JP4360319B2 - High tensile hot dip galvanized steel sheet and its manufacturing method - Google Patents

Description

  The present invention relates to a high-tensile hot-dip galvanized steel sheet suitable for applications such as press forming, bending, spot welding, and the like, and a manufacturing method thereof.

  Vehicle weight reduction is being promoted as a means of improving fuel consumption in automobiles for reducing environmental impact. Therefore, in steel sheets for automobiles, in order to achieve both weight reduction and safety, there is an increasing need for high-tensile steel sheets having a tensile strength (TS) of 750 MPa or more.

  In addition, high-strength steel sheets are used for various purposes, and materials with adjusted properties such as ductility, bendability, strength, yield ratio, and weldability are required depending on the parts used as automotive structural parts. In addition, there is an increasing need for use in parts where rust prevention is required, and in order to realize this, a high-tensile steel sheet subjected to hot dip galvanizing or alloying hot dip galvanizing is required.

  Steel strengthening techniques for achieving high tension include solid solution strengthening, precipitation strengthening, and transformation strengthening, and a predetermined tensile strength is usually achieved by combining a plurality of techniques. Depending on the balance of these combinations, the yield ratio, ductility, bendability, and hot dip plating adhesion differ even with the same tensile strength. Therefore, in order to satisfy the above-mentioned required performance for automobile applications, it is important to properly balance the strengthening methods.

  However, when hot-dip galvanizing is applied to the ultra-high-strength steel sheet created by balancing the strengthening mechanisms described above and bending is performed, streaks are formed along the ridgeline of the bending, resulting in adhesion and aesthetics of the plating. In addition, the problem of cracking starting from unevenness may occur.

As a conventional invention for improving bendability, for example, Patent Document 1 achieves bending workability and high strength by obtaining tempered martensite.
Patent Document 2 discloses a method for obtaining a high-strength steel sheet having good plating adhesion by paying attention to the C concentration immediately below the plating layer.
JP-A-6-108152 JP 2002-88459 A

  However, in patent document 1, bending workability is a characteristic by the hole expansion rate evaluated by a hole expansion test, and there is no description regarding the bending surface of a plated steel plate. Moreover, in patent document 2, it is necessary to make C density | concentration immediately under a plating layer 0.02% or less, and in order to implement | achieve with a steel grade with high C density | concentration, the process of producing | generating a decarburization layer on the surface is required. As an example of the manufacturing method, a method of decarburizing by annealing a steel sheet in a high dew point atmosphere is shown. However, in a currently used continuous annealing line, it is difficult to control the atmosphere, and the target decarburized layer Is difficult to obtain quantitatively and stably. In addition, the effect is focused only on the plating adhesion, and there is no description about the surface irregularities generated during bending.

  The problem of the present invention is that, as described above, in the high-tensile hot dip galvanized steel sheet having a tensile strength of 750 MPa or more, which has been difficult in the prior art, improvement in bending characteristics during bending, and high tension excellent in plating adhesion and surface properties. The object is to propose a hot-dip galvanized steel sheet and a method for producing the same.

  The present invention lies in a hot dip galvanized steel sheet having a tensile strength of 750 MPa or more, a good bendability, a surface property during bending, and a plating adhesion performance, and a method for producing the same, by reexamining the balance of the steel strengthening method.

  As a result of diligent research on the surface of the high-tensile steel sheet during bending, the inventors of the present invention assumed that the structure in the vicinity of the surface is composed of a ferrite phase, and that the C concentration ratio between the surface vicinity and the inside of the steel sheet is It has been found that if it is below a certain level, good bending properties and surface properties can be obtained. That is, the average C concentration in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is excluded from the surface from the surface of the steel plate to the depth of 0.1 mm. If a steel sheet having an inner average C concentration of 85% or less and a ferrite area ratio in the vicinity of the surface of 80% or more is used, it is good even if the C concentration directly below the plating layer is more than 0.02%. The inventors have found that bendability and plating adhesion can be obtained, and have completed the present invention.

  In addition, as a production method capable of stably obtaining such a steel sheet, attention is paid to the change in the surface C concentration of the slab, and as a means to realize economically and easily, the powder to be put into the mold at the time of continuous casting A method for adjusting the ingredients was found and the present invention was completed.

  Note that, as in the invention described in Patent Document 2, if the carbon content immediately below the plating film is 0.02% or less by only the decarburization process in the annealing step, the thickness of the decarburized layer is stable in the plate thickness direction. In some cases, surface irregularities during bending cannot be prevented. Further, as a result of excessive decarburization, although bending workability can be ensured, the tensile strength is less than 750 MPa, and the intended high strength may not be obtained.

