JP6354268B2 - High-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more excellent in punching hole expandability and low-temperature toughness, and a method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in punching hole expandability and low-temperature toughness, and a method for manufacturing the same, and in particular, a steel sheet excellent in hole expanding formability and stretch flange formability after punching and a method for manufacturing the same. It is about.

  In recent years, in order to improve the fuel efficiency of automobiles and reduce the amount of carbon dioxide emissions, the application of high-strength hot-rolled steel sheets to undercarriage parts has been promoted with the aim of reducing the weight of the vehicle body. Further, from the viewpoint of ensuring the safety of passengers, high strength steel plates having a maximum tensile strength of 980 MPa or more are often used in automobile bodies in addition to mild steel plates.

  In general, the formability (workability) deteriorates as the strength of steel sheets increases. However, since parts for automobile undercarriage require stretch flange molding or hole expansion molding, high punching hole expansion at the same time as high strength is required. Sex is required. How to increase the strength without deteriorating the material properties is important in the development of high-strength steel sheets.

  In addition, the steel plate used for such a member is difficult to break even after being impacted by a collision after being formed and attached to a car as a part. Especially, to ensure impact resistance in cold regions, low temperature toughness There is also a desire to improve. This low temperature toughness is specified by vTrs (Charpy fracture surface transition temperature) or the like.

  For this reason, it is also necessary to consider the impact resistance itself of the steel material. In addition, since increasing the strength makes it difficult to plastically deform the steel sheet, the fear of fracture increases, so toughness is required as an important characteristic.

  Methods for improving the low-temperature toughness of high-strength hot-rolled steel sheets are disclosed in Patent Documents 1 and 2, for example. A method in which a martensite phase having an adjusted aspect ratio is used as a main phase (see, for example, Patent Document 1) and a method in which carbide is finely precipitated in polygonal ferrite having an average particle diameter of 5 to 10 μm (for example, a patent) According to the literature 2, the low temperature toughness is improved.

  However, in Patent Document 1, no mention is made of stretch flangeability, and there is a concern that when it is applied to a member subjected to burring, a molding defect may occur. Patent Document 2 discloses a steel plate having a high hole expansibility of about 100% and a low temperature toughness of about −40 ° C. at vTrs, but has a tensile strength of about 540 to 780 MPa, and has been further strengthened in recent years. It does not meet the demands of

  Regarding a method for improving stretch flangeability of a high-strength steel sheet, a metal structure control method of a steel sheet that improves local ductility is disclosed. Non-Patent Document 1, inclusion control, single structure, hardness difference between structures Has been disclosed to be effective for bendability and stretch flangeability.

  In addition, by controlling the finishing temperature of hot rolling, the reduction ratio and temperature range of finishing rolling, promoting the recrystallization of austenite, suppressing the development of rolling texture, and randomizing the crystal orientation, the strength and ductility Non-patent document 2 discloses a technique for improving stretch flangeability.

  Patent Document 3 discloses the use of the metal structure control method described above. Patent Document 3 has a structure composed of ferrite and bainite and / or martensite second phase in which fine precipitates of 20 nm or less are present, and the proportion of ferrite in which fine precipitates are present is 40 to 95%. It is disclosed that by setting the ratio of the phase to 5% or less, the hardness difference between the ferrite and the second phase can be further reduced, and thus excellent stretch flangeability can be obtained.

  According to Non-Patent Documents 1 and 2, it is considered that stretch flangeability can be improved by making the metal structure and rolling texture uniform, but in Non-Patent Documents 1 and 2, low temperature toughness and stretch flange are considered. There is no consideration for gender compatibility.

  In Patent Document 3, there is no description regarding low temperature toughness, and furthermore, since precipitation strengthened ferrite is used, it is considered that it is difficult to ensure high strength of 980 MPa or more.

  Regarding the coexistence of stretch flangeability and low temperature toughness, Patent Document 4 discloses a technique in which an appropriate amount of retained austenite and bainite are dispersed in a ferrite phase whose hardness and particle size are controlled. However, since the structure is a structure containing 50% or more of soft ferrite, it is difficult for the above technique to meet the recent demand for higher strength.

JP 2011-52321 A JP 2011-17044 A JP 2004-339606 A Japanese Patent Application Laid-Open No. 07-252592

K. Sugimoto et al, “ISIJ International” (2000) Vol.40, p.920 Kishida, “Nippon Steel Technical Report” (1999) No.371, p.13

  The present invention provides a hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more, excellent punching hole expandability and low-temperature toughness, and a manufacturing method capable of stably manufacturing the steel sheet in view of the above-described problems of the prior art. The purpose is to do.

  The inventors of the present invention have intensively studied methods for achieving the above object. As a result, by optimizing the chemical composition and manufacturing conditions of high-strength hot-rolled steel sheets and controlling the steel sheet structure, a hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more and excellent in punching hole expandability and low-temperature toughness is provided. I found what I could do.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) The chemical composition is mass%,
C: 0.01-0.20%,
Si: 2.50% or less (excluding 0),
Mn: 4.00% or less (0 is not included),
P: 0.10% or less (excluding 0),
S: 0.03% or less (excluding 0),
Al: 0.001 to 2.00%,
N: 0.01% or less (excluding 0),
O: 0.01% or less (excluding 0),
One or two of Ti and Nb: 0.01 to 0.30% in total
Consisting of the balance iron and unavoidable impurities,
The microstructure contains one or both of tempered martensite and lower bainite in a volume fraction of 90% or more in total,
In one or both of the tempered martensite and the lower bainite, iron-based carbides are present in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more,
The effective crystal grain size of one or both of the tempered martensite and lower bainite is 10 μm or less,
The aspect ratio of the effective crystal grain of one or both of the tempered martensite and the lower bainite is 2 or less,
A high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punching hole expansibility and low-temperature toughness, characterized in that the standard deviation σ of Vickers hardness distribution is 15 or less.

