JP2011225976A - Ultrahigh strength steel sheet excellent in workability and delayed fracture resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrahigh strength steel sheet having 1,350 MPa or more of tensile strength, and excellent in workability and delayed fracture resistance.SOLUTION: The ultrahigh strength steel sheet contains C, Si, Mn, Al, Ti, Cu, Ni and B; the remainder is iron and inevitable impurities; the total of martensite, retained austenite, bainite and bainitic ferrite is 70 area% or more with respect to the whole composition, when observing a tissue by a scanning electron microscope, as to a surface layer portion 30 μm apart along a sheet thickness direction from the most surface layer part of the steel sheet; the remainder is polygonal ferrite; vickers hardness is 300-400 HV; the total of martensite, retained austenite, bainite and bainitic ferrite is 90 area% or more with respect to the whole composition, when observing a tissue by a scanning electron microscope, as to a portion of 1/4 of the sheet thickness; the remainder is polygonal ferrite, and remaining γ is 3 vol.% or more, when measuring the retained austenite by a X-ray diffraction method.

Description

  The present invention relates to an ultra-high-strength steel plate used as a steel plate for automobiles and steel plates for transportation machinery. Specifically, the tensile strength is 1350 MPa or more, ductility, bending workability, stretch flangeability workability, and delayed fracture resistance. The present invention relates to an ultra-high strength steel sheet having excellent properties.

  With the recent increase in strength of steel sheets for automobiles and the like, workability such as ductility, bending workability and stretch flangeability has been reduced, and it has been difficult to press-mold a member having a complicated shape. Moreover, with the super high strength of the steel sheet, there is a concern about the occurrence of delayed fracture, and its suppression is an issue. Delayed fracture means hydrogen generated in a corrosive environment or hydrogen in the atmosphere diffuses into embrittlement parts such as dislocations, vacancies, and grain boundaries in the steel sheet, embrittles them, degrades the ductility and toughness of the steel sheet, It is a phenomenon that causes destruction in the applied state. Thus, in addition to workability, the ultra high strength steel sheet needs not to cause delayed fracture due to hydrogen embrittlement (hereinafter also referred to as delayed fracture resistance).

  TRIP (Transformation Induced Plasticity) steel sheets containing retained austenite (residual γ) have attracted attention as steel sheets having both strength and ductility, and various techniques for improving delayed fracture resistance of TRIP steel sheets have been proposed.

  For example, Patent Documents 1 to 6 are documents disclosed by the present applicant, and each has a metal structure containing 1% or more of residual γ and 80% or more of bainitic ferrite and martensite in total. This document discloses that in order to improve ductility and delayed fracture resistance, the shape of the residual γ may be a lath shape.

  Patent Document 7 discloses a steel sheet having a metal structure containing 1% or more of residual γ and 80% or more of bainitic ferrite and martensite in total. In this document, in order to improve ductility and stretch flangeability, the shape of the residual γ should be a lath shape and the residual γ should be present at the grain boundary triple point where the prior austenite grain boundaries overlap. Is disclosed.

  Patent Documents 8 and 9 disclose steel sheets having a metal structure containing 50% or more of bainitic ferrite, 5 to 35% of polygonal ferrite, and 5% or more of residual γ. In this document, in order to improve ductility, stretch flangeability and delayed fracture resistance, the shape of residual γ is made lath and the average grain size of polygonal ferrite is 10 μm or less, or the average size of the second phase Has been disclosed to be 10 μm or less.

  Patent Document 10 discloses a steel sheet having a soft layer of carbon 0.1% by mass or less on the steel sheet surface layer portion on one side and 3 to 15% on both sides. This document discloses that in order to improve the bending workability and impact characteristics, a composite structure of 10% or less of residual γ and a low-temperature transformation phase or further ferrite other than the surface layer portion may be used. .

JP 2006-207016 A JP 2006-2007017 A JP 2006-207018 A JP 2007-197819 A JP 2008-169475 A JP 2008-127581 A JP 2009-256773 A JP 2007-32236 A JP 2007-32237 A JP-A-5-195149

  The required characteristics regarding ultra-high strength steel sheets are becoming higher and higher, and there is an urgent need to provide ultra-high-strength steel sheets of 1350 MPa or higher, which are excellent in both workability and delayed fracture resistance.

  The present invention has been made in view of the above circumstances, and the purpose thereof is a tensile strength of 1350 MPa or more, workability (specifically, ductility, bending workability, stretch flangeability) and delayed fracture resistance. An object of the present invention is to provide an ultra-high strength steel sheet that is superior to the above.

  An ultra-high strength steel sheet having a tensile strength of 1350 MPa or more and excellent in workability and delayed fracture resistance according to the present invention that has solved the above problems is C: 0.15 to 0.25% (meaning mass%). The same applies to the chemical component composition), Si: 1.0 to 2.5%, Mn: 1.0 to 3.0%, Al: 0.01 to 0.10%, Ti: 0.01 to 0.10 %, Cu: 0.01 to 1%, Ni: 0.01 to 1%, and B: 0.0005 to 0.005%, the balance being iron and inevitable impurities, the outermost layer portion of the steel plate When the structure is observed with a scanning electron microscope for the surface layer part in the plate thickness direction of 30 μm, the total of martensite, retained austenite, bainite, and bainitic ferrite with respect to the entire structure is 70 area% or more, and the remainder is polygonal ferrite As well as The Vickers hardness is 300 to 400 HV, and the structure of martensite, retained austenite, bainite, and bainitic ferrite with respect to the entire structure is observed when the structure is observed with a scanning electron microscope with respect to a quarter of the plate thickness. The total is 90 area% or more, the remainder is polygonal ferrite, and when residual austenite is measured by X-ray diffraction method, the volume fraction of residual γ is 3% or more.

