JP2017088944A - High strength steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance hole expandability even when process steep is high in a high strength steel sheet, which does not exist in conventional technology.SOLUTION: There is provided a high strength steel sheet having a component composition containing, by mass%, C:0.05% to 0.30%, Si:0.05% to 2.00%, Mn:1.00% to 10.00%, P:0.10% or less, S:0.01% or less, sol.Al:0.01 to 1.00% and N:0.01% or less and the balance Fe with inevitable impurities and having area percentage of ferrite of 10% to 92%, area percentage of martensite of 8% to 90% and standard deviation of line segment percentage of martensite on a line drawn vertical to a sheet thickness direction at each position in the sheet thickness direction at a position of 3/8 t to 1/2 t of depth from a steel sheet surface of 0.050 or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a high-strength steel sheet used for machine structural parts such as undercarriage parts of automobiles and a method for manufacturing the same. Specifically, the present invention relates to a high-strength steel sheet having excellent ductility and hole expansibility and a method for producing the same.

  Steel sheets used for materials such as automobiles and other transportation machinery and structural members of various industrial machines are required to have mechanical properties excellent in strength, workability, toughness, and the like. In recent years, the application of high-strength steel sheets has been expanded from the viewpoint of reducing the weight of automobiles. However, since many automotive parts are manufactured by press molding, high strength and excellent formability are required. In particular, high-strength steel plates applied to lower arms and wheels, which are automobile underbody members, are required to have not only good ductility but also excellent hole expansibility.

  However, in general, tensile strength and stretch flangeability are in a trade-off relationship, and elongation and hole expandability are significantly reduced as the tensile strength increases. For this reason, it is not easy to achieve both high tensile strength, excellent elongation and hole expansibility. For this reason, in a high strength steel plate, various measures are taken in order to improve the elongation and hole expansibility.

  In response to the problem that high tensile strength, excellent elongation, and hole expandability cannot all be realized, Patent Document 1 describes that the content ratio of Mn and B is optimized to (Mn + 1300 × B) ≧ 2, and the steel structure 340 to 440 MPa class composite structure excellent in workability by forming a ferrite phase having a volume ratio of 95.0 to 99.5% and a low-temperature generation phase having a volume ratio of 0.5 to 5.0%. It is disclosed that the type | mold high tension cold-rolled steel plate can be manufactured easily. Such a structure control is a method that achieves both elongation and hole expansibility.

  However, in recent years, a steel sheet having a high strength and a high strength of TS (tensile strength) of 590 MPa or more has been demanded, and the conventional technology represented by Patent Document 1 has a viewpoint of securing formability. Therefore, it is necessary to contain 95% or more of the ferrite phase, but there is a problem that it is difficult to ensure the above-mentioned strength and cannot meet such a demand.

  Therefore, it is necessary to consider the compatibility between the ductility and hole expansibility of the steel sheet after containing a low-temperature generation phase such as martensite in a volume ratio of 8% or more and securing a strength of 590 MPa or more with TS. In order to stably obtain such a low-temperature generation phase, it is necessary to contain 1.0% or more of high Mn, but a melting process when industrially producing a steel material containing such high Mn In this case, Mn segregates and the element segregation region is stretched in the rolling direction in the hot rolling process and the cold rolling process, so that the martensite is distributed in a band shape (hereinafter sometimes referred to as a “band-like structure”). .

  At the time of deformation, the band-like hard second phase becomes a stress concentration location, which promotes the generation of voids, and moreover, the generation locations of voids become dense and promotes the connection of voids. As a result, premature breakage is caused, and in particular, the hole expandability is significantly reduced.

  In order to solve such an essential problem, in Patent Document 2, as shown in the Examples, a steel sheet having a martensite fraction of 20% or more is used, and a steel sheet after cold rolling and pickling is used. A steel sheet having excellent formability is disclosed in which Mn concentrated in a band-like structure is dispersed once by heating to a temperature range of 750 ° C. or higher, and the band-like structure is thin and finely dispersed.

  Patent Document 3 discloses that the annealing is performed twice. Specifically, at the time of the first annealing, the heating temperature Ar3 to 1000 ° C is held for 3600 seconds, and then cooled at 50 ° C / second to obtain a steel structure. Has a homogeneous martensite structure, and further, a steel sheet having excellent stretch flangeability is disclosed in which the major axis direction of ferrite grains is isotropically dispersed in the second annealing.

  However, all of these methods have an effect of improving the hole expansion property, but they require a long heating process or a plurality of heat treatment processes. It is difficult.

  Furthermore, as in the technique described in Patent Document 2, it is not possible to suppress the generation of voids only by reducing the thickness of the band-like tissue. Further, since the occurrence points of voids are unevenly distributed, they are required. It is not possible to ensure the formability. Further, as in the technique described in Patent Document 3, it is not sufficient to control the form of the ferrite, and the distribution of martensite itself cannot be controlled. Therefore, the required hole expandability cannot be ensured.

  That is, these techniques have the problem that, in addition to the problem of productivity, generation of the band-like structure itself cannot be suppressed, and sufficient hole expanding property cannot be realized.

  On the other hand, in Patent Document 4, Mn is diffused by holding 0.5 h or more and 5 h or less in a temperature range of 1200 ° C. or more and 1300 ° C. or less before the hot rolling step, and the Mn concentration in the cross section in the plate thickness direction of the steel sheet. A steel sheet excellent in elongation and stretch flangeability, in which the ratio C1 / C2 of the upper limit C1 and the lower limit C2 is 2.0 or less, is disclosed.

