JP5979326B1 - High strength plated steel sheet and method for producing the same - Google Patents

Abstract

A high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and good workability and a method for producing the same are provided. A high-strength hot-dip galvanized steel sheet having a steel sheet and a plating layer formed on the steel sheet, comprising a specific component composition, the ferrite phase having an area ratio of 20% or less (including 0%), and the bainite phase having an area 35% or more and 90% or less in terms of percentage, martensite phase is contained in an area ratio of 10% or more and 65% or less, and the number density of inclusions having an equivalent circle diameter exceeding 5.0 μm contains 400 pieces / mm 2 or less, It has a steel structure in which the average particle size of granular martensite constituting the martensite phase is 4.0 μm or less and the maximum length of martensite is 5.0 μm or less. When the hardness at a position of 1/4 t in the direction (t is the thickness of the steel sheet) is 100%, it is a high-strength hot-dip steel sheet characterized by being 95% or less.

Description

  The present invention relates to a high-strength plated steel sheet and a method for producing the same. The high-strength plated steel sheet of the present invention has a high tensile strength (TS): 780 MPa or more and excellent formability. For this reason, the high-strength plated steel sheet of the present invention is suitable as a material for a structural part for automotive.

In recent years, from the viewpoint of protecting the global environment, the automobile industry as a whole has been directed to improving the fuel consumption of automobiles for the purpose of reducing CO ₂ emissions. The most effective way to improve automobile fuel efficiency is to reduce the weight of automobiles by reducing the thickness of parts used. For this reason, in recent years, the usage amount of high-strength steel sheets is increasing as a material for automobile parts.

  On the other hand, in general, the formability of steel sheets decreases with increasing strength, making it difficult to process. For this reason, in order to reduce the weight of automobile parts and the like, the steel sheet is required to have good workability in addition to high strength.

  From the above, the development of steel sheets that combine high strength and bendability (also called workability and formability) is required, and various high-strength cold-rolled steel sheets and plated steel sheets that have focused on workability have been developed so far. Technologies have been proposed.

  For example, in Patent Document 1, in a hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet, by mass%, C: more than 0.02% and 0.20% or less, Si: 0.01-2.0 %, Mn: 0.1-3.0%, P: 0.003-0.10%, S: 0.020% or less, Al: 0.001-1.0%, N: 0.0004-0 0.15%, Ti: 0.03 to 0.2%, or Nb: 0.1% or less, etc., and the balance is Fe and impurities, and the ferrite has an area ratio of 30 to 95. %, The remaining second phase is composed of one or more of martensite, bainite, pearlite, cementite and retained austenite, and the martensite area ratio is 0 to 50 when martensite is contained. % Steel structure (m the steel sheet contains Ti carbonitrides with a particle size of 2 to 30 nm and an average interparticle distance of 30 to 300 nm, and crystallized TiN with a particle size of 3 μm or more and an average interparticle distance of 50 to 500 μm. It is said that a high-yield ratio high-strength steel sheet excellent in bending workability and notch fatigue resistance with a tensile strength of 620 MPa or more is obtained.

  In Patent Document 2, by mass%, C: 0.05 to 0.20%, Si: 0.01 to less than 0.6%, Mn: 1.6 to 3.5%, P: 0.05% or less , S: 0.01% or less, sol. A steel sheet containing Al: 1.5% or less, N: 0.01% or less, the balance being iron and inevitable impurities, and having a polygonal ferrite structure and a low-temperature transformed phases (low-temperature transformed phases) The low-temperature transformation formation structure contains at least bainite and may further contain martensite. The plate surface in a depth of 0.1 mm from the surface of the steel plate is changed in the plate width direction position, and a total of 20 fields of view are observed with a microscope. Observing and analyzing the image of 50 μm × 50 μm area in each field of view, bending workability and fatigue strength by determining the maximum and minimum values of polygonal ferrite area ratio and martensite area ratio It is said that a hot-dip galvanized steel sheet having an excellent tensile strength of 780 MPa or more can be obtained.

JP 2006-063360 A JP 2010-209428 A

  In the technique proposed in Patent Document 1, the influence of the component composition on the steel structure is not disclosed in the examples, and the improvement by considering the steel structure is insufficient. However, the improvement is not enough.

  Further, even in the technique proposed in Patent Document 2, factors that should be considered in order to realize improvement in formability by high work hardenability as required in the present invention are not sufficiently grasped.

  This invention is made | formed in view of this situation, Comprising: It aims at providing the high strength plated steel plate which has tensile strength: 780 MPa or more and favorable workability, and its manufacturing method.

  In order to solve the above-mentioned problems, the present inventors diligently studied the requirements for a steel sheet having a tensile strength of 780 MPa and good workability. As a result, in order to obtain a high-strength steel sheet, we focused on reducing the soft ferrite phase as much as possible and utilizing a low-temperature transformation phase such as a bainite phase or a martensite phase. On the other hand, in the conventional technology, if the ferrite phase rich in formability is reduced, good formability cannot be obtained. Therefore, a means for improving the formability of a steel sheet not containing much ferrite phase was studied. As a result, when the fine granular martensite is the martensite phase dispersed in the bainite phase, the uniform deformation of the bainite phase is promoted, and as a result, the workability is increased and the formability is improved. . In order to disperse fine martensite into the bainite phase, it is effective to finely disperse cementite in the structure before the annealing process and to suppress coarsening of the austenite grain size during annealing. I found out. On the other hand, since the austenite grain interfacial area, which becomes the nucleation site of ferrite transformation, increases with the reduction of the austenite grain size during annealing, the ferrite phase is easily produced. In order to suppress ferrite transformation after refining the martensite phase, improve the hardenability of the steel sheet by adding appropriate elements, and reduce the inclusion density of 5.0 μm or more that becomes ferrite nucleation sites It turned out to be important.

