JP5206350B2 - 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 bendability and a method for producing the same, and in particular, press forming such as a car body of an automobile. The present invention relates to a high-strength hot-dip galvanized steel sheet suitable for the intended use and a method for producing the same. Here, “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, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment, and in order to reduce the weight of the vehicle body and ensure the safety of passengers, high-strength steel sheets with a tensile strength of 590 MPa or more, especially considering rust prevention In materials, there is an increasing need for high-strength hot-dip galvanized steel sheets.

  However, it is not sufficient for a steel sheet to be used for an automobile member to have high strength, and it must satisfy various performances required at the time of component molding, such as press formability and weldability. In particular, considering the part forming process, bending forming is most frequently used, and thus formed into various shaped parts. Therefore, a high-strength hot-dip galvanized steel sheet having excellent bendability is required. However, the bendability deteriorates as the steel plate strength increases.

  For improving the bendability of a high-strength steel sheet, conventionally, the approach of controlling the steel structure has been taken, and as described in Patent Document 1, the hardness of the low-temperature transformation generation phase is reduced and the hardness with the ferrite phase is reduced. It is considered good to reduce the difference. On the other hand, Patent Document 2 and Patent Document 3 describe that when the ferrite crystal grains are made ultrafine, it is possible to achieve both stretch flangeability that requires local deformability and high strength as well as bendability.

  However, in the case of a high-strength steel sheet containing a large amount of Mn for the purpose of increasing strength, local chemical composition fluctuations occur due to solidification segregation, and a heterogeneous structure corresponding to the fluctuations is formed. Therefore, in the technique disclosed in Patent Document 1, it is extremely difficult to precisely control the hardness of the ferrite phase and the low temperature transformation phase in the entire steel sheet, and it corresponds to local chemical composition fluctuations. Due to the non-uniform structure, as shown in FIG. 1, noticeable irregularities appearing on the surface of the processed part appear visually, the irregularities promote non-uniform deformation and induce cracks, and the bendability itself is improved. Deteriorate. Moreover, even if it does not lead to a crack, if unevenness exists in the processed part, the collision characteristics as a component deteriorate.

  In addition, the transformation phenomenon locally changes due to solidification segregation, and the crystal grain size becomes non-uniform, so even the techniques disclosed in Patent Document 2 and Patent Document 3 cannot improve the bendability. In particular, in the techniques described in these documents, a large amount of Mn and Ni that easily solidify and segregate is contained in the steel, so that it is easily expected to be inferior in bendability and impactability as a part as described above. The

  From the viewpoint of homogenizing the structure, there is an ultimate approach of a single-phase structure, and Patent Document 4 describes that the bendability can be improved by using a martensite single-phase structure that is the ultimate uniform structure. Has been. However, if the steel structure is made of a single martensite phase as in the technique disclosed in Patent Document 4, the flatness of the steel sheet is impaired, making it difficult to apply as an automobile part that requires parts accuracy.

Patent Document 5 describes that by making a ferrite single-phase structure, it is possible to achieve both hole expandability and high strength that require local deformability as well as bendability. However, when the technique disclosed in Patent Document 5 is applied to a process for manufacturing a high-strength cold-rolled steel sheet or a high-strength hot-dip galvanized steel sheet that requires a cold rolling process that improves surface roughness and sheet thickness accuracy. The addition of a large amount of carbonitride-forming elements raises the recrystallization temperature, necessitates high-temperature annealing at ₃ or more points on the Ac, and increases the coarsening of the precipitate, which makes it impossible to increase the strength. . Also, the crystal grain size becomes non-uniform, and the bendability cannot be improved.

  Therefore, in order to achieve both bendability and high strength, it is necessary to satisfy both seemingly contradictory conditions so that a uniform structure can be obtained even if a large amount of Mn is contained for high strength. There is an approach to resolve the solidification segregation itself, which is the origin of the heterogeneous structure, by diffusion. Patent Document 6 describes that the segregation is reduced and the steel material is homogenized by a solute treatment in which the steel material is held at a high temperature of 1250 ° C. or higher for a long time of 10 hours or longer. However, since the solidification segregation does not completely disappear only by the technique disclosed in Patent Document 6, a non-uniform structure is formed due to the remaining segregation, and unevenness cannot be removed from the processed portion, and the bendability is sufficient. Not.