  The present invention is based on the above-mentioned new knowledge, and the gist of the present invention is the high-tensile hot-dip galvanized steel sheet having a tensile strength of 750 MPa or more and (5) to (7) shown in the following inventions (1) to (4). It is in its manufacturing method shown.

(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: 0.05 to 0.20%, Si: 0.5% or less, Mn: 1.0-3.0%, P: 0.05% or less, S: 0.05% or less, sol. Al: 0.1% or less and N: 0.01% or less, and Ti: 0.5% or less and Nb: 0.5% or less, or a total of 0.05% or more And the balance of the steel composition comprising Fe and impurities, and the average C concentration ([C] _S ) in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness, and the surface of the steel plate The ratio ([C] _S / [C] _B ) to the internal average C concentration ([C] _B ) excluding the portion up to the depth of 0.1 mm from the center to the thickness center direction is 0.6 or more and 0 The tensile strength is 750 MPa or more, wherein the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is 80% or more. High tensile hot dip galvanized steel sheet.

  (2) The steel composition further contains one or two kinds selected from the group of V: 0.5% or less and W: 0.5% or less in mass% instead of part of Fe. (1) The high-tensile hot-dip galvanized steel sheet according to (1).

  (3) The steel composition is mass% instead of part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% The high-tensile hot-dip galvanized steel sheet according to (1) or (2), further comprising at least one selected from the group consisting of:

  (4) The steel composition is one type selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass% instead of part of Fe. Alternatively, the high-tensile hot-dip galvanized steel sheet according to any one of (1) to (3), further comprising two or more kinds.

(5) A method for producing a high-tensile hot-dip galvanized steel sheet comprising the following steps (1-1) to (1-4):
(1-1) By continuous casting, the molten steel having the steel composition according to any one of (1) to (4) is set to a C concentration contained in the powder for continuous casting charged into the mold and less than 1% by mass. Continuous casting process to slab;
(1-2) A hot rolling step in which the cast slab is hot rolled after the temperature is set to 1100 ° C. to 1350 ° C. to obtain a hot rolled steel sheet with a finishing temperature: Ar ₃ points or higher and a winding temperature: 700 ° C. or lower;
(1-3) A pickling process in which the hot-rolled steel sheet is pickled to remove the scale on the surface of the steel sheet to obtain a pickled steel sheet; and (1-4) the pickled steel sheet is Ac _{3 to} 1000 ° C. After maintaining in the temperature range for 5 seconds or more, it is cooled to 600 ° C. at an average cooling rate of 1 to 40 (° C./s), and further cooled at an average cooling rate of 1 to 70 (° C./s) to melt at 440 to 520 ° C. A continuous hot dip galvanizing process in which hot dip galvanizing is performed by dipping in a galvanizing bath.

(6) A method for producing a high-tensile hot-dip galvanized steel sheet comprising the following steps (2-1) to (2-5):
(2-1) By continuous casting, the molten steel having the steel composition according to any one of (1) to (4) is set to a C concentration contained in the powder for continuous casting charged into the mold and less than 1% by mass. Continuous casting process to slab;
(2-2) A hot rolling step in which the cast slab is hot rolled after the temperature is set to 1100 ° C. to 1350 ° C. to obtain a hot rolled steel sheet with a finishing temperature: Ar ₃ points or higher and a winding temperature: 700 ° C. or lower;
(2-3) A pickling process in which the hot-rolled steel sheet is subjected to a pickling treatment to remove scale on the surface of the steel sheet to obtain a pickled steel sheet;
(2-4) A cold rolling step in which the pickled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (2-5) the cold-rolled steel sheet is held in a temperature range of Ac _{3 to} 1000 ° C for 5 seconds or more. Then, it is cooled to 600 ° C. at an average cooling rate of 1 to 40 (° C./s), further cooled at an average cooling rate of 1 to 70 (° C./s), and immersed in a hot-dip galvanizing bath at 440 to 520 ° C. Continuous hot dip galvanizing process for hot dip galvanizing.

(7) The method for producing a high-tensile hot-dip galvanized steel sheet according to (5) or (6), wherein in the continuous hot-dip galvanizing process, the hot-dip galvanized steel sheet is subjected to alloying treatment.
Here, the “hot dip galvanized steel sheet” includes an alloyed hot dip galvanized steel sheet obtained by applying an alloying treatment after hot dip galvanizing. Further, the “hot dip galvanized layer” also includes an galvannealed layer obtained by performing an alloying treatment after hot galvanizing.