  In a preferred embodiment of the present invention, the steel sheet contains Cr: 1% or less (excluding 0%) as the other element.

  According to the present invention, since the metallographic structure of the steel sheet surface layer and the steel sheet is appropriately controlled, ductility, bending workability, stretch flangeability workability, even at ultra-high strength of 1350 MPa or more, In addition, an ultra-high strength steel sheet having excellent delayed fracture resistance could be provided.

FIG. 1 is a schematic view showing a heat pattern in the case of manufacturing the steel plate of the present invention.

  In order to provide a TRIP steel sheet in which both workability and delayed fracture resistance are improved even if it is an ultra-high strength steel sheet having a tensile strength of 1350 MPa or more, the present inventors have conducted intensive studies with a particular focus on the metal structure. It has been repeated. As a result, while controlling the metal structure (VS) and Vickers hardness (HV) in the part (henceforth a surface layer part) of 30 micrometers in the plate | board thickness direction from the outermost layer part of a steel plate, plate | board thickness 1/4 By controlling the metal structure (VI) of the part, it was found that an ultra-high strength steel sheet having all of strength, workability and delayed fracture resistance was obtained, and the present invention was completed.

  For the surface layer portion of the steel sheet, the surface layer portion has a Vickers hardness (HVS) of 300 to 400, and the total fraction (VS) of martensite, retained austenite, bainite, and bainitic ferrite in the surface layer portion is 70 areas. By setting the ratio to at least%, it is possible to achieve both high workability and high delayed fracture resistance in ultra-high strength steel sheets.

  Specifically, by lowering the Vickers hardness of the surface layer and softening it, the strain distribution during processing of the steel sheet becomes wider, so that all workability of ductility, bending workability and stretch flangeability can be improved. It was found that the delayed fracture resistance was also improved. As a result of further investigation, if the total fraction (VS) of martensite, retained austenite, bainite, and bainitic ferrite, which is the hard phase, is 70 area% or more, the improvement in workability due to softening of the surface layer is inhibited. It has been found that the delayed fracture resistance (DF) can be prevented from being lowered without any failure.

  As described above, in addition to the improvement of workability and softening of delayed fracture resistance due to softening, and the prevention of delayed fracture resistance reduction due to structure control, as described above, in the present invention, the steel sheet surface layer portion further includes a steel plate. Also for the interior (t / 4 part: t is the plate thickness), the fraction (VI) of martensite, retained austenite, bainite, and bainitic ferrite, which are hard phases, was increased to 90 area% or more. As a result, while ensuring the desired strength, the reduction and progress of microcracks generated at the time of punching are suppressed, and workability such as stretch flangeability is improved, and it is effective in improving delayed fracture resistance. Because it was found.

  In the present invention, when identifying the structure inside the steel sheet, the structure of t / 4 part is observed, but this is empirically that the structure inside the steel sheet can be identified by observing the structure of t / 4 part. Because it is known. Further, in the present invention, in identifying the structure of the surface layer part, the structure existing in the region of 30 μm in the plate thickness direction from the outermost layer part of the steel sheet is observed, and this is particularly the structure existing in the above region. This is because it has been found by the present inventors' basic experiment that it has a great influence on the delayed fracture resistance.

  In the present invention, the surface layer portion and the internal structure are identified by observation with a scanning electron microscope (SEM), but as will be described in detail later, only polygonal ferrite is identified, and the other structure (remainder) ) Is a hard structure of martensite, retained austenite, bainite, and bainitic ferrite. This is because, in the present invention, it has been found that the workability and the delayed fracture resistance can be combined even by such a simple tissue discrimination method. The structure of the steel sheet according to the present invention is a TRIP steel sheet mainly composed of martensite and containing retained austenite, and bainite and bainitic ferrite are not necessarily included. In order to obtain a desired strength, it is preferable that martensite is present at 60% or more, preferably 70% or more, more preferably 80% or more.

  In the present specification, the “high strength steel plate having excellent workability” means elongation (El) and bending workability (R / t: t is a plate thickness, R / t is a high strength steel plate having a tensile strength of 1350 MPa or more. Rmin divided by plate thickness t) and stretch flangeability (λ). Specifically, it is intended for high-strength steel sheets having a tensile strength (TS) of 1350 MPa or more, preferably 1370 MPa or more, more preferably 1400 MPa or more, and the elongation (El) is preferably 9% or more, more preferably 10% or more, It is desirable that 11% or more is satisfied. Further, the bending workability (R / t) is preferably 4 or less, more preferably 3 or less. The stretch flangeability (λ) is preferably 30% or more, more preferably 35% or more, and still more preferably 40% or more.

  In the present invention, the elongation (El), the bending workability (R / t), and the stretch flangeability (λ) may be collectively referred to as “workability”.

  Further, in the present invention, “excellent in delayed fracture resistance” refers to a case where the time until occurrence of cracking is 48 hours or more in the evaluation of delayed fracture resistance described in Examples.

  Hereinafter, the range of the tissue fraction that characterizes the present invention and the reason for setting it will be described in detail. Hereinafter, for convenience of explanation, “martensite, retained austenite, bainite, and bainitic ferrite” may be abbreviated as a hard structure.