Patent Document 7 and Patent Document 8, the average cooling rate at the position of (1/4) t _s the thickness t _s of the slab as 100 ° C. / min or higher, cooled to solidus temperatures from liquidus temperature It is described that the segregation is reduced and the steel material is homogenized by applying continuous casting conditions. However, the techniques disclosed in Patent Document 7 and Patent Document 8 require application of a thin slab continuous casting method with a slab thickness of 30 to 70 mm in order to achieve a desired cooling rate. It cannot be produced with equipment for continuous casting for ordinary thin steel sheets of 200 to 300 mm.
Japanese Patent Laid-Open No. 62-13533 Japanese Patent Laid-Open No. 2004-211126 JP 2004-250774 A JP 2002-161336 A JP 2002-322539 A JP-A-4-191322 JP 2007-70649 A JP 2007-70659 A

  An object of the present invention is to provide a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability, and a method for manufacturing the same, which have been difficult to manufacture with the conventional technology as described above. Here, “excellent bendability” means that the minimum bend radius of the 180 ° bend test is 1.0 t or less, and no irregularities appear on the surface after processing at the visual level. Therefore, unless otherwise specified, the bendability in this specification is evaluated by observing such physical properties and actual members. In addition, in order to apply the hot dip galvanized steel sheet according to the present invention to parts having more complicated shapes such as front side members and rear cross members which are typical examples of reinforcing members for automobiles, a 180 ° bending test is performed. It is preferable that the minimum bending radius is 0.5 t or less, no irregularities appear on the surface after processing at a visual level, and the (TS × El) value obtained by a tensile test is 12000 MPa ·% or more. Furthermore, in order to apply the hot dip galvanized steel sheet according to the present invention to a part that also requires ductility, such as a center pillar that is a typical example of an automotive reinforcing member, the minimum bending radius of the 180 ° bending test is 0. More preferably, it is 5 t or less, no irregularities appear on the surface after processing at the visual level, and the (TS × El) value obtained by a tensile test is 16000 MPa ·% or more. On the other hand, in order to further reduce the weight of these parts, the tensile strength is preferably 980 MPa or more.

  In the present invention, in a hot-dip galvanized steel sheet, a desired chemical composition and manufacturing conditions are reviewed and optimized so as to suppress the formation of a heterogeneous structure due to solidification segregation, which has been difficult with conventional techniques. This is based on the knowledge that a Mn concentration distribution can be obtained, whereby a uniform structure can be obtained, and a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability can be obtained.

  The present invention relates to a hot dip galvanized steel sheet provided with a hot dip galvanized layer on the surface of the steel sheet. This steel sheet is C: 0.03% or more and 0.20% or less (in this specification, unless otherwise specified, “ % "Means"% by mass "), Si: 0.005% to 2.0%, Mn: 1.2% to 3.0%, P: 0.1% or less, S : 0.01% or less, sol. Al: 0.003% or more and 1.0% or less, N: 0.01% or less, Bi: 0.0001% or more and 0.05% or less, with the balance being Fe and impurities, In the rolling direction of the Mn-concentrated portion, which has a steel structure containing 2.0 to 15 area% of retained austenite and extends in the rolling direction at a (1/20) depth position of the plate thickness from the steel sheet surface. On the other hand, the hot-dip galvanized steel sheet is characterized in that an average interval in a right-angle direction is 300 μm or less.

  In the hot-dip galvanized steel sheet according to the present invention, the chemical composition is replaced by a part of Fe, and the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.05% or less. It is preferable to contain the 1 type (s) or 2 or more types selected.

  In these hot-dip galvanized steel sheets according to the present invention, instead of part of Fe, the chemical composition is a group consisting of Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less. It is preferable to contain 1 type, or 2 or more types selected from.

  In these hot-dip galvanized steel sheets according to the present invention, the chemical composition is Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: It is preferable to contain one or more selected from the group consisting of 0.01% or less.

In these hot-dip galvanized steel sheets according to the present invention, the chemical composition preferably contains B: 0.01% or less instead of part of Fe.
From another viewpoint, the present invention is a method for producing a hot dip galvanized steel sheet, comprising the following steps (A) to (C).
(A) The molten steel having the chemical composition of the steel sheet in the above-described hot-dip galvanized steel sheet according to the present invention has a solidification rate at a depth of 10 mm from the surface of 100 ° C./min to 1000 ° C./min. Continuous casting process to cast into the slab;
(B) A hot rolling process and a cold rolling process in which the slab obtained by the continuous casting process is hot rolled to obtain a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet; And (C) the cold-rolled steel sheet is subjected to recrystallization annealing in a temperature range of 750 ° C. to 950 ° C., and then cooled to a temperature range of [zinc plating bath temperature−20 ° C.] to [zinc plating bath temperature + 100 ° C.]. Then, continuous hot dip galvanizing step that maintains the temperature range for 500 seconds or less including the time of immersion in the plating bath. From another viewpoint, the present invention is the hot dip galvanizing obtained by the manufacturing method according to the present invention described above. A method for producing a hot-dip galvanized steel sheet, characterized by subjecting a steel sheet to an alloying treatment in a temperature range of 430 ° C. to 600 ° C.

  According to the present invention, a hot dip galvanized steel sheet having a strength of 590 MPa or more and excellent bendability can be obtained. The hot-dip galvanized steel sheet according to the present invention can be used widely in industry, particularly in the automobile field.

