JPH05171270A - Production of hot rolled steel plate excellent in stretch-flange formability - Google Patents

Abstract

PURPOSE:To obtain the hot rolled steel plate without deteriorating external appearance characteristic and economical efficiency by subjecting a steel having a specific composition free from Nb and Ti to heavy draft in the latter stage of finish rolling, to rolling at high finishing temp., and to air cooling after rolling. CONSTITUTION:A steel having a composition consisting of, by mass, <=0.01% C, <=0.3% Si, 0.8-2.0% Mn, <=0.1% Al, <=0.01% S, <=0.03% P, and the balance Fe is formed into a slab, which is hot-rolled without delay or after heating up to 1000-1200 deg.C. At this time, rolling is done while regulating the effective strain [epsiloneff = final pass draft(%) + 1/2(draft(%) at the last pass but one) + 1/4(draft(%) at the last pass but two)] to >=40% and the steel is rolled at 860-960 deg.C finishing temp., air-cooled for >=1 sec after rolling, successively cooled at >=10 deg.C/sec average cooling rate, and coiled at <=700 deg.C. By this method, the recrystallization of gamma-grains can be accelerated and in-blane anisotropy can be reduced, by which the hot rolled steel plate excellent in spreadability and used for members for machine structural use, including automobile use, and for steels for general working can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot rolled steel sheet having excellent stretch flangeability, which is used for mechanical structural members such as automobiles and general processing.

[0002]

2. Description of the Related Art In recent years, there is a strong demand for reducing the plate thickness and increasing the strength in order to save energy and reduce fuel consumption. On the other hand, so-called Dual Phase steels and the like have been developed. However, in the case of members with severe stretch flange processing, cracks occur even in these high-strength hot-rolled steel sheets, and thus the applicable materials are currently limited, A steel plate with high strength and good workability is required.

In order to overcome such a situation, a method for producing a hot rolled steel sheet having excellent stretch flangeability is disclosed in JP-A-60-14.
9730 and JP-A-51-44508. However, the method of adding Si for high strength disclosed in JP-A-60-149730 has a problem of poor appearance due to generation of Si scale. Further, the method disclosed in JP-A-51-44508 is economically disadvantageous because it requires addition of Cr.

[0004]

DISCLOSURE OF THE INVENTION As a result of intensive studies, the inventors of the present invention found that there is a strong correlation with the r-value in each direction of stretch-flangeability in an extremely low carbon system which is not affected by the second phase. It was That is, in the hole expansion test for evaluating stretch flangeability, the test ends when the crack penetrates the plate thickness, but the crack occurs at this time in the direction of low r value. In order to improve the minimum r value, it is effective to reduce the difference between the characteristic values in each direction, that is, reduce the in-plane anisotropy. For that purpose, it is important to promote the recrystallization of γ grains.

That is, the present inventors (1) promote recrystallization of γ grains by large reduction in the latter stage of finishing rolling, high finishing temperature and air cooling after rolling. (2) The in-plane anisotropy is reduced by strictly determining the manufacturing conditions based on a method such as not adding Nb or Ti, which has a strong tendency to unrecrystallize γ grains,
We have completed a method for producing hot-rolled steel sheets with excellent hole expandability.

That is, the present invention provides a method for producing a hot-rolled steel sheet having excellent stretch-flange formability without impairing the appearance and economy.

[0007]

The main points of the present invention are as follows. (1) C: 0.01% or less by mass%, Si: 0.3
% Or less, Mn: 0.8 to 2.0%, Al: 0.1% or less, S: 0.01% or less, P: 0.03% or less,
Immediately or after heating to 1000 to 1250 ° C., a slab made of steel consisting of the balance Fe and unavoidable impurities,
In performing hot rolling, the reduction is effective strain [ε _eff = final pass reduction rate (%) + 1/2 (first stage first stage pass reduction rate (%)) +1/4 (final second stage pass reduction) rate(%))〕
At a finishing temperature of 860 ° C. to 960 ° C., followed by air cooling for 1 second or more, and then an average cooling rate of 10%.
Features excellent stretch-flangeability and tensile strength of 350 N / mm, characterized by cooling at ℃ / s or higher and winding at 700 ℃ or lower.
Manufacturing method of ^two or more hot rolled steel sheets.

