JP5440375B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a hot dip galvanized steel sheet and a method for producing the same. Specifically, the present invention relates to a high-strength hot-dip galvanized steel sheet having excellent hole expansibility and a method for producing the same, and in particular, press forming like a car body of an automobile, and among them, stretch flange forming, which has been conventionally difficult, is necessary. The present invention relates to a high-strength hot-dip galvanized steel sheet suitable for indispensable uses and a method for producing the same. Here, in the present invention, “hot-dip galvanized steel sheet” includes “alloyed hot-dip galvanized steel sheet”, and “high-strength hot-dip galvanized steel sheet” includes “high-strength galvanized steel sheet”. .

  In recent years, in order to protect the global environment, there has been a demand for improving the fuel efficiency of automobiles. To reduce the weight of the vehicle body and ensure the safety of passengers, steel sheets with a tensile strength of 590 MPa or more, especially parts that require corrosion resistance. On the other hand, the needs for hot dip galvanized steel sheets are increasing.

  In a high-strength steel sheet applied to automotive parts, not only strength characteristics but also various workability required at the time of forming parts, such as press formability and weldability, must be satisfied. In the molding process of automobile parts, stretch flange molding is very frequently used, and as a result, parts of various shapes are molded. Therefore, high strength steel plates with excellent resistance to stretch flange cracking, that is, hole expansibility are required. . However, generally, as the tensile strength increases, the hole expandability deteriorates. This is because the high-strength steel sheet is a structure strengthened with a hard phase such as martensite or bainite with ferrite as the parent phase, and the microvoids that are the origin of ductile fracture are the heterogeneous interface between the ferrite and the hard phase, or its This is because it tends to occur in the vicinity. Therefore, many researches and developments have been made to improve the hole expansibility of high-strength steel sheets, and a structure control method for realizing the research is being established.

  For example, Non-Patent Document 1 discloses the finding that, in a composite steel sheet of ferrite and martensite, the hole expandability is improved as the hardness difference (deformation stress difference) between ferrite and martensite decreases. Non-Patent Document 2 discloses a ferrite single-phase steel sheet excellent in hole expansibility that does not include a hard phase itself such as martensite.

  However, most of the conventional knowledge does not consider the manufacturing process of the hot dip galvanized steel sheet. Non-Patent Document 1 relates to a cold-rolled steel sheet manufactured by quenching and tempering processes, and Non-Patent Document 2 is manufactured by a heat treatment that temporarily dissolves microalloys such as Ti and Mo that contribute to precipitation strengthening. It was related to hot-rolled steel sheets.

  In the manufacturing process of high-strength hot-dip galvanized steel sheet, the recrystallization annealing temperature is 750 to 950 ° C., and the conditions for solid solution of an amount of microalloy necessary to achieve a tensile strength of 590 MPa or more, specifically, It is much lower than 1100 ° C. corresponding to the lower limit of the slab heating temperature.

  Moreover, when cooling from recrystallization annealing, the alloying process of being immersed in the hot dip galvanizing bath of about 460 degreeC and then reheating to 500-600 degreeC after immersion may be given. That is, in this manufacturing process, cooling is temporarily interrupted on the high temperature side of the bainite transformation temperature range, and the steel sheet is heat-treated in a state close to constant temperature. Therefore, when manufacturing a high-strength hot-dip galvanized steel sheet, the steel sheet tends to have a structure including upper bainite. In the upper bainite structure, coarse cementite is preferentially precipitated at the lath boundary, the packet boundary, the block boundary, and the prior austenite grain boundary. When cementite precipitates in this way, strain is unevenly distributed and the cementite itself is destroyed, or microvoids are generated in the vicinity thereof. Therefore, it is easily understood that a steel sheet having such a structure is inferior in hole expansibility. Furthermore, since the time required from the cooling stop to the alloying process is short, only a part of the steel becomes bainite and the remainder becomes austenite or martensite. These austenite and martensite are extremely hard structures because C (carbon) is distributed among the structures due to partial formation of bainite and their C concentration is extremely high. That is, it has been extremely difficult to produce a steel sheet that does not contain cementite or hard martensite and has excellent hole expandability under the heat treatment conditions for producing a high-strength hot-dip galvanized steel sheet. Therefore, a more advanced structure control method has been studied for improving the hole expandability of high-strength hot-dip galvanized steel sheets.

For example, Patent Document 1 adds Mn and Ti to a steel sheet, anneals at a recrystallization temperature or higher and at an Ac ₁ point or higher, and rapidly cools to a Ms point or lower until reaching a hot dip galvanizing bath. Or the steel plate which achieved the tensile strength of 590 Mpa or more and the hole expansion rate of 80% or more by producing tempered martensite entirely is disclosed. However, cooling below the Ms point is difficult to achieve in a normal hot dip galvanizing line, and in order to achieve this, a forced cooling device and a heating furnace are installed between the soaking zone where the steel sheet is annealed and the plating bath. It is necessary to establish a new one. Furthermore, energy for reheating the steel sheet from the Ms point or lower to the plating bath temperature is added, and the manufacturing cost becomes extremely high.

