JP2003342683A - High-strength hot rolled steel sheet excellent in press formability and blanking workability and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot rolled steel sheet excellent in press formability and blanking workability and its production method. <P>SOLUTION: This high-strength hot rolled steel sheet comprises 0.01-0.30% C, 0.01-2.0% Si, 0.5-3.0% Mn, 0.03%≥P, 0.009%≥S, 0.010%≥N, 0.002-0.70% Al, 0.03-0.40% Ti, and the balance being Fe and unavoidable impurities, contains Ti nitride particles with a particle size of 2.0 μm or more in a number of 160 or less per mm<SP>2</SP>, and has a ferrite-bainite structure mainly comprising ferrite. In the production method for this steel sheet, hot rolling is conducted after such heating that the highest heating temperature of steel is 1,200°C or higher and the heating temperature of 1,100°C or higher is kept for 300 min or less. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mainly applied to automobile underbody parts and the like to be pressed, and is 1.0 to 6.0 mm.
The present invention relates to a high-strength hot-rolled steel sheet having a good plate expandability with a moderate plate thickness, excellent press formability without cracks in punching and punching workability, and a method for producing the same.

[0002]

2. Description of the Related Art High-strength hot-rolled steel sheets used in automobiles and the like are required to have excellent press formability. As means for improving these properties, for example, a steel structure is
There are methods such as a ferrite / martensite structure, a bainite-based structure, a ferrite-based structure, and a ferrite / bainite structure, and a method of reducing S in steel and controlling the sulfide morphology by Ca and REM.

Further, Japanese Patent No. 3233743 discloses C
A method of improving the hole expandability by defining the amount of Ti relative to is disclosed. Although press formability can be improved by adding Ti in this way, when cutting (punching) a high-strength hot-rolled steel sheet containing Ti into a blank shape, roughening of the end surface easily occurs, and this roughening It often develops into cracks perpendicular to the thickness. Therefore, it is not provided with the characteristics that can sufficiently cope with the further weight reduction of parts and the complicated shape as in today's automobiles. Therefore, the conventional high-strength hot-rolled steel sheet has not been sufficiently satisfactory for applications such as underbody parts that require high press formability and punching workability.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above-mentioned conventional problems, and prevents deterioration of press formability and punching workability due to high strength, and press formability. And a high-strength hot-rolled steel sheet excellent in punching workability and a method for manufacturing the same.

[0005]

[Means for Solving the Problems] The high-strength hot-rolled steel sheet excellent in press formability and punching workability of the present invention made in order to solve the above-mentioned problems, in mass%, C: 0.01 to 0.30%, S
i: 0.01 to 2.0%, Mn: 0.5 to 3.0%, P ≤ 0.03%, S ≤
0.009%, N ≤ 0.010%, Al: 0.002 to 0.70%, Ti: 0.03
~ 0.40%, the balance is iron and unavoidable impurities, Ti-based nitride particles with a particle size of 2.0 μm or more per square mm
The feature is that the number is 160 or less.

The high-strength hot-rolled steel sheet described above has a Nb content of 0.
It can contain 01 to 0.10%, Ca: O.OOO5 to 0.0100%
And REM: O.OOO5 to 0.0100%, or both, and Mo: 0.01 to 0.5%, V: 0.01 to
0.2%, Zr: 0.01 to 0.2%, Cr: 0.01 to 2.0%, Cu: 0.2
.About.2.0% and Ni: 0.1 to 1.5% can be contained alone or in combination. The high-strength hot-rolled steel sheet preferably has a ferrite bainite structure mainly composed of ferrite and a strength of 780 N / mm ² or more.

The method for producing a high-strength hot-rolled steel sheet excellent in press formability and punching workability of the present invention produces a high-strength hot-rolled steel sheet excellent in press formability and punching workability as described above. When heating the steel with the above composition, the maximum heating temperature is 12
And 00 ° C. or higher, after and heated 1100 ° C. or more holding time as less 300 minutes, characterized in that hot rolling in the manufacturing process, the hot rolling finishing temperature Ar ₃
Hot rolling at a transformation point of ~ 950 ° C, followed by cooling to 650 ~ 800 ° C at a cooling rate of 20 ° C / sec or more, then 2 ~ 15
Air-cooled for 2 seconds, and 350 to 60 at a cooling rate of 20 ° C / sec
It is desirable to cool to 0 ° C and wind.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of intensive studies to solve the above problems, the present inventors have found that among Ti-based nitrides, the particle size is
The inventors have found that by preventing a large amount of large particles exceeding 2 μm from being formed, the press formability can be favorably maintained and the punching workability can be improved, and the above problems have been solved. In this specification, the Ti-based nitride is
It is a general term for TiN and a Ti composite compound containing TiN, and Ti (CN) is exemplified as the Ti composite compound.

