JP2014240510A - Galvanized steel sheet and production method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-tensile-strength galvanized steel sheet which combines a high tensile strength with a tensile strength of 980 MPa or higher and good baking hardenability, impact absorption characteristics after baking hardening, formability and plating adhesion and an alloyed galvanized steel sheet.SOLUTION: A base material steel sheet has a chemical composition consisting of, by mass%, 0.10-0.30% C, 0.05-0.40% Si, 1.8-3.0% Mn, 0.05% or less P, 0.0080% or less S, 0.50% or less Al, 0.03% or less N and additionally one or both selected from 0.001-0.10% Ti and 0.001-0.10% Nb. The increment of impact absorption energy after subjecting the base material steel sheet to impartment of a pre-strain of an elongation percentage of 4% and a heat treatment of holding at 170°C for 20 min is 10 J/cmor greater. The toughness deterioration due to decrease of the impact absorption energy with increasingly high strength of the steel sheet can be improved by a coating baking treatment.

Description

  The present invention relates to a hot dip galvanized steel sheet and a method for producing the same. The present invention is particularly suitable for applications such as press molding, bending, etc., mainly for automobile bodies, etc., and a high-tensile hot-dip galvanized steel sheet excellent in bake hardening characteristics and shock absorption characteristics after bake hardening and a method for producing the same About.

  In recent years, in order to protect the global environment, improvement in fuel efficiency of automobiles has been demanded. In automobile steel sheets, tensile strength (hereinafter also referred to as “TS”) is 980 MPa or more in order to reduce the weight of the vehicle body and ensure safety. There is a growing need for high-strength steel sheets. Furthermore, steel plates subjected to alloying hot dip galvanizing are widely used for members that require rust prevention.

  On the other hand, from the viewpoint of formability, if the strength of the steel sheet is increased, the ductility and bendability are lowered, so that cold press forming becomes difficult. Therefore, there is a demand for a material that is relatively soft during molding and easy to mold, and has a large amount of bake-hardening during paint baking after molding.

  However, it is difficult to say that the development of materials taking into account the bake-hardening characteristics has been conducted with high-strength hot-dip galvanized steel sheets having a TS of 980 MPa or more. As a relatively close technique, Patent Document 1 discloses a steel sheet having excellent bake hardening characteristics.

  However, in patent document 1, although the bake hardening characteristic of the steel plate which performed the continuous annealing is evaluated, the steel plate which performed the alloying hot dip galvanization process is not evaluated.

  As is well known, continuous hot dip galvanizing is more constrained in heat treatment from the viewpoint of plating treatment, and the technology related to continuous annealing cannot be directly transferred to continuous hot dip galvanizing. Therefore, new technology development is required for continuous hot dip galvanizing.

JP 2004-76114 A

  The present invention is a hot-dip galvanized steel sheet having excellent bake-hardening properties and good formability and plating adhesion, and its manufacture, in a high-tensile steel sheet having a TS of 980 MPa or higher, which has not been substantially studied so far. A method is provided.

  Furthermore, in general, steel sheets are deteriorated in toughness due to a decrease in impact absorption energy as the strength is increased, and the present invention improves toughness deterioration due to a decrease in impact absorption energy due to an increase in strength of the steel sheet by a coating baking process. The present invention provides a hot-dip galvanized steel sheet and a method for producing the same.

  The present inventors have examined a high-tensile steel sheet excellent in bake hardenability, impact absorption characteristics after bake hardening, formability, and plating adhesion. As a result, by controlling C, Si, Mn, Ti, Nb, and N within a predetermined range, high strength of 980 MPa or more and excellent bake hardenability, shock absorption characteristics after bake hardening, bendability and plating adhesion It was found that it can be combined.

  Based on the above knowledge, Cr, V, Mo, B for improving the strength, Cu, Ni for further improving the strength and ductility, and Ca, Bi for improving the bendability. I found a good thing.

  Further, it has been found that by subjecting the obtained steel sheet to a predetermined heat treatment, the bake hardenability, the shock absorption characteristics after bake harden and the bendability are further improved.

The present invention completed based on such new knowledge is as follows.
(1) A hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is in mass%, C: 0.10% to 0.30%, Si: 0.05% to 0 .40% or less, Mn: 1.8% or more and 3.0% or less, P: 0.05% or less, S: 0.0080% or less, Al: 0.50% or less, and N: 0.03% or less And a chemical composition containing one or two selected from the group consisting of Ti: 0.001% to 0.10% and Nb: 0.001% to 0.10%. The hot dip galvanized steel sheet has a tensile strength of 980 MPa or more, and an increase in impact absorption energy after pre-straining with an elongation of 4% and heat treatment held at 170 ° C. for 20 minutes is 10 J / cm ^2. A hot-dip galvanized steel sheet having the mechanical properties as described above.

