JP2761096B2 - Manufacturing method of high ductility and high strength alloyed hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a method for producing a high-strength alloyed hot-dip galvanized steel sheet excellent in workability, and more specifically, to a tensile strength of 60 to 60.
The present invention relates to a method for producing a 120 kgf / mm ² class composite structure alloyed hot-dip galvanized steel sheet.

(Prior Art) In recent years, high-strength cold-rolled steel sheets having excellent workability have been used as measures for reducing the safety and weight of automobiles. Further, it is strongly desired that cold-rolled steel sheets have an improved rust-preventive force in order to extend the life of automobiles.
There is also a need for the development of galvannealed steel sheets having excellent spot weldability and paintability. In particular, recently, there has been a demand for alloyed hot-dip galvanized steel sheets having a tensile strength of 60 to 100 kgf / mm ² for reinforcing members such as automobile bumpers and door impact bars.

(Problems to be Solved by the Invention) Conventionally, the tensile strength of 60 kgf / mm ² class is manufactured by solid solution strengthening and precipitation strengthening, so that there is a problem that the added element increases and the cost increases. was there. Further, in order to obtain a steel sheet having a weight of 80 kgf / mm ² or more, these strengthening methods not only greatly reduce ductility, but also make rolling difficult. Therefore, in order to obtain high ductility with such a high-strength thin steel sheet, it is known that strengthening of the transformation structure including a hard phase such as bainite or martensite is advantageous.

In order to solve such a problem, for example, JP-A-55-504
No. 55 discloses that a steel sheet to which Mn or the like is added is heated in a two-phase region of A _{c1 to} A _c3 , and a cooling rate from the heating temperature to a plating bath temperature and a cooling rate after the plating treatment are limited to thereby reduce the ferrite / martennel. There has been proposed a method of obtaining a high-strength galvanized steel sheet having excellent workability by forming a composite structure of a site.
However, in this method, pearlite or coarse bainite is generated by alloying treatment, and a high-strength galvannealed steel sheet excellent in workability cannot be obtained.

In Japanese Patent Application Laid-Open No. 55-122820, the alloying temperature is set to A _c1
It has been proposed to obtain a composite structure steel sheet by setting a two-phase region of ~ _Ac3 and controlling the cooling rate thereafter. However, in this method, there is a problem that since the alloying temperature is high, the iron concentration of the alloy layer is increased, and poor powdering properties and the like are caused to deteriorate the material.

On the other hand, in Japanese Patent Application Laid-Open No. 56-142821, heating of A _{c1 to} 900 ° C. and controlling the cooling rate suppress the formation of pearlite and bainite, and change the structure to ferrite / martensite (partially retained austenite). A method has been proposed for producing a hot-dip galvanized steel sheet having excellent workability by forming a composite structure of the present invention. However, in this method, the difference in hardness between ferrite and martensite is large, and local elongation and bending workability are low. In particular, a tensile strength of 70
At kgf / mm ² or more, the martensite volume ratio increases,
Since the hole expansion ratio is remarkably reduced, the workability is insufficient in severe bending performed in channel-type molding such as a bumper.

As described above, when manufacturing a high-ductility high-strength alloyed hot-dip galvanized steel sheet, it is necessary to strengthen the composite structure, which is advantageous in obtaining high strength.However, simply by focusing on chemical components and cooling rates, It is difficult to produce a high-ductility, high-strength alloyed hot-dip galvanized steel sheet.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for producing an alloyed hot-dip galvanized steel sheet having excellent high ductility and high strength by forming a composite structure.

(Means for Solving the Problems) As a result of intensive studies to solve the problems, the present inventors have found that the chemical composition of steel and the heating temperature of the continuous galvanizing line, from the heating temperature to the plating bath temperature. The microstructure of the ferrite, bainite, and martensite is fine and uniform by controlling the cooling of the alloy into two stages of slow cooling and quenching, and controlling the alloying temperature and the cooling rate from the alloying temperature to the Ms point. (Including some retained austenite)
The present inventors have found that a high ductility and high strength alloyed hot-dip galvanized steel sheet can be obtained in the composite structure of the present invention, which has led to the present invention.

