainite phase. In order to acquire such an effect, it is preferable to contain 0.005% or more, but inclusion exceeding 0.3% causes the surface property to deteriorate. For this reason, when it contains, it is preferable to limit Cu to 0.005 to 0.3% of range. In addition, More preferably, it is 0.01 to 0.2%.

  Ni is an element that has the effect of increasing the strength of steel by solid solution and improving the hardenability and facilitating the formation of a bainite phase. In order to acquire such an effect, it is preferable to contain 0.005% or more, but when it contains exceeding 0.3%, it will become easy to produce | generate a martensite phase and hole expansion workability will fall remarkably. For this reason, when it contains, it is preferable to limit Ni to 0.005 to 0.3% of range. In addition, More preferably, it is 0.01 to 0.2%.

Cr is an element that forms carbides and contributes to an increase in steel strength. In order to exhibit such an effect, it is preferable to contain 0.005% or more. On the other hand, an excessive content exceeding 0.3% lowers the corrosion resistance of the steel sheet. For this reason, when it contains, it is preferable to limit Cr to 0.005 to 0.3% of range. In addition, More preferably, it is 0.01 to 0.2%.
Mo is an element that has effects of improving hardenability, facilitating the formation of a bainite phase, and increasing the strength of steel. In order to acquire such an effect, it is preferable to contain 0.005% or more, but when it contains exceeding 0.3%, it will become easy to produce | generate a martensite phase and hole expansion workability will fall remarkably. For this reason, when it contains, it is preferable to limit Mo to 0.005 to 0.3% of range. In addition, More preferably, it is 0.01 to 0.2%.

One or two selected from Ca: 0.0003 to 0.01%, REM: 0.0003 to 0.01%
Both Ca and REM are elements that contribute to improving the hole expansion workability through shape control of inclusions, and can be selected as necessary to contain one or two kinds.
Ca is an element that controls the shape of sulfide inclusions and contributes effectively to the improvement of hole expansion workability. In order to express this effect, the content of 0.0003% or more is required. On the other hand, an excessive content exceeding 0.01% increases the amount of inclusions and causes frequent surface defects. For this reason, when it contains, it is preferable to limit Ca to 0.0003 to 0.01% of range.

  REM, like Ca, is an element that controls the shape of sulfide inclusions, improves the adverse effects of sulfide inclusions on hole expanding workability, and contributes to improving hole expanding workability. In order to express this effect, the content of 0.0003% or more is required. On the other hand, an excessive content exceeding 0.01% increases the amount of inclusions, deteriorates the cleanliness of the steel, and decreases the hole expansion workability. For this reason, when it contains, it is preferable to limit REM to 0.0003 to 0.01% of range.

The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include O (oxygen): 0.005% or less, W: 0.1% or less, Ta: 0.1% or less, Co: 0.1% or less, Sb: 0.1% or less, Sn: 0.1% or less, Zr: 0.1 % Or less is acceptable.
Next, the reason for the structure limitation of the hot-rolled steel sheet of the present invention will be described.
In the hot-rolled steel sheet of the present invention, the main phase is a bainite phase. The “main phase” here refers to a phase having an area ratio of 90% or more. If the phase other than the bainite phase is the main phase, the desired high strength and good hole expansion workability cannot be secured stably. Therefore, the main phase was a bainite phase having an area ratio of 90% or more. In addition, Preferably it is 92% or more, More preferably, it is 95% or more.

  The balance other than the bainite phase, which is the main phase, is composed of one or more selected from a martensite phase, an austenite phase (residual austenite phase), and a ferrite phase. The remaining phases other than the main phase are 10% or less in total (including 0%) in terms of area ratio. If the remaining phase other than the bainite phase exceeds 10%, the desired high strength and good hole expansion workability cannot be secured stably. In particular, when the martensite phase increases, the desired good hole expansion workability cannot be secured stably.

The hot-rolled steel sheet of the present invention has the above-described structure and exhibits a structure in which cementite is dispersed in the structure. Cementite exists mainly in a dispersed state in the bainite phase, but may exist in a phase other than bainite or at the boundary between phases. In the hot rolled steel sheet of the present invention, the cementite dispersed in the structure is 0.8% or less by mass% and the average particle size is 150 nm or less.
When cementite is dispersed in a large amount exceeding 0.8% by mass in the structure, the number of dispersed cementite increases, voids originating from cementite are easily connected during processing, local ductility is reduced, and holes are reduced. Spreading processability decreases. For this reason, cementite was limited to 0.8% or less by mass. In addition, Preferably it is 0.6% or less. More preferably, it is 0.5% or less.