(1) About the surface layer portion: When the structure is observed by SEM, the total fraction (VS) of martensite, retained austenite, bainite, and bainitic ferrite (hard structure): 70 area% or more, the remainder being polygonal ferrite And Vickers hardness (HVS): 300 to 400

(1-1) Total fraction of hard structure in surface layer part (VS): 70 area% or more, the remainder is the hard structure in the surface part of polygonal ferrite is stretch flangeability (λ), and delayed fracture resistance (DF) It is important as an organization that contributes to the combination of If the total fraction (VS) of the hard tissue is less than 70 area%, the stretch flangeability (λ) is deteriorated. In addition, since the interface between the polygonal ferrite and the hard microstructure that causes delayed fracture increases, and the hard microstructure that is excellent in hydrogen storage decreases, the accumulation of hydrogen proceeds at the interface, Delayed fracture resistance deteriorates. Therefore, in the present invention, the total fraction (VS) in the surface layer portion is set to 70 area% or more. Preferably it is 75 area% or more, More preferably, it is 80 area% or more.

  In addition, although the surface layer part is comprised from the said hard structure and polygonal ferrite (remainder structure), you may be comprised only from the hard structure. Considering delayed fracture resistance and the like, it is preferable that there are more hard structures.

(1-2) Vickers hardness (HVS) of surface layer portion: 300 to 400
The Vickers hardness (HVS) of the surface layer is important as a requirement that contributes to the combination of workability and delayed fracture resistance (DF) in an ultra-high strength steel sheet. In the present invention, the Vickers hardness (HV) of the surface layer portion may be expressed as HVS.

  If the Vickers hardness (HVS) is less than 300, the fatigue strength deteriorates. Therefore, in the present invention, the Vickers hardness (HVS) of the surface layer portion is set to 300 or more. Preferably it is 320 or more, More preferably, it is 340 or more. On the other hand, when Vickers hardness (HVS) exceeds 400, workability and delayed fracture resistance (DF) deteriorate. Therefore, the surface layer portion has a Vickers hardness (HVS) of 400 or less. Preferably it is 390 or less, More preferably, it is 380 or less.

  In the present invention, the hardness is defined only for the surface layer portion of the steel sheet, and the hardness inside the steel sheet is not particularly defined. This is because, according to the examination results of the present inventors, it has been clarified that the hardness of the surface layer portion is greatly involved in improving the desired characteristics. Therefore, as long as the hardness of the surface layer portion of the steel sheet satisfies the above requirements, the hardness inside the steel sheet can have any value, but if the preferred manufacturing method of the present invention is carried out, the strength (TS) can be secured. In this case, the hardness is preferably controlled within a range of about 400 to 600 (Vickers hardness).

(2) About the thickness t / 4 (t is the thickness): When the structure is observed by SEM, the total fraction (VI) of martensite, retained austenite, bainite, and bainitic ferrite (hard structure): 90 area% or more, balance is polygonal ferrite, and residual γ fraction (Vγ) by X-ray diffraction method: volume ratio of 3% or more

(2-1) Total fraction of hard structure at thickness t / 4 (VI): 90 area% or more Hard structure inside the steel sheet at thickness t / 4 is strength (TS), stretch flangeability (λ ), Which is important as an organization that contributes to the combination of delayed fracture resistance (DF). If the total fraction of the hard structure is less than 90 area%, the strength (TS) decreases, and microcracks develop at the interface between the relatively soft polygonal ferrite and other hard microstructures, and the stretch flange The property (λ) decreases. Therefore, in the present invention, the total fraction (VI) of martensite, retained austenite, bainite, and bainitic ferrite at a thickness of t / 4 is set to 90 area% or more. Preferably it is 95 area% or more, More preferably, it is 100 area%.

  In addition, although the inside of a steel plate is comprised from the said hard structure and polygonal ferrite (remainder structure), it may be comprised only from a hard structure. Considering stretch flangeability (λ) and the like, the inside of the steel plate is preferably composed of only the hard structure (100 area%).

  In the present invention, the fraction in each of the steel sheet surface layer part and the hard structure inside the steel sheet is defined, but according to the preferred manufacturing method of the present invention described later, generally the hard structure of the surface layer part is generated compared to the inside. However, the present invention is not limited to this. That is, any aspect in which the hard structure of the surface layer portion is larger than that in the inside and further, an aspect in which the hard structure exists to the same extent are included in the scope of the present invention.

(2-2) Residual γ fraction at plate thickness t / 4 (Vγ): The volume fraction of 3% or more The residual γ fraction at plate thickness t / 4 (Vγ) is an improvement in elongation (El) characteristics. It is important as an organization that contributes to In addition, the hydrogen trapping ability of residual γ is also effective in ensuring delayed fracture resistance (DF). In order to ensure elongation (El) and delayed fracture resistance, the residual γ fraction (Vγ) for the entire structure is set to 3% or more by volume. The volume ratio of residual γ is preferably 4% or more, more preferably 5% or more.

  In the present invention, the fraction of residual γ by the X-ray diffraction method is defined only for the inside of the steel sheet, and the fraction of the steel sheet surface layer portion is not particularly defined. This is because, according to the examination results of the present inventors, it has been clarified that the internal residual γ fraction is greatly involved in improving the desired characteristics. However, according to the examination results of the present inventors, it has been confirmed that the residual γ fraction in the steel sheet surface layer portion and the residual γ fraction in the steel plate are substantially the same. Therefore, as long as the fraction of residual γ in the steel sheet satisfies the above requirements, the residual γ fraction of the steel sheet surface layer portion can have any value, but according to the preferred production method of the present invention, it is generally 3 volumes. % Is preferably controlled within a range of at least%.

