JP2005272888A - Cold rolled steel sheet having excellent ductility, hot rolled steel sheet, and cast steel ingot - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having excellent ductility of the type entirely different from heretofore. <P>SOLUTION: The cold rolled steel sheet having the excellent ductility contains 0.001 to 0.3% C, (in terms of mass % and hereinafter the same), 0.05 to 2.5% Si, 0.1 to 3% Mn, ≤0.02% P, ≤0.02% S, and ≤0.02% N, contains ≤0.002% solid solution Al, and the balance Fe and inevitable impurities, wherein Mn and Si satisfy the following formula (1) and oxide-based inclusions of ≥5 μm in minor axis containing at least one kind selected from the group consisting of MnO, SiO<SB>2</SB>, and Al<SB>2</SB>O<SB>3</SB>are below 35 pieces per 1 cm<SP>2</SP>. [Mn]<SP>2</SP>/[Is]≥2 (1). In the formula, [Mn] represents the concentration of Mn in the steel and [Si] represents the concentration of Si in the steel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a cold-rolled steel sheet rich in ductility (particularly stretch flangeability), and a hot-rolled steel sheet and a cast steel ingot useful for producing the same.

As cold-rolled steel sheets excellent in ductility, for example, Ti-containing cold-rolled steel sheets excellent in stretch flangeability and deep drawability are known (Patent Documents 1 to 3, etc.). For example, Patent Document 1 discloses a technique for improving the stretch flangeability by reducing the starting point of cracks during hole expansion molding. In Patent Document 1, the size of oxide inclusions remaining in steel is disclosed. If the inclusions are made smaller and the composition of inclusions is CaO · REM: 5 to 50 wt%, Ti oxide: 90 wt% or less, and Al ₂ O ₃ : 70 wt% or less, the inclusions are finely dispersed without enlarging the inclusions in clusters. This teaches that stretch flangeability is improved. In Patent Document 1, Ti is the most important component, and it is said that oxide inclusions can be made a predetermined size by Ti deoxidation.

The Patent Document 2, MnO: 20~90%, TiO 2: MnO-Ti oxide inclusions of 80 to 10% is a low-melting liquid phase oxides, if such an oxide, the immersion nozzle It teaches that deposit deposition can be prevented and oxides coarsened by the deposition flow into the mold.

In Patent Document 3, an extremely low C, low Si Ti-containing cold rolled steel sheet (ie, C: 0.0002 to 0.0080%, Si: 0.001 to 0.04%, Mn: 0.05 to 1.0). %, P: 0.001 to 0.050%, S: 0.001 to 0.030%, Solid solution Al: 0.001 to 0.005%, Ti: 0.004 to 0.030%, N: 0.005 to 0.008% steel material), the inclusions include inclusions containing TiO ₂ and MnO in the same manner as the low melting point liquid phase oxide disclosed in Patent Document 2, that is, titanium oxide (TiOx, X = 1.5 to 2.0), manganese oxide (MnO), silicon oxide (SiO ₂ ), and alumina (Al ₂ O ₃ ). With such a composition, it is taught that inclusions have a relatively low melting point, do not generate a hard crystallized phase with a high melting point during cooling, can finely pulverize inclusions by rolling, etc., and can improve drawability. doing. However, in patent document 1, it is necessary to devise a rolling process in order to grind | pulverize inclusions finely.

On the other hand, as cold-rolled steel sheets that do not require Ti, aluminum killed steel sheets (Patent Document 4), steel sheets that reduce the Al concentration in molten steel as much as possible (Patent Document 5), and the like are known. The aluminum killed steel of Patent Document 4 teaches that by adding Mg, cracks generated in the cross section of the punched hole can be made fine and uniform, and the hole expandability can be improved. On the other hand, in the low-Al steel of Patent Document 5, by controlling the Al concentration and O concentration in the molten steel, can generate MnO · Al ₂ O _3, can be suppressed the formation of Al ₂ O ₃ which cause surface defects Teaches.
JP 2000-1748 A Japanese Patent Publication No. 7-47764 JP-A-11-279721 JP 2002-212673 A Japanese Patent Publication No. 7-30384

  The present invention has been made paying attention to the circumstances as described above, and the object thereof is a cold-rolled steel sheet excellent in ductility, which is completely different from conventional ones, and a hot-rolled steel sheet useful for producing the same. And providing a cast ingot.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have shown that the following is remarkably excellent in ductility in Si deoxidized steel instead of Al deoxidation and Ti deoxidation. The headline and the present invention were completed.

