JPH0819472B2 - Method for producing high-strength Zn-Al-based composite hot-dip galvanized steel sheet having good workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention has a tensile strength of 45 to 60 kgf / mm ² and a yield ratio of
High-strength Zn-Al with workability less than 0.85 and excellent corrosion resistance
The present invention relates to a method for manufacturing a composite hot-dip galvanized steel sheet.

<Prior art and its problems> In the automobile industry, there is a strong demand for high-strength steel sheets for weight reduction without sacrificing the safety of automobiles for the purpose of improving fuel efficiency, and also for improving rust prevention.

Further, there is a strong demand for a material having excellent strength and anticorrosiveness as a building material, and therefore, there is great expectation for a high strength hot-dip galvanized steel sheet having excellent anticorrosiveness.

Further, in recent years, there has been an increasing demand for workability improvement for this high-strength hot-dip galvanized steel sheet.

Conventionally, high-strength steel sheets using precipitation hardening elements such as Nb and Ti have a high yield ratio and have a drawback that sufficient cold formability such as stretch flange formability cannot be obtained. Also, with Si
High-strength steel sheets that use solid solution hardening elements such as Mn have a lower yield ratio and better cold formability than precipitation hardening steel,
In the production of galvanized steel sheets by hot dip galvanizing, for example, Cirrus type continuous galvanizing equipment and non-oxidizing furnace type continuous hot dip galvanizing equipment, in the case of solid solution hardening type high strength steel with Si exceeding 0.1%, the oxidation of Fe and Si Since a substance is generated on the steel surface, which is not sufficiently removed even in the reduction step, and the plating treatment is performed while remaining on the steel surface, there is a great disadvantage that non-plating occurs.

Further, with the recent deterioration of atmospheric environment, the galvanized steel sheets to date have not been sufficiently rustproof, and their corrosion resistance and weather life have been reduced.

<Means for Solving Problems> The inventors of the present invention have conducted extensive studies as a solution to such problems, and as a result, Zn-Al-based composite hot-dip galvanized Zn-Al-based composite hot-dip galvanized steel having superior rust-preventive power than zinc plating. The yield ratio is less than 0.85 and the tensile strength is reduced by reducing the Si content that inhibits the composite hot dipability as much as possible, adding P as an alternative strengthening element, and controlling the cooling conditions during hot rolling. 45-60kgf / mm
^In No. ² , a method for producing a high-strength Zn-Al composite hot-dip galvanized steel sheet having a good balance of strength and ductility, good cold formability and good corrosion resistance was found.

<Structure of Invention> That is, according to the present invention, C: 0.05 to 0.15%, Si: 0.10% or less, Mn: 0.60 to 1.50%,
P: 0.035 to 0.100%, S: 0.015% or less, if necessary Ca and / or REM containing 0.001 to 0.008%, the balance consisting of iron and other unavoidable impurities, steel at a finishing temperature of 850 ° C or higher After rolling, it is cooled at a cooling rate of 15 to 60 ° C / sec and wound at a temperature of 470 to 680 ° C to produce a hot rolled sheet having a structure of polygonal ferrite and pearlite. High strength with good workability and corrosion resistance by performing Zn-Al composite hot dip plating after pickling
It is characterized in that a Zn-Al hot-dip steel sheet is obtained. Here, the term "polygonal ferrite-perlite structure" means that the ferrite particles are dispersed as polycrystals of almost the same size and the pearlite particles are also uniformly dispersed. Speaking of which, it is distinguished from the structure having ferrite or pearlite that is spread like a strip, and is also distinguished from the bainite structure.

In the present invention, the Zn-Al system means an alloy mainly composed of Zn and containing 10% or less of Al and a small amount of Mg and / or REM.

Next, the reasons for limiting the steel composition and manufacturing conditions in the method of the present invention will be described.

C (Carbon) In the steel of the present invention, C is preferably as low as possible in order to improve its workability and weldability. However, if the C content is less than 0.05%, it is difficult to obtain a tensile strength of 45 kgf / mm ² or more after hot dip galvanizing. 0.15%
If added over, a large amount of bainite-like structure is produced within the hot rolling condition range in the present invention, the tensile strength exceeds 60 kgf / mm ² and the yield ratio is 0.85 or more, and cold workability is significantly reduced. Therefore, the C content is limited to 0.05 to 0.15%.

Si (Silicon) Si is a preferable element for the purpose of improving strength, but in the production of Cirrus type and non-oxidizing furnace type hot dip galvanized steel sheets, the occurrence of non-plating becomes significant when the Si content exceeds 0.10%. Therefore, the Si content is limited to 0.10% or less.

Mn (manganese) Mn is an important element to secure the strength of steel,
If the content is less than 0.60%, the desired strength cannot be obtained, while if it exceeds 1.50%, the strength of the welded joint increases greatly and the toughness deteriorates, so the Mn content is 0.60 to 1.
Limited to 50%.

