JPS6017004B2 - Manufacturing method of cold-rolled steel sheet for drawing with excellent bake hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cold-rolled steel sheet for drawing which has excellent dawn hardening properties.

The dovetail hardenability here refers to the hardenability that occurs during the paint baking process after press forming applied to cold rolled steel sheets or immersed Zn-plated steel sheets using cold rolled steel sheets as plating base sheets, and usually has a pre-strain of 2%. After a subsequent heat treatment of 17,000°C for 20 minutes, the yield strength is evaluated based on the degree of increase in yield strength.

This invention aims to propose a method for manufacturing a cold-rolled steel sheet for drawing which has excellent dovetail hardenability, high r, and a low aging index. It also includes steel plates, Zn-plated steel plates, alloy I#n-plated steel plates, etc.

Now, with recent advances in continuous steel casting technology, surface-treated steel sheets, typically used for automobiles, have changed from conventional rimmed steel to aluminum-killed steel, which has excellent formability, with almost no increase in cost. It was boiled. By the way, rimmed steel contains solid solution nitrogen, so it is resistant to aging at room temperature, but if it is press-formed within a short period of time after manufacturing, it will not be able to withstand stretching.
It has the advantage that it does not generate strain, strain aging occurs due to nitrogen during coating competition, and yield strength increases.

However, although aluminum killed steel has excellent deep drawability, it does not have such dawn hardenability because nitrogen is fixed by aluminum. Bake hardenability after press forming is a favorable phenomenon for dent resistance, especially when used for automobile exterior panels, and there is a strong demand for rolled steel sheets that have both deep drawability and dawn hardenability. .

In addition, the amount of high-strength steel plates used is increasing in order to reduce the weight and improve safety of automobiles, but the yield strength is low before press forming to compensate for the dent resistance that comes with the decrease in plate thickness. Therefore, a steel plate with increased yield strength during competitive painting is desired.

From such a Tsurumi point, a steel sheet with a composite structure consisting of ferrite and martensite is ideal in terms of dawn hardening properties, but
The value is 1. Since it is low as before and after , the drawability is poor and the parts to which it can be applied are limited. On the other hand, a steel plate with a high r value and dawn hardening property is produced by open-coil phosphor annealing of aluminum-killed steel strengthened by machine addition, and then taking advantage of the high cooling rate after phosphor annealing, hard twill carbon remains and strain is produced. A method has been proposed to exhibit aging properties, or to coarsen carbides by performing tight coil competitive hammering at high temperatures and prevent the precipitation of solid solution carbon, thereby leaving solid twill carbon and imparting adjoining hardenability. ing. However, the former requires a process of rewinding it into an oven coil and then converting it into a tight coil after being hammered, and the latter requires a process of fusion between the coil layers due to the high temperature of the hammer, and an inner cover (retort) inside the phosphor furnace.
deformation is unavoidable, and a significant increase in manufacturing costs is unavoidable. This invention provides a steel plate oval and its manufacturing method that is more improved than the conventional steel sheets and their manufacturing methods described above, and the gist thereof is as follows: C: 0.002
~0.008% by weight (hereinafter simply expressed as %), Mn: 0.
05 to 1.2%, Si: 0.5% or less, and P: 0.1
Contains the following, and further A〆: 0.01 to 0.08% and N% x 8 or more, Nb: C% x 3 or more, C% x 80.0
A hot-rolled steel sheet containing 2% or less Fe with the remainder being substantially Fe was cold-rolled at a rolling reduction of 60% or more, and subjected to Rentsugi firing at a temperature of 750 oo to 900° C. 1 inkstone or more. A method for producing a cold-rolled steel sheet for drawing with excellent bake hardenability, which comprises controlling the average cooling rate to at least 1000 qo per second in the subsequent cooling process to at least 650 qo.

It is. By the way, many methods have been proposed for producing deep drawable steel sheets using hardened low carbon steel, but to summarize them, CO. 0.05 to 0.02% of ultra-low carbon aluminum killed steel with 0.07 to 0.0% of NbC equivalent or more.
By adding 18%, preferably 0.08-0.12% Nb, r value 1.6-2.1 and elongation 40-48%
Non-aging steel sheets are the mainstream, while high tensile strength steel sheets are described in, for example, CO. 00
4%, Sil. 01%, Mno. 22%, Nbo. 04
Ultra-low carbon aluminum killed steel consisting of 9% was subjected to T.I. S. 42-4
6k9/I don't like it, Y. P. 28-30k9/abbreviated, rl. 6～
1. & It is disclosed that high tensile strength steel with an elongation of 32-35% can be obtained.