JP 2009-13488 A JP 2002-88447 A JP 2008-97411 A JP 2010-65307 A

  As described in Patent Document 4, in order to control the band-like structure, optimization by heat treatment at 1000 ° C. or higher is indispensable. On the other hand, the present inventors not only make this optimization, but also promotes the maximum diffusion of Mn by utilizing recrystallization that introduces lattice defects during deformation at high temperature, and further makes the Mn distribution network-like, It was found that it is important to control the structure to minimize the influence of the remaining Mn segregation.

  Specifically, the slab that is continuously cast by actively utilizing facilities capable of multi-axial deformation, such as a sizing mill that has not been used for a slab having a component strength having a tensile strength of 590 MPa or more, Compressive deformation from the direction perpendicular to the plate thickness direction, followed by a homogenization treatment that performs one or more multiaxial compression deformations rolled from the plate thickness direction, appropriately temperature of the hot rolling process in the austenite single phase region Controlling and recrystallizing the austenite grains into a spherical shape and controlling the hot-rolled sheet structure is a method for eliminating the band-shaped structure as much as possible.

  However, in patent document 4, a structure | tissue is controlled only by the homogenization heat processing by high temperature holding. In that case, the Mn concentration is somewhat suppressed, but the remaining Mn segregation affects the formation of the structure during annealing, and the band-like structure of the steel sheet cannot be eliminated, which is a problem in practical use, particularly in practical use. The hole expandability deteriorates when the deformation speed is high. Thus, the present condition is that the hot-rolled steel plate excellent in the ductility and impact characteristic whose tensile strength is 590 Mpa or more is not obtained.

  Hole expansibility is evaluated by the method defined in JIS T1 001, JIS Z 2256, or JFS T 1001. In recent years, due to the improvement in productivity due to advances in manufacturing technology, Therefore, it is required to test at a speed close to 1 mm / second, which is a specified upper limit, at a speed higher than the currently used 0.2 mm / second.

  However, the test speed during the hole expansion test causes an increase in the strain rate, which is considered to be different from the test speed of the conventional method. On the other hand, there is no example tested at a test speed that causes an increase in strain rate. As a result of intensive studies, the present inventors have found that the test value varies depending on the test speed, and that the test value when the test speed is high is important.

  Then, this invention makes it a subject to improve the hole expansibility in the case where processing speed is high in a high strength steel plate in the high strength steel plate, and provides the high strength steel plate and its manufacturing method which solve this subject. With the goal.

  The present inventors have intensively studied to solve the above problems, and as a result, in the component composition of the steel sheet, C, Si, and Mn are controlled to a limited range, and ultra-high temperature multiaxial compression deformation By applying the slab, the slab is remarkably homogenized, and by applying the optimum hot rolling conditions to the homogenized slab, it is a homogeneous structure that was difficult to manufacture with the prior art, and has a tensile strength of 590 MPa or more. It came to the new knowledge that it can obtain the steel plate which has the outstanding ductility and hole expansibility, having strength.

  This invention is made | formed based on the said new knowledge, The summary is as follows.

(1) By mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.00%, Mn: 1.00% to 10.00%, P: 0.00. 10% or less, S: 0.01% or less, sol.Al: 0.01-1.00%, and N: 0.01% or less, with the balance being Fe and inevitable impurities. Have
The area ratio of ferrite is 10% or more and 92% or less, the area ratio of martensite is 8% or more and 90% or less, and the depth is 3 / 8t to 1 / 2t from the steel sheet surface (t: High-strength steel sheet having a steel structure in which the standard deviation of the line segment ratio of martensite on the line drawn perpendicularly to the sheet thickness direction is 0.050 or less at each position in the sheet thickness direction) .

  (2) The component composition is mass%, and instead of a part of Fe, one or two of Ti: 0.20% or less, Nb: 0.20% or less, and V: 0.20% or less The high-strength steel plate according to (1) above, which contains more than seeds.

  (3) The component composition is mass%, and instead of part of Fe, Cr: 1.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, and Ni: 1. The high-strength steel sheet according to (1) or (2) above, comprising one or more of 00% or less.

  (4) The component composition is mass%, and in place of part of Fe, Ca 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% The high-strength steel sheet according to any one of (1) to (3), including one or more of the following:

(5) A production method for producing the high-strength steel plate according to any one of (1) to (4), comprising the following steps (A) to (C): Production method.
(A) The steel slab having the composition described in any one of (1) to (4) above is compressed by 3% or more and 50% or less from the sheet width direction at the time of product in a temperature range of 1000 ° C. to 1250 ° C. Homogenization process in which deformation and multiaxial compression deformation consisting of compression deformation of 3% to 50% from the direction perpendicular to the plate width direction at the time of product is performed once to 5 times (B) The homogenization process is completed. The steel slab is heated to a temperature range of 1200 ° C. or higher, finish rolling is started at a temperature range of 1000 ° C. or higher and 1150 ° C. or lower, and rolling at a reduction ratio of 3% or higher and 50% or lower is performed at an Ar 3 point or higher and 950 at the final stand. (C) After holding the steel sheet that has finished the hot rolling process for 1 second or more and 3 seconds or less, 600 ° C or more and 800 ° C or less at a cooling rate of 25 ° C / second or more. Forcibly cooled to a temperature of 2 to 10 seconds. Cooling step of cooling and winding up to a temperature range of 200 ° C. or lower at a cooling rate of 10 ° C./second or higher

  In the present invention, forced cooling is “actively cooling with gas or liquid, or a mixture thereof”, and natural cooling is “a phenomenon that is not actively cooled, generally used in air cooling”. Means.