That is, the cold-rolled steel sheet rich in ductility according to the present invention has C: 0.001 to 0.3% (meaning mass%, hereinafter the same), Si: 0.05 to 2.5%, Mn: 0.00. 1 to 3%, P: 0.02% or less, S: 0.02% or less, N: 0.02% or less,
Solid solution Al is 0.002% or less and the balance is Fe and inevitable impurities,
The Mn and Si satisfy the following formula (1),
The gist is that the number of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ and having a minor axis of 5 μm or more is 35 or less per 1 cm ² .

[Mn] ² / [Si] ≧ 2 (1)
(In the formula, [Mn] indicates the Mn concentration in the steel, and [Si] indicates the Si concentration in the steel)
The present invention also includes a hot-rolled steel sheet useful for improving ductility, and this hot-rolled steel sheet has the same components as the cold-rolled steel sheet,
The main point is that the number of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ and having a minor axis of 5 μm or more is 100 or less per 1 cm ² .

The present invention also includes a cast steel ingot useful for improving ductility, and this cast steel ingot has the same components as the cold-rolled steel plate,
When measuring the average composition of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ ,
(A) MnO, the sum of SiO ₂ and Al ₂ O _{3 is, MnO, SiO 2, Al 2} O 3, MgO, the total of TiO _2, and CaO, and 80 mass% or more,
(B) MnO, relative to the sum of SiO ₂ and Al ₂ O _3, MnO 20 to 80 wt%,
SiO ₂ is 20 to 70% by mass, and Al ₂ O ₃ is 35% by mass or less.

  According to the present invention, the composition of inclusions is appropriately controlled at the steel ingot stage by controlling the amount of Mn, the amount of Si, the amount of solute Al, etc., so when hot rolling and cold rolling, Inclusions can be made finer, and the ductility of the cold-rolled steel sheet can be significantly increased.

Inclusions are a major cause of hindering the ductility of cold-rolled steel sheets. Ductility can be improved by reducing the inclusions or reducing the number of inclusions. Specifically, oxide inclusions having a minor axis of 5 μm or more (meaning inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O _3, hereinafter the same) are 35 per cm ² . It was found that a cold-rolled steel sheet having a high ductility can be obtained by setting the number to 30 or less (preferably 30 or less, more preferably 25 or less. The number of oxide inclusions having a minor axis of 5 μm or more is small. The lower limit is not particularly limited, but may be, for example, about 1 or more (particularly 5 or more).

Further, in order to produce the cold-rolled steel sheet as described above under normal cold rolling conditions, at the stage of the hot-rolled steel sheet, 100 or less oxide inclusions having a minor axis of 5 μm or more per 1 cm ² (preferably Is recommended to be 90 or less, more preferably 80 or less. The lower limit of the number of oxide inclusions having a minor axis of 5 μm or more is desirably as small as possible, but the lower limit is not particularly limited, but may be, for example, about 10 or more (particularly 30 or more).

Furthermore, in order to produce the hot rolled steel sheet as described above under normal hot rolling conditions, the oxide inclusions in the steel ingot are MnO—SiO ₂ inclusions or MnO—SiO ₂ —Al ₂ O ₃ series. Inclusion is recommended. There may be many coarse inclusions (minor axis of 5 μm or more) in the steel ingot. For example, 100 or more (particularly 200 or more), 700 or less (preferably 600 or less) per 1 cm ^2. Further, it may be about 550 or less.

  This will be described in more detail below.

  Conventionally, inclusions are mainly generated during the deoxidation process in the molten steel stage. Therefore, in order to improve ductility, it is necessary to suppress the amount of inclusions generated or to coarsen the inclusions generated in the molten steel. Often finely dispersed.

  On the other hand, in the present invention, attention is not paid to the amount of inclusions and fine dispersion at the molten steel stage (and the steel ingot stage) (as is clear from the examples described later, rather, the steel ingot stage is coarse. If the inclusion composition is prepared as described above after making the Si deoxidized steel, an oxide is formed by the rolling when the steel ingot is hot-rolled and cold-rolled. It was found that the system inclusions were stretched, and the stretched inclusions were broken (crushed), and the inclusions were remarkably refined in the finally obtained cold-rolled steel sheet.