  (2) The chemical composition is selected from the group consisting of Cr: 0.5% or less, V: 0.1% or less, Mo: 0.3% or less, and B: 0.01% or less in terms of mass%. The hot-dip galvanized steel sheet according to (1), further containing one or more kinds.

  (3) The chemical composition further comprising one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less by mass%, The hot-dip galvanized steel sheet according to 2).

  (4) The above chemical compositions (1) to (1), wherein the chemical composition further contains one or two selected from the group consisting of Ca: 0.050% or less and Bi: 0.050% or less in mass%. The hot-dip galvanized steel sheet according to any one of 3).

(5) The method for producing a hot-dip galvanized steel sheet according to any one of the above (1) to (4), comprising the following steps (A) to (D):
(A) Hot-rolling a slab having the chemical composition according to any one of (1) to (4) above and rolling it in a temperature range of 400 ° C. to 750 ° C. to obtain a hot-rolled steel sheet Rolling process;
(B) A pickling / cold rolling step of obtaining a cold-rolled steel sheet by subjecting the hot-rolled steel sheet to pickling and cold rolling at a reduction rate of 30% to 80%;
(C) After allowing the cold-rolled steel sheet to stay in a temperature range of 750 ° C. or more and 950 ° C. or less for 5 seconds or more and 200 seconds or less, it is cooled to a temperature range of 400 ° C. or more and 600 ° C. or less and is kept in the temperature range for 5 seconds. And (D) a hot dip galvanizing step in which a hot dip galvanized steel plate is applied to the steel plate obtained by the heat treatment step.

(6) The method for producing a hot-dip galvanized steel sheet according to (5), further comprising the following step (E):
(E) An alloying treatment step of subjecting the hot dip galvanized steel sheet to an alloying treatment in a temperature range of 460 ° C. or more and 600 ° C. or less.

  (7) A steel material obtained by subjecting the hot-dip galvanized steel sheet according to any one of (1) to (4) to a forming process and further a paint baking process.

  Here, the paint baking process is not only performed when the steel material itself after the forming process is painted and subjected to the baking process, but the steel material itself after the forming process is not painted, but is also baked together with other painted steel materials. The case where is given is included.

  According to the present invention, a high-tensile hot-dip galvanized steel sheet having excellent plating adhesion and bendability and excellent in bake hardening characteristics and shock absorption characteristics after bake hardening and a method for producing the same are provided. This high-tensile hot-dip galvanized steel sheet is excellent in press formability, further strengthened during paint baking after press forming, and has excellent shock absorption characteristics, so it is a structural member in the fields of home appliances, building materials and automobiles. Suitable as

  The present invention will be described in detail below. In the following description, “%” that defines the chemical composition of a steel sheet, a plating layer, a plating bath, and the like is “mass%”.

1. Chemical composition of steel sheet C: 0.10% or more and 0.30% or less C has an effect of increasing strength by promoting the refinement of ferrite and generation of a second phase other than ferrite. Moreover, it has the effect | action which improves a bake hardening characteristic. If the C content is less than 0.10%, it is difficult to ensure a tensile strength of 980 MPa or more. In addition, it is difficult to obtain excellent bake-hardening characteristics and shock absorption characteristics after bake-hardening. Therefore, the C content is 0.10% or more. Preferably it is 0.12% or more. On the other hand, if the C content is more than 0.30%, the weldability is significantly reduced. Therefore, the C content is 0.30% or less. Preferably it is 0.20% or less.

Si: 0.05% or more and 0.40% or less Si promotes the diffusion of Fe from the grain boundary of the steel sheet into the plating layer in the alloying treatment process, and the Fe content in the plating layer from within the grain of the steel sheet It is an important element having an action of increasing the interfacial adhesion strength between the steel sheet and the plating layer by suppressing diffusion and increasing the unevenness at the interface between the steel sheet and the plating layer. If the Si content is less than 0.05%, the effect by the above action cannot be sufficiently obtained. Therefore, the Si content is set to 0.05% or more. Preferably it is 0.10% or more. On the other hand, when the Si content exceeds 0.40%, the alloying speed is remarkably reduced. Therefore, when producing an alloyed hot-dip galvanized steel sheet, it is necessary to lengthen the alloying treatment time. This will cause a drop and increase in equipment length. On the other hand, when the alloying treatment temperature is increased in order to shorten the alloying treatment time, the operability is lowered or the interface adhesion strength is lowered. Therefore, the Si content is 0.40% or less. Preferably it is 0.25% or less.

