JP2824136B2 - Hot rolled steel sheet with low plastic anisotropy for processing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a compacted hot-rolled steel sheet for processing having low plastic anisotropy, and more specifically, to mechanical properties, metal structure homogeneity, and isotropy. The present invention relates to a hot-rolled steel sheet having high tensile strength, excellent press formability, a tensile strength of 60 kgf / mm ² or more, a sheet thickness of 3.2 mm or more, and a low plastic anisotropy for processing.

[Prior Art] In recent years, press molding is increasingly used in the production of automobile parts and the like.

2. Description of the Related Art Conventionally, automobile parts and the like manufactured by a method such as hot forging, casting or sintering are manufactured from steel plates by press forming in order to reduce the weight and cost.

Then, in the conventional had been generally performed, tensile strength of a relatively low intensity of 45 kgf / mm ² or less 3.2mm or less in thickness thin Furthermore, the tensile strength 60~80kgf / mm ² With a class of high strength, the tendency to manufacture automotive parts and the like from steel plates about 2 mm thick began to change. That is, a steel plate having a tensile strength of 60 kgf / mm ² or more and a plate thickness of 3.2 mm or more has been used as a material for press forming of automotive parts and the like. Naturally, the large size of the press forming machine and the remarkable improvement of the press pressing ability have made it possible to form high-strength parts with thick materials.

Therefore, it has emerged as a major technical problem that there was no problem with low-strength thin materials until now.

That is, in the case of a thick pressed product which is a high-strength material obtained by adding a reinforcing element to the low-strength thin material, the plastic anisotropy of the material greatly affects the determination of the pressed shape, and isotropic dimensional accuracy and In order to do so, the number of press steps is increased, the number of work steps such as forming a mold into a special shape or material is increased, the work is complicated, and the cost may be significantly increased. all right.

In particular, at the shoulder portion of the drawing process, there is a problem that the production by press molding must be abandoned depending on the processing size of the subsequent process, such as the rising shoulder (R) greatly differs depending on the position.

Therefore, it can be said that it is an urgent need to solve such a problem of the prior art.

[Problems to be Solved by the Invention] In view of the problems that occur during press molding of high-strength thick parts such as automobile parts in the prior art described above, the present inventors have conducted intensive research and studied. As a result, the appropriate selection of the components and proportions of the steel to be used, inevitable impurities, and the microstructure
60 kgf / mm ² or more, plate thickness 3.2 mm or more, extremely low plastic anisotropy during cold working such as press forming, high strength and thick material that can ensure the dimensions and shape during forming isotropically Hot rolled steel sheets were developed.

[Means for Solving the Problems] The hot-rolled thick steel sheet for processing having low plastic anisotropy according to the present invention comprises: (1) 0.15 wt% or less of C, 0.01 to 0.50 wt% of Si, and 0.25 to 1.50 of Mn. wt%, Al 0.01 ~ 0.10wt%, Ca 0.0005 ~ 0.0050wt%, and one or more selected from Nb 0.01 ~ 0.10wt%, V 0.01 ~ 0.10wt%, Ti 0.01 ~ 0.10wt% Containing, the balance being Fe and having a maximum diameter of 1 μm
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less.

However, I = Pa / Bs Pa: Perlite area ratio in the cross section in the steel plate L direction Bs: Number of layered structures per 1 mm in the thickness direction in the cross section in the steel plate L direction is defined as the first invention, and (2) C 0.15 wt% or less , Si 0.01 to 0.50 wt%, Mn 0.25 to 1.50 wt%, Al 0.01 to 0.10 wt%, Ca 0.0005 to 0.0050 wt%, Cu 0.10 to 1.5 wt%, Cr 0.10 to 1.5 wt%, Mo 0.10 Steel containing at least one selected from the group consisting of 〜1.5 wt% and the balance being Fe, with a maximum diameter of 1 μm
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less.

However, I = Pa / Bs Pa: Perlite area ratio in the cross section in the steel plate L direction Bs: Number of layered structures per 1 mm in the thickness direction in the cross section in the steel plate L direction is defined as the second invention, and (3) C 0.15 wt% or less , Si 0.01 to 0.50 wt%, Mn 0.25 to 1.50 wt%, Al 0.01 to 0.10 wt%, Ca 0.0005 to 0.0050 wt%, Nb 0.01 to 0.10 wt%, V 0.01 to 0.10 wt%, Ti 0.01 0.10 wt% or more, and one or more selected from Cu 0.10-1.5 wt%, Cr 0.10-1.5 wt%, Mo 0.10-1.5 wt%, Steel consisting of the balance Fe, with a maximum diameter of 1 μm
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less.