In contrast, the inventors found that when hot rolling a material at high speed and high reduction rate, when the amount of Nb added to carbon is approximately equivalent to or less than NbC,
It was discovered that a non-aging deep drawing steel sheet with better ductility could be obtained, and an application has already been filed.

Furthermore, under such conditions, phosphorus as a solid solution strengthening element has a small effect on the deterioration of the r value, and it was found that it is advantageous for high-strength steel sheets with excellent deep drawability, and so they filed an application. The inventors further conducted research into methods for imparting dawn hardening properties to these steel sheets, and found that 1. In order to exhibit an r-value of 1.5 or more, which is necessary for a drawing steel sheet, 3 to carbon is required.
Although twice as much Nb is required, in order to obtain a knee hardness of 5k9/masu or higher, the amount of Nb relative to carbon must be approximately 8 times or less.

2. A high cold rolling reduction of 60% or more is effective in increasing the r value.

3. When annealing is performed continuously at a high temperature of 750 qC or higher, it is effective in exhibiting high r value and high ductility, and a portion of the previously fixed C or day dissolves into solid solution.

4 The redeposition rate of solid solution C or N is large at a temperature of 650 qo or more, and if the average cooling rate until reaching at least 65,000 qo is controlled to 10°C or more per second, preferably 5,000 qo or more, the dawn hardenability is increased. To become a.

This invention was completed based on this knowledge.

In addition, as a method for manufacturing cold-rolled steel sheets with competitive hardening properties using Nb-added ultra-low carbon aluminum killed steel, Japanese Patent Application Laid-Open No. 49-1
Publication No. 30819 states that Nbo at C0,004%
．． A hot rolled sheet containing an excess of 0.062% was heated at 800:010.
In the cooling process after annealing for 1 minute, the
A method of rapid cooling at a speed of 650 qo is described.
It will be clear that this invention has nothing to do with the knowledge that the above quenching is particularly effective. In this invention, CO. 005%, Aso 0.03%, Nbo. 03% and PO. A cold-rolled plate containing 0.07% was annealed at 3°C/850006 under sand conditions.
The degree of hardening with brazing when changing the temperature at which quenching starts at 50 qo seconds as shown in Figure 1 a while cooling at a rate of 1.
Shown in Figure b.

The degree of hardening BH of the vertical shaft in FIG. 1b is expressed as the difference between the yield stress and the initial yield stress when aged for 170q×2 after 2% tensile prestrain. According to Figure 1, the competitive hardening degree is 5K when rapidly cooled from a temperature higher than 65,000.
9/Indicates rocky shore or higher, while 3 indicates up to about 620℃ or below.
If the material is gradually cooled at a rate of .degree. C./second and then the lining is started at a timing that is too late, a BH of only 45k9/iso or less can be obtained. The reason for this is thought to be that C or N dissolved in solid solution at the dawn anchor temperature recombines with Nb or A and precipitates by slow cooling to a temperature lower than 650°C. In any case, unless the high-temperature region immediately after being hammered is rapidly cooled, the aging hardenability will not be fully exhibited no matter how rapidly the low-temperature castle of 650 qo or less is cooled. By the way, if the material is hot-rolled at low speed and low pressure, solid solution C or N will exist during the temperature rise during burning, and a steel plate with excellent r value and ductility will not be obtained, but instead it will be rolled in the low temperature range. The scale hardening property can be easily increased by rapidly cooling.

Next, the steel components of this invention will be explained.

If C is less than 0.002%, there will be insufficient solid solution C that contributes to bake hardening, so 0.002% or more is required.
If it exceeds 0.08%, the yield strength will be high and the ductility and r value will deteriorate, so the content should be 0.008% or less.

Mn is required to be 0.05% or more in order to prevent red brittleness by combining with S, and is an effective component for obtaining high tensile strength steel, but if it exceeds 1.2%, the r value will be 1.5 or more. Since it cannot be obtained, it is set at 0.05 to 1.2%. Si is effective for obtaining high-strength steel, but if it exceeds 0.5%, an oxide film will form and the chemical conversion properties will be impaired, so it should be kept at 0.5% or less.
Note that when used for a hot-dip Zn plated original plate, the content is preferably 0.3% or less. In order to fix A and N, 0.01% or more is required, and 8xN% or more is required. However, 0.08%
Exceeding this is not preferable because inclusions will occur frequently. Nb is a particularly important component for the steel of this invention, and unless it is added at least three times as much as C, a large amount of solid solute C will remain and a texture with excellent drawability will not develop during cold rolling recrystallization.