  The “steel plate surface” serving as a reference for the measurement range of the C concentration is a steel plate surface obtained by removing a plated layer of a hot dip galvanized steel plate by an antimony chloride method defined as an adhesion amount test method in JIS H0401.

  The steel sheet of the present invention is excellent in bending workability, plating adhesion, and corrosion resistance while ensuring high strength and workability. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members typified by automobile bumpers and suspension parts.

  In addition, since it can be manufactured stably and in large quantities at a low cost, it is possible to achieve a remarkable industrial effect.

First, the reasons for limiting the steel composition of the steel sheet will be described. Hereinafter,% indicating the steel composition is mass%.
C: 0.05% to 0.20%
C is an element necessary for ensuring the strength of the steel, and the lower limit of the content is set to 0.05% in order to ensure the strength of 750 MPa or more.

By the way, the average C concentration ([C] _S ) in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the thickness center direction is called hot rolling, cold rolling, pickling treatment, and annealing treatment. By passing through each treatment step prior to hot dip galvanization, and further performing hot dip plating treatment, the average C concentration in the vicinity of the surface portion, particularly the surface 1 to 10 μm deep from the steel sheet surface, is smaller than the central portion. About 90%. However, it has been difficult to stably achieve a steel plate of 85% or less without going through a special process. If the increase of the carbon concentration in the continuous casting stage, that is, carburization in the slab stage can be prevented, the average C concentration in the vicinity of the surface can be stably reduced.

Here, in the present invention, the average C concentration ([C] _S ) in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel sheet toward the thickness center direction and the thickness direction from the surface of the steel sheet. The ratio ([C] _S / [C] _B ) with the internal C concentration ([C] _B ) excluding the portion up to a depth of 0.1 mm toward the surface increases with the C content, and the C content Is more than 0.20%, the above [C] _s / [C] _B is more than 0.85, so the upper limit of the content is made 0.20%. Preferably, it is 0.05 to 0.18%.

Si: 0.5% or less Si is a solid solution strengthening element and is effective for strengthening steel sheets, but deteriorates the wettability and adhesion of plating. Therefore, 0.5% was made the upper limit of the content. Furthermore, in order to obtain stable and good adhesion of plating, the content is desirably set to 0.3% or less.

Mn: 1.0-3.0%
Mn is an element effective in increasing the strength of steel by transformation strengthening. It also has the effect of lowering the Ac ₃ point of steel and expanding the preferred annealing temperature range. Therefore, it is necessary to contain 1.0% or more. A preferred lower limit is 1.5%. On the other hand, excessive addition degrades the strength / ductility balance, so the upper limit of the content was made 3.0%. A preferred upper limit is 2.5%.

P: 0.05% or less P is a solid solution strengthening element and is effective in improving the tensile strength of the steel sheet, but excessive addition deteriorates the adhesion and weldability of the plating. Therefore, the upper limit of the content is set to 0.05%.

S: 0.05% or less S is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of workability and weldability. Therefore, the upper limit of the content is set to 0.05%.

Sol.Al: 0.1% or less
Al is added as a deoxidizing element, but if it exceeds 0.1%, inclusions increase and the cleanliness of the steel is impaired, so the content was made 0.1% or less.

N: 0.01% or less N is an impurity inevitably contained in the steel, and is preferably as low as possible from the viewpoint of workability and weldability. Therefore, the upper limit of the content is set to 0.01%.

Ti + Nb ≧ 0.05%, Ti ≦ 0.5%, Nb ≦ 0.5%
Ti and Nb are elements that form carbides, nitrides, and carbonitrides and are effective in increasing the strength of the steel sheet. Further, it has an effect of suppressing recrystallization of ferrite during annealing, promotes transformation to austenite, and significantly promotes ferrite transformation during cooling after annealing. In addition, the crystal grain size is extremely refined, and the stripe pattern generated on the surface is reduced. In order to express the above effects, the total content is 0.05% or more. Moreover, even if it adds excessively, since an effect is saturated, the upper limit of each content was made into 0.5%.

  According to another aspect of the present invention, 1 is selected from the group consisting of V: 0.5% or less and W: 0.5% or less for the purpose of increasing strength by precipitation hardening instead of part of Fe. It may contain seeds or two kinds.