  The identification of the hard tissue and residual γ, the measurement of the fraction and hardness may be performed by the method shown in the examples described later.

  Excellent properties (ultra high strength (TS), elongation (El), bending workability (R / t), stretch flangeability (λ), delayed fracture resistance (DF)) due to the above structure In order to exert it, it is necessary to control the chemical composition of the steel sheet as follows. Hereinafter, the chemical component composition will be described in detail.

C: 0.15-0.25%
C is an element necessary for ensuring the strength of the steel sheet. When the amount of C is insufficient, the strength is lowered and the fraction of the hard structure in the surface layer portion is lowered, making it difficult to achieve delayed fracture resistance (DF). Therefore, in the present invention, the C amount is set to 0.15% or more. Preferably it is 0.17% or more, More preferably, it is 0.19% or more. However, since the weldability deteriorates if contained excessively, the C content is made 0.25% or less. Preferably it is 0.24% or less, More preferably, it is 0.23% or less.

Si: 1.0-2.5%
Si is an element that contributes to increasing the strength of steel as a solid solution strengthening element. Further, it is an element that suppresses the generation of carbides and effectively acts on the generation of residual γ. In order to effectively exhibit such an action, the Si amount is 1.0% or more, preferably 1.3% or more, more preferably 1.6% or more. However, when it contains excessively, a remarkable scale will be formed at the time of hot rolling, a scale trace may be attached to the steel plate surface, and surface properties may worsen. In addition, the residual γ produced is saturated. Accordingly, the Si amount is set to 2.5% or less. Preferably it is 2.3% or less, More preferably, it is 2.1% or less.

Mn: 1.0-3.0%
Mn is an element that contributes to increasing the strength of the steel sheet by improving the hardenability. If the amount of Mn is insufficient, the fraction (VI) inside the steel plate (t / 4 part) becomes low, and ultrahigh strength (TS) cannot be achieved. Therefore, in the present invention, the amount of Mn is set to 1.0% or more. Preferably it is 1.5% or more, More preferably, it is 1.9% or more. However, if it is contained excessively, the workability may deteriorate. Accordingly, the Mn content is 3.0% or less. Preferably it is 2.7% or less, more preferably 2.5% or less.

Al: 0.01-0.10%
Al is an element that acts as a deoxidizer, and in order to effectively exhibit such an action, it is contained in an amount of 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.03% or more. However, when it contains excessively, many inclusions, such as an alumina, produce | generate in a steel plate, and the surface property of a steel plate may deteriorate. Therefore, the Al content is 0.10% or less. Preferably it is 0.08% or less, More preferably, it is 0.06% or less.

Ti: 0.01-0.10%
Ti is an element that contributes to securing high strength by precipitation of carbonitride and refinement of the structure. If Ti is not added, the fractions (VS, VI) at the surface layer of the steel sheet and inside the steel sheet (t / 4 part) will be low, ultra-high strength (TS), stretch flangeability (λ), and delayed fracture resistance. Sex (DF) cannot be achieved. Therefore, in the present invention, the amount of Ti is set to 0.01% or more. Preferably it is 0.015% or more, More preferably, it is 0.02% or more. However, if it is contained excessively, the carbonitride precipitates may be coarsened to cause deterioration of bending workability (R / t) and stretch flangeability (λ). Therefore, Ti is set to 0.10% or less. Preferably it is 0.09% or less, More preferably, it is 0.08% or less.

Cu: 0.01 to 1%, Ni: 0.01 to 1%
Cu and Ni are elements that suppress the generation of hydrogen that causes hydrogen embrittlement and suppress the intrusion of the generated hydrogen into the steel sheet, and have an effective action for ensuring delayed fracture resistance. Yes. In order to exert such an effect, Cu and Ni are each independently contained in an amount of 0.01% or more. Preferably it is 0.05% or more, More preferably, it is 0.1% or more. However, the above action is saturated even if contained excessively. Further, since the manufacturing cost also increases, Cu and Ni are each 1% or less. Preferably, any element is 0.8% or less, more preferably 0.6% or less.

B: 0.0005 to 0.005%
B is an element that improves hardenability and is an element that effectively acts to ensure delayed fracture resistance. In order to exert such an effect effectively, 0.0005% or more is contained. Preferably it is 0.0007% or more, More preferably, it is 0.0009% or more. However, if it is contained excessively, the delayed fracture resistance deteriorates, so B is contained in an amount of 0.005% or less. Preferably it is 0.003% or less, More preferably, it is 0.002% or less.

  The steel sheet of the present invention satisfies the above component composition, and the balance is iron and inevitable impurities.

  The steel plate of the present invention may contain Cr as another element as required.

Cr: 1% or less (excluding 0%)
Cr is an element that effectively acts to increase the strength of a steel sheet. In order to exert such an effect, the content is preferably 0.1% or more. More preferably, it is 0.2% or more, More preferably, it is 0.3% or more. However, if excessively contained, bending workability (R / t) and stretch flangeability (λ) deteriorate, so Cr is preferably 1% or less. More preferably, it is 0.9% or less, More preferably, it is 0.8% or less.

  Next, a method for producing the steel sheet of the present invention will be described.

  In order to produce the steel sheet of the present invention that satisfies the above requirements, particularly the coiling temperature (CT) after hot rolling, the subsequent holding time (ct), and the annealing process after cold rolling are appropriately controlled. Thus, an ultra-high-strength steel sheet that satisfies the above requirements can be obtained.

