JP2014009399A - High strength galvanized steel sheet and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength galvanized steel sheet having excellent bending formability and excellent plating adhesiveness in a formed portion and a method for manufacturing the steel sheet.SOLUTION: The galvanized steel sheet has a chemical composition containing, by mass%, C: 0.02% or more and 0.30% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.1% or more and 3.0% or less, P: 0.003% or more and 0.080% or less, S: 0.010% or less, Al: 0.001% or more and 0.200% or less, Ti: 0.03% or more and 0.40% or less and the balance being Fe and inevitable impurities, and the surface of the steel sheet is coated with a zinc plating layer, in which the coating weight is 20 to 120 g/mper side, and in which the depth of a crack, which is present at grain boundaries in the surface layer of the base steel directly under the coating layer, is 2 μm or less.

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet suitable as an automotive rust-proof surface-treated steel sheet and a method for producing the same.

  Conventionally, hot rolled steel sheets of TS440 MPa class or lower have been used for members such as automobile and truck frames and undercarriages. However, recently, for the purpose of improving the anti-collision properties of automobiles and preserving the global environment, high strength and thinning of steel sheets for automobiles has been promoted, and high strength hot rolled steel sheets of TS590MPa class, TS780MPa class, and more than TS980MPa class. The use of is beginning to be considered.

  Many members for automobiles have complicated shapes obtained by press molding, and a material having high strength and excellent workability is required. On the other hand, from the viewpoint of securing the rust prevention power of the vehicle body due to the thinning of the steel sheet, there is a demand for a surface-treated steel sheet imparted with rust resistance to the steel sheet, particularly a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance Yes.

  Conventionally, several high-tensile hot-rolled steel sheets excellent in workability, hot-dip galvanized high-tensile steel sheets, and methods for producing the same have been proposed. For example, in Patent Document 1, C: 0.02 to 0.06%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0 by mass%. 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially made of Fe By melting steel and performing hot rolling under conditions where the finish rolling finish temperature is 880 ° C. or higher and the coiling temperature is 570 ° C. or higher, Ti and Mo having a substantially single-phase ferrite structure and an average grain size of less than 10 nm are obtained. There is disclosed a high-tensile steel sheet excellent in workability having a tensile strength of 590 MPa or more, and a method for producing the same, characterized in that a carbide containing is dispersed and precipitated.

  Moreover, in patent document 2, C: 0.01-0.1%, Si <= 0.3%, Mn: 0.2-2.0%, P <= 0.04%, S <= 0 in mass%. 0.02%, Al ≦ 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0% Steel, the balance being Fe and inevitable impurities are melted, hot-rolled in the austenite single-phase region, wound at 550 ° C. or more, and after producing a single-phase ferrite hot-rolled steel sheet, the scale is further removed. By applying the hot dip galvanizing as it is, it is 2.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5 by mass%, the structure is ferrite with an area ratio of 98% or more, and the atomic ratio is (Mo + W) / Within a range satisfying (Ti + Mo + W) ≧ 0.2, Ti and one or more of Mo and W Wherein the precipitates of less than 10nm, including the presence dispersed manufacturing method of hot-dip galvanized high-strength hot-rolled steel sheet is disclosed.

  However, in Patent Documents 1 and 2, in order to precipitate fine carbides including Ti and Mo in the ferrite, the coiling temperature is 550 ° C. or higher after finishing rolling (hereinafter sometimes referred to as CT). It is necessary to perform winding. When a hot-rolling base material containing an element that is easier to oxidize than Fe, such as Si and Mn (hereinafter sometimes referred to as an easily oxidizable element), is subjected to a winding process under such a high CT condition In addition, an internal oxide containing an easily oxidizable element is generated in the grain boundary part of the steel sheet surface layer. Oxides are selectively removed to form voids. Thereby, in the subsequent hot dip galvanizing process, molten zinc enters into the voids of the grain boundaries, thereby causing embrittlement of the grain boundary parts and a problem that bending workability is remarkably deteriorated. Further, in the alloying process after the hot dip galvanizing process, there is a problem that the plating adhesion of the processed part deteriorates due to excessive acceleration of the Zn—Fe alloying reaction from the grain boundary part.

JP 2002-322543 A JP 2003-321736 A

  The present invention has been made in view of such circumstances, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet excellent in plating adhesion of a processed part and a method for producing the same while ensuring good bending workability. And

  As a result of intensive studies on the plating treatment of high-strength steel sheets, the present inventors have obtained the following knowledge.

