JPH04173945A - Manufacture of high strength hot-dip galvanized steel sheet excellent in bendability - Google Patents

Abstract

PURPOSE:To manufacture a high strength hot-dip galvanized steel sheet excellent in bendability by subjecting the cold rolled sheet of a low carbon steel to recrystallization annealing under specified temp. conditions and thereafter applying hot-dip galvanizing thereto. CONSTITUTION:The ingot of a low carbon steel having a compsn. contg., by weight, 0.06 to 0.2% C, <0.6% Si, 0.6 to 3.0% Mn, <0.1% P and 0.01 to 0.10% sol.Al, furthermore cong. at least one kind of 0.01 to 1.0% Mo and 0.1 to 1.5% Cr is subjected to hot rolling, pickling and cold rolling by ordinary methods to manufacture a cold rolled sheet. This cold rolled sheet is introduced into a continuous hot-dip galvanizing line and is subjected to recrystallization annealing in such a manner that it is held to (the Ac3 point -50) to 900 deg.C for >=10sec, is cooled from >=600 deg.C to the temp. range of the MS point to 480 deg.C at the cooling rate of the critical cooling rate CR ( deg.C/sec) or above expressed by the formula 1 and is held to the MS point to 480 deg.C for >=10sec. After that, it is hot-dip galvanized and, if required, is heated to the Ac1 point or below, and the galvanizing and Fe in the steel sheet are subjected to alloying treatment.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing hot-dip galvanized high-strength steel sheets with excellent bending workability, and more specifically,
The present invention relates to a method for producing a hot-dip galvanized high-strength steel sheet mainly composed of 0 kgf/mm'' class bainite or bainite-decaferrite.

(Prior Art) In recent years, high-strength cold-rolled steel sheets with excellent workability have come into use as a measure to improve safety and reduce weight of automobiles. Also,
In order to extend the lifespan of automobiles, it is strongly desired that cold-rolled steel sheets have improved anti-rust properties. Recently, 60 to 120 kgf/mm2 is used for automobile bumpers, door impact beams, etc.
There is also a demand for alloyed hot-dip galvanized steel sheets with excellent spot weldability and paintability for class reinforcement members.

Conventionally, bare steel sheets have a high strength-hole expansion rate (λ) balance by using a transformation structure strengthening method6.
It is known that high-strength thin steel sheets of 0 kgf/mm2 class or higher can be obtained. For example, in Japanese Patent Application Laid-Open No. 63-241115 proposed by the present inventors, a water quenching type continuous annealing method is used, the recrystallization heating temperature is set to Ac or higher, and after forced air cooling, the It was disclosed that a high-strength thin steel sheet with a high strength-λ balance can be obtained by overaging at a temperature of °C to obtain a composite structure consisting of ferrite and tempered martensite. However, in the case of hot-dip galvanized steel sheets, not only is water quenching performed after recrystallization annealing heating, but also the plating or alloying treatment is performed at a temperature higher than the Ms point, so the tempered martensite There is a problem that a high-strength thin steel plate with a high strength-to-edge balance cannot be obtained using the above method.

On the other hand, so far, for example, Japanese Patent Application Laid-Open No. 55-50455 discloses that the structure is improved by performing two-phase region heating and controlling the cooling rate from 700°C to the hot-dip plating temperature and from the hot-dip plating temperature to 300°C. It has been proposed to make hot-dip galvanized steel sheets with ferrite + martensite and excellent cold workability and age hardenability. However, in this method, the tensile strength is 40 to 70 kgf/mm2.
The target material is tensile strength 80kgf/mm2.
Above that, the amount of ferrite decreases and the elongation decreases significantly. In addition, when alloying treatment is performed, bainite or pearlite is generated, making it impossible to obtain the desired material.

In addition, Japanese Patent Application Laid-Open No. 56-142821 discloses that by heating Ac from a point to 900°C and regulating the cooling rate, the formation of pearlite and bainite is suppressed, and the structure is changed to ferrite and martensite (partly A method has been proposed for producing hot-dip galvanized steel sheets with excellent workability by creating a composite structure of retained austenite. However, in this method, the difference in hardness between ferrite and martensite is large, the hole expansion rate is low, and the bending workability is low. In particular, when the tensile strength is 70 kgf/mm2 or more, the martensite volume fraction increases and the hole expansion rate decreases significantly, resulting in insufficient workability in severe bending performed in channel molding of bumpers and the like.

