JP5516057B2 - High-strength hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength 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 a tensile strength of 980 MPa or more and excellent bendability, and a method for producing the same, and particularly, 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 applications in which bending forming is indispensable and a method for producing the same. Here, in the present invention, “high-strength galvanized steel sheet” includes “high-strength galvannealed steel sheet”.

  In recent years, improvement in fuel efficiency of automobiles has been demanded in order to protect the global environment. In order to reduce the weight of the vehicle body and ensure the safety of passengers, high-strength steel sheets having a tensile strength of 980 MPa or more, particularly considering rust prevention In members, there is an increasing need for high-strength hot-dip galvanized steel sheets. Furthermore, the application of a high-strength hot-dip galvanized steel sheet having a tensile strength of 1180 MPa or more in a skeleton member that protects passengers, such as a side sill, has also been studied.

  However, steel sheets used for automotive parts are not sufficient to have high strength, and satisfy various performances required when molding parts such as press formability, weldability, plating adhesion, etc. Must. In particular, considering the part forming process, bending molding is used most frequently, and thus formed into various shaped parts. Therefore, a high-strength hot-dip galvanized steel sheet having excellent bendability is required. However, bendability deteriorates as the tensile strength increases. Even if the desired bending molding is possible, the high-strength member has a large absolute value of the amount of spring back after molding, so that it is difficult to stably ensure component accuracy. In order to increase the accuracy of parts, a steel sheet is required which has a small variation in tensile strength with respect to slight variations in chemical composition and manufacturing conditions during mass production, that is, excellent material stability. However, as the tensile strength increases, not only bendability but also material stability deteriorates.

  On the other hand, the manufacturing process of the hot dip galvanized steel sheet is soaked in a hot dip galvanizing bath at 410 ° C. or higher when cooled from the recrystallization temperature, and then reheated after the immersion for the purpose of alloying as necessary Has a characteristic temperature history. In this manufacturing process, since cooling is temporarily interrupted at 400 ° C. or higher, when manufacturing a steel having a chemical composition suitable for a high-strength steel sheet, this temperature history is essentially a heat treatment condition in which bainite transformation is likely to proceed. I can say that. However, because the interruption time is relatively short, the bainite transformation is not complete in most steels. Thus, when bainite is generated in the middle, not only coarse cementite but also C is concentrated in austenite, and a structure containing island martensite is easily obtained. Coarse cementite or island-like martensite is a very strong hard phase that promotes non-uniform deformation. Therefore, it is extremely difficult to improve the bendability of the high-strength hot-dip galvanized steel sheet. Moreover, in the manufacturing process of the hot-dip galvanized steel sheet, the cooling rate from the recrystallization temperature is usually in the range of 0.1 to 50 ° C./second, which is smaller than the manufacturing process of the high-strength cold-rolled steel sheet and the tensile strength is 980 MPa or more. It is difficult to manufacture a high-strength hot-dip galvanized steel sheet.

The approach of optimizing the annealing conditions and the cooling rate for steel with a specific chemical composition is about the bendability of the high-strength hot-dip galvanized steel sheet with a tensile strength of 980 MPa or more, and further improvement of material stability. As described in Patent Document 1, it is preferable that Mn is positively added, and further that the steel added with a small amount of Ti and Nb is soaked at Ac ₃ point or higher and 900 ° C. or lower. Has been. On the other hand, in Patent Document 2, when steel with positive addition of Cr is annealed under specific conditions and the steel has a structure containing fine bainite and martensite, the bendability is not less than 980 MPa. It is described that weldability can be improved as well. However, as described above, in the case of a high-strength hot-dip galvanized steel sheet containing a large amount of Mn, the cooling is temporarily interrupted at 400 ° C. or more, whereby bainite is generated, austenite stabilization is promoted, and unstable. It becomes a structure containing a lot of austenite. Due to the shearing process, unstable austenite is transformed into extremely hard martensite on the shear-cut end face, and the bendability is significantly deteriorated. Therefore, in the technique disclosed in Patent Document 1, when the sheared shear-cut end steel plate is bent in accordance with an actual automobile member forming process, cracks are scattered and the bendability is poor. And easily expected. Furthermore, if the annealing time exceeds 150 seconds, the structure becomes coarse and the hydrogen embrittlement resistance deteriorates, so that it is not suitable as an automobile member as will be described later. Further, when Cr is positively added, grain boundary oxidation at the time of hot rolling or annealing is promoted, and the properties of the punched end face are deteriorated, and the bendability is improved even in the technique disclosed in Patent Document 2. Can not do it. Further, in the technique described in this document, since many voids are present in the vicinity of the steel plate and the plating interface due to grain boundary oxidation, it is easily expected that the plating adhesion is unsatisfactory.

