JP2008255441A - High-tensile-strength hot-dip galvanized steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-dip galvanized steel sheet which has a TS of 780 MPa or higher, and shows superior low-temperature toughness after having been press-worked, painted and baked, superior formability and a superior adhesiveness of a plated layer, and to provide a manufacturing method therefor. <P>SOLUTION: The steel sheet has a chemical composition comprising, by mass%, 0.035 to 0.150% C, 0.05 to 0.60% Si, 2.0 to 4.0% Mn, 0.015% or less P, less than 0.0015% S, 0.8% or less sol. Al, 0.0031 to 0.015% N, 0.0030% or less O, 0.005 to 0.130% Ti and 0 to 0.130% Nb so as to satisfy Ti+Nb≥0.055%, and the balance Fe with impurities; and has a metal structure comprising ferrite phase with an average crystal grain size of 5.0 μm or smaller and hard secondary phase with an average particle diameter of 5.0 μm or smaller. The manufacturing method comprises the steps of: starting cooling a plate within 4 seconds after having completed a hot rolling step; cooling the plate to 700°C or lower within 10 seconds after having finished the hot rolling step; taking up the plate at a temperature between 400 and 700°C; acid-pickling the steel plate; cold-rolling the steel plate at a rolling reduction of 30 to 80%; holding the cold-rolled steel sheet at a temperature between 750 and 950°C for 5 to 200 seconds; cooling the steel sheet to a temperature between 400 and 600°C; holding the steel sheet at a temperature between 400 and 600°C for 5 to 200 seconds; and hot-dip galvanizing the steel sheet. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a high-tensile hot-dip galvanized steel sheet and a method for producing the same. In particular, the present invention relates to a high-tensile hot-dip galvanized steel sheet excellent in low-temperature toughness, which is suitable for applications such as press molding and bending, such as automobile bodies, and a method for producing the same.

  In recent years, in order to protect the global environment, there has been a demand for improvement in fuel efficiency of automobiles. In steel sheets for automobiles, tensile strength (hereinafter also referred to as “TS”) is 780 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. However, it does not have to be just high strength. For example, high ductility and good bendability are required from the viewpoint of formability, and a hot-dip galvanized steel sheet is required from the viewpoint of rust prevention. In response to these demands, various high-strength steel plates have been developed. Such high-strength steel sheets are used for structural parts and various reinforcing materials in order to improve the shock absorption capacity of the vehicle body.

  When the collision of the vehicle body is considered, the collision under various conditions can be considered. For example, in a collision at low temperatures in winter, there is a possibility that a phenomenon that a part breaks without absorbing an impact, that is, a so-called low-temperature brittle fracture occurs. However, it is difficult to say that material development considering low temperature brittleness has been performed so far. As comparatively close characteristics, Patent Document 1 and Patent Document 2 disclose steel plates with excellent shock absorption capability, and Patent Document 3, Patent Document 4 and Patent Document 5 disclose steel plates with excellent secondary work brittleness resistance. .

However, Patent Document 1 and Patent Document 2 evaluate the crushing characteristics of the member, but the test temperature is not described, and no consideration is given to brittle cracking at low temperatures.
In Patent Document 4, secondary work brittleness resistance is evaluated by molding in a low temperature medium. Although the molding speed is not described, by analogy from the test method, it is considered that it is not a high speed that simulates a collision, and is not evaluated considering the collision of the vehicle body. Moreover, although it is performed with respect to the test piece after shaping | molding, since the painting baking process is further performed after shaping | molding in the case of a vehicle body, different evaluation is required.

  In Patent Document 5, evaluation is made by dropping a 5 kg weight from a height of 80 cm after molding. However, the impact energy is insufficient for simulating automobile collision, and the above-mentioned paint baking process is considered. It has not been evaluated properly.