Further, when the average particle size of cementite exceeds 150 nm and becomes coarse, coarse voids starting from cementite are likely to be generated during processing, and the hole expansion workability is lowered. For this reason, the average particle diameter of cementite was limited to 150 nm or less. In addition, Preferably it is 130 nm or less, More preferably, it is 110 nm or less.
Below, the preferable manufacturing method of this invention hot rolled sheet steel is demonstrated.

In the present invention, the steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, then subjected to cooling consisting of two stages of first stage cooling and second stage cooling, and then through a winding process, A hot-rolled steel sheet is used.
The manufacturing method of the steel material that is the starting material is to melt the molten steel having the above-described composition by a conventional melting method such as a converter, and by a conventional casting method such as a continuous casting method, Any conventional manufacturing method can be applied, and there is no particular limitation. It should be noted that there is no problem even if the ingot-bundling rolling method is used.

The obtained steel material is first heated to a heating temperature of 1200 ° C. or higher.
Heating temperature: 1200 ° C or higher The steel material used in the present invention contains carbonitride-forming elements such as Ti, but these carbonitride-forming elements are mostly coarse carbonitrides (precipitates). Exist as. In addition, when a carbonitride-forming element such as Ti is present as coarse precipitates, the amount of fine precipitates contributing to precipitation strengthening is reduced. For this reason, steel plate strength falls. In order to dissolve the coarse precipitates before hot rolling, the heating temperature was limited to 1200 ° C. or higher. In addition, Preferably it is 1220 to 1350 degreeC.

Next, the heated steel material is subjected to hot rolling consisting of rough rolling and finish rolling.
Rough rolling only needs to secure a desired sheet bar size, and the conditions are not particularly limited. Subsequent to rough rolling, finish rolling is performed at a finish rolling finish temperature of 850 to 950 ° C. Needless to say, descaling is performed before finish rolling or during rolling between finish rolling stands.

Finishing rolling finish temperature: 850-950 ° C
When the finish rolling finish temperature is less than 850 ° C., the finish rolling becomes a ferrite + austenite two-phase region rolling, and the processed structure remains after rolling, so that the hole expanding workability is lowered. On the other hand, when the finish rolling finish temperature is higher than 950 ° C., austenite grains grow, and the bainite phase of the hot-rolled sheet obtained after cooling becomes coarse. For this reason, hole expansion workability falls. For this reason, the finish rolling finish temperature was limited to the range of 850 to 950 ° C. In addition, Preferably it is 870-930 degreeC. As used herein, “finishing rolling finish temperature” is the surface temperature.

After finishing rolling, the cooling which consists of two steps of the first stage cooling and the second stage cooling is performed.
In the first stage cooling, after finishing rolling, cooling is started within 1.5 s, preferably immediately, and cooled to the first stage cooling stop temperature of 500 to 600 ° C. at an average cooling rate of 20 to 80 ° C./s. To do.
When the cooling start time of the first stage cooling exceeds 1.5 s, the austenite grains become coarse and the bainite phase becomes coarse. Further, when the austenite grains are coarse, the hardenability of the steel sheet is increased, the martensite phase is easily generated, and the desired excellent hole expanding workability cannot be ensured. For this reason, the cooling start time of the first stage cooling is limited to 1.5 s or less after finishing rolling.

  On the other hand, when the average cooling rate of the first stage cooling is less than 20 ° C./s and the cooling is slow, the formation of ferrite or coarse bainite is promoted, and the desired high strength or hole expansion workability cannot be ensured. On the other hand, when it is rapidly cooled at a temperature exceeding 80 ° C./s, martensite is easily generated and becomes hard, and the hole expansion workability is lowered. For this reason, the average cooling rate of the first stage cooling was limited to the range of 20 to 80 ° C./s. In addition, Preferably it is 25-60 degreeC / s.

  On the other hand, if the first stage cooling stop temperature is less than 500 ° C., the steel plate temperature varies greatly in the transition boiling region, the structure becomes non-uniform, and the desired excellent hole expanding workability cannot be ensured. On the other hand, when the first stage cooling stop temperature is higher than 600 ° C., the ferrite transformation is promoted and the desired high strength cannot be ensured. For this reason, the first stage cooling stop temperature was limited to 500 to 600 ° C. In addition, Preferably it is 520-580 degreeC.

Immediately after the completion of the first stage cooling, the second stage cooling is immediately started within 3 s, preferably immediately, and cooled to the second stage cooling stop temperature of 330 to 470 ° C. at an average cooling rate of 90 ° C./s or more.
If the cooling start time of the second stage cooling is longer than 3 s, the ferrite transformation starts and the desired high strength cannot be ensured. For this reason, the cooling start time of the second stage cooling is limited to within 3 s after the end of the first stage cooling.