  In Patent Document 3, it is described that, by adding Cr and Mo so as to have a specific ratio, plating adhesion such as powdering resistance can be improved while the tensile strength is 980 MPa or more. . However, as in the technique disclosed in Patent Document 3, if a large amount of Cr is contained, there is a concern about a decrease in bendability, and making Mo essential is not preferable from the viewpoint of economy. Furthermore, since the C content is relatively small, the material stability is deteriorated, and the tensile strength is often 980 MPa or less.

  On the other hand, high-strength steel sheets with a tensile strength of 980 MPa or more satisfy not only various performances required at the time of forming parts, but also performances required for automobile parts such as hydrogen embrittlement resistance and impact resistance characteristics. Must be something to do. In Patent Document 4, by adding Mn positively and controlling the temperature history from plating immersion to alloying treatment, the hydrogen embrittlement resistance and ductility are improved while the tensile strength is 980 MPa or more. It is stated that it can be done. However, as described above, containing a large amount of Mn is disadvantageous from the viewpoint of securing bendability.

On the other hand, there is an innovative approach that utilizes a tempered martensite structure. In Patent Document 5, by reheating after cooling to the _Ms point or lower, the bendability can be improved while the tensile strength is 980 MPa or higher. It is described. However, as in the technique disclosed in Patent Document 5, a manufacturing method that requires rapid cooling and reheating after the galvanizing step is inferior in productivity and not excellent in mass productivity. Moreover, according to the Example of patent document 5, in order to achieve a tensile strength of 980 MPa or more, it is necessary to make C substantially more than 0.20%, but such a high C content. Then, weldability will deteriorate remarkably.

  As is clear from the prior art disclosed in these patent documents, it is practically impossible to produce a steel sheet suitable for an automobile member having a tensile strength of 980 MPa or more in the existing hot-dip galvanized steel sheet manufacturing process. It was possible.

Japanese Patent Laid-Open No. 5-179402 JP 2008-280608 A JP 2006-283128 A JP-A-6-145893 JP-A-6-108152

  As described above, the present invention is a high-strength melt that has been difficult to manufacture by the conventional technology, has a tensile strength of 980 MPa or more, has excellent bendability, and satisfies the properties required for automotive parts such as weldability in a well-balanced manner. It aims at providing a galvanized steel plate and its manufacturing method. Here, “excellent bendability” means that the end surface of the test piece is left shear-cut, and no crack appears on the surface and end surface after processing at the visual level when the bending radius of the 90 ° V bending test is 2.0 t. Means that. 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 high-strength hot-dip galvanized steel sheet according to the present invention to a higher-strength automotive member such as a side sill, it is preferable that the bendability is excellent and the tensile test is 1180 MPa.

  The present invention controls the amount of C, Si, Mn, and B in the chemical composition within a very limited range and strictly applies the optimum production conditions therefor, so that the tensile strength is 980 MPa or more and the bending This is based on the knowledge that high-strength hot-dip galvanized steel sheets satisfying the necessary characteristics as automotive parts can be obtained. Conventionally, it has been impossible to provide a high-strength hot-dip galvanized steel sheet that simultaneously satisfies such characteristics.

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, and the steel sheet has C: 0.12% to 0.20%, Si: more than 0.10%, 0.40% or less Mn: 2.2% or more and 3.0 or less, P: 0.025% or less, S: 0.005% or less, sol. Al: 0.001% or more and 0.10%, B: more than 0.0010 and 0.010% or less, N: 0.01% or less , remaining Fe and impurities, and the area ratio of non-recrystallized ferrite is 0.5 %, The area ratio of retained austenite is 5.0% or less, and the tensile strength is 980 MPa or more.

  In the high-strength hot-dip galvanized steel sheet according to the present invention, the chemical composition is one selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less. It is preferable to further contain two or more kinds.

  In these high-strength hot-dip galvanized steel sheets according to the present invention, the chemical composition is from Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.5% or less, and Ni: 1.0% or less. It is preferable to further contain one or more selected from the group consisting of:

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

In these high-strength hot-dip galvanized steel sheets according to the present invention, the chemical composition preferably further contains Bi: 0.05% or less.
From another viewpoint, the present invention is the above-described method for producing a high-strength hot-dip galvanized steel sheet, comprising the following steps (A) to (C).
(A) A steel material having the chemical composition of a steel sheet in the hot-dip galvanized steel sheet according to the present invention described above, rolling start temperature: 1100 ° C. to 1300 ° C., finishing temperature: 800 ° C. to 1000 ° C., coiling temperature: 400 ° C. A hot rolling step of hot rolling at 750 ° C. or lower to obtain a hot rolled steel sheet;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to Ac ₃ points and a temperature range of 810 ° C. to 950 ° C. for 5 seconds. After annealing for not less than 150 seconds, the obtained cold-rolled annealed steel sheet has an average cooling rate from 800 ° C. to 580 ° C. of 3 ° C./second to 50 ° C./second, 400 ° C. to 560 ° C. A continuous hot dip galvanizing step of cooling to a temperature range of 400 ° C. to 600 ° C. and subsequently maintaining the temperature in a temperature range of 400 ° C. to 600 ° C. for 25 seconds to 500 seconds, including when immersed in the plating bath.

