JP2011080126A - Hot-dip galvannealed steel sheet and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a painted metal material which has superior corrosion resistance in a flat part and high electroconductivity, and besides can be manufactured by high-speed operation, and to provide a worked article by using the same. <P>SOLUTION: The hot-dip galvannealed steel sheet is provided with a hot-dip galvannealed layer on the surface of the steel sheet; has a chemical composition including, by mass%, 0.10-0.18% C, 0.10-0.60% Si, 2.2-3.0% Mn, 0.1% or less P, 0.01% or less S, 0.01-0.10% sol. Al, 0.01% or less N, and further one or two of 0.15% or less Ti and 0.15% or less Nb in the range satisfying the following expression (1): 0.05&le;Ti+Nb/2&le;0.15; also has a steel structure containing 5.0-15% of retained austenite by an area rate; and has such mechanical properties that the tensile strength is 980 MPa or more, the total elongation is 13% or more and the yield strength is 640 MPa or less. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an galvannealed steel sheet and a method for producing the same. In particular, the present invention relates to a high-strength alloyed hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more and a method for producing the same, which is suitable for applications such as car bodies of press forming.

  In recent years, improvement in fuel efficiency of automobiles has been demanded from the viewpoint of global environmental protection. For this reason, for the purpose of reducing the weight of automobile bodies and ensuring the safety of passengers, the steel sheets used for various members are used for high-strength steel sheets having a tensile strength of 980 MPa or more, particularly for members that require rust prevention. There is a growing need for high-strength galvannealed steel sheets as steel sheets.

  As a characteristic required for a steel sheet to be used for the above-mentioned purposes, it is not sufficient that it is simply high strength, and it has necessary performance at the time of forming into a member such as press workability, and a member after forming. It is required to have performance necessary for ensuring high dimensional accuracy.

  Specifically, it is required to have the performance necessary for molding into a member due to high ductility, and the performance necessary to ensure high dimensional accuracy for the molded member due to excellent shape freezing property. It is done.

  Especially for steel plates used for long, high-strength members such as center pillars, bumpers, and rocker reinforcements, the absolute value of the springback amount after forming increases, so a better shape freezing property is required. It is said.

  The shape freezing property is relatively evaluated by the yield ratio, and the lower the yield ratio, that is, the lower the yield strength at the same tensile strength, the better the shape freezing property. However, since the absolute value of the springback amount after forming is determined by the yield strength, the absolute evaluation of the shape freezing property is made by the yield strength. Therefore, even if the yield ratio is about the same, the yield strength increases as the tensile strength increases, so the problem of shape freezing that did not surface with a high strength steel plate with a tensile strength of 590 MPa is the tensile strength. May be a serious problem for high-strength steel sheets having a thickness of 980 MPa or more.

  By the way, there are several types of high-strength steel sheets, and as a typical example, a composite structure steel sheet in which a strengthening phase such as martensite or bainite coexists in a ferrite as a parent phase is known.

  The composite structure steel sheet is mainly composed of soft ferrite, so that the yield ratio can be lowered and the work hardening index can be increased. Therefore, in general, it has all of high strength, excellent ductility, and excellent shape freezing property. It is possible.

However, in the prior art, when the tensile strength is 980 MPa or more, the high ductility and the excellent shape freezing property, which are advantages of the composite structure steel plate, are impaired.
That is, in order to increase the strength of the composite structure steel plate, it is necessary to increase the strength of the strengthening phase or the volume fraction of the strengthening phase, but there is a limit to the strength of the strengthening phase, so to secure a tensile strength of 980 MPa or more. In this case, the volume fraction of the strengthening phase has to be increased. For this reason, the volume fraction of the soft ferrite is reduced, and the ductility and the shape freezeability are impaired.

  And when applying a composite structure steel plate to an galvannealed steel plate, it becomes not only more difficult to ensure high ductility and excellent shape freezing property, but also to ensure a tensile strength of 980 MPa or more. It was also difficult.

  That is, the manufacturing process of the alloyed hot-dip galvanized steel sheet is a temperature in which it is immersed in a hot-dip galvanizing bath at about 460 ° C. and then reheated to about 500 ° C. for alloying in the cooling process after recrystallization annealing. Requires history. In such a manufacturing process, since cooling is temporarily interrupted in a temperature range of 400 ° C. or higher, bainite transformation and cementite precipitation are promoted, and it is necessary to realize high strength, excellent ductility, and excellent shape freezing property. It is difficult to secure a strengthening phase such as martensite and retained austenite.

Under such circumstances, a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more has been proposed.
For example, Patent Document 1 discloses that a cold-rolled steel sheet having a predetermined component composition is subjected to reduction annealing in a two-phase temperature range in which ferrite and austenite coexist, thereby forming a composite structure mainly containing ferrite and containing 3% or more of austenite. A method is disclosed.