Patent Document 2 discloses that Ti and Nb are positively added to a steel sheet, annealed immediately above a single-phase region (austenite single-phase temperature region), pickled, heat treated at 500 ° C. or higher and A ₁ point or lower and then melted. By galvanizing, decomposing hard martensite and austenite, and making the second phase constituting the composite structure mainly composed of bainite or tempered martensite, it has a tensile strength of 590 MPa or more and excellent hole expansibility The steel plate is disclosed. However, since the annealing process of 2 times is required substantially, manufacturing cost becomes high. Furthermore, the second heat treatment temperature is significantly lower than the normal annealing temperature, and in order to realize this, the productivity of the line is hindered.

  On the other hand, Patent Document 3 has a tensile strength of 590 MPa or more by actively adding Si or Mn to a steel sheet, optimizing casting conditions, reducing segregation, and homogenizing the structure, and has excellent holes. An expansible steel sheet is disclosed. However, the desired solidification rate is not realized by a normal thin plate continuous casting facility in which the slab thickness is 200 to 300 mm, and in order to realize this, the thin slab continuous casting method in which the slab thickness is 30 to 70 mm. Therefore, the manufacturing cost becomes extremely high.

  As described above, the techniques disclosed in Patent Documents 1 to 3 require significant equipment remodeling, lengthy, and special manufacturing processes in order to realize a structure that achieves both a tensile strength of 590 MPa or more and excellent hole expandability. Therefore, it is not a realistic method. Therefore, it is necessary to devise a combination of chemical composition and thermomechanical treatment, and to examine a structure control method for producing a desired steel sheet in a normal manufacturing process.

As described above, since the hole expansibility deteriorates when a hard structure is mixed, an ultimate method of creating a single-phase structure that does not include a hard structure has been studied.
In Patent Document 4, Ti and Mo are positively added to a steel sheet, the coiling temperature condition of hot rolling is optimized, and the structure is made to be a ferrite single phase reinforced with fine carbonitride, so that it is 590 MPa or more. A steel sheet having tensile strength and excellent hole expansibility is disclosed. However, when the steel to which Ti is added is cold-rolled, the recrystallization temperature rises remarkably. Therefore, the carbonitride becomes unstable at the recrystallization temperature, and not only does the tensile temperature dependence of the annealing strength increase, but it also becomes difficult to increase the tensile strength itself.

  Patent Document 5 has a tensile strength of 900 MPa or more and excellent hole expansion by actively adding Mn and Mo to a steel sheet, optimizing the annealing temperature condition, and making the structure a martensite single phase. Steel sheet is disclosed. However, when the structure is a martensite single phase, the ductility is remarkably deteriorated and various molding defects are likely to occur. Therefore, a steel sheet having such a structure is not suitable for press forming. Furthermore, since a large amount of expensive Mo is added, the manufacturing cost becomes extremely high.

  On the other hand, Patent Document 6 actively adds Nb and Mo to a steel sheet, optimizes annealing conditions and cooling stop conditions after annealing, promotes bainite transformation, and increases the area ratio of bainite or bainitic ferrite structure. Accordingly, a steel sheet having a tensile strength of 590 MPa or more and excellent hole expansibility is disclosed. However, as described above, in the case of bainite, martensite and austenite having a high C concentration are inevitably generated in the case of coarse cementite and bainitic ferrite precipitated at the lath boundary or the like. Therefore, unless the plating bath temperature and the alloying treatment temperature that affect the transformation rate of bainite are strictly controlled, the hole expandability deteriorates. In a wide range of temperature control in which the alloying treatment temperature is set to 430 ° C. or higher and 580 ° C. or lower, it is difficult to stably ensure good hole expansibility, and such a technique is not suitable for mass production. As described above, the techniques disclosed in Patent Documents 4 to 6 are not realistic methods because it is not easy to create a hot-dip galvanized steel sheet having a single-phase structure or a bainite structure equivalent thereto.

  On the other hand, the addition of microalloys such as Ti and Nb manifests a refinement of the structure and a significant strengthening of ferrite. Therefore, methods for optimizing the optimum amount of these elements, and even combinations with other elements and manufacturing methods have been studied. It was.

  Patent Document 7 adds Ti and Si to a steel sheet, optimizes the hot rolling temperature and annealing temperature conditions, reinforces the recrystallized ferrite with fine precipitates, and the hardness difference between adjacent structures constituting the composite structure The steel sheet having a tensile strength of 590 MPa or more and excellent hole expansibility is disclosed. However, if the steel to which Ti is added is annealed in a two-phase region (two-phase coexistence temperature range of ferrite and austenite), the dependency of tensile strength on the annealing temperature increases. That is, it becomes difficult to stably secure desired strength characteristics, and such a technique is not suitable for mass production.

  Patent Document 8 not only positively adds Ti to the steel sheet, but also controls the ratio of C amount, Mn amount and Ti amount, optimizes the annealing temperature, and the second phase constituting the composite structure is bainite. A steel sheet having a tensile strength of 590 MPa or more and excellent hole expansibility is disclosed. Furthermore, since the annealing temperature condition includes a single-phase region, it is expected that a steel plate having a small dependence of the tensile strength on the annealing temperature can be mass-produced. However, as described above, in a composite structure steel plate that positively uses bainite or martensite, the structure change after cooling after annealing is remarkable, and the hole expandability is significantly deteriorated by the temperature history. In particular, when the Mn content of the steel sheet is 3.0% or less, since only a part of the steel tends to be bainite, the entire temperature history after cooling including the plating bath temperature and alloying temperature is strictly observed. If not controlled, the hole expandability deteriorates. That is, it is difficult to stably ensure good hole expansibility, and such a technique is not suitable for mass production.