FIG. 1 is a conceptual diagram showing the state of inclusions in a steel ingot, hot-rolled steel sheet, and cold-rolled steel sheet. In steel materials to which Al is added, such as Al deoxidized steel, the main component of inclusions is Al ₂ O ₃ , and as shown in the upper (I) of FIG. However, the oxide inclusions are MnO-SiO ₂ inclusions or MnO-SiO ₂ -Al ₂ O ₃ inclusions after Si-deoxidized steel. As shown in the lower part (II) of FIG. 1, the inclusions are elongated and elongated by hot rolling, and the elongated inclusions are extremely finely broken by cold rolling and are coarse (for example, the minor axis is 5 μm or more). It was found that the inclusions drastically decreased.

FIG. 2 shows an ingot stage, a hot-rolled steel sheet stage, and a cold-rolled stage when Al ₂ O ₃ inclusions, MnO—SiO ₂ inclusions, or MnO—SiO ₂ —Al ₂ O ₃ inclusions are used. It is a figure which shows the change of the coarse inclusion in this steel plate stage (in this example of illustration, a minor axis is 2 micrometers or more). As is apparent from FIG. 2, MnO—SiO ₂ inclusions and MnO—SiO ₂ —Al ₂ O ₃ inclusions are more coarse at the steel ingot stage than Al ₂ O ₃ inclusions. However, it can be seen that it is remarkably miniaturized by hot rolling and cold rolling.

More specifically, it is recommended that the average composition of oxide inclusions satisfies the following relationships (a) and (b) in the steel ingot stage.
(A) MnO, the sum of SiO ₂ and Al ₂ O _{3 is, MnO, SiO 2, Al 2} O 3, MgO, the total of TiO _2, and CaO, 80% by mass or more.
(B) MnO, relative to the sum of SiO ₂ and Al ₂ O _3, MnO 20 to 80 wt%,
SiO ₂ 20 to 70 wt%, Al ₂ O ₃ is not more than 35 wt%.

The relationship (a) means that the inclusion is substantially composed of MnO, SiO ₂ , Al ₂ O _3, etc. For example, the sum of MnO, SiO ₂ and Al ₂ O ₃ is 100 The mass% may be sufficient, and it is easy to set it to 100 mass% at the laboratory level. However, in actual melting, it is affected by impurities in a container such as a crucible or an alloy added for component adjustment, and for example, one or more of MgO, TiO ₂ , CaO, and the like may be inevitably mixed. . Thus MnO, the sum of SiO ₂ and _{Al 2 O 3, MnO, SiO} 2, Al 2 O 3, MgO, the total of TiO _2, and CaO, 80 wt% or more (preferably 85 wt% or more, further Preferably it was 90 mass% or more, and especially 95 mass% or more.

In the relationship (b), in terms of the ratio of MnO, SiO ₂ and Al ₂ O ₃ , inclusions substantially correspond to MnO—SiO ₂ inclusions or MnO—SiO ₂ —Al ₂ O ₃ inclusions. Is meant to do. Preferably, MnO: 25 to 75 wt%, SiO _2: 25 to 65 wt%, Al ₂ O _3: 30 wt% or less, more preferably MnO: 30 to 70 wt%, SiO _2: 30 to 60 wt%, Al ₂ O ₃ : 25% by mass or less.

In order to make the inclusion composition within the above range, it is important to deoxidize Si instead of deoxidizing Al. Doing Al deoxidation, 0.002% solid solution Al content in steel super, and the inclusions mainly composed of _{Al 2 O 3 (Al 2 O} 3 inclusions) are produced, the upper of FIG 1 As shown in (I), inclusions do not become finer. Therefore, the amount of dissolved Al is 0.002% or less, preferably 0.0015% or less, more preferably 0.0010% or less.

In order to make the inclusion composition within the above range, it is also important to appropriately control the Si concentration (may be expressed as [Si]) and the Mn concentration (may be expressed as [Mn]) of the steel material. It is. When Si is excessively compared to Mn, inclusions composed mainly of SiO ₂ (SiO ₂ inclusions) will be generated. The stretching and fracture behavior of SiO ₂ inclusions in hot rolling and cold rolling is the same as that of the Al ₂ O ₃ inclusions, and the ductility of the cold rolled steel sheet cannot be increased. Therefore, the ratio [Mn] ² / [Si] is set to 2 or more, preferably 2.3 or more, more preferably 2.5 or more. The upper limit of the ratio [Mn] ² / [Si] is naturally determined from the upper limit of Mn concentration and the lower limit of Si concentration, which will be described later, and may be about 180, for example, about 60 (particularly about 30). There are many.