On the other hand, a high-strength hot-dip galvanized steel sheet with a tensile strength of 980 MPa or more has been proposed, focusing on reducing the yield ratio.
For example, Patent Document 2 discloses that a cold-rolled steel sheet having a predetermined component composition is annealed to a single phase region of austenite and rapidly cooled to a temperature range of 450 ° C. or less, so that bainitic ferrite is mainly used, and residual austenite is 5%. A method of forming a composite tissue containing at least% is disclosed.
Japanese Patent Laid-Open No. 2004-2111157 JP-A-2005-240178

  As described above, high-strength hot-dip galvanized steel sheets having a tensile strength of 980 MPa or more have been proposed in the past, but it is said that practically sufficient high strength, excellent ductility, and excellent shape freezing properties have been obtained. hard.

  That is, when a steel sheet having a component composition containing a large amount of an alloy element as disclosed in Patent Document 1 is annealed in a two-phase temperature range, non-recrystallization tends to remain, and fluctuations in tensile strength and yield strength are likely to occur. Therefore, the amount of spring back at the time of press molding varies, and molding defects tend to occur.

  In addition, the technique disclosed in Patent Document 1 does not focus on lowering the yield strength, and the yield strength increases when unrecrystallized remains, so that the steel sheet disclosed in Patent Document 1 increases. The shape freezing property is assumed to be essentially bad.

Moreover, although the technique disclosed in Patent Document 2 aims at lowering the yield ratio, the yield strength is 750 MPa or more, and it cannot be said that excellent shape freezing property is obtained.
The present invention has been made in view of the above-described prior art, and provides an alloyed hot-dip galvanized steel sheet having high tensile strength of 980 MPa or more and excellent ductility and shape freezing property, and a method for producing the same, which has been difficult in the past. The task is to do.

  In the steel sheet of the present invention, the ductility is based on the total elongation in the tensile test as an index, and the target value is 13% or more. The shape freezing property uses the yield strength as an index, and the target value is 640 MPa or less. Preferably it is 600 MPa or less.

  In order to apply the steel plate of the present invention to a part having a higher strength and a complicated shape such as a cross member which is a representative member of an automobile reinforcing member, in addition to the above characteristics, the hole expansion defined in JFS T 1001 It is preferable to have excellent stretch flangeability with a rate of 25% or more.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have obtained the following new knowledge.
When steel sheet containing a large amount of Ti and / or Nb and containing a predetermined amount of Si is subjected to a predetermined heat treatment for annealing in the austenite region, strengthening action by Ti, and / or Nb carbides, nitrides and carbonitrides High tensile strength is obtained in combination with the refinement of the steel structure. Furthermore, the refining action of the steel structure and the residual austenite generating action due to the inclusion of a predetermined amount of Si promote the generation of massive retained austenite that causes work-induced transformation in the early stage of deformation, thereby High ductility and low yield strength are obtained. Furthermore, by the refinement effect of the steel structure, good plating wettability and alloying processability are ensured despite containing a predetermined amount of Si, thereby obtaining a good surface appearance.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.
(1) An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in mass%, C: 0.10% to 0.18%, Si: 0.00. 10% or more and 0.60% or less, Mn: 2.2% or more and 3.0% or less, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.01% or more and 0.10 % Or less and N: 0.01% or less, and further, one or two of Ti: 0.15% or less and Nb: 0.15% or less are contained within a range satisfying the following formula (i). The alloyed hot-dip galvanized steel sheet has a chemical composition and an area ratio of retained austenite of 5.0% or more and 15% or less. The alloyed hot-dip galvanized steel sheet has a tensile strength of 980 MPa or more and a total elongation of 13% or more. Alloyed molten alloy having a mechanical property of yield strength of 640 MPa or less Plated steel sheet.
0.05 ≦ Ti + Nb / 2 ≦ 0.15 (i)

  (2) The galvannealed steel sheet according to (1), wherein the chemical composition further contains B: 0.01% by mass or less.

(3) The galvannealed steel sheet according to (1) or (2) above, wherein the C concentration in the retained austenite is 0.85% by mass or less.
(4) The alloyed hot-dip galvanized steel sheet according to (3) above, which has mechanical properties such that a hole expansion ratio measured by a method defined in JFS T 1001 is 25% or more.

(5) A method for producing an alloyed hot-dip galvanized steel sheet comprising the following steps (A) to (C):
(A) The steel material having the chemical composition described in (1) or (2) above is set to 1120 ° C to 1300 ° C, finishing temperature: 800 ° C to 950 ° C, winding temperature: 450 ° C to 720 ° C Hot rolling process to hot rolled steel sheet by hot rolling;
(B) a cold rolling step in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to a temperature range of Ac ₃ points to 950 ° C. for 5 seconds to 200 seconds. It is annealed to stay below, and is cooled to 560 ° C. with an average cooling rate in the temperature range of 600 ° C. to 750 ° C. being 3 ° C./second to 50 ° C./second, and the residence time in the temperature range of 400 ° C. to 600 ° C. A continuous annealing-alloying hot dip galvanizing process in which galvanizing is performed for 500 seconds or less and alloying is performed in a temperature range of 430 ° C. to 600 ° C. and then cooled to room temperature.