P (phosphorus) P is a characteristic element of the steel of the present invention. That is, Zn−
Si content 0.10% to improve Al-based composite hot dip galvanizing
Since it is limited to the following, P is used as a reinforcing element instead of Si. In order to secure the desired tensile strength of 45 kgf / mm ² or more of the steel of the present invention, the P content must be at least 0.035%. In the range of 0.035 to 0.100%, since the ferrite transformation reaction is promoted by the increase of the P content, a polygonal ferrite and pearlite structure is formed and good ductility is exhibited.
However, if added over 0.100%, the impact transition temperature of the Zn-Al composite hot dip plated steel sheet rises and the embrittlement of the steel material becomes visible, so the P content was limited to the range of 0.035 to 0.100%. .

S (Iou) S is an element that is introduced into steel as an unavoidable impurity, but excess S forms expanded sulfide-based inclusions,
Since the cold workability of the Zn-Al composite hot-dip galvanized steel sheet, especially the stretch flangeability is deteriorated, the S content is limited to 0.015% or less.

Ca (calcium) and REM (rare earth elements) Addition of Ca and / or REM is effective in changing the form of expanded sulfide-based inclusions into spherical inclusions and improves material anisotropy. . Therefore, it is possible to improve cold workability such as stretch flange formability of Zn-Al composite hot dip galvanized steel sheet.
Add one of Ca or REM. Ca and / or R
If the total amount of EM added is less than 0.001%, there is no spheroidizing effect of inclusions, and even if added over 0.008%, the effect is saturated, so the amount of Ca and / or REM added is 0.001 to 0.008.
Limited to%. Ca and REM are equivalent for the above purposes.

Rolling finish temperature A rolling finish temperature of 850 ° C. or higher is usually performed and is not a limitation specific to the present invention.

Cooling rate The cooling rate from the finishing stand to the winder is important for controlling the tensile strength of the steel sheet of the present invention. When the cooling rate is slower than 15 ° C / sec, the obtained metallographic structure becomes a remarkable band structure of ferrite and pearlite, and the cold formability of the Zn-Al composite hot dip plated steel sheet deteriorates despite the low strength. To do. The cooling rate is 60 ℃ /
If the cooling rate is faster than 10 seconds, the bainite structure is often generated, and the ductility and the cold formability are significantly reduced. Therefore, the cooling rate after finish rolling was limited to 15 to 60 ° C / second.

Winding temperature The winding temperature strongly influences the cooling rate after finish rolling as well as the tensile strength and cold formability of the Zn-Al composite hot dip plated steel sheet. When the coiling temperature is coiled at a low temperature of less than 470 ° C, ferrite and pearlite transformation are not completed, the bainite structure is increased, and the tensile strength of the Zn-Al composite plated steel sheet exceeds 60 kgf / mm ² . The deterioration of elongation and the deterioration of cold formability are remarkable. On the other hand, when the metal structure obtained by coiling at a high temperature exceeding 680 ° C becomes a band structure with remarkable ferrite and pearlite, and embrittlement due to P is also recognized, the cold cooling of the Zn-Al-based composite plated steel sheet becomes possible. The inter-moldability and toughness are significantly reduced. Therefore, in order to obtain a polygonal ferrite or pearlite structure and to obtain good cold formability and toughness, the coiling temperature was limited to the temperature range of 470 to 680 ° C.

Next, the effects of the present invention will be further described with reference to examples. In each of these examples, hot dip Zn-Al plating was carried out in a Sendzimir type continuous hot dip galvanizing equipment. In particular, in-line annealing was not performed in the equipment and the hot rolled steel strip was heated to almost the bath temperature. It was continuously immersed in a Zn-Al bath.

Example 1 Steels having different Si contents shown in Table 1 were melted by a converter and made into a slab of about 11 tons by continuous casting. The finishing temperature during hot rolling was 880 ° C, and the final stand for finishing rolling was taken out. Then, it was cooled at an average cooling rate of 32 ° C./sec and wound at 625 ° C. to obtain a coil having a thickness of 3.0 mm. After pickling this hot-rolled steel strip, in a Zn-4% Al-0.1% Mg plating bath kept at 450 ° C by a continuous hot dip galvanizing system of non-oxidizing furnace system, the coating weight of one side 45g / m ² The target was plated.

This plated steel strip was cut to a length of 1829 mm with a shear and the non-plating occurrence rate was measured and shown in Table 1.

It is clear that when the Si content exceeds 0.10%, the second-grade downgrade rate due to the occurrence of non-plating increases rapidly.

Example 2 A steel having the components shown in Table 2 was melted in a converter and made into an 11 ton slab by continuous casting, and the finishing temperature during hot rolling was 860.
~ 890 ℃, after cooling from the final stand of finish rolling, cooled at an average cooling rate of 29 ~ 45 ℃ / sec, rolled up at 500 ~ 620 ℃, thickness 3.
A 0 mm hot rolled steel strip was produced.

After pickling this hot-rolled steel strip with a Cirrus-type hot dip galvanizing machine, in a molten Zn-4% Al-0.1% Mg plate kept at 450 ° C, plating treatment with a target of 45g / m ^{2 on} one side Was done.

