JP2886562B2 - Soft hot-rolled steel sheet with excellent deep drawability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial application field) The present invention has a tensile strength of 35 kgf / mm ² or less and a total elongation of 50 kgf / mm ² or less.
% Of soft hot-rolled steel sheet with excellent drawability, which has characteristics of more than 0.1%, especially by actively controlling the surface roughness of the steel sheet, to improve not only press formability but also bending workability and stretch formability. It is an improvement.

(Prior Art) In recent years, in order to reduce costs, automobile manufacturers, home appliance manufacturers, and the like tend to use thin hot-rolled steel sheets instead of cold-rolled steel sheets. However, conventional hot-rolled steel sheets are inferior in cold-rolled steel sheets in terms of deep drawability, so that the use of hot-rolled steel sheets in this field is currently limited.

By the way, the deep drawability of a steel sheet depends on the elongation characteristics, surface roughness and r-value of the material. One of the reasons that the conventional hot-rolled steel sheet is difficult to apply for deep drawing is that it is manufactured by a usual method. The hot-rolled steel sheet has a three-dimensional surface roughness SRa value of less than 1.0 μm and an r-value level of 1.0 or less, and it is difficult to obtain a characteristic of r value ≥2.15 like a cold-rolled steel sheet. Is mentioned. The deep drawability of the cold-rolled steel sheet almost depends on the r-value. However, since the hot-rolled steel sheet has a low r-value, it is covered by the total elongation and the surface roughness.

Means for improving such drawbacks related to the deep drawability of a hot-rolled steel sheet are roughly classified into three methods. The first
Is a method of bringing the r-value as close as possible to a cold-rolled steel sheet,
The third is a method for greatly improving elongation characteristics, and the third is a method for positively increasing the surface roughness.

Regarding the first method, see, for example, JP-A-55-97.
No. 431, Japanese Unexamined Patent Publication No. Sho 60-77927, etc., using a steel having a conventional component composition range, the rolling conditions during hot finish rolling are mainly under high pressure or high strain rate rolling, or specified. There has been proposed a method for improving the r value by reducing the pressure in a low temperature range.

However, even if such a method is performed, the obtained r
The value is at most in the range of 1.0 to 1.3, and does not reach the level of a cold-rolled steel sheet, so that at present the deep drawability cannot be satisfied. In addition, in such a method, the higher the r value is to be obtained, the more the hot rolling conditions need to be significantly changed to the high-pressure lower side and the high-speed rolling side from the range usually performed. The disadvantage of exceeding the allowable range in the hot rolling mill occurs.

Next, regarding the improvement of elongation properties, for example,
In 139849, a method of adding B to a chemical component in a normal soft hot-rolled steel sheet and controlling the weight ratio of each of B / N and Mn / S to a predetermined range has been proposed,
The total elongation of this hot-rolled steel sheet is limited to 48% or less, and the fact is that even when compared with the best properties, it reaches only the same level as that of the cold-rolled steel sheet.

Further, as a method of roughening the surface roughness, a method of hitting a shot grain on the surface of a steel sheet, a method of roughening the surface of a hot-rolled work roll with a shot dull or a laser dull and transferring the roll at the time of rolling, similarly a work roll of a skin pass For example, a method of imparting roughness to the material and transferring the material during rolling. Of the above methods, the method of hitting shot grains on the surface of the steel sheet is not a preferable method because the surface height becomes hard.

(Problems to be Solved by the Invention) The present invention relates to a soft hot-rolled steel sheet having improved elongation characteristics and a rough drawn surface to improve deep drawability, specifically, a tensile strength of 35 kgf / mm ² or less. And
An object of the present invention is to propose a soft hot-rolled steel sheet having extremely excellent formability, having a total elongation of 50% or more and a surface roughness of SRa of 1.0 or more.

Here, the tensile strength is the tensile strength when a JIS No. 5 tensile test piece is used, and the elongation characteristic is the value of the total elongation of a 1.4 mm thick JIS No. 5 tensile test piece, and the sheet thickness differs. In this case, the value of E1 ^{* obtained} by correcting the elongation by the following equation (1) is used.