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

[1] A high-strength plated steel plate having a steel plate and a plating layer formed on the steel plate, wherein the component composition of the steel plate is% by mass, C: 0.06% to 0.18%, Si : Less than 0.50%, Mn: 1.9% to 3.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.006% B: 0.0002% or more and 0.0030% or less, Nb: 0.007% or more and 0.030% or less, and Ti containing so as to satisfy the following formula (1), the balance being Fe and inevitable impurities The steel structure of the steel sheet has a ferrite phase area ratio of 20% or less (including 0%), a bainite phase area ratio of 35% to 90%, and a martensite phase area ratio of 10% to 65%. % Inclusions with an equivalent circle diameter exceeding 5.0 μm in number density 00 pieces / mm ² contained less, the average particle size of the martensite particulate constituting the martensite phase 4.0μm or less, is not less 5.0μm or less maximum length between martensite, the surface hardness of the steel sheet Is a high-strength plated steel sheet, which is 95% or less, assuming that the hardness at a position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%.
[% N] -14 [% Ti] / 48 ≦ 0 (1)
In the formula (1), [% N] means N content, and [% Ti] means Ti content.

  [2] The component composition further includes, by mass%, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.3% % Or less, Mo: 0.001% or more and 0.3% or less, W: 0.001% or more and 0.2% or less, Hf: 0.001% or more and 0.3% or less The high-strength plated steel sheet according to [1], wherein the high-strength plated steel sheet has a component composition.

  [3] The component composition further contains, in mass%, one or more of REM, Mg, and Ca in a total of 0.0002% to 0.01% [1] or The high strength plated steel sheet according to [2].

  [4] The high-strength plated steel sheet according to any one of [1] to [3], wherein the plated layer is any one of a hot-dip plated layer and an alloyed hot-dip plated layer.

  [5] A steel material having the composition described in any one of [1] to [3] is heated at 1000 ° C. or more and 1200 ° C. or less, and after finish rolling at a finish rolling temperature of 800 ° C. or more, finish rolling temperature To 720 ° C. at an average cooling rate of 10 ° C./s or more and winding at 580 ° C. or more and 720 ° C. or less, and cold rolling to cold-roll the hot-rolled sheet after the hot rolling step After the step and the cold rolling step, the cold-rolled sheet is heated to the Ac3 point, which is the maximum temperature, and the heated cold-rolled sheet is cooled at a cooling rate of 5 ° C / s or higher to 580 ° C. In this heating and cooling, the time for which the cold-rolled sheet stays at the Ac3 point or higher is 60 seconds or less, the dew point at the Ac3 point or higher is −45 ° C. or higher and −20 ° C. or lower, and the cooling is 440 ° C. or higher and 530 ° C. or lower. The time that the cold-rolled sheet stays in 2 The average heating rate from 100 ° C. to the highest achieved temperature of (Ac3 point−10) ° C. or higher is 3.0 ° C./s or higher in the pre-treatment heating step for 2 seconds or more and the cold-rolled sheet after the pre-treatment heating step. The cold-rolled sheet heated to the maximum temperature is cooled under the condition that the average cooling rate up to 560 ° C. is 10 ° C./s or more, and (Ac3 point−10) ° C. or more in the heating and cooling. An annealing process in which the time for which the cold-rolled sheet is retained is 60 seconds or less, and the time in which the cold-rolled sheet is retained at 440 ° C. or more and 530 ° C. or less in the cooling is 20 seconds or more and 180 seconds or less, and plating after the annealing process And a plating step of forming a plating layer on the annealed plate, and a method for producing a high strength plated steel sheet.

  [6] The plating layer contains, by mass%, Fe: 5.0 to 20.0%, Al: 0.001% to 1.0%, and Pb, Sb, Si, Sn, Mg, Contains one or more selected from Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0 to 3.5%, with the balance being Zn and inevitable impurities The method for producing a high-strength plated steel sheet according to [5].

  [7] The method for producing a high-strength plated steel sheet according to [5] or [6], wherein the plating treatment is any one of a hot dip galvanizing treatment and an alloyed hot dip galvanizing treatment.

  According to the present invention, the high-strength plated steel sheet of the present invention has a high tensile strength (TS): 780 MPa or higher and excellent formability. If the high-strength plated steel sheet of the present invention is applied to automobile parts, further weight reduction of the automobile parts can be realized.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<High strength plated steel plate>
The high-strength plated steel sheet of the present invention includes a steel sheet and a plating layer formed on the steel sheet. It demonstrates in order of a steel plate and a plating layer.

  The component composition of the steel sheet is mass%, C: 0.06% to 0.18%, Si: less than 0.50%, Mn: 1.9% to 3.2%, P: 0.03% S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less, B: 0.0002% or more and 0.0030% or less, Nb: 0.007% or more and 0.030% The component composition contains Ti so as to satisfy the following and the above expression (1). The following components will be described. In the following description, “%” representing the content of a component means “mass%”.

C: 0.06% or more and 0.18% or less C has a hardenability which increases the hardness of martensite and suppresses ferrite transformation. In order to obtain a steel sheet having a tensile strength of 780 MPa or more, it is necessary that at least the C content is 0.06% or more. On the other hand, when the C content exceeds 0.18%, the area ratio of the martensite phase exceeds 65% and the ductility and formability are lost. Therefore, the C content is set to 0.06% or more and 0.18% or less. Desirably, it is 0.07% or more and 0.18% or less.