According to the study by the present inventors, in each of the aforementioned inventions, it has been found that a low temperature brittleness test simulating a collision of a vehicle body does not ensure sufficient low temperature toughness, and a new technical development is necessary.
JP 2003-221623 A JP 2004-285435 A JP 2004-211140 A JP 2004-29281 A JP 2004-323958 A

  The present invention has been made for the purpose of solving such problems, particularly in a high-tensile steel sheet having a TS of 780 MPa or more, excellent in low-temperature toughness after press working and paint baking, and formability. The present invention provides a hot-dip galvanized steel sheet having excellent plating adhesion and a method for producing the same.

  The present inventors have studied a high-tensile steel sheet excellent in formability, plating adhesion, and low temperature toughness. As a result, by controlling C, Si, Mn, N, Ti, Nb within a predetermined range, the particle size of the ferrite and the hard second layer can be reduced to 5.0 μm or less, and a high strength of 780 MPa or more and It has been found that it is possible to combine excellent ductility, bendability and plating adhesion. Furthermore, it has been found that low temperature toughness is improved by strictly limiting S, P and O, and controlling N and Ti within an appropriate range.

  Based on the above knowledge, Si, Cu, and Ni are included for improving ductility, Cr, Mo, V, and B are included for improving strength, and Ca and REM are included for improving bendability. I found that I should do it.

In addition, the present invention was completed by finding that formability and low temperature toughness were further improved by subjecting the obtained steel sheet to a predetermined heat treatment.
The present invention has been completed based on such new findings, and the gist thereof is as follows.
(1) In a high-tensile 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: 0.035 to 0.150%, Si: 0.05 to 0.60%. , Mn: 2.0 to 4.0%, P: 0.015% or less, S: less than 0.0015%, sol.Al: 0.8% or less, N: 0.0031 to 0.015%, O : 0.0003% or less, Ti: 0.005 to 0.130% and Nb: 0 to 0.130%, the total amount of Ti and Nb is 0.055% or more, the balance being Fe and impurities A high-tensile hot-dip galvanized steel sheet having a chemical composition comprising: a ferrite having an average crystal grain size of 5.0 μm or less and a hard second phase having an average grain size of 5.0 μm or less .
(2) When the chemical composition is replaced by a part of Fe, in mass%, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, and B: 0.01% The high-tensile hot-dip galvanized steel sheet according to (1) above containing one or more selected from the following group.
(3) The chemical composition described above containing one or two kinds selected from the group consisting of Ca: 0.050% or less and REM: 0.050% or less in mass%, instead of a part of Fe. A high-tensile hot-dip galvanized steel sheet according to 1) or (2).
(4) The chemical composition described above containing one or two selected from the group consisting of Cu: 1.5% or less and Ni: 1.5% or less in mass%, instead of a part of Fe. A high-tensile hot-dip galvanized steel sheet according to 1), (2) or (3).
(5) Hot rolling is performed on the slab having the chemical composition according to any one of (1) to (4) above, cooling is started within 4 seconds after completion of hot rolling, and within 10 seconds after hot rolling. Then, the steel sheet is cooled to a temperature range of 700 ° C. or lower and wound up in a temperature range of 400 ° C. to 700 ° C. to form a hot rolled steel plate. To give a cold rolled steel sheet, the cold rolled steel sheet is allowed to stay in a temperature range of 750 to 950 ° C. for 5 to 200 seconds, then cooled to a temperature range of 400 to 600 ° C., and 400 to 600 ° C. A method for producing a high-tensile hot-dip galvanized steel sheet, which is allowed to stay in a temperature range for 5 to 200 seconds and then subjected to a hot-dip galvanizing treatment.
(6) The method for producing a high-tensile hot-dip galvanized steel sheet according to (5), wherein after the hot-dip galvanizing is performed, an alloying treatment is further performed in a temperature range of 600 ° C. or lower.
(7) A method for producing a high-tensile hot-dip galvanized steel sheet, characterized by subjecting the high-tensile hot-dip galvanized steel sheet obtained by the production method of (5) or (6) to a heat treatment that satisfies the following formula (1): .

t = max [10 ^{−10.4 + 5000 / T + log (T / 291)} , 5] (1)
Here, t in the formula represents a heat treatment time (unit: second), T represents a heat treatment temperature (unit: K), and max [] is a function that returns the maximum value of arguments in [].