El ^* = (1.4 / t) El _t (1) where t is the plate thickness (mm) of the tensile test piece, and El _t is the plate thickness t
Total elongation of tensile test specimens (%) (Means for solving the problem) The deep drawability is determined by the sheet thickness at the time of forming when the sheet thickness is less than 1.0 mm, such as a cold rolled steel sheet. Since the thickness reduction affects the forming limit, the ratio depending on the r value is particularly higher than the elongation characteristic. However, in the case of steel sheets with a thickness of 1.0 mm or more, such as hot-rolled steel sheets, the effect of the reduction in thickness is alleviated, so the ratio depending on the r value decreases, and the elongation characteristics and surface characteristics during pressing are reduced. It is known that the ratio depending on (surface roughness, rust prevention oil, etc.) becomes relatively high.

From the above viewpoints, the present inventors have worked on the development of means for significantly improving the deep drawability of hot-rolled steel sheets by improving the elongation properties and improving the formability (LDR) by the surface roughness. Although the value is the same as that of conventional steel, we have developed a hot-rolled steel sheet that has significantly improved deep drawability by improving the formability (LDR characteristic) by much higher elongation characteristics and surface roughness adjustment than conventional steel. The present invention has been completed.

That is, the soft hot-rolled steel sheet excellent in deep drawing formability according to the present invention is as follows: C: 0.10 wt% (hereinafter simply referred to as%), Mn: less than 0.20%, Cr: 0.10 to 2.0%, and Al: 0.002 to 0.100% The balance consists of Fe and unavoidable impurities, and the three-dimensional surface roughness SRa of the steel sheet is 1.0 μm or more, the tensile strength is 35 kgf / mm ² or less, and the total elongation is 50% or more. This is a soft hot-rolled steel sheet (first invention) excellent in deep drawability.

Also contains C: 0.10% or less, Mn: less than 0.20%, Cr: 0.10 to 2.0%, and Al: 0.002 to 0.100%, and one or more selected from Ti, Nb, and Zr: 0.005
0.10%, the balance consists of Fe and inevitable impurities. The three-dimensional surface roughness SRa of the steel sheet is 1.0 μm or more, and the tensile strength is 35 kgf / mm ² or less and the total elongation is 50% or more. It is a soft hot-rolled steel sheet excellent in deep drawing formability (second invention).

Furthermore, it contains C: 0.10% or less, Mn: less than 0.20%, Cr: 0.10 to 2.0 and Al: 0.002 to 0.100%, and B: 0.0004 to 0.0100%, the balance being Fe and unavoidable impurities. Soft hot-rolled steel sheet with excellent three-dimensional surface roughness SRa of 1.0 μm or more, tensile strength of 35 kgf / mm ² or less, and total elongation of 50% or more. Invention).

Further, C: 0.10% or less, Mn: less than 0.20%, Cr: 0.10 to 2.0%, and Al: 0.002 to 0.100%, and one or more selected from Ti, Nb, and Zr: 0.005
0.10% and B: 0.0004-0.0100%, the balance consists of Fe and unavoidable impurities, the three-dimensional surface roughness SRa of the steel sheet is 1.0 μm or more, and the tensile strength is 35 kgf / mm ² or less, A soft hot-rolled steel sheet having excellent total drawability of 50% or more and excellent in deep drawability (fourth invention).

(Operation) Hereinafter, the soft hot-rolled steel sheet of the present invention will be specifically described.

The soft hot-rolled steel sheet according to the present invention is a low-carbon aluminum-killed steel having a C content of 0.10% or less, containing a Cr content in a range of 0.10 to 2.0% while suppressing a Mn content to a low range of less than 0.20%. In addition, the surface roughness was positively increased. The chemical composition range of the soft hot-rolled steel sheet of the present invention is intended to ensure the properties of the target tensile strength: 35 kgf / mm ² or less and the total elongation: 50% or more, and the surface roughness range is the target. Limit drawing ratio (LDR): To secure the characteristic of 2.10 or more, it is determined by the following design concept.

