JP4544579B2 - Manufacturing method of high strength molten Zn-Al-Mg alloy plated steel sheet - Google Patents

Description

  The present invention relates to a method for producing a high-strength molten Zn—Al—Mg alloy-plated steel sheet exhibiting ductility that can withstand severe press forming and having excellent strength and corrosion resistance.

  High-strength steel sheets have been used for structural members, suspension members, safety members, and the like of automobiles with the aim of improving fuel efficiency and safety. In this type of application, high strength steel sheets are required to have good strength and corrosion resistance in addition to excellent workability such as stretch forming and stretch flange forming in response to the complexity of press forming. . In particular, ferrite and martensite composite structure steel has a lower yield ratio than other high-strength steel sheets such as precipitation-strengthened steel, and therefore has a small spring back after press forming and excellent shape freezing property.

  Corrosion resistance is improved by using alloyed hot-dip galvanized steel sheets instead of cold-rolled steel sheets, as is the case with high-strength steel sheets. Is also underway. An alloyed hot-dip galvanized steel sheet is manufactured by immersing and lifting a plating base plate in a hot-dip galvanizing bath having a bath temperature of 450 to 470 ° C., and then alloying at 500 to 550 ° C. However, pearlite transformation is likely to proceed during immersion in a hot dip galvanizing bath and alloying treatment, and the amount of martensite tends to decrease due to pearlite transformation. As a result, when the composite structure steel is used for a plating base plate, it tends to cause an increase in yield ratio and a decrease in strength.

It is known that the addition of quenching strengthening elements such as Cr and Mo suppresses pearlite transformation and prevents the yield ratio from increasing and the strength from decreasing (Non-Patent Documents 1 and 2). Increases the cost of steel. Mn is a relatively inexpensive quenching strengthening element. However, in a plating base plate to which a large amount of Mn is added, Mn is concentrated on the surface of the steel sheet to lower the plating property, and defects such as non-plating are likely to occur in the plating layer.
Kawatetsu Technical Report 31 (1999) 3, p.181-184 Kobe Steel Engineering Reports Vol.51, No.3 (2002), p.10

  By utilizing the manufacturing conditions of the hot-dip Zn-Al-Mg alloy-plated steel sheet that does not require alloying treatment, the present invention does not increase the yield ratio and decrease the strength due to pearlite transformation even when heated during hot-dip plating. An object of the present invention is to produce a high-strength molten Zn—Al—Mg alloy-plated steel sheet having a low yield ratio even when a simple C—Si—Mn-based plated original sheet is used.

  In the present invention, C: 0.05 to 0.20% by mass, Si: 1.5% by mass or less, Mn: 1.0 to 2.5% by mass, P: 0.05% by mass or less, S: 0.2% by mass. 01% by mass or less, Fe: Hot rolled steel strip manufactured by hot rolling a slab having a composition of the remainder, or manufactured by hot pickling followed by pickling and cold rolling Use a cold-rolled steel strip for the original plate.

The plating plate is passed through a continuous hot dipping line, heated to a temperature range of Ac ₁ to Ac ₃ in a reduction annealing furnace, then cooled to 650 ° C. at 3 to 10 ° C./s, and further cooled at 10 ° C. / Heat treatment is performed to cool to 440 ° C. over a second. Subsequently, by introducing a plating base plate into a molten Zn—Al—Mg alloy plating bath having a bath temperature of 440 ° C. or less and pulling it up, it is preferable that ferrite having an average crystal grain size of 15 μm or less and a martensity of 5 to 45%. A high-strength molten Zn—Al—Mg alloy-plated steel sheet that has been tempered into a composite structure at the site and suppressed in the yield ratio is manufactured.