  The high-tensile hot-dip galvanized steel sheet in the present invention includes not only hot-dip galvanized steel sheets but also alloyed hot-dip galvanized steel sheets subjected to alloying treatment.

  According to the present invention, it is possible to provide a high-tensile hot-dip galvanized steel sheet with excellent formability having excellent low-temperature toughness and a method for producing the same. This high-tensile alloyed hot-dip galvanized steel sheet is excellent in pressability and plating adhesion, and is suitable as a structural member in fields such as home appliances, building materials and automobiles.

The reason why the chemical composition of the steel sheet according to the present invention is defined as described above will be described. In the present specification, “%” defining the chemical composition is “mass%”.
(1) Chemical composition of steel plate as base material C: 0.035 to 0.15%
C is an element effective for improving strength at low cost. If the C content is less than 0.035%, the effect of improving the strength is not sufficient, and it becomes difficult to ensure the target strength, so the C content is set to 0.035% or more. Preferably it is 0.05% or more. On the other hand, if the C content exceeds 0.15%, the weldability deteriorates. For this reason, C content is made into 0.15% or less. Preferably it is 0.13% or less.

Si: 0.05-0.60%
Si promotes the diffusion of Fe from the steel plate grain boundary into the coating layer of the coating during the alloying process, but suppresses the diffusion of Fe from within the grain into the plating layer. It is an important element that increases the interfacial adhesion strength between the base steel sheet and the plating layer by increasing the unevenness of the interface.

  If the Si content is less than 0.05%, the effect of improving the interfacial adhesion strength is not sufficient, so the Si content is set to 0.05% or more. Preferably it is 0.06% or more. On the other hand, when the Si content exceeds 0.60%, 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 and productivity. Decrease and lengthening of equipment. Increasing the alloying treatment temperature in order to shorten the alloying treatment time results in a decrease in operability or a decrease in the interfacial adhesion strength. For this reason, Si content shall be 0.60% or less. Preferably it is 0.30% or less.

Mn: 2.0-4.0%
Mn is an element effective for increasing the strength of steel. It also has the effect of lowering the Ac ₃ point of steel and expanding the preferred annealing temperature range. Therefore, the Mn content is set to 2.0% or more. On the other hand, excessive content degrades the balance between strength and ductility, so the Mn content is 4.0% or less. Desirably, it is 3.0% or less.

P: 0.015% or less P is an element that deteriorates the toughness of steel. Therefore, the P content is set to 0.015% or less. Preferably it is 0.013% or less.

S: Less than 0.0015% S is an element that deteriorates the toughness of steel, and its content is preferably low. If the S content exceeds 0.0015%, the precipitation of sulfides becomes remarkable and the toughness deteriorates remarkably. For this reason, S content shall be less than 0.0015%.

sol.Al: 0.8% or less Al is contained for deoxidation of steel. In order to more reliably obtain the deoxidation effect, the sol.Al content is preferably set to 0.01% or more. In addition, Al promotes ferrite transformation during annealing and promotes concentration of elements in austenite, so that it also serves to increase strength. However, since the effect is saturated even if contained excessively, the sol.Al content is set to 0.8% or less.

N: 0.0031 to 0.015% or less N is an impurity element that is inevitably contained in general, but in the present invention, the effect of improving the low temperature toughness by coarsening TiN produced during steelmaking. Have. Therefore, the N content is set to 0.0031% or more. On the other hand, if it is excessively contained, the amount of TiN becomes excessive, leading to deterioration of moldability and toughness. Therefore, the N content is set to 0.015% or less. Preferably it is 0.0006% or less.