( 2 ) The chemical composition is further mass%,
Cu: 0.01-2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.01 to 0.30%,
Cr: 0.01-2.00%
The high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punched hole expansibility and low-temperature toughness as described in (1 ) above.

( 3 ) The chemical composition is further mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.10%
The high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punching hole expansibility and low-temperature toughness according to any one of (1) or ( 2 ).

( 4 ) A method for producing a high-strength steel sheet having a maximum tensile strength of 980 MPa or more and excellent in punched hole expansibility and low-temperature toughness according to any one of (1) to ( 3 ),
(I) The cast slab having the chemical composition according to any one of the above (1) to ( 3 ) is directly or once cooled, then heated to 1200 ° C. or higher and subjected to hot rolling, and roughened at 1050 to 1100 ° C. Complete rolling, finish rolling at 900 ° C or higher to make a hot-rolled steel sheet,
(Ii) The hot-rolled steel sheet is cooled from the finish rolling temperature to 300 ° C. at an average cooling rate of 50 ° C./second or more, and is cooled from 300 ° C. to room temperature at an average cooling rate of 40 ° C./second or less. A method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more, which is excellent in punching hole expandability and low-temperature toughness.

( 5 ) The method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MP or more excellent in punched hole expansibility and low-temperature toughness as described in ( 4 ), wherein galvanization is performed after the cooling.

  According to the present invention, it is possible to provide a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more and excellent punching hole expandability and low-temperature toughness, and a manufacturing method capable of stably manufacturing the steel sheet.

The high-strength hot-rolled steel sheet (hereinafter sometimes referred to as “the present invention steel sheet”) having a maximum tensile strength of 980 MPa or more and excellent in punched hole expansibility and low-temperature toughness of the present invention.
Chemical composition is mass%,
C: 0.01-0.20%,
Si: 2.50% or less (excluding 0),
Mn: 4.00% or less (0 is not included),
P: 0.10% or less (excluding 0),
S: 0.03% or less (excluding 0),
Al: 0.001 to 2.00%,
N: 0.01% or less (excluding 0),
O: 0.01% or less (excluding 0),
One or two of Ti and Nb: 0.01 to 0.30% in total
Consisting of the balance iron and unavoidable impurities,
The microstructure contains one or both of tempered martensite and lower bainite in a volume fraction of 90% or more in total,
In one or both of the tempered martensite and the lower bainite, iron-based carbides are present in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more,
The effective crystal grain size of one or both of the tempered martensite and lower bainite is 10 μm or less,
The aspect ratio of the effective crystal grain of one or both of the tempered martensite and the lower bainite is 2 or less,
The standard deviation σ of the Vickers hardness distribution is 15 or less.

The method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more excellent in punching hole expandability and low-temperature toughness according to the present invention (hereinafter sometimes referred to as “the present invention production method”) is as follows.
A method for producing the steel sheet of the present invention,
(I) A cast slab having a chemical composition of the steel sheet of the present invention is directly or once cooled, then heated to 1200 ° C. or higher and subjected to hot rolling, and rough rolling is completed at 1050 to 1100 ° C. and finished at 900 ° C. or higher. Complete the rolling to make a hot rolled steel sheet,
(Ii) The hot-rolled steel sheet is cooled from the finish rolling temperature to 300 ° C. at an average cooling rate of 50 ° C./second or more, and is cooled from 300 ° C. to room temperature at an average cooling rate of 40 ° C./second or less. To do.

  Hereinafter, this invention steel plate and this invention manufacturing method are demonstrated.

  As a result of intensive studies by the inventors, in the hot-rolled steel sheet, the microstructure is a structure containing one or both of tempered martensite and lower bainite in a total volume of 90% or more, and is a standard for Vickers hardness distribution. It has been found that when the deviation σ is 15 or less, a maximum tensile strength of 980 MPa or more, excellent punched hole expansibility and low temperature toughness can be secured.

Furthermore, the present inventors (a) if iron-based carbides are present in one or both of tempered martensite and lower bainite in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more, and / or (b) tempering. It has been found that if the effective crystal grain size of martensite or lower bainite is 10 μm or less, it is possible to ensure a maximum tensile strength of 980 MPa or more, better punched hole expandability and low temperature toughness.

  The effective crystal grain size is a crystal region surrounded by a grain boundary having an orientation difference of 15 ° or more, and can be measured using EBSD or the like. Details will be described later.

  First, the microstructure of the steel sheet of the present invention will be described.

  In the steel sheet of the present invention, the main phase is one or both of tempered martensite and lower bainite. A tensile maximum strength of 980 MPa or more can be achieved by securing one or both of the tempered martensite and the lower bainite in a total volume of 90% or more. The volume fraction is preferably 95% or more.

  In the steel sheet of the present invention, tempered martensite is an important microstructure in securing a maximum tensile strength of 980 MPa or more, excellent punched hole expansibility and low temperature toughness.

  Tempered martensite is an aggregate of lath-like crystal grains, and contains iron-based carbide having a major axis of 5 nm or more. The iron-based carbide belongs to a plurality of variants, that is, a plurality of iron-based carbide groups extending in different directions.