That is, in the present invention, C: 0.06 to 0.30%, Si: 0.6% or less, P: 0.1% or less, Nb: 0.01 to 0.20%, and sol.
Containing 0.10%, and further containing one or more of Mn: 0.6 to 3.0%, Cr: 0.1 to 1.5% and Mo: 0.1 to 1.0%, with the balance being iron and unavoidable impurities, After hot rolling, pickling and cold rolling in the usual manner, when performing recrystallization annealing in a continuous hot-dip galvanizing line, the heating temperature is set to A _c3 point −50.
To 900 ° C., and cooling to the plating bath is cooled to a temperature range of 500 to 650 ° C. at a cooling rate of 20 ° C./s or less, and then to the plating bath temperature lnCR = −1.18Meq + 3.37 Meq (%) = Mn + 1.52Mo + 1.10Cr + 0.10Si +
After cooling at a critical cooling rate CR (° C / s) or higher indicated by 2.1P, hot-dip galvanizing, and then performing an alloying treatment at a temperature between 500 ° C and _Ac1 point, then a cooling rate higher than CR The gist of the present invention is a method for producing a high ductility alloyed hot-dip galvanized steel sheet, characterized in that the steel sheet is cooled to an Ms point or lower.

 Hereinafter, the present invention will be described in more detail.

(Action) FIG. 1 shows the thermal history of the continuous galvanizing line in the present invention. Here, the cooling from the soaking temperature to 500 to 650 ° C. is the primary cooling, the cooling to the next plating bath temperature is the secondary cooling, and the cooling after the alloying treatment is the tertiary cooling. It is called the secondary cooling rate, the secondary cooling rate, and the tertiary cooling rate. The temperature at which the primary cooling is changed to the secondary cooling is referred to as a rapid cooling start temperature.

First, the reasons for limiting the chemical components in the present invention will be described.

C: C is an indispensable element for strengthening the steel sheet. To obtain a steel sheet having the desired strength, it is necessary to add at least 0.06%, but if it exceeds 0.30%, the volume fraction of hard martensite Not only deteriorates the workability but also decreases the spot weldability. Therefore, the amount of C is 0.
The range is from 06 to 0.30%.

Si: Since Si has an effect of discharging solid solution C in ferrite into austenite, ferrite ductility can be improved. However, excessive addition causes poor plating, so the Si content is set to 0.6% or less.

P: P has the same effect as Si when added in an amount of 0.02% or more, and is effective for ensuring a balance between strength and elongation. However, if added in excess of 0.1%, plating failure or the like will occur. , P content is 0.1% or less.

Nb: Nb is an effective element for increasing the strength of a steel sheet, and not only contributes to precipitation strengthening but also improves hardenability. Further, it has the effect of making the structure ultrafine and uniform, and has the effect of increasing local elongation without reducing uniform elongation.
In order to exhibit such effects, 0.01% or more is required. However, if it is added in excess of 0.20%, ductility is deteriorated, which is not preferable. Therefore, the Nb amount is 0.01 to
The range is 0.20%.

sol.Al: Al is added for deoxidation of steel and requires at least 0.01%. However, excessive addition not only saturates the effect but also causes inconvenience, which is not preferable.
Therefore, the amount of sol.Al is in the range of 0.01 to 0.1%.

Further, the steel used in the present invention must contain an appropriate amount of one or more of Mn, Cr and Mo, in addition to the above elements.

Mn: Mn is added to stabilize the austenite phase, facilitate the formation of a hard phase in the cooling process, and obtain high strength. However, if the addition amount is small, it is not possible to obtain a sufficient hard phase to achieve high strength, so the lower limit is 0.1.
6%. On the other hand, if it is added excessively, the band structure develops and not only the workability deteriorates, but also the cost increases, so the upper limit is made 3.0%.