Mn: 1.8% or more and 3.0% or less Mn has the effect of increasing the hardenability of the steel and increasing the strength of the steel sheet. If the Mn content is less than 1.8%, it is difficult to ensure a tensile strength of 980 MPa or more. In addition, since it is difficult to generate a second phase other than ferrite containing a lot of dislocations, it is difficult to obtain good bake hardening characteristics and shock absorption characteristics after bake hardening. Therefore, the Mn content is 1.8% or more. Preferably it is 2.0% or more. On the other hand, if the Mn content exceeds 3.0%, the hardenability becomes excessive and a lot of martensite is generated. Therefore, the bendability is significantly reduced. Therefore, the Mn content is 3.0% or less. Preferably it is 2.8% or less.

P: 0.05% or less P is an element generally contained as an impurity. However, P has an effect of increasing the strength of steel and may be positively contained. However, since P is an element that is easily segregated, if added in a large amount, the workability is deteriorated. In particular, if its content exceeds 0.05%, the segregation becomes remarkable and the workability is deteriorated. Become very large. Therefore, the P content is 0.05% or less. Preferably it is 0.02% or less.

S: 0.0008% or less S is an element generally contained as an impurity, and has the effect of reducing the bendability by forming sulfides in the steel, and when the content exceeds 0.0008%. The bendability is significantly reduced. Therefore, the S content is set to 0.0008% or less. Preferably it is 0.0050% or less.

Al: 0.50% or less Al has a function of making a steel plate sound by deoxidizing the steel. Moreover, since the ferrite transformation at the time of annealing is accelerated | stimulated and the concentration of the element in austenite is promoted, it also has the effect | action which raises the intensity | strength of a steel plate. Therefore, Al is contained. However, if the Al content exceeds 0.50%, the effect by the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Al content is 0.50% or less. Preferably it is 0.40% or less. In order to obtain the deoxidation effect more reliably, the Al content is preferably set to 0.01% or more. The Al content in the steel in the present invention means acid-soluble Al (sol. Al).

N: 0.03% or less N is generally contained as an impurity, and if its content exceeds 0.03%, a large amount of TiN is produced, leading to deterioration of moldability and toughness. Therefore, the N content is 0.03% or less. Preferably it is 0.01% or less.

One or two types selected from the group consisting of Ti: 0.001% or more and 0.10% or less and Nb: 0.001% or more and 0.10% or less Ti and Nb are used in the processes after hot rolling. It has the effect | action which produces | generates carbonitride and raises the intensity | strength of a steel plate. When the content of Ti is less than 0.001% and the content of Nb is less than 0.001%, it is difficult to obtain the effect by the above action. Accordingly, one or two selected from the group consisting of Ti: 0.001% or more and Nb: 0.001% or more are contained. The Ti content is preferably 0.010% or more, and the Nb content is preferably 0.010% or more. On the other hand, when the Ti content exceeds 0.10% or the Nb content exceeds 0.10%, not only the cost increases, but the amount of solute C decreases, and excellent bake hardening characteristics and baking are achieved. It becomes difficult to obtain impact absorption characteristics after curing. Therefore, the Ti content is 0.10% or less, and the Nb content is 0.10% or less. The Ti content is preferably 0.09% or less, and the Nb content is preferably 0.06% or less.

One or more selected from the group consisting of Cr: 0.5% or less, V: 0.1% or less, Mo: 0.3% or less, and B: 0.01% or less Cr, V, Mo and All of B have the effect of improving the strength. Therefore, you may contain 1 type, or 2 or more types chosen from the group which consists of Cr, V, Mo, and B. However, if the Cr content exceeds 0.5%, the V content exceeds 0.1%, and the Mo content exceeds 0.3%, the wettability of the hot dipping deteriorates. Further, if the B content exceeds 0.01%, the toughness deteriorates. Therefore, the Cr content is 0.5% or less, the V content is 0.1% or less, the Mo content is 0.3% or less, and the B content is 0.01% or less. It is preferable that the Cr content is 0.25% or less, the V content is 0.03% or less, the Mo content is 0.15% or less, and the B content is 0.0006% or less. In order to obtain the effect of the above operation more reliably, any of Cr: 0.01% or more, V: 0.005% or more, Mo: 0.01% or more and B: 0.0003% or more is satisfied. It is preferable to make it.