  On the other hand, if the average cooling rate of the second stage cooling is less than 90 ° C./s, the bainite to be produced becomes coarse, and the desired hole expansion workability cannot be ensured. For this reason, the average cooling rate of the second stage cooling was limited to 90 ° C./s or more. The upper limit of the average cooling rate of the second stage cooling is not particularly limited, but the upper limit is about 250 ° C./s in relation to the thickness of the plate to be cooled and the capacity of the cooling equipment. In addition, Preferably it is 100-200 degreeC / s.

  If the second stage cooling stop temperature is less than 330 ° C., a hard martensite phase or a retained austenite phase is formed in the steel sheet structure, and a desired structure cannot be secured, resulting in a decrease in hole expansion workability. On the other hand, when the temperature is higher than 470 ° C., the ferrite phase and martensite phase increase in the steel sheet structure, the desired structure cannot be secured, and the hole expansion workability is remarkably lowered. For this reason, the first stage cooling stop temperature was limited to 330 to 470 ° C. In addition, Preferably it is 350-450 degreeC.

After cooling to the second stage cooling stop temperature, the second stage cooling stop temperature is taken as the coiling temperature, and the coil is wound into a hot rolled steel sheet (hot rolled steel strip).
In addition, above-described temperature means the surface temperature of a steel plate.
Further, after the winding, the hot-rolled steel sheet may be further subjected to temper rolling according to a conventional method. Moreover, the obtained hot-rolled steel sheet may be pickled to remove the scale formed on the surface. Or after pickling, you may give plating processing, such as hot dip galvanization and electrogalvanization, and chemical conversion treatment.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material) by a continuous casting method. Subsequently, these steel materials are heated under the conditions shown in Table 2, hot-rolled comprising rough rolling and finish rolling under the conditions shown in Table 2, and after finishing rolling, cooled under the conditions shown in Table 2. The steel sheet was wound at the winding temperature shown in Table 2 to obtain a hot-rolled steel sheet having the thickness shown in Table 2. In some hot-rolled steel sheets, the cooling is one-step cooling.

Test pieces were collected from the obtained hot-rolled steel sheet and subjected to structure observation, tensile test, and hole expansion test. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation was collected from the obtained hot-rolled steel sheet, the cross section of the plate thickness parallel to the rolling direction was polished, and the microstructure was revealed with a corrosive solution (3% nital solution). Tissues were observed at 4 positions using a scanning electron microscope (SEM), tissues were imaged at 3 fields of view (magnification: 3000 times), and tissue fractions (area ratios) of each phase were identified by tissue identification and image analysis Was calculated.

  In addition, a specimen for replica collection (size: 10 mm x 15 mm) is collected from the 1/4 thickness position of the obtained hot-rolled steel sheet, a replica film is produced by the two-stage replica method, and cementite is collected for transmission. Using a scanning electron microscope (TEM), the collected cementite was observed, photographed for 5 fields of view (magnification: 50000 times), the particle size of each cementite was determined, and the average particle size of the cementite of the steel sheet was averaged It was. In the case of cementite having an aspect ratio, the average value of the major axis length and the minor axis length was used as the particle size of the cementite.

In addition, an electrolytic residue extraction test piece (size: t × 50 × 100 mm) was collected from the obtained hot-rolled steel sheet, and 10% AA electrolyte (10 vol% acetylacetone-1 mass% tetramethylammonium chloride / methanol). In particular, the current density was 20 mA / cm ² , and constant current electrolysis was performed on the entire thickness of the test piece. The obtained electrolytic solution was filtered, and the electrolytic residue remaining on the filter paper was analyzed using an ICP spectrometer, and the amount of Fe in the electrolytic residue was measured. Assuming that all of the quantified Fe is Fe ₃ C, the following formula
Fe ₃ C (mass%) = (1.0716 × [quantitative Fe (g)]) / [electrolytic weight (g)] × 100
The amount of precipitated cementite was calculated. The atomic weight of Fe is 55.85 (g / mol), and the atomic weight of C is 12.01 (g / mol). In addition, the electrolysis weight was calculated | required by wash | cleaning the test piece for electrolysis after electrolysis, measuring weight, and subtracting from the test piece weight before electrolysis.
(2) Tensile test JIS No. 5 test piece (GL: 50mm) was sampled from the obtained hot-rolled steel sheet so that the tensile direction was perpendicular to the rolling direction, and a tensile test was conducted according to JIS Z 2241. The yield strength (yield point) YP, the tensile strength TS, and the elongation El were determined.
(3) Hole expansion test A test piece for hole expansion test (size: t x 100 x 100 mm) is collected from the obtained hot-rolled steel sheet, and a 10mmφ punch is placed in the center of the test piece in accordance with the iron standard JFST 1001. Then, after punching a punch hole at a clearance of 12.5%, a 60 ° conical punch is inserted into the punch hole so as to push up from the punching direction, and the hole diameter dmm when the crack penetrates the plate thickness is obtained. Next formula
λ (%) = {(d−10) / 10} × 100
The hole expansion rate λ (%) defined in (1) was calculated.