  According to the present invention, a high-strength galvannealed steel sheet having a tensile strength of 980 MPa or more and excellent in ductility and shape freezing property can be obtained stably. The alloyed hot-dip galvanized steel sheet according to the present invention can be widely used industrially, particularly in the automobile field.

1. Steel composition The reason why the chemical composition of the steel sheet of the present invention is defined as described above will be described. In this specification, “%” defining the chemical composition of the steel sheet is “mass%”.

C: 0.10% or more and 0.18% or less C is an element having an effect of enhancing the tensile strength of the steel sheet and promoting the formation of retained austenite. If the C content is less than 0.10%, it may be difficult to ensure a tensile strength of 980 MPa or more and to secure retained austenite of 5.0 area% or more. Therefore, the C content is 0.10% or more. Preferably it is 0.12% or more. On the other hand, if the C content exceeds 0.18%, the weldability is significantly deteriorated. Therefore, the C content is 0.18% or less.

Si: 0.10% or more and 0.60% or less Si increases the tensile strength of the steel sheet by solid solution strengthening, further improves ductility by the action of increasing the area ratio of retained austenite, and Ti and / or Nb described later. Combined with the refinement of the steel structure by the element, it is an element having the action of reducing the yield strength. If the Si content is less than 0.10%, it may be difficult to obtain the effect of the above action. Therefore, the Si content is set to 0.10% or more. Preferably it is 0.20% or more. On the other hand, if the Si content exceeds 0.60%, the surface wettability of the galvannealed steel sheet is deteriorated by reducing the wettability of the plating and the alloying processability, and the C concentration in the retained austenite is reduced. It may be excessively increased and the hole expandability may be deteriorated. Therefore, the Si content is not more than 0.60%.

  In the present invention, as described later, by including Ti and / or Nb, a good surface appearance is secured for the galvannealed steel sheet despite containing 0.10% or more of Si. It is possible to do.

Mn: 2.2% or more and 3.0% or less Mn increases the tensile strength of the steel sheet by solid solution strengthening and expands the suitable annealing temperature range by lowering the Ac ₃ point of the steel to increase the production stability. It is an element having an action. If the Mn content is less than 2.2%, it may be difficult to obtain the effect by the above action. Therefore, the Mn content is set to 2.2% or more. Preferably it is 2.3% or more. On the other hand, if the Mn content exceeds 3.0%, the ductility may be degraded and the yield strength may be significantly increased. Therefore, the Mn content is 3.0% or less. Preferably it is 2.8% or less.

P: 0.1% or less P is an element that is generally contained as an impurity. However, since P is also an element that has an effect of increasing the tensile strength of a steel sheet by solid solution strengthening, it may be positively 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 more reliably obtain the effect of increasing the tensile strength by the solid solution strengthening, the P content is preferably set to 0.002% or more.

S: 0.01% or less S is an element contained as an impurity. If the S content exceeds 0.01%, the hole expandability and weldability are significantly deteriorated. Therefore, the S content is 0.01% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less.

sol. Al: 0.01 to 0.1%
Al is an element that has the effect of making the steel material sound by deoxidizing the steel, and that effectively acts to improve the yield of carbonitride-forming elements such as Ti. sol. If the Al content is less than 0.01%, it may be difficult to obtain the effect of the above action. Therefore, sol. The Al content is 0.01% or more. Preferably it is 0.02% or more. On the other hand, sol. If the Al content exceeds 0.1%, the oxide inclusions increase remarkably, and the surface properties and molding processability may be deteriorated. In addition, the manufacturing cost increases. Therefore, sol. Al content shall be 0.1% or less. Preferably it is 0.08% or less.

N: 0.01% or less N is an element generally contained as an impurity, but in the present invention in which Ti and / or Nb is contained as described later, in the steel sheet, Ti, Nb, or Ti- Since it is also an element having an effect of increasing the tensile strength of the steel sheet by forming Nb composite nitride or carbonitride, it may be actively contained. However, if the N content exceeds 0.01%, the formation of coarse TiN in the steel sheet may lead to significant deterioration of hole expansibility. Therefore, the N content is 0.01% or less. In order to more reliably obtain the effect of increasing the tensile strength, the N content is preferably set to 0.0005% or more.

Ti: 0.15% or less, Nb: 0.15% or less, 0.05 ≦ Ti + Nb / 2 ≦ 0.15
Ti and Nb are elements having an effect of increasing the tensile strength of the steel sheet by forming carbide, nitride or carbonitride in the steel sheet. Furthermore, it has the effect of refining the steel structure, and in combination with the above-described retained austenite promoting action by Si, the formation of massive retained austenite that causes work-induced transformation in the early stage of deformation is promoted, and thereby high ductility and Low yield strength is obtained. Furthermore, it is possible to improve the wettability of plating and alloying processability due to the above-mentioned refinement of the steel structure, and it is good for galvannealed steel sheets despite containing 0.10% or more of Si. It is possible to ensure the surface appearance.