Cu: 1.0% or less and Ni: 1 or 2 types selected from the group consisting of 1.0% or less Cu and Ni both have the effect of increasing strength and ductility. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the Cu content exceeds 1.0% or the Ni content exceeds 1.0%, the ductility is lowered. Therefore, the Cu content is 1.0% or less, and the Ni content is 1.0% or less. The Cu content is preferably 0.30% or less, and the Ni content is preferably 0.30% or less. In addition, in order to acquire the effect by the said action | operation more reliably, it is preferable to satisfy any of Cu: 0.01% or more and Ni: 0.01% or more.

One or two types selected from the group consisting of Ca: 0.050% or less and Bi: 0.050% or less Ca and Bi both have the effect of improving bendability. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if the content of Ca exceeds 0.050% and the content of Bi exceeds 0.050%, the effect of the above action is saturated, which is disadvantageous in cost. Therefore, the Ca content is 0.050% or less, and the Bi content is 0.050% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy one of Ca: 0.0003% or more and Bi: 0.0003% or more.

The remainder of the chemical composition other than the above elements is Fe and impurities.
2. Hot-dip galvanized layer There is no particular limitation on the chemical composition and the amount of adhesion of the hot-dip galvanized layer. The hot dip galvanized layer may be a pure galvanized layer that has not been subjected to an alloying treatment, but is preferably an alloyed hot dip galvanized layer that has been subjected to an alloying treatment. This is because the latter provides better plating adhesion. A preferable amount of adhesion of the hot dip galvanized layer will be described later. Below, the preferable chemical composition of a plating layer is demonstrated about the case where a hot dip galvanization layer is an alloying hot dip galvanization layer.

(Fe: 8% to 15%)
When the Fe content in the hot dip galvanized layer is less than 8%, a soft part tends to be formed on the surface layer portion of the plated layer after the alloying treatment, the slidability is lowered, and the plated layer of the coating becomes the base material. Flaking-like peeling due to peeling from the interface with the steel sheet increases. Therefore, the Fe content is preferably 8% or more. More preferably, it is 9.5% or more. On the other hand, if the Fe content exceeds 15%, when the steel sheet is subjected to bending, the amount of powdering peeling increases due to compression deformation of the galvannealed layer inside the bent portion. For this reason, Fe content shall be 15% or less. Preferably it is 14% or less.

(Al: 0.15% or more and 0.50% or less)
When the Al content in the hot dip galvanized layer is less than 0.15%, 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 coating. Therefore, the Al content is 0.15% or more. Preferably it is 0.20% or more, more preferably 0.25% or more. On the other hand, when the Al content exceeds 0.50%, the alloying speed decreases, so that at the normal line speed, the alloying temperature must be higher than 540 ° C. in order to realize the Fe content. In some cases, as will be described later, it becomes difficult to set the interfacial adhesion strength between the steel sheet and the galvannealed layer to 20 MPa or more. Therefore, the Al content is 0.50% or less. Preferably it is 0.45% or less, more preferably 0.40% or less.

(Other)
In the hot-dip galvanized layer, Si, Mn, P, S, Ti, Nb, Cr, Mo, V, B, Ca, etc. are incorporated from the base steel plate in the alloying process, but under normal conditions There is no problem because the plating quality is not adversely affected as long as it is within the range that can be incorporated into the plating layer when the hot dipping and alloying treatment is performed. The normal plating conditions here are, as described later, a plating bath temperature of 400 ° C. or higher and 500 ° C. or lower, a steel sheet penetration temperature of 400 ° C. or higher and 500 ° C. or lower, and an alloying temperature of 460 ° C. or higher and 600 ° C. or lower. is there.

3. Mechanical properties The hot-dip galvanized steel sheet according to the present invention is a high-strength steel sheet having a tensile strength of 980 MPa or more, and is also excellent in formability and plating adhesion, and further has a high bake hardenability and shock absorption characteristics after bake hardening. Have Therefore, it can be satisfactorily adapted to the manufacture of various structural members of automobiles in particular.

  The reason why the tensile strength is set to 980 MPa or more is to satisfy the demand for weight reduction of the automobile body. The tensile strength is preferably 1000 MPa or more, more preferably 1050 MPa or more, and most preferably 1100 MPa or more. In addition, it is preferable that the steel plate which is a plating base material is a cold-rolled steel plate from a viewpoint of ensuring the tensile strength of 980 MPa or more.