  From another point of view, the present invention provides a high strength characterized by subjecting the hot-dip galvanized steel sheet obtained by the production method according to the present invention to an alloying treatment in a temperature range of 430 ° C. or higher and 540 ° C. or lower. It is a manufacturing method of a hot-dip galvanized steel sheet.

  According to the present invention, a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, excellent bendability, and also characteristics such as weldability 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.

It is an observation image of the rolling direction cross section in an example of the steel plate containing the non-recrystallized ferrite concerning the present invention.

Hereinafter, modes for carrying out the present invention will be described.
1. Chemical Composition 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: 0.12% to 0.20%)
C is an element that contributes to strength improvement, and is contained in an amount of 0.12% or more in order to secure the material stability and to make the steel sheet have a tensile strength of 980 MPa or more. However, when C is contained exceeding 0.20%, the weldability deteriorates. For this reason, C content shall be 0.12% or more and 0.20% or less. Preferably, it is 0.19% or less. In addition, when 0.13% or more of C is contained, it becomes easy to make the tensile strength 1180 MPa or more. For this reason, the C content is preferably 0.13% or more.

(Si: more than 0.10% and less than 0.40%)
Si is an element that contributes to improving plating adhesion without significantly reducing ductility or improving ductility. In the present invention, Si is contained in an amount exceeding 0.10%. However, if Si exceeds 0.40%, the wettability of plating deteriorates, and non-plating defects occur frequently during production. For this reason, Si content shall be more than 0.10% and 0.40% or less. When 0.20% or more of Si is contained, the TRIP effect is promoted and the ductility is further improved. For this reason, it is preferable that Si content shall be 0.20% or more.

(Mn: 2.2% to 3.0%)
Mn is an element contributing to strength improvement, and is contained in an amount of 2.2% or more in order to make the steel sheet have a tensile strength of 980 MPa or more while ensuring material stability. However, if Mn exceeds 3.0%, not only melting and refining of the steel in the converter becomes difficult, but also a band structure develops and unstable austenite and MnS are generated, resulting in bendability. Deteriorates significantly. For this reason, Mn content shall be 2.2% or more and 3.0% or less. Preferably, it is 2.9% or less. In addition, when 2.4% or more of Mn is contained, it becomes easy to make the tensile strength 1180 MPa or more. For this reason, it is preferable that Mn content shall be 2.4% or more.

(P: 0.025% 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 0.025%, the weldability is significantly deteriorated. For this reason, the P content is set to 0.025% or less. Preferably, it is 0.015% or less. On the other hand, in order to strengthen the steel sheet more reliably, the P content is preferably set to 0.005% or more.

(S: 0.005% 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 0.005% or less. Preferably, it is 0.003% or less. More preferably, it is 0.0015% or less.

(Sol.Al: 0.001% or more and 0.10% or less)
Since Al is an element added to deoxidize steel and is an element that effectively acts to improve the yield of carbonitride-forming elements such as Ti, the sol.Al content is 0.001. % Or more. However, if the sol.Al content exceeds 0.10%, the weldability deteriorates and the oxide inclusions increase, so that the surface properties deteriorate. For this reason, sol.Al content shall be 0.001% or more and 0.10% or less. In addition, Preferably, they are 0.020% or more and 0.080% or less.

(B: more than 0.0010% and 0.010% or less)
B is an important element in the present invention. As will be described later, even if the tensile strength is increased by selecting the optimum production conditions, the grain boundary and the heterogeneous interface are strengthened, and the bendability and hydrogen resistance are increased. Deterioration of embrittlement characteristics is suppressed. Therefore, in order to achieve the target bendability and hydrogen embrittlement resistance, B is contained in excess of 0.0001%. However, if B is contained in an amount exceeding 0.010%, coarse precipitates are generated at the grain boundaries, and the bendability deteriorates. For this reason, B content shall be more than 0.0010% and 0.010% or less. In order to obtain the above effect more reliably, the B content is preferably 0.0015% or more in order to further increase the tensile strength to 1180 MPa or more.

(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 set to 0.01% or less. Preferably, it is 0.006% or less.

(Ti: 0.5% or less, Nb: 0.5% or less, and V: one or more selected from the group consisting of 0.5% 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 1180 MPa or more, it is effective to contain one or more of Ti, Nb and V. In order to acquire the said 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 0.5%, the inclusions containing Ti, Nb and V increase, and the surface properties deteriorate. For this reason, it is preferable that the content of Ti, Nb and V is 0.5% or less, respectively.

(Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.5% or less, and Ni: 1.0% or less selected from the group consisting of 1.0% 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 secure a tensile strength of 1180 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 0.005% or more. However, if Cr exceeds 0.5%, the bendability and plating adhesion deteriorate, and even if it exceeds 0.5%, Cu or Mo, or even if Ni exceeds 1.0%. The above effects are saturated and not only economically wasteful but also hot rolling and cold rolling become difficult. 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.