  In order to acquire the effect by the said effect | action, while containing 1 type or 2 types of Ti and Nb, it is set to 0.05 or more by the value of Ti + Nb / 2. Preferably, it is 0.06 or more. On the other hand, even if the Ti content exceeds 0.15%, the Nb content exceeds 0.15%, or the value of Ti + Nb / 2 exceeds 0.15, the effect of the above action is saturated and mischievous. Incurs an increase in cost. Therefore, the Ti content is 0.15% or less, the Nb content is 0.15% or less, and the value of Ti + Nb / 2 is 0.15 or less.

B: 0.01% or less B is an optional element and may be contained because it is an element having an effect of increasing the tensile strength of the steel sheet by transformation strengthening. However, if the B content exceeds 0.01%, the ductility of the steel sheet deteriorates significantly. Therefore, the B content is 0.01% or less. In order to more reliably obtain the effect of the above action, the B content is preferably set to 0.0003% or more.

2. Next, the reason why the steel structure of the steel sheet of the present invention is defined as described above will be described.

Area ratio of retained austenite: 5.0% to 15% The steel sheet of the present invention has a steel structure in which the area ratio of retained austenite is 5.0% to 15%.
In the present invention, as described above, the steel structure refinement effect by Ti and / or Nb and the residual austenite generation effect by Si promote the generation of massive retained austenite that undergoes work-induced transformation in the early stage of deformation. Not only is ductility improved, but yield strength is reduced. Therefore, the area ratio of retained austenite is set to 5.0% or more in order to obtain the effect by the above action. On the other hand, retained austenite, when processed, transforms to hard martensite by processing-induced transformation, has the effect of promoting microcracking and degrading hole expansibility. Therefore, the area ratio of retained austenite is set to 15% or less.

  Here, if the C concentration in the retained austenite is low, the hardness of martensite generated by the processing-induced transformation is lowered, so that it is possible to suppress the deterioration of the hole expansibility. Therefore, in order to ensure good hole expansibility, the C concentration in the retained austenite is preferably 0.85% by mass or less. The lower limit of the C concentration in the retained austenite is not particularly limited, but in order to make the C mass in the austenite less than 0.5%, the alloying treatment temperature is increased or the alloying treatment time is increased. Therefore, it becomes difficult to set the Fe content in the galvannealed layer to about 8 to 13% by mass, which is an appropriate degree of alloying. Therefore, the C concentration in the retained austenite is preferably 0.5% by mass or more.

3. Production Method Next, a preferred production method for the high-strength galvannealed steel sheet of the present invention will be described.
It is preferable that molten steel having the above chemical composition is melted by a known melting method such as a converter or an electric furnace, and a steel material such as a slab is formed 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.

(Hot rolling process)
The steel material thus obtained is hot-rolled to obtain a hot-rolled steel sheet. In hot rolling, cast steel material is charged directly into a heating furnace without being cooled to room temperature, heated in a heating furnace and then rolled, or rolled immediately after a little heat retention. Direct rolling may be performed, or rolling may be performed by reheating a cold piece once cooled to room temperature. At this time, when the hot rolling process is composed of a rough rolling process and a finish rolling process, the characteristics of the rough bar before the rough rolling after the rough rolling and the finish rolling are equalized by induction heating or the like. It is preferable because fluctuation can be suppressed.

Rolling start temperature of steel material: 1120 ° C. or higher and 1300 ° C. or lower In the present invention, Ti and / or Nb carbides, nitrides or carbonitrides are formed in the steel plate, thereby increasing the tensile strength of the steel plate and By remarkably reducing the particle size, the wettability and alloying processability of plating are enhanced. Therefore, when coarse TiC and NbC are present in the steel material to be subjected to hot rolling, it is important that these are brought into a solid solution state as much as possible before being subjected to hot rolling.

  If the temperature of the steel material used for hot rolling is less than 1120 ° C, it may be insufficient to make TiC or NbC into a solid solution state. Therefore, the temperature of the steel material used for hot rolling is preferably 1120 ° C. or higher. On the other hand, when the temperature of the steel material subjected to hot rolling exceeds 1300 ° C., the yield decreases due to scale loss. Therefore, the temperature of the steel material used for hot rolling is preferably 1300 ° C. or less.

  In addition, when reheating a cold piece and using it for hot rolling, in order to make TiC and NbC into a solid solution state more reliably, the time for staying in a temperature range of 1120 ° C. or higher is set to 10 minutes or more. Is preferred. More preferably, it is 30 minutes or more. When direct rolling or direct rolling is performed, rolling may be started as it is as long as TiC and NbC are dissolved, but in that case as well, the rolling start temperature is preferably set to the above temperature range. In addition, from the viewpoint of suppressing yield reduction due to scale loss, the time for which the temperature is maintained is preferably 3 hours or less. More preferably, it is 2 hours or less.