  According to the present invention, it is possible to provide a high-strength steel sheet having excellent ductility and hole expandability while having a tensile strength of 590 MPa or more. The high-strength steel sheet of the present invention is suitable for uses that require press forming, such as automobile undercarriage members, and in particular, applications that require ductility and stretch flange forming, which have been difficult to apply conventionally.

It is a figure which shows how to obtain | require the martensite line segment rate in steel structure.

The high-strength steel sheet of the present invention (hereinafter sometimes referred to as “the present invention steel sheet”)
In mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.00%, Mn: 1.00% to 10.00%, P: 0.10% or less , S: 0.01% or less, sol.Al: 0.01-1.00%, and N: 0.01% or less, with the balance being composed of Fe and inevitable impurities,
The area ratio of ferrite is 10% or more and 92% or less, the area ratio of martensite is 8% or more and 90% or less, and the depth is 3 / 8t to 1 / 2t from the steel sheet surface (t: plate of steel sheet) (Thickness) has a steel structure in which the standard deviation of the line segment ratio of martensite on the line drawn perpendicularly to the thickness direction is 0.050 or less.

  The method for producing a high-strength steel plate of the present invention (hereinafter sometimes referred to as “the present invention production method”) is a production method for producing the steel plate of the present invention, and comprises the following steps (A) to (C). It is characterized by.

(A) In the steel slab having the component composition of the steel sheet of the present invention, in a temperature range of 1000 ° C. or more and 1250 ° C. or less, compression deformation of 3% or more and 50% or less from the plate width direction at the product, and the plate width direction at the product Homogenization process in which multiaxial compression deformation consisting of 3% or more and 50% or less compression deformation from the direction perpendicular to the axis is applied once to 5 times or less (B) The steel slab after the homogenization process is subjected to a temperature range of 1200 ° C. or more And finish rolling at a temperature range of 1000 ° C to 1150 ° C, and finish rolling at a final stand at a rolling rate of 3% to 50% at a temperature range of Ar3 to 950 ° C. Rolling step (C) After holding the steel plate after the hot rolling step for 1 second or more and 3 seconds or less, it is forcibly cooled to a temperature of 600 ° C or more and 800 ° C or less at a cooling rate of 25 ° C / second or more and 2 seconds or more. After natural cooling for 10 seconds or less, cooling at 10 ° C / second or more Cooling and winding process that cools and winds up to a temperature range of 200 ° C or less at a rejection speed

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

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

Component Composition First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. Hereinafter, “%” relating to the component composition means “% by mass”.

C: 0.05% or more and 0.30% or less C is an important element for enhancing the hardenability and ensuring the strength of the steel sheet. If C is less than 0.05%, the required martensite fraction cannot be stably obtained, and it becomes difficult to ensure a tensile strength of 590 MPa or more. To do. Preferably it is 0.10% or more. On the other hand, if C exceeds 0.30%, the martensite becomes hard and the weldability is remarkably deteriorated, so C is made 0.30% or less. Preferably it is 0.20% or less.

Si: 0.05% or more and 2.00% or less Si is an element that contributes to improvement of tensile strength without deteriorating hole expansibility by solid solution strengthening, and contributes to improvement of ductility. It is an element that promotes the formation of a ferrite phase and suppresses the band-like distribution of martensite.

  If Si is less than 0.05%, the effect of addition is not sufficiently exhibited, so Si is made 0.05% or more. Si is preferably 0.50% or more in terms of ensuring the ductility improving effect and the band-like distribution suppressing effect. More preferably, it is 1.00% or more. On the other hand, if Si exceeds 2.00%, the effect of addition becomes saturated and disadvantageous economically, and the surface properties deteriorate, so Si is made 2.00% or less. Preferably it is 1.70% or less.

Mn: 1.00% or more and 10.00% or less Mn is an element that improves the hardenability of steel and contributes to the improvement of strength. If Mn is less than 1.00%, it becomes difficult to ensure a tensile strength of 590 MPa or more, so Mn is made 1.00% or more. Mn is preferably 1.50% or more, and more preferably 2.0% or more in that the strength can be increased without adding an expensive alloy element.

  On the other hand, when Mn exceeds 10.00%, the precipitation amount of MnS increases and the low-temperature toughness decreases, so Mn is made 10.00% or less. Mn is preferably 8.00% or less from the viewpoint of productivity during hot rolling and cold rolling. More preferably, it is 4.00% or less.

P: 0.10% or less P is usually an impurity element, but is also an element that acts to increase the tensile strength. If P exceeds 0.10%, the weldability deteriorates remarkably, so P is made 0.10% or less. Preferably it is 0.03% or less. The lower limit includes 0%, but if P is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet. P is preferably 0.01% or more from the viewpoint of reliably obtaining the effect of improving the strength of P.

S: 0.01% or less S is an impurity element, and the more preferable it is from the viewpoint of weldability. If S exceeds 0.01%, the weldability is significantly reduced, the amount of MnS precipitated is increased, and the low-temperature toughness is lowered. Therefore, S is made 0.01% or less. Preferably it is 0.003% or less, More preferably, it is 0.0015% or less. The lower limit includes 0%, but if S is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet. In terms of manufacturing cost, S is preferably 0.001% or more.

sol. Al: 0.01 to 1.00%
Al is an element that acts to deoxidize steel and to make the steel plate sound. sol. If the Al content is less than 0.01%, the effect of addition is not sufficiently exhibited. Al is 0.01% or more. Preferably it is 0.015% or more. On the other hand, sol. When Al exceeds 1.00%, the weldability is significantly lowered and the oxide inclusions are increased, so that the surface properties are significantly deteriorated. Al is 1.00% or less. Preferably it is 0.50% or less. Note that sol. Al means acid-soluble Al that is not an oxide such as Al ₂ O ₃ and is soluble in acid.