That is, in order to make the material as soft as possible when rolling is performed within the range of normal hot rolling conditions, it is necessary to thoroughly eliminate strength increasing factors such as structure strengthening, solid solution strengthening, and precipitation hardening, and Ferrite grain size as a structure
In order to achieve the two points of having a sizing structure in the range of 20 to 100 μm, to specify the chemical component composition range, to improve the drawability at the time of pressing, to give the surface roughness at the time of hot rolling or skin pass Specifies the surface roughness (SRa).

Hereinafter, the reasons for limiting the component composition and the surface roughness to the above ranges in the present invention will be described.

C: 0.10% or less The C content needs to be 0.10% or less in order to obtain a soft hot-rolled steel sheet. The reason for this is that if the C content exceeds 0.10%, the tensile strength exceeds 35 kgf / mm ² , so that it is not possible to obtain the characteristic of the total elongation: 50% or more which is the target of the present invention. is there. Since the lower the C content is, the softer the steel sheet becomes and the higher the elongation characteristics can be easily obtained, the C content is preferably set to 0.025% or less. However, the lower limit is substantially 0.0010% due to restrictions on steelmaking. It is. In addition, even if it is in the range of 0.025 to 0.10%,
By appropriately selecting the amount of Cr and the amount of Cr, the target tensile strength of the present invention can be obtained.

Mn: less than 0.20% The amount of Mn must be suppressed to a low range of less than 0.20% from the viewpoint of obtaining high elongation characteristics. The reasons are as follows. One is to suppress the increase in tensile strength due to the solid solution strengthening of Mn, and the other is to reduce the temperature of the onset of ferrite transformation due to the lowering of the Ar ₃ point of Mn. This is for preventing fineness and suppressing an increase in tensile strength due to fineness and roughness. When the amount of Mn is 0.20% or more, the effect of Mn cannot be suppressed,
The target tensile strength of the present invention: 35 kgf / mm ² or less, and the total elongation: 50% or more cannot be obtained. Mn content is 0.01%
When the content is less than the above, the effect of fixing S decreases, and there is a possibility that hot brittleness may be caused.

Cr: 0.10 to 2.0% The Cr content needs to be 0.10 to 2.0% in order to prevent the adverse effect of solid solution C and optimize the microstructure. Adjusting the Cr content to this range is one of the important requirements of the present invention. This point will be described in detail below based on the knowledge obtained by the research of the inventors.

Means adopted to make soft hot-rolled steel sheet include: methods to reduce as much as possible the strength-increasing components such as C and Mn, and increasing the hot finishing temperature or winding temperature to increase the ferrite grain size. Methods of doing so are generally well known. However, even if the present inventors proceeded with these measures, the resulting elongation characteristics of the obtained elongation characteristics due to residual solid solution C or generation of film-like cementite at grain boundaries and abnormal coarsening of ferrite crystal grain size, etc. There are limits to the level, and this method alone is the target of the present invention at 50%
It was found that it was never possible to obtain the high elongation characteristics described above.

Now, in the conventional hot-rolled steel sheet, the reason that the solid solution C tends to remain as the C content is reduced is that the supersaturation degree of the solid solution C in the steel decreases, so that the driving force for precipitating as carbide is reduced. This is because the number of precipitation nuclei decreases due to the coarsening of the crystal grain size. When the precipitation of cementite grain boundaries is forcibly promoted, particularly when the C content is as low as 0.02% or less, a film-like cementite is formed at the ferrite crystal grain boundaries, which also deteriorates the elongation characteristics.

By the way, as a means for preventing such residual of solid solution C or formation of film-like cementite, a method of containing a carbide-forming component such as Ti, Nb, Zr or the like is also known. This method can provide a quite effective result particularly when applied to ultra-low carbon steel having a C content of 0.0050% or less, but still does not improve the elongation characteristics remarkably. The cause is that in order to sufficiently eliminate the adverse effect of solid solution C by containing these carbide-forming components, the ratio between the carbide-forming components and the C content needs to be at least 10 or more in terms of atomic%. This is because, when such a carbide-forming component is contained, the effect of increasing the strength of the contained component itself occurs, and as a result, the improvement effect due to the reduction of solid solution C disappears.

In this regard, the inventors have found that the above disadvantages can be advantageously improved by the effect of Cr.