With respect to the impact absorption characteristics after bake hardening, the increase in impact absorption energy after pre-straining with an elongation of 4% and heat treatment held at 170 ° C. for 20 minutes is 10 J / cm ² or more. This increase amount is preferably 15 J / cm ² or more, more preferably 20 J / cm ² or more. This is because the toughness deterioration due to the reduction in impact absorption energy accompanying the increase in strength of the steel sheet is substantially improved by the coating baking process.

  The reason for the heat treatment conditions of 170 ° C. × 20 minutes in the evaluation of the impact absorption characteristics after bake hardening is that the bake conditions after electrodeposition coating are general. By evaluating under these conditions, it is possible to appropriately evaluate the impact absorption characteristics of a baked coated product manufactured industrially from the hot dip galvanized steel sheet of the present invention.

  Excellent formability is achieved by having high ductility and excellent bendability. The ductility is preferably 3% or more, more preferably 5% or more, and particularly preferably 7% or more. Regarding the bendability, the critical bending radius determined by the test method described later in the examples is preferably 4 times or less, more preferably 3.5 times or less, particularly preferably 3 times or less of the plate thickness.

  About bake hardening ability, the increase amount of the tensile strength after baking calculated | required by the test method mentioned later in an Example becomes like this. Preferably it is 100 Mpa or more, More preferably, it is 140 Mpa or more, Most preferably, it is 160 Mpa or more.

4). Production Conditions The hot-dip galvanized steel sheet according to the present invention is not particularly limited as long as it has the chemical composition of the base steel sheet and the tensile strength as the plated steel sheet. explain.

  This manufacturing method is a hot rolling process for hot rolling a slab to obtain a hot rolled steel sheet, pickling and cold rolling to obtain a cold rolled steel sheet by pickling the hot rolled steel sheet and then cold rolling Alloying treatment including a step, a heat treatment step of heat-treating the cold-rolled steel plate, and a hot-dip galvanizing step of hot-dip galvanizing the heat-treated cold-rolled steel plate, preferably further subjecting the hot-dip galvanized steel plate to an alloying treatment Process.

(Hot rolling process)
The molten steel adjusted to the chemical composition is made into a slab by, for example, continuous casting or casting and ingot rolling, and then hot rolled. Hot rolling usually consists of a rough hot rolling process using a slab as a rough bar and a finishing hot rolling process using a rough bar as a hot rolled steel sheet. It is preferable to equalize the temperature of the entire length of the coarse bar by induction heating or the like before the finish hot rolling with respect to the bar, since it is possible to suppress characteristic fluctuations in the coil. Moreover, it is desirable to perform finish rolling at Ar ₃ points or more.

  As for the coiling temperature, if it is less than 400 ° C., it is extremely hardened and cold rolling becomes difficult. Preferably it is 500 degreeC or more. On the other hand, when it exceeds 750 ° C., the yield deteriorates due to scale loss. Therefore, the winding temperature is set to 750 ° C. or less. Preferably it is 700 degrees C or less.

(Pickling / Cold rolling process)
What is necessary is just to implement about the pickling and cold rolling performed after hot rolling by a conventional method. 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. In that sense, it is preferable to carry out mild rolling before pickling.

  For cold rolling, the rolling reduction is 30% or more and 80% or less. When the rolling reduction is increased within the above range, the transformation to austenite during annealing is promoted, so that the effect of expanding the preferred range of annealing is obtained.

(Heat treatment process)
The cold-rolled steel sheet obtained in the pickling / cold rolling step is allowed to stay in a temperature range of 750 ° C. to 950 ° C. for 5 seconds to 200 seconds, and then cooled to a temperature range of 400 ° C. to 600 ° C. Then, after performing heat treatment for staying in the temperature range for 5 seconds or more and 200 seconds or less, hot dip galvanization is performed.

The heat treatment and hot dip galvanizing are preferably performed in a continuous hot dip galvanizing line.
In the heat treatment, first, a soaking treatment is performed in which the temperature stays in the temperature range of 750 ° C. to 950 ° C. for 5 seconds to 200 seconds. If the soaking temperature at this time is less than 750 ° C., the austenite transformation is insufficient, and it becomes difficult to ensure the target ductility and bendability. Therefore, the soaking temperature is set to 750 ° C. or higher. Preferably it is Ac ₃ points or more. On the other hand, when the soaking temperature exceeds 950 ° C., the austenite grain growth is excessively promoted and the structure becomes coarser, so that it is difficult to ensure strength and bendability. Therefore, the soaking temperature is 950 ° C. or lower. Preferably it is 900 degrees C or less.