  Tempered martensite can be obtained by slowing the cooling rate below the Ms point (martensitic transformation start temperature) or by tempering martensite at 100 to 600 ° C.

  Lower bainite is also an aggregate of lath-like crystal grains and contains iron-based carbide having a major axis of 5 nm or more inside. The iron-based carbide belongs to a single variant, that is, a group of iron-based carbides extending in the same direction. In addition, the iron-type carbide group extended | stretched in the same direction means the iron-type carbide group whose difference of an extension direction is less than 5 degrees.

  By observing the extension direction of carbide, tempered martensite and lower bainite can be easily distinguished. If the volume fraction of one or both of tempered martensite and lower bainite is less than 90%, the maximum tensile high strength of 980 MPa or more cannot be secured, so the volume fraction is 90% or more. Preferably it is 95% or more.

  Even when the volume fraction is 100%, it is possible to ensure a maximum tensile strength of 980 MPa or more and excellent punched hole expansibility and low temperature toughness.

  The microstructure of the steel sheet of the present invention is, as another structure, one or more of ferrite, fresh martensite, upper bainite, pearlite, and retained austenite as long as the properties of the steel sheet of the present invention are not impaired. Or 10% or less. However, the smaller the volume fraction of these other tissues, the better, so 5% is the upper limit.

  Ferrite has a massive crystal grain structure and does not contain a substructure such as lath. Since ferrite causes a decrease in strength in a soft structure, the volume fraction is limited to 10% or less in order to ensure the maximum tensile strength of 980 MPa or more.

  In addition, if ferrite is present, deformation concentrates on the interface between ferrite and tempered martensite or lower bainite and ferrite and becomes a starting point of fracture, thereby inhibiting low temperature toughness. If the volume fraction exceeds 10%, the degree to which low-temperature toughness is inhibited becomes significant, so the volume fraction of ferrite is limited to 10% or less. Preferably it is 5% or less.

  Fresh martensite is martensite containing no carbide. Fresh martensite has high strength but is inferior in low-temperature toughness, so the volume fraction is limited to 10% or less. Preferably it is 5% or less.

  The upper bainite is an aggregate of lath-like crystal grains, and an aggregate of lath containing carbides between the laths. The carbide contained between the laths is a starting point of fracture, and thus inhibits low temperature toughness. Moreover, since the upper bainite is produced at a higher temperature than the lower bainite, it has a low strength.

  Therefore, excessive formation of lower bainite makes it difficult to ensure the maximum tensile strength of 980 MPa or more. If the volume fraction of the upper bainite exceeds 10%, it becomes difficult to ensure the maximum tensile strength of 980 MPa or more, so the volume fraction is limited to 10% or less. Preferably it is 5% or less.

  Pearlite, like ferrite, reduces strength and deteriorates low temperature toughness, and therefore limits the volume fraction to 10% or less. Preferably it is 5% or less.

  Residual austenite is transformed into fresh martensite by plastic deformation at the time of press molding or plastic deformation of an automobile member at the time of a collision, and thus adversely affects the properties of the steel sheet of the present invention in the same manner as fresh martensite. Therefore, the volume fraction of retained austenite is limited to 10% or less. Preferably it is 5% or less.

  Identification of main structure tempered martensite and lower bainite, other ferrites, fresh martensite, upper bainite, pearlite, retained austenite, and remaining structure, confirmation of location, and area fraction ( The volume fraction) is measured by corroding a steel plate rolling direction cross section or a rolling direction perpendicular direction cross section using the Nital reagent and the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473, and increasing the corrosion cross section by 1000 to 100000 times. This is possible by observing with scanning and transmission electron microscopes.

  The structure can also be discriminated from crystal orientation analysis using the FESEM-EBSP method and micro region hardness measurement by micro Vickers hardness measurement or the like.

  For example, as described above, tempered martensite, upper bainite, and lower bainite have different carbide generation sites and crystal orientations (elongation directions), so a field emission scanning electron microscope (FE-SEM: Field Emission Scanning). It can be easily discriminated by observing the elongation direction of the iron carbide inside the lath-like crystal grains using an Electron Microscope).

  The volume fraction of ferrite, pearlite, bainite, tempered martensite, and fresh martensite is obtained by taking a sample with the cross section parallel to the rolling direction of the steel sheet as the observation surface, polishing the observation surface, and corroding with the nital reagent. Then, an area fraction is measured by observing a range of 1/8 to 3/8 thickness centered on 1/4 of the plate thickness with an FE-SEM, and this is used as a volume fraction. The area ratio was measured at 10 fields at a magnification of 5000 times, and the average value was defined as the area ratio.

  Since fresh martensite and residual austenite are not sufficiently corroded by the nital reagent, they can be clearly distinguished from ferrite, bainitic ferrite, bainite, and tempered martensite when observed by FE-SEM.

  Therefore, the area fraction (volume fraction) of fresh martensite is the difference between the area fraction of the uncorroded area observed with FE-SEM and the area fraction of retained austenite measured with X-rays. Can be obtained as

  In the steel sheet of the present invention, the standard deviation σ of the Vickers hardness distribution of the microstructure having the volume fraction is set to 15 or less in order to ensure excellent punch hole expansibility. In general, when the difference in hardness between structures is large, deformation concentrates on a portion with low hardness and cracks tend to occur, so that excellent hole expansibility cannot be ensured. Therefore, in the steel sheet of the present invention, excellent punching hole expandability was ensured by setting the cooling condition after hot rolling, in particular, the average cooling rate from 300 ° C. to room temperature was 40 ° C./second or less.