First, the adverse effect of solid solution C is eliminated by the inclusion of Cr, and the shape of the film-like cementite is prevented. The details of these mechanisms are not clear, but are presumed to be due to the following reasons. That is, Cr is not a strong carbide-forming component like Ti, Nb, and Zr, but has a strong tendency to form carbides compared to Fe and Mn.
Interaction between the two atoms works to form solid solution C around Cr atoms.
It is considered that the state where atoms are unevenly distributed is maintained. By the way, when the above state is taken, C is localized in the vicinity of Cr atom despite being in a solid solution state, and therefore, on one side, the same state as precipitated as carbide, that is, In most positions, the amount of solid solution C is reduced and solid solution strengthening is not involved, and on the other hand, carbide is not formed, so that the formation of film-like cementite is prevented. As described above, the presence state of C is extremely advantageously maintained in terms of mechanical properties due to the inclusion of Cr. Note that, as is well known, the amount of solid solution strengthening of Cr itself is much smaller than that of Mn or the like, so that the point that the increase in strength due to its inclusion is small produces more favorable results.

The second effect containing Cr is that it is easy to obtain a ferrite sized structure having a good particle size. In order to promote the softening of the hot-rolled steel sheet and the improvement of the elongation properties, the grain size of the ferrite must be a grain-sized structure sufficiently grown within a range that does not cause surface roughness. In order to obtain such a structure, it is desirable that the hot-rolling finishing temperature and the winding temperature are higher. Among them, a problem with the conventional steel in particular is that the range of the optimum winding temperature is narrow.

FIG. 1 schematically shows the difference in the relationship between the winding temperature and the ferrite crystal grain size of a conventional steel and a Cr-containing steel within the scope of the present invention.

As is clear from the figure, in the conventional steel, when the temperature exceeds a certain winding temperature, a sudden abnormal grain growth phenomenon occurs,
A mixed-grain structure and a coarse-grained structure with rough skin come to be exhibited. Therefore, in order to avoid this danger and set the winding temperature range in which a sized structure can be obtained, the winding temperature is limited to a relatively low temperature in a narrow region, and the obtained ferrite crystal grain size is also insufficient. In contrast, in the steel of the present invention containing Cr, abnormal grain growth is suppressed, and grain growth proceeds smoothly with an increase in the winding temperature, so that it is easy to obtain a grain-sized structure having an optimal grain size. It is.

To achieve the above effects, Cr should be at least 0.
When the content exceeds 2.0%, the tensile strength increases, and it becomes difficult to achieve the target tensile strength of the present invention: 35 kgf / mm ² or less.
2.0%.

Al: 0.002 to 0.100% Al is a useful component that improves the detergency of steel as a deoxidizing component, fixes N, and prevents strain aging deterioration of elongation characteristics.

To achieve the above effects, Al should be at least 0.
002% is required. On the other hand, if the content exceeds 0.100%, this effect not only saturates, but also causes an increase in AlN and inhibits the growth of ferrite grains. Therefore, the content of Al was limited to the range of 0.002 to 0.100%.

In the present invention, the object can be basically achieved by regulating the amounts of C, Mn, Cr and Al, but by adjusting other chemical components to the following ranges. Further, the deep drawability is improved.

One or more of Ti, Nb, and Zr: 0.005 to 0.10% In the second invention and the fourth invention, one or more of Ti, Nb, and Zr are selected from 0.005 to 0.10%. It is contained in the range. Ti, Nb, and Zr form carbides and reduce solid solution C. Therefore, if they are contained in an appropriate amount, deep drawability can be improved. In order to exert this effect, the C content is set to 0.020% or less, and the atomic weight% of the total amount of one or more of the above components and the C content is determined.
Preferably, the ratio is in the range of 1-5. This is because if the ratio is less than 1, the effect of the added element is less likely to be exerted, while if the ratio exceeds 5 the tensile strength tends to increase and elongation properties tend to deteriorate. Therefore, these elements are contained in the range of 0.005 to 0.10%.