Cr: Cr has the same effect as Mn, stabilizes the austenite phase, facilitates the formation of a hard phase, and obtains high strength.
It is added as needed. However, at least 0.1% is required to obtain the effect. However, if added excessively, uniform elongation and local elongation are reduced.
1.5%.

Mo: Mo is added to significantly stabilize the austenite phase, facilitate the formation of a hard phase in the cooling process, and increase the strength. However, if the addition amount is small, a hard phase for achieving high strength cannot be obtained, so the lower limit is set to 0.1.
1%. On the other hand, if it is added in excess of 1.0%, a large amount of martensite is formed in a band-like form, thereby deteriorating workability. Therefore, the upper limit is 1.0%.

Next, the manufacturing conditions in the method of the present invention will be described.

The steel having the above-mentioned chemical composition is formed into a slab by a normal process through steelmaking, lumping or continuous casting, and then into a hot coil through hot rolling. With regard to hot rolling, the conditions do not need to be particularly limited.However, in order to obtain a hot-dip galvanized high-strength steel sheet having a composite structure of fine and uniform ferrite and martensite, the winding temperature of hot rolling should be lowered. It is preferable to have a uniform ferrite and bainite structure.

After hot rolling, according to a conventional method, pickling and cold rolling are performed to obtain a thin steel sheet. The cold working rate is desirably 30% or more.

Next, the thin steel sheet is led to a continuous hot-dip galvanizing line, and subjected to recrystallization annealing, hot-dip galvanizing, and alloying under the following conditions.

In the recrystallization annealing, it is necessary to set the heating temperature to an _Ac3 point of −50 ° C. to 900 ° C. If the heating temperature is lower than the _Ac3 point −50 ° C., a large amount of untransformed recrystallized ferrite cannot be obtained to form a uniform and fine structure, so that it is difficult to obtain high local elongation. On the other hand, when the heating temperature is higher than 900 ° C., the austenite grains are coarsened, the nuclei of ferrite in the primary and secondary cooling processes are reduced, and the ferrite grains are coarsened.
Due to the non-uniform structure, local elongation deteriorates.

Cooling from the recrystallization annealing to the plating bath is first performed by 50
Cool to a temperature range of 0 to 650 ° C at a cooling rate of 20 ° C / s or less.
This primary cooling step is an important step for increasing elongation and local elongation. That is, due to the transformation inhibiting effect and the grain growth inhibiting effect by the addition of Nb, ferrite is finely and uniformly precipitated at a low temperature, and the C concentration of the remaining austenite is increased. For this reason, austenite is stabilized, and the formation of bainite is suppressed at a plating bath temperature or higher. However, if the cooling rate exceeds 20 ° C / s, ferrite cannot be sufficiently precipitated, and the C concentration of austenite is low, so that the amount of bainite generated in the cooling process increases, and uniform elongation and local elongation deteriorate. Not preferred.

Then, from the quenching start temperature to the temperature of the plating tank, lnCR = −1.18Meq + 3.37 where Meq (%) = Mn + 1.52Mo + 1.10Cr + 0.10Si +
Cool at a critical cooling rate CR (° C / s) or higher indicated by 2.1P.
Next, after hot-dip galvanizing, after alloying at a temperature of 500 ° C. to A _c1 point, by cooling to a Ms point or less at a cooling rate of the critical cooling rate CR or more, a fine uniform composite structure Steel sheet can be obtained. If both the cooling rates of the secondary cooling and the tertiary cooling are smaller than CR, the generation of pearlite or the bainite volume ratio increases, and it is difficult to obtain high ductility and high strength.

 An embodiment of the present invention will be described below.

(Example) 40 kg of steel having the chemical components shown in Table 1 was vacuum-melted,
The slab was 20 mm thick. Heat this slab to 1200 ° C,
Hot rolling was performed at a finishing temperature of 850 ° C. and a winding temperature of 560 ° C. to obtain a 3.2 mm-thick hot-rolled steel sheet. The obtained steel sheet was pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm (a reduction ratio of 62.5%).