  Further, Patent Literature 9 adds Ti, Nb, and B to a steel sheet, optimizes the hot rolling temperature and annealing temperature, and increases the crystal grain size of the austenite low-temperature transformation phase constituting the composite structure. A steel sheet having excellent tensile strength and hole expansibility is disclosed. Furthermore, since the annealing temperature condition includes a single-phase region and the Mn content of the steel sheet exceeds 3.0% (Mn content: 1.4% to 3.5%), the tensile strength and hole of the steel sheet Expandability is not only affected by the annealing temperature but also the temperature history after cooling, and is expected to be a technology suitable for mass production. However, even if the Mn content of the steel sheet exceeds 3.0%, as will be described later, [Ti] + [Nb] / 2 value of the steel sheet ([Ti] is the Ti content% contained in the steel, When [Nb] is contained in steel by 0.14% or less, the structural change from the cooling stop after annealing to the alloying treatment is remarkable, and a very hard structure is formed by the temperature history, Hole expandability changes significantly. Therefore, after annealing and cooling, it is impossible to stably ensure good hole expansibility only by holding at 450 to 600 ° C. for 10 to 120 seconds.

Japanese Patent Laid-Open No. 6-93340 Japanese Patent Laid-Open No. 2004-211126 JP 2007-70649 A JP 2002-322540 A JP 2004-315882 A JP 2003-193190 A JP 2002-69574 A JP 2005-220417 A JP 2007-9317 A

ISIJ International, 44 (2004), no. 3, p. 603-609 ISIJ International, 44 (2004), no. 11, p. 1945-1951

  As described above, the subject of the present invention is to provide a hot dip galvanized steel sheet that has been difficult to manufacture by conventional techniques and has excellent hole expansibility with a tensile strength of 590 MPa or more, preferably 780 MPa or more, and a method for producing the same. It is to be. In the steel plate of the present invention, the target value of hole expandability (hole expansion rate described later) is 50% or more.

  In the present invention, a larger amount of Mn is added to the steel than the conventional steel plate so that the formation of a hard structure composed of coarse cementite that deteriorates hole expansibility, martensite and austenite with high C concentration can be suppressed. In addition, by controlling the C amount, Ti amount and Nb amount to a specific range and finding the optimum production conditions for the chemical composition, a hot dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent hole expansibility Based on the knowledge that it can be obtained. Furthermore, the present invention is a technique suitable for mass production because a steel sheet having a desired performance is manufactured without requiring remodeling of a hot dip galvanizing line or a long manufacturing process. In the prior art, it has been difficult to stably manufacture such a steel plate.

The present invention provides a hot-dip galvanized steel sheet comprising a hot-dip galvanizing layer on the surface of the steel sheet, the steel sheet, C: 0.0 45% or more 0.075% or less (on the composition unless otherwise stated in this specification “%” Means “mass%”), Si: 0.001% to 0.2%, Mn: 3 . 0% more than 4.5% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.001% or more and 0.2% or less, N: 0.01% or less, O: 0.01% or less, and within a range satisfying the following inequality with one or two of Ti and Nb A hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and a hole expansion ratio of 76% or more measured by the method defined in JFST1001, characterized by containing a chemical composition comprising Fe and impurities .

0.1 8 ≦ Ti + Nb / 2 ≦ 0.30
Here, Ti and Nb in the above formula mean the contents of Ti and Nb (unit: mass%), respectively.

In the hot dip galvanized steel sheet according to the present invention, the chemical composition is mass%, Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1% or less. And V: It is preferable to further contain one or more selected from the group consisting of 0.1% or less.
In the hot-dip galvanized steel sheet according to the present invention, the chemical composition is mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less. And Bi: It is preferable to further contain one or more selected from the group consisting of 0.01% or less.

In the hot-dip galvanized steel sheet according to the present invention, the chemical composition preferably further contains B: 0.002% or less in terms of mass%.
The hot dip galvanized steel sheet according to the present invention preferably has a steel structure containing 2.0% or less of retained austenite in terms of area ratio.

From another viewpoint, the present invention is the above-described method for producing a hot-dip galvanized steel sheet, comprising the following steps (A) to (C).
(A) The steel material having the chemical composition of the hot-dip galvanized steel sheet according to the present invention described above is subjected to hot rolling at a rolling start temperature of 1100 ° C. or higher and 1300 ° C. or lower, and a coiling temperature: 530 ° C. or higher and 600 ° C. or lower. Hot rolling process to make a rolled steel sheet;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is held for 90 seconds or less in a temperature range of Ac _{3 to} 950 ° C. And then cooled to a temperature range of [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less, and then the temperature range including the time of immersion in the plating bath is 500. Continuous hot dip galvanizing process that holds for less than a second.

