JPS62130268A - Production of hot dip zinc coated mild steel sheet for working subjected to alloying treatment - Google Patents

Abstract

PURPOSE:To inexpensively produce the titled mild steel sheet having excellent workability by subjecting a hot rolled steel sheet contg. specific ratios of C, Mn, Si, P, S, Al, and N to hot dip zinc coating then to quick heating and quick cooling under specific conditions. CONSTITUTION:The hot rolled steel sheet or cold rolled steel sheet contg., per wt%, 0.01-0.08% C, 0.05-0.4% Mn, <=0.1% Si, <=0.02% P, <= 0.02% S, 0.005-0.1% Al, <=0.015 N, and others such as inevitable impurities is subjected to hot dip zinc coating. The steel sheet is then quickly heated up to the temp. region of >=670 deg.C and <= the A3 transformation point at a heating rate of >=15 deg.C/sec and is held at said temp. for >=1sec, by which the steel sheet is subjected to an alloy treatment. The steel sheet is quickly cooled in 670-400 deg.C temp. region at a quick cooling rate of >=30 deg.C/sec. The alloyed hot dip zinc coated mild steel sheet having good workability at a simple heat cycle regardless of the heat history of the steel sheet is thus inexpensively produced. Such steel sheet has good aging resistance for about a few months and is satisfactorily usable as the steel sheet for press working.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Application in Industry H> The present invention relates to a method for manufacturing a hot-dip galvanized 4Ii steel sheet for processing used in automobiles, electrical products, etc.

<Prior art and its problems> As a method for gM production of alloyed hot-dip galvanized steel sheets,
Generally, cold-rolled steel sheets are recrystallized and annealed by heating to above the recrystallization temperature in the same line as hot-dip galvanizing.
Next, after applying Ilk lead plating by lowering the temperature to 400-450°C and immersing it in a lead bath,
It was common to perform alloying treatment by reheating to 0°C (for example, Furuki Tetsu to Sm1l Vol. 84 No.
, 9 (197B) p1457-+4137).

In addition, as a special usage, hot-rolled steel sheets or cold-rolled steel sheets that have been recrystallized annealed on a separate line are used, and in the galvanizing line, galvanization is performed without recrystallization annealing, and then alloying treatment is performed. There are also many.

In such a conventional hot-dip galvanizing line, the alloying treatment temperature is 550 to 600°C, and recrystallization and grain growth of the steel cannot be expected. Furthermore, this is the temperature range in which about several tens of ppm of C dissolves (solid solution) in the steel, so it will never impair the workability of steel, such as press forming.
In general, the quality of the material deteriorates more often.

In the case of low C steel like the present invention, residual solid solution C is removed by Fe3
As a method for precipitation treatment as C, over-aging treatment is carried out (for example, Nakamura et al. Steel Manufacturing Research Vol. 1.308 (1)
982) p14313), since the cooling start temperature (=alloying treatment temperature) before performing the overaging treatment is low, an excessive amount of solid solution C remains, and the workability is also poor.

Regarding the thermal history of steel sheets, hot-rolled steel sheets and cold-rolled steel sheets that have undergone recrystallization annealing in advance do not require any annealing equipment in the galvanizing line; on the other hand, when using cold-rolled steel sheets, zinc It becomes necessary to perform recrystallization annealing within the plating line. In other words, it becomes necessary to be able to reproduce various heat cycles within the same line depending on the pre-T degree of the steel plate, and as a result, in-line annealing equipment becomes essential. This is undesirable because it leads to an increase in manufacturing cost due to an increase in equipment cost.

The inventors have developed various methods to improve both these problems, namely the deterioration of workability due to alloying treatment and the complicated heat cycle in the galvanizing line for steel sheets with various heat histories in the previous process. We examined the following. As a result, by raising the alloying temperature significantly higher than conventional methods and rapidly cooling the subsequent cooling, we were able to promote grain growth and improve workability in hot-rolled steel sheets and cold-rolled steel sheets recrystallized by Tomae. Moreover, for the most common cold-rolled steel sheets, recrystallization annealing can be performed simultaneously with the alloying process, making it possible to monocycle the galvanizing line. It turned out that.