Finishing temperature: 800 ° C. or higher and 950 ° C. or lower The hot rolling finishing temperature is preferably 800 ° C. or higher and 950 ° C. or lower.
When the finishing temperature is less than 800 ° C., deformation resistance during hot rolling becomes excessive, and operation may be difficult. Accordingly, the finishing temperature is preferably 800 ° C. or higher. On the other hand, when the finishing temperature is higher than 950 ° C., it is difficult to control the coiling temperature described later, and the operation may be difficult. Therefore, the finishing temperature is preferably 950 ° C. or lower.

Winding temperature: 450 ° C. or higher and 720 ° C. or lower The hot rolling winding temperature is preferably 450 ° C. or higher and 720 ° C. or lower.
When the coiling temperature is less than 450 ° C., hard bainite and martensite are generated, and the subsequent cold rolling may be difficult. Therefore, the winding temperature is preferably 450 ° C. or higher. More preferably, it is 520 degreeC or more. On the other hand, when the coiling temperature exceeds 720 ° C., the precipitates of Ti and Nb are coarsened, and the effect of increasing the tensile strength of the steel sheet and the effect of increasing the wettability of plating and the alloying processability can be obtained. It can be difficult. Therefore, the winding temperature is preferably 720 ° C. or lower. More preferably, it is 690 degrees C or less.

(Cold rolling process)
The hot-rolled steel sheet obtained by the hot rolling process is subjected to descaling treatment by a conventional method such as pickling, and then cold-rolled to obtain a cold-rolled steel sheet.

  The cold rolling process may be performed by a conventional method, but in order to further refine the steel structure of the steel sheet after being subjected to the continuous annealing described later, it is preferable that the rolling reduction of the cold rolling is 30% or more. Further, from the viewpoint of avoiding an excessive load on the cold rolling equipment, the rolling reduction of the cold rolling is preferably 70% or less.

Incidentally, modifying the shape by performing a mild rolling about 0-5% before or after pickling, from the viewpoint of flatness securing. In addition, the mild rolling improves pickling properties, promotes removal of surface enriching elements, and provides an effect of improving the plating adhesion of the alloyed hot dip galvanizing.

(Continuous annealing-alloying hot dip galvanizing process)
The cold-rolled steel sheet obtained by the cold rolling process is annealed to stay in a temperature range of Ac ₃ points or more and 950 ° C. or less for 5 seconds or more and 200 seconds or less, and the average cooling rate in the temperature range of 600 ° C. or more and 750 ° C. or less is set. It is cooled to 560 ° C. at 3 ° C./second or more and 50 ° C./second or less, and a hot-dip galvanizing treatment is performed with a stay time in the temperature range of 400 ° C. or more and 600 ° C. or less being 500 seconds or less, It is preferable to apply a crystallization treatment and then cool to room temperature to obtain an alloyed hot-dip galvanized steel sheet. Hereinafter, the case where the said process is performed in a continuous hot dip galvanizing line is demonstrated as an example.

  In the present invention, since Ti and / or Nb are contained in a large amount, recrystallization of the processed ferrite is remarkably suppressed. Therefore, processing strain remains up to the austenite region at the time of temperature rise during annealing, and the phase transformation to austenite is significantly promoted. Therefore, a desired structure is achieved under the following continuous annealing conditions.

Annealing: Ac in a temperature range of not lower than ₃ points 950 ° C. or less than 200 seconds 5 seconds or more stay annealing is preferably carried out by stay 200 seconds or more for 5 seconds or less in a temperature range above ₃ points 950 ° C. or less of Ac. If the annealing temperature is less than Ac ₃ points, an unrecrystallized structure remains and a uniform steel structure cannot be obtained, and it may be difficult to ensure high ductility and low yield strength. On the other hand, if the annealing temperature exceeds 950 ° C., the precipitates become coarse and fine precipitates cannot be obtained, and it may be difficult to ensure the target tensile strength after continuous annealing.

  Further, if the time for staying in the above temperature range is less than 5 seconds, the transformation from the processed ferrite, which is a cold-rolled structure, to austenite does not proceed sufficiently, and an unrecrystallized structure remains and a uniform steel structure is obtained. In some cases, it becomes difficult to ensure stable mechanical properties. On the other hand, if the time for staying in the temperature range exceeds 200 seconds, the grain growth proceeds excessively, the steel structure becomes coarse, and it may be difficult to ensure the target tensile strength after continuous annealing. The time for staying in the temperature range is preferably 120 seconds or less from the viewpoint of productivity.

Average cooling rate in the temperature range above 600 ° C. and below 750 ° C .: 3 ° C./second or more and 50 ° C./second or less After annealing, it is cooled to a temperature range near the hot dip galvanizing bath in order to perform hot dip galvanizing treatment and alloying treatment However, this cooling is preferably performed at an average cooling rate in the temperature range of more than 600 ° C. and not more than 750 ° C. at 3 ° C./second or more and 50 ° C./second or less.

  In the steel type containing a large amount of Ti and / or Nb as in the present invention, the transformation from austenite to ferrite proceeds most in the temperature range of more than 600 ° C. and 750 ° C. or less. For this reason, by controlling the cooling rate in the temperature range, it is possible to effectively control the steel structure in order to obtain the target mechanical characteristics.