Table 3 shows the mechanical properties, cold formability (hole expansion ratio), fracture surface transition temperature of 2 mm V notch test piece, and second-grade downgrade rate due to non-plating of this plated steel sheet.

No. 5 steel in which C, Mn, and P are lower than the range of the method of the present invention has a tensile strength of 45 kgf / mm ^{2 which} is the target in the method of the present invention, even when rolled under the hot rolling conditions of the method of the present invention. I can't.

Steel No. 9 in which P is higher than the range of the method of the present invention has a structure in which ferrite transformation is promoted and the amount of ferrite is large and shows good elongation, but the fracture surface transition temperature by the impact test is −40 ° C.,
The P content is higher than that of steel types within the range of the method of the present invention, and embrittlement due to P is observed.

Steel No. 10 having a C content higher than the range of the method of the present invention produced a large amount of bainite structure even under hot rolling conditions within the range of the method of the present invention. As a result, the tensile strength exceeded 60 kgf / mm ² and the yield ratio was 0.8.
As high as 7, the hole expansion ratio is small and the cold formability is significantly reduced. Furthermore, the fracture surface transition temperature is also high.

On the other hand, each of No. 6, 7, and 8 steels within the scope of the method of the present invention has a yield ratio lower than 0.8 and a good elongation, resulting in a high hole expansion ratio and good cold formability. Show. The transition temperature of the impact fracture surface is also low. Among the methods of the present invention, particularly Ca was added
In No. 8 steel, the morphology of non-metallic inclusions became spherical, the anisotropy of mechanical properties was improved, and the hole expansion ratio was increased to 2.1, and the cold formability was remarkably improved.

In addition, since the Si content of each of the No. 5 to 10 steels was 0.10% or less, no plating occurred and good plating properties were exhibited.

Example 3 Using a No. 8 steel type slab shown in Table 2, hot rolling finish temperature
After leaving the final stand for finish rolling at 825 to 880 ° C, it was cooled at an average cooling rate of 17 to 62 ° C / sec and wound at a temperature of 400 to 690 ° C to produce a hot rolled steel strip having a thickness of 3.0 mm.

After pickling this hot-rolled steel strip with a Cirrus-type hot dip galvanizing machine, in a Zn-4% Al-0.1% Mg plating bath maintained at 450 ° C, plating treatment with a target of 45g / m ^{2 on} one side Was done. The material properties of the obtained plated steel sheet are shown in Table 4.

In No. 11, the winding temperature is higher than that of the method of the present invention, so that the tensile strength cannot exceed the strength of 45 kgf / mm ² targeted by the method of the present invention. Further, the obtained metallographic structure also shows a striped structure of ferrite and pearlite, and although the elongation is good, the hole expansion ratio is not good.

In the case of the comparative method in which the cooling rate of No. 15 is as high as 62 ° C / sec, the coiling temperature is low due to the high cooling rate, and the bainite structure is often generated, so the tensile strength also exceeds 60 kgf / mm ² . As a result, the yield ratio becomes 0.85 or more, the hole expansion ratio drops sharply, and the cold formability deteriorates significantly.

On the other hand, in the case of the method of the present invention in which the cooling rate of Nos. 12 to 14 is 20 to 60 ° C. and the winding temperature is 470 to 680 ° C., a structure composed of ferrite and pearlite is shown, the elongation is good, and the hole expansion ratio is Also shows high cold formability.

<Effects> The method of the present invention shows good plating properties in the Zn-Al composite hot dip coating process and has a tensile strength of 45 to 60 kgf / mm ² grade and high strength Zn-Al excellent in corrosion resistance and corrosion resistance. The composite hot-dip galvanized steel sheet can be provided as a material for automobiles and building materials.

Claims (2)

[Claims] 1. A steel containing C: 0.05 to 0.15%, Si: 0.10% or less, Mn: 0.60 to 1.50%, P: 0.035 to 0.100%, S: 0.015% or less, balance Fe and inevitable impurities at 850 ° C. Hot rolling at the above finishing temperature, cooling at a cooling rate of 15 to 60 ° C / sec, and winding at a temperature of 470 to 680 ° C to produce a hot rolled sheet having a structure consisting of polygonal ferrite and pearlite, A method for producing a high-strength Zn-Al-based composite hot-dip steel sheet having excellent workability and corrosion resistance, which comprises pickling this hot-rolled sheet and then performing Zn-Al-based composite hot-dip plating. 2. C: 0.05 to 0.15%, Si: 0.10% or less, Mn: 0.60 to 1.50%, P: 0.035 to 0.100%, S: 0.015% or less, Ca and / or REM total 0.001 to 0.008%, balance Steel consisting of Fe and unavoidable impurities is hot-rolled at a finishing temperature of 850 ℃ or higher, cooled at a cooling rate of 15 to 60 ℃ / sec, and wound at a temperature of 470 to 680 ℃ to remove from polygonal ferrite / pearlite. A hot-rolled sheet with the following structure is produced, and the hot-rolled sheet is pickled, and then Zn-Al-based composite hot-dip plating is performed. Manufacturing method of plated steel sheet.