The coiling temperature is set to 600 ° C. or higher (CT), and then held at a temperature range of 500 ° C. or higher for 3 hours or longer (ct)
In the present invention, a slab satisfying the above chemical composition is hot-rolled according to a conventional method, and then the obtained hot-rolled steel sheet is wound. The winding temperature (CT) at this time is set to 600 ° C. or higher, and then 500 Hold (ct) for 3 hours or more in a temperature range of ℃ or higher. In addition, when hold | maintaining in the temperature range of 500 degreeC or more, it is not necessarily required to hold | maintain (isothermal holding) at the same temperature. In the present invention, the coiling temperature (CT) and the holding time (ct) are requirements that affect the softening of the steel sheet surface layer after annealing, and when the coiling temperature (CT) is low or the holding time (ct) is low. When it is short, the steel sheet surface layer cannot be softened, and excellent bending workability, stretch flangeability, and delayed fracture resistance cannot be obtained. Therefore, in the present invention, the coiling temperature (CT) is set to 600 ° C. or higher, and the holding time (ct) in the temperature range of 500 ° C. or higher is set to 3 hours or longer. A preferable winding temperature (CT) is 620 ° C. or higher, more preferably 640 ° C. or higher. The preferred holding time (ct) is 4 hours or longer, more preferably 5 hours or longer.

  The upper limit of the coiling temperature (CT) is 750 ° C. If the coiling temperature (CT) is too high, the heating temperature in the previous process needs to be increased, which is undesirable because it increases costs. A preferable winding temperature (CT) is 730 ° C. or lower, more preferably 710 ° C. or lower. The upper limit of the holding time (ct) is 10 hours. If the holding time (ct) is too long, it is not desirable because the cost increases such as the need for a heat insulating cover. Preferably, it is 8 hours or less, more preferably 6 hours or less.

  By setting it as the said winding temperature (CT) and holding time (ct), the Vickers hardness (HVS) of the steel plate surface layer part after annealing can be softened to 300-400.

  Next, although cold rolling is performed, the cold rolling conditions are not particularly limited, and the cold rolling rate may be set so that a steel sheet having a predetermined thickness is obtained.

  Hereinafter, the annealing process after cold rolling that characterizes the present invention will be described in order with reference to FIG.

Keeping soaked for 100 to 500 seconds (ts) in a temperature range (Ts) of 880 ° C or higher
In the present invention, when the steel sheet is heated to reach a temperature range of 880 ° C. or higher (Ts), the temperature is maintained at the temperature range for a predetermined time (ts). Here, “the temperature range” means a temperature range of 880 ° C. or higher, and it is not always necessary to hold at the same temperature (isothermal holding) as long as this requirement is satisfied. In the present invention, the soaking temperature (Ts) is a requirement that affects the surface structure of the steel sheet and the metal structure inside the steel sheet. When the soaking temperature (Ts) is less than 880 ° C., the inside of the steel sheet (t / 4 part). The fraction (VI) at the time becomes low, and the tensile strength (TS) decreases. Therefore, in the present invention, the soaking temperature (Ts) is set to 880 ° C. or higher. The upper limit of the soaking temperature (Ts) is not particularly limited, but it is preferably about 950 ° C. or less for the operation.

  In the present invention, the soaking time (ts) is also a requirement that affects the metal structure of the steel sheet surface layer and the steel sheet. If the soaking time (ts) is less than 100 seconds, the inside of the steel sheet (t / 4 parts) ) Is reduced, and the tensile strength (TS) is reduced. Therefore, in the present invention, the soaking time (ts) is set to 100 seconds or more. A preferable soaking time (ts) is 150 seconds or longer, more preferably 200 seconds or longer. On the other hand, if the soaking time exceeds 500 seconds, the alloy elements have an excessively uniform distribution, resulting in an increase in Vickers hardness (HVS), bending workability (R / t), stretch flangeability (λ ) And delayed fracture resistance (DF). Therefore, in the present invention, the soaking time (ts) is set to 500 seconds or less. A preferable soaking time (ts) is 450 seconds or shorter, more preferably 400 seconds or shorter.

Cooling temperature range (Ts → T1) from soaking temperature range (Ts) to 150-230 ° C (T1) at an average cooling rate of 5-70 ° C / sec (CR)
After soaking under the above conditions, a range (Ts → T1) from a soaking temperature range (Ts) to a temperature range of 150 to 230 ° C. (cooling stop temperature: T1) is set to an average cooling rate of 5 to 70. Cool (CR) at ° C / second. When the cooling stop temperature (T1) is lower than 150 ° C., the amount of residual γ decreases, and the elongation (El) and delayed fracture resistance (DF) decrease. In the present invention, the cooling stop temperature (T1) is set to 150 ° C. or higher. Preferably it is 160 degreeC or more, More preferably, it is 170 degreeC or more. On the other hand, if the cooling stop temperature is too high, a high strength of 1350 MPa or more cannot be secured, so the temperature was set to 230 ° C. or lower. A preferable cooling temperature range (T1) is 220 ° C. or lower, more preferably 210 ° C. or lower.

  On the other hand, if the average cooling rate (CR) is less than 5 ° C./second, the fractions (VS, VI) in the steel sheet surface layer part and the steel sheet inside (t / 4 part) will be low. When VS is low, the delayed fracture resistance is deteriorated, and when VI is low, a desired strength cannot be obtained. Therefore, in the present invention, the average cooling rate (CR) is set to 5 ° C./second or more. The average cooling rate is preferably 20 ° C./second or more, more preferably 30 ° C./second or more. On the other hand, when the cooling rate exceeds 70 ° C./second, it becomes difficult to control the cooling stop temperature (T1). Therefore, in the present invention, the average cooling rate (CR) is set to 70 ° C./second or less. A preferable average cooling rate is 60 ° C./second or less, more preferably 50 ° C./second or less.

