JPH01177321A - Manufacture of cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PURPOSE:To obtain fine hot-rolled structure and to improve deep drawability by specifying respective ranges of hot rolling-finishing temp., cooling velocity, and cooling temp. at the time of rolling a dead soft C steel in which specific amounts of Ti and Nb are added. CONSTITUTION:A steel which has a composition consisting of, by weight, <=0.005% C, <=1.0% Si, <=2.0% Mn, <0.03% P, <=0.1% SolAl, <=0.05% S, <=0.008% N, 0.004-0.2% Ti and/or 0.004-0.05% Nb, and the balance Fe with inevitable impurities and containing, if necessary, 0.0005-0.003% B is cast. The resulting steel slab is hot-rolled, and finish rolling is finished at a temp. of the Ar3 point or above, and then the steel sheet is cooled through a temp. region from >=Ar3 point to <=(Ar3-30 deg.C) at 80-400 deg.C/sec cooling rate. Subsequently, the above steel sheet is wound up at 650-750 deg.C and then subjected to cold rolling and annealing. When high tensile strength is required, P is incorporated by 0.03-0.15%.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a cold-rolled steel sheet with excellent deep drawability.

[Conventional technology] Because ultra-low carbon steel has good workability, it is expected that it will have high elongation and stretchability when made into cold-rolled steel sheets for press forming. It easily becomes a rolled structure, and cold rolling,
Deep drawability after annealing tends to decrease accordingly. Therefore, technology that improves deep drawability is important.

A method for manufacturing a steel plate with good deep drawability from steel with carbon content of o, oos% or less is already known. For example, JP-A-61-
No. 276930 discloses extremely low C-Ti-
The hot rolling finishing temperature, cooling start time, cooling rate, coiling temperature, cold rolling reduction ratio, heating temperature range and heating rate, holding temperature and holding time of Nb-based steel are controlled within specific ranges,
This is a method for producing cold-rolled sheets with good elongation and deep drawability.

However, this method starts cooling immediately after hot rolling to suppress the growth of γ grains and cause α transformation, thereby obtaining a fine hot rolled structure. However, in suppressing the growth of γ grains, there is a lower limit to the α grain size that can be obtained in a hot rolled sheet, and a very fine hot rolled structure cannot be obtained, making it impossible to produce a cold rolled steel sheet with very high deep drawability. In this invention, the rolled material is cooled at an average cooling rate of 10° C./S or more, but there is no specific description of the limitation on the cooling rate, so 10° C./S or more refers to, for example, 30° C./S in the examples. .

Furthermore, JP-A-61-110722 discloses a method for manufacturing hot-rolled steel sheets with excellent workability by controlling the hot finish rolling temperature, cooling conditions, and winding conditions of ultra-low C-low Mn-low N steel. It is. However, this method relates to hot-rolled steel sheets and does not relate to the press formability of cold-rolled steel sheets. In this invention, after hot rolling, the rolled material is cooled at a cooling rate of at least (Ar3+10°C) and at least 30°C/S. 10
~b This is based on the fact that the cooling rate is significantly saturated in a high cooling rate region. Therefore, 30°C/S or higher refers to, for example, 45°C/S in Examples, and the hot-rolled structure does not become very fine.

[Problems to be Solved by the Invention] The present invention aims to disclose a method for manufacturing a cold-rolled steel sheet that uses ultra-low carbon steel, obtains a fine hot-rolled structure, and has excellent deep drawability.

[Means for Solving the Problems] The present invention provides (1) weight %, C: 0.005 or less, Si: 1.0
Below, Mn: 2.0 or less, P: less than 0.03, So
L A Q: 0.1 or less, s: o, os or less,
N: 0.008 or less, Ti: 0.004 to 0
．． 2. Contains at least one kind of Nb: 0.004 to 0.05, and further contains B: 0.000 as necessary.
5 to 0.003, with the remainder consisting of Fe and unavoidable impurities, finish rolling is completed at a temperature of Ar 3 or higher during hot rolling, and then finish rolling at a temperature of Ar 3 or higher (Ar
Deep drawing, characterized by cooling at a cooling rate of 80-400°C in a temperature range of 3-30°C or lower, coiling at a temperature of 650-750°C, and then cold-rolling for 8 hours and annealing according to a conventional method. It is a method for producing cold-rolled steel sheets with excellent properties, and (2) in weight%, C: 0.005 or less, Si: 1.0 or less, M
n: 2.0 or less, P: 0.03-15, s: o,
os or less, Sol AQ: 0.1 or less, N: 0
．． 008 or less, Ti: 0.004 to 0.2, Nb
: Contains at least one or more of the following: 0.004 to 0.05, and optionally B: 0.0005 to 0.003
When hot rolling a steel containing Fe and unavoidable impurities, finish rolling is completed at a temperature of Ar3 or higher, and then finish rolling at a temperature of Ar3 or higher (Ar3-30°C).
This is a method for producing a cold-rolled steel sheet with excellent deep drawability, which is characterized in that it is rolled in the following temperature range from 80 to 650 to 750°C, and then cold rolled and annealed according to a conventional method.