V: 0.5% or less, W: 0.5% or less V and W are elements that increase the strength by precipitation strengthening in the same manner as Nb and Ti. In order to ensure this effect, each content is preferably 0.01% or more. However, even if the content exceeds 0.5%, the strengthening ability is saturated, so that the cost of raw materials increases. Therefore, the upper limit of the content is 0.5%.

  Furthermore, in another aspect of the present invention, instead of a part of Fe, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1. It may contain one or more selected from the group consisting of 0% or less, Ni: 1.0% or less, and B: 0.01% or less.

Cr, Mo, Cu, Ni: 1.0% or less, B: 0.01% or less Each of the above elements from Cr to B has the effect of further enhancing the strength by solid solution strengthening, so each is contained alone You may make it contain, and may contain 2 or more types in combination.

  In order to reliably obtain the effect of solid solution strengthening, it is preferable that Cr, Mo, Cu, and Ni each have a content of 0.05% or more. About B, it is preferable to make it contain 0.0003% or more.

  However, even if Cr, Mo, Cu, and Ni are contained in excess of 1.0%, the strengthening ability is saturated and the cost of raw materials increases, so the upper limit of each content is set to 1.0%. . Further, when B is contained in an amount exceeding 0.01%, the ductility is lowered, and further, the cost of raw materials is significantly increased due to the reduction in yield. Therefore, the upper limit of the content is 0.01%.

  In still another aspect of the present invention, REM: 0.1% or less, Mg: for the purpose of improving bending workability by controlling the form of oxides, sulfides, etc., instead of a part of the above-mentioned Fe It may contain one or more selected from the group consisting of 0.01% or less and Ca: 0.01% or less.

REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less The above-mentioned REM (rare earth element), Mg, and Ca are formed into fine spheroids of oxides and sulfides and bent. Has the effect of improving the performance. Each of them may be contained alone or in combination of two or more.

  Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In addition, content of REM said by this invention points out the total content of the said element. In order to reliably obtain the effect of spheroidizing oxides and sulfides, it is preferable that REM, Mg, and Ca each have a content of 0.0005% or more. However, when REM exceeds 0.1% and Mg and Ca exceed 0.01%, oxides and sulfides are excessively present in the steel, so that bending workability deteriorates. Therefore, REM: 0.1% or less, Mg: 0.01% or less, Ca: 0.01% or less.

The steel composition of the steel sheet, which is the plating base of the high-tensile hot-dip galvanized steel sheet according to the present invention, is composed of the above elements and the balance consisting of Fe and impurities.
<Ratio of the C vicinity of the surface near the surface of the steel sheet and the area ratio of ferrite near the surface>
In order to obtain a steel sheet with excellent bending properties, plating adhesion and surface properties with a tensile strength of 750 MPa or more, in the vicinity of the surface at a depth of 1 to 10 μm from the surface of the steel sheet toward the center of the plate thickness. Ratio ([C] of average C concentration ([C] _s ) and internal average C concentration ([C] _B ) excluding a portion from the surface of the steel plate to a depth of 0.1 mm toward the center of the plate thickness. _S / [C] _B ) is 0.85 or less, and the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is 80% or more. is necessary.

When the ratio ([C] _s / [C] _B ) exceeds 0.85 or the area ratio of the ferrite is less than 80%, the bending characteristics and the surface properties during bending are remarkably deteriorated.
The lower limit of the ratio ([C] _s / [C] _B ) is not particularly limited. However, the lower limit is set to 0.6 from the viewpoint of realizing a stable C concentration distribution within a range where actual operation is easy. It is preferable to do.

  Here, in the present invention, the ferrite area ratio can be adjusted by a cooling rate from soaking in the pre-plating annealing. For example, to make it 80% or more, as described later, up to 600 ° C. The average cooling rate may be 40 ° C./s or less, and the average cooling rate from 600 ° C. to immersion in the plating bath may be 70 ° C./s or less.

<Plating>
The present invention is a hot-dip galvanized steel sheet and a method for producing the same, but the plating method and the amount of adhesion thereof are not particularly limited. Generally, 3 to 800 g / m ² per side is applied to the steel sheet surface as a plating adhesion amount. When the plating adhesion amount is less than 3 g / m ² , the anticorrosion action by plating is not sufficiently exhibited, and the purpose of galvanization cannot be achieved. On the other hand, if the adhesion amount exceeds 800 g / m ² , defects such as blow holes tend to occur remarkably during welding. From the viewpoint of the cost of plating adhesion, 10 to 200 g / m ² is a more preferable range. Moreover, after performing said plating, the adhesiveness of plating can be improved by performing an alloying process. The alloying degree of the plating film is preferably in a range of 3% to 20% in terms of Fe content. If it is less than 3%, the degree of alloying is low, and there is no effect of the alloying treatment. On the other hand, when the degree of alloying exceeds 20%, powdering deteriorates remarkably. From the viewpoint of securing plating adhesion and powdering properties by alloying treatment, a more preferable range of the degree of alloying is 7 to 15%.