(One or two or more selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 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 either element 0.0005% or more.

(Bi: 0.05% or less)
Bi is an element that further improves the bendability and is an optional element that can be contained as necessary. However, if excessively contained, hot workability deteriorates and hot rolling becomes difficult, so the Bi content is preferably 0.05% or less. In order to acquire the said effect more reliably, it is preferable that Bi content shall be 0.0005% or more.

2. Next, the reason for limiting the steel structure of the high-strength hot-dip galvanized steel sheet according to the present invention will be described.
The high-strength hot-dip galvanized steel sheet of the present invention having the above composition has a steel structure in which the area ratio of non-recrystallized ferrite is less than 0.5% and the area ratio of retained austenite is 5.0% or less.

(Area ratio of non-recrystallized ferrite: less than 0.5%)
In order to achieve the desired bendability in a region where the tensile strength is 980 MPa or more, the area ratio of non-recrystallized ferrite is set to less than 0.5% (including the case of 0%). The non-recrystallized ferrite described here is a structure elongated in the rolling direction as observed in FIG. 1 by a scanning electron microscope (SEM).

(Austenite area ratio: 5% or less)
In order to achieve the desired bendability in a region where the tensile strength is 980 MPa or more, the area ratio of austenite is set to 5% or less (including the case of 0%). In addition, when the cooling stop temperature is set to a range described later, the average C concentration of austenite becomes 0.4 mass% or more and 1 mass% or less. Austenite having an average C concentration of 0.4% by mass or more and 1% by mass or less is unstable and effective in increasing tensile strength, but deteriorates bendability.

3. Plating film The chemical composition of the plating film of the high-strength hot-dip galvanized steel sheet according to the present invention is not particularly limited. Suitable conditions when the plating film is alloyed hot dip galvanizing are shown below.

(Fe: 8% to 15% by mass)
If the Fe content in the galvanized layer to be the coating is less than 8% by mass, a soft part is likely to be formed on the surface layer portion of the plated layer after the alloying treatment, and the slidability is lowered and the coating layer of the coating is reduced. Flaking-like peeling due to peeling from the interface with the base steel sheet increases. Therefore, the Fe content is preferably 8% by mass or more. More preferably, it is 9.5 mass% or more. On the other hand, when the Fe content exceeds 15% by mass, when the steel sheet is bent, the amount of powdering peeling due to the alloyed hot-dip galvanized layer undergoing compressive deformation inside the bent portion increases. For this reason, it is preferable that Fe content shall be 15 mass% or less. More preferably, it is 14 mass% or less.

(Al: 0.15 mass% or more and 0.50 mass%)
When the Al content in the galvanized layer to be a coating is less than 0.15% by mass, the effect of suppressing the development of the alloy layer in the plating bath becomes insufficient, and it becomes difficult to control the amount of plating adhesion. Therefore, the Al content is preferably 0.15% by mass or more. More preferably, it is 0.20 mass% or more, Most preferably, it is 0.25 mass% or more. On the other hand, when the Al content exceeds 0.50% by mass, the alloying speed is lowered, so that the alloying temperature must be higher than 540 ° C. in order to realize the Fe content at a normal line speed. In some cases, as will be described later, it becomes difficult to set the tensile strength of the steel sheet to 980 MPa or more. Therefore, the Al content is preferably 0.50% by mass or less. More preferably, it is 0.45 mass% or less, Most preferably, it is 0.40 mass% or less.

(Other)
In the alloying process, Si, Mn, P, S, Ti, Nb, V, Cr, Mo, Cu, Ni, B, Ca, REM, etc. are incorporated into the galvanized layer that forms the coating. However, there is no problem because the plating quality is not adversely affected as long as it is within the range incorporated in the plating layer when hot-dip plating and alloying are performed under normal conditions. The normal plating conditions here are, as described later, a plating bath temperature of 410 ° C. or higher and 490 ° C. or lower, a steel sheet penetration temperature of 410 ° C. or higher and 500 ° C. or lower, and an alloying temperature of 430 ° C. or higher and 540 ° C. or lower. is there.

4). Manufacturing method Next, the reason for limitation of the manufacturing method of the high intensity | strength hot-dip galvanized steel sheet of this invention is demonstrated.
It is preferable to melt the molten steel having the above-described steel composition by a generally known melting method such as a converter or an electric furnace, and to produce a steel material such as a slab by a continuous casting method. In place of the continuous casting method, an ingot casting method, a thin slab casting method, or the like may be employed. This steel material is hot rolled to obtain a hot rolled steel sheet. In hot rolling, cast steel material is not cooled to room temperature but directly fed into a heating furnace while being heated and heated and then rolled, or directly after a little heat retention Rolling may be performed, or the steel material may be once cooled and then reheated for rolling. At this time, it is preferable to equalize the entire length of the rough bar after the rough rolling and before the finish rolling by induction heating or the like because the characteristic variation can be suppressed.