(2) C in mass%: 0.01% or less,
Si: 0.3% or less, Mn: 0.1 to 2%, Al: 0.
1% or less, S: 0.01% or less, P: 0.03% or less, and Cu: 0.8 to 2.0%, balance Fe
Immediately after heating the steel made of unavoidable impurities into a slab or heating it to 1000 to 1200 ° C. and performing hot rolling, the reduction of the effective strain [ε _eff = final pass reduction (%) + 1/2 ( First-stage pass reduction rate (%) before final +1
/ 4 (the second pass pass reduction rate (%) before final)] at 40% or more, rolling at a finishing temperature of 860 ° C. to 960 ° C., air cooling for 1 second or more after rolling, and then an average cooling rate of 10 ° C. / A method for producing a hot-rolled steel sheet having excellent stretch-flangeability and having a tensile strength of 400 N / mm ² or more, which comprises cooling at s or more and winding at 400 ° C. or less.

(3) C in mass%: 0.01% or less,
Si: 0.3% or less, Mn: 0.1 to 2%, Al: 0.
1% or less, S: 0.01% or less, P: 0.03% or less, and Cu: 0.8 to 2.0%, Ni: 0.3 to
Immediately after the steel containing 1.0% and the balance Fe and unavoidable impurities was made into a slab or 1000 to 1
When performing hot rolling by heating to 250 ° C., the reduction is effective strain [ε _eff = final pass reduction (%) + 1/2 (1 before final)
The finishing temperature is 860 ° C. at the finishing pass reduction rate (%)) + 1/4 (the second-stage rolling reduction rate before the final stage (%)) of 40% or more.
Rolling at ~ 960 ° C., air cooling after rolling for 1 second or more, followed by cooling at an average cooling rate of 10 ° C./s or more and winding at 400 ° C. or less, excellent stretch flangeability, tensile strength of 400 N / A method for manufacturing a hot rolled steel sheet having a size of mm ² or more.

[0010]

Next, the reasons for limiting the constituents of the present invention will be described in detail. C is 0.01% or less. If it exceeds this, carbides are generated, and the stretch flangeability deteriorates.

Mn is an element that imparts strength, and is 0.8
Add in the range of ~ 2.0%. If it is less than the lower limit, the target strength cannot be obtained. In addition, when adding Cu, the lower limit was made into 0.1%. The upper limit was set to a range that can be manufactured in an actual machine while keeping C: 0.01% or less (high technology is required to have a C pickup and to make it a high Mn type and an extremely low C type).

Since Si causes Si scale and deteriorates the chemical conversion treatment property, it is made 0.3% or less.

Al is necessary as a deoxidizer, but 0.1%
If it exceeds, alumina inclusions increase and the stretch flangeability of steel is impaired.

[0014] S increases the amount of A-type inclusions extending in the rolling direction, and cracks propagate from there as a starting point, so the hole expandability decreases. Therefore, the upper limit value is set to 0.01%.

P is an element that imparts corrosion resistance. However, if it exceeds 0.03%, the ductility decreases and the stretch flangeability decreases.

Cu may be added as an element which imparts strength. In that case, less than 0.8% has no effect and 2.0%
If the addition exceeds the range, the effect is saturated and a surface defect called Cu heggling may occur during hot rolling. Further, it is desirable to add Ni to prevent the Cu hegging. 0.3
If it is less than 1.0%, there is no effect, and if it exceeds 1.0%, not only is the effect saturated, but the economy is impaired.

Next, the hot rolling conditions will be described. The above-mentioned steel is usually melted in a converter and made into a slab by continuous casting. After the converter is melted, various secondary refining may be performed. This slab is inserted into the heating furnace as cold, hot or hot pieces.