  When the average cooling rate in the temperature range of more than 600 ° C. and less than 750 ° C. is less than 3 ° C./second, the grain growth proceeds excessively and the steel structure becomes coarse, and it is difficult to ensure the target tensile strength after continuous annealing. It may become. Therefore, it is preferable that the average cooling rate in the temperature range of more than 600 ° C. and not more than 750 ° C. is 3 ° C./second or more. On the other hand, if the average cooling rate in the temperature range of more than 600 ° C. and less than 750 ° C. is more than 50 ° C./second, it is difficult to control the cooling rate over the entire coil, and operation may be difficult. Therefore, it is preferable that the average cooling rate in the temperature range of more than 600 ° C. and 750 ° C. or less is 50 ° C./second or less.

Cooling stop temperature: 560 ° C or less Usually, the heat input to the hot dip galvanizing bath, which is 410 ° C or more and 490 ° C or less, is avoided and the operation becomes difficult, and the purpose is to suppress bainite transformation and cementite precipitation. The above cooling is preferably performed up to 560 ° C. in order to ensure the strength of the above, and to secure a retained austenite of 5.0 area% or more to ensure high ductility and low yield strength. From the viewpoint of ensuring good hole expansibility by suppressing the precipitation of coarse cementite and suppressing the increase in the C concentration in the retained austenite, it is more preferable to perform the cooling to 530 ° C.

  The lower limit of the cooling stop temperature is not particularly specified, but the steel plate before dipping in the hot dip galvanizing bath is avoided in order to avoid excessive heat removal from the hot dip galvanizing bath, which is usually 410 ° C. or higher and 490 ° C. or lower. It is preferable that the cooling is stopped in a temperature range of 400 ° C. or higher because the temperature is restricted, and it is disadvantageous in terms of cost to perform the heating treatment in the process from cooling to immersion in the hot dip galvanizing bath.

Residence time in a temperature range of 400 ° C. or more and 600 ° C. or less: 500 seconds or less After the cooling, hot dip galvanizing treatment and alloying treatment are performed. 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 400 ° C. or higher. The alloying treatment temperature is preferably 430 ° C. or more and 600 ° C. or less as will be described later. For this reason, in order to perform a series of processes of the hot dip galvanizing process and the alloying process, it is unavoidably kept in a temperature range of 400 ° C. or more and 600 ° C. or less. However, since the temperature range is a temperature range in which bainite transformation and cementite precipitation proceed, if the stay in the temperature range for a long time, the intended tensile strength becomes difficult and more than 5.0 area% retained austenite It is difficult to ensure the high ductility and low yield strength. Accordingly, the residence time in the temperature range of 400 ° C. or more and 600 ° C. or less, that is, stay in the temperature range of 400 ° C. or more and 600 ° C. or less in the process of cooling to normal temperature through the hot dip galvanizing treatment and alloying treatment after the start of cooling after annealing. The time is 500 seconds or less. From the viewpoint of productivity, it is preferably 200 seconds or shorter. If the hot-dip galvanizing treatment and alloying treatment are possible, the shorter the staying time is preferable, but in order to ensure a good appearance, it is preferable to make the temperature before immersion in the hot-dip galvanizing bath constant, From such a point of view, the staying time is preferably 20 seconds or longer.

Alloying temperature: 430 ° C. or higher and 600 ° C. or lower The alloying temperature is preferably 430 ° C. or higher and 600 ° C. or lower.
When the alloying treatment temperature is less than 430 ° C., unalloyed treatment occurs, and the surface properties of the steel sheet may deteriorate. Therefore, the alloying treatment temperature is preferably 430 ° C. or higher. On the other hand, when the alloying treatment temperature exceeds 600 ° C., bainite transformation and cementite precipitation are promoted, and it becomes difficult to secure the target tensile strength, and it is possible to secure a retained austenite of 5.0 area% or more. It may become difficult to secure the desired yield strength. Therefore, the alloying treatment temperature is preferably 600 ° C. or lower. In order to set the Fe content in the alloyed hot-dip galvanized layer to an appropriate degree of alloying of 8 to 13% by mass, the alloying treatment temperature is set to a temperature range of 500 ° C. or higher and 530 ° C. or lower, and stays in the temperature range. More preferably, the alloying treatment time is 10 seconds or more and 60 seconds or less.

  In addition, it is preferable that the average temperature increase rate at the time of annealing shall be 1 degree-C / sec or more. By setting the average heating rate to 1 ° C./second or more, non-uniform grain growth during heating can be suppressed, and the steel structure can be made more uniform, thereby further improving the hole expandability. Can do.

  Moreover, after the alloying treatment, it is preferable to perform skin pass rolling with an elongation of 0.05% or more and 1% or less. Yield point elongation is suppressed by skin pass rolling, and the yield strength can be easily adjusted, and the desired mechanical properties can be easily secured.