(Rolling start temperature of steel material: 1100 ° C or higher and 1300 ° C or lower)
In the case of reheating, it is necessary to re-dissolve the B-based compound in order to ensure tensile strength. In the present invention, such an effect is recognized by heating to 1100 ° C. or higher, but when heated to over 1300 ° C., not only the effect is saturated but also the scale loss increases. For this reason, the reheating temperature of a steel 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. Moreover, in order to perform the said solid solution by reheating reliably, it is preferable to make this heating time into 10 minutes or more, and in order to suppress an excessive scale loss, it is preferable to set it as 10 hours or less. More preferably, it is 30 minutes or more and 5 hours or less. Of course, when performing direct feed rolling or direct rolling, as long as the B compound is in solid solution, the rolling may be started as it is. In this case, the rolling start temperature is preferably 1100 ° C. or higher and 1300 ° C. or lower. To do.

(Finish temperature: 800 ° C to 1000 ° C)
The finishing temperature of hot rolling is set to a range of 800 ° C. or higher and 1000 ° C. or lower. When the finishing temperature is less than 800 ° C., the deformation resistance during rolling is large and the operation is impossible. On the other hand, when the temperature exceeds 1000 ° C., the grain boundary oxidation becomes remarkable, and the plating adhesion deteriorates.

(Winding temperature: 400 ° C or higher and 750 ° C or lower)
The winding temperature is in the range of 400 ° C. or higher and 750 ° C. or lower. When the coiling temperature is less than 400 ° C., hard bainite and martensite are generated, and subsequent cold rolling becomes difficult and operation is impossible. On the other hand, when the coiling temperature exceeds 750 ° C., grain boundary oxidation becomes remarkable, and the plating adhesion deteriorates. Preferably, it is 500 degreeC or more and 700 degrees C or less.

  The hot-rolled steel sheet is pickled by a normal method and then cold-rolled to obtain a cold-rolled steel sheet. In order to refine the structure of the steel sheet after the continuous annealing, it is preferable that the rolling reduction of the cold rolling is 30% or more. On the other hand, when the rolling reduction exceeds 70%, the steel sheet is easily broken by cold rolling. In addition, before or after pickling, it is advantageous in terms of ensuring flatness if mild rolling of about 0 to 5% is performed and the shape is corrected. In addition, the mild rolling improves pickling properties, promotes removal of surface concentrating elements, and has an effect of improving the adhesion of hot dipping.

According to the present invention, the cold-rolled steel sheet thus obtained is subjected to annealing that is held for 5 seconds to 150 seconds at a temperature of Ac ₃ point and 810 ° C. to 950 ° C., and then from 800 ° C. to 580 ° C. The average cooling rate to 0 ° C. is set to 3 ° C./second or more and 50 ° C./second or less, and after cooling to 400 ° C. or more and 560 ° C. or less, hot dip galvanization is performed. It is preferable to perform the annealing heat treatment and hot dip galvanizing treatment by soaking for 5 seconds or more and 150 seconds or less in a temperature range of Ac ₃ points or more and 810 ° C. or more and 950 ° C. or less in a continuous hot dip galvanizing line. Hereinafter, the case where this process is performed in a continuous hot dip galvanizing line will be described as an example.

  In the present invention, since Mn is contained in a large amount and B is further contained, recrystallization of the processed ferrite is remarkably suppressed. Therefore, processing strain remains at the time of temperature rise during annealing, the remaining of non-recrystallized grains is remarkably accelerated, and bendability and toughness are affected by the annealing conditions. Therefore, the target performance is achieved under the following continuous annealing conditions.

(Annealing temperature: Ac ₃ points and 810 ° C. or higher and 950 ° C. or lower)
Annealing is performed at _three points Ac and a temperature range of 810 ° C. or more and 950 ° C. or less. If the annealing temperature is less than Ac ₃ points or less than 810 ° C., unrecrystallized remains, a uniform structure cannot be obtained, and bendability deteriorates. Moreover, by setting the annealing temperature to 950 ° C. or lower, it is possible to suppress damage to the annealing furnace and improve productivity.

(Annealing time: 5 seconds or more and 150 seconds or less)
Annealing is performed by holding at the annealing temperature for 5 seconds to 150 seconds. If the annealing time is less than 5 seconds, it becomes difficult to control the annealing temperature over the entire coil, and the transformation from processed ferrite to austenite is not sufficient, so that unrecrystallized remains, and bendability and toughness deteriorate. However, if the annealing time exceeds 150 seconds, the structure becomes coarse and the hydrogen embrittlement resistance deteriorates.