O: 0.0003% or less O is an impurity element and is an element that deteriorates the toughness of steel. For this reason, the content of O is set to 0.0003% or less. Preferably it is 0.025% or less.

Ti: 0.005-0.130%, Ti + Nb ≧ 0.055%
Ti is an element necessary for producing coarse TiN that does not adversely affect toughness as described above during steelmaking, and is contained by 0.005% or more. Ti is effective in increasing the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides in the processes after the hot rolling process. Further, Ti has an effect of suppressing recrystallization of ferrite during annealing, promotes transformation to austenite, and significantly promotes ferrite transformation during cooling after annealing. It also has the effect of remarkably reducing the crystal grain size. In order to exhibit such an effect, 0.055% or more of Ti is contained in total with Nb. Moreover, even if it contains excessively, an effect will be saturated and a cost increase will be caused, Therefore Ti content shall be 0.130% or less. Preferably it is 0.10% or less.

Nb: 0 to 0.130%, Ti + Nb ≧ 0.055%
Nb, together with Ti, is an element effective in increasing the strength of the steel sheet by forming carbide, nitride, or carbonitride in the steps after the hot rolling step. Nb has an effect of suppressing recrystallization of ferrite during annealing, promotes transformation to austenite, and significantly promotes ferrite transformation during cooling after annealing. Furthermore, it has the effect of remarkably reducing the crystal grain size. In order to develop such an effect, Nb is contained in an amount of 0.055% or more in total with Ti. Moreover, even if it contains excessively, since an effect will be saturated and a cost increase will be caused, Nb content shall be 0.13% or less. Preferably it is 0.10% or less.

Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, B: 0.01% or less The present invention is 780 MPa or more by precipitation strengthening with Ti and Nb and transformation strengthening with Mn. High strength can be achieved. However, since Mn makes the structure of a steel plate into a band shape, if bendability is required, Cr, Mo, V, or B may be included to replace a part of Mn. preferable. Further, even when the strength is further increased to 980 MPa or more, it is effective to contain one or more of Cr, Mo, V, and B.

  However, if Cr, Mo and V are excessively contained, the wettability of hot dipping is deteriorated, so the respective contents are set to 1.0% or less. Further, if B is contained excessively, the toughness deteriorates, so the B content is set to 0.01% or less. In order to obtain the effect of increasing the strength more reliably, it is preferable to contain 0.03% or more of Cr, Mo, and V and 0.0003% or more of B, respectively.

  In addition, B has the effect | action which suppresses a ferrite transformation and promotes the production | generation of a hard 2nd phase and strengthens a steel plate, but when it is made to contain especially with Mo, the effect which achieves high strengthening, improving bendability remarkably. Therefore, it is preferable to contain Mo and B in combination for bending applications.

Cu, Ni: 1.5% or less each The present invention combines corrosion resistance by applying hot dip galvanization. However, Cu and / or Ni to be contained as necessary is concentrated on the surface to increase the surface concentration of Si. Therefore, it has the effect of improving the wettability of plating and alloying processability. For that purpose, it is preferable to contain Cu and / or Ni and to make the total content 0.05% or more. Desirably, the total content is 0.10% or more. If each content exceeds 1.5%, the effect is saturated and the cost is increased. Therefore, each content is 1.5% or less.

  Further, since Cu has an effect of improving the corrosion resistance at the end portion not subjected to hot dipping, the Cu content is preferably set to 0.03% or more, and if it exceeds 0.5% The effect is saturated.

Ca: 0.050% or less, REM: 0.050% or less These elements have the effect of changing the precipitation form of sulfides and improving the bendability. it can. Even if contained excessively, the effect is saturated and the cost is increased, so the respective contents are set to 0.050% or less. In order to obtain the effect of the above action more reliably, the total content is preferably 0.0005% or more, and more preferably 0.0010% or more.