  If the soaking time is less than 5 seconds, processing strain remains, and the ductility and bendability of the product deteriorate. Therefore, the soaking time is 5 seconds or more. Preferably it is 10 seconds or more. On the other hand, if the soaking time exceeds 200 seconds, the crystal grains grow excessively in the soaking process, and the strength and bendability deteriorate. Therefore, the soaking time is 200 seconds or less. From the viewpoint of productivity, it is preferably 180 seconds or shorter.

  After the soaking process, the temperature is lowered to a temperature range of 400 ° C. or more and 600 ° C. or less, and stays in the temperature range for 5 seconds or more and 200 seconds or less. The cooling rate at this time is not particularly limited, but is preferably 40 ° C./second or less up to 700 ° C. The cooling rate from 700 ° C. to 400 ° C. to 600 ° C. is not particularly limited, but there is no problem if it is, for example, 70 ° C./second or less.

  In this way, the temperature is cooled to a temperature range of 400 ° C. or more and 600 ° C. or less, and stays in the temperature range for 5 seconds or more and 200 seconds or less. Therefore, it is possible to effectively increase the strength. On the other hand, by setting the cooling stop temperature to a temperature range of 400 ° C. or higher and lower than 500 ° C., the bendability can be further improved instead of suppressing the increase in strength. A staying time of 5 to 200 seconds is sufficient. Preferably, it is 10 to 150 seconds.

(Hot galvanizing process)
The hot dip galvanizing treatment may be carried out according to a conventional method, and suitable conditions and the like are shown below.

  When the Al concentration in the plating bath is less than 0.08%, an excessive Fe—Zn interface alloy layer is already formed in the plating bath before the alloying treatment, making it difficult to control the amount of adhesion. Therefore, the Al concentration in the plating bath is preferably 0.08% or more. More preferably, it is 0.09% or more.

  On the other hand, if the Al concentration in the plating bath exceeds 0.20%, the concentration of Al in the plating film proceeds excessively, resulting in a decrease in the alloying speed, and the above-mentioned Fe content is realized at a normal line speed. For this reason, the alloying treatment temperature may be forced to exceed 600 ° C., and it becomes difficult to make the interfacial adhesion strength between the steel sheet and the galvannealed layer 20 MPa or more as will be described later. Therefore, the Al concentration in the plating bath is preferably 0.20% or less. More preferably, it is 0.15% or less.

As long as the immersion time in the plating bath is within 5 seconds, the performance and operability are not particularly impaired. Other plating conditions may be in a generally adopted range, and there is no particular problem as long as the plating bath temperature is 400 ° C. or higher and 500 ° C. or lower and the penetration plate temperature is 400 ° C. or higher and 500 ° C. or lower. As a component other than Al in the plating bath, even if each of the inevitable elements Fe and Pb, Cd, Cr, Ni, W, Ti, Mg, Si is contained in an amount of 0.1% or less, this performance is achieved. Has no effect. Coverages generally may be single-sided per 25 g / m ² or more 70 g / m ² or less of the range being used as a product.

(Alloying process)
When alloying a hot-dip galvanized steel sheet to obtain an alloyed hot-dip galvanized steel sheet, the alloying temperature is 460 ° C. or higher and 600 ° C. or lower. The treatment time is not particularly limited, but is usually in the range of 5 to 60 seconds.

  When the alloying treatment temperature exceeds 600 ° C., the diffusion of Fe from within the grain of the steel sheet into the plating layer becomes dominant, promoting the diffusion of Fe from the grain boundary of the steel sheet into the plating layer, and from within the grain of the steel sheet. By suppressing the diffusion of Fe into the plating layer and increasing the unevenness at the interface between the steel plate and the plating layer, the effect of the above-described Si-containing effect of increasing the interfacial adhesion strength between the steel plate and the plating layer is achieved. In some cases, the interfacial adhesion strength between the steel sheet and the galvannealed layer may be reduced. Therefore, the alloying treatment temperature is set to 600 ° C. or less. Preferably it is 580 degrees C or less. As a heating means in the alloying treatment, any means such as radiant heating, high frequency induction heating, energization heating and the like may be used.

