JPH03100125A - Production of hot rolled steel plate excellent in deep drawability - Google Patents

Abstract

PURPOSE:To precipitate nitride, to increase r-value, and to improve deep drawability by successively subjecting an aluminum-killed steel in which respective contents of C, Mn, Si, Sol.Al, N, and Fe are specified to thermomechanical treatment by the prescribed stages. CONSTITUTION:An aluminum-killed steel having a composition consisting of, by weight, <=0.05% C, 0.01-0.4% Mn, <=0.3% Si, 0.01-0.08% Sol.Al. <=0.61% N, and the balance Fe is refined. This steel is subjected to rolling of final pass at 900-1100 deg.C at >=30% rolling reduction, by which nitride is precipitated. Subsequently, after the above primary rolling, the above steel is held in the above state at 900-1050 deg.C for 1-60min, by which the precipitation of the nitride is accelerated. Then, after the above precipitation treatment, the above steel is cooled down to the the secondary rolling initiating temp. at >=5 deg.C/sec cooling rate and subjected to secondary rolling under the condition of 450-800 deg.C and 150% total rolling reduction, followed by recrystallization treatment.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing a hot rolled steel sheet with excellent deep drawability, specifically a hot rolled steel sheet with a high Rankford value (r value). .

(Prior Art) Hot-rolled steel sheets are widely used as relatively inexpensive structural materials in automobiles, home appliances, building materials, various industrial equipment, and the like. In addition, hot-rolled steel sheets are rarely used as they are, and are often used after being formed into a predetermined shape using a press. Therefore, hot-rolled steel sheets are required to have excellent workability. However, hot-rolled steel sheets have inferior workability compared to cold-rolled steel sheets. This is because hot-rolled steel sheets have inferior deep drawability compared to cold-rolled steel sheets.For example, the r value, which is used as an indicator of deep drawability, is 1.5 for cold-rolled steel sheets.
~1.8, whereas for hot rolled steel sheet it is 0.5~0.9
and low.

As described above, hot-rolled steel sheets are inferior in workability compared to cold-rolled steel sheets, so for applications that require workability such as automobiles and other home appliances, they are preferred over cheaper hot-rolled steel sheets. Cold-rolled steel sheets, which are also more expensive, are more commonly used.

Therefore, it would be very desirable industrially if it were possible to manufacture a hot rolled steel sheet having deep drawability equal to or better than that of a cold rolled steel sheet. In recent years, various methods of manufacturing hot rolled steel sheets with excellent deep drawability have been studied, and patents related to these methods have also been filed.

For example, in Japanese Patent Application Laid-open No. 59-153836, C,
Mn.

5oi1. A hot rolled steel sheet with excellent deep drawability is produced by hot rolling aluminum killed steel containing ^2 under specific conditions after precipitation treatment of 8 IN, followed by pickling, light reduction rolling and recrystallization treatment. A manufacturing method is disclosed. However, in this method, since there are no regulations regarding specific conditions before the precipitation treatment, heating for the precipitation treatment requires a long time, such as 100 minutes or more, resulting in extremely low production efficiency.

(Problem B to be Solved by the Invention) The problem of the present invention is to develop a method that can efficiently precipitate nitrides and efficiently produce hot-rolled steel sheets with a high r value and excellent deep drawability. It is about providing.

(Means for Solving the Problems) The present inventors have developed a hot rolled steel sheet with excellent deep drawability by using aluminum killed steel with a specific chemical composition as a material, and subjecting it to primary rolling including large reduction rolling, and precipitation treatment. discovered that it could be produced by secondary rolling and recrystallization, and filed a patent application (Japanese Patent Application No. 177776/1983).

In the method of this prior invention, ferrite grains are refined in the primary rolling before precipitation treatment, and working strain is accumulated in the steel to introduce nitride precipitation sites. The rapid precipitation of nitrides significantly improves the deep drawability of the final product after secondary rolling and recrystallization.