(2) Base material structure The metal structure of the steel sheet according to the present invention is defined as follows.
In order to achieve good bendability in a region where the tensile strength is 780 MPa or more, the average crystal grain size of ferrite and the average grain size of the hard second phase are each set to 5.0 μm or less. Further, it is desirable that the thickness is 3.0 μm or less.

The hard second phase described here is 1-5 μm martensite, bainite, residual austenite or mixtures thereof observed at the SEM level.
In the case of the present invention, austenite grains are refined during annealing (particle size 1 to 5 μm), and a part of the fine austenite grains are transformed into fine ferrite during subsequent cooling. Of the remaining fine austenite grains, some are transformed into bainite, some are transformed into martensite, and some are transformed into a martensite / austenite mixture. As shown in the SEM observation structure photograph of FIG. 1 described later, they are collectively referred to as “hard second phase”. Their particle size can be determined from the SEM observation photograph by a cutting method.

(3) Chemical composition of the plating layer to be a coating The chemical composition of the plating coating is not particularly limited, but suitable conditions when the plating coating is alloyed hot dip galvanizing are shown below.

Fe: 8-15%
When the Fe content in the galvanized layer to be the coating is less than 8%, 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% 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 to 0.50%
When the Al content in the galvanized layer to be a coating is less than 0.15%, the effect of suppressing the development of the alloy layer in the plating bath is insufficient, and the control of the coating amount becomes difficult. 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 alloying process, Si, Mn, P, S, Ti, Nb, Cr, Mo, V, B, Ca, REM, etc. are usually taken into the galvanized layer as the coating, If it is within the range that is incorporated in the plating layer when the hot dipping and alloying treatment is performed under the above conditions, there is no problem because the plating quality is not adversely affected. The normal plating conditions here are, as will be described later, a plating bath temperature of 400 ° C. to 500 ° C., a steel plate penetration temperature of 400 ° C. to 500 ° C., and an alloying temperature of 460 to 600 ° C.
(4) Manufacturing conditions of steel plate used as base material The steel plate according to the present invention is manufactured through hot rolling, cold rolling, and hot dip galvanizing. Suitable manufacturing conditions are shown below.

For example, after the molten steel adjusted to the above chemical composition is made into a slab by continuous casting or casting and ingot rolling, hot rolling is performed. 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. At this time, finishing hot rolling after rough hot rolling is performed. It is preferable to equalize the temperature of the entire length of the coarse bar by induction heating or the like with respect to the previous coarse bar, because the characteristic fluctuation in the coil can be suppressed. Moreover, it is desirable to perform finish rolling at ₃ or more points of Ar.

  Cooling is started within 4 seconds after completion of hot rolling, cooling is performed to a temperature range of 700 ° C. or lower within 10 seconds after completion of hot rolling, and winding is performed at 400 to 700 ° C. When the cooling start time exceeds 4 seconds after completion of hot rolling, or when cooling is not performed to a temperature range of 700 ° C. or lower within 10 seconds after completion of hot rolling, the structure becomes coarse and bendability deteriorates.

  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 700 ° C., the yield deteriorates due to scale loss. Therefore, the winding temperature is set to 700 ° C. or less.

  Conventional methods may be used for pickling and cold rolling performed after hot rolling. 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.

  As for cold rolling, there is no particular problem when the rolling reduction is in the range of 30 to 80%. However, when the rolling reduction is increased, the transformation to austenite at the time of annealing is promoted, which has the effect of expanding the preferred range of annealing.

  The cold rolled steel sheet thus obtained is allowed to stay in a temperature range of 750 to 950 ° C. for 5 to 200 seconds, and then hot dip galvanized. It is preferable that the soaking process and the hot dip galvanizing treatment for staying in a temperature range of 750 to 950 ° C. for 5 to 200 seconds are performed in a continuous hot dip galvanizing line.

