JP6209175B2 - Manufacturing method of hot-dip Zn-Al-Mg-based plated steel sheet with excellent plating surface appearance and burring properties - Google Patents

Description

  The present invention is excellent in workability such as plating surface appearance and burring property, which is suitable for a material of a member to be used after being subjected to some processing such as press processing and stretch flange processing, in applications where high corrosion resistance is required. Further, the present invention relates to a method for producing a molten Zn—Al—Mg based steel sheet.

  In recent years, as a social trend, in the field of automobiles and building materials, there has been a demand for higher strength for the purpose of weight reduction and resource saving. In addition, since various processes such as press working and stretch flange processing are performed, a material steel plate excellent in workability such as high burring property in addition to high strength is required. Furthermore, from the viewpoint of saving resources and omitting processes, it is required to extend the service life, omit post-plating, and the like, and thus a high-rust-proof high-strength steel sheet is required.

  Patent Document 1 discloses a molten Zn—Al—Mg-based plated steel sheet excellent in burring properties, molten metal embrittlement cracking resistance, and corrosion resistance of welds, and a method for producing the same. However, there is a problem that non-plating or the like occurs in the final product and a good plated surface appearance is not necessarily obtained, and the yield decreases.

JP 2012-193451 A

  An object of this invention is to provide the manufacturing method of the hot-dip Zn-Al-Mg type plated steel plate which has the tensile strength of 400 Mpa or more and was excellent in the plating surface external appearance and burring property in view of the above-mentioned problem.

In order to solve the above problems, the configuration of the present invention is as follows.
1) Material steel plate is C: 0.005-0.08%, Si: 0.8% or less, Mn: 0.1-1.8%, P: 0.05% or less, S: 0.02% Hereinafter, hot rolled into a steel material containing Ti: 0.02 to 0.2%, B: 0.0005 to 0.01%, Al: 0.1% or less, the balance being Fe and inevitable impurities In the process of sequentially performing pickling, annealing in a continuous hot dipping line and hot-dip Zn—Al—Mg plating, the coiling temperature in hot rolling is 500 ° C. to 650 ° C., with light pressure treatment after pickling treatment High-strength molten Zn-Al-Mg-based plating with excellent plating surface appearance and burring characteristics, characterized in that the rolling reduction rate is 2 to 8% and the annealing temperature in a continuous hot dipping line is 600 ° C to 750 ° C A method of manufacturing a steel sheet.

2) The method for producing a high-strength molten Zn—Al—Mg-based plated steel sheet according to 1), wherein the rolling reduction in the light rolling treatment is 4 to 6%.

3) The tensile strength is 400 MPa as described in 1) or 2), wherein the material steel plate further has a composition containing at least one of Nb: 0.1% or less and V: 0.1% or less in terms of mass%. The manufacturing method of the high intensity | strength hot-dip Zn-Al-Mg type plated steel plate excellent in the above plating surface external appearance and burring property.

4) The plating composition of the hot-dip Zn—Al—Mg based steel sheet is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10. %, B: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and inevitable impurities, 1) to 3) The manufacturing method of the high intensity | strength fusion | melting Zn-Al-Mg type plated steel plate as described in a crab.

  ADVANTAGE OF THE INVENTION According to this invention, the high intensity | strength fusion | melting Zn-Al-Mg type plated steel plate excellent in the favorable plating surface external appearance and burring property can be manufactured.

It is an example which shows a favorable plating external appearance. This is an example of poor appearance in which non-plating has occurred. It is a figure which shows the relationship between the reduction rate of the light reduction process in which a favorable plating external appearance is obtained, and the annealing temperature in a continuous hot dipping line. It is a figure which shows the relationship between the rolling reduction of a light rolling process, and the TSxlambda balance of a plated steel plate.

A mode for carrying out the present invention will be described. The chemical composition of the steel material and the reasons for limitation are as follows.
<C: 0.005-0.080%>
C forms an carbide containing Ti and finely precipitates in bainitic ferrite or ferrite structure, and thus is an element effective for increasing the strength. If the C content is less than 0.005%, it is difficult to obtain a strength of 400 MPa or more. If the C content exceeds 0.08%, the precipitates become coarse and the formation of hard second phase and cementite causes burring. At this time, the burring property is lowered because it becomes a starting point of cracking.

<Si: 0.8% or less>
Si is an element effective for solid solution strengthening. However, if it is added excessively, it is desirable to reduce it as much as possible because it forms an oxide on the surface of the steel sheet during heating in the continuous hot dipping line and inhibits the plating property, and the upper limit of the addition amount is 0.8%.