Hereinafter, the best mode for carrying out the present invention will be described.
First, the reason why the chemical composition of the hot-dip galvanized steel sheet according to the present invention is specified as described above will be described.

(C: 0.03% to 0.20%)
C is an element contributing to strength improvement, and is contained in an amount of 0.03% or more in order to make the tensile strength of the steel plate 590 MPa or more. However, if the C content exceeds 0.20%, the weldability deteriorates. For this reason, C content shall be 0.03% or more and 0.20% or less. The C content is preferably 0.06% or more. By doing so, the tensile strength can be easily made 980 MPa or more.

(Si: 0.005% to 2.0%)
Si is an element that contributes to improving the strength without deteriorating the ductility so much or improving the ductility. In the present invention, Si is contained in an amount of 0.005% or more. However, if the Si content exceeds 2.0%, the wettability of plating and the adhesion of plating deteriorate. For this reason, Si content shall be 0.005% or more and 2.0% or less. Note that when 0.2% or more of Si is contained, the ductility is further improved by the TRIP effect. For this reason, the Si content is preferably 0.2% or more, and more preferably 0.6% or more.

(Mn: 1.2% to 3.0%)
Mn is an element that contributes to strength improvement, and is contained in an amount of 1.2% or more in order to make the tensile strength of the steel plate 590 MPa or more. However, when Mn is contained exceeding 3.0%, not only the melting and refining of the steel in the converter becomes difficult, but also the weldability deteriorates. For this reason, Mn content shall be 1.2% or more and 3.0% or less. Here, Mn is an element that promotes a heterogeneous structure. However, as will be described later, by adding Bi, such an adverse effect of Mn is alleviated, the structure becomes uniform, and deterioration of bendability is suppressed. Thus, an improvement in strength is achieved. In addition, in order to make tensile strength 980 MPa or more, it is preferable to contain Mn 1.8% or more.

(P: 0.1% or less)
In general, P is an unavoidable impurity, but it is also a solid solution strengthening element and is effective for strengthening the steel sheet. Therefore, P may be actively contained. However, when the P content exceeds 0.1%, the weldability is significantly deteriorated. Therefore, the P content is 0.1% or less. In order to strengthen the steel sheet more reliably, the P content is preferably set to 0.005% or more.

(S: 0.01% or less)
S is an impurity inevitably contained in steel, and is preferably as low as possible from the viewpoints of bendability and weldability. For this reason, S content shall be 0.01% or less. Preferably it is 0.005% or less. More preferably, it is 0.003% or less.

(Sol.Al: 0.003% to 1.0%)
Al is an element added for deoxidizing steel, and is an element that effectively acts to improve the yield of carbonitride-forming elements such as Ti. Al content shall be 0.003% or more. However, sol. If the Al content exceeds 1.0%, the weldability deteriorates and the oxide inclusions increase, so the surface properties deteriorate. For this reason, sol. Al content shall be 0.003% or more and 1.0% or less. In addition, Preferably it is 0.02% or more and 0.2% or less.

(N: 0.01% or less)
N is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of bendability. Therefore, the N content is 0.01% or less. Preferably it is 0.006% or less.

(Bi: 0.0001% to 0.05%)
Bi is an important element in the present invention, and its inclusion makes the solidified structure finer, and even if Mn is contained in a large amount, the structure becomes uniform and the deterioration of bendability is suppressed. Therefore, in order to ensure a desired bendability, Bi is contained 0.0001% or more. However, when Bi is contained exceeding 0.05%, hot workability deteriorates and hot rolling becomes difficult. For this reason, Bi content shall be 0.0001% or more and 0.05% or less. In order to further improve the bendability, it is preferable to contain 0.0010% or more of Bi.

(One or more selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.05% or less)
Ti, Nb, and V are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain one or more of Ti, Nb and V. In order to obtain the above effect more reliably, it is preferable to contain 0.003% or more of any element of Ti, Nb, and V. However, if each content exceeds 0.05%, inclusions containing Ti, Nb, and V increase, and the surface properties deteriorate. For this reason, it is preferable that the contents of Ti, Nb, and V are each 0.05% or less.

(Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less selected from the group consisting of 1% or less)
Cr, Mo, Cu, and Ni are all elements that contribute to strength improvement, and are optional elements that can be contained as necessary. In order to ensure a tensile strength of 980 MPa or more, it is effective to contain one or more of Cr, Mo, Cu and Ni. In order to acquire the said effect more reliably, it is preferable to contain any element 0.01% or more. However, even if Cr, Mo, Cu, and Ni are contained in excess of 1%, the above effect is saturated and not only economically wasteful, but the hot-rolled steel sheet becomes hard and cold rolling is performed. It becomes difficult to do. For this reason, it is preferable to contain 1 type (s) or 2 or more types of Cr, Mo, Cu, and Ni by said quantity.

(Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: one or more selected from the group consisting of 0.01% or less)
Ca, Mg, REM, and Zr are all elements that contribute to inclusion control, particularly fine dispersion of inclusions, and further improve bendability, and can be included as necessary. . However, since the surface properties are deteriorated if excessively contained, the content of each element is preferably 0.01% or less. In order to acquire the said effect more reliably, it is preferable to contain any element 0.001% or more.

(B: 0.01% or less)
B is an element that contributes to strength improvement, and is an optional element that can be contained as necessary. However, if it exceeds 0.01% and contains B, the hot-rolled steel sheet becomes hard and it becomes difficult to perform cold rolling. For this reason, it is preferable that B content shall be 0.01% or less. In order to acquire the said effect more reliably, it is preferable to make it contain 0.0005% or more.

The balance other than the components described above is Fe and impurities.
Next, the reasons for limiting the Mn distribution and the steel structure of the steel sheet constituting the hot-dip galvanized steel sheet according to the present invention will be described.

(Average interval in the plate width direction of Mn enriched part: 300 μm or less)
The Mn distribution of the steel sheet is an average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion extended in the rolling direction at the (1/20) depth position of the sheet thickness from the steel sheet surface, that is, 300 μm in the sheet width direction. The following. By setting the average interval in the plate width direction of the Mn-concentrated portion to 300 μm or less, solidification segregation is eliminated, a uniform structure is obtained, unevenness is hardly generated in the bent portion, and bendability is improved. In addition, setting the average interval in the plate width direction of the Mn-concentrated portion to 300 μm or less means that the slab thickness (1/20) from the slab surface, which is the source of a non-uniform structure in the slab subjected to hot rolling. ) The dendrite primary arm interval at the depth position is 300 μm or less. In order to set the primary arm interval to 300 μm or less by a normal continuous casting method, it is effective to contain Bi and set the solidification rate at a depth of 10 mm from the slab surface to 100 ° C./min or more.

It should be noted that at the (1/20) depth position of the plate thickness from the steel plate surface, the Mn segregation ratio (Mn _co ) calculated from the Mn concentration (Mn _st ) in the stationary part and the Mn concentration (Mn _co ) in the Mn concentrated part. / Mn _st ) is preferably 1.20 or less. By setting the Mn segregation ratio to 1.20 or less, a more uniform structure is obtained, unevenness in the bent portion is further less likely to occur, and the bendability is further improved. In order to make the Mn segregation ratio 1.20 or less, it is effective to perform a homogenization treatment for a predetermined time as will be described later.

(Area ratio of retained austenite: 2.0% to 15%)
The steel structure of the steel sheet is a fraction evaluated by area ratio, and the retained austenite is 2.0% or more and 15% or less. Residual austenite suppresses non-uniform deformation by the TRIP effect, and contributes to strength improvement without reducing bendability and ductility. However, when the area ratio of retained austenite becomes an excessive steel structure, the amount of martensite generated by TRIP increases, microcracks tend to occur at the structure interface, and the bendability deteriorates. For this reason, the area ratio of a retained austenite shall be 2.0% or more and 15% or less.

  Furthermore, in order to obtain excellent ductility of (TS × El) value of 12000 MPa ·% or more, the steel structure of the steel sheet is preferably a fraction evaluated by area ratio, and the area ratio of ferrite is preferably 30% or more. . By including ferrite in an area ratio of 30% or more, an excellent ductility with a (TS × El) value of 12000 MPa ·% or more can be achieved. For this reason, the area ratio of ferrite is preferably set to 30% or more.

  Furthermore, in order to obtain excellent ductility with a (TS × El) value of 16000 MPa ·% or more, the steel structure of the steel sheet is a fraction evaluated by area ratio, and the area ratio of retained austenite is 5% or more, and ferrite The area ratio is preferably 30% or more. By including residual austenite in an area ratio of 5% or more and ferrite in an area ratio of 30% or more, an excellent ductility of (TS × El) value of 16000 MPa ·% or more can be achieved. Therefore, the area ratio of residual austenite is More preferably, it is 5% or more, and the area ratio of ferrite is 30% or more.

Next, the suitable manufacturing method of the hot dip galvanized steel plate concerning this invention is demonstrated.
[Continuous casting process]
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 the solidification rate at a depth of 10 mm from the slab surface is set to 100 ° C./min or more and 1000 ° C./min or less to 200 mm or more. Continuous casting is performed on a slab having a thickness of 300 mm or less.

(Coagulation rate: 100 ° C / min to 1000 ° C / min)
The solidification rate at a position 10 mm deep from the slab surface in the continuous casting process is 100 ° C./min to 1000 ° C./min. When the solidification rate is less than 100 ° C./min, it becomes difficult to set the dendrite primary arm interval at the (1/20) depth position of the slab thickness from the slab surface to 300 μm or less, and the bendability of the steel sheet cannot be improved. There is a case. On the other hand, when the solidification rate exceeds 1000 ° C./min, surface cracking of the slab may be induced.

