JPS5849623B2 - Method for manufacturing high-strength galvanized steel sheet with excellent drawability and shapeability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a high tensile strength galvanized steel sheet with excellent drawability and shapeability.

In recent years, the use of high-tensile steel sheets for the purpose of increasing the strength or reducing the weight of automobile bodies, and the use of galvanized steel sheets for the purpose of corrosion resistance, has become remarkable. development is desired.

Since steel sheets for automobiles are generally used after being press-formed, they must have properties suitable for this press-forming.

In particular, parts that undergo severe processing such as car body panels require excellent drawability and shapeability.

The higher the r-value (Lankford value) of a steel sheet, the better the drawability is, and various types of steel sheets with higher r-values have been developed by improving manufacturing conditions for conventional soft steel sheets.

However, the r value tends to decrease as the strength of the steel plate increases, and the r value of the high tensile strength steel plate was around 1.0.

The lower the yield strength and yield ratio of the steel sheet, the better the shape properties will be.However, in general steel with ferrite + pearlite structure, the yield ratio is 0.65 to 0.85, so as the tensile strength of the steel increases, The yield strength also increased, and generally speaking, the formability of high-strength steel sheets was extremely poor, so they were not used much for automobiles.

In recent years, a steel with finely dispersed martensite in ferrite has been developed, but in this steel, the martensite becomes a source of dislocations and uniform deformation occurs relatively easily, so the yield strength is low and 0. This results in a low yield ratio of .60 or less.

The results of various press formability tests conducted on steel with such a low yield ratio confirmed that the formability was better than that of conventional steel sheets with a ferrite + pearlite structure (yield ratio of 0.65 to 0.85). ing.

Now, in order to obtain a steel plate with such a low yield ratio, the method of continuous annealing after cold rolling was considered to be the most advantageous.

The metallurgical background is that the steel plate is heated to the coexistence temperature of ferrite (α) and austenite (γ), and then rapidly cooled to transform the r phase into martensite and obtain the low yield ratio. The amount of C in steel, etc.
The Mn amount, Si amount, etc. are adjusted.

The low-yield-ratio high-strength steel sheets manufactured by this manufacturing method have good shape properties as described above, but the r value is low at 1.0 or less, and a large amount of Mn or Si is added to obtain a martensitic structure. There were two problems with requiring .

In the continuous hot-dip Zn plating line, a kind of continuous annealing and Zn
Since plating is done at the same time, it is basically possible to obtain a high-strength steel plate with a low yield ratio using the above concept, but since the thermal history during cooling is significantly different from normal continuous annealing, it is necessary to obtain a low yield ratio. It is necessary to add a large amount of Si and Mn, and as a result, the r value is low and the adhesion of the zinc film is also deteriorated.

The reason why a larger amount of Si and Mn is required than in the normal continuous annealing method is that in a continuous molten Zn plating line, after heating to the α+γ phase, the molten Zn is heated for a certain period of time in a low temperature holding zone of about 500°C, and then the molten Zn is heated to about 450°C. When galvanizing by immersing in a bath or performing alloying treatment, heating at about 560°C is additionally required, so the Si ,M
This is because it is necessary to stabilize the γ phase even at low temperatures by adding n or the like.

Even small amounts of Si and Mn addition can stabilize the γ phase, and if a steel plate with a low yield ratio can be obtained in a continuous hot-dip Zn plating line, it will improve the alloy cost, improve the r value, and improve the adhesion of the zinc coating. The benefits are great.

As a result of various experimental studies, the inventors found that Mn, S in steel
By adding an appropriate amount of olAl,N and box annealing in a temperature range of 650 to 800 ° C before the hot-dip Zn plating line, it had a yield ratio of 0.60 or less and an r value of 1.2 or more.
It has been found that a high tensile strength galvanized steel sheet with extremely good workability can be obtained.

In other words, when a cold-rolled steel sheet is box annealed, AlN precipitates during the heating process and forms a recrystallized texture favorable for the r value, which is not destroyed in the next continuous hot-dip Zn plating process, so the r value is 1.2 or more. A high r value can be obtained.

In addition, during the silent process during box annealing, the steel plate is heated to a temperature of 650℃ to 800℃.
As the steel is heated to
, Mn is significantly enriched.

When this is cooled and then heated again to 600 to SOO℃ in the heating furnace soaking zone of the continuous hot-dip galvanizing line, the part where C and Mn are concentrated becomes a very stable γ phase, and this γ phase
It transforms into martensite at around 200°C without undergoing pearlite transformation at temperatures up to 600°C.

Since the temperature of molten zinc is usually around 460'C, martensite is formed in galvanized steel sheets during the cooling process after plating.

Furthermore, even in alloyed galvanized steel sheets that include a step of continuously heating up to around 560°C after galvanizing, the steel sheets are not cooled below 250°C after the zinc bath.
Martensite is formed during the cooling process after alloying treatment.

In this way, a ferrite + martensite structure can be obtained in any galvanized steel sheet.

Since this manufacturing method utilizes the polarization of Mn and C, the average amount of Mn in the steel can be small, and it does not require the addition of Si, resulting in lower costs, improved r-value, and better adhesion of the Zn film. Contributes to improving sexual performance.