<Mn: 0.1 to 1.8%>
Mn is an element effective for increasing the strength. If it is less than 0.1%, it is difficult to obtain a strength of 400 MPa or more, and if added over 1.8%, segregation tends to occur, and the burring property is lowered because it becomes a starting point of cracking during burring. .

<P: 0.05% or less>
P is an element effective for solid solution strengthening, but if added over 0.05%, segregation tends to occur, and the burring property is lowered because it becomes a starting point of cracking during burring.

<S: 0.02% or less>
S forms sulfides with Ti and Mn, and becomes a starting point of cracking during burring, so that burring properties are lowered. For this reason, S is an element which should be reduced as much as possible.

<Ti: 0.02 to 0.2%>
Ti combines with C and precipitates as fine Ti carbides, and is an element effective for increasing the strength. Further, Ti has a high affinity with N and fixes N in the steel as TiN. Therefore, the addition of Ti is extremely effective in securing the B content of the structure strengthening element. In order to obtain these effects sufficiently, addition of 0.02% or more is necessary, and if added over 0.2%, workability is reduced.

<B: 0.0005 to 0.01%>
B is an element that suppresses the austenite-ferrite transformation of the steel, and by suppressing the austenite-ferrite transformation, it lowers the precipitation temperature of the Ti-based carbide and contributes to the refinement of the Ti-based carbide. If it is less than 0.0005%, the effect is not obtained. If it is added in excess of 0.01%, a boride is formed and the workability is deteriorated.

<Al: 0.1% or less>
Al is added as a deoxidizer during steelmaking. If it exceeds 0.1%, ductility is reduced.

<One or more of V: 1.0% or less, Nb: 0.1% or less>
Nb and V prevent the coarsening of γ grains during heating and hot rolling, and are effective in making ferrite grains fine. Further, similarly to Ti, a composite carbide containing C is formed, which contributes to an increase in strength. For this reason, it can contain 1 or more types of these elements as needed.

Next, manufacturing conditions and the reasons for limitation are as follows.
<The coiling temperature in hot rolling is 500 ° C. to 650 ° C.>
When the coiling temperature is less than 500 ° C., the amount of precipitation of carbide containing Ti becomes insufficient and the strength is lowered. On the other hand, if the coiling temperature exceeds 650 ° C., the carbide containing Ti is coarsened, and the strength and burring properties are reduced.

<Rolling ratio in light rolling treatment: 2 to 8%>
When pickling is performed on a hot-rolled steel sheet, smut is generated on the surface of the steel sheet. When plating is performed with the smut remaining on the surface of the steel sheet, the reaction between the plated metal and the steel sheet is hindered, and thus non-plating is likely to occur. The smut that causes non-plating can suppress the occurrence of non-plating by crushing the smut generated on the surface of the steel sheet by light pressure treatment after pickling or pulling it away from the surface of the steel sheet. However, when the rolling reduction is less than 2%, the pressure on the smut is small, so the smut is not effectively removed and non-plating is likely to occur. On the other hand, if the rolling reduction exceeds 8%, work hardening occurs and the burring property decreases, so the content was set to 8% or less.
Moreover, it is preferable that the rolling reduction in the light rolling treatment is 4 to 6%. By setting the rolling reduction to 4% or more, the smut on the steel sheet surface can be more reliably removed, and the occurrence of non-plating can be suppressed. On the other hand, by setting the rolling reduction to 6% or less, work hardening can be reduced and deterioration of burring properties can be suppressed. In particular, by setting the rolling reduction to 4 to 6%, it is possible to stably obtain a good plated surface appearance and high burring properties.

<Annealing temperature in continuous hot dipping line: 600-750 ° C.>
If the annealing temperature is less than 600 ° C., the surface of the steel sheet is not sufficiently reduced and the plateability is lowered. On the other hand, if the annealing temperature exceeds 750 ° C., the carbides become coarse, leading to a decrease in strength and a decrease in burring properties.