As mentioned above, when manufacturing hot-dip galvanized high-strength steel sheets with excellent bending workability, it is necessary to strengthen the composite structure, which is advantageous in terms of obtaining high strength. It is difficult to produce a hot-dip galvanized high-strength steel sheet with excellent bending workability using this method.

The present invention solves the problems of the prior art described above.

The object of the present invention is to provide a method for manufacturing a high-strength galvanized steel sheet that has high strength through composite texture and has excellent bending workability.

(Means for Solving the Problems) As a result of extensive research in order to solve the above-mentioned problems in the production of hot-dip galvanized high-strength steel sheets with excellent bending workability, the present inventors have developed a continuous hot-dip galvanizing line. The recrystallization annealing heating temperature is 1480℃ above the Ms point from this heating temperature.
By controlling the cooling rate and holding time at that temperature, and further controlling the alloying temperature, the structure can be changed to bainite or a uniformly fine composite structure of bainite, ferrite, and martensite mainly composed of ferrite and bainite. The present invention was developed based on the discovery that a hot-dip galvanized high-strength steel sheet with excellent bending workability can be obtained.

That is, in the present invention, C: 0.06 to 0.2%, Si:
0.6% or less, Mn: 0.6-3.0%, P: 0°1%
Below and sol. Contains Al: 0.01 to 0.10%, and further contains Mo: 0.01 to 1.0% and Cr as necessary.
: After hot rolling, pickling, and cold rolling a steel containing at least one of 0 and 1 to 1.5%, with the remainder consisting of iron and unavoidable impurities, it is passed through a continuous galvanizing line. When re-crystallizing annealing at A c 3, the heating temperature is A c 3
Hold at a temperature of -50℃ to 9o○℃ for more than 10 seconds, and
From the temperature above 00℃ to the temperature range above 480'C below the Ms point, lnCR=-1,18Mneq+3.37 where, Mn
eq=Mn+1.52Mo+1.10Cr+0.10S
After cooling at a cooling rate equal to or higher than the critical cooling rate cR (℃/s) represented by i+2.1P, 1 at a temperature of 1480℃ or less above the Ms point.
After holding for more than 0 seconds, hot-dip galvanizing produces bainite, ferrite, and bainite-based materials.
The gist of this invention is a method for producing a high-strength galvanized steel sheet with excellent bending workability, which is characterized by obtaining a martensitic composite steel sheet.

Further, in another aspect of the present invention, after applying the hot-dip galvanizing,
The steel sheet is characterized by performing alloying treatment at an Ac point of 1 or less to produce a steel sheet with a bainite-ferrite-martensitic composite structure mainly composed of bainite.

The present invention will be explained in more detail below.

(Function) First, the reason for limiting the chemical composition of steel in the present invention will be explained.

C: C is an essential element for strengthening steel sheets, and in order to obtain steel sheets with the desired strength, it is necessary to add at least 0.06%. However, when it exceeds 0.2%, the volume fraction of hard martensite becomes high, which not only deteriorates bending workability but also deteriorates spot weldability. Therefore, the amount of C is set in the range of 0.06 to 0.2%.

Sj: Since Si has the effect of discharging solid solution C in ferrite into austenite, it can improve the ductility of ferrite. However, adding too much will cause plating defects, so it should be added at 0.6% or less.

Mn: Mn is added to stabilize the austenite phase, facilitate the formation of a hard phase during the cooling process, and obtain high strength. However, if the amount added is less than 0.6%, sufficient hard phase cannot be obtained to achieve high strength. Moreover, if it is added in excess of 3.0%, a band structure develops, which not only deteriorates bending workability but also increases cost. Therefore, the amount of Mn is set in the range of 0.6 to 3.0%.

P: P has the same effect as Si when added in an amount of 0.02% or more, and is effective in ensuring a balance between strength and elongation, but if added in an amount exceeding 0.1%, it may cause plating defects, etc. Therefore, it should be added at 0.1% or less.

sol, AQ: AQ is added to deoxidize steel, but adding too much will not only saturate the effect but also cause poor plating, so the amount added should be limited to sol. The content of Al should be 0.1% or less.

The above elements are essential components, but if necessary, at least one of MO and Cr may be contained in an appropriate amount.

MO = Mo significantly stabilizes the austenite phase, facilitates the formation of a hard phase during the cooling process, and has the effect of increasing strength. However, if it is less than 0.01%, it will not be possible to obtain the hard phase needed to achieve high strength, and if it is added more than 1.0%, bainite will be suppressed and martensite will be formed in large amounts in bands. As a result, bending workability deteriorates. Therefore, the amount of MO is in the range of 0.01 to 1.0%.