<Production conditions>
(Steel making)
In the refining stage, it is not necessary to adjust the components by a special method, and the steel composition specified in the present invention may be adjusted by a conventional method.

  In the continuous casting stage, the components of the powder for continuous casting put into the mold at the time of continuous casting are limited. The powder for continuous casting is melted slag on the surface of molten steel to prevent oxidation of the molten steel surface in the mold, lubrication between the mold and slab, supplementation of inclusions floating on the meniscus, heat insulation of the molten steel surface in the mold, etc. It is essential for continuous casting operations.

The melting temperature and viscosity of the powder for continuous casting are adjusted by the mixing ratio of SiO ₂ , CaO, and other oxides as main components. Further, in order to adjust the melting rate and ensure heat retention, the aggregate usually contains about 4% by mass of C.

  Therefore, a normal continuous casting powder generates a concentrated layer of C on the surface of a slab by contacting with molten steel in a mold. This C-enriched layer remains through hot rolling and cold rolling, thereby increasing the carbon concentration from the surface of the sheet product to 10 μm.

As a method for preventing the influence of C enrichment generated on the surface of the slab, there is surface cutting (scarfing) at the slab stage. However, surface cutting causes a reduction in slab yield and a reduction in work efficiency. Depending on the components of the slab, cracking defects may occur on the surface of the slab due to abrupt heating during cutting. Therefore, while omitting surface cutting, C concentration on the surface of the slab is prevented, and the average C concentration in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness in the thin plate product stage ( [C] _s ) and the ratio ([C] _s / [) of the internal average C concentration ([C] _B ) excluding the portion from the surface of the steel plate to the depth of 0.1 mm toward the center of the plate thickness. In order to make C] _B ) 0.85 or less, the C concentration contained in the powder for continuous casting needs to be less than 1% by mass.

(Hot rolling)
The slab cast by the above method before hot rolling is set to a temperature of 1100 ° C. or higher. When the temperature of the slab before hot rolling is less than 1100 ° C, Ti and Nb carbides, nitrides, and carbonitrides precipitated during casting solidification do not re-dissolve, so precipitation strengthening of Ti and Nb is not possible. It will be enough. Moreover, when the temperature of a slab exceeds 1350 degreeC, it will deform | transform with the dead weight of a slab, and heat processing will become difficult.

  Thereafter, hot rolling is performed according to a conventional method. At this time, if the temperature of the entire length of the rough bar before rough rolling after rough rolling is equalized by induction heating or the like, characteristic fluctuation due to temperature fluctuation can be suppressed.

Finish rolling is performed at ₃ or more points of Ar. If the Ar is less than ₃ points, volumetric expansion caused by ferrite transformation occurs during finish rolling, resulting in deterioration of sheet thickness accuracy and operation trouble.
As for the coiling temperature, when the temperature exceeds 700 ° C., the generation of scale flaws on the coil surface is remarkable, leading to deterioration in quality and a decrease in yield, so the upper limit is set to 700 ° C. When it is desired to suppress the amount of alloying element added, it is preferable to increase the winding temperature to 550 ° C. or lower, since it is possible to effectively increase the strength. The lower limit is not particularly limited, but when cold rolling is performed in the subsequent steps, if the coiling temperature is less than 400 ° C., it is hardened and cold rolling becomes difficult, so the lower limit should be 400 ° C. Is preferred. When there is no need to limit the amount of alloying element added, the winding temperature is preferably set to 550 ° C. or higher from the viewpoint of rollability.

(Pickling)
The pickling and cold rolling after hot rolling may be performed in a conventional manner. 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-enriched elements, and has the effect of expanding the preferred range of Si and P that are restricted from the viewpoint of hot-plated adhesion.

(Cold rolling)
Cold rolling may be performed by a conventional method as necessary for adjusting the plate thickness and surface roughness. There is no particular problem with the rolling reduction in the range of 35 to 80%. A higher rolling reduction is preferable because it promotes transformation to austenite during annealing, and thus has the effect of expanding the preferred range of annealing conditions.