However, I = Pa / Bs Pa: the pearlite area ratio in the cross section in the steel sheet L direction Bs: the number of layered structures per 1 mm in the thickness direction in the cross section in the steel sheet L direction is comprised of three inventions as the third invention. .

BEST MODE FOR CARRYING OUT THE INVENTION The hot rolled steel sheet for processing having small plastic anisotropy according to the present invention will be described in detail below.

First, the components contained in the hot rolled steel sheet for processing having small plastic anisotropy according to the present invention and the component ratio thereof will be described.

C is an element effective for increasing the strength of the steel sheet,
If the content exceeds 0.15% by weight, the effect of weldability and the like after press forming as a steel material for automobiles deteriorates. Therefore, the C content is set to 0.15 wt% or less.

Si is an element necessary both for deoxidation during steelmaking and for improving the strength of steel by solid solution. If the content is less than 0.01 wt%, deoxidation is insufficient, and clean steel cannot be obtained. Also,
When the content exceeds 0.50 wt%, the cold workability is reduced, or so-called Si red scale is easily generated at the time of hot rolling, and the notch effect due to the deterioration of the surface properties of the steel sheet is increased, and the ductility is increased. Decrease. Therefore, the Si content is 0.01 to 0.50
wt%.

Mn is an element that improves hardenability and increases strength, and at the same time, prevents hot brittleness due to S during hot rolling. If the content is less than 0.25 wt%, the strength as a high-strength steel is insufficient.
On the other hand, when the content exceeds 1.50 wt%, the strength becomes too high, and the cold workability decreases with an increase in segregation of Mn during production. Therefore, the Mn content is 0.25 to 1.50 wt%.

Al is at least 0.01wt% as a deoxidizing element
Is necessary, and inclusions exceeding 0.10 wt% increase inclusions. Therefore, the Al content is set to 0.01 to 0.10 wt%.

Ca is an element that imparts the effect of reducing mechanical anisotropy and improving ductility and toughness by controlling the morphology of sulfides from expanded to spherical, and when the content is less than 0.0005 wt%, such No effect can be expected, and 0.
If the content exceeds 0050 wt%, nonmetallic inclusions in steel increase, and ductility and toughness decrease. Therefore, the Ca content is 0.00
05 to 0.0050 wt%.

Nb, V, and Ti are effective elements for increasing the strength of the steel in combination with the hot rolling conditions. If the Nb content, V content, and Ti content are less than 0.01 wt%, sufficient strength is obtained. If it is not obtained, and if the content exceeds 0.10 wt%, the anisotropy of strength increases, and the plastic anisotropy increases. Therefore,
Nb content is 0.01 ~ 0.10wt%, V content is 0.10 ~ 0.10wt
%, And the Ti content is 0.10 to 0.10 wt%.

Cu, Cr, and Mo are elements that increase the strength of steel in a solid solution or precipitation state, and have Cu content, Cr content, and Mo content.
If it is less than 0.10 wt%, sufficient strength cannot be obtained.
%, The cold workability decreases. Therefore, Cu content 0.10-1.5wt%, Cr content 0.10-1.5wt%,
Mo content is 0.10 to 1.5 wt%.

In the hot work rolled steel sheet having a small plastic anisotropy according to the present invention, in order to maintain excellent cold workability, P, as an element other than the components described above,
Since S, O, N, etc. are harmful to the cold workability, it is desirable that they are as low as possible.

Next, the nonmetallic inclusions and the band structure will be described.

Non-metallic inclusions with a maximum diameter of 1μm or more have a density of 10 pieces / m
although for the m ² or less, when it is contained Ca and the density of the non-metallic inclusions of the maximum diameter of more than 1 [mu] m 10 pieces / mm ² or less, the total elongation in the rolling direction (ELL) and perpendicular to the rolling direction of the total The difference in elongation (ElT), ie, △ El = ElL-ElT, is almost gone,
This greatly contributes to the reduction of plastic anisotropy (see FIG. 1 described later).

To reduce non-metallic inclusions with a large maximum diameter, P,
The content of impurity elements such as S, N, O, etc. is reduced as much as possible, and degassing is sufficiently performed by a DH degassing method or a RH degassing method in the steel making process.

By ensuring the uniformity of the microstructure, the anisotropy of the yield strength is reduced, which significantly contributes to the reduction of the plastic anisotropy.

Then, the pearlite area ratio on the measurement surface of the cross section in the rolling direction of the steel sheet is Pa, and the
When the number of sheets per 1 mm length in the thickness direction is Bs, I = Pa / Bs
When the band-structure index I defined by is 10 ⁻⁴ or less, ΔYS = YS T−YS L becomes 1 kgf / mm ² or less, and the plastic anisotropy decreases (YS T: yield strength in the direction perpendicular to rolling, YS L : Yield strength in the rolling direction, see FIG. 2 described later).