Note that Japanese Patent Application No. 103844/1983 discloses a method of adjusting the structure of the galvanized alloy layer by increasing the alloying temperature to improve the plating powdering resistance. There is no mention of the T tendency or monocycling, and this is a completely different technology from the present invention.

<Objective of the Invention> Therefore, the object of the present invention is to solve the drawbacks of the prior art described above, and to provide a processing material that can inexpensively produce an alloyed hot-dip galvanized mild steel sheet with excellent workability through a simple heat cycle. An object of the present invention is to provide a method for producing an alloyed hot-dip galvanized mild steel sheet.

<Structure of the invention> These objects are achieved by the following invention.

That is, the first invention of the present invention is C: 0.01 to 0.08wt%1Mn: 0.05
~0.4 wt%Si: 0.1 wt% or less, P
: 0.02wt% or less S: 0.02wt% or less, Al: 0.005-0.1wt% N: 0.01
wt% or less.

Hot-dip galvanizing is applied to the F8 rolled steel sheet or cold-rolled steel sheet containing the inevitable fine impurities, and then 15"C/sec
After rapidly heating at the above heating rate to a temperature in the range of 670 ° C to A3 transformation point and holding at this temperature for 1 second or more,
The present invention provides a method for manufacturing an alloyed hot-dip galvanized mild steel sheet for processing, which involves rapid cooling in the temperature range of 670 to 400°C at a cooling rate of 30'C/sec or more.

The second invention is: C: 0.01-0.08wt%, Mn: 0.0
5-0.4 wt%Si: 0.1 wt% or less,
P: 0.02wt% or less S: 0.02wt% or less, l: 0.005-0.1wt% N: 0
．． 01wt% or less.

Hot-rolled steel sheets or cold-rolled steel sheets containing unavoidable impurities are hot-dip galvanized and then rapidly heated at a heating rate of 15°C/sec or higher to a temperature in the range of 670°C or higher and below the A3 transformation point. , after holding at this temperature for more than 1 second, 30
Alloying for processing characterized by rapid cooling in the temperature range of 670 to 400 °C at a cooling rate of ℃/sec or more, followed by overaging treatment at a temperature in the range of 300 to 400'C for 30 seconds. A method of manufacturing a treated hot-dip plated 111i steel plate is provided.

be.

The third invention is.

C: 0.01-0.08vt%, Mn: 0.0
5-0.4 wt%Si: 0.1 wt% or less,
P: 0.02wt% or more FS: 0.02wt% or less, An: 0.005-0.1 wt%N・0
．． O11 days or less.

Hot-dip galvanizing is applied to the #+ rolled steel sheet or cold-rolled steel sheet containing inevitable fine impurities, and then 15'C/sec
Rapidly heat the A3 transformation to a temperature in the following range from 670 °C to A3 transformation at the heating rate below, hold at this temperature for 1 second, and then heat from 670 to 400 °C at a cooling rate of 30 °C/sec or higher.
Rapidly cool the steel plate to a temperature range of 150 to 300℃.
To provide a method for manufacturing an alloyed hot-dipped mild steel sheet for processing, which comprises winding the sheet into a coil while maintaining the temperature in the range of be.

Hereinafter, the method for manufacturing a hot-dip galvanized mild steel sheet for processing according to the present invention will be explained in detail.

The steel plate in the present invention has C: 0.01 to 0.08wt%, Mn: 0.0
5-0.4 wt%Si: 0.1 wt% or less,
P: 0.02wt% or less S: 0.02wt% or less, Al: O, OQ5~0.1wt% N: 0.0
1 day t% or less.

Its fine stones contain inevitable impurities. The reason for limiting the content of each of these elements will be explained below.

About IrIC When the content exceeds 0.08 wt%, processability deteriorates.

If the carbon content is less than 0.01 wt%, the workability will be affected, but the dissolution of solid solution C will proceed during the alloying process, and a large amount will remain after cooling. Therefore, strain aging deterioration becomes easier to control.