  In order to make the inclusion composition within the above range, it is also important to control the dissolved oxygen concentration in the molten steel before casting the steel ingot. Even if Al is not substantially used as Si deoxidized steel, if the dissolved oxygen concentration is low, Al contained in the container refractory that receives the molten steel will be eluted into the molten steel, and the Al ratio in the inclusions Becomes higher. Moreover, the amount of solid solution Al in steel may become larger than the said range. It is recommended that the dissolved oxygen concentration be 0.0005% or more, for example. On the other hand, if the dissolved oxygen concentration is too high, the number of coarse oxide inclusions becomes too large, and it is difficult to sufficiently reduce even if hot rolling and cold rolling are performed. Therefore, it is recommended that the dissolved oxygen concentration be 0.005% or less, for example.

  In addition, the component of Si deoxidation steel made into the object of this invention is as follows.

C: 0.001 to 0.3%
In order to improve ductility (especially stretch flangeability), the smaller the amount of C, the better. However, if the amount is too small, the strength is significantly reduced. Therefore, the C amount is 0.003% or more, preferably 0.004% or more. In the present invention, the ductility (stretch flangeability) is remarkably enhanced by appropriate inclusions, so even if the amount of C is increased to some extent, high ductility (stretch flangeability) comparable to the case of low C is achieved. Can be achieved. However, if C becomes excessive, ductility (stretch flangeability) starts to decrease. Accordingly, the C content is 0.3% or less, preferably 0.25% or less, more preferably 0.20% or less, and particularly 0.18% or less.

Si: 0.05 to 2.5%
Si is not only used as a deoxidizing element for molten steel, but is an indispensable element for controlling the amount and composition of inclusions. When there is too little Si, deoxidation will become inadequate. Therefore, the Si content is 0.05% or more, preferably 0.06% or more. On the other hand, when Si is excessive, SiO ₂ inclusions are easily generated. Accordingly, the Si content is 2.5% or less, preferably 2.3% or less, more preferably 2.0% or less, and particularly 1.5% or less.

Mn: 0.1 to 3%
Mn, together with Si, is an important element for controlling the inclusion composition. When Mn is too small it tends to generate the SiO ₂ inclusions. Therefore, the amount of Mn is 0.1% or more, preferably 0.3% or more, more preferably 0.5% or more. On the other hand, when Mn is excessive, Mn is segregated at the time of casting and workability is likely to deteriorate. Therefore, the amount of Mn is 3% or less, preferably 2.8% or less.

P: 0.02% or less P has an effect of strengthening steel, but reduces ductility due to embrittlement. Therefore, the P content is 0.02% or less, preferably 0.017% or less, more preferably 0.015% or less. The lower limit of P is not particularly limited, but may be, for example, about 0.0005% or more (particularly 0.001% or more).

S: 0.02% or less S is more preferable as it is less generated because it produces sulfide inclusions and deteriorates workability and weldability. Therefore, the S content is 0.02% or less, preferably 0.015% or less, more preferably 0.0010% or less. The lower limit of the amount of S is not particularly limited, but may be, for example, about 0.0005% or more (particularly 0.001% or more).

N: 0.02% or less N has an effect of strengthening steel, but when it is excessive, workability is deteriorated. Therefore, the N content is 0.02% or less, preferably 0.017% or less, more preferably 0.015% or less. The lower limit of the N amount is not particularly limited, but may be, for example, about 0.0005% or more (particularly 0.001% or more).

  The balance is Fe and inevitable impurities.

  Since the cold-rolled steel sheet obtained as described above is excellent in ductility (particularly stretch flangeability), it can be advantageously used in various applications utilizing this characteristic.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental Example The following experiment was performed at the laboratory level. That is, 150 kg of electrolytic iron was melted in a high frequency induction melting furnace, and alloyed iron was added thereto to adjust the composition shown in Table 1 below. The molten steel was cast into a mold having a diameter of 220 mm and a height of 500 mm, and then forged into a 155 mm square. Hot-rolled to a thickness of 3 mm under normal conditions (rolling rate = 98%), then cold-rolled to a thickness of 1.7 mm under normal conditions (rolling rate = 45%) to produce a cold-rolled steel sheet .

The size and minor axis of oxide inclusions (inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ ) in the ingot, hot-rolled steel sheet and cold-rolled steel sheet are 5 μm. Average composition of oxide inclusions [(ΣA _i × a _i ) / Σa _i ; A is a composition of oxide inclusions having a minor axis of 5 μm or more. a is the area of oxide inclusions] measured by EPMA (Electron Probe X-ray Microanalyzer), and a test piece (plate) was taken from the cold-rolled steel sheet, and the stretch flangeability was evaluated as follows.