(Average cooling rate from 800 ° C to 580 ° C: 3 ° C / second to 50 ° C / second)
About the cooling after annealing, the average cooling rate from 800 degreeC to 580 degreeC shall be 3-50 degreeC / sec. The reason for prescribing the average cooling rate in the temperature range from 800 ° C. to 580 ° C. is that austenite is easily transformed into ferrite in that temperature range, and it is effective for strength adjustment by controlling the cooling rate in that temperature range. This is because the properties of ferrite can be controlled, and the tensile strength can be increased to 980 MPa or more while ensuring material stability. However, when the average cooling rate is less than 3 ° C./second, it is difficult to ensure the tensile strength. On the other hand, if it exceeds 50 ° C./sec, it becomes difficult to control the cooling rate and cooling stop temperature over the entire coil, and it cannot be operated in the continuous hot dip galvanizing line. In addition, when the average cooling rate from 800 ° C. to 580 ° C. is 6 ° C./second or more, it becomes easy to make the tensile strength 1180 MPa or more. For this reason, the average cooling rate from 800 ° C. to 580 ° C. is preferably 6 ° C./second or more and 50 ° C./second. More preferably, it is 6 ° C./second or more and 30 ° C./second or less.

(Cooling stop temperature: 400 ° C or more and 560 ° C or less)
In the present invention, the cooling stop temperature is set to a temperature range of 400 ° C. or more and 560 ° C. or less. When the cooling stop temperature is less than 400 ° C., heat removal at the time of entering the plating bath is large, and operation cannot be performed. On the other hand, when the cooling stop temperature exceeds 560 ° C., the operation becomes difficult and the precipitation of cementite that does not contribute to the strength is accelerated, so that it is difficult to ensure the strength. Furthermore, in order to suppress coarse cementite precipitation and unstable austenite formation, the temperature is preferably 540 ° C. or lower. In hot dip galvanization, the annealed cold-rolled steel sheet is immersed in a hot dip galvanizing bath at 410 ° C. or higher and 490 ° C. or lower according to a conventional method.

(Dwelling time in the temperature range of 400 ° C. or more and 600 ° C. or less: 25 seconds or more and 500 seconds or less, but also during plating immersion)
After the cooling, a hot dip galvanizing treatment and further an alloying treatment are performed as necessary. Here, since the bath temperature of the hot dip galvanizing bath is usually 410 ° C. or higher and 490 ° C. or lower, in order to avoid excessive heat removal from the hot dip galvanizing bath and difficult operation, The temperature before immersion is usually 410 ° C. or more and 500 ° C. or less. The alloying treatment temperature is preferably 430 ° C. or higher and 540 ° C. or lower as will be described later. For this reason, in order to perform a hot dip galvanization process and also an alloying process as needed, it will inevitably stay in the temperature range of 400 degreeC or more and 600 degrees C or less. However, since the temperature range is a temperature range where bainite transformation and cementite precipitation proceed most, control of the residence time in the temperature range is extremely important.

  If the residence time in the temperature range of 400 ° C. or more and 600 ° C. or less is less than 25 seconds, a large amount of the above-described unstable austenite remains and the bendability deteriorates. On the other hand, if the staying time exceeds 500 seconds, it is difficult to ensure the tensile strength. In addition, it is more preferable that the residence time in the temperature range of 400 ° C. or more and 600 ° C. or less is 30 seconds or more and 180 seconds or less from the viewpoint of securing material stability.

(Alloying temperature: 430 ° C or higher and 540 ° C or lower)
When the alloying treatment is performed after immersion in the plating bath, the alloying treatment is performed at 430 ° C. or more and 540 ° C. or less. When the alloying treatment temperature is less than 430 ° C., unalloyed treatment occurs, and the surface properties of the steel sheet deteriorate. On the other hand, when the alloying treatment temperature exceeds 540 ° C., the alloying treatment is reheated from the time of immersion in the plating bath, the bainite transformation and the cementite precipitation are accelerated, and it becomes difficult to ensure the target tensile strength. The alloying treatment conditions are preferably a temperature of 500 ° C. to 530 ° C. and a treatment time of 5 seconds to 60 seconds. Thereby, the degree of alloying (Fe content of the plating layer) is preferably about 8 to 15%.

  In addition, when heating to the annealing temperature, it is preferable that the average temperature rising rate is 1 ° C./second or more. If the average rate of temperature rise is less than 1 ° C./second, non-uniform grain growth occurs during temperature rise, resulting in a non-uniform structure, and toughness decreases.

  After the continuous hot dip galvanizing treatment, it is preferable to further perform temper rolling in the range of 0.05% to 1% elongation. Yield point elongation is suppressed by temper rolling, and yield strength is adjusted.

  In this way, by adjusting the steel composition, optimizing the continuous annealing-hot galvanizing conditions after hot rolling and cold rolling, the tensile strength is 980 MPa or more, high strength, bendability, plating adhesion, weldability, By optimizing the annealing time, a hot-dip galvanized steel sheet having good hydrogen embrittlement resistance can be obtained.

Furthermore, the present invention will be described more specifically with reference to examples.
Steels having chemical components shown in Table 1 were melted in a converter and slabs having a thickness of 245 mm were obtained by continuous casting.