(Slab thickness: 200mm to 300mm)
The slab thickness is 200 mm or more and 300 mm or less. When the slab thickness is less than 200 mm, it is difficult to set the total reduction ratio in hot rolling and cold rolling described later to 99.0% or more. On the other hand, when the slab thickness exceeds 300 mm, the average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion extended in the rolling direction is (300 μm or less) at the (1/20) depth position from the steel plate surface. Difficult to do.

[Hot rolling process and cold rolling process]
The slab obtained by the continuous casting process is hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.

  Preferably, the slab obtained by the above continuous casting process is subjected to a homogenization treatment for 20 minutes or more in a temperature range of 1200 ° C. to 1350 ° C., and then a finishing temperature: 800 ° C. to 950 ° C., a coiling temperature : Hot rolling at 400 ° C. or higher and 750 ° C. or lower to obtain a hot rolled steel sheet, and cold rolling to the hot rolled steel sheet to obtain a cold rolled steel sheet. The total rolling reduction in hot rolling and cold rolling is 99.0. % Or more.

(Homogenization treatment temperature: 1200 ° C to 1350 ° C, homogenization treatment time: 20 minutes or more)
The slab to be subjected to hot rolling is preferably subjected to a homogenization treatment in which the slab is kept in a temperature range of 1200 ° C. or higher and 1350 ° C. or lower for 20 minutes or longer. By holding the slab used for hot rolling in a temperature range of 1200 ° C. or higher for 20 minutes or more, the non-uniform structure due to segregation of Mn is further eliminated, and the bendability can be further improved. The homogenization temperature is preferably 1350 ° C. or lower from the viewpoints of suppressing scale loss, preventing heating furnace damage, and improving productivity.

  More preferably, the homogenization time is 1.0 hour or more and 3 hours or less. By making the homogenization time 1.0 hour or more, the Mn segregation ratio can be 1.20 or less, and the bendability of the steel sheet can be further improved. On the other hand, by setting the homogenization time to 3 hours or less, scale loss can be suppressed, productivity can be improved, and manufacturing costs can be reduced.

(Finish temperature: 800 ° C or higher and 950 ° C or lower)
The finishing temperature is preferably 800 ° C. or higher and 950 ° C. or lower. By setting the finishing temperature to 800 ° C. or higher, deformation resistance during hot rolling is reduced, and operation can be performed more easily. In addition, by setting the finishing temperature to 950 ° C. or less, wrinkles due to scale can be more reliably suppressed, and good surface properties can be ensured.

(Winding temperature: 400 ° C or higher and 750 ° C or lower)
The winding temperature is preferably 400 ° C. or higher and 750 ° C. or lower. By setting the coiling temperature to 400 ° C. or higher, generation of hard bainite and martensite is suppressed, and subsequent cold rolling becomes easy. Moreover, by setting the coiling temperature to 750 ° C. or lower, oxidation of the steel sheet surface is suppressed, and good surface properties can be ensured.

  In the hot rolling process, it is preferable to equalize the temperature of the entire length of the rough bar by induction heating or the like with respect to the rough bar after the rough rolling and before the finish rolling, because the characteristic variation can be suppressed.

(Total reduction ratio in hot rolling and cold rolling: 99.0% or more)
The hot-rolled steel sheet obtained by the hot rolling step is usually descaled by a conventional method such as pickling, and then cold-rolled to obtain a cold-rolled steel sheet. In this case, it is preferable that the total rolling reduction in hot rolling and cold rolling is 99.0% 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
The surface unevenness generated after bending is affected not only by the fluctuation of the Mn concentration expanded in the rolling direction in the plate width direction but also by the thickness of the Mn concentrated portion in the plate thickness direction. Therefore, by reducing the thickness of the Mn enriched zone, surface irregularities after processing can be more reliably suppressed, and as a result, bendability is improved. In order to obtain such an effect, it is effective to set the total rolling reduction to 99.0% or more.

  In order to make the structure of the steel sheet after continuous annealing uniform, it is preferable that the rolling reduction of cold rolling is 30% or more. Moreover, it is preferable from the viewpoint of ensuring flatness to correct the shape by performing mild rolling with a rolling reduction of 5% or less before or after pickling. In addition, when such mild rolling is performed before pickling, pickling performance is improved, removal of surface concentrating elements is promoted, and surface properties can be improved.