Si: Less than 0.50% Si is an element contributing to high strength by solid solution strengthening. On the other hand, since Si raises the transformation point (Ac ₃ point) from the ferrite phase to the austenite phase, it makes it difficult to remove the ferrite phase during annealing. Furthermore, since Si reduces the wettability between the plating layer and the steel sheet surface, excessive inclusion of Si causes defects such as non-plating. In this invention, Si content is accept | permitted if it is less than 0.50% of range. Desirably, it is less than 0.30%. Although the lower limit is not particularly defined, 0.01% Si may inevitably be mixed into the steel.

Mn: not less than 1.9% and not more than 3.2% Mn contributes to high strength by solid solution strengthening, and lowers the Ac ₃ transformation point to facilitate removal of the ferrite phase during annealing. It also has the effect of improving the hardenability of the steel sheet. In order to obtain the target steel structure, the Mn content needs to be 1.9% or more. On the other hand, when the Mn content exceeds 3.2%, the bainite transformation does not proceed, and as a result, the area ratio of the martensite phase exceeds 65%. For this reason, the upper limit of the Mn content is set to 3.2%. A preferable range of the Mn content is 2.0% or more and 3.0% or less.

P: 0.03% or less P is an element that has an adverse effect on formability because it segregates at grain boundaries and becomes the starting point of cracking during molding. Therefore, it is preferable to reduce the P content as much as possible. In the present invention, in order to avoid the above problem, the P content is set to 0.03% or less. Preferably it is 0.02% or less. Although it is desirable to reduce it as much as possible, 0.002% may be inevitably mixed in manufacturing.

S: 0.005% or less S exists in the state which became inclusions, such as MnS, in steel. This inclusion becomes wedge-shaped by hot rolling and cold rolling. In such a form, it tends to be a starting point for void generation, and the moldability is also adversely affected. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and set it to 0.005% or less. Preferably it is 0.003% or less. It is desirable to reduce the S content as much as possible, but 0.0005% may be inevitably mixed in production.

Al: 0.08% or less When Al is added as a deoxidizer in the steelmaking stage, it is preferable to contain Al in an amount of 0.02% or more. On the other hand, if the Al content exceeds 0.08%, the ferrite transformation is promoted by the influence of inclusions such as alumina, and the tensile strength is less than 780 MPa. Therefore, the Al content is 0.08% or less. Preferably it is 0.07% or less.

N: 0.006% or less In the present invention, N combines with Ti and precipitates as coarse Ti-based nitride. Since this coarse Ti-based nitride serves as a nucleation site for ferrite transformation, the N content needs to be reduced as much as possible, and the upper limit is made 0.006%. A preferable N content is 0.005% or less. Although it is desirable to reduce the N content as much as possible, 0.0005% may be inevitably mixed in production.

B: 0.0002% or more and 0.0030% or less B has an effect of segregating at the grain boundary of the austenite before transformation to significantly delay the nucleation of the ferrite phase and to suppress the formation of the ferrite phase. In order to obtain this effect, the B content needs to be 0.0002% or more. On the other hand, if the B content exceeds 0.0030%, not only the hardenability effect is saturated, but also the ductility is adversely affected. From the above, the B content is set to 0.0002% or more and 0.0030% or less. Desirably, it is 0.0005% or more and 0.0020% or less.

Nb: 0.007% or more and 0.030% or less Nb is an important element for suppressing coarsening of austenite grains during annealing. When the Nb content is excessive, coarse carbonitrides containing Nb (generic names for carbides, nitrides, and carbonitrides; hereinafter the same applies to the present invention) are precipitated, and the area ratio of the ferrite phase increases. In order to suppress the austenite grain coarsening, the Nb content needs to be 0.007% or more. On the other hand, when the Nb content exceeds 0.030%, coarse Nb-based carbonitrides precipitate under the production conditions specified in the present invention. Therefore, the upper limit of Nb content is 0.030%. A preferable Nb content is 0.012% or more and 0.027% or less.

Ti: [% N] -14 [% Ti] / 48 ≦ 0
When the Ti content does not satisfy the above inequality and [% N] -14 [% Ti] / 48> 0, N binds to B, so that the hardenability is lowered and the area ratio of the ferrite phase is 20%. It will exceed. If it is the range of [% N] -14 [% Ti] / 48 <= 0, since N is the state couple | bonded with Ti, the hardenability of a steel plate is not lost. On the other hand, if Ti is excessively contained, carbides are formed by combining with C. This carbide precipitates on the dislocations and significantly inhibits the movement of the dislocations, which causes a decrease in moldability. From this viewpoint, it is preferable that the left side of the formula (1) is −0.010 or more. More preferably, it is -0.006 or more.

  Further, the high-strength plated steel sheet of the present invention is, in mass%, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.00%. 1% or more of 3% or less, Mo: 0.001% or more and 0.3% or less, W: 0.001% or more and 0.2% or less, Hf: 0.001% or more and 0.3% or less You may contain.

  Cr, Ni, V, Mo, W and Hf have the effect of delaying the start of the ferrite transformation. If there is an effect of these elements in addition to the effect of hardenability by B, it becomes easy to stably obtain a desired steel structure. On the other hand, if the Cr content exceeds 0.9%, the plating property is adversely affected. Further, when Ni is 0.5%, V is 0.3%, Mo is 0.3%, W is 0.2%, and Hf exceeds 0.3%, the effect of hardenability is saturated. From the above, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.3%, Mo: 0.001% to 0 .3% or less, W: 0.001% to 0.2%, and Hf: 0.001% to 0.3%.