After holding (t2) for 200 to 3000 seconds in the temperature range (T2) of 180 to 230 ° C., cooling After cooling to the temperature range of the cooling stop temperature (T1) as described above, the temperature range of 180 to 230 ° C. After holding at (T2) for 200 to 3000 seconds (t2), it is cooled to room temperature. The holding temperature (T2) is not necessarily held at the same temperature (isothermal holding). When the holding temperature (T2) is lower than 180 ° C., the amount of residual γ is reduced and the Vickers hardness (HVS) is increased, so that the elongation (El) and delayed fracture resistance (DF) are lowered. Therefore, in the present invention, the holding temperature (T2) is set to 180 ° C. or higher. Preferably it is 185 degreeC or more, More preferably, it is 190 degreeC or more. On the other hand, if the holding temperature (T2) is too high, the amount of residual γ decreases and the workability deteriorates. Accordingly, the holding temperature (T2) is set to 230 ° C. or lower. A preferable holding temperature (T2) is 225 ° C. or lower, more preferably 220 ° C. or lower.

  When the holding time (t2) at the holding temperature (T2) is less than 200 seconds, the Vickers hardness (HVS) of the steel sheet surface layer portion is increased and the fraction (Vγ) of the residual γ is decreased. ), Bending workability (R / t), stretch flangeability (λ), and delayed fracture resistance are reduced. Therefore, in the present invention, the holding time (t2) is set to 200 seconds or more. Preferably it is 250 seconds or more, More preferably, it is 300 seconds or more. On the other hand, when the retention time (t2) exceeds 3000 seconds, the fraction of residual γ (Vγ) decreases, and elongation (El) and delayed fracture resistance (DF) also decrease. Therefore, the holding time (t2) is set to 3000 seconds or less. Preferably it is 2500 seconds or less, More preferably, it is 2000 seconds or less.

  The average cooling rate from the holding temperature (T2) to room temperature is generally preferably about 0.5 to 30 ° C./second, whereby a predetermined steel plate can be produced. What is necessary is just to perform the cooling method by a conventional method, for example, air-water cooling is mentioned.

  The technique of the present invention can be suitably used particularly for a thin steel plate having a plate thickness of 0.8 to 2.5 mm.

  The steel sheet of the present invention thus obtained is a material for parts that require high strength, such as parts such as seat rails, pillars, reinforcements, members, and reinforcing parts such as bumpers and impact beams. Can be suitably used.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  A slab steel having the composition shown in Table 1 below (the balance is iron and inevitable impurities: plate thickness 30 mm) is melted and cast according to a normal melting method, and then hot rolled to a plate thickness of 2.0 mm. (The finish rolling temperature is 950 ° C., and the winding temperature (CT) is shown in Table 2). Subsequently, the obtained hot-rolled steel sheet was pickled and then cold-rolled to a thickness of 1.4 mm. Subsequently, the annealing process was performed on the annealing conditions shown in Table 2. In addition, ct was hold | maintained within the range of 500-700 degreeC.

  Each steel plate obtained as described above was examined for the Vickers hardness, metal structure, mechanical properties, bending workability, stretch flangeability, and delayed fracture resistance of the steel sheet surface layer.

≪Measurement of Vickers hardness (HVS) of surface layer of steel sheet≫
Vickers hardness was measured at a location 30 μm below the surface of the steel plate. Specifically, a load of 3 gf is applied at a position of 30 μm from the surface of the steel plate to the center of the plate thickness, and the Vickers hardness is measured at 20 arbitrary locations. The average value of the measured values is the Vickers hardness (HVS) of the surface layer portion. It was.

≪Observation of metal structure≫
For polygonal ferrite, the fraction was measured by the following method. After polishing the cross section parallel to the rolling direction of the steel plate obtained above and performing nital corrosion, SEM (scanning electron microscope) at 30 μm below the surface of the steel plate and t / 4 (t: plate thickness), The structure was observed at a magnification of 3000 times in a measurement region having a field of view of about 20 μm × 20 μm. Observation was performed for 10 visual fields at each location, and the arithmetic average of the area ratio measured by the point arithmetic method was obtained.

The martensite, retained austenite, bainite, and bainitic ferrite were obtained by the following formulas for the 30 μm fraction (VS) and t / 4 fraction (VI) below the steel sheet surface.
(Fraction of martensite, retained austenite, bainite, and bainitic ferrite) = 100− (fraction of polygonal ferrite)

≪Volume ratio of retained austenite (Vγ) ≫
Vγ was measured by X-ray diffractometry after being ground to ¼ the thickness of the plate and then chemically polished (ISIJ Int. Vol. 33. (1933), No. 7, P.776).

<< Evaluation of mechanical properties >>
As for the mechanical properties of the test material, a tensile test was performed using a No. 5 test piece defined by JIS Z2201, and tensile strength (TS) and elongation (El) were measured. The test piece was cut out from the specimen so that the direction perpendicular to the rolling direction was the longitudinal direction. The measurement results are shown in Table 3 below. In the present invention, the case where TS is 1350 MPa or more was evaluated as ultra-high strength (pass), and the case where TS was less than 1350 MPa was evaluated as insufficient strength (fail). Moreover, the case where El was 9% or more was evaluated as a pass, and the case where it was less than 9% was evaluated as a disqualification.