[Continuous hot dip galvanizing process]
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 750 ° C. or more and 950 ° C. or less, and then [zinc plating bath temperature −20 ° C.] or more [zinc plating bath] [Temperature + 100 ° C.] The temperature is lowered to a temperature range of not higher than 100 ° C., and continuous hot dip galvanizing treatment is performed in this temperature range for 500 seconds or less, including when immersed in the plating bath. When the alloying treatment is performed, it is performed in a temperature range of 430 ° C. or more and 600 ° C. or less.

(Recrystallization annealing temperature: 750 ° C. or higher and 950 ° C. or lower)
The annealing temperature is preferably 750 ° C. or higher and 950 ° C. or lower. By setting the annealing temperature to 750 ° C. or higher, the remaining of non-recrystallized is suppressed, a uniform structure can be reliably obtained, and the bendability can be further improved. Moreover, by setting the annealing temperature to 950 ° C. or lower, it is possible to suppress damage to the annealing furnace and improve productivity.

  In order to completely remove unrecrystallized crystals and to ensure good bendability stably, the annealing time is preferably set to 10 seconds or more. Further, from the viewpoint of productivity, it is preferable that the annealing time is within 300 seconds.

  In addition, when 0.2% or more of Si is contained in order to improve ductility, in order to ensure the wettability and alloying processability of the plating and to make the hot-dip galvanized steel sheet have a good appearance, it is being annealed. The dew point is preferably −30 ° C. or higher.

  After recrystallization annealing, cooling is performed in the process of immersing in a galvanizing bath. In this case, the average cooling rate is from 1 ° C./s to 50 ° C./s from the highest temperature to 700 ° C. Subsequently, 700 The temperature from 0 ° C. to the cooling stop temperature is preferably 3 ° C./s or more and 50 ° C./s or less. The area ratio of ferrite can be easily adjusted by cooling to 700 ° C. at 1 ° C./s or more and 50 ° C./s or less. On the other hand, by cooling from 700 ° C. to the cooling stop temperature at 3 ° C./s or more, pearlite transformation leading to strength reduction can be suppressed. Further, in order to cool the cooling stop temperature to more than 50 ° C./s, it is necessary to remodel the continuous hot dip galvanizing equipment, and the manufacturing cost is 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 preferably in a temperature range of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less. When the cooling stop temperature is less than [zinc plating bath temperature −20 ° C.], heat removal at the time of entering the plating bath is large, and operation becomes difficult. On the other hand, if the cooling stop temperature is higher than [zinc plating bath temperature + 100 ° C.], the operation becomes difficult as the temperature of the plating bath increases. Hot dip galvanization is performed by immersing the annealed steel sheet in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower according to a conventional method.

(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.)
The holding time of [zinc plating bath temperature −20 ° C.] or more and [zinc plating bath temperature + 100 ° C.] or less is set to 500 seconds or less including the time of plating immersion. When the holding time exceeds 500 seconds, most of the austenite disappears and coarse cementite that hardly contributes to strength is generated, so that a desired tensile strength cannot be obtained. In order to improve ductility, the holding time is preferably set to 10 seconds or more.

(Alloying temperature: 430 ° C or higher and 600 ° C or lower)
When the alloying treatment is performed, it is performed in a temperature range of 430 ° C. or more and 600 ° C. or less after immersion in the plating bath. When the alloying treatment temperature is less than 430 ° C., unevenness of the unalloyed part occurs, and the surface properties of the steel sheet deteriorate. On the other hand, when the alloying temperature exceeds 600 ° C., most of the austenite disappears, and a desired tensile strength cannot be obtained. If the alloying treatment temperature is 500 ° C. or more and 530 ° C. or less and the alloying treatment time is 10 seconds or more and 60 seconds or less, the degree of alloying (Fe content of the plating layer) is 8 mass% or more and 13 mass%. Since it becomes easy to improve the adhesiveness of plating as below, it is preferable.

  After the continuous hot dip galvanizing treatment, it is preferable to perform temper rolling with an elongation of 0.05% or more and 1% or less. Temper rolling can suppress the occurrence of yield point elongation and can prevent seizure and galling during pressing.

  According to the manufacturing method described above, at the (1/20) depth position of the plate thickness from the steel plate surface, the average interval in the plate width direction of the Mn-concentrated portion expanded in the rolling direction is 300 μm or less, and the area ratio of retained austenite Can be easily produced.

  Thus, the present invention provides a hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more and excellent bendability, which has been difficult to manufacture by conventional techniques, and a method for manufacturing the same.

Furthermore, 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 continuously cast so that the solidification rate at a depth of 10 mm from the surface of the slab would be the conditions shown in Table 2 to produce a slab having a thickness of 245 mm. .

  Furthermore, hot rolling was performed under the conditions shown in Table 2, and then pickling was performed, and further cold rolling was performed under the conditions shown in Table 2 to obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm.

  A heat treatment test material was collected from the obtained cold-rolled steel sheet and subjected to heat treatment corresponding to a heat pattern in a continuous hot dip galvanizing facility as shown in Table 3.