  Thus, the area ratio of retained austenite is 5.0% or more and 15% by adjusting the chemical composition of steel, hot rolling, cold rolling, and appropriate conditions of continuous annealing-alloyed hot dip galvanizing. A steel structure can be obtained, and an alloyed hot-dip galvanized steel sheet having a high tensile strength of 980 MPa or more and excellent in ductility and shape freezing property is obtained. Furthermore, by optimizing the cooling stop temperature in the continuous annealing, the C concentration in the retained austenite can be made 0.85% by mass or less, and an alloyed hot-dip galvanized steel sheet with good stretch flangeability can be obtained.

  Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace to produce a 20 mm thick slab. The obtained slab was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 3.5 mm. The obtained hot-rolled steel sheet was pickled and cold-rolled to produce a 1.2 mm-thick cold-rolled steel sheet. The obtained cold-rolled steel sheet was subjected to heat treatment under the conditions shown in Table 2 simulating the heat treatment conditions of the continuous hot-dip galvanizing equipment. That is, it is heated to the annealing temperature shown in Table 2 at the heating rate shown in Table 2, and held at that temperature for the annealing time shown in Table 2, and then cooled to the cooling stop temperature shown in Table 2 at the cooling rate shown in Table 2. Then, the temperature is held for the cooling stop holding time shown in Table 2, and then cooled to the bath temperature shown in Table 2 over 7 seconds and held at the temperature for the bath temperature holding time shown in Table 2, Heating to the alloying treatment temperature shown in Table 2 over 5 seconds, holding at that temperature for 14 seconds, then cooling to room temperature at a cooling rate of 5 ° C./second, and then partially temper rolling shown in Table 2 Annealed cold-rolled steel sheet was prepared by temper rolling at an elongation rate.

  The annealed cold rolled steel sheet produced in this example is not hot dip galvanized, but receives the same thermal history as the alloyed hot dip galvanized steel sheet, so the structure and mechanical properties of the steel sheet have the same thermal history. It is substantially the same as the alloyed hot-dip galvanized steel sheet.

In addition to measuring Ac ₃ points of steel slabs having the chemical composition shown in Table 1, the annealed cold-rolled steel sheet was analyzed for steel structure by X-ray diffraction, and subjected to a tensile test and a hole expansion test to obtain mechanical properties. Evaluated. The results are also shown in Table 2.

  Furthermore, for steel grades that have achieved the desired mechanical properties for annealed cold-rolled steel sheets, hot-dip galvanized steel sheets are manufactured using the cold-rolled steel sheets, and the plating wettability is evaluated. The obtained galvanized steel sheet was evaluated for alloying processability. The results are shown in Table 3.

[Test method]
(Ac ₃ points)
By using a cold-rolled steel plate having a chemical composition shown in Table 1, the change in the expansion coefficient when heated at a heating rate of 10 ° C./second was measured to measure Ac ₃ points of each test steel.

(Residual austenite area ratio and C concentration in retained austenite)
One side of each 25 mm × 25 mm × 1.2 mm annealed cold-rolled steel sheet is chemically polished to reduce the thickness by 0.3 mm, and X-ray diffraction is performed three times on the steel sheet surface after chemical polishing. Analysis was performed, and the average value of the retained austenite area ratio (residual γ area ratio) and the C concentration in the retained austenite (residual γ C concentration) was calculated.

(Mechanical properties)
JIS No. 5 tensile test specimens were taken from the direction perpendicular to the rolling direction, and the yield strength (YS), tensile strength (TS), and elongation (El) were measured. Moreover, the hole expansion rate (HER) was measured by the method prescribed | regulated to JFST1001.

(Plating wettability)
Steel plates No. 2 having the chemical compositions of steel types A, C to G and M shown in Table 1 are shown in Table 2. Cold-rolled steel sheets of 1, 5, 6, 10, 12, 13 and 20 are 10 ° C./second in a mixed gas atmosphere having a dew point of −30 ° C., containing 10% by volume of hydrogen, and the balance being nitrogen. To 880 ° C., hold at 880 ° C. for 60 seconds, cool to 460 ° C. at 10 ° C./second, hold at 460 ° C. for 20 seconds, and hot-dip galvanizing at 460 ° C. with an Al concentration of 0.13 mass% A hot-dip galvanized steel sheet was prepared by immersing in a bath for 3 seconds and cooling to room temperature at 10 ° C./second. The appearance of the obtained hot-dip galvanized steel sheet was visually observed. Those with no plating were considered good wettability, and those with no plating were considered poor wettability.

(Alloyability)
By using a hot dip galvanized steel sheet that was confirmed to have good plating wettability in the above test, it was immersed in a salt bath at 400 ° C. and 500 ° C. for 40 seconds, thereby simulating hot galvanizing at each temperature. An alloyed hot-dip galvanized steel sheet in which the Fe concentration in the alloyed hot-dip galvanized film is 10% by mass or more is good, and an alloyed hot-dip galvanized steel sheet in which the Fe concentration is less than 10% by weight is poor in alloying processability. did.