The heating temperature at this time is 1000 to 1250 ° C.
And The upper limit is a value that can be taken with the current equipment. The lower limit was set to the range where the current continuous hot rolling equipment can operate without lowering productivity. When Cu is added, the upper limit of the heating temperature is 1200 ° C. in order to prevent Cu hegging. Further, when Ni is added within the above range, the upper limit of the heating temperature is set to 1250 ° C. The upper limit is improved by adding Ni, but if it is more than this, it is unavoidable to cause Cu hegging. Similarly, the lower limit is set to a value that can be taken by the current equipment. The rolling reduction is an important condition in the present invention. In particular, since the reduction ratio in the latter stage of finishing is effective, the present inventors have used ε _eff = final pass reduction ratio (%) + 1/2 (first stage first stage pass reduction ratio (%)) + 1/4 (final pre-final stage). The second pass rolling reduction rate (%) is defined as the effective strain. This effective strain is 40% or more. If it is less than this, unrecrystallized γ grains remain and a texture develops, the in-plane anisotropy increases and the stretch-flangeability deteriorates.

The finishing temperature is a very important condition in the present invention. That is, the in-plane anisotropy is reduced by randomizing the texture of the hot rolled steel sheet. Therefore, the finishing temperature is 880 to 960 ° C. If it is less than the lower limit, the in-plane anisotropy increases and the stretch flangeability decreases. The upper limit was set to a value that can be manufactured on an actual machine in consideration of the heating temperature.
After finish rolling, air cool for 1 second or longer. If it is less than this, unrecrystallized γ grains remain, a texture develops, and stretch-flangeability deteriorates.

The average cooling rate after air cooling is 10 ° C./s or more. If it is less than this, the productivity is lowered and the production cost is increased. The winding temperature is 700 ° C or lower. If it exceeds this, cooling will take a lot of time, productivity will fall and production cost will increase. When Cu is added, the coiling temperature is 400 ° C or lower. If it exceeds this, the strength increases due to the precipitation of Cu or the strengthening of clusters, and the strength of 400 N / mm ² class cannot be obtained.

[0021]

EXAMPLE Steels having the components shown in Table 1 were tapped in a converter and subjected to secondary refining such as vacuum degassing to obtain slabs.

[0022]

[Table 1] Steels indicated by symbols A to F in Table 1 are within the scope of the present invention, and steels indicated by G to N are outside the present invention. G steel has C above the upper limit, H steel has Si above the upper limit, I steel has C above the upper limit, and J steel has S.
i is above the upper limit, and for K and L steels, P, Al and S are above the upper limit.
Cu is above the upper limit for M steel, and Ni is below the lower limit for N steel.

These steels were hot rolled under the hot rolling conditions shown in Table 2. The characteristic values of the obtained hot rolled steel sheet are also shown in Table 2. As the tensile test piece, a No. 5 test piece according to JIS Z 2201 was used, and the tensile test was performed according to the method described in JIS Z2241.
The in-plane anisotropy was evaluated by Δr. Δr is the rolling direction,
Measure the r value (10% strain) in the 90 ° and 45 ° directions,
Δr = [{r value in rolling direction + r value in 90 ° direction− (2 ×
The r value in the 45 ° direction)} / 2].

The stretch flangeability was evaluated by the hole expansion ratio in the punching hole expansion test. As the test piece, a 250 mm square steel plate punched out with a hole having a diameter d ₀ (= die diameter) by a die having a 20 mm diameter punch and a die having a clearance (one side) of 10% of the plate thickness was used. In the hole expansion test, the above test piece was punched out by a press tester and expanded with a 30 ° conical punch from the side without the hole burr (opposite to the burr) (at this time, there was no inflow of material into the expanded part). 60 on the flange
Ton's crease is applied), and it is stopped when the crack penetrates the plate thickness, and the ratio (d / d ₀ ) of the hole diameter (d) and the original hole diameter (d ₀ ) at this time is calculated as the hole expansion ratio. And

[0025]

[Table 2]

[Table 3] In Table 2, No. 1-No. No. 6 is the steel of the present invention, which has the strength intended for the present invention and a good hole expansion ratio, does not generate Si scale and Cu heggins, and has a good surface property.