However, the present inventors were not satisfied with this result, and as a result of further examination of the manufacturing method of the prior invention, they found that if the precipitation treatment is started before the processing strain obtained in the primary rolling is released, nitrides will be produced. It was discovered that precipitation is further promoted and deep drawability is further improved, leading to the present invention.

Here, the gist of the first invention of the present application is ``in weight percent, CiO
, 05% or less, Mn: 0.01-0.4%, S!
! 0.3% or less, Sol, Al: 0.01
~0.08%, N: 0.01% or less, and the balance consists of Fe and unavoidable impurities. A hot-rolled steel sheet with excellent deep drawability, which is characterized by sequentially processing and heat-treating it in the following steps. "Manufacturing method".

■ A step of performing primary rolling in which the final pass is rolled at a temperature range of 1100°C or lower and 900°C or higher, and with a rolling reduction of 30% or higher; [2] Immediately after the primary rolling, rolling is carried out at 1050°C or lower 90°C.
A step of holding at a temperature range of 0℃ or higher for 1 to 60 minutes to perform precipitation treatment, or after primary rolling, up to the next precipitation treatment start temperature.
Cooled at a cooling rate of ℃/sec or more, 900℃ below 1050℃
A step of performing precipitation treatment by holding the temperature in a temperature range of 1 to 60 minutes above ℃, (2) After the precipitation treatment, cooling to the next secondary rolling start temperature at a cooling rate of 5 ℃/second or more, and 450 ℃ below 800 ℃. A step of performing secondary rolling at a total reduction rate of 50% or more in the above temperature range; (2) A step of performing recrystallization treatment after secondary rolling.

The second invention of the present application further provides Tis in addition to the above alloy components.
Using aluminum killed μ containing a total of 0.001 to 0.150% of one or more components selected from Nbs Zrs V, the third invention further contains B in addition to the above alloy components. The fourth invention uses aluminum killed steel containing 0.0001 to 0.0050%, and in addition to the above alloy components, one or more of Ti, Nb% Zr, and V are added together. 0.001~0.150% and B
A method for producing a hot-rolled steel sheet with excellent deep drawability, characterized in that aluminum killed steel containing 0.0001-0.0050% of are the gist of each.

(Function) Hereinafter, each component of the present invention and its function and effect will be explained.

First, the reason for limiting the composition of aluminum killed steel, which is the material steel used in the present invention, as described above will be described.

Note that "%" regarding steel composition is "% by weight".

C: Since C is an element that has a negative effect on deep drawability, it is desirable that its content be small.If C is contained in excess of 0.05%, deep drawability will be significantly degraded.

Moon: Mn is an effective element that fixes S mixed as an impurity in steel as MnS and improves hot workability.

However, if the content exceeds 0.4%, deep drawability will be significantly deteriorated. On the other hand, due to recent advances in S removal technology, extremely low S
It is now possible to manufacture steel, even with a Mn content of 0.01%.
can be sufficiently fixed as MnS, and hot working cracks can be prevented. For this reason, the Mn content is set to 0.01 to 0.4%.

Sl: In order to improve deep drawability, it is better to have as little St as possible. If the content of Si increases, not only the deep drawability will deteriorate, but also the scale properties will deteriorate, resulting in a loss of product quality, so the content should be 0.3% or less.

Soj! , Ag: Sol, ^l are important elements in the present invention. This element is used for deoxidation treatment, and at the same time has the effect of fixing N in the steel as AIN and improving deep drawability. However, if the content is less than 0.01%, the above effects cannot be obtained sufficiently, and if the content exceeds 0.08%, the effect will be saturated and it will be economically disadvantageous. The content is 0.08%.

N: N forms a nitride with 8i and Tt, Nb, Zr, and V, which will be described later, and contributes to the refinement of ferrite. However, on the other hand, it is also an element that adversely affects deep drawability.

If the N content increases, solid solution N remains and the deterioration of deep drawability becomes more significant than the effect of making the ferrite finer. Therefore, the content is set to 0.01% or less.

One of the raw material steels used in the present invention, aluminum-killed steel, has the above-mentioned components, with the remainder consisting of Fe and unavoidable impurities. In addition to these components, one or more of 0.001 to 0.150% of Ti in total may be added.
, Nb, Zr, ■, or/and 0.0001-0
．． Aluminum killed steel containing 0.050% B can also be used as the raw steel.