≪Evaluation of bending workability (R / t) ≫
About bending workability, based on the V block method prescribed | regulated by JISZ2248, the bending test was done using the 1st test piece prescribed | regulated to JISZ2204, and the presence or absence of the crack was observed. The test piece was cut out from the specimen so that the direction perpendicular to the rolling direction was the longitudinal direction. The measurement results are shown in Table 3 below. In the present invention, the value R / t obtained by dividing the minimum bending radius (Rmin) by the plate thickness t is 4 or less without changing the radius of the front end of the metal fitting and the valley of the V block and generating cracks (including hair cracks). The case where it was able to be evaluated as a pass, and the other cases were evaluated as a failure. The presence or absence of cracks was observed using a loupe.

≪Evaluation of stretch flangeability (λ) ≫
The stretch flangeability test was conducted to evaluate stretch flangeability. Specifically, by creating a disk-shaped test piece having a diameter of 100 mm and a plate thickness of 1.4 mm, punching out a hole of φ10 mm with a punch, and then expanding the hole with a burr up using a 60 ° conical punch, The hole expansion rate (λ) at the time of crack penetration was measured (Iron and Steel Federation Standard JFST 1001). A case where the hole expansion ratio (λ) was 30% or more was regarded as acceptable.

≪Evaluation of delayed fracture resistance (DF) ≫
For the delayed fracture resistance of the test material, after cutting a strip of 150 mm × 33 mm cut so that the direction perpendicular to the rolling direction is the longitudinal direction, both ends of the test piece are milled so that the length is further 33 mm to 30 mm. Then, using a test piece whose end face shape is adjusted, U-bending is performed so that the value obtained by dividing the radius R of the bent portion by the thickness t is 7, and then stress of 1000 MPa (strain is converted into stress by a strain gauge). The time until cracking was measured by immersion in a 5% aqueous hydrochloric acid solution. In the present invention, the case where the time until crack generation was 48 hours or more was evaluated as being excellent in delayed fracture resistance (pass), and the case where it was less than 48 hours was evaluated as being inferior in delayed fracture resistance (fail). In Table 3 below, when the delayed fracture resistance is excellent, it is indicated by ○, and when it is inferior in delayed fracture resistance, the time until occurrence of cracking is indicated.

  The chemical composition of the test material is shown in Table 1, the heat treatment conditions are shown in Table 2, and the test results are shown in Table 3.

From Table 3, it can be considered as follows.
No. 1 to 17 and 41 to 45 are manufactured by heat treatment 2 to 5 and 18 of Table 2 satisfying the requirements of the present invention, using steel types 2 to 6 and 15 to 19 of Table 1 satisfying the composition of the present invention, respectively. Vickers hardness (HVS), surface layer portion fraction (VS), plate thickness inside (t / 4) fraction (VI), and residual γ fraction (Vγ). In order to satisfy the requirements of the present invention, the tensile strength (TS) is 1350 MPa or more, and the elongation (El), the bending workability (R / t), and the stretch flangeability (λ) are excellent. Good delayed fracture resistance (DF) is also obtained.

  On the other hand, the following examples that do not satisfy any of the requirements of the present invention have the following problems.

  No. 18 and 19 are examples produced by heat treatments 2 and 4 in Table 2 using the steel type 1 in Table 1 with a small amount of C and satisfying the requirements of the present invention. In either case, the tensile strength (TS) is low, Moreover, since the fraction (VS) of the surface layer portion does not satisfy the requirements of the present invention, the delayed fracture resistance (DF) could not be achieved.

  No. 20 is an example manufactured by heat treatment 5 in Table 2 that uses the steel type 7 in Table 1 to which Ti is not added and satisfies the requirements of the present invention, and the fraction (VS) of the surface layer portion and the thickness inside (t / 4) fraction (VI) and residual γ (Vγ) do not satisfy the requirements of the present invention, so tensile strength (TS), elongation (El), stretch flangeability (λ), and delayed fracture resistance Sex (DF) could not be achieved.

  No. No. 21 is an example manufactured by heat treatment 2 in Table 2 using the steel type 8 in Table 1 with a small amount of Mn and satisfying the requirements of the present invention, and the fraction (VI) in the plate thickness inside (t / 4) is Since the requirements of the invention were not satisfied, the tensile strength (TS) could not be achieved.

  No. No. 22 is an example manufactured by the heat treatment 3 of Table 2 using the steel type 9 of Table 1 with a large amount of Ti and satisfying the requirements of the present invention. The elongation (El), the bending workability (R / t), and the elongation The flangeability (λ) could not be achieved.

  No. No. 23 is an example manufactured by the heat treatment 4 of Table 2 using the steel type 10 of Table 1 having a large amount of Cr and satisfying the requirements of the present invention. The elongation (El), the bending workability (R / t), and the elongation The flangeability (λ) could not be achieved.

  No. No. 24 is an example manufactured by the heat treatment 2 of Table 2 using the steel type 11 of Table 1 to which Cu is not added and satisfying the requirements of the present invention, and could not achieve delayed fracture resistance (DF).

  No. No. 25 is an example manufactured by heat treatment 2 in Table 2 that uses the steel type 12 in Table 1 to which Ni is not added and satisfies the requirements of the present invention, and delayed fracture resistance (DF) could not be achieved.

  No. No. 26 is an example manufactured by the heat treatment 2 of Table 2 using the steel type 13 of Table 1 to which B is not added and satisfying the requirements of the present invention, and the fraction (VS) of the surface layer portion and the inside of the thickness (t Since none of the / 4) fractions (VI) satisfied the requirements of the present invention, tensile strength (TS), stretch flangeability (λ) and delayed fracture resistance (DF) could not be achieved.