In order to prevent aging deterioration, a carbonitride forming element such as T1°Nb is required, which increases the cost.

(ri)About Mn When the content exceeds 0.4 wt%, processability deteriorates.

When the content is less than 0.05 wt%, S in a solid solution state increases, and cracking defects are likely to occur during hot rolling.

(White) When the content of Si exceeds 0.1 wt%, processability deteriorates.

(4) When the P content exceeds 0.02% #t%, processability deteriorates.

! 6) Regarding S, S is rendered harmless by Mn as MnS, but if S increases, that is, the S content exceeds 0.02 wt%, a large amount of n is required and workability deteriorates.

tg) Regarding Al, An impairs its important role in fixing 0 and N, but its effectiveness becomes more effective when the Al content exceeds 0.1 wt%.

In addition, if the content is less than 0.005 wt%, there will be many 0 items during casting, many blow holes will occur, and when pressurized; I:
This may cause blurring.

When the content of (γ)N exceeds 0.01 wt%, it becomes difficult to ensure workability.

In the uninvention, a hot-rolled steel plate produced by melting steel containing the above components and unavoidable impurities, or a cold-rolled steel plate produced by descaling and cold-rolling the steel plate, or a cold-rolled steel plate produced by melting steel containing the above components and unavoidable impurities. The material is a cold-rolled steel plate that has been recrystallized by box annealing or freeze-continuation annealing. In addition, hot rolling conditions,
Cold rolling conditions, annealing conditions, etc. are not particularly limited.

Next, these steel plates are heated to 400 to 500°C or directly immersed in a galvanizing bath without being heated, thereby applying a predetermined hot-dip galvanizing process.

After performing such hot-dip galvanizing, 1-alloying treatment is performed. The method will be detailed below.

670℃- for steel plate at a heating rate of 15℃/see or higher
Rapid bulb heating is performed to a temperature within the range of Ac3 transformation point (A3 transformation point during heating). If the heating rate is 15℃/sec, carbide (Fe3G) will dissolve during heating, and the temperature will rise from 670℃ to 670℃.
Disadvantageous drawing property during holding at the temperature of Ac3 transformation point (100
), (110) orientation becomes easier to develop. The reason why the alloying treatment temperature was set to 670° C.-AC3 transformation point will be explained. For as-cold-rolled steel sheets, a minimum temperature of 670°C is required to complete recrystallization, and for hot-rolled steel sheets and cold-rolled steel sheets that have been annealed before rolling, sufficient grain growth is promoted during holding. This is because at least 670'c is required to make Is. At the end of a hot-rolled steel sheet or a pre-annealed cold-rolled steel sheet in the width direction, the grain size may be smaller than at the center of the width direction, or conversely, the grain size may be smaller than at the center of the width direction, or conversely, at the end of the width direction of a hot-rolled steel sheet or a pre-annealed cold-rolled steel sheet. Sometimes, it is rolled below the Ar3 transformation point, leaving coarse grains and unrecrystallized structures, and the workability is significantly inferior to that at the center in the width direction. In such a case, if the temperature is maintained at 670° C. or higher during the alloying process, the grain growth will be accelerated (U) and the workability of the widthwise end portion will be impaired.

to

Further, the reason why the h-limit temperature is set as the AC3 transformation point is because if the temperature is maintained at a temperature exceeding this, the texture becomes random and the workability is significantly deteriorated.

The holding time at the temperature in the range listed above is 1 second. This is because if the time is less than 1 second, no improvement in workability can be expected.

After such alloying treatment, 30”C/Sec
A temperature range of 670 to 400°C is rapidly cooled at a cooling rate as follows. This is because the solid solution C dissolved during the alloying process is replaced with Fe.
This is done in order to precipitate it as 3C. solid solution C
In order to extract a sufficient amount of , rapid cooling must be carried out at a temperature range of 670°C to 400°C, even at relatively low temperatures. Even if rapid cooling is performed in the temperature range below 670°C or above 400°C, solid solution C remains.
is not sufficiently precipitated, resulting in poor workability and aging resistance.