  Furthermore, from another point of view, the present invention is characterized by subjecting the hot dip galvanized steel sheet obtained by the manufacturing method according to the present invention to an alloying treatment in a temperature range of 430 ° C. to 600 ° C. It is a manufacturing method of a galvanized steel sheet.

  According to the present invention, it is possible to mass-produce hot dip galvanized steel sheets having a tensile strength of 590 MPa or more and excellent hole expansibility. The hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

The reason why the chemical composition of the hot dip galvanized steel sheet according to the present invention is defined as described above will be described.
(C: 0.02% to 0.075%)
C is an element contributing to strength improvement, and is contained in an amount of 0.02% or more in order to make the tensile strength of the steel plate 590 MPa or more. Preferably it is 0.035% or more, More preferably, it is 0.045% or more. However, when C is contained exceeding 0.075%, the hard structure is likely to be mixed, and the hole expandability is deteriorated. For this reason, C content shall be 0.075% or less. Preferably, it is 0.065% or less. By controlling the amount of C in this way, the amount of expensive alloy elements such as Mn, Ti, Nb, etc. added can be suppressed, and the manufacturing cost can be reduced.

(Si: 0.001% to 0.2%)
Si is an element contributing to strength improvement, and in the present invention, Si is contained in an amount of 0.001% or more. However, when Si is contained exceeding 0.2%, a non-plating part will generate | occur | produce in a steel plate and corrosion resistance will deteriorate. For this reason, Si content shall be 0.001% or more and 0.2% or less. Preferably, the Si content is 0.05% or more and 0.15% or less. By controlling the Si amount in this way, the adhesion of plating is improved, and powdering and flaking occur due to press molding. Can be prevented.

(Mn: 2.0% to 4.5%)
Mn delays the bainite transformation, and further suppresses the formation of coarse cementite and hard structure by the balance of C content, Ti content and Nb content, and is an element that contributes significantly to improving the strength. In order to make it 590 MPa or more, it is made to contain 2.0% or more. Preferably it is more than 3.0%. However, when Mn is contained exceeding 4.5%, not only refining and casting in a converter become extremely difficult, but also weldability deteriorates. For this reason, Mn content shall be 4.5% or less.

(P: 0.1% or less)
P is an element contained as an impurity, but may also be positively incorporated because it is also an element contributing to strength improvement. However, when P is contained exceeding 0.1%, the weldability is remarkably deteriorated. Therefore, the P content is 0.1% or less. Preferably, the P content is 0.005% or more and 0.025% or less. By controlling the P content in this way, the steel sheet can be strengthened more reliably and plating defects such as powdering can be prevented. It becomes possible to do.

(S: 0.01% or less)
S is an element that is unavoidably contained as an impurity and significantly deteriorates the hole expansibility. For this reason, S content shall be 0.01% or less. In addition, as the content is lower, the hole expanding property is improved, and is preferably 0.005% or less. More preferably, it is 0.0015% or less.

(Sol.Al: 0.001% to 0.2%)
Al is an element that deoxidizes steel and improves the yield of carbonitride-forming elements such as Ti. In order to suppress the formation of Ti-based, Nb-based, or Ti—Nb composite-based oxides, sol. The Al content is 0.001% or more. Preferably it is 0.02% or more. However, the sol. When Al is contained, a non-plated portion is generated in the steel sheet, and the corrosion resistance is deteriorated. For this reason, sol. The Al content is 0.2% or less. Preferably it is 0.1% or less.

(N: 0.01% or less)
N is an element that is unavoidably contained as an impurity and significantly deteriorates the hole expandability. For this reason, N content shall be 0.01% or less. In addition, as the content is lower, the hole expanding property is improved, and is preferably 0.005% or less. More preferably, it is 0.003% or less.

(O: 0.01% or less)
O is unavoidably contained as an impurity, and is an element that significantly deteriorates hole expansibility. For this reason, the O content is set to 0.01% or less. In addition, as the content is lower, the hole expanding property is improved, and is preferably 0.005% or less. More preferably, it is 0.002% or less.

(Ti and Nb: 0.14% ≦ Ti + Nb / 2 ≦ 0.30%)
Ti and Nb are elements that form fine carbides, nitrides, or carbonitrides and contribute significantly to improving the strength. In addition, as described above, by balancing the amount of C and the amount of Mn, and further combining annealing conditions as described later, coarse cementite and hard structure become difficult to generate, while the tensile strength is 590 MPa or more, Excellent hole expandability is also achieved. In order to express such an effect, at least one or two of Ti and Nb are contained, and the value of Ti + Nb / 2 (where Ti and Nb are the contents of Ti and Nb, respectively (unit: mass%)) ) 0.14 or more. However, if the Ti + Nb / 2 value exceeds 0.3 and one or two of Ti and Nb are contained, the effect is saturated and the manufacturing cost only increases. For this reason, the value of Ti + Nb / 2 is set to 0.14 or more and 0.3 or less. When Ti + Nb / 2 is contained in an amount of 0.18 or more, it becomes easy to obtain a steel structure having an area ratio of retained austenite of 2.0% or less, and a good hole expansibility is secured more stably. It becomes possible to do.

(Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1% or less, and V: 0.1% or less, or one selected from the group consisting of 2 types or more)
Cr, Mo, Cu, Ni and V are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. However, even if each content exceeds 0.1%, the effect is saturated and the manufacturing cost is increased. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Cr, Mo, Cu, Ni, and V by the said quantity. In addition, in order to acquire the said effect more reliably, it is preferable to contain any element 0.01% or more.

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less, and Bi: 0.01% or less, or one selected from the group consisting of 2 types or more)
Ca, Mg, REM, Zr, and Bi are all elements that improve hole expansibility and are optional elements that can be included as necessary. However, if each content exceeds 0.01%, the surface properties deteriorate. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Ca, Mg, REM, Zr, and Bi with the said quantity. In addition, in order to acquire the said effect more reliably, it is preferable to contain any element 0.0005% or more.

(B: 0.002% or less)
B is an element that contributes to strength improvement, and is an optional element that can be contained as necessary. However, even if it contains B exceeding 0.002%, the said effect is saturated. For this reason, it is preferable that B content shall be 0.002% or less. In addition, in order to acquire the said effect more reliably, it is preferable to make it contain 0.0002% or more.

(Area ratio of retained austenite: 2.0% or less)
Next, a suitable steel structure of the hot dip galvanized steel sheet according to the present invention will be described.
The hot-dip galvanized steel sheet according to the present invention having the above-described chemical composition has a structure mainly composed of ferrite, bainitic ferrite and bainite, and preferably contains no martensite and retained austenite as much as possible. Among them, austenite becomes the hardest structure due to processing-induced transformation. Therefore, when a steel plate containing retained austenite that has been cut or punched is stretched and flange-formed, the hole expandability is significantly deteriorated. For this reason, it is preferable that the retained austenite is 2.0% or less (including the case of 0%) in the fraction evaluated by the area ratio. The total area ratio of martensite and retained austenite is preferably 30% or less. In the hot dip galvanized steel sheet according to the present invention, it is difficult to clearly distinguish between ferrite, bainitic ferrite and bainite, so it is difficult to define each area ratio. The total area ratio of ferrite, bainitic ferrite and bainite is preferably 70% or more.

Next, the suitable manufacturing method of the hot dip galvanized steel plate concerning this invention is demonstrated.
It is preferable that the molten steel having the above-described chemical composition is melted by a known melting method such as a converter or an electric furnace, and is made into a steel material such as a slab by a continuous casting method. In place of the continuous casting method, casting may be performed by an ingot casting method, a thin slab casting method, or the like. This steel material is hot-rolled to obtain a hot-rolled steel sheet. Hot rolling does not cool the cast steel material to room temperature, inserts it into a heating furnace in the form of a hot piece, direct feed rolling to roll after heating, or direct rolling to roll immediately after holding a little heat Alternatively, the steel material may be once cooled and then reheated and rolled.

(Starting temperature of hot rolling: 1100 ° C or higher and 1300 ° C or lower)
The tensile strength of the hot dip galvanized steel sheet according to the present invention is increased by the dispersion of fine precipitates such as Ti and Nb. Therefore, in order to make the tensile strength of the steel plate 590 MPa or more, it is necessary to once dissolve Ti and Nb before hot rolling. For this reason, the heating temperature before hot rolling shall be 1100 degreeC or more. However, if it heats exceeding 1300 degreeC, the internal oxidation of a steel raw material will be accelerated | stimulated and surface properties will deteriorate remarkably. For this reason, the heating temperature of a steel raw material shall be 1100 degreeC or more and 1300 degrees C or less. In other words, the hot rolling start temperature is 1100 ° C. or higher and 1300 ° C. or lower. The starting temperature of hot rolling is preferably 1200 ° C. or higher and 1270 ° C. or lower, and the effect can be obtained more reliably by limiting the temperature in this way. Moreover, it is preferable to hold | maintain a steel raw material in the temperature range of 1200 degreeC or more for 30 minutes or more before starting hot rolling. By holding the steel material at a high temperature in this way, the non-uniform structure due to solidification segregation of Mn is eliminated, and the hole expandability is improved. However, even if it is kept for more than 180 minutes, the above effect is saturated and the manufacturing cost only becomes high, so it is preferable to keep it for 180 minutes or less.

(Finishing rolling temperature: 800 ° C or higher and 1000 ° C or lower)
In order to reduce the deformation resistance during hot rolling and make the operation easier, the finish rolling temperature is preferably set to 800 ° C. or higher. However, when finish rolling is performed at a temperature exceeding 1000 ° C., scale wrinkles are easily generated, and the surface properties are significantly deteriorated. For this reason, it is preferable that finish rolling temperature shall be 800 degreeC or more and 1000 degrees C or less. More preferably, it is 850 degreeC or more and 950 degrees C or less.