  The high-strength plated steel sheet of the present invention may further contain 0.0002% or more and 0.01% or less of REM, Mg, and Ca in total by mass%.

  REM (REM: lanthanoid element having atomic numbers 57 to 71), Mg and Ca spheroidize cementite precipitated in bainite. As a result, the stress concentration around the cementite is reduced, and the formability is improved. On the other hand, when the total content of REM, Mg, and Ca exceeds 0.01%, the effect of changing the shape of cementite is saturated and the ductility is adversely affected. As mentioned above, when these are contained, it is preferable to contain 0.0002% or more and 0.01% or less of REM, Mg, and Ca in total. Desirably, one or more of REM, Mg, and Ca is 0.0005% or more and 0.005% or less in total.

  Components other than the above components are Fe and inevitable impurities.

Next, the steel structure of the high strength plated steel sheet of the present invention will be described. The steel structure of the high-strength plated steel sheet of the present invention has a ferrite phase area ratio of 20% or less (including 0%), a bainite phase area ratio of 35% or more and 90% or less, and a martensite phase area ratio of 10%. The number density of inclusions containing 65% or less and having an equivalent circle diameter exceeding 5.0 μm contains 400 pieces / mm ² or less. And the average particle diameter of the granular martensite which comprises the said martensite phase is 4.0 micrometers or less, and the maximum length between martensites is 5.0 micrometers or less.

Ferrite phase The ferrite phase is a soft structure, and when the content of the ferrite phase exceeds 20%, the tensile strength is less than 780 MPa. In addition, since the ferrite phase has a low element solubility, if the ferrite phase content is excessive, the arrangement of cementite finely dispersed in the structure before annealing is changed, and a fine martensite phase cannot be obtained. Therefore, it is desirable to reduce the content of the ferrite phase as much as possible. In the present invention, the content of the ferrite phase needs to be suppressed to 20% or less. Desirably, it is 15% or less.

Bainite phase The bainite phase is higher in hardness than the ferrite phase and is effective for finely forming the martensite phase. In order to obtain a desired steel structure, the content of the bainite phase needs to be 35% or more. On the other hand, if the content of the bainite phase exceeds 90%, the maximum length (maximum distance) between the martensites exceeds 5.0 μm, and good moldability cannot be obtained. The preferred bainite phase content is 40% or more and 80% or less in terms of area ratio.

Martensite phase The content of the martensite phase and the form of the martensite phase have a great influence on the strength and formability. When the content of the martensite phase is less than 10% in terms of area ratio, the tensile strength is less than 780 MPa. On the other hand, when the content of the martensite phase exceeds 65% by area ratio, ductility and formability are lost. The preferred martensite phase content is 20% or more and 55% or less in terms of area ratio.

  In the high strength plated steel sheet of the present invention, the martensite phase is composed of granular martensite. When the average particle size of martensite exceeds 4.0 μm, deformation in the vicinity of coarse martensite is constrained and the steel sheet deforms unevenly during forming. In this case, cracks are likely to occur in the preferentially deformed portion, and good moldability cannot be obtained. The average particle size of martensite is preferably 3.0 μm or less. In addition, the lower limit of the average particle size of martensite is not particularly limited, but 0.5 μm or more is preferable from the viewpoint of stably obtaining a martensite fraction of 10% or more.

  Moreover, the maximum length of the space | interval between martensites is 5.0 micrometers or less. If the maximum length of the interval between martensites is within this range, many bainite phases are in contact with the martensite phase. The bainite phase in contact with the martensite phase is liable to cause dislocations and work hardening. As a result, the work hardening exponent increases and deforms uniformly, so that good moldability is obtained. The maximum distance between martensites (maximum length) is preferably 4.0 μm or less. The lower limit of the maximum distance between martensites (maximum length) is not particularly limited, but when the distance between martensites is too close, dislocations are introduced near the martensite due to transformation strain caused by the occurrence of martensite transformation. By doing so, the occurrence of new dislocations between martensite is hindered and work hardening is difficult, so 1.0 μm or more is preferable.

Inclusions In the steel structure of the high-strength plated steel sheet of the present invention, the number density of inclusions having an equivalent circle diameter and a particle size exceeding 5.0 μm is 400 pieces / mm ² or less. Inclusions having a particle size exceeding 5.0 μm are likely to become nucleation sites of the ferrite phase, and the content of the ferrite phase in the area ratio is not in the desired range. Here, the inclusions exceeding 5.0 μm include oxides containing Al or Ti, nitrides containing Ti, and carbonitrides containing Nb.

  Then, the property of the steel plate in the high strength plated steel plate of the present invention will be described.

Hardness In the high-strength plated steel sheet of the present invention, the surface hardness of the steel sheet is 95% or less when the hardness at the position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%. It is. Many cracks at the time of forming occur on the surface of the steel sheet. In the present invention, the formability can be improved by improving the ductility of the steel sheet surface layer part by adjusting the coiling temperature and decarburizing the steel sheet surface layer part in the annealing step. Since the hardness and the ductility are in a trade-off relationship, the ductility of the steel sheet surface layer can be estimated by the decrease in the hardness of the steel sheet surface. If the hardness of the steel sheet surface is 95% of the hardness inside the steel sheet (position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface), the formability is improved. Preferably, the surface hardness of the steel sheet is 90% or less when the hardness at a position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%. In many cases, defects at the time of forming by bending process cause a crack starting point in the range of 100 μm from the surface. Therefore, the steel sheet surface layer is in the range from the steel sheet surface to 100 μm in the thickness direction.