The effects of the above additionally added elements are as follows.

Tis Nb, Zr, V: These elements are expensive, but they have the effect of fixing N more firmly as a nitride than A1, and improving deep drawability by refining the ferrite grain size before secondary rolling. It is a very preferable element. However, if the content is 1 or 2
If the total content is less than 0.001%, the above-mentioned effects cannot be obtained, and even if the content exceeds 0.150%, the effect is saturated, which is economically disadvantageous.

B: B is an effective element that has the effect of preventing "vertical cracking" which is a problem in drawn parts. However, 0.0001%
If the content is less than 0.00, the above effect cannot be obtained sufficiently.
Even if the content exceeds 50%, the effect will be saturated and it will be economically disadvantageous.

Using aluminum killed steel with such a chemical composition as the material steel,
This is processed and heat treated in sequence according to the steps [1] to [4] above to obtain a hot rolled steel plate. The attached FIG. 1 is a schematic diagram showing an example of the manufacturing process.

Hereinafter, the processing heat treatment process will be explained with reference to this figure.

[Primary rolling]

The raw material steel (slab) of aluminum-killed steel having the above chemical composition to be subjected to primary rolling may be either directly delivered from continuous casting and still at high temperature, or it may be cooled after casting and then reheated.

The purpose of the primary rolling is to introduce precipitation sites for rapidly precipitating nitrides in the next precipitation treatment, and to obtain fine ferrite grains.

To achieve this, the primary rolling must be rolled at 1100 in the final bus.
It is necessary to carry out the process at a temperature range of 900°C or lower and at a large reduction rate of 30% or more. Preferably, the rolling reduction is carried out at a large reduction rate of 45% or more.

If the reduction ratio of the final bath is less than 30%, the resulting machining strain is small, so nitride precipitation sites are not introduced, and in the next precipitation treatment, the temperature range of 1050°C or less and 900°C or less is held for 1 to 60 minutes. It is also difficult to efficiently precipitate nitrides. Further, if the final bath temperature is higher than 1100°C, the effect of refining the ferrite cannot be obtained, and if it is lower than 900°C, it becomes difficult to secure the precipitation treatment temperature in the next step.

[Processing after primary rolling]

What is important in the treatment after the primary rolling is to immediately send the rough rolled material to the next precipitation treatment step before the processing strain is released. If the working strain is released after the primary rolling and before the precipitation treatment, the precipitation of nitrides will be reduced in the precipitation treatment and the improvement in deep drawability will be small. , it is better to suppress the release of processing strain.

In other words, if the finishing temperature of the primary rolling is the predetermined precipitation treatment temperature, it is immediately sent to the precipitation treatment process, and if the finishing temperature after the primary rolling is higher than the predetermined precipitation treatment temperature, it is sent as shown in Figure 1. 2. After primary rolling, the cooling rate is 5°C/sec or more. After cooling and lowering the temperature of the rough rolled material to the precipitation treatment temperature, it is sent to the precipitation treatment step. In this way, the processing strain accumulated during primary rolling can be carried through to the precipitation treatment process without being released during cooling.
In the precipitation treatment step, precipitation of nitrides due to processing strain is sufficiently promoted, the precipitates become coarse, and deep drawability is improved. Further, when the finishing temperature is higher than the predetermined precipitation treatment temperature, the production time can be shortened by rapidly cooling the rough rolled material at a cooling rate of 5° C./sec or more.

[Precipitation treatment]

The purpose of this treatment is to improve deep drawability by precipitating N in the steel as nitrides such as Aj2NSTiN.

For this purpose, it is necessary to hold the rough rolled material after the primary rolling in a temperature range of 1050° C. or lower and 900° C. or higher for 1 to 60 minutes immediately after rolling without cooling it to room temperature. Preferably, the treatment is carried out in a temperature range of 1050°C or lower and 950°C or higher. A precipitation nose in the austenite region exists in this temperature range.