(Hot rolled coiling temperature: 530 ° C or higher and 600 ° C or lower)
The hot dip galvanized steel sheet according to the present invention contains a large amount of Mn and Ti and / or Nb. Since these are easily oxidizable elements, the steel plate surface and its vicinity are easily oxidized in the winding process. Therefore, in order to suppress oxidation of the steel sheet and ensure good surface properties, the hot rolling coiling temperature is set to 600 ° C. or lower. On the other hand, these elements have the effect of increasing the strength of the hot-rolled steel sheet. In particular, when it is wound at a temperature lower than 530 ° C., hard bainite and martensite are generated, and thereafter it is difficult to cold-roll. Furthermore, the plate thickness accuracy of the steel plate deteriorates. For this reason, a hot rolling coiling temperature shall be 530 degreeC or more and 600 degrees C or less. Preferably, it is 540 degreeC or more and 590 degrees C or less.
In the hot rolling process, in order to suppress fluctuations in characteristics, it is preferable to achieve uniform temperature of the entire length of the rough bar by performing induction heating or the like on the rough bar before the finish rolling after the rough rolling.

The hot-rolled steel sheet obtained by the hot rolling process is descaled by a conventional method such as pickling, and then cold-rolled to obtain a cold-rolled steel sheet. In this case, the total rolling reduction in hot rolling and cold rolling is preferably 95% or more. Here, the total rolling reduction is calculated by the following equation.
Total rolling reduction (%) = {1− (thickness of cold-rolled steel sheet) / (thickness of slab used for hot rolling)} × 100

  When the total rolling reduction is increased, the Mn segregation band distributed in the plate thickness direction becomes thin, and the hole expandability is improved. In addition, in order to make the steel structure after continuous annealing uniform, it is preferable that the total rolling reduction of cold rolling is 30% or more. In order to ensure the flatness of the steel sheet, it is preferable to correct the shape by rolling at a reduction rate of 5% or less before or after pickling. Moreover, by performing such mild rolling before pickling, pickling performance is improved, removal of surface concentrating elements is promoted, and surface properties are improved.

The cold-rolled steel sheet obtained by the hot rolling process and the cold rolling process is subjected to recrystallization annealing in a temperature range of Ac ₃ points or more and 950 ° C. or less, and then [zinc plating bath temperature −20 ° C.] or more [ It is preferable to cool to a temperature range of galvanizing bath temperature + 100 ° C. or lower, and then to perform a continuous hot dip galvanizing treatment for 500 seconds or less including the time of plating bath immersion in the temperature range to obtain a hot dip galvanized steel sheet. . Moreover, after immersing in a galvanizing bath, it is preferable to perform an alloying process in a temperature range of 430 ° C. or higher and 600 ° C. or lower to obtain an alloyed hot-dip galvanized steel sheet. These steel plates are preferably manufactured by annealing, hot dip galvanizing and alloying in a continuous hot dip galvanizing line.

(Recrystallization annealing temperature: Ac maintained for ₃ seconds or more and 950 ° C. or less for 90 seconds or less)
As described above, when a steel sheet containing a large amount of Ti or Nb is annealed in the two-phase region, non-recrystallized ferrite remains, and not only the annealing temperature dependency on the tensile strength increases, but also the bendability deteriorates significantly. To do. For this reason, annealing temperature shall be Ac ₃ points or more. However, if the annealing temperature exceeds 950 ° C., the annealing furnace is rapidly damaged, and its repair is required, and the productivity is deteriorated. For this reason, recrystallization annealing temperature shall be Ac ₃ points or more and 950 degrees C or less.

The time for holding in the temperature range of Ac ₃ points or more and 950 ° C. or less is 90 seconds or less. If the time for which the temperature is maintained in the temperature range of Ac ₃ points or more and 950 ° C. or less exceeds 90 seconds, the grain boundary and the precipitates in the vicinity thereof become coarse, and the deterioration of toughness may be remarkable. Although the lower limit of time is not specifically limited for said temperature range holding | maintenance, It is preferable to set it as 10 seconds or more. By controlling the annealing time in this way, it is possible to stably ensure good hole expansibility.

Moreover, in order to improve the wettability and alloying processability of plating, it is preferable that the dew point during annealing is −40 ° C. or higher.
After recrystallization annealing, the steel sheet is cooled in the process of being immersed in a galvanizing bath. In this case, the average cooling rate is from 1 ° C./second to 50 ° C./second from the highest temperature to 700 ° C., and then from 3 ° C./second to 50 ° C./second from 700 ° C. to the cooling stop temperature. It is preferable. By cooling to 700 ° C. at 1 ° C./second or more and 50 ° C./second or less, it becomes possible to easily adjust the area ratio of ferrite, bainitic ferrite and bainite and the area ratio of martensite and retained austenite. . On the other hand, by cooling from 700 ° C. to the cooling stop temperature at 3 ° C./second or more, it becomes possible to suppress pearlite transformation leading to strength reduction. Further, when cooling to the cooling stop temperature at more than 50 ° C./second, it is necessary to significantly modify the continuous hot dip galvanizing equipment and the manufacturing cost is remarkably increased.

(Cooling stop temperature: [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less)
The cooling stop temperature is set to [zinc plating bath temperature −20 ° C.] or higher in order to reduce the heat removal upon entering the plating bath and facilitate the operation. However, if the cooling of the steel sheet is stopped exceeding [zinc plating bath temperature + 100 ° C.], the temperature change of the plating bath becomes remarkable, and the operation becomes difficult. For this reason, the cooling stop temperature is set to [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less. The hot dip galvanizing treatment follows a conventional method of immersing the annealed steel sheet in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower.