  As described above, as a cause of deterioration of the bending workability after plating, a fine crack is caused by excessive pickling at the grain boundary portion of the steel sheet surface layer during pickling after hot rolling, and subsequent hot dip galvanizing. It is thought that the brittleness of the grain boundary portion occurs due to the molten zinc entering the cracks in the grain boundary during the treatment. For this reason, it discovered that it was very important to control the crack depth of the grain-boundary part of the steel plate surface just under a plating layer.

The present invention is based on the above findings, and features are as follows.
[1] By mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.50%, Mn: 0.1% to 3.0%, P: 0.0. 003% or more and 0.080% or less, S: 0.010% or less, Al: 0.001% or more and 0.200% or less, Ti: 0.03% or more and 0.40% or less, with the balance being Fe and The steel sheet surface having an inevitable impurity composition has a galvanized layer with a plating adhesion amount of 20 to 120 g / m ^{2 on} one surface, and the crack depth at the grain boundary portion on the surface of the base material steel plate immediately below the plated layer. A high-strength hot-dip galvanized steel sheet characterized by having a thickness of 2 μm or less.
[2] The steel sheet further has a component composition of mass%, Nb: 0.001% to 0.200%, V: 0.001% to 0.500%, Mo: 0.01% or more. The high-strength hot-dip galvanized steel sheet according to [1], containing 0.50% or less and W: one or more of 0.001% to 0.200%.
[3] The high-strength melt according to [1] or [2], wherein the steel sheet further contains B: 0.0002% or more and 0.0050% or less as a component composition by mass%. Galvanized steel sheet.
[4] The high-strength hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the steel sheet is a hot-rolled steel sheet.
[5] After hot rolling the steel having the component composition described in any one of [1] to [3], cooling and winding are performed, and then pickling to obtain a hot rolled steel sheet, and then continuous annealing And a method of producing a high-strength hot-dip galvanized steel sheet that is subjected to hot dip galvanizing treatment, the finish rolling end temperature in the hot rolling is 850 ° C. or higher, the coiling temperature is 540 ° C. or lower, and the steel plate and pickling liquid during pickling The relative flow velocity of the steel sheet is set to 1.2 m / sec or less, and the maximum reached temperature of the steel sheet during continuous annealing is set to 600 ° C. or higher and 800 ° C. or lower.
[6] After the hot dip treatment, the steel sheet is further heated to a temperature of 450 ° C. or higher and 550 ° C. or lower to perform alloying treatment, and the Fe content in the plating layer is in the range of 7 to 15%. The method for producing a high-strength hot-dip galvanized steel sheet according to [5].
In the present invention, high strength means that the tensile strength TS is 590 MPa or more.

  According to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet that is excellent in plating adhesion at a processed portion while ensuring good bending workability.

  Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element of the steel component composition is “mass%”, and hereinafter, simply indicated by “%” unless otherwise specified.

Hereinafter, the present invention will be described in detail.
(1) Component composition C of steel sheet: 0.02% or more and 0.30% or less C is an element necessary for precipitating carbide in the steel sheet, and for that purpose, 0.02% or more is necessary. On the other hand, if it exceeds 0.30%, the weldability deteriorates, so the upper limit is made 0.30%.

Si: 0.01% or more and 2.50% or less Si is effective as a solid solution strengthening element, and in order to exhibit the strengthening effect, it is necessary to contain 0.01% or more. On the other hand, if it exceeds 2.50%, the oxide of Si is concentrated on the surface of the steel sheet during the annealing process, causing non-plating defects and plating adhesion deterioration, so the upper limit is made 2.50%.

Mn: 0.1% or more and 3.0% or less Mn is added to increase the strength, and it is necessary to contain 0.1% or more for the strengthening effect to appear. On the other hand, if it exceeds 3.0%, the Mn oxide is concentrated on the surface of the steel sheet in the annealing process, causing non-plating defects and plating adhesion deterioration, so the upper limit is made 3.0%.

P: 0.003% to 0.080% P is one of the elements inevitably contained. If it is less than 0.003%, there is a concern about an increase in cost. On the other hand, when P exceeds 0.080%, weldability deteriorates. Furthermore, since P raises the alloying temperature, the surface quality deteriorates. Specifically, a desired degree of alloying cannot be achieved unless the alloying temperature is increased during the alloying process. When the alloying treatment temperature is raised to obtain a desired degree of alloying, the ductility deteriorates and at the same time the adhesion of the alloyed plating film deteriorates. From the above, in order to achieve a desired degree of alloying, good ductility, and an alloyed plating film, the content is made 0.080% or less.