Therefore, if held at a temperature higher than 1050°C, not only will precipitation of nitrides not proceed quickly due to high solubility, but also austenite grains will grow and become coarser, and ferrite grains before secondary rolling will become coarser. Therefore, the deep drawability of the final product cannot be improved. On the other hand, if the temperature is maintained at a temperature in the austenite range lower than 900° C., the precipitation rate is extremely slow, so that the precipitation of nitrides also does not proceed rapidly and no improvement in deep drawability can be obtained. Further, if the holding time is less than 1 minute, the amount of nitride precipitation is small, while if it is longer than 60 minutes, the nitride precipitation is saturated, leading to an increase in manufacturing costs.

It is desirable to maintain the rough rolled material after the primary rolling in the precipitation treatment temperature range within the rolling line.

For example, this can be done by using a recently developed coil box and winding the roughly rolled material into a coil within the line.

[Cooling treatment after precipitation treatment]

After the precipitation treatment, it is necessary to cool the rough rolled material to lower the temperature to the temperature at which secondary rolling starts. This cooling is also preferably performed at a cooling rate of 5° C./second or more.

Cooling at a cooling rate of 5°C/second or higher will shorten manufacturing time, and the transformation from austenite to ferrite will progress rapidly, preventing coarsening of ferrite grains, improving deep drawability. will improve.

[Secondary rolling]

The purpose of the secondary rolling is to process the roughly rolled material to the final thickness and to provide processing strain necessary for recrystallization of ferrite. To achieve this, the secondary rolling must be carried out at temperatures below 800°C and 450°C without cooling the rough rolled material after precipitation treatment to room temperature.
It is necessary to perform rolling in the above temperature range under conditions where the total rolling reduction is 50% or more.

If the temperature exceeds 800°C or the total rolling reduction is less than 50%, the processing strain required for recrystallization of ferrite will not accumulate sufficiently, and good deep drawability will not be obtained after the recrystallization process. At lower temperatures, the deformation resistance increases significantly, making secondary rolling practically difficult.

Note that if this secondary rolling is performed using rolling lubricating oil, the processing deformation in the plate thickness direction is made uniform, so that the r value including the plate surface layer is improved. Therefore, there is an effect of improving the r value of the entire board.

[Recrystallization treatment]

The hot-rolled steel sheet that has been processed to a predetermined thickness through the above process is wound into a coil and annealed using its own heat (self-annealing) to proceed with recrystallization. This recrystallization improves the deep drawability of the steel sheet. Although it is extremely important, if the hot-rolled steel sheet has gone through the steps described above, even the above-mentioned self-annealing can sufficiently obtain a recrystallized texture favorable for improving deep drawability. A separate annealing process may be provided to promote thermal and recrystallization. In this case, 55
It is preferable to carry out at a temperature range of 0 to 900°C.

If the final product is to be galvanized after hot rolling, it is also advantageous to recrystallize it in the continuous annealing process of the hot-dip galvanizing line.

Next, the present invention will be further explained with reference to Examples.

(Example) 50 kg of aluminum killed steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace and cast into a 60 mm thick slab. However, some of the steel types C were 8 mm thick.

These slabs were subjected to primary rolling, cooling, precipitation treatment, cooling, and secondary rolling under the conditions shown in Table 2, then wound into a coil, and then subjected to any of the following conditions [1] to [4]. Recrystallization treatment was carried out.

“Recrystallization treatment conditions” ■After winding into a coil, recrystallization treatment using its own retained heat during slow cooling (treatment A).

■ After winding, cool down to room temperature and then
Recrystallization treatment (processing B) by imparting a thermal history equivalent to 2 min of continuous annealing.

■Same (850℃ x 10 seconds after cooling to room temperature)
Recrystallization treatment (treatment C) by imparting a thermal history equivalent to continuous annealing on a hot-dip galvanizing line.

■After cooling to room temperature, a thermal history equivalent to batch annealing at 700°C for 5 hours is applied to recrystallize (processing D
).