In more detail, C0.07, Si0.07φ, M
n 1.4 2%, P0.011%, SoilAll
0.038%, NO. After melting the test steel containing 0.062% into a slab in a converter, the heating temperature was 1220
℃, finishing temperature of 860℃, and coiling temperature of 560℃.3. After finish pickling on 2 inch thick steel plate,
0.0 by normal cold rolling. A cold-rolled steel plate rolled to a thickness of 8 mm was heated in a mixed gas atmosphere of H2 and N2 at a heating rate of 40
℃/hr1 Cooling rate 40℃/hr, Soaking time 16hr
The heat pattern is the same as when producing alloyed galvanized steel sheets on a normal continuous hot-dip galvanizing line, using various test steel sheets box-annealed under the same annealing conditions and varying the soaking temperature between 500 and 800 degrees Celsius. The heat pattern shown in Figure 1,
That is, the temperature increase rate is 10°C/sec. Soaking temperature 7 5 0'C3
0SeC, primary cooling rate 10℃/sec, low temperature holding zone 5
0 0 'C 2 0SeC, secondary cooling rate 10°C
/sec, l and the lowest temperature (T') point in secondary cooling 300
A heat treatment test was conducted under the conditions of soaking in an alloying furnace at 560°C, 10SeC, and a tertiary cooling rate of 10°C/SeC.

In an actual hot-dip galvanizing line, the steel plate is kept at a low temperature of about 500°C and then immediately placed in a zinc bath set at about 460°C with about 2 SeC. The sample was immersed in the zinc bath for a short time, and then further cooled to 250 to 4008 C by gas wiping or the like, but since the immersion time in the zinc bath was short, it was omitted in the main heat treatment.

Note that the Ms point of the above-mentioned box-annealed steel is approximately 120°C, so during the cooling in the secondary cooling, the lowest temperature (τ point) is maintained at or above the Ms point, and γ phase transformation does not occur and the temperature is 560°C.
, 10SeC, the temperature of the steel sheet becomes below the Ms point during the cooling process, and martensitic transformation of the γ phase occurs.

FIG. 2 is a chart showing the relationship between the box annealing temperature and the yield value and r value determined from the characteristic values of the tensile test based on JI 85 of each test steel sheet obtained in the above heat treatment test.

In the figure, a characteristic curve P shows the relationship between the box annealing temperature and the r value, and a characteristic curve Q shows the relationship between the box annealing temperature and the yield ratio.

As shown in FIG. 2, if the box annealing temperature is in the range of 650 to 800°C, a steel plate can be obtained that satisfies both the r value of 1.20 or more and the yield ratio of 0.60 or less.

Figure 3 shows C0.04-0.08, Si0.05-0.
25%, SoAAA'0.03-0.08%, NO. 0
03~o. oos% and Mn is 0. 2-3.
Various test steels with varying amounts of 3% added were treated in the same manner as described above. After finishing the cold-rolled steel plate with a thickness of 8 mm, the heating rate was 400C/hr and the cooling rate was 40°C/hr.
r. Soaking temperature 700℃, soaking time 16hr1H2 and N
Yield determined from the characteristic values of the tensile test based on JISS No. 2 of each test steel sheet obtained by box annealing under the annealing conditions of a mixed gas atmosphere in 2 and further heat treatment test with the same heat pattern as shown in Fig. 1 above. It is a chart showing the relationship between the ratio, r value, and Mn content.

In the figure, a characteristic curve R shows the relationship between the r value and the amount of Mn, and a characteristic curve S shows the relationship between the yield ratio and the amount of Mn.

As shown in Figure 3, the r value is 1.20 or more, and the yield ratio is 0.60.
The Mn content that satisfies both of the following is in the range of 0.8 to 2.5.

The heat pattern shown in FIG. 1 is a heat pattern simulated in a laboratory for manufacturing an alloyed galvanized steel sheet.

Using this diagram to compare the case of a normal galvanized steel sheet and the case of an alloyed galvanized steel sheet, in the latter case, as shown in Figure 1, there is an alloying process, so the alloying ingredients are necessary to create a ferrite + martensite structure. Although efforts such as adjustment are required, the former is usually galvanized (without alloying treatment)
In this case, since the steel plate does not pass through the alloying treatment furnace, in the heat cycle shown in FIG. 1, the temperature of the steel plate is simply cooled to room temperature after γ.

Therefore, it is easier to obtain a ferrite + martensite structure than in the case of alloyed steel sheets.

Therefore, if the ingredients and box annealing conditions are limited so that the desired structure and mechanical properties can be obtained in the heat cycle shown in FIG. 1, the desired structure and mechanical properties can usually be obtained in galvanized steel sheets.

The present invention provides a new method for producing high-strength galvanized steel sheets based on the above findings, with improvements made in terms of both alloy content and heat treatment. 2oφ or less, Si0.6
0% or less, Mn 0.8 to 2.5%, P 0.10% or less,
SolAAi'0.01-0.20%, NO. 0015
After hot rolling and cold rolling, a steel containing ~0.0150 yen and the remainder substantially consisting of Fe is box annealed in a temperature range of 650 to 800°C, cooled, and then placed in a heating furnace of a continuous hot-dip galvanizing line. The gist is to perform galvanization or alloyed zinc plating after heating at a temperature range of 600 to 800°C in a soaking zone.