N: 0.01% or less N is usually an impurity element, and a smaller element is preferable from the viewpoint of weldability. If N exceeds 0.01%, the weldability deteriorates remarkably, so N is made 0.01% or less. Preferably it is 0.006% or less. The lower limit includes 0%, but if N is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is a practical lower limit on a practical steel sheet.

Inevitable impurities Inevitable impurities are elements that are inevitably mixed from raw materials and / or elements that are inevitably mixed in the production process, and are elements that are acceptable within a range that does not impair the properties of the steel sheet of the present invention.

  The component composition of the steel sheet of the present invention includes, in addition to the above elements, (a) Ti: 0.20% or less, Nb: 0.20% or less, and V: 0.20% or less, b) Cr: 1.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, and Ni: 1.00% or less, and (c) Ca0. One or two or more element groups of 01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less may be included. .

(a) Group element Ti: 0.20% or less Nb: 0.20% or less V: 0.20% or less These elements are elements that contribute to ensuring stable strength. If any element exceeds 0.20%, the strength increases so much that rolling becomes difficult. Therefore, any element is preferably 0.20% or less. More preferably, it is 0.15% or less. In terms of obtaining the effect of addition reliably, any element is preferably 0.003% or more.

(b) Group element Cr: 1.00% or less Mo: 1.00% or less Cu: 1.00% or less Ni: 1.00% or less All of these elements contribute to stable securing of strength. It is. If any element exceeds 1.00%, the effect of addition may be saturated, which may be economically disadvantageous. Therefore, any element is preferably 1.00% or less. More preferably, it is 0.50% or less. In order to ensure the effect of addition, any element is preferably 0.005% or more.

(c) Group element Ca: 0.01% or less Mg: 0.01% or less REM: 0.01% or less Zr: 0.01% or less All of these elements control inclusions, in particular intervening. It is an element that finely disperses materials and contributes to improved toughness. If any element exceeds 0.01%, surface property deterioration may be manifested. Therefore, any element is preferably 0.01% or less. More preferably, it is 0.005% or less. In terms of obtaining the effect of addition reliably, any element is preferably 0.0003% or more. Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid, and is at least one of them. The amount of REM means the total amount of at least one of these elements. In the case of a lanthanoid, it is added industrially in the form of misch metal.

Steel structure Next, the steel structure of the steel sheet of the present invention will be described.

  The steel sheet of the present invention has a ferrite area ratio of 10% to 92%, a martensite area ratio of 8% to 90%, and a depth of 3/8 t to 1/2 t from the steel sheet surface. At each position in the plate thickness direction in (t: plate thickness of the steel plate), it has a steel structure in which the standard deviation of the martensite line segment on the line drawn perpendicular to the plate thickness direction is 0.050 or less.

  In the steel plate of the present invention, at a position of 1/4 of the steel plate width, the plate thickness cross section in the direction parallel to the rolling direction and the direction perpendicular to the rolling direction is corroded by repeller etching, and is an image taken at 200 times using an optical microscope. The ferrite area ratio measured and the area ratio of martensite are defined.

  Martensite is a region colored white by repeller etching, and ferrite is a region colored black. In the black area, the particularly heavily etched area is a pearlite or carbide area.

Area ratio of ferrite: 10% or more and 92% or less If the area ratio of ferrite is less than 10%, it becomes difficult to ensure a total elongation of 10% or more, so the area ratio of ferrite is 10% or more. Preferably it is 20% or more. On the other hand, if the area ratio of ferrite exceeds 92%, it becomes difficult to ensure a tensile strength of 590 MPa or more, so the area ratio of ferrite is set to 92% or less. Preferably it is 82% or less. In addition, it is preferable that all the ferrites are recrystallized grains.

Martensite area ratio: 8% or more and 90% or less When the martensite area ratio is less than 8%, it becomes difficult to secure a tensile strength of 590 MPa or more, so the martensite area ratio is 8% or more. Preferably it is 10% or more. On the other hand, if the area ratio of martensite exceeds 90%, the ductility decreases, so the area ratio of martensite is 90% or less. Preferably it is 80% or less.

Standard martensite line segment on the line drawn perpendicular to the thickness direction at each position in the thickness direction at a depth of 3 / 8t to 1 / 2t from the steel sheet surface (t: thickness of the steel sheet) Deviation: 0.050 or less In a test in which large local deformation is performed, such as a hole expansion test, fracture occurs through necking of the steel sheet and generation / connection of voids in the material structure. In the tensile deformation in which the steel plate is constricted, the central portion of the steel plate becomes a stress concentration location, and the void is usually generated around a position 1/2 t from the steel plate surface (t: plate thickness of the steel plate).

  Voids are connected before the material breaks. However, when the voids are coarsened to a size of 1/8 t or more, fractures occur starting from the coarsened voids. Thus, since the void generation position that is connected to the void generated at the position of 1 / 2t and is the starting point of the destruction is martensite existing at the position of 1 / 2t to 3 / 8t, this range is It was defined as a position having a depth of 3/8 t to 1/2 t from the steel plate surface (t: plate thickness of the steel plate).