Further, the faster the cooling rate is, the better in terms of workability and aging resistance, and rapid cooling requires a cooling rate of 30° C./sec or more to exhibit the above effects.

In such a cooling method, a small band of solid solution C of about lop per million remains in the steel. With this amount of solid solute C, it has good aging resistance for about 2 to 3 months from the time it is made into a product by heat treatment pressing and subsequent temper rolling until it undergoes processing such as pressing, so press r... It can be fully used as a steel plate.

However, among steel sheets for T-processing, those that require a long period of time to be processed for export overseas are required to have good workability even after 2 to 3 months.

If you want to obtain such a non-aging steel plate, it is necessary to
It is possible to carry out an overaging treatment for 30 seconds at a temperature in the range of 00'C, or to wind the steel plate into a coil at a temperature in the range of 150 to 300'C, and then allow it to cool to below 100'C. It becomes necessary.

First, the overaging treatment conditions will be explained. Processing temperature is 4
If the temperature exceeds 00°C, non-aging property cannot be ensured because the equilibrium solid solubility is high even if solid solution C is precipitated. Also, 300
If it is less than 'C, overaging treatment cannot be carried out in a short period of time. Therefore, in the temperature range of 300 to 4000 (:!), solid solution C
precipitates, and sufficient anti-aging properties are obtained. As for the processing time, sufficient non-aging can be achieved if the processing time is 30 seconds or less. The upper limit is not particularly limited, but in consideration of production efficiency, equipment cost, etc., about 10 minutes is preferable.

Next, conditions for winding into a coil will be described.

If the holding temperature is 300"C#fi, it will be difficult to handle the coil during or after winding, and it will take a long time to let it cool. If the holding temperature is less than 150℃, solid solution C will be formed during cooling after winding the coil. Precipitation does not proceed and aging properties are not improved.

Therefore, if it is wound into a coil at a temperature in the range of 150 to 300'C, the precipitation of solid solution C will proceed, and non-aging properties can be achieved. The temperature at which the coil is left to cool is set to 100° C. or less in order to allow sufficient precipitation of solid solution C and to take into consideration the temper rolling properties of the next step.

<Example> Examples of the present invention will be described below.

(Example 1) C = 0.031 wt%, Si = 0.01 wt%
, Mn=0.18wt%, P=0.0+3wt%
, S = 0.014 wt%, Al = 0.039
wt%, N = 0.0038wt%, steel having inevitable impurities ★ was tapped in a converter and made into a slab by continuous casting. Heat the slab to 1220°C with
Stir rolling was completed at 70°C and coiled at 620°C. Hot rolled steel plate after descaling (plate thickness 2.8mm) (Steel A
A cold 1. [: Steel plate (steel B), followed by Ritsutsugi annealing (800”C1l win
A large number of four types of coils were prepared: a steel coil (steel C) subjected to aging treatment at 350°C after homogenization and a steel coil (mD) subjected to box annealing at 650°C for 15 hours.

These coils i! I! 460℃ with continuous heating
After hot-dip galvanizing at a rate of 70 g/m' per side, the material properties were examined by varying the subsequent alloying treatment conditions (heating rate, soaking temperature, cooling temperature). Note that cooling was performed at a rate of 5°C/sec from 400°C to 250°C, and water cooling treatment was performed from 250°C.

The results are shown in Table 1. No in Table 1, 1°3.4
．． 9.10.11 is an example of the present invention, and other No. 2
．． 5.6.7.8 lacks the conditions of the present invention. The deviating conditions are shown by underlining the numerical values in Table 1.

As shown in the results in Table 1, No. 2 (Wf4 plate B) and No. 8 (steel plate A) had low heating rates and therefore had inferior 〒 values. The -r value of hot-rolled steel sheets is usually 0.7 to 1, O, which is inferior to cold-rolled steel sheets, but No. 1 has a faster heating rate.
The value is better than 8. No. 5 and No.