(Holding time of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less: 500 seconds or less, but also includes plating immersion.)
In order to suppress softening of ferrite and bainitic ferrite and to secure a desired tensile strength, the holding time of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less is also maintained during plating immersion. Including 500 seconds or less. Preferably, the holding time is 10 seconds or longer. By giving such a holding time, the coating amount of the steel sheet can be adjusted, and good corrosion resistance can be secured stably.

(Alloying temperature: 430 ° C or higher and 600 ° C or lower)
When the alloying treatment is performed, the alloying treatment temperature after immersion in the plating bath is set to 430 ° C. or more in order to suppress the occurrence of unalloyed treatment and improve the corrosion resistance. However, when alloying is performed at a temperature exceeding 600 ° C., ferrite and bainitic ferrite are softened, and the tensile strength is remarkably lowered. For this reason, the alloying treatment temperature is set to 430 ° C. or more and 600 ° C. or less. Preferably, the alloying treatment temperature is 500 ° C. or more and 560 ° C. or less, and the degree of alloying (Fe content of the plating layer) is 8 mass% or more and 13 mass% or less by controlling the temperature in this way. It becomes easy to improve the adhesion of plating. On the other hand, when the alloying treatment temperature is set to [zinc plating bath temperature + 40 ° C.] or higher, austenite is decomposed, and it becomes easy to obtain a steel structure containing 2.0% or less of retained austenite by area ratio, and good hole expansion. It becomes easy to ensure stability stably. For this reason, it is preferable to make alloying processing temperature into 430-600 degreeC and [zinc plating bath temperature +40 degreeC] or more.
In order not only to suppress the occurrence of yield point elongation, but also to prevent seizure and galling during pressing, it is preferable to perform temper rolling at an elongation of 0.05% or more and 1% or less after continuous hot dip galvanizing treatment.

Moreover, in order to improve the wettability and alloying processability of plating, the steel plate before annealing may be plated with one or more of Ni, Cu, Co, and Fe.
By the said manufacturing method, the hot dip galvanized steel plate which is excellent in the hole expansibility whose tensile strength is 590 Mpa or more can be manufactured.

  Thus, by devising the chemical composition of steel and optimizing the conditions for continuous annealing after hot rolling and cold rolling, hot dip galvanizing, and alloying treatment, the tensile strength is excellent in hole expansibility of 590 MPa or more. A hot-dip galvanized steel sheet and a method for producing the same are provided. About hole expansibility, when the hole expansion ratio (HER) measured by the method prescribed | regulated to JFST1001 is 50% or more, hole expansibility is favorable. When the HER value is 80% or more, the hole expandability is better.

The present invention will be described more specifically with reference to examples.
Steel having the chemical composition shown in Table 1 was melted in a converter, and a 245 mm thick slab was cast by continuous casting.

Ac ₃ shown in Table 1 was obtained by regression analysis of literature values and experimental values, and was obtained from the following equation.
Ac ₃ = 910-203 × (C ^1/2 ) −15.2 × Ni + 44.7 × Si + 104 × V + 31.5 × Mo-30 × Mn-11 × Cr-20 × Cu + 700 × P + 400 × Al + 400 × Ti

  The obtained slab was hot rolled under the conditions shown in Table 2 to produce a 2.6 mm thick hot rolled steel sheet. The obtained hot-rolled steel sheet was pickled and cold-rolled to produce a 1.2 mm-thick cold-rolled steel sheet.

  The obtained cold-rolled steel sheet was heated to 700 ° C. at a heating rate of 10 ° C./second, heated from 700 ° C. to the annealing temperature shown in Table 3, which was the highest temperature, at a heating rate of 3 ° C./second, The temperature was maintained for the time shown in Table 3 and annealed. Cooling is performed at an average cooling rate of 8 ° C./second from the annealing temperature to the cooling stop temperature. Further, in order to simulate the thermal history during the hot dip galvanizing process, the cooling stop temperature is maintained for the time shown in Table 3, and an assumed plating bath It cooled to 460 degreeC which is temperature over 5 second, it hold | maintained at the temperature for 10 second, it cooled at the cooling rate of 10 degreeC / second to room temperature, and produced the annealed cold rolled steel plate. Further, when simulating the heat history during the alloying treatment, after holding at 460 ° C. for 10 seconds, further heating to the alloying treatment temperature shown in Table 3 over 5 seconds, and holding at that temperature for the time shown in Table 2 Then, it was cooled to room temperature at a cooling rate of 10 ° C./second to produce an annealed cold rolled steel sheet. The holding time of the cooling stop temperature is the total of the holding time at the cooling stop temperature, the time for cooling to the plating bath temperature, and the time for holding at the plating bath temperature.

  The annealed cold-rolled steel sheet produced in this example is not hot-dip galvanized, but receives the same thermal history as the hot-dip galvanized steel sheet, so the mechanical properties of the steel sheet are hot-dip galvanized with the same thermal history. It is substantially the same as a steel plate.

  The obtained annealed cold-rolled steel sheet was analyzed for structure with an optical microscope or an electron microscope, and further with an X-ray diffraction method, and subjected to a tensile test and a hole expansion test to evaluate its mechanical properties. The results are shown in Table 4.