(Annealing before plating and hot dip galvanizing)
About the manufacturing conditions in a continuous hot dip galvanizing line, it is common in the case where a plating base material is a hot-rolled steel plate, and the case where it is a cold-rolled steel plate, and it is set as the following conditions.

First, the steel sheet is held in the temperature range of Ac _{3 to} 1000 ° C. for 5 seconds or more and soaked. If the soaking temperature is less than Ac ₃ points or the soaking time is less than 5 seconds, the phase transformation to austenite does not proceed sufficiently, so that many phases other than ferrite formed in the hot rolling process remain in the sheet product stage. As a result, it becomes difficult to make the ferrite area ratio 80% or more in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness in the thin plate product stage. On the other hand, when the temperature exceeds 1000 ° C., crystal grains grow excessively and the strength is insufficient.

  About cooling after soaking, the average cooling rate is set to 40 ° C./s or less up to 600 ° C. The lower limit of the cooling rate is 1 ° C./s from the viewpoint of operation efficiency. In the case of the present invention, a large amount of Ti and Nb is added. Therefore, by setting the cooling rate to 1 to 40 ° C./s, the amount of ferrite necessary to ensure the desired bending workability, that is, a thin plate In the product stage, the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness can be 80% or more.

  In this regard, in the above-mentioned Patent Documents 1 and 2, the structure in the vicinity of the surface portion has a ratio of the second phase centered on tempered martensite of 20% or more, and the ferrite area ratio is 80%. % Or less.

  Regarding hot dip galvanization, it is immersed in a hot dip galvanizing bath at 440 to 520 ° C. according to a conventional method. About cooling from the above-mentioned 600 degreeC to this temperature range of 440-520 degreeC, if an average cooling rate is 70 degrees C / s or less, there is no problem. When the cooling rate is higher than 70 ° C./s, the martensite area ratio increases, and the ferrite area ratio in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel sheet toward the center of the plate thickness in the thin plate product stage. It is because it becomes less than 80% and bending workability deteriorates. Also here, the lower limit of the cooling rate is 1 ° C./s from the viewpoint of operation efficiency. Further, by maintaining the stay time in the temperature range of 400 to 600 ° C. for 5 to 100 s before plating immersion, it is possible to further increase the area ratio of the ferrite and further improve the bending workability.

(Alloying treatment)
After immersion in the plating bath, an alloying treatment may be performed both when the plating substrate is a hot-rolled steel plate and when it is a cold-rolled steel plate. In the case of the present invention, the alloying processability is very high due to the large addition of Ti + Nb. Furthermore, the alloying treatment is promoted under light pressure before or after pickling, or by addition of Cu and Ni, and the preferred range of Si and P can be expanded.

Specific examples of the present invention will be described below.
Steels having chemical components A to L shown in Table 1 were melted in a converter and continuously cast by a test continuous casting machine to obtain a slab having a width of 1000 mm and a thickness of 250 mm. At that time, continuous casting powders having different C concentrations shown in Table 2 were used.

  Using the test rolling mill, the obtained slab was heated under the conditions shown in Tables 3 to 4, followed by hot rolling, and then pickling. Thereafter, some of the steel sheets were cold-rolled at a cold pressure rate of 40 to 60%.

The hot-rolled steel sheet and the cold-rolled steel sheet thus obtained were annealed in the laboratory under the conditions shown in Table 3, and then hot-dip galvanized on the continuous hot-dip galvanizing test line. . The amount of plating was 20 to 150 (g / m ² ). Some steel plates were also alloyed at a furnace temperature of about 800-1300 ° C. after plating.

1) Evaluation of C concentration of hot-rolled steel sheet and cold-rolled steel sheet A sample was cut out from a quarter part of the sheet width, and the plating layer of the hot-dip galvanized steel sheet was removed by the antimony chloride method specified as an adhesion amount test method in JIS H0401. After that, the C concentration in the depth direction was measured with a 1 μm pitch by glow discharge spectroscopy (GDS), and the average value was defined as the surface C concentration ([C] _S ).

That is, if the C concentration at the depth of _i μm is [C] _i

である。
また、同様にめっき層を除去した後にサンプルの表裏面をそれぞれ０．１ｍｍづつ研削し、燃焼法にて分析し、バルクＣ濃度（［Ｃ］_Ｂ）とした。

It is.
Similarly, after removing the plating layer, the front and back surfaces of the sample were each ground by 0.1 mm and analyzed by a combustion method to obtain a bulk C concentration ([C] _B ).