  The smaller the standard deviation σ of the hardness distribution, the better the punched hole expansibility, so it is preferably 13 or less, more preferably 11 or less. The hardness distribution was measured according to the JIS Z 2244 Vickers hardness test method. The measurement was performed at 300 or more points at equal intervals in the range of plate thickness × 1 mm in a cross section parallel to the rolling direction. The measurement load was 10 gf. A standard deviation σ was calculated based on the measurement result.

In the steel sheet of the present invention, when iron carbide is present in tempered martensite or lower bainite in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more, the low-temperature toughness of the parent phase is improved, and the balance between excellent strength and low-temperature toughness is achieved. Can be obtained.

That is, as-quenched martensite is excellent in strength but poor in toughness, it is necessary to improve toughness. Accordingly, 1 × 10 ⁶ (pieces / mm ² ) or more of iron carbide was precipitated in the tempered martensite to improve the toughness of the main phase.

When the present inventors investigated the relationship between the low temperature toughness and the number density of iron-based carbides, in the tempered martensite and the lower bainite, if there are iron-based carbides of 1 × 10 ⁶ (pieces / mm ² ) or more, It was found that excellent low temperature toughness can be secured. For this reason, the number of iron-based carbides is set to 1 × 10 ⁶ (pieces / mm ² ) or more. Preferably it is 5 × 10 ⁶ (pieces / mm ² ) or more, more preferably 1 × 10 ⁷ (pieces / mm ² ) or more.

  The size of the iron-based carbide is as small as 300 nm or less, and most of the iron-based carbide was precipitated in the tempered martensite and the lath of the lower bainite. Therefore, it is estimated that the iron-based carbide does not inhibit the low temperature toughness.

  The number density of the iron-based carbide is measured by taking a sample with the plate thickness section parallel to the rolling direction of the steel plate as the observation surface, polishing the observation surface, corroding with the Nital reagent, and centering on 1/4 of the plate thickness. The range of 1/8 to 3/8 thickness was observed by FE-SEM. Ten fields of view were observed at 5000 times, and the number density of the iron-based carbide was measured.

In the steel sheet of the present invention, in order to further improve the low temperature toughness, the main phase is tempered martensite or lower bainite, and the effective crystal grain size is 10 μm or less. Preferably it is 8 micrometers or less. The effective crystal grain size means the size of a crystal region surrounded by the crystal orientation difference 15 ° or more grain boundaries, the tempered martensite and lower bainite, which corresponds to the block diameter.

Next, an effective crystal grain size and a structure identification method will be described. In the steel sheet of the present invention, the effective crystal grain size, ferrite, and retained austenite are defined using EBSP-OIM ^™ (Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy).

The EBSP-OIM ^TM method irradiates a highly inclined sample with an electron beam in a scanning electron microscope (SEM), images the Kikuchi pattern formed by backscattering with a high-sensitivity camera, It consists of a device and software that measure the crystal orientation of the glass in a short time.

  In the EBSP method, the microstructure and crystal orientation of the bulk sample surface can be quantitatively analyzed. The analysis area is an area that can be observed with an SEM. Depending on the resolution of the SEM, the analysis area can be analyzed with a minimum resolution of 20 nm.

  In the steel sheet of the present invention, the grain region is visualized from an image mapped by defining the difference in crystal grain orientation as 15 °, which is a threshold of a large-angle grain boundary generally recognized as a grain boundary, and effective grain The diameter was determined.

  The aspect ratio of the effective crystal grains of tempered martensite and lower bainite (grain region surrounded by a grain boundary of 15 ° or more) is preferably 2 or less.

  Crystal grains flattened in a specific direction have a large anisotropy, and cracks propagate along grain boundaries in the Charpy test, so that toughness often decreases. Therefore, the effective crystal grains need to be equiaxed grains as much as possible. In the steel sheet of the present invention, the cross section in the rolling direction of the steel sheet was observed, and the ratio (= L / T) of the length (L) in the rolling direction and the length (T) in the thickness direction was defined as the aspect ratio.

  Next, the reason for limiting the chemical composition of the steel sheet of the present invention will be described. In addition,% is the mass%.

C: 0.01 to 0.20%
C is an element that contributes to an increase in the strength of the base material and an improvement in bake hardenability. On the other hand, C is an element that forms iron-based carbides such as cementite (Fe ₃ C) that is the starting point of cracks during hole expansion. is there. If C is less than 0.01%, the effect of improving the strength due to the low temperature transformation generation phase cannot be obtained, so the content is made 0.01% or more. Preferably it is 0.04 or more.

On the other hand, if it exceeds 0.20%, the ductility is lowered, and the amount of iron-based carbides such as cementite (Fe ₃ C), which becomes the starting point of cracks on the secondary shear surface during punching, increases, and the hole expandability. Therefore, the content is set to 0.20% or less. Preferably it is 0.16% or less.

Si: 2.50% or less (excluding 0)
Si is an element that contributes to an increase in the strength of the base metal, and is an element that also functions as a deoxidizer for molten steel. In order to obtain the effect of addition, the required amount is added, but if it exceeds 2.50%, the strength increasing effect is saturated, so the content is made 2.50% or less. The lower limit is not particularly limited, but 0.001% or more of addition is necessary to obtain the addition effect.

  Moreover, the addition of 0.10% or more suppresses the precipitation of iron-based carbides such as cementite and contributes to the improvement of strength and the ability to expand holes. If it exceeds 1.20%, the precipitation suppression effect of iron-based carbide is saturated. 0.10 to 1.20% is a preferable range.

