JP2002356740A - Hot rolled steel sheet for press working excellent in workability and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet which has excellent formability. SOLUTION: The hot rolled steel sheet for press working excellent in workability has a composition containing, by mass, <=0.15% C, <=1.5% Si, <=1.5% Mn, <=0.2% P, <=0.1% S, 0.001 to 0.10% Al and <=0.01% N, and the balance Fe with inevitable impurities, and has a roughness of the surface of ferrite in the steel sheet of 0.8 to 1.6 μm by surface roughness. The steel sheet further contains one or more kinds selected from 0.1 to 1.5% Cr, 0.1 to 1.5% Ni, 0.1 to 1.5% Mo, 0.1 to 1.5% Cu, 0.01 to 0.05% Nb and 0.01 to 0.1% Ti in addition to the above components.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet which is excellent in workability and is particularly used for an automobile chassis, underbody parts and the like.

[0002]

BACKGROUND OF THE INVENTION Hot rolled steel sheets are often supplied as coils or cut sheets, then blanked, pressed and finally used in many applications. Therefore, in order to use the steel sheet according to the application, it is necessary to form the steel sheet into a required shape, and it is desired that the steel sheet has good formability.

[0003] The main forms of forming thin steel sheets are stretch forming,
There is deep drawing. In order to improve these formability, it is desired to improve the n value and r value of the material, respectively. However, it is often the case that the addition of alloying elements to steel and the complicated production conditions often lead to an increase in cost, which is not always a preferred method for consumers. On the other hand, improving the lubricity between the steel sheet and the mold at the time of forming also contributes to the improvement of the formability, and therefore, a high lubricating press oil has been developed and used so far. However, this method also makes the lubricating oil expensive, which is not preferable in terms of cost.

[0004] Therefore, techniques for adjusting the surface roughness of a steel sheet to improve the lubrication characteristics of the steel sheet have been studied. For example, Japanese Patent Application Laid-Open No. 2-280903 discloses a technique for improving the lubrication characteristics of a steel sheet by forming a recess having a depth of 1 μm or more on the surface of the steel sheet. However, as disclosed in the gazette, this technique has a “surface roughness of 0.2 μm.
This is a technique for a steel sheet in which a portion having a flatness of “m or less” exists over 30% or more of the steel sheet surface, and for a cold-rolled steel sheet having a small surface roughness. In a hot-rolled steel sheet, the surface becomes rough due to the scale being caught between the roll and the steel sheet during hot rolling, so that the surface roughness is a value much larger than 0.2 μm. For this reason, the above technology for cold-rolled steel sheets cannot be applied to hot-rolled steel sheets.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above,
It is an object of the present invention to provide a low-cost hot-rolled steel sheet having high strength and excellent formability.

[0006]

The present invention has the following features to solve the above-mentioned problems. (1) In mass%, C ≦ 0.15%, Si ≦ 1.5%, Mn ≦ 1.5%, P ≦ 0.2%, S ≦ 0.1%, Al: 0.001-0. 10%, N ≦ 0.01%, the balance being Fe and unavoidable impurities, and the surface roughness of the steel plate ground iron is 0.8 μm or more and 1.6 μm in surface roughness.
A hot-rolled steel sheet characterized by the following. (2) In mass%, Cr: 0.1 to 1.5%, Ni: 0.1 to 1.5%, Mo: 0.1 to 1.5%, Cu: 0.1 to 1.5 %, Nb: 0.01 to 0.05%, and Ti: 0.01 to 0.1%. The hot-rolled steel sheet according to the above (1), comprising one or more of the following: (3) The slab comprising the components described in the above (1) or (2) is reheated to 1100 ° C. or higher, subjected to rough rolling / descaling, and then held at 880 ° C. or higher for 1 second or more until the start of finish rolling And then start finish rolling at 880 ° C or higher,
A method for producing a hot-rolled steel sheet, comprising finishing finish rolling in a temperature range of 800 to 880 ° C., and then performing cooling and winding.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, in order to improve the lubricating properties of a hot-rolled steel sheet without using expensive lubricating oil,
The present inventors diligently studied a method for improving the lubrication characteristics of a hot-rolled steel sheet by improving the surface properties of the hot-rolled steel sheet. for that reason,
The relationship between the surface properties of the hot-rolled steel sheet and the friction coefficient was investigated.