  When the alloying treatment temperature is less than 460 ° C., coarse crystals of ζ phase are likely to be formed on the surface layer portion of the alloyed hot-dip galvanized layer, and the Fe content in the galvanized layer may be less than 8%. . Therefore, the alloying treatment temperature is set to 460 ° C. or higher. Preferably it is 470 degreeC or more, More preferably, it is 480 degreeC or more.

(Other)
Temper rolling:
It is preferable to subject the hot dip galvanized steel sheet to temper rolling after the alloying treatment step or the hot dip galvanizing step (when not alloying treatment). The temper rolling is not particularly limited, but it is desirable that the elongation is 0.5% or less from the viewpoint of rolling load.

Post-processing:
The product surface after hot dip galvanization may be untreated, but may be subjected to post-treatments such as known chromic acid treatment, phosphate treatment, chromium-free chemical conversion treatment, resin coating application, and lubrication treatment. In addition, rust preventive oil may be applied, and the rust preventive oil used for the application may be a commercially available general one, but it is highly lubricious and contains S or Ca which are extreme pressure additives. Rust oil may be applied. Such treatment can be carried out on both sides or one side of the plated steel sheet as required.

  Steel having the chemical composition shown in Table 1 was melted in a converter, and a slab having a thickness of 245 mm was obtained by continuous casting. The obtained slab was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet having the indicated thickness. The hot-rolled steel sheet was pickled according to a conventional method, and cold-rolled at the cold rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet having the indicated thickness. This cold-rolled steel sheet was subjected to heat treatment and hot-dip galvanizing (including alloying treatment) under the conditions shown in Table 2 to obtain an alloyed hot-dip galvanized steel sheet.

  The obtained alloyed hot-dip galvanized steel sheet was examined for mechanical properties (tensile characteristics, bendability, bake hardening characteristics, shock absorption characteristics after bake hardening), and plating characteristics. The results are summarized in Table 3.

  Regarding mechanical properties, first, using a No. 5 test piece specified in JIS Z 2201 sampled in the direction perpendicular to the rolling direction, a tensile test was performed according to the method specified in JIS Z 2241 to obtain YP (yield stress), TS ( Tensile strength) and EL (total elongation) were measured.

The impact absorption characteristics after bake hardening were evaluated by measuring the impact absorption energy in a Charpy impact test. That is, 4% pre-strain was applied to the steel sheet collected in the direction perpendicular to the rolling, and then unloaded, and subjected to heat treatment at 170 ° C. for 20 minutes, which is a baking condition after general electrodeposition coating. Thereafter, 8 to 6 steel plates were overlapped and screwed to obtain a test piece having a plate thickness of 10 mm. The specimen shape was a V-notch Charpy specimen described in JIS Z2202. In accordance with the method described in JIS Z 2242, the test method uses shock absorption energy at −80 ° C. as an index, and those that have increased by 10 J / cm ² or more after pre-straining and heat treatment are judged as good. did.

  Bake hardening (BH, bake hardening) characteristics are as follows: 4% pre-strain is applied to the JIS No. 5 test piece, then unloaded, and heat treatment is performed at 170 ° C for 20 minutes, which is a general baking condition after electrodeposition coating. The TS was measured by the same tensile test as described above, and the difference (BHΔTS) from the TS of the original plate before the heat treatment was determined and evaluated. If BHΔTS is 100 MPa or more, it can be determined that the bake hardenability is good.

  The bending test was carried out by taking the No. 1 test piece described in JIS Z 2204 so that the bending ridge line was in the rolling direction, and using this test piece by the V block method described in JIS Z 2248. . In this method, bending is repeated while changing a metal fitting having a different radius of curvature (R) at the tip, and after the bending test, the crack of the bent portion is determined by an optical microscope or SEM, and the limit bending R / t (R : Bending radius, t: plate thickness of test piece). If R / t is 4 or less, it can be determined that the bendability is good.

The plating characteristics were investigated as follows.
(1) Composition analysis and adhesion amount of plating layer A sample piece having a diameter of 25 mm was taken from the galvanized steel sheet after the alloying treatment, and the plating layer was coated with a 10% HCl aqueous solution containing an inhibitor at a concentration of 0.5% vol%. The composition of the plating layer was analyzed by dissolving the resulting solution and analyzing the resulting solution by the ICP method. Table 3 shows the contents (%) of Fe and Al in the plating layer. The amount of adhesion was determined from the total amount of Zn, Fe and Al in the solution and the area of the sample piece.