Mn: 4.00% or less (excluding 0)
Mn is an element that contributes to improving strength by hardening strengthening in addition to solid solution strengthening. However, if it exceeds 4.00%, the effect of addition is saturated, so 4.00% or less. Preferably it is 3.00% or less. The lower limit is not particularly limited, but if it is less than 1.00%, the effect of suppressing ferrite transformation and bainite transformation is difficult to be exhibited, so 1.00% or more is preferable. More preferably, it is 1.40% or more.

P: 0.10% or less (excluding 0)
P is an impurity element and is an element that segregates at the grain boundary and inhibits low temperature toughness. The smaller the content, the better. However, if it exceeds 0.10%, workability and weldability as well as low-temperature toughness are impaired, so the content is made 0.10% or less. In particular, considering weldability, 0.03% or less is preferable. The lower limit is not particularly limited, but if it is reduced to less than 0.001%, the production cost is increased, so 0.001% is a practical lower limit on the practical steel sheet.

S: 0.03% or less (excluding 0)
S is an impurity element, which forms inclusions such as MnS and causes cracking during hot rolling, as well as an element that hinders hole expandability. If it exceeds 0.03%, the adverse effect of MnS becomes remarkable, so the content is made 0.03% or less. Preferably it is 0.02% or less. When securing a certain degree of hole expansion property, 0.01% or less is preferable. More preferably, it is 0.005% or less.

Al: 0.001 to 2.00%
Al is an element that functions as a deoxidizing material, suppresses the formation of coarse cementite, and contributes to the improvement of low-temperature toughness. If it is less than 0.001%, the effect of addition does not appear, and if it is reduced to less than 0.001%, the production cost is increased, so it is made 0.001% or more.

  On the other hand, if it exceeds 2.00%, the production cost increases, and Al-based coarse inclusions are generated, which causes deterioration of hole expansibility and surface scratches. Preferably it is 1.50% or less.

N: 0.01% or less N is an element that forms a blow hole during welding and acts to reduce the joint strength of the weld. If it exceeds 0.01%, the above action becomes remarkable, so the content is made 0.01% or less. The lower limit is not particularly limited, but if it is reduced to less than 0.0005%, the production cost is increased, so 0.0005% is a practical lower limit on a practical steel sheet.

O: 0.01% or less (excluding 0)
O is an element that forms an oxide and contributes to refinement of martensite crystal grains by suppressing coarsening of austenite grains during hot rolling. However, if it exceeds 0.01%, the refinement of crystal grains proceeds excessively, and the oxide becomes the starting point of cracking, and the punching hole expandability and elongation decrease. Preferably it is 0.006% or less. Although a minimum is not specifically limited, In order to acquire an addition effect stably, 0.001% or more is added. Preferably it is 0.005% or more.

Ti and Nb 1 type or 2 types: 0.01 to 0.30% in total
Ti and Nb are elements that contribute to both improvement of low-temperature toughness and high strength of 980 MPa or more. These carbonitrides are formed and contribute to high strength of 980 MPa or more, and solute Ti and solute Nb delay grain growth during hot rolling to refine the grain size of the hot-rolled steel sheet. It is an element that contributes to the improvement of low temperature toughness.

  In particular, Ti is an element that exists as TiN in addition to the delay of grain growth due to solute N and contributes to the improvement of low-temperature toughness by refining the crystal grain size during slab heating. In order to make the particle diameter of the hot-rolled steel sheet 10 μm or less, one or two of Ti and Nb are made 0.01% or more in total. Preferably it is 0.02% or more. More preferably, it is 0.04% or more.

  On the other hand, if the total of one or two of Ti and Nb exceeds 0.30%, the effect of addition is saturated and the economic efficiency is lowered, so the content is made 0.30% or less. Preferably it is 0.25% or less, More preferably, it is 0.20% or less.

  In addition to the above elements, the steel sheet of the present invention is (1) one or more of Cu, Ni, Mo, V, Cr, and / or (2) Mg, as long as the properties of the steel sheet of the present invention are not impaired. You may contain 1 type, or 2 or more types of Ca and REM.

Cu: 0.01 to 2.00%
Ni: 0.01-2.00%
Mo: 0.01 to 1.00%
V: 0.01 to 0.30%
Cr: 0.01-2.00%
Cu, Ni, Mo, V, Cr suppresses ferrite transformation at the time of cooling, makes the microstructure tempered martensite or lower bainite structure, and improves the strength of hot-rolled steel sheet by precipitation strengthening or solid solution strengthening Is an element that contributes to If any of Cu, Ni, Mo, V, and Cr is less than 0.01%, the effect of addition is not sufficiently exhibited, so that any element is made 0.01% or more. Preferably, any element is 0.04% or more.

  On the other hand, if any of Cu, Ni, and Cr exceeds 2.00%, the effect of addition is saturated and the economic efficiency is lowered, so that any element is made 2.00% or less. Preferably, any element is 1.50% or less.

  If Mo exceeds 1.00%, and if V exceeds 0.30%, the effect of addition is saturated and the economic efficiency is lowered. Therefore, Mo is set to 1.00% or less, and V is 0.30%. The following. Preferably, Mo is 0.60% or less, and V is 0.10% or less.

Mg: 0.0005 to 0.01%
Ca: 0.0005 to 0.01%
REM: 0.0005 to 0.10%
Mg, Ca, and REM (rare earth elements) are elements that contribute to the improvement of workability by controlling the form of non-metallic inclusions that become the starting point of destruction and cause deterioration of workability. If Mg, Ca, and REM are less than 0.0005%, the effect of addition is not sufficiently exhibited, so that any element is made 0.0005% or more. Preferably, any element is 0.0010% or more.