FIG. 1 shows the relationship between the steel plate surface roughness [surface roughness, Ra (μm)] and the friction coefficient when ordinary rust preventive oil is used as a lubricant. Here, the surface roughness is a value obtained from the following equation.

(Equation 1)

The surface roughness Ra is determined by applying a probe to the surface of a steel sheet and measuring the surface irregularities while moving the probe in one direction on the surface. It is calculated in such a manner. F in the above equation (2)
(X) is the height of the unevenness at a distance x from the point at which the probe began to move, and L is the distance at which the probe was moved. The average value Z of x) is obtained, and the surface roughness Ra is obtained from f (x) and Z according to the above equation (1). L was about 10 mm.

FIG. 1 shows that the surface roughness of the steel sheet surface is 0.8 μm.
It was found that good friction was obtained when the thickness was not less than m and not more than 1.6 μm. The reason is considered as follows. That is, in press forming, in order to obtain good lubricity between the steel sheet and the mold, it is necessary to always supply lubricating oil to the contact portion between the steel sheet and the mold to prevent metal contact. If the surface roughness is too small, the lubricating oil on the surface of the steel sheet is easily extruded from the portion in contact with the mold during press forming, so that the lubricity deteriorates. On the other hand, if the surface roughness is too large, the true contact area between the steel plate and the mold is reduced, thereby increasing the local surface pressure at the contact portion, and the lubricating property is reduced due to the absence of lubricating oil between the steel plate and the mold. It is considered to have deteriorated. From the above, it was found that by adjusting the surface roughness to 0.8 μm or more and 1.6 μm or less, the lubricity of the hot-rolled steel sheet can be improved.

The present invention has been made based on the results of such studies, and will be described in detail below. First, the composition of the steel sheet of the present invention will be described. C is an element necessary for the strengthening element, but an excessive addition degrades the workability of steel, so the upper limit is made 0.15%. Although the effect of the present invention can be obtained without specifically setting the lower limit, the cost is significantly increased if the amount of C is extremely reduced.
It is preferably at least 0.02%.

Si is also a necessary element as a strengthening element, but if it is excessive, the strength increases and the workability is impaired.
The upper limit is set to 1.5% in order to maintain good workability of steel.
The effect of the present invention can be obtained without any particular lower limit. However, extremely reducing the amount of Si causes an increase in cost, so that it is preferably 0.01% or more.

Mn is an element required as a strengthening element.
Excessive addition degrades the workability of the steel, so the upper limit is 1.
5%. The effect of the present invention can be obtained without any particular lower limit. However, if the amount of Mn is extremely reduced, the cost is increased. Therefore, the amount is preferably 0.01% or more.

P is an impurity, and if its content increases, it causes a decrease in ductility and also causes grain boundary embrittlement. Therefore, it is better to reduce P as much as possible, and to avoid extreme decrease in ductility and grain boundary embrittlement. Therefore, the upper limit is set to 0.2%.

Since S is an impurity and adversely affects ductility due to the formation of MnS, the upper limit is set to 0.1%.

Al is necessary as a deoxidizing element in order to reduce inclusions in steel. If the content is excessively small, deoxidation becomes insufficient, so the lower limit is made 0.001%. However, an excessive amount will increase the cost, so the upper limit is set to 0.1%.

N causes precipitation (nitride) formation,
The upper limit of N is set to 0.01% to cause deterioration of workability.
And

In addition to the above basic components, the following components can be contained as required. Cr, Ni, Mo, C
u is preferably added as at least one or more reinforcing elements. In order to obtain the effect, each of Cr, Ni, and Mo is set to 0.1.
1% or more is required. However, excessive addition degrades the workability of steel, so that the upper limit of each element content is set to 1.
5%.

