JPH0759734B2 - Low carbon aluminum killed cold-rolled steel sheet excellent in workability, surface roughening property and earring property, and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a low carbon aluminum killed cold rolled steel sheet having excellent press workability, rough surface after press working, and earring property, and a method for producing the same.

(Prior Art) Cold-rolled steel sheet that is used after being subjected to press forming such as deep drawing is required to have press workability: it can be formed without causing defects such as cracks during processing, and rough surface:
The surface roughness after pressing is small, the finished appearance and corrosion resistance are good, and the earring properties are small anisotropy of the material and small ears after deep drawing.

As a conventional cold-rolled steel sheet having good press workability and less likely to cause surface roughening, JP-A-54-135615 discloses an Al core-killed steel sheet in which a steel sheet surface layer is formed of fine grains and a center layer is formed of elongated grains, No. 61-264135 proposes an Al-killed steel sheet utilizing a continuous casting-direct rolling technique, in which the inside of the steel sheet has a coarse expanded grain structure and the surface layer has equiaxed fine grains.

(Problems to be Solved by the Invention) However, in both cases, since the central portion of the steel sheet has a drawn grain structure and the r value of the central portion is high, workability and skin roughness are good, but the anisotropy of the r value, that is, Δ The r value was large, and there was a problem in terms of earring properties after pressing.

Further, the former Al core killed steel sheet is not general because it cannot be manufactured by the continuous casting method because it uses the Al core killed steel, and the latter Al killed steel sheet is premised on continuous casting-direct rolling, and the temperature control during casting etc. It was difficult and lacked stable productivity.

The present invention provides a low carbon aluminum killed cold rolled steel sheet excellent in press workability, surface roughness after press working, and earring property, and a method for stably producing the cold rolled steel sheet.

(Means for Solving the Problems) The features of the low-carbon aluminum-killed cold-rolled steel sheet of the present invention are:
From the steel sheet surface layer to the central layer, the Δr is small and the equiaxed crystal structure is excellent in earring property, and the steel sheet surface layer is made to have the equiaxed crystal fine crystal structure to secure the rough surface, and the inner layer is press workable as the equiaxed crystal coarse grain structure. Is improved.

Although the Al killed steel sheet having an equiaxed structure is smaller than 1.4 and an Al killed steel sheet having a drawn grain structure, the present inventors have noted that press workability has a large correlation with the elongation of the steel sheet, Press workability was secured by coarsening the equiaxed crystal grains in the inner layer of the steel sheet to improve elongation,
As shown in FIG. 1, the larger the steel sheet crystal grains, the better the elongation of the steel sheet. Further, the equiaxed structure has a small Δr as shown in FIG. 2 as compared with the stretched structure Al killed steel and is excellent in earring property. Contrary to the press workability, the rougher the surface, the better the smaller the grain size of the steel sheet, and the relationship is shown in FIG.

As described above, in general, the press workability and the surface roughening property are contradictory properties, and since the conventional inner layer has a problem in earring property in the steel sheet having a stretched structure, the present inventors have to satisfy all of them in a well-balanced manner. As a result of diligent examination,
The present invention has been achieved.

Next, the reason for limiting the invention of claim 1 will be described.

The present invention has an equiaxed structure from the steel plate surface layer to the inner layer,
The reason for limiting the crystal grain axial ratio to 1.5 or less is that when the axial ratio exceeds 1.5, the value becomes relatively large, and the Δr value increases and the earring property deteriorates. Further, the crystal grain size of the surface layer from the steel plate surface to 10 to 30 μm is set to ASTM grain size No. 10.5 or more, because it is only the surface layer from the steel plate surface to 30 μm at the maximum that is involved in the rough skin after press forming, This is because even if the inner layer is made finer, not only the rough surface is not improved, but also the workability is deteriorated. If the thickness of the surface layer having a grain size of ASTM grain size No. 10.5 or more is less than 10 μm, the rough skin improving effect can be obtained. Because you can't.