In addition, underlined contents, conditions, characteristic values, and the like in each table indicate that they are outside the specified range of the present invention or when desired characteristics are not obtained.
As shown in Table 2, the steel plate Nos. 2, 8, 14 and 21 of the comparative examples are out of the scope of the present invention and the desired tensile strength cannot be obtained. Steel plate Nos. 3, 11 and 22 have manufacturing conditions that are out of the scope of the present invention, and not only the desired tensile strength can be obtained, but also the desired austenite area ratio cannot be obtained, so that the desired yield strength can be obtained. I can't. Steel plate No. 7 has manufacturing conditions outside the scope of the present invention, and a desired yield strength cannot be obtained, and a desired ductility cannot be obtained. Steel plate No. 4 has a chemical composition that is out of the scope of the present invention, and not only a desired tensile strength cannot be obtained, but also a desired austenite area ratio cannot be obtained, so that a desired yield strength cannot be obtained. Steel plates No. 5, 15, 17, and 19 are out of the scope of the present invention in chemical composition, and a desired tensile strength cannot be obtained. Steel plate No. 18 has a chemical composition that is out of the scope of the present invention, and a desired yield strength cannot be obtained, and a desired ductility cannot be obtained. As shown in Table 3, the steel plate No. Nos. 5 and 12 have chemical compositions that are out of the scope of the present invention, have poor plating wettability, and are difficult to perform hot dip galvanizing treatment.

  On the other hand, as shown in Table 2, the steel sheet of the present invention example has a steel structure containing 5.0 to 15% of retained austenite in area% and despite having a high tensile strength of 980 MPa or more. In addition to having excellent ductility with an elongation of 13% or more, it has excellent shape freezing property with a yield strength of 640 MPa or less. Moreover, as shown in Table 3, the galvannealed steel sheet simulating the alloying treatment at 500 ° C., which has good plating wettability in the hot dip galvanizing treatment and the alloying treatment conditions are within the scope of the present invention, is an alloy. The chemical conversion is also good.

  Further, among the steel plates of the present invention examples, the steel plates No. 1, 6, 9 and 20 have a preferable Si amount, a yield strength of 600 MPa or less, and a good shape freezing property. . Steel plates No. 1, 6, 10, 13, and 20 have a C concentration in the retained austenite of 0.85% by mass or less, and further have good hole expandability.

  However, as shown in Table 3, even when the chemical compositions are steel types A, D, E, G and M within the scope of the present invention, alloying at 400 ° C. where the alloying treatment conditions are outside the scope of the present invention. An alloyed hot-dip galvanized steel sheet that simulates the treatment has poor alloying processability.

Claims (5)

An alloyed hot-dip galvanized steel sheet provided with an alloyed hot-dip galvanized layer on the surface of the steel sheet,
The steel sheet is, in mass%, C: 0.10% to 0.18%, Si: 0.10% to 0.60%, Mn: 2.2% to 3.0%, P: 0 0.1% or less, S: 0.01% or less, sol.Al: 0.01% or more and 0.10% or less and N: 0.01% or less, and Ti: 0.15% or less and Nb : A steel structure having a chemical composition containing one or two of 0.15% or less in a range satisfying the following formula (1) and an area ratio of retained austenite of 5.0% or more and 15% or less. Have
The alloyed hot dip galvanized steel sheet has mechanical properties such that a tensile strength is 980 MPa or more, a total elongation is 13% or more, and a yield strength is 640 MPa or less.
0.05 ≦ Ti + Nb / 2 ≦ 0.15 (1)   The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the chemical composition further contains B: 0.01 mass% or less.   The galvannealed steel sheet according to claim 1 or 2, wherein a C concentration in the retained austenite is 0.85% by mass or less.   The alloyed hot-dip galvanized steel sheet according to claim 3, which has mechanical properties such that a hole expansion ratio measured by a method defined in JFS T 1001 is 25% or more. A method for producing an alloyed hot-dip galvanized steel sheet, comprising the following steps (A) to (C):
(A) The steel material having the chemical composition according to claim 1 or 2 is made 1120 ° C. or higher and 1300 ° C. or lower, finishing temperature: 800 ° C. or higher and 950 ° C. or lower, and winding temperature: 450 ° C. or higher and 720 ° C. or lower. Hot rolling process to give hot-rolled steel sheet;
(B) a cold rolling step in which the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; and (C) the cold-rolled steel sheet is subjected to a temperature range of Ac ₃ points to 950 ° C. for 5 seconds to 200 seconds. It is annealed to stay below, and is cooled to 560 ° C. with an average cooling rate in the temperature range of 600 ° C. to 750 ° C. being 3 ° C./second to 50 ° C./second, and the residence time in the temperature range of 400 ° C. to 600 ° C. A continuous annealing-alloying hot dip galvanizing process in which galvanizing is performed for 500 seconds or less and alloying is performed in a temperature range of 430 ° C. to 600 ° C. and then cooled to room temperature.