  On the other hand, if Mg exceeds 0.01%, Ca exceeds 0.01%, and REM exceeds 0.10%, the effect of addition is saturated and economic efficiency is lowered. %, REM is 0.10% or less. Preferably, Mg and Ca are 0.006% or less, and REM is 0.06% or less.

  The steel sheet of the present invention may contain, in addition to the above elements, 1.0% or less of Zr, Sn, Co, Zn, and W in total within a range that does not impair the characteristics of the steel sheet of the present invention. In addition, since Sn is an element which generates a flaw at the time of hot rolling, 0.05% or less is preferable.

  The steel plate of the present invention may be provided with a hot dip galvanized layer or an alloyed galvanized layer on the surface of the steel plate in order to improve corrosion resistance. The plating layer is not limited to a pure zinc plating layer, and may contain Si, Mg, Zn, Al, Fe, Mn, Ca, Zr, and the like. By containing these elements, the corrosion resistance of the plating layer is further improved. A plating layer does not impair the characteristic of this invention steel plate.

  Moreover, even if this invention steel plate has a surface treatment layer by organic film formation, film lamination, organic salt / inorganic salt treatment, non-chromic treatment, etc., the characteristics of this invention steel plate are not impaired.

  Next, the manufacturing method of the present invention (the manufacturing method of the steel plate of the present invention) will be described.

  In the steel sheet of the present invention, in order to achieve a maximum tensile strength of 980 MPa or more and excellent punch hole expansibility and low temperature toughness, a total volume fraction of one or both of tempered martensite and lower bainite is ensured to be 90% or more in total. In addition, the standard deviation σ of the Vickers hardness distribution needs to be 15 or less.

Furthermore, in the steel sheet of the present invention, in order to achieve better punched hole expansibility and low temperature toughness, the standard deviation σ of the Vickers hardness distribution is set to 15 or less, and it exists in one or both of tempered martensite and lower bainite. The number of iron-based carbides to be processed must be 1 × 10 ⁶ (pieces / mm ² ) or more, and / or the effective crystal grain size of one or both of tempered martensite and lower bainite must be 10 μm or less.

In the steel sheet of the present invention, the maximum production strength of 980 MPa or more, and the production method of the present invention that realizes excellent punched hole expansibility and low temperature toughness,
(I) A cast slab having a chemical composition of the steel sheet of the present invention is directly or once cooled, then heated to 1200 ° C. or higher and subjected to hot rolling, and rough rolling is completed at 1050 to 1100 ° C. and finished at 900 ° C. or higher. Complete the rolling to make a hot rolled steel sheet,
(Ii) The hot-rolled steel sheet is cooled from the finish rolling temperature to 300 ° C. at an average cooling rate of 50 ° C./second or more, and is cooled from 300 ° C. to room temperature at an average cooling rate of 40 ° C./second or less. To do.

  The production of the cast slab preceding the hot rolling is not limited to a specific production method. Subsequent to smelting with a blast furnace, electric furnace, etc., various secondary smelting may be performed to adjust the chemical composition, and then casting may be performed by ordinary continuous casting or ingot method. Moreover, you may cast by methods, such as thin slab casting. Although scrap may be used as a raw material for the cast slab, adjustment of the chemical composition is necessary.

  The cast slab is directly or once cooled, and then heated to 1200 ° C. or higher for hot rolling. A cast slab of 1200 ° C. or higher is continuously subjected to hot rolling without cooling to room temperature. The cast slab of less than 1200 ° C is heated to 1200 ° C or higher and subjected to hot rolling.

  In the steel sheet of the present invention, since austenite grain coarsening is suppressed using solute Ti or Nb, it is necessary to redissolve NbC or TiC precipitated during casting.

  When the temperature of the casting slab is less than 1200 ° C., it takes a long time to dissolve the carbides of Nb and Ti, and subsequent refinement of crystal grains and the effect of improving low-temperature toughness due to this cannot be obtained. The heating temperature of the slab is 1200 ° C. or higher. The upper limit of the heating temperature of the casting slab is not particularly defined, but an excessively high temperature is not preferable from an economical viewpoint, and is preferably less than 1300 ° C.

  The rough rolling in the hot rolling is completed at 1050 to 1100 ° C. Since the next finish rolling needs to be completed at 900 ° C. or higher, the rough rolling is completed at 1050 to 1100 ° C.

  Finish rolling in hot rolling is completed at 900 ° C. or higher.

  The steel sheet according to the present invention contains a large amount of Ti and Nb in order to refine the austenite grain size. Therefore, if the completion temperature of the finish rolling is less than 900 ° C., the austenite is difficult to recrystallize, the crystal grains are stretched in the rolling direction, and the toughness is liable to be lowered, and the martensite or bainite transformation from unrecrystallized austenite Therefore, the finish rolling temperature is set to 900 ° C. or higher.

  The hot-rolled steel sheet that has been finish-rolled at 900 ° C. or higher is cooled. At the time of cooling, from the finish rolling temperature to 300 ° C. is cooled at an average cooling rate of 50 ° C./second or more, and from 300 ° C. to room temperature is cooled at an average cooling rate of 40 ° C./second or less. By this cooling, it is possible to impart excellent punching hole expandability and low temperature toughness to a high strength hot rolled steel sheet having a maximum tensile strength of 980 MP or more.