  Also, a test piece for hole expansion test (size: t × 100 × 100 mm) was taken from the obtained hot-rolled steel sheet, and a punch hole was punched out with a 10 mmφ punch at the center of the test piece with a clearance of 25.0%. Thereafter, a 60 ° conical punch is inserted into the punch hole so as to push it up from the punching direction, and the hole diameter dmm when the crack penetrates the plate thickness is obtained. Was calculated. The clearance is a ratio (%) to the plate thickness.

In addition, λ obtained in a hole expansion test performed on a punch hole punched with a clearance of 12.5% was obtained in a hole expansion test performed on a punch hole punched with a clearance of 60% or more and clearance of 25.0%. When λ was 40% or more, the hole expansion workability was evaluated as good.
The obtained results are shown in Table 3.

  Each of the examples of the present invention is a high strength hot rolled steel sheet having high tensile strength: 980 MPa or more and excellent hole expansion workability. On the other hand, in the comparative example that is out of the scope of the present invention, the desired tensile strength is not ensured or the hole expansion workability is lowered.

Claims (6)

% By mass
C: more than 0.1% and 0.2% or less, Si: 1.0% or less,
Mn: 1.5 to 2.5%, P: 0.05% or less,
S: 0.005% or less, Al: 0.10% or less,
N: 0.007% or less, Ti: 0.07 to 0.2%,
V: contains more than 0.1% and 0.3% or less, has a composition composed of the balance Fe and inevitable impurities, and has a bainite phase of 90% or more in area ratio as the main phase and the balance other than the main phase in area ratio 10% or less of the martensite phase, austenite phase, or ferrite phase selected from one or more of the structures, and the cementite dispersed in the structure is 0.8% by mass or less, average grain size A high-strength hot-rolled steel sheet excellent in hole expansion workability, characterized in that the diameter is 150 nm or less and the tensile strength TS is 980 MPa or more.   In addition to the above composition, Nb: 0.005-0.1%, B: 0.0002-0.002%, Cu: 0.005-0.3%, Ni: 0.005-0.3%, Cr: 0.005-0.3%, Mo: 0.005- The high-strength hot-rolled steel sheet according to claim 1, comprising one or more selected from 0.3%.   The composition according to claim 1 or 2, further comprising one or two selected from Ca: 0.0003 to 0.01% and REM: 0.0003 to 0.01% by mass% in addition to the composition. High strength hot rolled steel sheet. After the steel material is heated and subjected to hot rolling consisting of rough rolling and finish rolling, it is subjected to cooling consisting of two stages of first stage cooling and second stage cooling, and then to a wound hot rolled steel sheet,
The steel material in mass%,
C: more than 0.1% and 0.2% or less, Si: 1.0% or less,
Mn: 1.5 to 2.5%, P: 0.05% or less,
S: 0.005% or less, Al: 0.10% or less,
N: 0.007% or less, Ti: 0.07 to 0.2%,
V: A steel material containing more than 0.1% and not more than 0.3% and having the balance Fe and inevitable impurities,
The heating is a treatment of heating the steel material to 1200 ° C. or higher;
The finish rolling is a finish rolling finishing temperature: 850 to 950 ° C,
The first stage cooling starts cooling within 1.5 s after finishing the finish rolling, and cools to the first stage cooling stop temperature of 500 to 600 ° C. at an average cooling rate of 20 to 80 ° C./s. And
The second-stage cooling is cooling to the second-stage cooling stop temperature of 330 to 470 ° C. at an average cooling rate of 90 ° C./s or more within 3 s after the completion of the first-stage cooling;
After completion of the second stage cooling, a method for producing a high-strength hot-rolled steel sheet excellent in hole expansion workability, wherein the second stage cooling stop temperature is taken up as a winding temperature.   In addition to the above composition, Nb: 0.005-0.1%, B: 0.0002-0.002%, Cu: 0.005-0.3%, Ni: 0.005-0.3%, Cr: 0.005-0.3%, Mo: 0.005- The method for producing a high-strength hot-rolled steel sheet according to claim 4, comprising one or more selected from 0.3%.   6. The composition according to claim 4, further comprising one or two selected from Ca: 0.0003 to 0.01% and REM: 0.0003 to 0.01% by mass% in addition to the composition. Manufacturing method of high strength hot rolled steel sheet.