S: 0.010% or less Since S segregates at the grain boundaries, the toughness decreases. Moreover, when MnS produces | generates abundantly, toughness will fall. From the above, it is necessary to make it 0.010% or less. The lower limit of the S content is not particularly limited, and may be about the impurity level.

Al: 0.001% or more and 0.200% or less Al is added for the purpose of deoxidation of molten steel, but when the content is less than 0.001%, the purpose is not achieved. On the other hand, if it exceeds 0.200%, a large amount of inclusions are generated, which causes wrinkling of the steel sheet. From the above, Al is made 0.001% or more and 0.200% or less.

Ti: 0.03% or more and 0.40% or less Ti is an element necessary for increasing the strength by precipitating carbide in the steel sheet, and is also an effective element from the viewpoint of cost. However, if it is less than 0.03%, the amount of precipitates necessary for increasing the strength is insufficient. On the other hand, if it exceeds 0.40%, the effect is saturated and the cost is increased. Accordingly, Ti is set to 0.03% or more and 0.40% or less.

Moreover, in order to control intensity | strength, you may contain the following elements other than the said element.
Nb: 0.001% to 0.200%, V: 0.001% to 0.500%, Mo: 0.01% to 0.50%, W: 0.001% to 0.200% One or more of the following, Nb, V, Mo, W are elements that are effective for precipitating fine carbides in a stable manner by precipitating as a composite carbide containing Ti in the steel sheet. 1 type or 2 types or more may be contained. However, if the content is less than the specified value, the effect of increasing the strength by precipitation is insufficient, and if the content exceeds the specified value, the effect is saturated and the cost is increased.

Furthermore, in addition to the above elements, the following elements may be contained.
B: 0.0002% or more and 0.0050% or less B is an effective element for improving the hardenability, but if it is less than 0.0002%, it is difficult to obtain the quenching promoting effect. On the other hand, if it exceeds 0.0050%, the effect is saturated and the cost is increased. Therefore, when contained, B is made 0.0002% or more and 0.0050% or less.

  The balance other than the above is Fe and inevitable impurities.

(2) Surface of the base steel plate immediately below the plating layer In the high-strength hot-dip galvanized steel plate of the present invention, the crack depth of the grain boundary portion on the base steel plate surface needs to be 2 μm or less. By controlling the crack depth of the grain boundary part within this range, penetration of hot dip galvanizing into the grain boundary part is suppressed, and good bending workability is obtained due to an increase in grain boundary strength. Moreover, the excessive Zn-Fe alloying reaction after an alloying process is suppressed, and the plating adhesiveness of a process part improves. More preferably, it is 1.2 μm or less.

  In addition, the crack depth of the steel plate surface layer part just under a plating layer can be confirmed with the following method. For example, after dissolving and removing the plating layer, embedding and polishing the steel sheet in a resin to obtain a cross section of the steel sheet and performing SEM observation of the steel sheet surface layer part can confirm the presence and depth of cracks in the grain boundary part. it can.

(3) Manufacturing method of high-strength hot-dip galvanized steel sheet Next, the manufacturing method of the high-strength hot-dip galvanized steel sheet according to the present invention and the reason for limitation will be described.

In the present invention, the steel having the above component composition is hot-rolled, and after finishing rolling, cooling and winding are performed, then pickling is performed to obtain a hot-rolled steel sheet, and then continuous annealing and hot-dip galvanizing are performed. In this case, the finish rolling finish temperature in the hot rolling is 850 ° C. or more, the coiling temperature is 540 ° C. or less, the relative flow rate of the steel plate and pickling solution during pickling is 1.2 m / sec or less, and during continuous annealing. The steel sheet maximum temperature is 600 ° C. or higher and 800 ° C. or lower.
First, hot rolling conditions will be described.

If the finish rolling finish temperature is 850 ° C. or higher and the finish rolling finish temperature is less than 850 ° C., the cumulative amount of strain that occurs due to the progress of rolling without recrystallization increases, leading to an increase in rolling load. Therefore, the finish rolling end temperature is set to 850 ° C. or higher. Although it does not specifically limit about an upper limit, 1100 degrees C or less is preferable.

When the coiling temperature is 540 ° C. or less, when the coiling temperature exceeds 540 ° C., an internal oxide due to an easily oxidizable element is formed at the grain boundary portion of the steel sheet surface layer, and the oxide is selectively dissolved during the subsequent pickling treatment. Thereby, a fine crack arises in a grain boundary part. Thereby, molten zinc permeates into the grain boundary during the subsequent hot dip galvanizing treatment, causing a decrease in grain boundary strength, leading to deterioration of the bent portion. Furthermore, the Zn—Fe alloying reaction is excessively promoted during the alloying treatment, thereby causing a decrease in the plating adhesion of the processed part. Therefore, the coiling temperature is 540 ° C. or lower.