  Subsequently, the plating layer will be described. In the high-strength plated steel sheet of the present invention, the components constituting the plating layer are not particularly limited and may be general components. For example, the plating layer contains Fe: 5.0 to 20.0%, Al: 0.001% to 1.0% by mass%, and Pb, Sb, Si, Sn, Mg, Mn, One or two or more selected from Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM are contained in a total of 0 to 3.5%, and the balance is made of Zn and inevitable impurities. Further, the plating layer may be an alloyed plating layer (a plating layer mainly including an Fe—Zn alloy formed by diffusing Fe in steel during galvanization by an alloying reaction).

  Next, the manufacturing method of the high strength plated steel plate of this invention is demonstrated. The manufacturing method of the high strength plated steel sheet of the present invention includes a hot rolling process, a cold rolling process, a pretreatment heating process, an annealing process, and a plating process. Moreover, you may have an alloying process after a plating process as needed. Hereinafter, each step will be described. In the following description, the temperature is the surface temperature unless otherwise specified. The average heating rate is ((surface temperature after heating−surface temperature before heating) / heating time), and the average cooling rate is ((surface temperature before cooling−surface temperature after cooling) / cooling time).

  The hot rolling process means that the steel material having the above composition is heated at 1000 ° C. or more and 1200 ° C. or less, and after finishing rolling at a finish rolling temperature of 800 ° C. or more, the average cooling rate from the finish rolling temperature to 720 ° C. Is a step of cooling at 10 ° C./s or more and winding at 580 ° C. or more and 720 ° C. or less.

  The melting method for manufacturing the steel material is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Then, it is preferable to use a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality. Further, the slab may be formed by a known casting method such as ingot casting and blooming or thin slab continuous casting.

Heating temperature of steel material: 1000 ° C. or more and 1200 ° C. or less In the present invention, it is necessary to heat the steel material prior to rough rolling so that the steel structure of the steel material becomes a substantially homogeneous austenite phase. Moreover, in order to suppress the formation of coarse inclusions, it is important to control the heating temperature. If the heating temperature is lower than 1000 ° C, the hot rolling cannot be completed at a finish rolling temperature of 800 ° C or higher. On the other hand, when the heating temperature exceeds 1200 ° C., the formation of particularly nitrides containing Ti is promoted, and the number density of inclusions exceeding 5.0 μm increases. Therefore, the heating temperature of the steel material is set to 1000 ° C. or more and 1200 ° C. or less. Desirably, it is 1020 degreeC or more and 1150 degreeC or less. In addition, it does not specifically limit about the rough rolling conditions of the rough rolling after the said heating.

Finishing rolling temperature: 800 ° C. or more When the finishing rolling temperature is lower than 800 ° C., the ferrite transformation starts during finishing rolling, and a structure in which ferrite grains are expanded and a mixed grain structure in which ferrite grains partially grow ( duplex grain microstructure). For this reason, the thickness accuracy at the time of cold rolling is adversely affected, and cementite is not finely dispersed in the steel structure before annealing. Accordingly, the finish rolling temperature is 800 ° C. or higher. Preferably it is 820 degreeC or more. Further, if the finish rolling temperature is excessively high, the surface quality is preferably lowered to 940 ° C. or less because the surface properties are deteriorated due to the biting of the scale.

The average cooling rate from the finish rolling temperature to 720 ° C. is 10 ° C./s or more. After the finish rolling, the product is cooled to just above the coiling temperature by forced cooling. The length of the forced cooling zone (runout table) is limited, and when it is cooled at less than 10 ° C./s, the desired winding temperature cannot be achieved. Therefore, an average cooling rate shall be 10 degrees C / s or more. The upper limit is not particularly limited, but is substantially 200 ° C./s. When the coiling temperature is less than 720 ° C, the average cooling rate from 720 ° C to the cooling stop temperature may be 10 ° C / s or more or less than 10 ° C / s.

Winding temperature: 580 ° C. or more and 720 ° C. or less By winding, decarburization from the scale is promoted, and the C concentration in the steel sheet surface layer portion is reduced. When the coiling temperature is lower than 580 ° C., decarburization does not proceed and the hardness of the steel sheet surface layer portion does not decrease. On the other hand, when the coiling temperature exceeds 720 ° C., an internal oxide layer is formed on the surface layer portion of the steel sheet, which causes cracks during forming. For this reason, the coiling temperature was set to 580 ° C. or higher and 720 ° C. or lower. Preferably they are 600 degreeC or more and 690 degrees C or less.

  The subsequent cold rolling step is a step of cold rolling the hot-rolled sheet after the hot rolling step. In order to obtain a desired sheet thickness, it is necessary to cold-roll the hot-rolled sheet after the hot rolling process. Although there is no restriction on the cold rolling rate, the cold rolling rate is often set to 30% or more and 80% or less due to restrictions on the production line.

  The pretreatment heating step to be performed subsequently refers to heating the cold-rolled plate to the Ac3 point which is the highest temperature after the cold-rolling step, and the cooling rate to 580 ° C is 5 ° C / cooling after the heating. In the cooling and cooling, the time during which the cold-rolled sheet stays at the Ac3 point or higher is 60 seconds or less and the dew point at the Ac3 point or higher is −45 ° C. or higher and −20 ° C. or lower. In this step, the time during which the cold-rolled sheet stays at 440 ° C. or higher and 530 ° C. or lower is set to 20 seconds or longer.