No. 7-No. 18 is a comparative example steel. No.
In No. 7, since C was too high, carbide was generated and the hole expansion ratio was lowered. No. In No. 8, since Si was too high, Si scale was generated and the surface quality was deteriorated. No. In No. 9, since C was too high, carbide was generated and the hole expansion ratio was lowered. No. In No. 10, since Si was too high, Si scale was generated and the surface quality was deteriorated. No. In No. 11, since P, S and Al were too high, inclusions were generated and the hole expansion ratio was lowered. No. In No. 12, since P, S and Al were too high, inclusions were generated and the hole expansion ratio was lowered. N
o. In No. 13, since Cu was too high, Cu hegging was generated and the surface quality was deteriorated. No. In No. 14, since Ni was too low, Cu hegging was generated and the surface quality was deteriorated. No. In No. 15, since the heating temperature was too high, Cu hegging occurred and the surface quality deteriorated. No. In No. 16, since the finishing temperature was too low, the texture of the hot rolled steel sheet developed, the in-plane anisotropy increased, and the hole expansion ratio decreased.

No. In No. 17, since the finishing temperature was too low, the texture of the hot rolled steel sheet developed, the in-plane anisotropy increased, and the hole expansion ratio decreased. No. In No. 18, since the air-cooling time after finishing was short, unrecrystallized γ remained and the texture developed, the in-plane anisotropy increased, and the hole expansion ratio decreased. No. In No. 19, since the effective strain was small, unrecrystallized γ remained, the texture developed, the in-plane anisotropy increased, and the hole expansion ratio decreased.

[0028]

According to the manufacturing method of the present invention, it is possible to provide a hot-rolled steel sheet excellent in stretch-flange formability without impairing its appearance and economical efficiency. Thus, by applying the steel sheet of the present invention to a member that requires severe stretch-flange processing, it is possible to easily reduce the weight of, for example, an automobile, improve fuel efficiency, save resources, and the like, and have an extremely high industrial value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masanori Nishimoto 1 Kimitsu, Kimitsu City Nippon Steel Corporation Kimitsu Steel Works

Claims (3)

[Claims] 1. Mass%: C: 0.01% or less, Si: 0.3% or less, Mn: 0.8 to 2.0%, Al: 0.1% or less, S: 0.01% or less. , P: 0.03% or less, steel made of balance Fe and unavoidable impurities is made into a slab, and immediately or thereafter heated to 1000 to 1250 ° C.,
In performing hot rolling, the reduction is effective strain [ε _eff = final pass reduction rate (%) + 1/2 (first stage first stage pass reduction rate (%)) +1/4 (final second stage pass reduction) rate(%))〕
At a finishing temperature of 860 ° C. to 960 ° C., followed by air cooling for 1 second or more, and then an average cooling rate of 10%.
A method for producing a hot-rolled steel sheet having excellent stretch-flangeability, which comprises cooling at 700 ° C / s or more and winding at 700 ° C or less. 2. Mass% C: 0.01% or less, Si: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1% or less, S: 0.01% or less, P : 0.03% or less, Cu: 0.8 to 2.0%, steel made of balance Fe and unavoidable impurities is made into a slab and then immediately or heated to 1000 to 1200 ° C.,
In performing hot rolling, the reduction is effective strain [ε _eff = final pass reduction rate (%) + 1/2 (first stage first stage pass reduction rate (%)) +1/4 (final second stage pass reduction) rate(%))〕
At a finishing temperature of 860 ° C. to 960 ° C., followed by air cooling for 1 second or more, and then an average cooling rate of 10%.
A method for producing a hot-rolled steel sheet having excellent stretch-flange formability, which comprises cooling at a rate of ℃ / s or higher and winding at a temperature of 400 ℃ or lower. 3. Mass% C: 0.01% or less, Si: 0.3% or less, Mn: 0.1 to 2%, Al: 0.1% or less, S: 0.01% or less, P : 0.03% or less, Cu: 0.8 to 2.0%, Ni: 0.3 to 1.0%, balance steel and unavoidable impurities are used as a slab and then immediately or 1000 to 1250 ° C. Heated to
In performing hot rolling, the reduction is effective strain [ε _eff = final pass reduction rate (%) + 1/2 (first stage first stage pass reduction rate (%)) +1/4 (final second stage pass reduction) rate(%))〕
At a finishing temperature of 860 ° C. to 960 ° C., followed by air cooling for 1 second or more, and then an average cooling rate of 10%.
A method for producing a hot-rolled steel sheet having excellent stretch-flange formability, which comprises cooling at a rate of ℃ / s or higher and winding at a temperature of 400 ℃ or lower.