  Next, conditions for pickling after hot rolling will be described.

In the present invention, at the time of pickling after hot rolling, the relative flow rate of the steel plate and the pickling solution is set to 1.2 m / sec or less, so that the grain boundary Excess pickling in the part is suppressed, and bending workability and plating adhesion of the processed part are improved. More preferably, it is 1.0 mm / sec or less.

  Examples of the method for controlling the relative flow rate of the steel plate and the pickling solution include a method for controlling the traveling speed of the steel plate in the pickling treatment facility (hereinafter also referred to as LS). For example, when the pickling process is performed with LS being 72 m / min or less, the relative flow velocity is 1.2 m / sec or less. However, LS in a normal pickling line is operated in the range of 100 to 200 m / min. That is, lowering the LS to reduce the relative flow velocity causes a reduction in productivity. For this reason, as a method for controlling the relative flow rate of the steel plate and the pickling solution, it is effective to give a jet in the same direction as the traveling direction of the steel plate in the pickling solution to reduce the relative flow rate. At this time, it is desirable to use a slit nozzle having a constant width so that a uniform flow velocity is given to the entire width of the steel plate.

  Next, continuous annealing conditions and hot dip galvanizing conditions will be described.

In the method for producing a high-strength hot-dip galvanized steel sheet according to the present invention, it is necessary to control hot-rolling CT to be low in order to suppress the formation of internal oxides during hot rolling. However, when CT is low, fine carbides in the steel sheet structure are not sufficiently precipitated during winding, making it difficult to obtain desired strength and workability. For this reason, by controlling the annealing temperature in the subsequent continuous hot dip galvanizing equipment, Ti or the like that has not yet precipitated during hot rolling and is in a solid solution state is precipitated as carbide. Thereby, the intensity | strength of a steel plate and workability improve.
When the steel plate maximum temperature is less than 600 ° C, fine carbides are not sufficiently precipitated in the steel sheet structure, not only causing a decrease in strength but also reducing the natural oxide film present on the steel sheet surface, and subsequent melting. In the galvanizing process, non-plating occurs. On the other hand, when the temperature exceeds 800 ° C., carbides precipitated in the steel sheet structure are coarsened, leading to deterioration of strength and workability, and oxidizable elements such as Si and Mn are concentrated on the surface to form oxides. Causes non-plating.

  The hot dip galvanizing treatment conditions in the present invention may be conventional methods. Moreover, when alloying treatment is performed after the hot dip galvanizing treatment to obtain an alloyed hot dip galvanized steel sheet, the Fe content in the plating layer is set to 7 to 15%. If it is less than 7%, not only a uniform surface appearance cannot be obtained due to the occurrence of uneven alloying, but also the Zn-Fe alloying reaction is insufficient, so that a soft ζ phase is formed thick on the plating surface layer, and plating is performed during bending. The flaking that causes the layer to peel off in the form of flakes is caused. On the other hand, if it exceeds 15%, the Zn—Fe alloying reaction proceeds excessively, a brittle Γ phase is generated in the vicinity of the interface between the plating layer and the steel sheet, and the plating adhesion deteriorates.

As described above, the high-strength hot-dip galvanized steel sheet of the present invention is obtained. The high-strength hot-dip galvanized steel sheet of the present invention is characterized by having a galvanized layer having a plating adhesion amount of 20 to 120 g / m ^{2 on} one surface of the steel sheet. If it is less than 20 g / m ² , it will be difficult to ensure corrosion resistance. On the other hand, when it exceeds 120 g / m < ² >, plating adhesiveness will fall.

  After heating the slab having the composition shown in Table 1 at 1250 ° C., hot rolling is performed under the conditions shown in Table 2, and further black scale removal is performed by pickling, and a hot-rolled steel sheet having a thickness of 2.3 mm is obtained. did.

Next, continuous annealing treatment was performed on the CGL line, and then immersed in an Al-containing Zn bath having a bath temperature of 460 ° C. to perform hot dip galvanizing treatment. Coating weight at this time was adjusted to per side ^{45g / m 2, 70g / m} 2, 140g / m 2 by gas wiping. The alloyed hot dip galvanized steel sheet was subjected to an alloying treatment after the hot dip galvanizing treatment.