  When the average cooling rate from the finish rolling temperature to 300 ° C. is less than 50 ° C./second, ferrite is generated during the cooling, and the volume fraction of one or both of the tempered martensite and the lower bainite of the main phase is 90 in total. Since it is difficult to secure at least%, the average cooling rate is set to 50 ° C./second or more. If ferrite is not generated during the cooling process, air cooling may be performed in the middle temperature range.

When the average cooling rate from 300 ° C. to room temperature exceeds 40 ° C./second, the standard deviation σ of the Vickers hardness distribution exceeds 15, and tempering in which iron-based carbides exist in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more. Since martensite and lower bainite cannot be obtained, and excellent punching hole expandability and low temperature toughness cannot be obtained, the average cooling rate is set to 40 ° C./second or less. It is preferably 1 ° C./second or less, more preferably 0.01 ° C./second or less.

  The cooling rate of 40 ° C./second or less does not mean only the cooling rate in ROT (Run out table), but is a cooling rate including isothermal holding and winding.

  Since the purpose of controlling the cooling rate in the above temperature range is to control the number density of iron-based carbides in the steel sheet structure, the temperature is once cooled to the martensite transformation start temperature (Ms point) or lower. Even if it is raised and reheated, it is possible to impart excellent punching hole expandability and low temperature toughness to a high strength hot rolled steel sheet having a maximum tensile strength of 980 MP or more.

  Generally, in order to obtain martensite, it is necessary to suppress ferrite transformation, and a cooling rate of 50 ° C./second or more is required. In addition, in the low temperature range, the heat transfer coefficient is relatively low and it is difficult to cool down (film boiling region), and the heat transfer coefficient is large and the temperature range is easy to cool (nuclear boiling temperature range). When the temperature is 300 ° C. or lower, the temperature during cooling is likely to change, and the material also changes accordingly.

  For this reason, the cooling stop temperature is usually set to 300 ° C. or higher and the winding temperature is set to room temperature or higher in many cases. Therefore, it is presumed that the knowledge that the maximum tensile strength of 980 MPa or more, excellent punched hole expansibility and low temperature toughness can be secured by controlling the cooling rate at 300 ° C. or lower was not found.

  Even if it heat-processes 100-600 degreeC on-line or offline for the purpose of precipitation of a carbide | carbonized_material after manufacture to this invention steel plate, the characteristic of this invention steel plate is maintained. In addition, the steel plate of the present invention may be subjected to skin pass rolling with a rolling reduction of 0.1 to 2.0% after the completion of all the steps in order to improve the ductility by correcting the shape of the steel plate or introducing movable dislocations.

  Moreover, you may perform pickling to the steel plate of this invention for the purpose of removing the scale adhering to the steel plate surface after completion | finish of all the processes. Further, the steel plate of the present invention after pickling may be subjected to skin pass or cold rolling with a rolling reduction of 10% or less online or offline.

  The maximum tensile strength of the steel sheet of the present invention is 980 MPa or more, and the maximum tensile stress by a tensile test conducted in accordance with JIS Z 2241 using a JIS No. 5 test piece cut in the direction perpendicular to the rolling direction of hot rolling is 980 MPa or more. It means that.

  The excellent punching hole expandability of the steel sheet of the present invention means that the hole expanding ratio (λ) in a hole expanding test performed in accordance with JIS Z 2256 is 50% or more. The hole expansion rate (λ) is preferably 80% or more.

  The excellent low temperature toughness of the steel sheet of the present invention means that the fracture surface transition temperature (vTrs) in the Charpy test conducted according to JIS Z 2242 is -40 ° C or lower. Since the steel sheet of the present invention is mainly used for automobiles, the thickness of the sheet is often about 3 mm. Therefore, when preparing a test piece, the surface of the steel sheet of the present invention is ground to obtain a 2.5 mm sub-size test piece. Was made.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

(Example)
Cast slabs having the chemical compositions of invention steels A to N and comparative steels a to g shown in Table 1 were cast, directly or once cooled to room temperature, and then Table 2 and Table 3 (continuation of Table 2). ), The rough rolling is completed at 1050 to 1100 ° C., and then finish rolling is performed at the temperatures (820 to 1010 ° C.) shown in Table 2 and Table 3 (continuation of Table 2). Was completed to obtain a hot-rolled steel sheet having a thickness of 2.6 to 3.4 mm.

  The hot-rolled steel sheet was cooled at an average cooling rate shown in Table 2 and Table 3 (continuation of Table 2), and wound at a temperature shown in Table 2 and Table 3 (continuation of Table 2).

  Thereafter, the wound hot-rolled steel sheet is rewound, pickled and subjected to 0.5% skin pass rolling, and then a JIS No. 5 test piece is cut out from the hot-rolled steel sheet in a direction perpendicular to the rolling direction. A tensile test was performed in accordance with Z2242.

  The hardness was measured according to JIS Z 2244 to obtain a hardness distribution by measuring the Vickers hardness. The measurement was performed at 300 or more points at equal intervals in the range of plate thickness × 1 mm in a cross section parallel to the rolling direction. The measurement load was 10 gf. The standard deviation σ was calculated from the Vickers hardness wrinkle distribution.

  The hole expansion ratio λ was obtained by cutting a test piece from the center and the left and right 1/4 positions with respect to the plate width and performing a test in accordance with JIS Z 2256.

  A Charpy test was performed in accordance with JIS Z 2242 to measure the fracture surface transition temperature. Since the thickness of the hot-rolled steel sheet was less than 10 mm, the front and back of the hot-rolled steel sheet were ground to 2.5 mm, and then a test piece was collected and a Charpy test was performed.