Cr: Cr has the same effect as Mn and MO, and has the effect of stabilizing the austenite phase, facilitating the formation of a hard phase, and obtaining high strength. To achieve this effect, at least 0.1
%, but if it is added in excess of 1.5%, the elongation will be reduced, so the amount of Cr is set in the range of 0.1 to 1.5%.

Next, manufacturing conditions in the method of the present invention will be explained. Note that FIG. 1 shows the thermal history of the continuous galvanizing line in the present invention.

First, steel having the above-mentioned chemical composition is made into a slab through normal steel manufacturing steps or continuous casting, and then hot rolled into a hot coil. There is no need to particularly limit the conditions for hot rolling, but in order to obtain a hot-dip galvanized high-strength steel sheet with a uniform fine composite structure of ferrite and bainite, it is necessary to lower the coiling temperature during hot rolling. It is preferable to have a uniform structure of ferrite and bainite.

After hot rolling, a thin steel plate is obtained by pickling and cold rolling according to a conventional method. It is desirable that the cold working rate is 30% or more.

Next, this thin steel sheet is led to a continuous hot-dip galvanizing line, where it is subjected to recrystallization annealing and hot-dip galvanizing, and is further subjected to alloying treatment if necessary.

For recrystallization annealing, the heating temperature is Ac, the point -50 to 900
It is necessary to bring the temperature to ℃ and hold it for 10 seconds or more. When the heating temperature is lower than the Ac3 point -50'C, the volume fraction of austenite decreases, and its C concentration increases, resulting in stabilization, suppressing the formation of bainite, and increasing the martensite volume fraction.

Furthermore, since the recrystallized grains of ferrite become coarse, the bending workability deteriorates.

Next, as cooling from the above heating temperature to the hot dip galvanizing process, from a temperature of 600°C or higher to 1480°C above the Ms point
In the temperature range below ℃, Q nCR=-1,18Mneq+3.37, where, M
neq=Mn+1.52Mo+1.10Cr+0.10
After cooling at a critical cooling rate CR ("C/s)" indicated by Si+2.1 F', the temperature is maintained at 1480° C. or lower above the Ms point for 10 seconds or more, and then hot-dip galvanizing is applied.

If the cooling rate is slower than CR, pearlite transformation will occur, making it impossible to obtain the desired strength and bending workability.

In addition, regarding the process of holding at a temperature of 480'C above the Ms point for 10 seconds or more, when the temperature is below the Ms point, a large amount of austenite transforms into martensite, resulting in a decrease in bending workability6, while at temperatures above 480'C. In this case, fine bainite that is effective for bending workability cannot be obtained. In addition, if the holding time is less than 10 seconds, sufficient bainite will not be obtained and austenite will transform into martensite in the subsequent process, resulting in a marked decrease in the punched hole expansion rate (λ), as shown in Figure 2. , excellent bending workability cannot be obtained.

By applying hot-dip galvanizing, a bainite-ferrite-martensitic composite structure consisting mainly of bainite is obtained, and a high-strength steel plate with excellent bending workability is obtained.

In addition, after hot-dip galvanizing, A c 1 point or less,
A bainite-ferrite-martensitic composite structure mainly composed of bainite can be obtained by alloying at a temperature of preferably 500° C. to the Acm point and cooling. 1. A high-strength steel plate with excellent bending workability. is obtained. This is because since the alloying treatment temperature is below the Ac1 point, the appropriate bainite-based structure obtained by cooling after recrystallization annealing is maintained without re-austenite transformation.

Next, one embodiment of the present invention will be described.

(Example) Steel having the chemical composition shown in Table 1 was melted and made into a 20 mm thick slab. This was hot rolled at a finishing temperature of 850'C and a coiling temperature of 560'C to obtain a 3.2 mm thick hot rolled steel plate. The obtained steel plate was pickled and cold rolled to obtain a cold rolled steel plate with a thickness of 1.2 mm (rolling ratio: 62.5%).

These cold-rolled steel sheets were subjected to hot-dip galvanizing or further alloying treatment under the conditions shown in FIG. 1 and Table 2.

The tensile properties and bending properties of the obtained steel plate were investigated. The bending properties were evaluated based on the expansion ratio of a punched hole of 10φmm. The results are also listed in Table 2.

The following considerations can be made from Table 2.