  As the standard deviation of the martensite line segment ratio increases, the steel structure becomes a noticeable band-like structure, and the density of stress-concentrated locations increases locally, resulting in a decrease in hole expansibility. In order to ensure excellent hole expandability, the standard deviation of the martensite line segment ratio is set to 0.050 or less. Preferably it is 0.040 or less.

  The area ratio of each phase in the steel structure and the line segment ratio of martensite can be measured by the method described in Examples.

Mechanical Properties The steel sheet of the present invention preferably has a tensile strength (TS) of 590 MPa or more as a sufficient strength that contributes to reducing the weight of an automobile.

  As for the hole expandability, it is preferable that the hole expansion ratio (HER) of the steel sheet measured at a hole expansion test speed of 1 mm / second in the method defined in JIS T 1001 is 50% or more.

  As for ductility, it is preferable that the elongation at break El measured by a method defined in JIS Z 2241 is 17% or more by taking a JIS No. 5 tensile test piece from a steel plate so that the tensile direction is orthogonal to the rolling direction.

  Next, the manufacturing method of the present invention will be described.

  In order to obtain excellent hole expansibility while securing a tensile strength of 590 MPa or more, the steel sheet of the present invention has a steel structure with a ferrite area ratio of 10% to 92% and a martensite area ratio of 8%. 90% or more, and a line drawn perpendicularly to the plate thickness direction at each position in the plate thickness direction at a position (t: plate thickness of the steel plate) at a depth of 3 / 8t to 1 / 2t from the steel plate surface It is necessary to have a steel structure in which the standard deviation of the fraction of martensite on the minute is 0.050 or less.

  This invention manufacturing method which manufactures this invention steel plate which has the said steel structure comprises the process of following (A)-(C), It is characterized by the above-mentioned.

(A) In the steel slab having the component composition of the steel sheet of the present invention, in a temperature range of 1000 ° C. or more and 1250 ° C. or less, compression deformation of 3% or more and 50% or less from the plate width direction at the product, and the plate width direction at the product A homogenization step in which a multiaxial compression deformation comprising a compression deformation of 3% to 50% from a direction perpendicular to the vertical direction is applied at least once and no more than 5 times. (B) The steel slab after the homogenization step has a temperature of 1200 ° C or higher. Heat that heats the zone, starts finish rolling in the temperature range of 1000 ° C to 1150 ° C, and finishes rolling in the final stand at a reduction rate of 3% to 50% in the temperature range of Ar3 to 950 ° C. (C) The steel plate that has finished the hot rolling process is held for 1 second or more and 3 seconds or less, and then forcedly cooled to a temperature of 600 ° C. or more and 800 ° C. or less at a cooling rate of 25 ° C./second or more. 10 ℃ / second or more after natural cooling for 10 seconds or less Cooling and winding process for cooling and winding up to 200 ° C. or less at a cooling rate of

  Hereinafter, conditions for defining each process will be described.

(A) Homogenization process Multiaxial compression deformation: (A1) In the temperature range from 1000 ° C to 1250 ° C
3% or more and 50% or less of compressive deformation, and the vertical direction of the plate width direction at the time of product
3% to 50% compression deformation
(A2) 1 to 5 times

  A molten steel having the component composition of the steel sheet of the present invention is cast into a steel slab. The casting method is preferably a continuous casting method, but may be an ingot casting method or a thin slab casting method. The steel slab is heated to a temperature range of 1000 ° C. or higher and 1250 ° C. or lower and subjected to multiaxial compression deformation. The holding time after heating is not particularly defined, but is preferably 30 minutes or more in order to improve the hole expanding property. In order to suppress excessive scale loss, 10 hours or less is preferable, and 5 hours or less is more preferable. Direct feed rolling or direct rolling may be performed and subjected to multiaxial compression deformation as it is without heating and holding.

  By subjecting the steel slab to multi-axial compression deformation at least once from the plate width direction and the direction perpendicular to the plate width direction at the time of the product, the alloy element enriched portion in the steel slab is subdivided to make the alloy elements homogeneous In addition to enabling dispersion, it is possible to promote the diffusion of the alloy element and suppress the formation of a band-like structure in a later step, thereby obtaining a more homogeneous steel structure.

  In particular, compression deformation from the plate width direction is effective. By this compression deformation, the alloy element enrichment portion that is connected in the plate width direction can be finely divided to uniformly disperse the alloy element. Tissue homogenization that cannot be achieved can be achieved in a short time.

  If the temperature range of multiaxial compression deformation is less than 1000 ° C, the subdivision of the alloy element enriched part in the steel slab does not proceed sufficiently, making it difficult to uniformly disperse the alloy element and delaying the diffusion of the alloy element. And since formation of a band-like structure | tissue cannot be suppressed, the temperature range of multiaxial compression deformation shall be 1000 degreeC or more. Preferably it is 1050 degreeC or more.

  On the other hand, if the temperature range of multiaxial compression deformation exceeds 1250 ° C., the manufacturing cost increases and the yield decreases due to an increase in scale loss. Therefore, the temperature range of multiaxial compression deformation is set to 1250 ° C. or less. Preferably it is 1200 degrees C or less.

  The amount of compressive deformation from the sheet width direction at the time of product is set to 3% or more and 50% or less per multiaxial compression deformation. When the amount of compressive deformation from the sheet width direction during product is less than 3%, defects introduced by plastic deformation at high temperatures become insufficient, and diffusion of alloy elements cannot be promoted, and the formation of a band-like structure is suppressed. Therefore, the amount of compressive deformation from the width direction of the product is 3% or more. Preferably it is 13% or more.