  If the soaking temperature is less than 750 ° C., the austenite transformation is insufficient, so that it is difficult to ensure the target ductility and bendability. If the temperature exceeds 950 ° C., the austenite grain growth is excessively promoted and the structure becomes coarse. Therefore, it becomes difficult to ensure the intended low temperature toughness, strength and bendability. Preferably, it is Ac3 point or more and 910 ° C or less.

  In the present invention, since a large amount of Ti + Nb is contained, recrystallization of ferrite processed by cold rolling is remarkably suppressed. Therefore, if the soaking time is less than 5 seconds, processing strain remains and the ductility of the product deteriorates, so the soaking time is set to 5 seconds or more. Preferably it is 10 seconds or more.

  On the other hand, the soaking time is 200 seconds or less. In the present invention, since Ti + Nb is contained in a large amount, grain growth during soaking can be effectively suppressed. Therefore, as an upper limit, there is no problem until 200 seconds. However, from the viewpoint of productivity, it is desirable to set it within 180 seconds. When the soaking time exceeds 200 seconds, the soaking crystal grains grow excessively, and fine grains, and hence good low temperature toughness and bendability cannot be obtained. Therefore, the soaking time is set to 200 seconds or less.

  After soaking, it is cooled to a temperature range of 400 to 600 ° C. The cooling at this time is not particularly limited, but is preferably 40 ° C./second or less up to 700 ° C. Due to the combined effect of a cooling rate of 40 ° C./second or less and a large amount of Ti + Nb, the ferrite transformation is remarkably promoted, and the ferrite particle size can be reduced to 5.0 μm or less. There is no particular limitation on cooling from 700 ° C. to a temperature range of 400 to 600 ° C., but there is no problem if it is, for example, 70 ° C./second or less.

  Thus, after cooling to 400-600 degreeC, it is made to stay at 400-600 degreeC for 5-200 second. At this time, by setting the cooling stop temperature in the temperature range of 500 to 600 ° C., it is possible to effectively increase the strength. On the other hand, by setting the cooling stop temperature to a temperature range of 400 to 500 ° C., it is possible to further improve the bendability instead of suppressing the increase in strength. A staying time of 5 to 200 seconds is sufficient. Preferably, it is 10 to 150 seconds.

(5) Plating conditions After the cooling and staying as described above, hot dip galvanization is performed. Although the hot dip galvanizing may be performed continuously after cooling and staying, the hot dip galvanizing may be performed after being once cooled and then heated again to a plating temperature of 400 to 600 ° C.

The plating conditions at this time are not particularly limited, but are exemplified as follows.
Al concentration in the plating bath: 0.08 to 0.20%
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. Therefore, the alloying treatment temperature may be forced to exceed 540 ° 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.

About immersion time, if it is less than 5 second, a performance and operativity will not be inhibited especially. About other plating conditions, the range generally employ | adopted may be sufficient, and if a plating bath temperature is 400-500 degreeC and an intrusion board temperature is the range of 400-500 degreeC, there will be no problem in particular. Even if 0.1% or less of Fe and Pb, Cd, Cr, Ni, W, Ti, Mg, and Si, which are inevitable elements, are contained as components other than Al in the plating bath, this performance is affected. Absent. What is necessary is just to let the adhesion amount be the range of 25-70 g / m < ² > generally used as a product.

(6) Alloying treatment When the alloying treatment is carried out following the plating treatment, it is preferably carried out under the following conditions.

Alloying temperature: 460-600 ° C
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%. is there. Therefore, the alloying treatment temperature is preferably set to 460 ° C. or higher. More preferably, it is 470 degreeC or more, Most preferably, it is 480 degreeC or more.

  On the other hand, when the alloying treatment temperature exceeds 600 ° C., the effect of promoting the diffusion of Zn in the plating film due to the inclusion of Si into the steel plate described above to the grain boundary of the steel plate that is the plating base material is weakened. Since the diffusion into the grains becomes dominant, the interfacial adhesion strength between the steel sheet and the galvannealed layer is lowered. Therefore, the alloying treatment temperature is set to 600 ° C. or less. Preferably it is 560 degrees C or less, More preferably, it is 540 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.