The crystal grain size of the surface layer is set to ASTM grain size No. 10.5 or more because the surface roughness after tensile processing of the steel sheet exceeds 20% as shown in FIG. The surface roughness of more than 0.65 μRa in the above experiment clearly impairs the aesthetics after pressing, corrosion resistance and other properties.

The grain size of the inner layer excluding the surface layer of the steel sheet was limited to ASTM grain size No. 9.5 or less because the elongation of the steel sheet was 35% in the experiment of the present inventors.
This is because if the above is not secured, cracks and other defects will occur during press molding, and this is determined from the relationship shown in FIG.

Although the steel sheet of the present invention has been described above, the present invention includes all low carbon aluminum killed steel sheets having the structure described in claim 1, and the manufacturing method thereof is not limited, but the present inventors have found out. The most rational manufacturing method is shown in claim 2.

The contents will be described below. In the present invention, by weight% as a steel component, C: 0.02-0.06%, Si ≦ 0.03%, Mn ≦ 0.60
%, P ≦ 0.050%, S ≦ 0.05%, Al: 0.060 to 0.090%, N:
Using a continuously cast slab consisting of 0.003 to 0.007%, the balance Fe and unavoidable impurities, hot rolling at a heating furnace extraction temperature of 1120 ° C or less and a winding temperature of 600 to 750 ° C in the hot rolling process
The hot-rolled coil has a N precipitation rate (N as AlN / Total N) of 80% or more and solutionized Al (sol.Al) of 0.020 wt% or more.

Here, the point is AlN, which is necessary to make equiaxed grain structure.
Is to be sufficiently precipitated in the hot rolling process, and the AlN precipitation rate is 8
It is achieved by setting it to 0% or more. Even after the required AlN is precipitated, a certain amount of sol.Al remains in the structure during the hot rolling process.The sol.Al content of 0.020% by weight or more is the first grain refinement of the surface layer only. Will be realized. In order to achieve this, first, the amounts of Al and N in the steel components are
It is necessary to control l: 0.060 to 0.090% by weight and N: 0.003 to 0.007% by weight.If the amount of Al is less than 0.060% by weight, sufficient sol.Al cannot remain in the hot rolled coil, and The same applies when the N content exceeds 0.007% by weight. Al in steel
FIG. 4 shows the influence of the above on the grain sizes of the surface layer and the inner layer.

If the amount of N is less than 0.003% by weight, it is not possible to secure the amount of AlN precipitation necessary for making the surface layer crystal grains fine in the product plate.

The upper limit of 0.09% of Al amount is limited because the effect is saturated even if added over that amount, inclusions such as Al ₂ O ₃ increase, and the material such as hardness is rather lowered. .

Needless to say, it is advantageous that P and S are as low as possible in order to improve workability, and the amounts of P and S are each 0.05 wt% or less. Desirably, each is 0.01% by weight or less.

The upper limits of the Si content and the Mn content are set to 0.03% by weight and 0.60% by weight, respectively, because if the amounts are exceeded, the material hardens, the elongation decreases, and the workability decreases.

The lower limit of the amount of C is set to 0.02% by weight, because if it is less than this amount, it becomes too hard and the strength after processing becomes insufficient and internal defects due to inclusions occur. The upper limit is set to 0.06% by weight because if it exceeds this amount, it hardens, the elongation decreases, and the workability decreases.

The hot rolling conditions were limited to the above-mentioned conditions because the slab heating temperature was
Above 1120 ℃, the AlN generated in the continuous casting process becomes a solution,
For the same reason, it was not possible to secure sufficient AlN precipitation in the hot rolling process, and the winding temperature was limited to 600 to 750 ° C.

If the hot rolling coiling temperature exceeds 750 ° C, coarse Fe ₃ C (cementite) will be deposited, and the variation of the material in the coil will be large, resulting in deterioration of workability. If the coiling temperature is less than 600 ° C, the hot rolling process required for equiaxed graining of the product sheet
AlN precipitation rate cannot be secured.