  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. In the laboratory, the obtained cold-rolled steel sheet was heated, annealed and cooled under the conditions shown in Table 3. Furthermore, after that, a predetermined time (holding time before immersion in Table 3) is maintained after cooling to the start of immersion at the cooling stop temperature shown in Table 3 so as to simulate the thermal history during the alloying hot dip galvanizing process. Then, it is cooled to 460 ° C., which is an assumed plating bath temperature, over 4 seconds, held at that temperature for 2 seconds, and then heated to the alloying treatment temperature shown in Table 2 over 4 seconds to simulate the alloying treatment. Thus, it hold | maintained at the temperature for 5 second, it cooled to room temperature with the average cooling rate of 20 degree-C / sec, and produced the annealing cold-rolled steel plate. Further, the annealed cold-rolled steel sheet was temper-rolled at an elongation rate of 0.1% to prepare various test pieces for evaluation.

  The annealed cold-rolled steel sheet produced in this example is not hot dip galvanized, but receives the same thermal history as the galvannealed steel sheet, so the mechanical properties, weldability and hydrogen embrittlement resistance of the steel sheet The properties are substantially the same as galvannealed steel sheets with the same thermal history.

  The annealed cold-rolled steel sheet obtained under various production conditions was subjected to a tensile test and a bending test in which the bending ridge line is in the rolling direction, and mechanical properties were evaluated. Further, as described later, weldability and hydrogen embrittlement resistance were evaluated. Furthermore, after applying the hot dip galvanization in the continuous hot dip galvanizing line under the conditions shown in Table 4 to the cold rolled steel sheet that was the target tensile strength, bendability, weldability, and hydrogen embrittlement resistance, While confirming the presence or absence of non-plating, the plating adhesion was evaluated.

[Test method]
(Ac ₃ point measurement)
By analyzing 32 types of hot-rolled steel sheets (H1 to H32 in the table) having different chemical compositions and hot-rolling conditions shown in Table 2, and analyzing the change in expansion coefficient when heated at a temperature rising rate of 10 ° C / second, _Three points of Ac of each test steel were measured.

(Tensile test)
From each annealed cold-rolled steel sheet, a JIS No. 5 tensile specimen was taken from the direction perpendicular to the rolling direction, and TS (tensile strength) and EL (total elongation) were measured.

(Bending test)
A bending test piece (width 40 mm × length 60 mm × sheet thickness 1.2 mm) in which the direction perpendicular to the rolling direction was the longitudinal direction was taken from each annealed cold-rolled steel sheet so that the bending ridge line was in the rolling direction. The end face of the test piece was left shear cut. The tip was pushed with a 90 ° punch having a radius of 2.4 mm, a bending test was performed, and the presence or absence of cracks on the surface and the end face was visually confirmed. Those having no cracks on either the surface or the end face were considered good, and those having cracks on at least one of the surface and the end face were judged as bad.

(Weldability evaluation test)
For spot weldability, the welding electrode has a dome-shaped tip diameter of 6 mm, the applied pressure is 3.6 kN, the welding current is 7.8 kA, the pressing time is 30 cyc, the welding time is (plate thickness (mm) /0.1+3) cyc, The holding time was 5 cyc. After welding, the tensile load (TSS) by the tensile shear test of JIS Z 3136 and the tensile load (CTS) by the cross tensile test of JIS Z 3137 are measured, satisfying the TSS defined in JIS Z 3140, and ductility Those satisfying a ratio (CTS / TSS) of 0.3 or more were considered good, and those not satisfying any one were considered defective.

(Hydrogen embrittlement resistance evaluation test)
The hydrogen embrittlement resistance was evaluated by a test piece in which the same strip test piece as in the bending test was bent at an inner circumference of 10 mm, the spring back portion was tightened with a bolt, and a stress was applied to the bent portion. The test piece was immersed in 0.1 N hydrochloric acid, and a sample that did not crack even after 100 hours was considered good, and a sample that cracked within 100 hours was regarded as defective.

(Plating adhesion evaluation)
The sample subjected to the alloying treatment is 20 mm × 100 mm so that the longitudinal direction is the rolling direction.
Into a one-component epoxy structural adhesive (trade name: E-6773, manufactured by Sunstar Co., Ltd.)
) As an adhesive, stacking margin: 12.5 mm, adhesive film thickness: 200 μm, baking condition: 1
A tensile test was carried out in the longitudinal direction under conditions of 80 × 20 minutes, tensile speed: 5 mm / min, and room temperature.
The interfacial adhesion strength in this test is affected by the substrate strength due to the deformation of the base material, but is almost negligible for the base material having a YP of 350 MPa or more as in this case. As a result of the test, the strength is 20 MPa
The above were considered good and those less than 20 MPa were considered bad.