In the present invention, C in the high strength hot rolled steel sheet is
0.01 to 0.30%. C is an element necessary to secure the strength by precipitating carbides, and if less than 0.01%, it becomes difficult to secure the desired strength. On the other hand, if it exceeds 0.30%, the ductility is greatly reduced and the punching workability is deteriorated.

Si is an element useful as a deoxidizing agent, and is important for suppressing the formation of harmful carbides and forming a composite structure of mainly bainite with ferrite as the main component. The press formability and ductility can be improved. To obtain such an effect, addition of 0.01% or more is necessary. However, if the amount of addition increases, the chemical conversion treatability deteriorates and the spot weldability also deteriorates, so the upper limit is 2.0%. The range of Si is 0.9-
It is desirable to set it to 1.2% because it can effectively have both press formability and ductility.

Mn is an element necessary to secure the strength, and for this purpose, 0.5% or more is required to be added. But 3.0
%, If added in a large amount, micro-segregation and macro-segregation are likely to occur, deteriorating press formability. In addition,
It is desirable to add Mn in an amount of 1.0 to 1.5% because high strength can be secured and press moldability can be improved.

Since P forms a solid solution in ferrite and reduces its ductility, its content is made 0.03% or less. Also, S
Since MnS forms MnS and acts as a starting point of fracture and markedly deteriorates press formability and punching workability, it is set to 0.009% or less. In addition, N forms a nitride with Ti to increase the amount of TiN produced, which deteriorates punching workability and can combine with C.
Since it becomes difficult to secure the strength by reducing the amount of Ti, 0.01
0% or less.

Al is an element effective as a deoxidizing agent, and is an element effective for forming a composite structure mainly composed of ferrite and the balance bainite like Si, but when it is used as a deoxidizing agent, its content is 0.002% or more. Requires addition. On the other hand, if it exceeds 0.70%, the cleanliness of steel will decrease. Therefore, Al
The range is 0.002 to 0.70%.

[0014] Ti is a fine grain of Ti
It is an important element in the present invention because it precipitates C to secure the strength and improves the press formability. Ti is 0.
If it is less than 03%, it is difficult to obtain the above-mentioned effects. On the other hand, if Ti exceeds 0.40%, a large amount of Ti-based carbides are generated, the ductility decreases, and the TiN production increases. The punching workability deteriorates. Therefore, the range of Ti is 0.03 ~
0.40%

Like Ti, Nb is an element useful for refining crystal grains and precipitating fine carbide such as NbC to secure strength. To do this, add Nb 0.01 to 0.10
% Addition is desirable. This is because if Nb is less than 0.01%, the strength cannot be sufficiently increased, and if Nb exceeds 0.10%, a large amount of precipitates are formed, the ductility is lowered and the punching workability is deteriorated.

Further, Ca and REM (rare earth elements) are elements effective for controlling the form of sulfide inclusions and improving press formability. In order to make this morphological control effect effective, Ca,
It is desirable to add 0.0005% or more of either one or both of REM. On the other hand, addition of a large amount leads to coarsening of sulfide inclusions, deteriorating cleanliness and reducing punching workability, so the upper limit is preferably made 0.0100%.

In the present invention, Mo:
0.01 to 0.5%, V: 0.01 to 0.2%, Zr: 0.01 to 0.2%, C
One or more of r: 0.01 to 2.0%, Cu: 0.2 to 2.0%, and Ni: 0.1 to 1.5% can be added to the steel.
Here, Mo is an element effective in enhancing the hardenability of the steel and increasing the strength of the hot rolled steel sheet, and in order to exert this effect, addition of 0.01% or more is required. But 0.5%
If it is added in excess, the effect will be saturated, and since Mo is an expensive element, the manufacturing cost will rise. Therefore, the amount of Mo is
It is desirable to set it to 0.01 to 0.5%.

Cr is also an element for improving hardenability, and in order to exert this effect, addition of 0.01% or more is required. However, addition of more than 2.0% not only saturates the effect but also increases the cost, so the Cr content is preferably 0.01 to 2.0%.

Ni is also an element for improving hardenability, and in order to exert this effect, it is necessary to add 0.1% or more. However, addition of more than 1.5% not only saturates the effect but also increases the cost, so the range of Ni is preferably 0.1 to 1.5%.