Nb and Ti are at least 1 to secure strength.
More than one addition is preferred. To obtain the effect, Nb
Is required to be 0.01% or more, and Ti is required to be 0.01% or more.
However, excessive addition degrades the workability of the steel, so the upper limits are 0.05% for Nb and 0.1% for Ti, respectively.

Next, the manufacturing conditions will be described. In order to set the finish rolling temperature to a predetermined value, the reheating temperature of the slab must be 1100 ° C. or higher. The effect of the present invention can be obtained without any particular upper limit of the heating temperature.
The temperature is preferably set to 0 ° C. or lower.

Next, a method for controlling the surface roughness of the hot-rolled steel sheet within the above range was examined. The surface roughness of the hot-rolled steel sheet is relatively rough because the hot-rolled steel sheet is rolled with the scale attached in the finish rolling stage. However, in a normal hot-rolled steel sheet, the surface roughness (surface roughness) is in a range smaller than the above-described range in which lubricity is good (surface roughness ≦ 0.6 μm). In the present invention, the purpose is to improve the lubrication characteristics by increasing the roughness, so that a method for obtaining the above roughness was studied.

C: 0.04%, Si: 0.02%, M
n: 0.25%, P: 0.01%, S: 0.003%,
Using steel having a composition of Al: 0.015% and N: 0.0020%, hot rolling was performed under various hot rolling conditions, and the winding temperature was 6 ° C.
The results of a trial production test of a hot-rolled steel sheet at 00 ° C. will be described.

FIG. 2 shows that the finish rolling start temperature is 900 ° C.
The relationship between the time from the last descaling to the start of finish rolling (hereinafter abbreviated as “t _df ”) and the surface roughness (surface roughness) when the finish rolling end temperature is 820 ° C. is illustrated. It has been found that the time from the last descaling to the start of finish rolling needs to be 1 second or more. It is considered that this is because the scale generated during the period from the descaling to the start of the finish rolling is bitten at the time of the finish rolling to increase the roughness of the steel sheet surface. Although the upper limit of the time from de-scaling before finish rolling to the start of finish rolling is not particularly defined, it is preferably 5 seconds or less from the viewpoint of preventing the rolling start temperature from excessively lowering.

Next, the start and end temperatures of the finish rolling were examined. FIG. 3 shows the relationship between the finish rolling start temperature and the surface roughness when t _df = 1.1 seconds and finish rolling end temperature: 820 to 850 ° C. From this, it was found that the finish rolling start temperature needs to be 880 ° C. or higher. This is presumably because when the finish rolling start temperature is lower than 880 ° C., sufficient scale is not generated in the holding stage before the start of finish rolling. Although the upper limit of the finish rolling start temperature is not defined, if the finish rolling start temperature is too high, the finish rolling end temperature becomes too high and the surface roughness becomes too large. Here, the finish rolling start temperature is the temperature of the material immediately before the finish rolling, and is synonymous with the temperature at which the material is held before the start of the finish rolling, that is, the holding temperature until the start of the finish rolling.

FIG. 4 shows the relationship between the finish rolling temperature and the surface roughness when t _{df is} 1.1 seconds and the finish rolling start temperature is 880 to 1000 ° C. From this, the lower the finish rolling finish temperature, the lower the finish rolling temperature. The surface roughness increases and the surface roughness is reduced to 0.
In order to attain 8 μm or more, the finish rolling end temperature is 880.
It was also found that the temperature had to be lower than or equal to ℃, and in order to reduce the thickness to 1.6 μm or less, it was necessary to be higher than or equal to 800 ° C. The lower the finish rolling end temperature, the larger the surface roughness is,
It is considered that the lower the finish rolling temperature, the more the scale loses plasticity and is pushed into the steel sheet surface as foreign matter during rolling. After finishing rolling, cool down by the usual method,
What is necessary is just to wind up.