6 is an example where the soaking temperature was incorrect, and the total elongation was significantly inferior (
No. 5) Poor workability. No. 7 is an example in which the cooling rate is slow, the total elongation is slightly inferior, and the yield point elongation after aging is significantly high.

(Example 2) Steel B (cold-rolled steel sheet) similar to Example 1 was used, and the hot-dip galvanizing conditions were 70 g/m' (per single side) at 460°C, followed by 20°C/see tare 80 It was rapidly heated to 0.degree. C. for alloying treatment (holding time: 20 seconds) and then rapidly cooled from 675.degree. C. at 65.degree. C./sec to the overaging treatment temperature or the temperature for winding into a coil.

Table 2 shows the properties of steel B when various overaging treatment conditions are changed.
Table 3 shows the properties of mH when the coil winding temperature was variously changed. In Table 2, No. 2 to 4 are examples of the present invention, and the others lack the conditions of the present invention. In Table 3, No. 1 and No. 2 are examples of the present invention, and the others lack article f [of the present invention. In Tables 2 and 3, the values outside the conditions of the present invention are underlined.

As shown in Tables 2 and 3, the steel of the present invention has Y after aging.
It can be seen that not only is there no generation of EL and non-aging property, but also that almost no solid solution C remains, the cooling strength is low, the material has good flowability, and has sufficient workability.

Our ``effects'' According to an uninvented method, alloyed hot-dip galvanizing IJ with good workability in a short heat cycle without knowing the thermal history of the steel sheet! It is possible to manufacture li' steel plates at low cost, and industrial H
The usefulness of is extremely large.

1. −7

Claims (3)

[Claims] (1) C: 0.01-0.08wt%, Mn: 0.05
~0.4wt%Si: 0.1wt% or less, P:0.02
wt% or less S: 0.02wt% or less, Al: 0.005
~0.1wt% N: 0.01wt% or less, hot-rolled steel sheet or cold-rolled steel sheet containing other unavoidable impurities is hot-dip galvanized, and then heated to 670°C or higher at a heating rate of 15°C/sec or moreE, A_3 After rapidly heating to a temperature in the range below the transformation point and holding at this temperature for more than 1 second,
A method for producing an alloyed hot-dip galvanized mild steel sheet for processing, characterized by rapidly cooling a temperature range of 670 to 400°C at a cooling rate of 30°C/sec or more. (2) C: 0.01-0.08wt%, Mn: 0.05
~0.4wt%Si: 0.1wt% or less, P:0.02
wt% or less S: 0.02wt% or less, Al: 0.005
~0.1wt% N: 0.01wt% or less, hot-rolled steel sheet or cold-rolled steel sheet containing other unavoidable impurities is hot-dip galvanized, and then heated to 670℃ or higher at a heating rate of 15℃/sec or higher to the A_3 transformation point. After rapidly heating to a temperature in the following range and holding at this temperature for more than 1 second,
Alloying treatment for processing, characterized by rapid cooling in the temperature range of 670 to 400 °C at a cooling rate of 0 °C / sec or more, followed by overaging treatment at a temperature in the range of 300 to 400 °C for 30 seconds or more A method for producing hot-dipped mild steel sheets. (3) C: 0.01-0.08wt%, Mn: 0.05
~0.4wt%Si: 0.1wt% or less, P:0.02
wt% or less S: 0.02wt% or less, Al: 0.005
~0.1wt% N: 0.01wt% or less, hot-rolled steel sheet or cold-rolled steel sheet containing other unavoidable impurities is hot-dip galvanized, and then heated to 670℃ or higher at a heating rate of 15℃/sec or higher to the A_3 transformation point. After rapidly heating to a temperature in the following range and holding at this temperature for more than 1 second,
The steel plate is rapidly cooled in the temperature range of 670 to 400 °C at a cooling rate of 0 °C/sec or more, then wound into a coil while maintaining the steel plate at a temperature in the range of 150 to 300 °C, and then allowed to cool to 100 °C. A method for producing a hot-dip galvanized mild steel plate for processing, characterized by subjecting it to a temperature of ℃ or less.