JP2001279376A - Hot rolled steel sheet excellent in galling resistance and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet free from the generation of galling at the time of press working. SOLUTION: This steel sheet has a steel composition containing, by mass, <=0.08% C, <=0.05% Si, <=0.60% Mn, <=0.060% Al, 0.003 to 0.012% and also 0.003 to 0.012% N in a state of solid solution, and the balance iron with inevitable impurities and a steel structure of a ferritic single phase, and in which surface roughness Ra after the application of strain by 3% is >=2.0 μm.

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 used for a formed product by press working, and in particular, to prevent mold seizure (mold seizure) occurring between a press die and a hot-rolled steel sheet during press forming. It is assumed that.

[0002]

2. Description of the Related Art Among the surface damages that occur in a sliding portion between a die and a workpiece during press forming, the phenomenon of sticking and seizure of the workpiece to the die is particularly described as "mold galling". "is called. This mold seizure is a phenomenon that occurs in a situation where unsteady plastic deformation during molding and plastic deformation of the surface layer of the workpiece due to sliding are combined, and the mechanism thereof is not sufficiently elucidated. At present, factors affecting mold seizure include (1) surface roughness of the work material, plastic deformability, (2) mold surface roughness, material, surface treatment method, and (3) Lubrication with the mold, surface pressure, etc. are considered.

[0003] Cold rolled steel sheets and hot rolled steel sheets are typical examples of the material to be subjected to press forming. Of these, press forming using cold rolled steel sheets has good formability of the steel sheets themselves. , R value is increased, the forming load in the press working is reduced, and the surface pressure is reduced, so that mold seizure can be eliminated.

[0004]

On the other hand, the plate thickness is 1.4 mm.
For those having a relatively thick plate thickness, a hot-rolled steel sheet that is advantageous in terms of production cost is often used, but a hot-rolled steel sheet is used in a cold-rolled steel sheet because the r-value is low in the first place. The method cannot be applied. Therefore, there is a problem that the rate of occurrence of mold galling is high in press forming of a hot-rolled steel sheet. Accordingly, an object of the present invention is to provide a hot-rolled steel sheet which does not depend on the r-value in press working using a hot-rolled steel sheet as a workpiece and which does not generate galling during press working.

[0005]

In order to suppress mold seizure, it is effective to maintain a good lubrication state on the sliding surface by the effect of sealing oil on the surface of the workpiece during pressing. The inventors have found that such a state can be achieved by increasing the surface roughness of the work material at the time of pressing by positively utilizing the stretcher strain due to solid solution N in steel. The present invention has been completed based on the above findings, and the gist configuration thereof is as follows.

(1) In mass%, C: 0.08% or less, Si: 0.05
%, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003
0.010.012% and N in the solid solution state: 0.003 to 0.012%, the balance being a steel composition of iron and unavoidable impurities, having a ferrite single-phase steel structure, and having a strain of 3%. A hot-rolled steel sheet having excellent galling resistance, characterized by having a surface roughness Ra of 2.0 μm or more.

(2) In mass%, C: 0.08% or less, Si: 0.05
%, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003
0.010.012% and N in the solid solution state: 0.003 to 0.012%, and further contains one or two of Ti and Nb within a range satisfying the following formula (1), with the balance being iron and unavoidable impurities. Having a steel structure of ferrite single phase, and having a surface roughness of 2.20 Ra after applying a strain of 3%.
A hot-rolled steel sheet with excellent mold resistance, characterized in that the thickness is 0 μm or more. Notation N (mass%) ≧ 0.003 + Ti (mass%) / 3.4 + Nb (mass%) / 6.6 (1)

(3) In mass%, C: 0.08% or less, Si: 0.05
%, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003
A steel slab containing -0.012%, the balance being iron and unavoidable impurities, is hot-rolled at a finish rolling end temperature of at least the Ar ₃ transformation point, and 15 ° C / s after the completion of hot rolling.
A method for producing a hot-rolled steel sheet having excellent anti-galling properties, comprising cooling at the above cooling rate and winding at a winding temperature of 550 ° C. or less.