These cold-rolled steel sheets were subjected to continuous galvanization under the conditions shown in Table 2 to obtain alloyed hot-dip galvanized steel sheets.

The obtained alloyed hot-dip galvanized steel sheet was examined for tensile properties and 10 mm hole punching hole expansion ratio. The results are shown in Table 2.

 The following is considered from Table 2.

No. 1 and No. ^{2 of the} present invention material have high strength of 85 kgf / mm ² or more, high elongation of 22% or more and high hole expansion ratio (λ) of 40% or more.

On the other hand, comparative material N having the same chemical components as No. 1 and No. 2
In the case of o.3, since the quenching start temperature was as high as 800 ° C., precipitation of ferrite was insufficient and elongation was poor.

Since the comparative materials No. 4, No. 7, and No. 9 did not contain Nb, the structure was coarser than that of the material of the present invention, and the strength and the hole expansion ratio were inferior.

The comparative material No. 6 has a cooling rate lower than CR, so that pearlite is generated, and the strength and the hole expanding ratio are inferior to the material No. 5 of the present invention.

Comparative material No. 10 has a large Mn content of 3.51%, so that a large amount of martensite is generated, and the elongation and hole expansion rate are low.

Comparative material No. 11 had a low C content and a lower cooling rate than CR, so that martensite was not obtained and the desired strength was not obtained.

Further, the material No. 8 of the present invention has a high strength of 80 kgf / mm ² and is excellent in elongation and hole expansion rate.

(Effects of the Invention) As described above in detail, according to the present invention, by controlling the cooling rate from the heating temperature of the recrystallization annealing to the galvanizing temperature in two stages of slow cooling and rapid cooling, uniform elongation and local cooling are achieved. Precipitates ferrite, which contributes to the improvement of elongation, finely and uniformly, and also increases the C concentration of the remaining austenite in this process and stabilizes it, thereby suppressing the generation of pearlite from galvanization to alloying. A large amount of bainite is prevented, and austenite is transformed into martensite by cooling after the alloying treatment, whereby a fine and uniform structure mainly composed of ferrite and martensite can be obtained.

Therefore, it is possible to obtain a high ductility and high strength galvannealed steel sheet with 60~120kgf / mm ² grade, Mekkimura, Parada rings, etc., in addition to the improvement of surface properties, reduced is also possible energy costs .

[Brief description of the drawings]

FIG. 1 is a diagram showing the thermal history of a continuous galvanizing line in the present invention.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-39770 (JP, A) JP-A-63-195222 (JP, A) JP-A-4-128320 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C23C 2/02 C23C 2/06 C21D 9/46

Claims (1)

(57) [Claims] (1) C: 0.06-0.30% by weight (hereinafter the same)
%, Si: 0.6% or less, P: 0.1% or less, Nb: 0.01 to 0.20% and
sol.Al: 0.01 to 0.10%, Mn: 0.6 to 3.0%, C
r: 0.1-1.5% and Mo: 0.1-1.0% steel containing one or more kinds, the balance being iron and unavoidable impurities, hot rolling, pickling, cold rolling in the usual way After that, when recrystallization annealing in a continuous hot-dip galvanizing line, the heating temperature is set to _Ac3 point -50 ℃ ~ 900 ℃, cooling to the plating bath,
Cool to a temperature range of 500 to 650 ° C. at a cooling rate of 20 ° C./s or less, and then up to the plating bath temperature lnCR = −1.18Meq + 3.37 where Meq (%) = Mn + 1.52Mo + 1.10Cr + 0.10Si + 2. 1
After cooling at a critical cooling rate CR (° C / s) or higher indicated by P, hot-dip galvanizing, and then performing alloying treatment at a temperature of 500 ° C to A _c1 point, A high ductility and high strength alloyed hot-dip galvanized steel sheet, wherein the steel sheet is cooled to an Ms point or lower.