Embodiment

In an ordinary alloyed hot-dip galvanized steel sheet manufacturing process, the plating original sheet is heated along the heat pattern shown in FIG. That is, the plating base plate that has been solution-treated and surface-activated by reduction annealing in the temperature range: Ac _{1 to} Ac ₃ is immersed in a hot dip galvanizing bath having a bath temperature of 450 to 470 ° C. Subsequently, the adhesion amount of the hot dip plating metal adhering to the plating original plate pulled up from the hot dip galvanizing bath is adjusted, and alloying is performed in a temperature range of 500 to 550 ° C. In the heat pattern of FIG. 1, pearlite transformation is unavoidable, and as a result, the yield ratio increases and the shape freezing property of the steel sheet decreases.

On the other hand, in the molten Zn—Al—Mg alloy plated steel sheet manufacturing process, the plating original sheet is heated along the heat pattern of FIG. The heat pattern of FIG. 2 is different from the heat pattern of FIG. 1 in that the plating temperature is as low as 400 to 440 ° C. and does not require an alloying treatment at 500 to 550 ° C., and the pearlite transformation is difficult to generate or the pearlite transformation. It can be said that the heat pattern is delayed.
In the present invention, in order to take advantage of the advantages of the heat pattern of FIG. 2, various influences of thermal history on the hot-dip Zn—Al—Mg alloy plated steel sheet were investigated and examined. As a result, by properly managing the cooling conditions from the reduction heating to the introduction of the plating bath, even if a simple C-Si-Mn plating plate is used, it has a composite structure with the proper amount of ferrite and martensite. It was clarified that a high-strength molten Zn—Al—Mg alloy-plated steel sheet with a low yield ratio was obtained.

Hereinafter, alloy components, contents, production conditions, and the like included in the plating base plate used in the present invention will be described.
[Alloy components]
C: 0.05 to 0.20 mass%
It is an alloy component effective for improving the strength, and an effect of improving the strength is seen at 0.05% by mass or more. However, excessive addition reduces weldability and ductility, so 0.20% by mass was made the upper limit. Preferably, the C content is selected in the range of 0.08 to 0.15% by mass.
Si: 1.5% by mass or less Si is an alloy component that increases the strength of steel by solid solution strengthening, and the effect of adding Si becomes remarkable at 0.05% by mass or more. However, if an excessive amount of Si is contained, a Si concentrated layer harmful to the plating property is generated on the steel material surface. Preferably, the Si content is selected in the range of 0.05 to 0.5 mass%.

-Mn: 1.0-2.5 mass%
It is an alloy component that increases the strength of steel by solid solution strengthening, stabilizes austenite, and promotes the generation of low-temperature transformation phases such as martensite. Such an effect is observed with 1.0% by mass or more of Mn, but if an excessive amount of Mn is contained, workability and plating properties are lowered, so the upper limit was made 2.5% by mass. Preferably, the Mn content is selected in the range of 1.5 to 2.5 mass%.
-P: 0.05 mass% or less Although it is a component that increases the strength of steel by solid solution strengthening, if it is excessively contained, it segregates at the grain boundaries and decreases workability, so 0.05 mass% or less (preferably P content was regulated to 0.02 mass% or less.

-S: 0.01 mass% or less Since it is a component which produces | generates a sulfide harmful to workability, it is preferable to reduce S content as much as possible. In this component system, the S content is regulated to 0.01% by mass or less, but it is preferable to lower the S content to 0.005% by mass or less in applications where severe processing conditions are assumed.
Al can be added as a deoxidizer, but adding an excessive amount of Al adversely affects press formability. Therefore, when Al is added, it is 0.06% by mass or less (preferably from 0.01 to The Al content is selected in the range of 0.05 mass%.

〔Heat treatment〕
The plating original plate is subjected to reduction annealing in a reducing atmosphere prior to hot dipping so as to solidify carbides and the like deposited on the matrix and activate the surface.
・ Reduction annealing: Ac _{1 to} Ac ₃
In the reduction annealing, precipitates such as carbides are completely dissolved in the matrix and finally martensite is obtained as the second phase. Therefore, the annealing temperature is in the temperature range of Ac ₁ to Ac ₃ (specifically, 750 to 850 ° C. , Preferably 780 to 820 ° C.). At an annealing temperature that does not reach Ac ₁ , austenite formation is insufficient, so that it is difficult to obtain a sufficient amount of martensite. On the other hand, at a heating temperature exceeding Ac ₃ , the C concentration of austenite decreases and pearlite is easily generated.