In the present invention, at the time of this hot rolled steel sheet, the steel sheet grain size, AlN
The state of precipitation and the amount of sol.Al remaining in the steel are the same for both the surface and inner layers of the steel sheet.

Next, the hot-rolled steel sheet is subjected to pickling, cold rolling, electrolytic cleaning steps in a usual manner, and then annealed at a recrystallization temperature or higher and an Ac ₃ transformation point or lower in a box-type annealing furnace. In this case, the annealing atmosphere should be an inert gas atmosphere of N ₂ 90% or more, and the dew point should be −20 ° C. or lower.
It is essential to create an atmosphere in which nitrogen absorption occurs on the surface of the steel sheet. As a result, nitrogen absorption occurs in the range of 10 to 30 μm from the surface of the steel sheet, and by combining with sol.Al remaining in the steel, fine AlN of 0.05 μm or less only in the steel sheet surface layer is generated, so only the steel sheet surface layer has crystal grains. The growth is suppressed and the crystal grains become fine. On the other hand, the inner layer of the steel sheet is not affected by nitrogen absorption in this annealing process, grain growth is not suppressed because it is only coarse AlN precipitated in the hot rolling process, and grain growth to the appropriate size required from the viewpoint of workability. To do. Plate thickness of 0.25 mm investigated by the present inventors
Moreover, the state of AlN precipitation of the surface layer of the steel sheet of the present invention (at a position of 10 μm from the outermost surface) and the inner layer (position of sheet thickness 1/2) after annealing is shown in FIGS. 5 and 6, and the surface layer contains many fine AlN. It can be seen that it is observed, but it is hardly observed in the inner layer. After being annealed, the steel sheet of the present invention is temper rolled by a usual method to obtain a product.

Although the manufacturing method of the present invention has been described above, its characteristic is that only the surface layer of the steel sheet is nitrified in the annealing step, and only the surface layer is formed by combining nitriding only the surface layer and sol.Al left in the hot rolling step. The point is that fine AlN, which is effective in suppressing grain growth, is generated.

(Example) Next, the Example of this invention is described.

Molten steels 1, 2 and Al of the limited range components of the present invention shown in Table 1
The slabs obtained by continuously casting molten steels 3 and 4 having amounts outside the range of the present invention were hot-rolled under the hot-rolling conditions shown in Table 2 to obtain hot-rolled coils. And pickling by the usual method, cold reduction
It was cold-rolled so that the plate pressure was 0.25 mm at 87% and annealed in a box annealing furnace. Annealing conditions were N ₂ : 95% .H ₂ : 5% (dew point −30 ° C.) in a soaking furnace temperature of 640 ° C., soaking for 5 hours and then cooling for 32 hours. Then, 1% temper rolling was performed to investigate various characteristics of the product, and the investigation results are shown in Table 2. The survey items are the following (A) to (E).

(A) AlN precipitation rate in hot-rolled sheet, solid solution Al amount investigation AlN precipitation rate was shown by the N amount corresponding to AlN with respect to the total N amount.

(B) Evaluation of metallographic structure The cross section of the product was examined with a microscope after polishing. The survey items are the crystal grain axial ratio (average crystal grain axial ratio from the outermost surface to the plate pressure 1/2 position), ASTM at the 10 μm position from the outermost surface and the plate thickness 1/2 position
It is the grain size No. In addition, Example 2 and Comparative Example 1 in Table 2
7 and 8 show examples of photographs of the structure.

(C) Mechanical test A tensile test of the product was carried out based on JIS. Table 2 shows only the elongation value (El%) and the value, and the Δr value.

(D) Evaluation of surface roughness The surface roughness of the steel sheet after the 20% tensile test was measured with a roughness meter. The surface roughness of the product before processing was the same and was 0.22 μRa. Further, it has been confirmed by experiments by the present inventors that a problem due to surface roughness after pressing does not occur when the surface roughness after 20% tension is 0.65 μRa or less.