In order to keep the band structure index I at 10 ^-4 or less, it is necessary to set the cooling rate of the steel sheet on the hot rolling run-out table to 40 ° C./sec or more and the winding temperature to 550 ° C. or less (described below). (See FIG. 3).

The numbers in FIGS. 1, 2 and 3 indicate the steel numbers in Table 1.

[Example] An example of a hot-rolled thick steel sheet for processing with small plastic anisotropy according to the present invention will be described.

Examples The melting of steel with the components and component ratios shown in Table 1
The slab can be produced by a usual steel making method, but the slab may be produced by any of ingot making, slab rolling and continuous forging.

Steel Nos. 1 to 9 are hot rolled steel sheets for processing having low plastic anisotropy according to the present invention and steel Nos. 10 to 11 are comparative steels.

These steels were subjected to hot rolling under the conditions shown in Table 2 to produce steels having the mechanical properties and internal quality shown in Table 2.

In order to determine the plastic anisotropy of these steel sheets, as shown in FIG.
An overhang of 30 mm in height was performed.

In this case, the steel sheet having the thickness t has a plastic anisotropy and has a hole entrance as shown in FIG. 4 (b) (A-in FIG. 4 (a)).
A ') is a rectangle.

Then, the roundness of the droop shape from the steel plate surface is evaluated by the ratio of ds / dr in FIG. 4 (b).

If ds / dr <1.1, the plastic anisotropy is good, the judgment is ○, ds / dr ≧ 1.1 is inferior, and the judgment is x.

 That is, ds / dr = perfect circle ds / dr <small plastic anisotropy (good) ds / dr ≧ large plastic anisotropy (poor).

[Effects of the Invention] As described above, the hot-rolled thick steel sheet for processing with low plastic anisotropy according to the present invention has the above-described configuration. This has the effect that the shape and dimensional accuracy after cold working are excellent.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the density of nonmetallic inclusions and ΔEl (%), FIG. 2 is a diagram showing the relationship between the band index and ΔYS,
FIG. 3 is a diagram showing the relationship between the cooling rate and the band structure index, and FIG. 4 is a schematic diagram of a plastic anisotropy test device.

Claims (3)

(57) [Claims] C. 0.15 wt% or less, Si 0.01 to 0.50 wt%, Mn 0.25 to 1.50 wt%, Al 0.01 to 0.10 wt%, Ca 0.0005 to 0.0050 wt%, and Nb 0.01 to 0.10 wt% , V 0.01 ~ 0.10wt%, Ti 0.01 ~ 0.10wt%, containing at least one selected from the group consisting of the balance Fe and a maximum diameter of 1μm
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less. Here, I = Pa / Bs Pa: Perlite area ratio in cross section of steel sheet L direction Bs: Number of layered structures per 1 mm in thickness direction in cross section of steel sheet L direction 2. The composition contains 0.15 wt% or less of C, 0.01 to 0.50 wt% of Si, 0.25 to 1.50 wt% of Mn, 0.01 to 0.10 wt% of Al, 0.0005 to 0.0050 wt% of Ca, and 0.10 to 1.5 wt% of Cu. , Cr 0.10-1.5wt%, Mo 0.10-1.5wt%, containing at least one selected from the group consisting of the balance Fe and a maximum diameter of 1 μm
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less. Here, I = Pa / Bs Pa: Perlite area ratio in cross section of steel sheet L direction Bs: Number of layered structures per 1 mm in thickness direction in cross section of steel sheet L direction C. 0.15 wt% or less, Si 0.01 to 0.50 wt%, Mn 0.25 to 1.50 wt%, Al 0.01 to 0.10 wt%, Ca 0.0005 to 0.0050 wt%, and Nb 0.01 to 0.10 wt% , V 0.01 to 0.10 wt%, Ti 0.01 to 0.10 wt%, and one or more selected from Cu 0.10 to 1.5 wt%, Cr 0.10 to 1.5 wt%, and Mo 0.10 to 1.5 wt%. Steel containing at least one selected from the group consisting of
A hot working rolled steel sheet having low plastic anisotropy, characterized in that the density of the nonmetallic inclusions is 10 or less / mm ² or less and the band structure index I is 10 ⁻⁴ or less. Here, I = Pa / Bs Pa: Perlite area ratio in cross section of steel sheet L direction Bs: Number of layered structures per 1 mm in thickness direction in cross section of steel sheet L direction