Heating temperature (maximum temperature reached): Ac3 point or higher It is necessary to further advance decarburization in a continuous annealing line to reduce the hardness of the steel sheet surface layer. For this reason, it is necessary to pass through the continuous annealing line before the continuous hot dipping line (the processing performed in the continuous annealing line is pretreatment heating, and the processing performed in the furnace in the continuous hot dipping line is in the present invention. Annealing treatment). Ac3 point is the temperature at which the two-phase region of ferrite and austenite changes to the austenite single-phase region. In the treatment in the continuous annealing line, it is necessary to promote bainite transformation and finely disperse cementite. However, if the formation of ferrite phase is not sufficiently suppressed, the ferrite phase has low element (C (carbon)) solubility, so that cementite does not precipitate inside the ferrite phase, and the dispersion form of cementite becomes non-uniform, resulting in a final product. And martensite is not finely dispersed. Therefore, in this step, it is necessary to sufficiently remove the ferrite phase. Therefore, the heating temperature was set to Ac3 point or higher. The upper limit of the heating temperature is not particularly limited, but the temperature exceeding 1000 ° C. is preferably 1000 ° C. or less because the load due to the heat of the annealing furnace is large and the equipment life is shortened.

Cooling rate from the highest temperature to 580 ° C .: 5 ° C./s or more When the cooling rate to 580 ° C. is slow, ferrite transformation starts in the cooling process, and ferrite grain growth proceeds. When ferrite grains grow, the arrangement of finely dispersed cementite changes, and a fine martensite structure cannot be obtained. For this reason, it is necessary to suppress excessive ferrite grain growth. For this, it is necessary to perform forced cooling at an average cooling rate from the cooling start temperature (maximum temperature reached) to 580 ° C. of 5 ° C./s or more. Preferably it is 7 degrees C / s or more. In addition, the cooling stop temperature of this cooling shall be 100 degrees C or less from controlling an average heating rate from 100 degrees C in the annealing process which is the next process.

Dew point in the temperature range of Ac3 point or higher: -45 ° C or higher and -20 ° C or lower Since the decarburization does not proceed when the dew point in the temperature range of Ac3 point or higher falls below -45 ° C in heating and cooling, the hardness of the steel sheet surface layer part Will not drop. On the other hand, when the dew point is higher than −20 ° C., an internal oxide layer and iron oxide are generated on the surface layer of the steel sheet, so that formability and surface properties are impaired. For this reason, the said dew point was made into -45 degreeC or more and -20 degreeC or less. Preferably, it is -45 degreeC or more and -25 degreeC or less. The dew point in the temperature range below the Ac3 point is not particularly limited and may be determined as appropriate.

Residence time in the temperature range above the Ac3 point: 60 seconds or less When the cold-rolled sheet is in the state above the Ac3 point in heating and cooling for a long time, the austenite grains become coarse and a steel structure in which martensite is finely dispersed is obtained. It becomes impossible. In the present invention, it is acceptable up to 60 seconds.

Residence time in the temperature range of 440 ° C. or more and 530 ° C. or less: 20 seconds or more In the temperature range of 440 ° C. or more and 530 ° C. or less, fine cementite is formed in the steel sheet by the progress of bainite transformation. It becomes possible to obtain martensite. The temperature range in which the bainite transformation proceeds most in the present invention is a temperature range of 440 ° C. or more and 530 ° C. or less. Preferably they are 460 degreeC or more and 520 degrees C or less. In order to advance the bainite transformation, it is necessary to set the residence time in the above temperature range for cooling to 20 seconds or more, so the lower limit of the residence time was set to 20 seconds. Preferably it is 30 seconds or more. Although the upper limit of residence time is not specifically limited, 900 seconds or less are preferable for the reason of equipment restrictions.

  In addition, in order to further reduce the hardness of the steel sheet surface layer portion, the steel sheet may be passed twice or more through a continuous annealing line for performing the pretreatment heating step. However, from the viewpoint of coarsening of austenite grains, it is desirable that the number of sheet passing be 4 or less.

  The subsequent annealing step is the heating of the cold-rolled sheet after the pretreatment heating step under the condition that the average heating rate from 100 ° C. to the highest attained temperature of (Ac3 point−10) ° C. or higher is 3.0 ° C./s or higher. Then, the cold-rolled sheet heated up to the highest temperature is cooled under the condition that the average cooling rate up to 560 ° C. is 10 ° C./s or more, and in this heating and cooling, it is cold-rolled to (Ac3 point−10) ° C. or more. In this step, the time for which the plate is retained is 60 seconds or less, and the time for which the cold-rolled plate is retained at 440 ° C. or more and 530 ° C. or less in the cooling is 20 seconds or more and 180 seconds or less.

Average heating rate from 100 ° C. to the highest temperature: 3.0 ° C./s or more 100 ° C. is a temperature at which C begins to diffuse, and an average heating rate of 100 ° C. or more at which C or Fe diffuses is 3.0 ° C. / Under heating conditions below s, finely dispersed cementite becomes coarse. Cementite becomes a martensite formation site, but fine cementite cannot be obtained when cementite is coarsened. In order to obtain finer martensite, it is necessary to suppress austenite coarsening during annealing. When the average heating rate is less than 3.0 ° C./s, austenite is coarsened and a desired average diameter of the martensite phase cannot be obtained. As described above, the average heating rate from 100 ° C. to the highest temperature reached 3.0 ° C./s or higher. A preferable heating rate is 4.0 ° C./s or more. Here, the highest temperature reached is (Ac3 point−10) ° C. or higher. Unless heated to at least (Ac3 point-10) ° C., the area ratio of the ferrite phase does not become 20% or less. A preferable maximum temperature is Ac3 point or higher.