(4) In mass%, C: 0.08% or less, Si: 0.05
% Or less, Mn: 0.60% or less, Al: 0.060% or less, N: 0.0
A steel slab containing at least one of Ti and Nb within a range satisfying the following formula (1), and the balance being iron and unavoidable impurities. After hot rolling at a transformation point of Ar ₃ or more, cooling at a cooling rate of 15 ° C./s or more after the completion of hot rolling, 55
A method for producing a hot-rolled steel sheet having excellent mold resistance, characterized by winding at a winding temperature of 0 ° C or lower. Notation N (mass%) ≧ 0.003 + Ti (mass%) / 3.4 + Nb (mass%) / 6.6 (1)

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The main feature of the present invention is that a steel containing a predetermined N is hot-rolled and then rapidly cooled, so that 0.003% by mass of solid solution N in a hot-rolled steel sheet (hereinafter, referred to as "H"). (%) Or more. When the work material in which a large amount of solid solution N is positively left is deformed by press working, the surface roughness of the work material increases, and a mold is formed to create a state suitable for oil pool. The galling is improved. The mechanism by which such a phenomenon appears is not necessarily clear, but a large amount of solid solution N causes a non-uniform portion of the Cottrell atmosphere (a portion having a strong dislocation fixing force) to be formed, and the start of plastic deformation differs for each ferrite grain. It is considered that this resulted in an increase in the surface roughness.

Next, the reasons for limiting the composition of each component of the hot-rolled steel sheet according to the present invention, which has excellent resistance to galling, will be described.
Hereinafter, the content (%) of each component means mass%. C: 0.08% or less C is an element necessary for securing strength, particularly after molding. However, if it exceeds 0.08%, a hard second phase appears in the structure, which increases the number of mobile dislocations. Letcher strain (yield point elongation) does not occur. Therefore, the content of C is set to 0.08% or less. The preferred C content range is 0.
01 to 0.08%.

Si: 0.05% or less Si is added to secure a desired strength.
If the addition exceeds 3, the surface quality after press molding is deteriorated. Therefore, the addition amount of Si is set to 0.05% or less.

Mn: 0.60% or less Mn is an element that contributes to securing the strength. However, when it exceeds 0.60%, a second phase (pearlite) appears, which increases the number of mobile dislocations and generates stretcher strain. Since it disappears, the upper limit is set to 0.60%.

Al: 0.060% or less Al is an element that is effective for deoxidation and control of the grain size of the structure, but is also an element that fixes N as AlN. If the Al content exceeds 0.060%, N is fixed and no stretcher strain is generated, so the upper limit is made 0.060%.

N: 0.003 to 0.012%, N: 0.3 in solid solution state.
003 to 0.012% N is an important component that is responsible for increasing the surface roughness using a stretcher strain. 2.0 μm surface roughness
m or more, the N in the solid solution state is 0.003% or more,
Therefore, it is necessary that 0.003% or more of N (total N) be present. However, since it is difficult to form a solid solution in an amount exceeding 0.012%, N in the solid solution state is 0.012%.
The following is assumed. In addition, since N other than that existing in a solid solution state has a bad influence on formability, the amount of N (total N) is also 0.012%.
The following is assumed.

Ti and Nb Ti and Nb can be added for adjusting the strength.
Is also an element having the effect of fixing and precipitating it as nitride. Therefore, it is necessary to add Ti and / or Nb in order to secure necessary solid solution N within a range satisfying N (mass%) ≧ 0.003 + Ti (mass%) / 3.4 + Nb (mass%) / 6.6. .

The steel slab adjusted to the above-mentioned composition can be produced, for example, by continuous casting, and the obtained steel slab is heated by a conventional method and subjected to hot rolling. In the hot rolling, when the end temperature of the finish rolling is _lower than the _Ar3 transformation point, rolling is performed in a ferrite region, the grain size becomes coarse, and cracks are easily generated during press forming.
The rolling finishing temperature is _{equal to} or higher than the _Ar3 transformation point. After the completion of hot rolling, at 15 ° C. to avoid precipitation of AlN during cooling.
/ S or more. And even if the winding temperature exceeds 550 ° C., the precipitation of AlN is promoted, so it is necessary to set the winding temperature to 550 ° C. or less and wind the film.