[0026]

Next, an example in which good moldability is obtained by the present invention will be described. A slab having the composition shown in Table 1
Table 2 shows the mechanical properties, surface roughness, coefficient of friction, and deep drawing height of the hot rolled steel sheet manufactured under the hot rolling conditions shown in Table 1. As shown in FIG. 5, the friction coefficient is such that the tool is pressed with a predetermined load from both sides of the steel sheet, and the steel sheet is further pulled upward (in the direction of the arrow), and the pulling load at that time / (2 × the pressing load) is defined as the friction coefficient. It is a measured value.

At this time, the sample size of the steel sheet was 30 mm.
× 300mm, contact area between steel plate and tool is 1200mm ²
And the pressing load was changed between 500 kgf and 2000 kgf. In addition, rust-preventive oil was used as a lubricant between the tool and the steel plate. The surface roughness is a value obtained by using the above-described equations (1) and (2). The deep drawing height was rustproof oil as lubricating oil for the steel plate and the mold, blank diameter: 230 mm, wrinkle holding load: 3.5 tonf, 100 mmφ cylindrical punch (shoulder R: 10 mm), clearance: 2.5 mm, This is a result of performing a cylindrical deep drawing test under the condition of a die shoulder R: 10 mm and measuring a forming height at the time when the blank is broken.

As can be seen from Tables 1 and 2, steels A, B,
C satisfies the requirements of the present invention, and has a low coefficient of friction and good formability. On the other hand, in comparative steel D,
Since the finish rolling start temperature is too low, the amount of scale formed before the start of finish rolling is small, and the surface roughness of the steel sheet is small. For this reason, the coefficient of friction of the steel sheet is large and the formability of the steel sheet is not good. In the comparative steel E, since the holding time from the final descaling to the start of the finish rolling is too short, the amount of scale generated before the start of the finish rolling is similarly small, and the surface roughness of the steel sheet is small. For this reason, the coefficient of friction of the steel sheet is large and the formability of the steel sheet is not good. In comparative steel F, the finish rolling start temperature is low and the finish rolling end temperature is too low, so that the surface roughness of the steel sheet is excessively large, the friction coefficient of the steel sheet is increased, and the formability is deteriorated. In comparative steel G, the finish rolling end temperature is too high, so that the surface roughness of the steel sheet is small, the coefficient of friction of the steel sheet is large, and the formability is deteriorated.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

According to the present invention, a hot-rolled steel sheet having excellent formability can be manufactured at a low cost, which can greatly contribute to a reduction in vehicle weight and a reduction in fuel consumption.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the surface roughness (surface roughness) of a steel sheet and the coefficient of friction.

FIG. 2 is a diagram showing the relationship between the time from the last descaling to the start of finish rolling in the inter-rolling process and the surface roughness.

FIG. 3 is a diagram showing a relationship between a finish rolling temperature and a steel sheet surface roughness (surface roughness).

FIG. 4 is a diagram showing a relationship between a finish rolling finish temperature and a steel sheet surface roughness (surface roughness).

FIG. 5 is a schematic view of a test for measuring a coefficient of friction.

Claims (3)

[Claims] 1. Mass%, C ≦ 0.15%, Si ≦ 1.5%, Mn ≦ 1.5%, P ≦ 0.2%, S ≦ 0.1%, Al: 0.001 to 0.001% 0.10%, N ≦ 0.01%, the balance being Fe and unavoidable impurities, and the surface roughness of the steel plate ground iron is 0.8 μm or more and 1.6 μm in surface roughness.
A hot-rolled steel sheet for press working excellent in workability, characterized in that: 2. In mass%, Cr: 0.1-1.5%, Ni: 0.1-1.5%, Mo: 0.1-1.5%, Cu: 0.1-1. 5%, Nb: 0.01 to 0.05%, and Ti: 0.01 to 0.1%. Hot rolled steel sheet for press working. 3. A slab made of the component according to claim 1 or 2 is reheated to 1100 ° C. or more, and after rough rolling and descaling, is held at 880 ° C. or more for 1 second or more until finish rolling is started. Next, finish rolling is started at 880 ° C. or higher, and finish rolling is completed in a temperature range of 800 to 880 ° C.
Next, a method for producing a hot-rolled steel sheet for press working, which is excellent in workability, characterized by cooling and winding.