・ Cooling rate to 650 ° C .: 3 to 10 ° C./second In order to secure a necessary amount of martensite and advance sufficient ferrite transformation, the solution-treated plating original plate has a cooling rate of 3 to 10 ° C./second. It is cooled to 650 ° C. (preferably 5 to 10 ° C./second). In slow cooling that does not reach 3 ° C / second, the amount of martensite is small, and in rapid cooling exceeding 10 ° C / second, the ferrite transformation does not proceed sufficiently.
-Cooling rate to 440 ° C: 10 ° C / second or more The cooling process from 650 ° C to 440 ° C has a dominant influence on the progress of pearlite transformation. By cooling the temperature range at a cooling rate of 10 ° C./second or more (preferably 15 to 20 ° C./second), pearlite transformation can be suppressed. On the other hand, in the case of slow cooling that does not reach 10 ° C./second, pearlite transformation occurs and the amount of martensite finally obtained decreases, so an increase in yield ratio and a decrease in strength are inevitable.

[Hot plating]
The pearlite transformation can also be caused by a thermal history that the plating original plate receives by immersion in a hot dipping bath. For example, in conventional alloyed hot dip galvanizing, the bath temperature is as high as 450 to 470 ° C., and therefore, an environment in which pearlite transformation is likely to occur. On the other hand, hot-dip Zn—Al—Mg alloy plating uses a hot-dip galvanizing bath having a lower bath temperature than alloyed hot-dip galvanizing, so that pearlite transformation hardly occurs.

  The hot dip galvanizing bath has, for example, Al: 4 to 10% by mass, Mg: 1 to 4% by mass, Zn: substantially the balance, Ti: 0.002 to 0.2% by mass as necessary, B :: Prepared by adding 0.001 to 0.1% by weight. The bath temperature is controlled to 440 ° C. or lower, preferably 400 to 420 ° C.

[Structure of molten Zn-Al-Mg alloy-plated steel sheet]
The hot-dip Zn-Al-Mg alloy-plated steel sheet preferably does not undergo pearlite transformation due to thermal history during immersion and pulling in a hot-dip galvanizing bath, and preferably has an average crystal grain size of 15 μm or less and a volume ratio of 5-45. % Tempered into a martensitic composite organization. Since the composite structure hardly changes before and after hot dip galvanizing, it can be formed by heat treatment prior to hot dip galvanizing.

  Since the volume ratio of martensite is in the range of 5 to 45% (preferably 10 to 40%), the yield ratio is low and high strength of 590 MPa or more is exhibited. The strength increases as the amount of martensite increases, and the effect of martensite is observed at 5% by volume or more. However, since an excessive amount of martensite deteriorates moldability, the upper limit is set to 45% by volume. Refinement of ferrite grains is also effective in increasing the strength, and the effect of refinement of ferrite grains can be seen when the average crystal grain size is 15 μm or less.

  A slab having the composition shown in Table 1 was heated to 1250 ° C., and then hot-rolled to a finishing temperature: 880 ° C., a winding temperature: 600 ° C., and a sheet thickness: 2.4 mm. After pickling the hot-rolled steel strip, it was cold-rolled to a plate thickness of 1.2 mm at a reduction ratio of 50%.

Each cold-rolled steel strip is subjected to heat treatment under the conditions shown in Table 2 and then introduced into a hot dipping bath (Al: 6% by mass, Mg: 3% by mass, Zn: balance), and the amount of plating deposited per side: 50 g / Mm ² hot-dip Zn—Al—Mg alloy-plated steel sheet was produced. For comparison, steel types A and B were used as a plating base plate, annealed at 800 ° C. for 30 seconds, cooled to 650 ° C. at 7 ° C./second, 460 ° C. at 13 ° C./second, and bath temperature: 460 ° C. An alloyed hot-dip galvanized steel sheet was manufactured by immersing in a hot-dip galvanizing bath of, and subjecting it to an alloying treatment at 530 ° C. for 10 seconds after pulling.