[0018]

EXAMPLE A steel slab having the composition shown in Table 1 was hot-rolled to a thickness of 2.0 mm and a width of 900 mm under the conditions shown in Table 2, and then pickled. Oil was applied on the outlet side of the pickling line. JIS from the oiled hot rolled steel sheet thus obtained
A No. 5 tensile test piece was sampled, subjected to a 3% elongation strain, and then unloaded. The unloaded test piece was measured for surface roughness Ra and subjected to a sliding test by the following method. Was.

Sliding test conditions: Surface condition of test piece: Oiled without degreasing. Tool: Tool material is SKD11, tool shape is shown in Fig. 1. Surface pressure (linear pressure): 100 N / mm. Sliding speed: 20 mm / s ・ Coefficient of friction: Pull-out load / (Pressing load × 2) The sliding test is repeated 5 times each. “Good” means that no mold seizure (seizure of the mold) occurs at all. The occurrence was evaluated as "poor". These results are also shown in Table 2.

As shown in Table 2, the invention examples of the ferrite single phase structure manufactured by optimizing the steel composition and the manufacturing conditions are as follows:
N in the solid solution state is contained in the range of 0.003 to 0.012%,
The surface roughness Ra after applying% strain was 2.0 μm or more, and there was no mold galling. On the other hand, in the comparative examples in which these conditions were out of the range of the invention, mold seizure occurred.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

As described above, according to the present invention,
It is possible to provide a hot-rolled steel sheet which is excellent in slidability with a metal mold at the time of press molding and does not generate galling during press molding. That is, even when a member using a hot-rolled steel sheet having a thickness of more than 1.4 mm is formed, it is possible to prevent the occurrence of mold seizure, which can contribute to an improvement in yield during press forming. Therefore, according to the present invention, it is possible to realize the processing of a deep drawn product without reducing the press productivity.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a tool shape used for a sliding test.

Claims (4)

[Claims] 1. In mass%, C: 0.08% or less, Si: 0.05% or less, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003 to 0.012%, and N: 0.003 to 0 in solid solution state .
012%, the balance being a steel composition of iron and unavoidable impurities, having a ferrite single phase steel structure, and
A hot-rolled steel sheet excellent in mold resistance, characterized in that the surface roughness after applying a strain of 2.0% or more in Ra is 2.0 μm or more. 2. In mass%, C: 0.08% or less, Si: 0.05% or less, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003 to 0.012%, and N: 0.003 to 0 in solid solution state .
012%, and further contains one or two of Ti and Nb within a range satisfying the following formula (1), and the balance consists of a steel composition of iron and unavoidable impurities. A hot-rolled steel sheet having excellent mold resistance, characterized by having a surface roughness Ra of 2.0 μm or more after applying a strain of 3%. Notation N (mass%) ≧ 0.003 + Ti (mass%) / 3.4 + Nb (mass%) / 6.6 (1) 3. In mass%, C: 0.08% or less, Si: 0.05% or less, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003 to 0.012%, the balance being iron and unavoidable impurities A steel slab consisting of
r Hot rolling at ₃ transformation points or more, 1 after hot rolling is completed
A method for producing a hot-rolled steel sheet having excellent anti-galling properties, wherein the hot-rolled steel sheet is cooled at a cooling rate of 5 ° C / s or more and wound at a winding temperature of 550 ° C or less. 4. In mass%, C: 0.08% or less, Si: 0.05% or less, Mn: 0.60% or less, Al: 0.060% or less, N: 0.003 to 0.012%, and further satisfies the following formula (1). Contains one or two of Ti and Nb in the range,
The rest is hot-rolled at a finish rolling end temperature of Ar ₃ transformation point or higher, and then cooled at a cooling rate of 15 ° C./s or higher after the completion of hot rolling. A method for producing a hot-rolled steel sheet having excellent mold resistance, which is characterized by winding at a winding temperature. Notation N (mass%) ≧ 0.003 + Ti (mass%) / 3.4 + Nb (mass%) / 6.6 (1)