The reason why the components of the steel are limited as described above in the present invention will be explained.

C: If it exceeds 0.20%, spot weldability deteriorates and the r value decreases.

Si: Si is an inexpensive reinforcing element, but if it exceeds 0.60 t, the adhesion of zinc to the steel plate will deteriorate significantly.

Mn: If it is less than 0.80, the Mn concentration in the Mn enriched section is insufficient and the effect of lowering the yield ratio is insufficient. and 2.5
When it exceeds excellent, the r value decreases to 1.2b or less.

゛・SoAAA: 0. If the diameter is less than 0.01φ, the cleanliness of the steel decreases and the r value also decreases.

If the diameter exceeds 0.20φ, it becomes difficult to melt the steel.

N: If it is less than 0.0015φ, the formation of A7N to obtain the recrystallized texture of steel is insufficient, and if it is less than 0.0150
%, ductility deteriorates.

P: Although it is an inexpensive reinforcing element, adding 0.10φ or more tends to cause brittle fracture.

In the method of the present invention, the soaking temperature of box annealing is set to 650 to 800.
The reason for limiting the heating temperature to 600 to 800 °C is that a yield ratio of 0.60 or less cannot be obtained if the temperature is lower than 650 °C or higher than 800 °C. The reason for limiting the temperature is that below 600°C, the formation of stable austenite phase in the C and Mn enriched section described above is insufficient, and when it exceeds 800°C, too much austenite phase is formed, so box annealing is not possible. This is because sometimes the effect of concentrating C and Mn in austenite is not exhibited.

Next, examples of the present invention will be described.

Table 1 shows the composition of the test steels (4), (B), (C), and CD).

The sample steels shown in Table 1) to 0 were each melted in a converter to form a slab, then hot rolled into a 3.2mm thick steel plate, pickled, and then cold rolled to a 0.8mmrn thick steel plate. Finished into a rolled steel plate and heated it in a mixed gas atmosphere of H2 and N2 at a heating rate of 40°C/hr.
, a cooling rate of 40°C/hr, a soaking temperature of 700°C, and a soaking time of 16 hours were used for box annealing, and the line speed was 8o.
Galvanized through a continuous hot dip galvanizing line at rrL/mln.

In the galvanizing line mentioned above, the plate temperature is 75% in the heating furnace.
After heating to 0℃, the plate temperature was raised to 500℃ in a low temperature holding zone for 20Se.
After holding the plate at 460°C, it was immersed in a galvanizing bath at 460°C to be galvanized. Immediately after adjusting the thickness of the zinc coating to 40g/rI1″ on one side by gas wiping, it was charged into an alloying furnace and the plate temperature was 460°C. The galvanized test steel sheet and the alloyed galvanized test steel sheet were divided into two methods: one was held for 10 seconds and then allowed to cool to room temperature, and the other was allowed to cool to room temperature without passing through the alloying furnace. Obtained.

The yield strength, tensile strength, and
The yield ratio, r value, and zinc adhesion results are shown in Table 2.

The above tensile test is the result of the L direction tensile test according to JI35, and the zinc adhesion is the result of investigating the peeling state of the film after adhesion bending. ○ mark indicates good adhesion, and × mark indicates poor adhesion.

Further, GI indicates a galvanized steel sheet, and GA indicates an alloyed galvanized steel sheet.

As shown in Table 2, even if the steel plate (B) has the composition range of the present invention, the box annealed one has a yield ratio of 0.60 or less and an r value of 1.
．． Although it shows a score of 20 or more, those without box annealing have insufficient yield ratio and r value.

Comparative steels (4) and (C) all did not satisfy the above-mentioned required level in either yield ratio or r value, and comparative steel (D)
This indicates that the zinc adhesion is poor and the steel sheet is unsuitable for use as a high-tensile galvanized steel sheet.

[Brief explanation of the drawing]

Fig. 1 is a chart showing an example of the heat pattern of a normal continuous hot-dip galvanizing line, Fig. 2 is a chart showing the relationship between yield ratio and r value and box annealing temperature for test steel sheets subjected to heat treatment tests. FIG. 3 is a chart showing the relationship between yield ratio, r value, and Mn content in the same test steel sheets.

Claims (1)

[Claims] IC0.201% or less, SiO. 60% or less, Mn
o. s ~2. 5%, po. to% or less, Sol
k1 0. 0 1~0.20 excellent, N0.0015~0
．． After hot rolling and cold rolling, a steel containing 0.0150% and the remainder substantially consisting of Fe is box annealed in a temperature range of 650 to 800°C, cooled, and placed in a soaking zone in a heating furnace of a continuous hot-dip galvanizing line. A method for producing a high-tensile galvanized steel sheet with excellent drawability and shapeability, characterized by carrying out galvanization or alloyed galvanization after heating in a temperature range of 600 to 800°C.