[Stretch flangeability]
A test piece (plate) was punched to form a hole having a diameter D ₀ (D ₀ = 10 mm). Push the hole by inserting the conical punch vertex angle 60 ° to the punched hole, the diameter of the hole when cracks generated around the hole penetrating the plate in the thickness direction was measured D _f, the following formula Based on this, the critical hole expansion rate λ (%) was calculated.

λ (%) = {(D _f −D ₀ ) / D ₀ } × 100
The results are shown in Table 1 below.

As is apparent from Table 1, No. 10, no. 13, no. In example 16, since Si is excessive compared to Mn, the SiO ₂ concentration of inclusions becomes too high, and the inclusion cannot be refined by hot rolling and cold rolling. As a result, stretch flangeability ( The hole expansion rate has decreased. No. In the example 14, the dissolved oxygen concentration of the molten steel is too low, so that Al ₂ O ₃ is easily eluted from the refractory, and the Al ₂ O ₃ concentration in the inclusion becomes too high. As a result, hot rolling and cold rolling are performed. Thus, the inclusions cannot be refined, and stretch flangeability (hole expansion rate) is lowered. No. In the examples of 12, 15, 17 and 18, not only the dissolved oxygen concentration of the molten steel is too low, but also the solute Al concentration becomes too high, and the Al ₂ O ₃ concentration in the inclusions becomes extremely high, resulting in hot rolling. And the inclusion cannot be refined by cold rolling, and stretch flangeability (hole expansion rate) is lowered. No. In the example 11, since the dissolved Al concentration is too high and the dissolved oxygen concentration of the molten steel before casting is too high, the Al ₂ O ₃ concentration of inclusions is too high and the amount of coarse inclusions is too large. As a result of the inclusions not being sufficiently refined by hot rolling and cold rolling, stretch flangeability (hole expansion rate) is reduced.

  On the other hand, no. In the examples 1 to 9, since the steel components and the melting method are appropriate, the inclusion composition is appropriate, the inclusion can be refined by hot rolling and cold rolling, and stretch flangeability (hole expansion) Rate) has improved.

FIG. 1 is a conceptual diagram for explaining the state of refinement of oxide inclusions. FIG. 2 is a graph showing the number of oxide inclusions having a minor axis of 2 μm or more.

Claims (4)

C: 0.001 to 0.3% (meaning mass%, hereinafter the same),
Si: 0.05 to 2.5%,
Mn: 0.1 to 3%
P: 0.02% or less,
S: 0.02% or less,
N: 0.02% or less,
The balance of solute Al being 0.002% or less is a cold-rolled steel sheet with Fe and inevitable impurities,
The Mn and Si satisfy the following formula (1),
A cold-rolled steel sheet rich in ductility, characterized in that the number of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ and having a minor axis of 5 μm or more is 35 or less per 1 cm ^2. .
[Mn] ² / [Si] ≧ 2 (1)
(In the formula, [Mn] indicates the Mn concentration in the steel, and [Si] indicates the Si concentration in the steel) C: 0.001 to 0.3%,
Si: 0.05 to 2.5%,
Mn: 0.1 to 3%
P: 0.02% or less,
S: 0.02% or less,
N: 0.02% or less,
The balance where the solid solution Al is 0.002% or less is a hot rolled steel sheet with Fe and inevitable impurities,
The Mn and Si satisfy the above formula (1),
Useful for improving ductility, characterized in that the number of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ and having a minor axis of 5 μm or more is 100 or less per 1 cm ^2. Hot rolled steel sheet. C: 0.001 to 0.3%,
Si: 0.05 to 2.5%,
Mn: 0.1 to 3%
P: 0.02% or less,
S: 0.02% or less,
N: 0.02% or less,
The balance where the solid solution Al is 0.002% or less is a cast steel ingot with Fe and inevitable impurities,
The Mn and Si satisfy the above formula (1),
When measuring the average composition of oxide inclusions containing at least one selected from MnO, SiO ₂ and Al ₂ O ₃ ,
(A) MnO, the sum of SiO ₂ and Al ₂ O _{3 is, MnO, SiO 2, Al 2} O 3, MgO, the total of TiO _2, and CaO, and 80 mass% or more,
(B) MnO, relative to the sum of SiO ₂ and Al ₂ O _3, MnO 20 to 80 wt%,
Useful cast ingot to improve the ductility, characterized in that SiO ₂ is 20 to 70 wt%, Al ₂ O ₃ is not more than 35 wt%.   The method for producing a cast steel ingot according to claim 3, wherein the dissolved oxygen concentration in the molten steel before casting is controlled within a range of 0.0005 to 0.005%.

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