  With respect to the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) obtained as described above, bending workability and work part plating adhesion were evaluated. Moreover, the crack depth of the grain boundary part by the cross-sectional observation of the base-material steel plate surface just under a plating layer was measured. The measurement method and evaluation criteria are shown below.

<Processability>
A bending workability test and a tensile test were performed to evaluate workability.
Based on JIS Z 2248, the bending workability test cut out a strip test piece perpendicular to the rolling direction, changed the bending radius, bent 180 ° U, and evaluated the critical bending radius. At this time, when the critical bending radius was 4.5 mm or less, the bending workability was good (◯), and when it exceeded 4.5 mm, the bending workability was poor (x).
In the tensile test, a JIS No. 5 tensile test piece was sampled from the sample at a direction of 90 ° with respect to the rolling direction, and the tensile test was performed at a constant crosshead speed of 10 mm / min in accordance with JIS Z 2241. MPa)) and elongation (El (%)). TS having a TS of 590 MPa or more and TS × El ≧ 15000 was judged good, and TS × El <15000 was judged poor.

<Plating adhesion of hot-dip galvanized steel sheet>
About the hot dip galvanized steel sheet which does not give an alloying process, the plating adhesiveness of the bending process part was evaluated. As an evaluation method, after bending the steel plate to 180 °, the outside of the bent portion was peeled off with tape, and the presence or absence of peeling of the plating layer was visually determined. The evaluation criteria are as follows.
○: Plating layer is not peeled ×: Plating layer is peeled <Plating adhesion of galvannealed steel sheet>
The plating adhesion of the galvannealed steel sheet was evaluated by the powdering test shown below.
An adhesive tape was affixed to the plated steel sheet, the tape was applied to the inside, and the tape was bent back 90 ° with a bending radius of 5 mm, and the peeled tape was analyzed with fluorescent X-rays. The Zn count number per unit length at this time was determined as the amount of plating peeling. The anti-powdering resistance is particularly good for rank 1 (◎), 2 for good (O) and 3 for the amount of plating peeling obtained in the above-mentioned powdering test according to the following criteria. Evaluated as defective (x). ◎ and ○ are acceptable.
Plating separation amount: Rank 0 to less than 3000: 1 (particularly good (（))
3000 or more and less than −6000: 2 (good (◯))
6000 or more: 3 (defect (x))
The results obtained as described above are shown in Table 2 together with the production conditions.

  From Table 2, the example of the present invention has good workability and good adhesion of the processed part (◯). On the other hand, the comparative example which does not satisfy the scope of the present invention is inferior in either evaluation result.

Claims (6)

In mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.50%, Mn: 0.1% to 3.0%, P: 0.003% or more 0.080% or less, S: 0.010% or less, Al: 0.001% or more and 0.200% or less, Ti: 0.03% or more and 0.40% or less, with the balance being Fe and inevitable impurities The surface of the steel sheet having a component composition comprising a galvanized layer having a coating adhesion amount of 20 to 120 g / m ² per one surface, and the crack depth of the grain boundary portion on the surface of the base steel sheet immediately below the plated layer is 2 μm. A high-strength hot-dip galvanized steel sheet characterized by:   The steel sheet further has a component composition of mass%, Nb: 0.001% to 0.200%, V: 0.001% to 0.500%, Mo: 0.01% to 0.50. % Or less, W: 0.001% or more and 0.200% or less of 1 type or 2 types or more are contained, The high-strength hot-dip galvanized steel sheet of Claim 1 characterized by the above-mentioned.   The high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein the steel sheet further contains B: 0.0002% or more and 0.0050% or less in terms of component composition.   The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel sheet is a hot-rolled steel sheet.   After hot-rolling the steel comprising the component composition according to any one of claims 1 to 3, cooling and winding treatment are performed, and then pickling to obtain a hot-rolled steel plate, followed by continuous annealing and hot-dip galvanizing treatment In the method for producing a high-strength hot-dip galvanized steel sheet, the finish rolling finish temperature in the hot rolling is set to 850 ° C. or higher, the coiling temperature is set to 540 ° C. or lower, and the relative flow rate of the steel plate and pickling solution during pickling is 1 A method for producing a high-strength hot-dip galvanized steel sheet, characterized in that the maximum steel sheet temperature during continuous annealing is 600 ° C. or higher and 800 ° C. or lower.   6. The steel sheet is further heated to a temperature of 450 ° C. or higher and 550 ° C. or lower after the hot dip plating treatment to give an alloying treatment, and the Fe content in the plating layer is in the range of 7 to 15%. A method for producing a high-strength hot-dip galvanized steel sheet as described in 1.