(5) Cylindrical processing test In order to investigate the workability, skin roughness, and earring property during deep drawing, some samples were evaluated by forming a circular blank with a diameter of 100 mm into a cylindrical shape with a drawing ratio of 2.4. The crack occurrence state, rough skin state and earring occurrence state during processing are as follows: ○: No occurrence or minute, △: Mild to moderate, ×: Not practical 3
The grade was evaluated.

The above evaluation results are shown in Table 2. In Examples, the steel sheet elongation,
All of the workability, the surface roughening property, and the earring property are satisfied, and the comparative examples are inferior to any of the properties.

(Effects of the Invention) The low-carbon aluminum killed cold-rolled steel sheet of the present invention is excellent in workability, surface roughening property, and earring property, and is suitable as a material for products to be subjected to press forming such as deep drawing,
Its applications are wide, ranging from various metal cans, dry battery interior cans, to various home appliances, electric potential parts, automobile parts, etc., and activity in a wide range can be expected.

Further, the method for producing a low carbon aluminum killed cold rolled steel sheet of the present invention is a steel composition, by means such as precipitating fine AlN only on the surface layer by the surface layer nitrification phenomenon during box annealing by optimization of hot rolling conditions, earrings, etc. The grain size of the surface layer and inner layer of low carbon aluminum killed cold rolled steel sheet with equiaxed crystal structure with excellent workability is controlled to satisfy workability and rough surface, and stable workability and rough surface are obtained. It is possible to manufacture a low carbon aluminum killed cold rolled steel sheet which has excellent properties and earring properties.

[Brief description of drawings]

Fig. 1 is an explanatory diagram of the relationship between grain size No. and elongation, Fig. 2 is an explanatory diagram of the relationship between axial ratio and Δr, and Fig. 3 is a grain size No. and 20%.
Fig. 4 is an explanatory diagram of the relationship with the roughness after tension, Fig. 4 is an explanatory diagram of the relationship between Al in steel and the grain sizes of the surface layer and the inner layer, and Figs. 5 and 6 are metallographic photographs showing the AlN precipitation state of the surface layer and the inner layer. ,
7 and 8 are photomicrographs showing the metal cross-sectional structures of the products obtained in Example 2 and Comparative Example 1, respectively.

Claims (2)

[Claims] 1. A steel sheet having a fine grain equiaxed grain structure (crystal grain axis ratio of 1.5 or less) having a surface grain size of 10 to 30 μm from the surface of the steel sheet and having an ASTM grain size of No. 10.5 or more. Low carbon aluminum killed with excellent workability, rough surface resistance and earring characteristics, characterized by a coarse equiaxed grain structure (grain grain axis ratio of 1.5 or less) with an inner layer grain size of ASTM grain size No. 9.5 or less. Cold rolled steel sheet. 2. C: 0.02 to 0.06% by weight as a steel component,
Si ≦ 0.03%, Mn ≦ 0.60%, P ≦ 0.050%, S ≦ 0.05%, A
l: 0.060 to 0.090%, N: 0.003 to 0.007%, continuous casting slab consisting of balance Fe and unavoidable impurities in a furnace with extraction temperature 1120 ° C
Below, the hot rolling at a coiling temperature of 600 to 750 ℃, the AlN precipitation rate (N as AlN / Total N) is 80% or more, and the solid solution Al amount (sol.Al) is 0.020% by weight or more. Rolled coil was then pickled in the usual process, cold rolled, and then box annealed in an atmosphere containing 90% or more of N ₂ at a temperature not lower than the recrystallization temperature and not higher than the Ac ₃ transformation point. A method for producing a low carbon aluminum killed cold rolled steel sheet excellent in workability, surface roughening property and earring property, which comprises performing quality rolling.