Average cooling rate from the highest temperature to 560 ° C: 10 ° C / s or more In the cooling after the above heating, if the cooling rate to 560 ° C is slow, ferrite transformation starts in the cooling process, and the ferrite phase is excessively generated The In order to avoid this, the average cooling rate up to 560 ° C. needs to be 10 ° C./s or more. Moreover, the cooling stop temperature in this cooling is not particularly limited, but the cooling stop temperature is normally 460 to 540 ° C. The cooling rate to the cooling stop temperature after reaching 560 ° C. is not particularly limited, and may be 10 ° C./s or more and less than 10 ° C./s.

(Ac3 point-10) Time to be retained in the temperature range above 60 ° C: 60 seconds or less In heating and cooling, the time for which the cold rolled sheet is retained in the temperature range above (Ac3 point-10) ° C is over 60 seconds. The austenite during annealing becomes coarse and fine martensite cannot be obtained. From the above viewpoints, the time of staying in the temperature range of (Ac3 point−10) ° C. or higher is 60 seconds or less, and preferably 50 seconds or less.

Time of residence in a temperature range of 440 ° C. or more and 530 ° C. or less: 20 seconds or more and 180 seconds or less To promote bainite transformation and obtain a bainite structure containing fine martensite, cooling is performed at a temperature of 440 ° C. or more and 530 ° C. or less. It is necessary to retain the cold-rolled plate in the region for 20 seconds or more. On the other hand, when the residence time exceeds 180 seconds, a bainite phase is excessively generated, and the bainite phase not in contact with the martensite phase increases. The preferred residence time is 25 seconds or more and 150 seconds or less.

  The subsequent plating step is a step of performing plating after the annealing step and forming a plating layer on the annealed plate. For example, when performing hot dipping frequently used for automotive steel sheets as plating, the annealing is performed in a continuous hot dipping line, followed by cooling after annealing and dipping in a hot dipping bath, and a plating layer on the surface. May be formed. Moreover, you may provide the alloying process which performs the alloying process of a plating layer as needed after the said plating process.

A steel material having a thickness of 250 mm having the composition shown in Table 1 is subjected to a hot rolling process under the hot rolling conditions shown in Table 2 to form a hot rolled sheet, and then subjected to a cold rolling process under the cold rolling conditions shown in Table 2. A cold-rolled sheet was prepared, and the pretreatment heat treatment under the conditions shown in Table 2 was applied on a continuous annealing line, and the annealing conditions shown in Table 2 were applied on a continuous hot dipping plating line. Then, the plating process was performed and the alloying process was further performed as needed. Here, the temperature of the plating bath (plating composition: Zn-0.13 mass% Al) immersed in the continuous hot dipping line is 460 ° C., and the coating adhesion amount is GI material (hot-plated steel plate), GA material (alloyed) Both the hot-dip galvanized steel sheets) were 45 to 65 g / m ² per side, and the Fe content contained in the plated layer was in the range of 6 to 14% by mass. Ac3 point was measured using a thermal expansion measuring device. The Ac3 point was measured at an average heating rate of 5 ° C./s.

  Test pieces were collected from the hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet obtained as described above, and evaluated by the following methods.

(I) Structure observation image The area ratio of each phase was evaluated by the following method. Cut out from the steel plate so that the cross section parallel to the rolling direction (the cross section perpendicular to the rolling direction when the steel plate is placed and parallel to the rolling direction) becomes the observation surface, and the center of the plate thickness is corroded with 1% nital. Then, the image was magnified 2000 times with a scanning electron microscope, and a 1/4 t portion of the plate thickness was photographed for 10 fields of view. The ferrite phase is a structure having a form in which corrosion marks and cementite are not observed in the grains, and the bainite phase is a structure in which corrosion marks and large carbides are recognized in the grains. The martensite phase is a structure in which no carbide is observed in the grains and is observed with white contrast. The bainite phase, bainite phase and martensite phase were separated from each other by image analysis, and the area ratio relative to the observation field was obtained. As for the average diameter of the martensite phase, the area occupied by each grain of the martensite phase was determined by image analysis, and an equivalent circular diameter equal to the area was determined. For the portions where the martensite phases are connected with a length of 0.5 μm or less, the martensite connected to the portions is regarded as two, and the respective equivalent circle diameters were determined. The maximum distance between the martensites was determined by taking the longest portion in 10 fields as the maximum length. The said space | interval means the distance of the part where the outer periphery of a martensite and the outer periphery of an adjacent martensite are the nearest.

(Ii) Tensile test A JIS No. 5 tensile test piece was produced from the obtained steel sheet in a direction perpendicular to the rolling direction (perpendicular direction), and a tensile test in accordance with the provisions of JIS Z 2241 (2011) was performed five times. Yield strength (YS), tensile strength (TS), and total elongation (El). The crosshead speed in the tensile test was 10 mm / min. In Table 3, the tensile strength: 780 MPa or more and the work hardening index (n value): 0.16 or more were set as the mechanical properties of the steel sheet required for the steel of the present invention. Here, the work hardening index is a value determined according to the method defined in JIS Z 2253 (1996), and the true strain range was determined from 0.02 to 0.05. This is because this region is the most sensitive region regarding the crack generation phenomenon due to the effect of work hardening in press working.