  On the other hand, if the amount of compressive deformation from the width direction of the product exceeds 50%, slab cracking occurs or the slab shape becomes non-uniform and the dimensional accuracy in subsequent hot rolling decreases, The amount of compressive deformation from the plate width direction is 50% or less. Preferably it is 40% or less.

  The amount of compressive deformation from the direction perpendicular to the sheet width direction at the time of product is set to 3% to 50% per multiaxial compression deformation. If the amount of compressive deformation from the direction perpendicular to the sheet width direction during the product is less than 3%, diffusion of the alloy elements cannot be promoted, and it is caught in the rolling roll in the subsequent hot rolling process due to shape defects. Since defects occur, the amount of compressive deformation from the direction perpendicular to the plate width direction at the time of product is set to 3% or more. Preferably it is 13% or more.

  On the other hand, if the amount of compressive deformation from the direction perpendicular to the sheet width direction at the time of product exceeds 50%, the surface of the steel slab will crack, so the amount of compressive deformation from the direction perpendicular to the sheet width direction at the time of product Is 50% or less. Preferably it is 40% or less.

  If multiaxial compression deformation is performed in the above temperature range and the above amount of compression deformation, the formation of band-like tissue can be suppressed, but by repeating multiaxial compression deformation, the effect of suppressing the formation of band-like tissue is Since it becomes remarkable, it is preferable to carry out twice. At this time, the steel slab may be charged into a heating furnace and reheated in the middle of multiaxial compression deformation.

  When the multiaxial compression deformation is performed 6 times or more, the manufacturing cost increases, the scale loss increases, the yield decreases, and further, the thickness of the steel slab used for the hot rolling process becomes non-uniform, Since hot rolling becomes difficult, the multiaxial compression deformation is set to 5 times or less.

(B) Hot rolling process The steel slab which finished the homogenization process is used for a hot rolling process. In the hot rolling step, the amount of reduction and the rolling temperature at the final stand are appropriately controlled. A more uniform steel structure can be obtained in combination with the subsequent cooling control.

Heating temperature range: 1200 ° C. or higher A steel slab to be subjected to hot rolling after finishing the homogenization step is heated to a temperature range of 1200 ° C. or higher. If the heating temperature range is less than 1200 ° C, the temperature decreases until the finish rolling and the finish rolling start temperature: 1000 ° C or more may not be secured. The steel slab to be used is heated to a temperature range of 1200 ° C. or higher. In addition, if the temperature of the steel slab which finished the homogenization process is 1200 degreeC or more, you may use for hot rolling as it is.

Finish rolling start temperature: 1000 ° C. or higher and 1150 ° C. or lower In order to achieve the refinement of the steel structure and the elimination of the band-like structure, it is necessary to repeatedly perform recrystallization before the finish rolling. When the finish rolling start temperature is less than 1000 ° C., recrystallization becomes insufficient, and finally martensite is coarsened, and unrecrystallized austenite is transformed into martensite to form a band-like structure. The finish rolling start temperature is 1000 ° C. or higher. Preferably it is 1050 degreeC or more.

  On the other hand, if the finish rolling start temperature exceeds 1150 ° C., the roll may be damaged and the productivity may be lowered. Therefore, the finish rolling start temperature is set to 1150 ° C. or less. Preferably it is 110 degrees C or less.

Rolling rate at the final stand: 3% to 50% Rolling end temperature range at the final stand: Ar3 point to 950 ° C or less If the rolling rate at the final stand is less than 3%, the steel structure can be sufficiently homogenized. Therefore, the required hole expansibility and elongation cannot be ensured, so the rolling reduction at the final stand should be 3% or more. Preferably it is 10% or more.

  On the other hand, when the rolling reduction at the final stand exceeds 50%, the austenite is stretched to form a band-like structure, and the hole expandability and elongation decrease. Therefore, the rolling reduction at the final stand is 50% or less. . Preferably it is 30% or less.

  If the rolling end temperature range at the final stand is less than the Ar3 point, a band-like structure is formed and recrystallized ferrite remains, and the hole expandability deteriorates. Therefore, the rolling end temperature range at the final stand is Ar3. Do not exceed the point. Preferably, it is Ar3 point + 20 ° C. or higher.

  On the other hand, if the rolling end temperature range in the final stand exceeds 950 ° C., ferrite and martensite are coarsened and the low temperature toughness is lowered. Therefore, the rolling end temperature range in the final stand is set to 950 ° C. or less. Preferably it is 930 degrees C or less.

The Ar3 point is a temperature that can be calculated by the following formula from the component composition.
Ar3 (° C.) = 870-390.C + 24.Si-70.Mn-50.Ni
-5 ・ Cr-20 ・ Cu + 80 ・ Mo
An element means the content of the element.

(C) Cooling and winding process The cooling of the steel sheet that has finished the hot rolling process is appropriately controlled to form and wind a more homogeneous steel structure.

Holding time until forced cooling after completion of hot rolling: 1 second or more and 3 seconds or less Forcibly cooling the steel sheet that has finished the hot rolling process, but the austenite that has been reduced in the final stand until forced cooling is started Recrystallize. If the holding time until the forced cooling is less than 1 second after the hot rolling is finished, ferrite is generated from the flat austenite and a band-like structure is formed. Therefore, the holding time until the forced cooling after the hot rolling is finished Is 1 second or longer.

  On the other hand, if the holding time exceeds 3 seconds, ferrite and martensite are coarsened and the low temperature toughness is lowered, so the holding time is set to 1 second or more and 3 seconds or less.