The materials of the present invention, Nα1 to Nα2, have a high strength of nearly 80 kgf/mm” and a high punching hole expansion rate (λ) of 60% or more, but the comparative materials Ma3 and Nα9 have a holding time of 5 at 460°C.
Due to the short time of seconds, the amount of bainite produced is small and the hard martensite structure increases, so the strength is high, but the hole expansion rate is inferior to the material of the present invention.

In the comparison material Na 4, the recrystallization annealing heating temperature is as low as 730° C., so the volume of austenite is small, and the C concentration is high, so the material does not undergo bainite transformation and produces hard martensite. For this reason, the difference in altitude from ferrite becomes large, and as a result, the hole expansion rate is low, which is inferior to the material of the present invention.

Comparative material &5 has a low quenching start temperature of 500'C, so the amount of ferrite produced increases, and the C concentration in austenite increases and becomes stable, making it difficult to produce bainite. This results in a composite structure consisting mainly of ferrite and hard, coarse martensite, resulting in a low hole expansion rate.

Since the comparative material Nα6 has a low holding temperature of 600° C., pearlite is produced, and as a result, sufficient strength and hole expansion rate are not obtained.

In the comparative material NCL7, since the holding temperature is 200° C., which is below the Ms point, most of the austenite transforms into martensite. Therefore, although it has high strength, the hole expansion rate is inferior to the material of the present invention.

Comparative materials Nα11 to Nα14 undergo pearlite transformation because the cooling rate is slower than that of CR, and therefore excellent bending workability with high strength cannot be obtained.

The present invention material Nci15 is an example without alloying treatment, but
High strength and excellent bending workability are obtained.

[Left below] (Effect of the invention) As detailed above, according to the method of the present invention.

Control the cooling from the recrystallization annealing heating temperature to a temperature range below 1480°C above the Ms point, and during the cooling process, 1480°C above the Ms point.
By carrying out the alloying treatment in the following temperature range for a holding time of Acm or less, a fine and uniform structure of bainite, ferrite, and martensite (with some retained austenite) mainly composed of bainite can be obtained. Furthermore, since the alloying treatment can be performed at low temperatures, it is possible to improve surface properties such as plating unevenness and powdering properties, and also to reduce energy costs.

Further, in the case of a normal hot-dip plated steel sheet, it is the same as that of an alloyed steel sheet, and a fine and uniform composite structure mainly composed of bainite can be obtained.

Therefore, according to the present invention, 60 to 120 kgf/a
It is possible to manufacture hot-dip galvanized high-strength steel sheets with excellent bending workability up to the m'' class.

[Brief explanation of drawings]

Figure 1 is a diagram showing the thermal history of alloyed hot-dip galvanizing and hot-dip galvanizing. Figure 2 is the holding time at 460°C of the alloyed hot-dip galvanized steel sheet obtained in the example (see Figure 1). It is a figure which shows the relationship between and a punched hole expansion rate ((lambda)). Patent applicant Hisashi Nakamura, patent attorney representing Kobe Steel, Ltd.

Claims (3)

[Claims] (1) In weight% (the same applies hereinafter), C: 0.06 to 0.2
%, Si: 0.6% or less, Mn: 0.6 to 3.0%, P
: 0.1% or less and sol. Al: 0.01-0.10
%, with the remainder consisting of iron and unavoidable impurities, is hot-rolled, pickled, and cold-rolled in the usual manner, and then annealed for re-pure crystallization on a continuous galvanizing line. Hold the temperature at the Ac_3 point -50℃ to 900℃ for 10 seconds or more, and from the temperature of 600℃ or higher to the Ms point or higher of 480℃
lnCR=-1.18Mneq+3.3 in the following temperature range
7 Here, Mneq=Mn+1.52Mo+1.10Cr
After cooling at a cooling rate equal to or higher than the critical cooling rate CR (°C/s) expressed by +0.10Si+2.1P, 1
After holding for more than 0 seconds, hot-dip galvanizing produces bainite, ferrite, and bainite-based materials.
A method for producing a high-strength hot-dip galvanized steel sheet with excellent bending workability, which is characterized by obtaining a martensitic composite steel sheet. (2) After applying the hot-dip galvanizing, alloying treatment A
A method for producing a high-strength hot-dip galvanized steel sheet with excellent bending workability, characterized by forming a steel sheet with a bainite-ferrite-martensitic composite structure mainly composed of bainite by applying the galvanized steel sheet at a point of c_1 or less. (3) The steel further includes Mo: 0.01 to 1.0% and Cr
The method according to claim 1 or 2, wherein the method contains at least one of: 0.1 to 1.5%.