(Area ratio of non-recrystallized ferrite)
Test specimens were taken from the rolling direction of each annealed cold-rolled steel sheet and the direction perpendicular to the rolling direction, the cross section in the rolling direction and the structure of the cross section perpendicular to the rolling direction were observed with an electron microscope, and a region of 8 mm ² was photographed. Images were taken and the area ratio of unrecrystallized ferrite (non-recrystallized area ratio in Table 5) was investigated by image analysis.

(Austenite area ratio)
Each annealed cold-rolled steel sheet of 25 mm x 25 mm x 1.2 mm is chemically polished to reduce the thickness by 0.3 mm, X-ray diffraction is performed three times on the surface of the steel sheet after chemical polishing, and the resulting profile is analyzed Then, the average value of the retained austenite area ratio (residual γ area ratio in Table 5) was calculated.

(Explanation of test results)
These results are shown in Tables 5 and 6.
In addition, the numerical value underlined in Tables 1-6 has shown that content, conditions, or a mechanical characteristic shown by the numerical value is outside the range of this invention.

  Specimen Nos. 1-4, 8-10, 12, 14, 14-18, 20, 22, 26, 29, 31, 33, 34, 37 and 40 in Table 4 satisfy all the conditions of the present invention. It is the steel plate of the example of this invention.

On the other hand, the test materials No. 5, 23, 24, 30 and 35 did not have the desired tensile strength because the manufacturing conditions were outside the range defined in the present invention.
Specimens No. 7 and 28 have the target tensile strength, bendability, weldability and hydrogen embrittlement resistance because the chemical composition is outside the range specified in the invention, but the non-plating is produced by hot dip galvanizing. Occasional occurrence or poor adhesion.

Since the test materials No. 6 and 25 were out of the range defined in the present invention, the intended tensile strength could not be obtained.
Specimen Nos. 11, 13, 21, 32, and 38 are out of the range defined by the present invention in terms of manufacturing conditions, and Specimen No19 has an upper limit of the range defined by the present invention for B content. Therefore, the specimen material No. 36 had poor bendability because the Mn content exceeded the upper limit of the range defined in the present invention.

Specimen No. 27 was poor in bendability and hydrogen embrittlement resistance because the B content was below the lower limit of the range defined in the present invention.
Specimens No. 39 and 41 had poor weldability because the chemical composition was outside the range defined in the present invention.

  Among the steel plates of the present invention, the contents of C and Mn are within the above-described preferred ranges, and effective for strengthening from Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less. One or more selected from the group consisting of Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.5% or less, and Ni: 1.0% or less Specimens No. 1, 2, 8, 9, 14, 15 containing one or more selected from the group consisting of an average cooling rate from 800 ° C. to 580 ° C. of 6 ° C./second or more. , 18, 31, 37 and 40 are preferable steel plates having a tensile strength of 1180 MPa or more and excellent bendability.

Claims (6)

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: 0.12% or more and 0.20% or less, Si: more than 0.10%, 0.40% or less, Mn: 2.2% to 3.0%, P: 0.025% or less, S: 0.005% or less, sol. Al: 0.001% to 0.10% or less, B: 0.0010% greater than 0.010%, N: 0.01% or less, has a Ru chemical composition Na balance being Fe and impurities, non-recrystallized A high-strength hot-dip galvanized steel sheet characterized in that the area ratio of ferrite is less than 0.5%, the area ratio of retained austenite is 5.0% or less, and the tensile strength is 980 MPa or more.   The chemical composition further contains, in mass%, one or more selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less. The high-strength hot-dip galvanized steel sheet according to claim 1.   The chemical composition is one selected from the group consisting of, by mass%, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.5% or less, and Ni: 1.0% or less The high-strength hot-dip galvanized steel sheet according to claim 1 or 2, further comprising two or more kinds.   The chemical composition is, in mass%, one selected from the group consisting of Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.01% or less, and Zr: 0.01% or less The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3, further comprising two or more kinds. The chemical composition, in mass%, Bi: further containing 0.05% or less, high-strength galvanized steel sheet according to any one of claims 1 to 4. The method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 5, comprising the following steps (A) to (C):
(A) The steel material having the chemical composition according to any one of claims 1 to 5 has a rolling start temperature of 1100 ° C to 1300 ° C, a finishing temperature of 800 ° C to 1000 ° C, and a winding temperature of 400. A hot rolling step of hot rolling at 750 ° C to obtain a hot rolled steel sheet;
(B) a cold rolling process in which the hot-rolled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to Ac ₃ points and a temperature range of 810 ° C. to 950 ° C. for 5 seconds. After annealing for 150 seconds or less, the average cooling rate from 800 ° C. to 580 ° C. is set to 3 ° C./second or more and 50 ° C./second or less to 400 ° C. or more and 560 ° C. or less, and subsequently, A continuous hot dip galvanizing step for holding in a temperature range of 400 ° C. or more and 600 ° C. or less, including immersion in the plating bath, for 25 seconds or more and 500 seconds or less.