Cooling rate after holding: 25 ° C./second or more Cooling temperature range after holding: 600 ° C. or more and 800 ° C. or less After the above holding, the steel plate is temperature range of 600 ° C. or more and 800 ° C. or less at a cooling rate of 25 ° C./second or more. Cool until forced. When the cooling rate is less than 25 ° C./second, ferrite is excessively generated, C concentration in martensite becomes remarkable, the steel structure is hardened, and the hole expandability is lowered. The cooling rate is 25 ° C./second or more. Preferably, it is 35 ° C./second or more. Since the cooling rate may be set as appropriate within the range of the cooling capacity of the cooling facility, the upper limit of the cooling rate is not particularly limited.

  When the cooling temperature range is less than 600 ° C., ferrite is excessively generated, the concentration of C in martensite becomes remarkable, the steel structure hardens, and the hole expandability decreases, so the cooling temperature range is Set to 600 ° C. or higher. Preferably it is 630 degreeC or more. On the other hand, if the cooling temperature range exceeds 800 ° C., the driving force for ferrite transformation decreases, and a sufficient ferrite fraction cannot be secured, so the cooling temperature range is set to 800 ° C. or less. Preferably it is 770 degrees C or less.

Natural cooling after forced cooling: 2 seconds to 10 seconds After forced cooling to a temperature range of 600 ° C. to 800 ° C. at a cooling rate of 25 ° C./second, natural cooling is performed for 2 seconds to 10 seconds. If the natural cooling time is less than 2 seconds, the driving force for ferrite transformation becomes low and a sufficient ferrite fraction cannot be secured, so the natural cooling time is set to 2 seconds or more. Preferably it is 4 seconds or more.

  On the other hand, when the natural cooling time exceeds 10 seconds, ferrite is excessively generated, the concentration of C in martensite becomes remarkable, the steel structure hardens, and the hole expandability decreases. The cooling time is 10 seconds or less. Preferably it is 8 seconds or less.

Cooling rate after natural cooling: 10 ° C / second or more Cooling temperature range after natural cooling: 200 ° C or less 200 ° C at a cooling rate of 10 ° C / second or more after natural cooling in order to transform austenite into martensite. The steel sheet is wound up by forced cooling to the following temperature range.

  When the cooling rate after natural cooling is less than 10 ° C./second, ferrite is excessively generated, C concentration in martensite becomes remarkable, the steel structure is hardened, and the hole expandability is lowered. Therefore, the cooling rate after natural cooling is set to 10 ° C./second or more. Preferably, it is 30 ° C./second or more. Since the cooling rate may be set as appropriate within the range of the cooling capacity of the cooling facility, the upper limit of the cooling rate is not particularly limited.

  The cooling temperature range after natural cooling is 200 ° C. or less. If the cooling temperature range after natural cooling, that is, the coiling temperature exceeds 200 ° C., the steel structure softens due to the formation of bainite or self-tempering of martensite and the strength decreases, so natural cooling The subsequent cooling temperature range, that is, the coiling temperature is set to 200 ° C. or lower.

  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 1
A steel slab having the composition shown in Table 1 is cast, and the steel slab is homogenized under the conditions shown in Table 2, and then hot-rolled under the conditions shown in Table 2, cold-rolled and wound up. It was. In Table 1, the underline means outside the scope of the present invention. The same applies to Tables 2 and 3 below.

  That is, in the homogenization step, after the steel slab is heated at 1250 ° C. for 1 hour, the steel slab is heated to a temperature not lower than the temperature indicated by the “multiaxial compression deformation lower limit temperature” and not higher than the temperature indicated by the “multiaxial compression deformation upper limit temperature”. In the temperature range, compressive deformation is performed from the plate width direction at the compression deformation rate shown in “Compression deformation rate from plate width direction”, and then in the same temperature range, from the plate thickness direction, “compression deformation rate from plate thickness direction”. After performing the multiaxial compression deformation for compressive deformation at the compressive deformation rate shown in FIG. 1 for the number of times shown in “Number of repetitions of compression deformation”, it was subjected to hot rolling.

  In the hot rolling process, the steel slab (coarse bar) after multiaxial compression deformation was reheated to the temperature indicated by “Solution temperature”, and then finish-rolled at the temperature indicated by “Finish rolling start temperature”.

  In the finish rolling, among the seven rollings in total, in the final stage rolling, rolling is performed at the temperature shown in “Final Stand Rolling Temperature” and the rolling reduction shown in “Final Stand Rolling Amount”, and then “Retained after final stand rolling” Hold for the time shown in “Time”, then cool to the temperature shown in “Natural cooling start temperature” at the cooling rate shown in “Cooling rate after final stand rolling”, hold for the time shown in “Natural cooling time”, and then The product was cooled to the temperature indicated by “winding temperature” at the cooling rate indicated by “cooling temperature after natural cooling”, and air-cooled to room temperature after 60 minutes.

  The steel plate thus manufactured was subjected to the following tests, the steel structure was observed, and the mechanical properties were measured.

(1) Observation of steel structure At the position of 1/4 of the plate width of the steel plate, the plate thickness section in the direction parallel to and perpendicular to the rolling direction is corroded by repeller etching and imaged at 200 times using an optical microscope. The obtained images were subjected to image analysis, and the area ratio of ferrite, the area ratio of martensite, and the presence or absence of unrecrystallized ferrite were measured.

  The martensite line segment ratio at a position of depth 3 / 8t to 1 / 2t from the steel plate surface (t: plate thickness of the steel plate) was calculated by the following procedure. FIG. 1 shows how to determine the martensite line segment ratio in the steel structure.