(2) Measurement of interfacial adhesion strength between steel plate and alloyed hot-dip galvanized layer From a hot-dip galvanized steel plate treated with alloying, two 20 mm x 100 mm sample pieces were collected so that the longitudinal direction was the rolling direction. One-component epoxy structural adhesive (trade name: E-6973) manufactured by Star Co., Ltd. was used as the adhesive, and the short sides of the two sample pieces were overlapped: 12.5 mm, adhesive film thickness : 200 μm, baking condition: 170 ° C. × 20 minutes. A tensile test was performed on the obtained adhesive specimen in the longitudinal direction under the conditions of a tensile speed of 5 mm / min and room temperature, and the peel strength (tensile stress at the time of peel) was measured. As a result of the test, those having a peel strength of 20 MPa or more were considered good, and those having a peel strength of less than 20 MPa were bad.

As shown in Table 3, the alloyed hot-dip galvanized steel sheets of Experiment Nos. 1 to 35 according to the present invention are excellent in extensibility and bending workability while having a high tensile strength (TS) of 980 MPa or more. (R / t is 4 or less), good plating adhesion, and good press formability. Moreover, the impact absorption energy was increased by 14 to 33 J / cm ² , and the impact absorption characteristics after bake hardening were excellent. Furthermore, BHΔTS was as high as 104 to 258 MPa, and the bake hardenability was also excellent.

  On the other hand, in the comparative example in which the chemical composition of the base steel plate is outside the scope of the present invention, the TS does not reach 980 MPa (experiment No. 37, 38, 43), or even if the strength is high Problems that workability is low (Experiment No. 36), plating adhesion is poor (Experiment No. 39, 40), or bake hardening ability and shock absorption characteristics after bake hardening are insufficient (Experiment No. 41 to 43). Was holding.

Claims (7)

A hot-dip galvanized steel sheet provided with a hot-dip galvanized layer on the surface of the steel sheet,
The steel sheet is, by mass%, C: 0.10% to 0.30%, Si: 0.05% to 0.40%, Mn: 1.8% to 3.0%, P: 0 0.05% or less, S: 0.0080% or less, Al: 0.50% or less, and N: 0.03% or less, and Ti: 0.001% to 0.10% and Nb: Having a chemical composition containing one or two selected from the group consisting of 0.001% or more and 0.10% or less, and the hot-dip galvanized steel sheet has a tensile strength of 980 MPa or more and an elongation of 4 %, And having an increase in impact absorption energy of 10 J / cm ² or more after heat treatment for 20 minutes at 170 ° C.
A hot-dip galvanized steel sheet.   The chemical composition is one selected from the group consisting of Cr: 0.5% or less, V: 0.1% or less, Mo: 0.3% or less, and B: 0.01% or less in terms of mass%. The hot-dip galvanized steel sheet according to claim 1, further comprising two or more kinds.   3. The chemical composition according to claim 1, wherein the chemical composition further contains one or two selected from the group consisting of Cu: 1.0% or less and Ni: 1.0% or less in mass%. Hot dip galvanized steel sheet.   The chemical composition further includes one or two selected from the group consisting of Ca: 0.050% or less and Bi: 0.050% or less in mass%. The hot-dip galvanized steel sheet according to any one of the above. The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 4, comprising the following steps (A) to (D):
(A) Heat obtained by hot rolling a slab having the chemical composition according to any one of claims 1 to 4 and winding the slab in a temperature range of 400 ° C to 750 ° C to obtain a hot rolled steel sheet Hot rolling process;
(B) A pickling / cold rolling step of obtaining a cold-rolled steel sheet by subjecting the hot-rolled steel sheet to pickling and cold rolling at a reduction rate of 30% to 80%;
(C) After allowing the cold-rolled steel sheet to stay in a temperature range of 750 ° C. or more and 950 ° C. or less for 5 seconds or more and 200 seconds or less, it is cooled to a temperature range of 400 ° C. or more and 600 ° C. or less and is kept in the temperature range for 5 seconds. And (D) a hot dip galvanizing step in which a hot dip galvanized steel plate is applied to the steel plate obtained by the heat treatment step. Furthermore, the manufacturing method of the hot dip galvanized steel plate of Claim 5 which has the following process (E):
(E) An alloying treatment step of subjecting the hot dip galvanized steel sheet to an alloying treatment in a temperature range of 460 ° C. or more and 600 ° C. or less.   A steel material obtained by subjecting the hot-dip galvanized steel sheet according to any one of claims 1 to 4 to a forming process and a paint baking process.