[In other words, the present invention applies to ultra-low carbon steel that exhibits good deep drawability,
By adding one or more types of carbonitride-forming elements, Ti or Nb, most of the C and N are precipitated and fixed, further improving deep drawability. To produce a cold-rolled steel sheet with excellent deep drawability by finishing the process at a specific temperature or higher, then ultra-rapidly cooling it in a specific temperature range, and then coiling it at a predetermined temperature to form an extremely fine hot-rolled structure. It is characterized by:

The present invention will be specifically explained below.

C is o, oos weight% or less. In order to improve deep drawability, it is better to have less C. Moreover, if C exceeds 0.005% by weight, the amount of Ti or Nb added increases to fix this, resulting in an increase in cost.

SL is 1.0% by weight or less. Si is an effective element for increasing strength and can be added depending on the required tensile strength, but if it exceeds 1.0%, hot-dip galvanizing properties and chemical conversion treatment properties are impaired.

Like Si, Mn can be added depending on the required tensile strength, but steel with extremely low carbon content and Mn of 2.0% or more requires high refining costs.

P is contained in steel as an impurity element in an amount of less than 0.03%. P is an element effective in increasing strength, and is actively added when high tensile strength is desired. However, if it exceeds 0.15%, secondary processing embrittlement is likely to occur.

Sol AΩ is contained in order to deoxidize molten steel and improve the yield of Ti and Nb. However, if added in excess, the press formability of the steel plate will be impaired, so the upper limit is set at 0.1%.

Since the smaller the amount of S as an impurity, the higher the press formability can be obtained, it is preferably set to 0.03% or less.

N is 0.008% by weight or less. If N is too high, Ti
The addition amount of Ti and Nb increases, which increases the cost.
When the nitrides of Nb and Nb increase, press formability is impaired.

In the present invention, most of the C and N in the steel are precipitated and fixed, and Ti and Nb are added in order to obtain good deep drawability.

If Ti is 0.004% or less, C and N are not sufficiently precipitated and fixed, resulting in a decrease in press formability. Ti content is 0
．． 2% is sufficient, and excessive addition is not preferable from the economic point of view.

Further, Nb is also contained in an amount of 0.004 to 0.05% by weight for the same reason.

In the present invention, B is added when suppressing secondary work brittleness. When added in an amount of 0.0005% by weight or more, secondary work brittleness is significantly improved. However, the effect does not change even if 0.003% by weight or more is added.

The finish rolling temperature of the hot rolling of the present invention is Ar3 point or higher. If the Ar point is 3 or less, coarse grains are generated in the hot rolled sheet or a processed structure remains, reducing the deep drawability after cold rolling and annealing.

Next, the cooling rate of the present invention will be explained. In the present invention, the temperature range from Ar3 points to (Ar3-30℃) is 80℃/S to
Cool at a cooling rate of 400°C/S. Ar after finish rolling
It has been found that cooling at a very high cooling rate in the temperature range of 3 points to (Ar3-30°C) has a remarkable effect of making the crystal grains of the hot-rolled sheet finer. FIG. 1 is a diagram showing the relationship between the cooling rate and the grain size of a hot-rolled sheet, and the relationship between deep drawability (Lankford value) after cold rolling and annealing. C in weight%: 0.002.
Si:0.01. Mn: 0.09. P:0.0
8, S: 0. O05, sol AQ: 0.025
．． N: 0.0025. Finish rolling of steel containing Ti: 0.04 was completed at 920°C, and the steel was rolled from 910°C to 85°C.
It was cooled down to 0°C at various cooling rates and wound up at 680°C.

As seen in Figure 1, the grain size of the hot-rolled sheet becomes finer,
The effect of improving deep drawability after cold rolling and annealing becomes noticeable at a very high cooling rate.

Next, the cooling start temperature may be equal to or higher than the Ar3 transformation point.

It has also been found that if the cooling end temperature is below (Ar3 transformation point -30°C), an extremely fine hot-rolled sheet structure can be obtained and the deep drawability will also be extremely good. (Ar3-50℃) or lower, a finer hot-rolled plate structure can be obtained and the deep drawability is also better. Ar3～(Ar3
This is thought to be because when the temperature range (-30°C) is cooled at a very high cooling rate, the number of alpha grain nuclei generated increases due to supercooling of the transformation point. Therefore, this effect cannot be achieved with the conventional cooling rate of 30° C./s or 45° C./s, and becomes noticeable at a cooling rate of 80° C./s or higher.

In the present invention, the upper limit of the cooling rate is 400°C/S. Although the cooling rate may be increased, this range is easily achieved.