(Iii) Hardness test The hardness of the steel sheet surface and the steel sheet interior was determined by the Vickers hardness test. The hardness of the steel sheet surface was determined by measuring a total of 20 points with a test load of 0.2 kgf from the steel sheet surface from which the plating layer had been removed by pickling, and the average value was determined. As for the hardness inside the steel plate, a total thickness of 5 points was measured at a test load of 1 kgf at a thickness of 1/4 t in a cross section parallel to the rolling direction, and an average value was obtained. When the average value of the hardness of the steel sheet surface was 95% or less (0.95 or less in the table) of the average value of the hardness inside the steel sheet, the steel sheet characteristics required in the present invention were obtained.

  The results obtained as described above are shown in Table 3.

  It can be seen that all the inventive examples obtained a steel sheet having a tensile strength TS of 780 MPa or more and a high work hardening index. On the other hand, a comparative example out of the scope of the present invention, particularly a steel sheet in which a desired ferrite area ratio has not been obtained, has a low tensile strength. When the area ratio and form of the martensite phase were not desired, the work hardening index was low. Further, in some cases where the coiling temperature or the continuous annealing line did not satisfy the range defined by the present invention, the hardness of the steel sheet surface was almost the same as that inside the steel sheet.

Claims (7)

A high-strength plated steel sheet having a steel sheet and a plating layer formed on the steel sheet,
The composition of the steel sheet is, by mass, C: 0.06% to 0.18%, Si: less than 0.50%, Mn: 1.9% to 3.2%, P: 0.03. %: S: 0.005% or less, Al: 0.08% or less, N: 0.006% or less, B: 0.0002% or more and 0.0030% or less, Nb: 0.007% or more and 0.030 %, And so as to satisfy the following formula (1), the balance is Fe and inevitable impurities,
The steel structure of the steel sheet contains a ferrite phase in an area ratio of 15 % or less (including 0%), a bainite phase in an area ratio of 35% to 90%, and a martensite phase in an area ratio of 10% to 65%. And the total area ratio of the bainite phase and the martensite phase is 85% or more, and inclusions having a circle equivalent diameter exceeding 5.0 μm are contained in a number density of 400 pieces / mm ² or less,
The average particle size of the granular martensite constituting the martensite phase is 4.0 μm or less, and the maximum length between martensites is 5.0 μm or less,
High strength plating characterized in that the surface hardness of the steel sheet is 95% or less when the hardness at the position of 1/4 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%. steel sheet.
[% N] -14 [% Ti] / 48 ≦ 0 (1)
In the formula (1), [% N] means N content, and [% Ti] means Ti content.   The component composition is further, in mass%, Cr: 0.001% to 0.9%, Ni: 0.001% to 0.5%, V: 0.001% to 0.3%, Component composition containing one or more of Mo: 0.001% to 0.3%, W: 0.001% to 0.2%, Hf: 0.001% to 0.3% The high-strength plated steel sheet according to claim 1, wherein   3. The component composition according to claim 1, further comprising, by mass%, one or more of REM, Mg, and Ca in total of 0.0002% to 0.01%. High strength plated steel sheet.   The high-strength plated steel sheet according to any one of claims 1 to 3, wherein the plating layer is either a hot-dip plating layer or an alloyed hot-dip plating layer. A method for producing a high-strength plated steel sheet according to any one of claims 1 to 4,
The steel material having the component composition according to any one of claims 1 to 3 is heated at 1000 ° C. or more and 1200 ° C. or less, and after finish rolling is finished at a finish rolling temperature of 800 ° C. or more, from the finish rolling temperature to 720 ° C. A hot rolling step of cooling at an average cooling rate of 10 ° C./s or more and winding up at 580 ° C. or more and 720 ° C. or less;
A cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step;
After the cold rolling step, the cold-rolled sheet is heated to Ac3 or higher, which is the highest temperature, and the heated cold-rolled sheet is cooled under the condition that the cooling rate to 580 ° C is 5 ° C / s or more, The time during which the cold-rolled sheet stays at the Ac3 point or higher during heating and cooling is 60 seconds or less, the dew point at the Ac3 point or higher is −45 ° C. or higher and −20 ° C. or lower, and the cooling is performed at 440 ° C. or higher and 530 ° C. or lower. A pretreatment heating step in which the plate is retained for 20 seconds or longer;
The cold-rolled sheet after the pretreatment heating step is heated under the condition that the average heating rate from 100 ° C. to the maximum temperature of (Ac3 point−10) ° C. or higher is 3.0 ° C./s or higher, and is heated to the maximum temperature. The obtained cold-rolled sheet is cooled under the condition that the average cooling rate up to 560 ° C. is 10 ° C./s or more, and the time during which the cold-rolled sheet stays at (Ac3 point−10) ° C. or more in the heating and cooling is 60 An annealing step in which the time during which the cold-rolled sheet is retained at 440 ° C. or more and 530 ° C. or less in the cooling is 20 seconds or more and 180 seconds or less,
A method for producing a high-strength plated steel sheet, comprising: plating after the annealing step and forming a plating layer on the annealed plate.   The plating layer contains Fe: 5.0 to 20.0%, Al: 0.001% to 1.0% by mass%, and Pb, Sb, Si, Sn, Mg, Mn, and Ni. , Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM containing at least 0 to 3.5% in total, with the balance consisting of Zn and inevitable impurities The method for producing a high-strength plated steel sheet according to claim 5.   The method for producing a high-strength plated steel sheet according to claim 5 or 6, wherein the plating treatment is any one of a hot dip galvanizing treatment and an alloying hot dip galvanizing treatment.