The composition of the plating layer and the reasons for limitation are as follows.
<Fused Zn-Al-Mg-based plating>
In the present invention, a known hot-dip Zn—Al—Mg-based plating method can be applied.
Al in a plating layer has the effect | action which improves the corrosion resistance of a plated steel plate. Moreover, it has the effect | action which suppresses generation | occurrence | production of Mg oxide type dross by containing Al in a plating bath. In order to obtain these effects sufficiently, the Al content of hot-dip plating needs to be 3.0% or more, and more preferably 4.0% or more. On the other hand, if the Al content exceeds 22.0%, the growth of the Fe—Al alloy layer becomes remarkable at the interface between the plating layer and the material steel plate, resulting in poor plating adhesion. In order to ensure excellent plating adhesion, the Al content is preferably 15.0% or less, and more preferably 10.0% or less. Mg in the plating layer exhibits a function of generating a uniform corrosion product on the surface of the plating layer and remarkably increasing the corrosion resistance of the plated steel sheet. In order to fully exert its action, the Mg content of the hot-dip plating needs to be 0.05% or more, and it is desirable to ensure 2.0% or more. On the other hand, when the Mg content exceeds 10.0%, an adverse effect that Mg oxide-based dross is likely to occur increases. In order to obtain a higher quality plating layer, the Mg content is preferably 5.0% or less, and more preferably 4.0% or less.

When Ti and B are contained in the hot dipping bath, the generation and growth of the Zn ₁₁ Mg ₂ phase that gives speckled appearance defects in the hot-dip Zn—Al—Mg based steel sheet are suppressed. Even if Ti and B are each contained alone, the effect of suppressing the Zn ₁₁ Mg ₂ phase is produced, but it is desirable to contain Ti and B in combination in order to greatly relax the degree of freedom of the production conditions. In order to sufficiently obtain these effects, it is effective that the Ti content of the hot dipping is 0.0005% or more and the B content is 0.0001% or more. However, if the Ti content is excessively large, Ti—Al-based precipitates are generated in the plating layer, and irregularities called “bumps” are generated in the plating layer, thereby impairing the appearance. For this reason, when adding Ti to a plating bath, it is necessary to set it as the content range of 0.10% or less, and it is more preferable to set it as 0.01% or less. On the other hand, when the B content is excessively large, Al—B or Ti—B based precipitates are generated and coarsened in the plating layer, and irregularities called “bumps” are also generated to impair the appearance. For this reason, when adding B to a plating bath, it is necessary to make it the content range of 0.05% or less, and it is more preferable to set it as 0.005% or less.

  When Si is contained in the hot dipping bath, the growth of the Fe—Al alloy layer is suppressed, and the workability of the hot-dip Zn—Al—Mg plated steel sheet is improved. Further, Si in the plating layer is effective in preventing the black change of the plating layer and maintaining the gloss of the surface. In order to sufficiently bring out such an action of Si, it is effective to set the Si content of the hot dipping to 0.005% or more. However, if Si is added excessively, the amount of dross in the hot dipping bath increases, so when Si is contained in the plating bath, the content range is 2.0% or less.

  A certain amount of Fe is mixed into the hot dipping bath from a raw steel plate or a pot component. In Zn—Al—Mg based plating, Fe in the plating bath is allowed to be contained up to about 2.0%. In the plating bath, as other elements, for example, one or more of Ca, Sr, Na, rare earth elements, Ni, Co, Sn, Cu, Cr, Mn may be mixed, but their total content Is desirably 1% by mass or less. The hot dip bath composition is almost directly reflected in the hot dip plated steel plate composition.

Each steel having the composition shown in Table 1 is melted and the slab is heated to 1250 ° C., then hot-rolled at a finish rolling temperature of 880 ° C. and a winding temperature of 520 to 680 ° C., and hot rolled to a thickness of 2.6 mm. A steel strip was obtained. The coiling temperature of each hot-rolled steel strip is shown in Table 2, respectively.
After pickling the hot-rolled steel strip, it is subjected to a light reduction treatment at a reduction ratio of 0 to 10%, and annealing is performed at 570 to 780 ° C. in a hydrogen-nitrogen mixed gas in a continuous hot dipping line, and an average cooling rate is obtained. The steel plate is cooled to about 420 ° C. at 5 ° C./sec to form a raw steel plate (plating original plate), and then in a molten Zn—Al—Mg plating bath having the following plating bath composition while the steel plate surface is not exposed to the atmosphere. After being immersed, the steel sheet was pulled up, and a hot-dip Zn—Al—Mg-based plated steel sheet having a plating adhesion amount adjusted to about 90 g / m ² per side by a gas wiping method was obtained. The plating bath temperature was about 410 ° C. The rolling reduction and annealing temperature of each steel are also shown in Table 2.

The composition (mass%) of the hot dipping bath is as follows.
Al: 6.0%, Mg: 3.0%, Ti: 0.002%, B: 0.0005%, Si: 0.01%, Fe: 0.1%, Zn: balance

  As for tensile properties, JIS No. 5 test piece taken so that the longitudinal direction of the test piece was perpendicular to the rolling direction of the raw steel plate was used, and tensile strength TS, total elongation T.sub. El was determined.