(7) Temper rolling About the hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet obtained by the present invention, temper rolling may be performed as necessary, and no particular limitation is imposed on the temper rolling at that time. From the viewpoint of rolling load, the elongation is preferably 0.5% or less.

(8) Post-treatment The product surface after plating may be untreated, but may be subjected to post-treatment such as known chromic acid treatment, phosphate treatment, and resin film coating. 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 an extreme pressure additive such as S or Ca. Rust oil may be applied.

(9) Heat treatment When the high-tensile hot-dip galvanized steel sheet is subjected to a heat treatment satisfying the following formula (1), the formability and low-temperature toughness of the steel sheet are further improved. When the galvanized steel sheet is subjected to temper rolling, the heat treatment may be performed either before or after temper rolling, or both before and after temper rolling. When performing both before and after temper rolling, the sum total of the heat processing before temper rolling and the heat processing after temper rolling should just satisfy the following Formula (1).

t = max [10 ^{−10.4 + 5000 / T + log (T / 291)} , 5] (1)
Here, t in the formula represents a heat treatment time (unit: second), T represents a heat treatment temperature (unit: K), and max [] is a function that returns the maximum value of arguments in [].

  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. The obtained hot-rolled steel sheet was pickled and cold-rolled at the cold pressure rate shown in Table 2. The obtained cold-rolled steel sheet was annealed and hot-dip galvanized under the conditions shown in Table 2, and the obtained hot-dip galvanized steel sheet was examined for tensile tests, limit bending, low temperature toughness, and plating characteristics. The results are shown in Table 3.

An example of the SEM observation structure | tissue photograph of the steel plate which concerns on this invention is shown in FIG. It can be seen that the hard second phase is precipitated at the grain boundaries of the ferrite phase.
For mechanical properties, YS, TS, and El were measured by the method specified in JIS Z 2241 using No. 5 test piece specified in JIS Z 2201 collected in the direction perpendicular to the rolling.

  In the bending test, No. 3 test piece defined in JIS Z2204 was used, and bending was performed by 180 ° by a push bending method defined in JIS Z 2248, and evaluation was performed with a limit bend r that does not cause cracking.

  The low temperature brittleness was investigated by the following method. First, a 90 mmφ test piece was collected from the steel plate, cup-formed with a 50 mmφ cylindrical punch, and then heat treated at 170 ° C. for 20 minutes. After the obtained cup was held at various temperatures for 5 minutes, it was placed on a truncated cone as shown in FIG. 2 and a 52.6 kg weight was dropped from a height of 1.901 m. The fracture surface morphology was investigated, the critical temperature at which brittle fracture did not occur was defined as the brittle transition temperature, and the brittle transition temperature of −80 ° C. or lower was considered good.

The plating characteristics were investigated as follows.
(1) Collection of sample piece A sample piece of 25 mmφ was collected from the alloyed sample, and the plating layer was coated with a 10% HCl aqueous solution containing 0.5 vol% inhibitor (trade name: “Ibit 710N” manufactured by Asahi Chemical). This was dissolved and subjected to composition analysis of the plating layer by the ICP method.

(2) Measurement of interfacial adhesion strength between steel plate and alloyed hot-dip galvanized layer The alloyed sample was cut into 20 mm × 100 mm so that the longitudinal direction was the rolling direction, and manufactured by Sunstar Co., Ltd. Liquid type epoxy structural adhesive (trade name: E-6973) is used as an adhesive, overlap margin: 12.5 mm, adhesive film thickness: 200 μm, baking conditions: 180 × 20 minutes, tensile speed: 5 mm / minute A tensile test was carried out in the longitudinal direction under conditions at 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, those having a strength of 20 MPa or more were considered to have good adhesion strength, and those having a strength of less than 20 MPa were judged to be defective.