2) Characteristic evaluation The obtained steel sheet was subjected to a tensile test, a limit bending test, a plating adhesion measurement, a plating adhesion test, and a surface property evaluation. The results are shown in Tables 5-6.

2-1) Tensile test A JIS No. 5 test piece was taken from the direction perpendicular to the rolling direction of each steel plate. The test method conformed to JIS Z2241. Yield point YP, tensile strength TS, and elongation El were measured.

2-2) Limit bending test A test piece having a width of 40 mm and a length of 200 mm was taken from the direction perpendicular to the rolling direction of each steel plate. The test shape and test method conformed to JIS Z2248. The bending radius was 1 to 2, 3 times, and 4 times the plate thickness from the close contact, and the bending radius at which no crack occurred visually at the bending ridge was defined as the limit bending radius.

2-3) Amount of plating adhesion Three test pieces of 57.2 mm square were collected from each steel plate, and the amount of adhesion was measured. The method for measuring the amount of adhesion was in accordance with JIS H0401.

2-4) Plating adhesion After collecting three test pieces from each steel plate and cylindrically forming them at a drawing ratio of 1.8, the presence or absence of peeling was evaluated with respect to plating adhesion in a plating peeling test using an adhesive tape. did.

2-5) Surface properties The same sample as that in the limit bending test was bent to a bending radius of 2.0 t, and the surface was observed and judged for occurrence of streaks with irregularities on the surface. At that time, a surface property defect was defined as 5 or more streaks crossing the specimen in the bending ridge line direction.

<Invention>
Specimen No. which is the present invention. In Nos. 1 to 21, the critical bending radius was close to 1.5 t, and the plating adhesion was also excellent. The surface properties were all good.

<Comparative example>
Specimen No. In Nos. 22 to 27, the C concentration contained in the powder during continuous casting was outside the scope of the present invention. Regardless of whether or not the surface of the slab is cut by 1.0 mm, the average C concentration ([C] _s ) in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is measured. The ratio ([C] _s / [C] _B ) with the internal C concentration ([C] _B ) excluding the portion from the surface toward the center of the plate thickness up to a depth of 0.1 mm exceeds 0.85. Bending radius ≥ 2.0t, bending workability deteriorated. A streak pattern was also generated on the surface.

  Specimen No. No. 28 was outside the scope of the present invention, with the heating temperature before hot rolling being 1080 ° C. Therefore, Ti and Nb-based carbides and nitrides were not sufficiently precipitated during slab heating, and the final strength was less than 750 MPa. Further, since the generation of ferrite starting from carbide was small and the ferrite area ratio on the surface was lowered, bending workability was deteriorated.

Specimen No. No. 29 was outside the scope of the present invention because the finishing temperature during hot rolling was 720 ° C. and became Ar ₃ or less of the material D. Therefore, during rolling, volume fluctuation due to ferrite generation occurred and rolling trouble occurred, so that the steel sheet could not be collected.

  Specimen No. No. 30 was outside the present invention, with the finishing temperature during hot rolling being 720 ° C. Therefore, scale generation occurred on the surface even after coil winding, and scale wrinkles occurred even after pickling, so that plating could not be performed and the steel sheet could not be evaluated.

Specimen No. Nos. 31 to 32 had low soaking temperatures before plating and were outside the scope of the present invention. Since the heating temperature was ₃ points or less for each material, the austenite single phase was not achieved, and the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the thickness center was less than 80%. As a result, bending workability deteriorated.

  Specimen No. Nos. 33 to 34 had high soaking temperatures before plating and were outside the scope of the present invention. Therefore, the strength was less than 750 MPa due to excessive grain growth. Also, due to coarse grain growth, a sufficient ferrite area ratio in the vicinity of the surface could not be obtained, and bending workability deteriorated.

  Specimen No. 35 was outside the present invention, soaking time before plating was 4 seconds. Therefore, since austenite generation during soaking is insufficient, the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is less than 80%, and the bendability workability Worsened.

  Specimen No. 36 was outside the scope of the present invention, with a cooling rate from soaking to 600 ° C. of 42 (° C./s). Therefore, the generation of ferrite was insufficient, and the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness was less than 80%, and the bendability was deteriorated.

  Specimen No. No. 37 was outside the present invention, with a cooling rate of 72 (° C./s) until dipping in the plating bath. Therefore, the martensite which is a hard phase is excessively generated, and the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness becomes less than 80%, and the bendability is deteriorated. did.