  Some hot-rolled steel sheets are heated to 660 to 720 ° C., subjected to galvanizing treatment or alloying heat treatment at 540 to 580 ° C. after the galvanizing treatment, and galvanized steel sheets (GI) or alloyed hot dip zinc After the plated steel sheet (GA), a material test was performed.

  For the microstructure, the volume fraction of each structure, the number density of iron-based carbides, the effective crystal grain size, and the aspect ratio were measured.

  Table 4 and Table 5 (continuation of Table 4) show the measurement results.

  It can be seen that the invention steel whose conditions are within the scope of the present invention has a maximum tensile strength of 980 MPa or more, excellent punched hole expansibility, and low temperature toughness.

  On the other hand, steel A-5, steel C-5, steel D-6, steel E-4, steel J-3, and steel M-3 with a slab heating temperature of less than 1200 ° C. are Ti and Nb precipitated during casting. Therefore, even if the hot rolling conditions are within the scope of the present invention, the volume fraction of the structure and the effective crystal grain size are outside the scope of the present invention, and the strength and low temperature toughness are inferior. .

  Steel A-6, Steel C-6, Steel D-7, Steel E-5, and Steel J-4 are rolled in the non-recrystallized austenite region because the finish rolling temperature is too low. Therefore, the punching hole expandability is inferior, and the crystal grains are elongated in the rolling direction, so that the aspect ratio is large and the toughness is inferior.

  Steel A-7, Steel C-7, Steel D-8, Steel E-6, and Steel M-4 have a cooling rate from the finish rolling temperature to 300 ° C. of less than 50 ° C./sec. A large amount of ferrite is generated, and it is difficult to ensure strength, and the interface between ferrite and martensite is the starting point of fracture, so that the low temperature toughness is inferior.

  Steel A-8, Steel C-8, Steel D-9, Steel E-7, Steel J-5, and Steel M-5 have a cooling rate from 300 ° C. to room temperature exceeding 40 ° C./second, and are carbides. Insufficient precipitation amount and low temperature toughness.

  Steel A-9, Steel A-10, Steel C-9, Steel C-10, Steel D-10, Steel D-11, Steel E-8, Steel E-9, Steel J-6, Steel J-7, As shown in Steel M-6 and Steel M-7, the properties of the steel sheet of the present invention are maintained even when alloyed hot dip galvanizing or alloying hot dip galvanizing is performed.

  On the other hand, steels a to g whose chemical compositions do not satisfy the scope of the present invention do not have a maximum tensile strength of 980 MPa or more, excellent punched hole expansibility, and low temperature toughness.

  As described above, according to the present invention, a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more and excellent in punching hole expandability and low-temperature toughness, and a manufacturing method capable of stably manufacturing the steel sheet are provided. be able to. The high-strength hot-rolled steel sheet of the present invention is easy to process and can withstand use in extremely cold regions, so the present invention has extremely high industrial applicability.

Claims (5)

Chemical composition is mass%,
C: 0.01-0.20%,
Si: 2.50% or less (excluding 0),
Mn: 4.00% or less (0 is not included),
P: 0.10% or less (excluding 0),
S: 0.03% or less (excluding 0),
Al: 0.001 to 2.00%,
N: 0.01% or less (excluding 0),
O: 0.01% or less (excluding 0),
One or two of Ti and Nb: 0.01 to 0.30% in total
Consisting of the balance iron and unavoidable impurities,
The microstructure contains one or both of tempered martensite and lower bainite in a volume fraction of 90% or more in total,
In one or both of the tempered martensite and the lower bainite, iron-based carbides are present in an amount of 1 × 10 ⁶ (pieces / mm ² ) or more,
The effective crystal grain size of one or both of the tempered martensite and lower bainite is 10 μm or less,
The aspect ratio of the effective crystal grain of one or both of the tempered martensite and the lower bainite is 2 or less,
A high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punching hole expansibility and low-temperature toughness, characterized in that the standard deviation σ of Vickers hardness distribution is 15 or less. The chemical composition is further mass%,
Cu: 0.01-2.00%,
Ni: 0.01 to 2.00%,
Mo: 0.01 to 1.00%,
V: 0.01 to 0.30%,
Cr: 0.01-2.00%
The high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punching hole expansibility and low-temperature toughness according to claim 1, comprising one or more of the following. The chemical composition is further mass%,
Mg: 0.0005 to 0.01%,
Ca: 0.0005 to 0.01%,
REM: 0.0005 to 0.10%
The high-strength hot-rolled steel sheet having a tensile maximum strength of 980 MPa or more excellent in punching hole expansibility and low-temperature toughness according to claim 1 or 2 , comprising one or more of the following. A method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more excellent in punched hole expansibility and low-temperature toughness according to any one of claims 1 to 3 ,
(I) After the casting slab having the chemical composition according to any one of claims 1 to 3 is directly or once cooled, the cast slab is heated to 1200 ° C. or more and subjected to hot rolling, and is roughly rolled at 1050 to 1100 ° C. Then, finish rolling is completed at 900 ° C. or higher to form a hot-rolled steel sheet,
(Ii) The hot-rolled steel sheet is cooled from the finish rolling temperature to 300 ° C. at an average cooling rate of 50 ° C./second or more, and is cooled from 300 ° C. to room temperature at an average cooling rate of 40 ° C./second or less. A method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa or more, which is excellent in punching hole expandability and low-temperature toughness. The method for producing a high-strength hot-rolled steel sheet having a maximum tensile strength of 980 MPa and excellent in punching hole expansibility and low-temperature toughness according to claim 4 , wherein galvanizing is performed after the cooling.