  As shown in Table 3, when satisfying the scope of the present invention, it has good low temperature toughness, strength / ductility balance, good bendability and plating characteristics. On the other hand, Experiment Nos. 19 to 31 in which the components depart from the present invention have poor low temperature toughness, No. 32 has lower strength, No. 33 has poor plating adhesion strength, and No. 34 has strength. The ductility balance and bendability were poor.

  The hot dip galvanized steel sheet obtained in Table 2 was subjected to heat treatment at 70 ° C. for 6 hours (t0 = 4.9 hours), and then the tensile test, limit bending, low temperature toughness, and plating characteristics were investigated. The results are shown in Table 4.

  As shown in Table 4, the low temperature toughness, bendability, and ductility were improved by heat treatment when the range of the present invention was satisfied.

It is an example of the SEM observation structure | tissue photograph of the steel plate which concerns on this invention. It is explanatory drawing of the test point of the low temperature brittleness in an Example.

Claims (7)

  In a high-tensile 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: 0.035 to 0.150%, Si: 0.05 to 0.60%, Mn: 2.0 to 4.0%, P: 0.015% or less, S: less than 0.0015%, sol. Al: 0.8% or less, N: 0.0031 to 0.015%, O: 0.0. Containing 0030% or less, Ti: 0.005 to 0.130% and Nb: 0 to 0.130%, the total amount of Ti and Nb being 0.055% or more, the balance being Fe and impurities A high-tensile hot-dip galvanized steel sheet having a composition and having a metal structure in which an average crystal grain size of ferrite is 5.0 μm or less and an average grain size of a hard second phase is 5.0 μm or less.   The chemical composition is a group of mass%, Cr: 1.0% or less, Mo: 1.0% or less, V: 1.0% or less, and B: 0.01% or less in place of part of Fe. The high-tensile hot-dip galvanized steel sheet according to claim 1, comprising one or more selected from:   The said chemical composition contains 1 type or 2 types chosen from the group of Ca: 0.050% or less and REM: 0.050% or less in the mass% instead of a part of Fe. The high-tensile hot-dip galvanized steel sheet described in 1.   The said chemical composition contains 1 type or 2 types chosen from the group of Cu: 1.5% or less and Ni: 1.5% or less by mass% instead of a part of Fe. Or the high-tensile hot-dip galvanized steel sheet according to 3.   The slab having the chemical composition according to any one of claims 1 to 4 is hot-rolled, starts cooling within 4 seconds after completion of the hot rolling, and has a temperature of 700 ° C or less within 10 seconds after the hot rolling. The steel sheet is cooled to a region, wound up in a temperature range of 400 ° C. to 700 ° C. to form a hot rolled steel plate, and after cold pickling of the hot rolled steel plate, it is cold rolled at a reduction rate of 30 to 80% and cold rolled. It is made into a steel plate, the cold-rolled steel plate is allowed to stay in a temperature range of 750 to 950 ° C. for 5 to 200 seconds, then cooled to a temperature range of 400 to 600 ° C., and 5 to 200 in a temperature range of 400 to 600 ° C. A method for producing a high-tensile hot-dip galvanized steel sheet, wherein the hot-dip galvanized steel sheet is subjected to a hot-dip galvanizing treatment.   The method for producing a high-tensile hot-dip galvanized steel sheet according to claim 5, wherein after the hot-dip galvanizing, an alloying treatment is further performed in a temperature range of 600 ° C or lower. A method for producing a high-tensile hot-dip galvanized steel sheet, wherein the high-tensile hot-dip galvanized steel sheet obtained by the production method according to claim 5 or 6 is subjected to a heat treatment satisfying the following formula (1).
t = max [10 ^{−10.4 + 5000 / T + log (T / 291)} , 5] (1)
Here, t in the formula represents a heat treatment time (unit: second), T represents a heat treatment temperature (unit: K), and max [] is a function that returns the maximum value of arguments in [].

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