  In the heat treatment after holding at the cooling stop temperature, the assumed plating bath temperature is set to 460 ° C., the temperature is cooled to this temperature for 5 seconds, held at this temperature for 10 seconds, and then cooled to room temperature at a cooling rate of 10 ° C./s. Until cooled. In order to simulate the alloying treatment, after maintaining the above-mentioned assumed plating bath temperature, the temperature is further raised to the alloying treatment temperature over 5 seconds, kept at this temperature for 10 seconds, and then a cooling rate of 10 ° C./s. At room temperature.

  The Mn distribution was investigated by EPMA analysis for cold-rolled steel sheets obtained under various production conditions. In addition, a tensile test and a bending test in which the bending ridge line is in the rolling direction were performed to evaluate the mechanical characteristics. In addition, the appearance after bending deformation was visually checked for irregularities after molding such that the bending radius was 1.0 t (= 1.2 mm). When the appearance with a bending radius of 1.0 t was good, molding was further performed so that the bending radius became 0.5 t (= 0.6 mm), and then the presence or absence of irregularities was confirmed visually.

(experimental method)
(Average solidification rate)
The cross section of the obtained slab was etched with picric acid, and at the position of 10 mm from the slab skin, five dendrite secondary arm intervals λ (μm) were measured. The cooling rate A (° C./min) within the liquidus temperature to the solidus temperature of the slab was calculated.

λ = 710 × A ^−0.39
(EPMA analysis)
The rolled surfaces of various annealed plates were ground and buffed to prepare analytical samples in which the analysis surface at the (1/20) depth position of the plate thickness was exposed from the surface, and the Mn distribution was investigated by EPMA. The beam diameter was 10 μm, a region of 500 μm in the rolling direction and a total of 8 mm in the direction perpendicular to the rolling direction was measured, and the Mn concentration distribution in the direction perpendicular to the rolling direction averaged with a width of 500 μm was analyzed. From the obtained Mn concentration distribution, the maximum value was taken as the Mn enriched part, the minimum value was taken as the stationary part, and the average interval and Mn segregation ratio of the Mn enriched part were calculated.

(Measurement of area ratio of retained austenite)
Various annealed plates are subjected to chemical polishing to reduce the thickness by (1/4), X-ray diffraction is applied to the surface after chemical polishing, the profile obtained is analyzed, and the area ratio of residual austenite is determined. It was measured.

(Measurement of ferrite area ratio)
Specimens were taken from the rolling direction of various annealed plates and the direction perpendicular to the rolling direction, and the cross section in the rolling direction and the structure of the cross section perpendicular to the rolling direction were photographed with an optical microscope or an electron microscope. The area ratio was measured.
(Tensile test)
From various annealed plates, JIS No. 5 tensile test specimens were collected from the direction perpendicular to the rolling direction, and the tensile strength (TS) and total elongation (El) were measured.
(Bending test)
Bending test pieces (width 40 mm × length 100 mm × sheet thickness 1.2 mm) having a longitudinal direction in the direction perpendicular to the rolling direction were collected from various annealed plates so that the bending ridge line was in the rolling direction. A 180 ° bending test with a 2.4 mm steel plate sandwiched was performed, and the presence or absence of cracks was confirmed visually. A 180 ° bending test with a 1.2 mm steel plate thinner by 1.2 mm than the previous test was performed on the test piece having no crack, and the presence or absence of a crack was similarly confirmed. When there was no crack, the contact bending which did not pinch | interpose a steel plate was performed, and the presence or absence of the crack was confirmed similarly.

  The minimum bend radius standardized by the plate thickness (t) by dividing the plate thickness of the steel plate that is not cracked after the test by twice the plate thickness of the bent specimen (2.4 mm) (indicated as Rmin in Table 4) Was calculated.

(Surface properties after bending deformation)
Bending test pieces (width 40 mm × length 60 mm × sheet thickness 1.2 mm) having a longitudinal direction perpendicular to the rolling direction were collected from various annealed plates so that the bending ridge line was in the rolling direction. When the minimum bending radius was 1.0 t or less, the tip was pushed with a 90 ° punch having a radius of 1.2 mm, a bending test was performed, and the presence or absence of surface irregularities was visually confirmed. Those with irregularities were judged as bad and those with no irregularities as good. Appearance is good, and when the minimum bending radius is 0.5t or less, push it in with a 90 ° punch with a radius of 0.6mm at the tip, perform a bending test, and visually check for surface irregularities did. The thing without an unevenness | corrugation was made favorable.
(Explanation of test results)
These results are shown in Table 4.

  Sample No. in Table 4 1, 2, 4, 6, 8, 10-16, 18-20, and 22 are steel plates of the present invention that satisfy all the conditions of the present invention. 3, 5, 7, 9, 17, and 21 are comparative steel plates that do not satisfy at least one of the conditions of the present invention.