B: 0.0004 to 0.0100% In the third invention and the fourth invention, B is contained in the range of 0.0004 to 0.0100%. B has beneficial effects such as preventing strain aging due to N and acting as a nucleus for precipitating supersaturated solid solution C when precipitated as BN. If contained in an appropriate amount, deep drawing Formability is improved. If the amount of B is less than 0.0004%, this effect is not exhibited. on the other hand
If it exceeds 0.0100%, there is a concern that the elongation properties will be degraded.

N, like C, causes solid solution strengthening, causes strain aging deterioration, and inhibits the growth of ferrite crystal grains as AlN. Preferably
It is better to be 20 ppm or less.

P segregates at the grain boundaries and tends to cause secondary work brittleness, and increases the strength of the base material by solid solution strengthening.
It is better to be 012% or less.

S is desirably in the range of 0.010% or less because it has an adverse effect on elongation properties as a nonmetallic inclusion and has an effect of promoting hot brittleness and secondary working brittleness.

The soft hot-rolled steel sheet according to the present invention can be manufactured by hot-rolling steel having the above-mentioned chemical components in accordance with a conventional method, and the manufacturing conditions are not particularly limited. The most common finish rolling conditions are:
The finish rolling is performed at a temperature of _three or more points of Ar. In this case, a higher finish rolling temperature can provide better mechanical properties. The soft hot-rolled steel sheet according to the present invention has a recrystallized ferrite structure by adjusting the rolling reduction in the final pass to 20% or more even when the finish rolling temperature is a low temperature in the range of 750 ° C. to Ar ₃ points. And the same good mechanical properties as in the case of high temperature finishing can be obtained. The reason for this is that the recrystallization of the processed ferrite crystal grains tends to proceed due to the inclusion of Cr, and recrystallization occurs at a final pass rolling reduction of 20% or more. The point that good mechanical properties can be obtained even by such a low finish rolling temperature of ₃ Ar or less is suitable for producing a thin hot-rolled steel sheet having a thickness of about 1.0 to 2.0 mm, which is in a trend of increasing demand in recent years. is there.

The winding temperature is preferably set to 550 ° C. or higher in order to improve the softness and elongation characteristics of the hot-rolled steel sheet. The upper limit of the winding temperature is not particularly limited from the viewpoint of mechanical properties, but is preferably about 750 ° C. or less from the viewpoint of pickling properties.

Surface roughness SRa: 1.0 μm or more.

To increase the drawability of soft hot-rolled steel sheets, SRa
It is necessary to be 1.0 μm or more. The reason is that, as described above, in the case of a hot-rolled steel sheet, the r-value is low and the elongation property alone has a limit in drawability. However, if the surface roughness is increased to 1.0 μm or more, the lubricating oil has an oil pooling effect. This is because draw formability is improved.

Although the drawability improves as the surface roughness increases, the three-dimensional surface roughness SRa: about 3.0 μm is optimal. This is because, in order to obtain a three-dimensional surface roughness SRa: 3.0 μm or more, there are other restrictions to control the roughness (for example, in the case of a method using skin pass pressure, the elongation is significantly reduced due to viscous work hardening). It is.

On the other hand, if the three-dimensional surface roughness SRa is less than 1.0 μm, improvement in drawing formability due to the lubricating oil accumulation effect cannot be expected.

(Example) Next, the present invention will be described with reference to examples. In the examples described below, rolling conditions for producing the hot-rolled steel sheet of the present invention are described. However, the hot-rolled steel sheet of the present invention is not limited by the rolling conditions.

After rolling steel having various chemical components shown in Table 1 with a hot-rolled steel sheet shown in Table 2 to a thickness of 1.40 mm, the surface shown in Table 2 was obtained by skin pass using rolls having variously adjusted surface roughness. A hot-rolled steel strip with roughness was used.

Table 2 also shows the results obtained by examining the tensile properties, elongation properties, and critical draw ratio (LDR) of the hot-rolled steel strip thus obtained.

As is evident from Table 2, all of the inventive steels have excellent properties of not only a tensile strength of 35 kgf / mm ² or less and a total elongation of 50% or more, but also an LDR of 2.06 or more.

FIG. 3 shows the relationship between the tensile strength and the total elongation, the tensile strength and the limit drawing ratio (LD
R.), but the difference in characteristics between the two is obvious.