The finished plate thickness after the secondary rolling is 3II1 m, and the secondary rolling is carried out using lubricated oil rolling (friction coefficient μ: 0.10 to 0.15).
).

A test piece was taken from the steel plate thus obtained, and the yield strength (YP), elongation (Effi), Lankford value (r value), and warp cracking transition temperature were examined.

The results are shown in Table 3.

Note that the vertical crack transition temperature here refers to a drawing ratio of 2.0.
It means the brittle crack stop temperature of a cup squeezed at . Further, treatments A to D in the recrystallization treatment in the table correspond to the above-mentioned [1] to [4].

(hereinafter referred to as "Kinpaku") Table 3 As is clear from Table 3, 14IIL14~
Hot-rolled steel sheets manufactured under conditions in which any of the primary rolling, precipitation treatment, and secondary rolling are outside the range specified by the present invention, as in No. 21, all have a low r value, and also have a low r value and a low warp crack transition temperature. is higher than that of N112 or No. 5 of the present invention, which is made of the same steel type and underwent the same recrystallization treatment.

Comparative Examples Magnet 22 and I41123 were manufactured by the method of the previous invention (Japanese Patent Application No. 153836/1982). This product has a significantly improved r value compared to other comparative examples. However, the r value is lower than that of the examples of the present invention. In contrast, all of the examples No. 1 to No. 13 of the invention have high ductility and a much higher r value. Among them, aluminum killed aluminum containing B Items 11 to 13, which are made of steel, have a significantly low warp cracking transition temperature.

(Effects of the Invention) As explained above, according to the method of the present invention, it is possible to produce a hot rolled steel sheet with an even higher r value and excellent deep drawability. Therefore, the present invention greatly contributes to the expansion of the use of hot-rolled steel sheets in fields such as automobiles, home appliances, and building materials.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of the manufacturing process of the present invention. Easy! figure

Claims (4)

[Claims] (1) In weight%, C: 0.05% or less, Mn: 0.01 to 0.4%, Si
: 0.3% or less, Sol. Al: 0.01-0.08%
, N: 0.01% or less, with the balance consisting of Fe and unavoidable impurities. A method for producing a hot plate steel sheet with excellent deep drawability, which comprises sequentially heat-processing aluminum in the following steps. [1] A step of performing primary rolling in which the final pass of rolling is carried out at a temperature range of 1100°C or lower and 900°C or higher, and with a rolling reduction of 30% or more; [2] Immediately after the primary rolling, the temperature is 1050°C or lower 90°C.
A step of holding at a temperature range of 0℃ or higher for 1 to 60 minutes to perform precipitation treatment, or after primary rolling, up to the next precipitation treatment start temperature.
Cooled at a cooling rate of ℃/sec or more, 900℃ below 1050℃
[3] After the precipitation treatment, cool to the next secondary rolling start temperature at a cooling rate of 5°C/second or more, and then maintain the temperature at a temperature of 800°C or higher for 450°C or higher. A step of performing secondary rolling with a total rolling reduction of 50% or more in a temperature range of ℃ or higher; [4] A step of performing recrystallization treatment after the secondary rolling. (2) In addition to the components of claim (1), Ti, Nb,
One or two or more selected from Zr, V in total 0
．． Aluminum killed steel containing 001 to 0.150% is added to the
The method for producing a hot-rolled steel sheet with excellent deep drawability according to claim 1, wherein the heat-processing process is carried out sequentially according to steps 1] to 4). (3) In addition to the components of claim (1), 0.00 of B is added.
The aluminum killed steel containing 0.01 to 0.0050% is
The method for manufacturing a hot plate steel sheet with excellent deep drawability according to claim (1), wherein the heat treatment is carried out sequentially according to the steps of [4]. (4) In addition to the components of claim (1), Ti, Nb,
One or two or more selected from Zr, V in total 0
．． 001~0.150% and B 0.0001~0.00
The method for manufacturing a hot plate steel sheet with excellent deep drawability according to claim (1), characterized in that the aluminum-killed steel containing 50% aluminum is subjected to processing heat treatment in sequence according to the steps [1] to [4].