About each manufactured hot dip galvanized steel plate, the test piece was cut out along the direction orthogonal to a rolling direction in the location of width / 4 from the both ends of a coil, and it used for the room temperature tension test. In the room temperature tensile test, No. 5 test piece of JIS Z2201 was used, and tensile properties were investigated according to JIS Z2241.
Moreover, the base steel of each plated steel plate was observed with a microscope, and the ferrite grain size and martensite volume fraction were determined. The ferrite particle size was calculated by a cutting method, and the martensite volume fraction was calculated by image analysis.

  As can be seen from the investigation results in Table 3, the test Nos. 1, 2, 4 and 5 satisfying the conditions specified in the present invention in terms of the components, composition and heat treatment have a ferrite grain size of 15 μm or less and a martensite content of 5 It had a composite structure in the range of ˜45% by volume, and showed a low yield ratio of 0.65 or less despite its high tensile strength at room temperature of 590 MPa. Since it has high strength and low yield ratio, it can be seen that it can be processed into complex product shapes by press working, roll forming, etc.

On the other hand, in test No. 3 in which the temperature range of 650 to 440 ° C. was slowly cooled, the amount of martensite was insufficient, and in test Nos. ₆ and 7 where the annealing temperature was too low to deviate from Ac ₁ to Ac ₃ , ferrite grains grew greatly. In Test No. 8 in which the steel material E with insufficient C content was used as the plating base plate, the martensite content was insufficient, and none of the yield ratios were reduced and the strength of 590 MPa or higher was not achieved. Even when the same steel materials A and B as the test Nos. 1 and 2 were used for the plating base plate, the alloyed test Nos. 9 and 10 were significantly reduced in martensite because of the pearlite transformation. The yield ratio increased and the strength decreased compared to.

  As is clear from this comparison, when performing hot-dip Zn-Al-Mg alloy plating of a C-Si-Mn based plating plate, heat management from the reduction annealing process to introduction into the hot dip galvanizing bath should be optimized. Thus, it was confirmed that a high-strength molten Zn—Al—Mg alloy-plated steel sheet having a low yield ratio, which was adjusted to a composite structure of ferrite and martensite with suppressed pearlite transformation, was obtained.

  As explained above, by utilizing the characteristics of hot-dip Zn-Al-Mg alloy plating that does not include an alloying process after hot-dip plating, and by appropriately managing the heat treatment of the plating base plate introduced into the hot-dip bath, Even when a C-Si-Mn based plating original plate is used, a hot dip plated steel sheet having a low yield ratio and a high strength of 590 MPa or more can be obtained. Moreover, since the Zn—Al—Mg alloy plating layer exhibiting high corrosion resistance is formed, it can be processed into a highly accurate product shape even under severe conditions, and is useful as a material excellent in strength and corrosion resistance.

Heat pattern of alloyed hot-dip galvanized steel sheet manufacturing process Heat pattern of manufacturing process of hot-dip Zn-Al-Mg alloy-plated steel sheet

Claims (2)

C: 0.05-0.20 mass%, Si: 1.5 mass% or less, Mn: 1.0-2.5 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less , Fe: After hot rolling a slab having substantially the remaining composition, or after hot rolling, pickling and cold rolling, and then passing through a continuous hot dipping line,
After heating to a temperature range of Ac ₁ to Ac ₃ in a reduction annealing furnace of a continuous hot dipping plating line, cooling rate: 3 to 10 ° C./second to 650 ° C., cooling rate: 10 ° C./second or more to 440 ° C. Heat treatment
Next, a method for producing a high-strength molten Zn—Al—Mg alloy-plated steel sheet, which is introduced into a molten Zn—Al—Mg alloy plating bath having a bath temperature of 440 ° C. or less.   The manufacturing method according to claim 1, wherein the steel strip is tempered by a composite structure of ferrite having an average crystal grain size of 15 µm or less and a martensite having a volume ratio of 5 to 45% by heat treatment in a reduction annealing furnace.

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