  An observation image at a depth of 3 / 8t to 1 / 2t from the steel sheet surface is binarized by image processing and divided into two areas of ferrite and martensite. It is divided into 15 t / 4r points at intervals of 30, and the martensite line segment in a line segment region having a width r / 30 in the divided plate thickness direction and a length 50 r in the direction perpendicular to the plate thickness direction is 15 t / It calculated | required about 4r point and calculated the standard deviation.

(2) Tensile test A JIS No. 5 tensile test piece with the direction perpendicular to the rolling direction as the longitudinal direction is taken from the steel sheet, and the tensile properties (yield strength YS, tensile strength TS, total elongation El) conform to JIS Z 2241. And measured.

(3) Hole expansion test A 90 mm square test piece was sampled from a steel plate, conformed to the provisions of JIS Z 2256 (or JIS T 1001), the test speed of the hole expansion test was 1 mm / second, and the hole expansion rate λ Was measured. Table 3 shows the measurement results.

  In Table 3, the test material No. 1, no. 12, no. 15, no. 16, no. 18, no. 19, no. 21, no. 22, no. 23, no. 26, no. 28 and No. 29 is an invention example that satisfies all the conditions of the present invention. In the invention example, the standard deviation of the martensite line segment along the plate thickness direction at a depth of 3 / 8t to 1 / 2t from the steel plate surface is 0.050 or less, and a high strain rate of 1 mm / sec. Excellent hole expansion in the hole expansion test.

  Specimen No. 9 and no. No. 10, the component composition of the steel sheet is out of the range of the component composition of the present invention, and the tensile strength of 590 MPa or more is not obtained. Specimen No. No. 11 is that the component composition of the steel sheet is out of the range of the component composition of the present invention, the area ratio of ferrite is higher than the range of the present invention, the area ratio of martensite is lower than the range of the present invention, and the tensile strength is 590 MPa or more. Is not obtained.

  Specimen No. 13, no. 14, no. 17, no. 20 and No. In No. 27, since the manufacturing conditions are out of the range of the manufacturing conditions of the present invention, the standard deviation of the martensite line segment exceeds the range of the present invention, and the hole expandability is inferior. Specimen No. 24 and no. No. 25, because the production conditions are out of the range of the production conditions of the present invention, pearlite is generated, the martensite area ratio is out of the range of the present invention, the tensile strength is low, and the hole expandability is inferior. .

  Specimen No. No. 3 had a low compressive deformation rate from the plate thickness direction in the homogenization process, and hot rolling was impossible due to a shape defect.

  As described above, according to the present invention, it is possible to provide a high-strength steel sheet having excellent ductility and hole expansibility while having a tensile strength of 590 MPa or more. The high-strength steel sheet of the present invention is suitable for uses that require press forming, such as automobile undercarriage members, and in particular, applications that require ductility and stretch flange forming, which have been difficult to apply conventionally. Therefore, the present invention has high applicability in the steel plate manufacturing industry and the automobile industry.

Claims (5)

In mass%, C: 0.05% to 0.30%, Si: 0.05% to 2.00%, Mn: 1.00% to 10.00%, P: 0.10% or less , S: 0.01% or less, sol.Al: 0.01-1.00%, and N: 0.01% or less, with the balance being composed of Fe and inevitable impurities,
The area ratio of ferrite is 10% or more and 92% or less, the area ratio of martensite is 8% or more and 90% or less, and the depth is 3 / 8t to 1 / 2t from the steel sheet surface (t: High-strength steel sheet having a steel structure in which the standard deviation of the line segment ratio of martensite on the line drawn perpendicularly to the sheet thickness direction is 0.050 or less at each position in the sheet thickness direction) .   The component composition is 1% or more of Ti: 0.20% or less, Nb: 0.20% or less, and V: 0.20% or less in place of part of Fe in mass%. 2. The high-strength steel sheet according to claim 1, comprising:   The component composition is mass%, and instead of part of Fe, Cr: 1.00% or less, Mo: 1.00% or less, Cu: 1.00% or less, and Ni: 1.00% or less The high-strength steel sheet according to claim 1 or 2, comprising one or more of the following.   The component composition is 1% by mass, in place of part of Fe, 0.01% or less of Ca, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. The high-strength steel sheet according to any one of claims 1 to 3, comprising seeds or two or more kinds. It is a manufacturing method which manufactures the high strength steel plate of any one of Claims 1-4, Comprising: The manufacturing method of the high strength steel plate characterized by including the process of following (A)-(C).
(A) The steel slab having the component composition according to any one of claims 1 to 4, wherein the compression deformation is 3% or more and 50% or less from a sheet width direction at the time of product in a temperature range of 1000 ° C to 1250 ° C. And a homogenization step in which a multiaxial compression deformation comprising a compression deformation of 3% or more and 50% or less from the direction perpendicular to the sheet width direction at the time of product is applied once to 5 times or less (B) Steel that has finished the homogenization step The slab is heated to a temperature range of 1200 ° C. or higher, finish rolling is started in a temperature range of 1000 ° C. or higher and 1150 ° C. or lower, and rolling at a reduction ratio of 3% or higher and 50% or lower is performed at an Ar 3 point or higher and 950 ° C. (C) After holding the steel sheet that has finished the hot rolling process for 1 second or more and 3 seconds or less, it is 600 ° C or more and 800 ° C or less at a cooling rate of 25 ° C / second or more. Forced cooling to temperature and natural cooling for 2 to 10 seconds Thereafter, a cooling and winding step of cooling and winding up to a temperature range of 200 ° C or lower at a cooling rate of 10 ° C / second or higher