The tensile properties and elongation properties were measured using JIS No. 5 tensile test specimens taken from a direction parallel to the rolling direction after subjecting a hot-rolled steel sheet having a thickness of 1.4 mm to skin pass rolling at 1.0% and then rolling in a direction parallel to the rolling direction. The draw ratio was measured by subjecting a 1.4 mm thick hot-rolled steel sheet to descaling by pickling and then taking a test specimen using a cylindrical flat-bottom drawing shown in FIG. 2 under the conditions shown in Table 3. It is.

(Effect of the Invention) The hot-rolled steel sheet according to claim 1 reduces Mn from the chemical composition range of the conventional soft hot-rolled steel sheet and contains an appropriate amount of Cr, so that the adverse effect of solid solution C disappears, and Since the ferrite crystal grains can be made to have the optimal grain size structure, they are not only softer than conventional steel but also have much better elongation characteristics,
By increasing the surface roughness to 1.0 μm or more in terms of the three-dimensional surface roughness SRa, improvement in drawability due to the lubricating oil accumulation effect can be expected. Therefore, such a hot-rolled steel sheet is particularly excellent in deep drawability.

The hot-rolled steel sheet according to claim 2 is characterized in that Ti, N
Since one or more of b and Zr are contained, the deep drawability is further improved.

Since the hot-rolled steel sheet of claim 3 contains B in addition to the components of claim 1, deep drawability is further improved.

The hot-rolled steel sheet according to claim 4 is characterized in that Ti, N
Since one or more of b and Zr and B are contained, the deep drawability is further improved.

[Brief description of the drawings]

FIG. 1 is a graph showing the difference in the ferrite crystal grain size with respect to the winding temperature of the invention steel and the conventional steel, FIG. 2 is a schematic diagram of a cylindrical flat bottom drawing used for measuring the limiting drawing ratio, and FIG. 7 is a graph showing the relationship between the tensile strength and the total elongation and the relationship between the tensile strength and the limit drawing ratio in the inventive steel and the comparative steel.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] C: 0.10 wt% or less, Mn: less than 0.20 wt%, Cr: 0.10 to 2.0 wt% and Al: 0.002 to 0.100 wt%, the balance being Fe and unavoidable impurities. A soft hot-rolled steel sheet excellent in deep drawability, characterized in that the three-dimensional surface roughness SRa is 1.0 μm or more, the tensile strength is 35 kgf / mm ² or less, and the total elongation is 50% or more. 2. One kind selected from Ti, Nb, and Zr, wherein C: 0.10 wt% or less, Mn: less than 0.20 wt%, Cr: 0.10 to 2.0 wt%, and Al: 0.002 to 0.100 wt%. Or two or more: 0.005 ~
Containing 0.10 wt%, the balance being Fe and unavoidable impurities, with the three-dimensional surface roughness SRa of the steel sheet 1.0μm or more, yet the tensile strength and the 35 kgf / mm ² or less, the total elongation of 50% or more A soft hot-rolled steel sheet with excellent deep drawability, characterized by the fact that: 3. C: 0.10 wt% or less, Mn: less than 0.20 wt%, Cr: 0.10 to 2.0 wt% and Al: 0.002 to 0.100 wt%, B: 0.0004 to 0.0100 wt%, the balance Is composed of Fe and unavoidable impurities, has a three-dimensional surface roughness SRa of at least 1.0 μm, a tensile strength of at most 35 kgf / mm ² , and a total elongation of at least 50%. Soft hot rolled steel sheet with excellent formability. 4. One kind selected from Ti, Nb and Zr, containing C: 0.10 wt% or less, Mn: less than 0.20 wt%, Cr: 0.10 to 2.0 wt%, and Al: 0.002 to 0.100 wt%. Or two or more: 0.005 ~
0.10 wt% and B: containing 0.0004～0.0100Wt%, balance being Fe and unavoidable impurities, with the three-dimensional surface roughness SRa of the steel sheet 1.0μm or more, yet the tensile strength and at 35 kgf / mm ² or less A soft hot-rolled steel sheet excellent in deep drawability, characterized in that the total elongation is 50% or more.