Specimen No. No. 38, the C concentration of the base material was outside the present invention. Therefore, although manufactured according to the manufacturing method of the present invention, ([C] _s / [C] _B ) exceeded 0.85, the limit bending radius ≧ 2.0 t, and bending workability deteriorated. A streak pattern was also generated on the surface.

  Specimen No. In No. 39, the Si concentration of the base material was outside the scope of the present invention. Therefore, although manufactured according to the manufacturing method of the present invention, the surface properties were poor, and the plating adhesion was also poor.

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 being in mass%, C: 0.05 to 0.20%, Si: 0.5% or less, Mn: 1.0 -3.0%, P: 0.05% or less, S: 0.05% or less, sol. Al: 0.1% or less and N: 0.01% or less, and Ti: 0.5% or less and Nb: 0.5% or less, or a total of 0.05% or more And the balance of the steel composition comprising Fe and impurities, the average C concentration ([C] _S ) in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel sheet toward the thickness center, and the surface of the steel sheet The ratio ([C] _S / [C] _B ) to the internal average C concentration ([C] _B ) excluding the portion up to the depth of 0.1 mm from the center to the thickness center direction is 0.6 or more and 0 The tensile strength is 750 MPa or more, wherein the area ratio of ferrite in the vicinity of the surface having a depth of 1 to 10 μm from the surface of the steel plate toward the center of the plate thickness is 80% or more. High tensile hot dip galvanized steel sheet.   The steel composition further includes one or two kinds selected from the group of V: 0.5% or less and W: 0.5% or less in mass% instead of a part of Fe. The high-tensile hot-dip galvanized steel sheet according to claim 1.   The steel composition is replaced by a part of Fe in mass%, Cr: 1.0% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, and B The high-tensile hot-dip galvanized steel sheet according to claim 1 or 2, further comprising one or more selected from the group of 0.01% or less.   The steel composition is one or two selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass% instead of part of Fe. The high-tensile hot-dip galvanized steel sheet according to any one of claims 1 to 3, further comprising the above. A method for producing a high-tensile hot-dip galvanized steel sheet comprising the following steps (1-1) to (1-4):
(1-1) A slab formed by continuous casting with a molten steel having the steel composition according to any one of claims 1 to 4 having a C concentration of less than 1% by mass contained in powder for continuous casting charged into a mold. Continuous casting process;
(1-2) A hot rolling step in which the cast slab is hot rolled after being made 1100 ° C. to 1350 ° C., and finished at a finishing temperature: Ar ₃ points or more and a winding temperature: 700 ° C. or less;
(1-3) A pickling process in which the hot-rolled steel sheet is pickled to remove the scale on the surface of the steel sheet to obtain a pickled steel sheet; and (1-4) the pickled steel sheet is Ac _{3 to} 1000 ° C. After maintaining in the temperature range for 5 seconds or more, it is cooled to 600 ° C. at an average cooling rate of 1 to 40 (° C./s), and further cooled at an average cooling rate of 1 to 70 (° C./s) to melt at 440 to 520 ° C. A continuous hot dip galvanizing process in which hot dip galvanizing is performed by dipping in a galvanizing bath. A method for producing a high-tensile hot-dip galvanized steel sheet comprising the following steps (2-1) to (2-5):
(2-1) A slab formed by continuous casting with the molten steel having the steel composition according to any one of claims 1 to 4 contained in a continuous casting powder charged into a mold with a C concentration of less than 1% by mass. Continuous casting process;
(2-2) A hot rolling step in which the cast slab is hot rolled after the temperature is set to 1100 ° C. to 1350 ° C. to obtain a hot rolled steel sheet with a finishing temperature: Ar ₃ points or higher and a winding temperature: 700 ° C. or lower;
(2-3) A pickling process in which the hot-rolled steel sheet is subjected to a pickling treatment to remove scale on the surface of the steel sheet to obtain a pickled steel sheet;
(2-4) a cold rolling process in which the pickled steel sheet is cold-rolled to form a cold-rolled steel sheet; and (2-5) the cold-rolled steel sheet is held in a temperature range of Ac _{3 to} 1000 ° C for 5 seconds or more. Then, it is cooled to 600 ° C. at an average cooling rate of 1 to 40 (° C./s), further cooled at an average cooling rate of 1 to 70 (° C./s), and immersed in a hot-dip galvanizing bath at 440 to 520 ° C. Continuous hot dip galvanizing process for hot dip galvanizing.   The method for producing a high-tensile hot-dip galvanized steel sheet according to claim 5 or 6, wherein, in the continuous hot-dip galvanizing step, an alloying treatment is performed on the hot-dip galvanized steel sheet.