[Test method]
(Total area ratio of ferrite, bainitic ferrite and bainite)
Test specimens were taken from each annealed cold rolled steel sheet in the rolling direction and the perpendicular direction of rolling, and the cross-sectional structure in the rolling direction and the cross-sectional structure in the direction perpendicular to the rolling were photographed with an optical microscope or electron microscope, and ferrite and bainitic ferrite were analyzed by image analysis. And the total area ratio of bainite was measured. The obtained area ratio is shown in the area 1 area column in Table 4.

(Area ratio of retained austenite)
Each annealed cold-rolled steel sheet is subjected to chemical polishing to reduce the thickness by ¼, X-ray diffraction is applied to the surface after chemical polishing, the obtained profile is analyzed, and the area ratio of residual austenite is calculated. did. The obtained area ratio is shown in the area ratio 2 column in Table 4.

(mechanical nature)
A JIS No. 5 tensile specimen was taken from the direction perpendicular to the rolling direction, and the tensile strength was measured. The hole expansion rate was measured by the method prescribed in JFST1001.

  In addition, the numerical value underlined in Tables 1-4 has shown that content, manufacturing conditions, or a mechanical characteristic shown by the numerical value is outside the scope of the present invention.

Sample No. in Table 4 Nos. 1, 3 to 5, 8 to 11, 13, 14 and 16 to 18 are steel plates of examples of the present invention that satisfy all the conditions of the present invention. 2, 6, 7, 12, 15, 19, and 20 are comparative steel plates that do not satisfy at least one of the conditions of the present invention.
Specimen No. The steel plates of the present invention examples of 1, 3 to 5, 8 to 11, 13, 14 and 16 to 18 are high strength steel plates excellent in hole expansibility with a tensile strength of 590 MPa or more.

  On the other hand, steel plate No. of the comparative example. 2, 7 and 15 are out of the scope of the present invention, and the desired strength cannot be obtained. Steel plate No. 6, 19 and 20 are out of the scope of the present invention, and the desired strength cannot be obtained. Steel plate No. No. 12 has a chemical composition that is out of the scope of the present invention, and has poor hole expandability.

  Among the steel plates of the present invention example, steel plate No. 1 having an area ratio of retained austenite of 2.0% or less. 1, 3, 5, 8, 9, 11, 13, 14 and 16 to 18 were preferable steel sheets having a tensile strength of 590 MPa or more and a hole expansion ratio of 80% or more.

  Moreover, among the steel plates of the present invention, the steel plate No. 1 in which the C amount and the Mn amount are in the preferred ranges. 1, 3, 4, 8, 10, 11, 13, 14, 16 and 17 became preferable steel plates excellent in hole expansibility having a tensile strength of 780 MPa or more.

Claims (7)

In galvanized steel sheet on the surface of the steel sheet comprising a hot-dip galvanizing layer, wherein the steel sheet contains, by mass%, C: 0.0 45% or more 0.075% or less, Si: 0.001% to 0.2% or less , Mn: 3 . 0% more than 4.5% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.001% or more and 0.2% or less, N: 0.01% or less, O: 0.01% or less, and within a range satisfying the following inequality with one or two of Ti and Nb A hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and a hole expansion ratio of 76% or more measured by a method prescribed in JFST1001, characterized by containing a chemical composition comprising Fe and impurities .
0.1 8 ≦ Ti + Nb / 2 ≦ 0.3
Here, Ti and Nb in the above formula mean the contents of Ti and Nb (unit: mass%), respectively.   The chemical composition is, in mass%, Cr: 0.1% or less, Mo: 0.1% or less, Cu: 0.1% or less, Ni: 0.1% or less, and V: 0.1% or less. The hot-dip galvanized steel sheet according to claim 1, further comprising one or more selected from the group.   The chemical composition is, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, Zr: 0.01% or less, and Bi: 0.01% or less. The hot dip galvanized steel sheet according to claim 1 or 2, further comprising one or more selected from the group.   The hot dip galvanized steel sheet according to any one of claims 1 to 3, wherein the chemical composition further contains, by mass%, B: 0.002% or less.   The hot-dip galvanized steel sheet according to any one of claims 1 to 4, having a steel structure in which the area ratio of retained austenite is 2.0% or less. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 5, comprising the following steps (A) to (C):
(A) Hot rolling at a rolling start temperature of 1100 ° C. or higher and 1300 ° C. or lower and a winding temperature of 530 ° C. or higher and 600 ° C. or lower is applied to the steel material having the chemical composition according to any one of claims 1 to 4. Hot rolling process to give hot-rolled steel sheet;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is held for 90 seconds or less in a temperature range of Ac _{3 to} 950 ° C. And then cooled to a temperature range of [Zinc plating bath temperature −20 ° C.] or more and [Zinc plating bath temperature + 100 ° C.] or less, and then the temperature range including the time of immersion in the plating bath is 500. Continuous hot dip galvanizing process that holds for less than a second.   A method for producing a hot-dip galvanized steel sheet, comprising subjecting the hot-dip galvanized steel sheet obtained by the production method according to claim 6 to an alloying treatment in a temperature range of 430 ° C to 600 ° C.