In the present invention, cooling can be started at an appropriate temperature of Ar3 or higher. That is, the timing to start cooling does not have to be immediately after finish rolling, and cooling can be started at an appropriate position on the runout table. Therefore, in the present invention, the thickness and temperature of the rolled material can be measured without being affected by steam due to cooling even in a normal rolling mill in which a thickness gauge and a thermometer are arranged after the finishing rolling mill. Hot rolling control is easy.

Further, it is desirable that the cooling device be placed as close to the finishing rolling mill as possible without interfering with the operation of the thermometer and plate thickness gauge that are normally placed after the finishing rolling mill. This is Ar3
This is because cooling starts from the point above. Even when finish rolling is terminated near the Ar point 3, cooling can be performed from the Ar point 3 or higher.

Next, in the present invention, the winding temperature is 650 to 750°C.

At temperatures below 650°C, deep drawability becomes extremely low. Also 750℃
If this is the case, the pickling properties will be impaired.

The hot-rolled slab heating temperature is not particularly limited, but 1
A good material can be obtained if the temperature is set to 000°C or more and 1300°C or less. If the temperature is 1000° C. or more and 1100° C. or less, even better material quality can be obtained, which is preferable. Also, good material quality can be obtained even when continuous casting is followed by direct rolling.

The hot rolled steel sheet produced by this method is cold rolled and annealed by conventional methods. The cold rolling and annealing conditions are not particularly limited, but the cold rolling rate is 40-95%, preferably 70-95%.
When it is 0%, a cold rolled steel sheet with very high deep drawability can be obtained. Also, for annealing, it is not preferable to use an annealing temperature that is too high or a temperature that is too low below the recrystallization temperature, but any annealing method such as continuous annealing or box annealing may be used, and depending on the usual annealing conditions for each, it has excellent deep drawability. A cold-rolled steel sheet is obtained.

Cooling after finish rolling may be performed by any method such as cooling with water or cooling with gas.

Various platings such as zinc plating, tin plating, and chrome plating may be performed during continuous annealing or in subsequent steps depending on the use.

Further, skin pass rolling, rust prevention treatment, application of lubricant, etc. may be performed as necessary.

[Example 1] A slab having a thickness of 245111111 mm was made by continuous casting of steel melted according to a normal process. The chemical composition of the steel is shown in Table 1.

Thereafter, after soaking at 1150° C. for 1.5 hours, rough rolling and finish rolling were performed, and the product was wound at a predetermined temperature to form a hot coil. Thereafter, after pickling, 80% cold rolling was performed, continuous annealing was performed at 760° C. for 40 seconds, and 0.6% temper rolling was performed to produce a cold rolled steel sheet.

Table 2 Table 2 shows the r value of the cold rolled steel sheet and the grain size of the hot coil, that is, the hot rolled sheet.

As shown in Table 2, by using steel with a chemical composition within the range of the present invention, and performing hot rolling at the end temperature, cooling start temperature, cooling end temperature, and cooling rate within the range of the present invention, deep drawing is possible. It can be seen that a cooling steel sheet with excellent properties can be manufactured.

The Lankford value (r) was used as an index of deep drawability. The r value is the rolling direction, a direction tilted by ±45° from the rolling direction,
It is the average of the values in the direction perpendicular to the rolling direction.

[Effects of the Invention] By doing so, a cold rolled steel sheet with excellent deep drawability can be manufactured.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between cooling rate, hot-rolled sheet grain size number, and r value after cold rolling and annealing. Patent applicant Nippon Steel Corporation

Claims (2)

[Claims] (1) C: 0.005 or less, Si: 1.0 or less, Mn: 2.0 or less, P: less than 0.03 SolAl: 0.1 or less, S: 0.05 or less N: 0. 0
08 or less, Ti: 0.004 to 0.2, Nb: 0.004 to 0.0
5, further containing B: 0.0005 to 0.003 as necessary, and the remainder consisting of Fe and unavoidable impurities, during hot rolling at a temperature of Ar 3 or higher. Finish rolling is finished,
After that, the temperature range from Ar3 point to (Ar3-30℃) was cooled at a cooling rate of 80 to 400℃/s to 650 to 7
Coiled at a temperature of 50°C, then cold rolled according to conventional methods,
A method for producing cold-rolled steel sheets with excellent deep drawability, characterized by annealing. (2) C: 0.005 or less, Si: 1.0 or less, Mn: 2.0 or less, P: 0.03 to 0.15 solAl
: 0.1 or less, S: 0.05 or less, N: 0.008 or less, Ti: 0.004 to 0.2, Nb: 0.004 to 0.0
5 and further contains B: 0.0005 to 0.003 as necessary, with the remainder consisting of Fe and unavoidable impurities, at a temperature of Ar 3 or higher during hot rolling. Finish rolling is completed, and then cooling is performed in the temperature range of Ar3 point or higher to (Ar3-30°C) or lower at a cooling rate of 80 to 400°C/s to 650°C to
A method for producing a cold-rolled steel sheet with excellent deep drawability, which comprises coiling at a temperature of 750°C, followed by cold rolling and annealing according to a conventional method.