About burring property, the sample of 70x70 mm was extract | collected from the hot-dip Zn-Al-Mg type plated steel plate, and this was made into the base plate (blank material) for a burring property test. A punched hole was made in the center of the base plate using a punch and a die. The diameter _D of the punch ₀ 10.0mm, dice chose what clearance is 12% of the plate thickness. A punch with an apex angle of 60 ° was pushed into the punched hole from the burr side to enlarge the initial hole. At that time, the moving speed of the punch was set to 10 mm / min. Stopped punch when cracks in the thickness direction to expand the hole of the steel sheet was penetrated was measured internal diameter D _b of the hole. _Then, determine the _{(D b -D 0) / D} 0 × 100 hole expansion ratio defined by (%) lambda.
A TS × λ balance of 40000 MPa ·% or more was determined to be acceptable.

[Appearance of plating surface]
Evaluation of the plated surface appearance, a case where there is no occurrence of non-plating product 50cm in ² as "○", was a case where the non-plated product 50cm within ² has occurred more than one place as "×".

No. of the present invention. Nos. 1 to 15 have a tensile strength TS of 400 MPa or more and a TS × λ balance of 40000 MPa ·% or more. Further, as shown in FIG. 1, there is no occurrence of non-plating, and high strength melting excellent in plating surface appearance and burring properties. It is a Zn—Al—Mg-based plated steel sheet.
In contrast, no. No. 16 has a high winding temperature in hot rolling. Since No. 17 has a high annealing temperature in the continuous hot dipping line, the TS × λ balance is low. No. Nos. 18 and 19 have low annealing temperatures in the continuous hot dipping line and the steel sheet surface is not sufficiently reduced. 20, 21, and 22 have a low reduction ratio in the light reduction process, and the smut is not sufficiently removed. Therefore, non-plating as shown in FIG. 2 occurs. No. Nos. 23 and 24 have a high reduction ratio in the light reduction process, so that work hardening occurs and TS × λ is low.

  FIG. 3 shows the relationship between the reduction rate in the light reduction treatment and the annealing temperature in the continuous hot dipping line on the plating properties, and FIG. 4 shows the relationship between the reduction rate in the light reduction treatment and the TS × λ balance. By satisfying the rolling reduction ratio in the light rolling treatment: 2% or more and the annealing temperature in the continuous hot dip plating line: 600 ° C. or more, non-plating does not occur and a good plated surface appearance can be obtained. Moreover, the target of TS × λ balance is satisfied by satisfying the reduction ratio in light reduction treatment: 8% or less. From the above, high-strength molten Zn-Al excellent in plating surface appearance and burring properties by satisfying the rolling reduction in light reduction treatment: 2-8% and the annealing temperature in continuous hot dipping line: 600-750 ° C -It turns out that a Mg type plated steel plate is obtained.

Claims (4)

The material steel plate is C: 0.005 to 0.08%, Si: 0.8% or less, Mn: 0.1 to 1.8%, P: 0.05% or less, S: 0.02% or less, A steel material containing Ti: 0.02-0.2%, B: 0.0005-0.01%, Al: 0.1% or less, the balance being Fe and inevitable impurities, hot rolling, acid In the process of sequentially performing washing, annealing in a continuous hot dipping line, and hot-dip Zn-Al-Mg plating, the coiling temperature in hot rolling is 500 ° C to 650 ° C, and the reduction in the light reduction treatment after the pickling treatment Of high-strength hot-dip Zn-Al-Mg-based plated steel sheet excellent in plating surface appearance and burring characteristics, characterized in that the rate is 2 to 8% and the annealing temperature in a continuous hot dipping line is 600 ° C to 750 ° C Method.   The method for producing a high-strength molten Zn-Al-Mg-based steel sheet according to claim 1, wherein the rolling reduction in the light rolling treatment is 4 to 6%.   The material steel plate further has a composition containing at least one of Nb: 0.1% or less and V: 0.1% or less in terms of mass%, and the tensile strength is 400 MPa or more. A method for producing a high-strength hot-dip Zn—Al—Mg-based plated steel sheet excellent in plating surface appearance and burring properties.   The plating composition of the hot-dip Zn—Al—Mg based steel sheet is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, 4. B: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and unavoidable impurities. The manufacturing method of the high intensity | strength molten Zn-Al-Mg type plated steel plate.