However, the presence of excessive Nb impairs the ductility of the steel sheet, so the upper limit is set to C% x 80.02%. P is usually 0.01 to 0
．． Although it exists at 0.2%, it is a preferable element because there is little deterioration of the r value when obtaining high tensile strength steel sheets, and it may be added depending on the required strength, but if it exceeds 0.10%, the steel sheet will become stiff. .10% or less.

Next, regarding the manufacturing method, first, the steel may be manufactured by an open hearth furnace, a top-blown converter, a bottom-blown converter, or an electric furnace, but a bottom-blown furnace is advantageous in order to obtain ultra-low carbon steel.

Vacuum degassing treatment such as RH or DH is performed as necessary.
After blending the necessary alloy components, casting is performed, and either zero ingot or continuous casting may be used. Hot rolling may be carried out under conditions commonly used in hot strip mills, typically with a total reduction of 90% or more from the slab to the hot-rolled sheet, and a rolling speed of 40 skin/roll. It becomes min. Tandem rolling is suitable industrially. The winding temperature is not particularly limited, although the scale hardening property is greater at lower temperatures, and the drawing property is better at higher temperatures. It is necessary to perform the cold rolling after pickling at a reduction rate of 60% or more in order to ensure an r value of 1.5 or more, but if it exceeds 90%, anisotropy increases. 70-85% is suitable.

Renzuki shogun is heated to a temperature range of 750 to 90,000 degrees Celsius, maintained for more than 1 hour, and then rapidly cooled to at least 650 degrees Celsius at an average speed of more than 10 minutes per second. Of course, it is possible to rapidly cool to room temperature, but when using a gas jet, it requires a large amount of energy to rapidly cool a low-temperature castle, so slow cooling is effective below 650 pm. Also, depending on the conditions, the room temperature aging property may increase, but if the aging index must be less than 4k9/pillow, it is sufficient to slowly cool or maintain the temperature at 300 to 450 qo. The steel sheet according to the present invention is also suitable for hot-dip Zn plating treatment, and in particular, when Sj is 0.3% or less, a Zn-plated steel sheet or high-strength steel sheet with excellent adhesion and corrosion resistance and dawn hardening properties can be obtained. It will be done. Approximately 60 minutes after roughening
Even if the plated layer is alloyed by heating to 0°C, the bake hardenability is not lost. These characteristics are that the steel plate of this invention is 650q
This is due to the fact that the dawn hardening property can be controlled by rapid cooling at a temperature of 0 or higher. The steel plate after anchoring is subjected to 0.2 to 2% skin pass rolling according to a conventional method. Next, examples will be shown.

Slabs having the components shown in Table 1 were hot rolled in a 7-stand tandem rolling mill, scales were removed by pickling, then cold rolled, annealed in a continuous Akatsuki hammer line, and skin pass rolled.

The results are shown in Table 1. Both cold-rolled* steel sheets (1, 2, 6) and 35-40kg high r-value cold-rolled steel sheets (1, 2, 6) have an aging index of 4kg/Iso or less and an excellent degree of dawn hardening of 6-9k9/. High-strength steel plates (3, 4, 5) of the same class were tied up. Table 1 *AI: Increase in yield stress (aging index) when subjected to artificial aging treatment at 100°C for 30 minutes after 7.5% tensile prestrain*
* BH: Sum of work hardening stress increment due to 2% tension and strain aging hardening stress increment due to subsequent baking coating equivalent treatment (170°C, 20 minutes) (degree of hardening with soot) According to the invention as described above, drawing Since it has particularly excellent bake hardenability at a low aging index and an r value required for a rolled steel sheet for general use, it can be advantageously used in applications such as automobile steel sheets, and the method of the present invention can improve the above properties. In the actual process, the steel plate for drawing is 650q.
It can be accurately obtained by simple and easy operations performed until reaching o.

[Brief explanation of the drawing]

FIGS. 1a and 1b are a temperature diagram for one hour of continuous brazing and a relation diagram between the quenching start temperature after moth dulling and the degree of bake hardening. Figure 1

Claims (1)

[Claims] 1 C: 0.002 to 0.008% by weight, Mn: 0.0
5 to 1.2 wt%, Si: 0.5 wt% or less, P: 0.10 wt% or less, and Al: 0.01 to 0.08 wt%, and N weight% x 8
Above, Nb: C weight % x 3 or more, C weight % x 8 + 0.
After cold-rolling a hot-rolled steel sheet containing 0.02% or less and the remainder substantially Fe at a rolling reduction of 60% or more, and continuously annealing at 750% to 900°C for 10 seconds or more, At least 650℃ during the cooling process
A method for producing a cold-rolled steel sheet for drawing with excellent bake hardenability, characterized by controlling the cooling rate to an average cooling rate of 10° C. per second or more.