  Specimen No. 1, 2, 4, 6, 8, 10-16, 18-20, and 22 steel sheets of the present invention examples all extend from the steel sheet surface in the rolling direction at a (1/20) depth position of the plate thickness. The average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion is 300 μm or less, the area ratio of retained austenite is 2.0% or more and 15% or less, the tensile strength is 590 MPa or more, and the bendability is excellent. It was.

  On the other hand, the test material No. No. 3 exceeded the upper limit of the range defined by the present invention at the cooling stop temperature in continuous hot dip galvanizing. In No. 21, the alloying temperature exceeded the upper limit of the range defined in the present invention, so the area ratio of retained austenite was low and the desired tensile strength could not be obtained.

Specimen No. Since No. 5 does not contain Bi, the average interval was over 300 μm, and the bendability was poor.
Specimen No. In No. 7, since the solidification rate in continuous casting was below the lower limit of the range defined in the present invention, the average interval was over 300 μm and the bendability was poor.

  Specimen No. No. 9 is less than the lower limit of the range specified by the present invention for the Mn content. Since the C content was below the lower limit of the range defined in the present invention, the desired tensile strength could not be obtained.

  Among the steel plates of the present invention example, the Bi content is 0.0010% or more and 0.05% or less which is the preferable range described above, and the homogenization time is 20 minutes or more which is the preferable range described above, and Mn segregation. The test material No. having a ratio of 1.20 or less and a ferrite area ratio of 30% or more is used. 1, 2, 4, 6, 8, 10-13, 15, 16, 18, and 20 have a tensile strength of 590 MPa or more, a TS × El value of 12000 MPa ·% or more, excellent ductility, and bendability. It is a very preferable steel plate.

  Among these preferable steel plates having excellent ductility and very good bendability, the test material No. No. 1 has a chemical composition in a more preferable range and an area ratio of retained austenite in a preferable range. Nos. 4, 6 and 18 are preferable steel plates having a tensile strength of 590 MPa or more, a TS × El value of 16000 MPa ·% or more, excellent ductility, and excellent bendability.

  On the other hand, the test materials 6, 13, and 22 in which the C amount and the Mn amount are in the preferred ranges are preferable steel plates that are extremely excellent in bendability with a tensile strength of 980 MPa or more.

It is explanatory drawing which shows the surface property after bending deformation.

Claims (7)

  In the hot dip galvanized steel sheet provided with a hot dip galvanized layer on the surface of the steel sheet, the steel sheet is in mass%, C: 0.03 to 0.20%, Si: 0.005 to 2.0%, Mn: 1.%. 2 to 3.0%, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.003 to 1.0%, N: 0.01% or less, Bi: 0.0001 to 0.05%, with the balance being a chemical composition composed of Fe and impurities, and residual austenite of 2 An average interval in the direction perpendicular to the rolling direction of the Mn-concentrated portion having a steel structure containing 0.0 to 15 area% and extending in the rolling direction from the steel sheet surface at a (1/20) depth position of the plate thickness. Is a hot-dip galvanized steel sheet characterized by being 300 μm or less.   The chemical composition is one selected from the group consisting of Ti: 0.05% or less, Nb: 0.05% or less, and V: 0.05% or less in mass%, instead of a part of the Fe. Or the hot dip galvanized steel plate of Claim 1 containing 2 or more types.   The chemical composition is one selected from the group consisting of Cr: 1% or less, Mo: 1% or less, Cu: 1% or less, and Ni: 1% or less in mass% instead of part of the Fe. Or the hot-dip galvanized steel sheet of Claim 1 or Claim 2 containing 2 or more types.   Instead of a part of the Fe, the chemical composition is, in mass%, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, comprising one or more selected from the group consisting of:   The hot dip galvanized steel sheet according to any one of claims 1 to 4, wherein the chemical composition contains B: 0.01% or less in mass% instead of a part of the Fe. A method for producing a hot-dip galvanized steel sheet comprising the following steps (A) to (C):
(A) A molten steel having the chemical composition according to any one of claims 1 to 5 is formed into a slab having a thickness of 200 to 300 mm by setting a solidification rate at a depth of 10 mm from the surface to 100 to 1000 ° C / min. Continuous casting process for casting;
(B) A hot rolling process and a cold rolling process in which the slab obtained by the continuous casting process is hot rolled to obtain a hot rolled steel sheet, and the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet; And (C) the cold-rolled steel sheet is subjected to recrystallization annealing in a temperature range of 750 to 950 ° C., and then cooled to a temperature range of [zinc plating bath temperature −20 ° C.] to [zinc plating bath temperature + 100 ° C.] A continuous hot dip galvanizing step that continues to be held for 500 seconds or less including the time of immersion in the plating bath in the temperature range.   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 to 600 ° C.