  No. No. 27 is an example manufactured by heat treatment 3 in Table 2 that uses the steel type 14 in Table 1 with a large amount of B and satisfies the requirements of the present invention, and delayed fracture resistance (DF) could not be achieved.

  No. 28 is an example produced by heat treatment 1 of Table 2 using a steel type 5 of Table 1 that satisfies the composition of the present invention and having a low coiling temperature (CT), and Vickers hardness (HVS) satisfies the requirements of the present invention. Therefore, bending workability (R / t), stretch flangeability (λ) and delayed fracture resistance (DF) could not be achieved.

  No. 29 is an example manufactured by heat treatment 6 in Table 2 with a short winding time (ct) using steel type 5 in Table 1 that satisfies the composition of the present invention, and Vickers hardness (HVS) satisfies the requirements of the present invention. Therefore, bending workability (R / t), stretch flangeability (λ), and delayed fracture resistance (DF) could not be achieved.

  No. 30 is an example manufactured by heat treatment 7 in Table 2 having a low soaking temperature (Ts), using steel type 5 in Table 1 that satisfies the composition of the present invention, and the fraction (t / 4) of the thickness inside (t / 4) Since VI) does not satisfy the requirements of the present invention, the tensile strength (TS) could not be achieved.

  No. No. 31 is an example manufactured by heat treatment 8 in Table 2 with a short soaking time (ts) using the steel type 5 in Table 1 that satisfies the composition of the present invention, and the fraction (t / 4) inside the plate thickness (t / 4) Since VI) does not satisfy the requirements of the present invention, the tensile strength (TS) could not be achieved.

  No. No. 32 is an example of using the steel type 5 in Table 1 that satisfies the composition of the present invention and manufactured by the heat treatment 9 in Table 2 having a low cooling rate (CR), and the fraction (VI) inside the plate thickness (VI) ) Does not satisfy the requirements of the present invention, the tensile strength (TS) could not be achieved.

  No. 33, 34, and 36 use the steel type 5 in Table 1 that satisfies the composition of the present invention, heat treatment 10 (No. 33) in Table 2 having a low cooling stop temperature (T1), and a low cooling stop temperature (T1). This is an example manufactured by heat treatment 11 (No. 34) and heat treatment 13 (No. 36) in Table 2 having a high holding temperature (T2). Since the requirements for the rate (Vγ) were not satisfied, elongation (El) and delayed fracture resistance (DF) could not be achieved.

  No. 35 is an example manufactured by heat treatment 12 of Table 2 using a steel type 5 of Table 1 that satisfies the composition of the present invention and having a low holding temperature (T2), and has a Vickers hardness (HVS) and a fraction of residual γ ( Since Vγ) does not satisfy the requirements of the present invention, elongation (El), bending workability (R / t), stretch flangeability (λ), and delayed fracture resistance (DF) could not be achieved.

  No. 37 is an example of the steel type 5 of Table 1 that satisfies the composition of the present invention, manufactured by the heat treatment 14 of Table 2 with a short holding time (t2), Vickers hardness (HVS), and residual γ fraction ( Since Vγ) does not satisfy the requirements of the present invention, elongation (El), bending workability (R / t), stretch flangeability (λ), and delayed fracture resistance (DF) could not be achieved.

  No. No. 38 is an example of using the steel type 3 in Table 1 that satisfies the composition of the present invention and the heat treatment 15 in Table 2 having a slow cooling rate (CR), and the surface layer fraction (VS) is a requirement of the present invention. The delayed fracture resistance (DF) could not be achieved.

  No. No. 39 is an example of using the steel type 4 in Table 1 that satisfies the composition of the present invention and manufactured by the heat treatment 16 in Table 2 having a long soaking time (ts), and the Vickers hardness (HVS) satisfies the requirements of the present invention. Therefore, bending workability (R / t), stretch flangeability (λ), and delayed fracture resistance (DF) could not be achieved.

  No. 40 is an example of using the steel type 4 of Table 1 that satisfies the composition of the present invention and manufactured by the heat treatment 17 of Table 2 having a long holding time (t2), and the requirement for the fraction of residual γ (Vγ) of the present invention. Is not satisfied, the elongation (El) and delayed fracture resistance (DF) could not be achieved.

Claims (2)

C: 0.15 to 0.25% (meaning mass%, the same applies to the chemical composition)
Si: 1.0-2.5%,
Mn: 1.0 to 3.0%
Al: 0.01 to 0.10%,
Ti: 0.01-0.10%,
Cu: 0.01 to 1%,
Ni: 0.01 to 1%, and B: 0.0005 to 0.005%
The balance is iron and inevitable impurities,
When the structure is observed with a scanning electron microscope for the surface layer portion in the thickness direction of 30 μm from the outermost layer portion of the steel sheet, the total of martensite, residual austenite, bainite, and bainitic ferrite with respect to the entire structure is 70 area% or more, The balance is polygonal ferrite, the Vickers hardness is 300 to 400 HV, and
When the structure was observed with a scanning electron microscope for a quarter of the plate thickness, the total of martensite, retained austenite, bainite, and bainitic ferrite with respect to the entire structure was 90% by area or more, and the balance was polygo In addition to being null ferrite, the residual austenite is measured by X-ray diffractometry. The volume fraction of residual γ is 3% or more, which is characterized by ductility, bending workability, stretch flangeability, and delayed fracture resistance. Super high strength steel plate with excellent tensile strength of 1350 MPa or more.   Furthermore, the ultra-high-strength steel plate according to claim 1, which contains Cr: 1% or less (not including 0%) as another element.

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