Similar to Ni, Cu is also effective for enhancing the hardenability of steel and strengthening the hot-rolled steel sheet, but in order to exert this effect, addition of 0.2% or more is required. But,
The effect not only saturates when added over 2.0%,
The hot ductility is lowered and the occurrence of surface defects becomes remarkable. Therefore, the range of Cu is preferably 0.2 to 2.0%.

V, like Nb, is effective in precipitating fine carbides to enhance the strength of the hot-rolled steel sheet, and in order to exert this effect, addition of 0.01% or more is required. But,
The effect is saturated even if added over 0.2%, so the range of V is preferably 0.01 to 0.2%.

Zr is also effective for increasing the strength of hot-rolled steel sheets by precipitating fine carbides like Ti, and it is necessary to add 0.01% or more in order to exert this effect. However, the effect saturates even if added over 0.2%,
The range of Zr is preferably 0.01 to 0.2%.

A steel having the chemical composition as described above is prepared by a converter or the like, and then a slab is formed by continuous casting or the like. The slab is heated and hot rolled to produce a high strength hot rolled steel sheet. However, in order to make the high-strength hot-rolled steel sheet have both excellent press formability and punching workability, it is necessary to keep the number of hard and large Ti-based nitrides small. That is, the number of Ti-based nitrides having a particle size (circular equivalent particle size) of 2.0 μm or more is 16 per square mm.
If the number exceeds 0, cracks tend to occur starting from the Ti-based nitride. Therefore, it is necessary to perform hot rolling so that the number of Ti-based nitrides having a particle size of 2.0 μm or more is 160 or less per 1 mm 2.

The steel structure of the high-strength hot rolled steel sheet is preferably a ferrite bainite structure containing 80% or more of ferrite. By making the steel structure a ferrite bainite structure in which ferrite is 80% or more, a high strength hot rolled steel sheet having good hole expandability and ductility can be obtained.
The amount of bainite is set to 20% or less because if the amount of bainite is larger than this, the ductility is greatly reduced.

When manufacturing the above high-strength hot rolled steel sheet by hot rolling, it is necessary to set the maximum heating temperature of the steel to 1200 ° C. or higher and the heating time of 1100 ° C. or higher to 300 minutes or less. If the maximum heating temperature of steel is less than 1200 ° C, coarse TiC that has precipitated during cooling of the slab manufactured in continuous casting cannot be melted into the steel, and strengthening and press formability of the steel are not possible. Fine TiC that is effective for improvement cannot be precipitated. In addition, when the heating time at 1100 ° C or higher becomes longer than 300 minutes, small Ti-based nitrides grow due to coalescence, etc., and their size is increased to a large Ti-based nitriding particle size exceeding 2 μm. 16 items per square mm
Because it will exceed 0.

In hot rolling, the finish rolling end temperature is
If it is less than the Ar ₃ transformation point, excessive formation of ferrite cannot be suppressed and press formability deteriorates. However, if the finish rolling end temperature becomes higher than 950 ° C, the strength and ductility will be deteriorated due to the coarsening of the structure. Therefore, the finish rolling end temperature is from the Ar ₃ transformation point to 950.
It is desirable to set the temperature to ° C.

Further, rapid cooling of the steel sheet immediately after the completion of rolling is preferable in order to obtain high press formability, and the cooling rate is preferably 20 ° C./sec or more. This is because if it is less than 20 ° C./sec, it becomes difficult to suppress the formation of a carbide that is harmful to press formability.

It is desirable to temporarily stop the rapid cooling of the steel sheet and perform air cooling in order to precipitate ferrite and increase its occupancy rate and improve ductility. However, if the air cooling start temperature is less than 650 ° C, pearlite, which is harmful to press formability, is generated from an early stage. On the other hand, when the air cooling start temperature is higher than 800 ° C, not only the ferrite formation is slow and it is difficult to obtain the effect of air cooling, but also pearlite is likely to be generated during the subsequent cooling. Therefore, it is desirable to set the air cooling start temperature to 650 to 800 ° C. Moreover, if the air cooling time is less than 2 seconds, ferrite cannot be sufficiently precipitated, while if the air cooling time exceeds 15 seconds, not only the increase in ferrite saturates, but also the cooling rate and coiling temperature are controlled thereafter. Will be overloaded. Therefore, it is desirable to set the air cooling time to 2 to 15 seconds.

After air cooling, the steel sheet is rapidly cooled again, and the cooling rate is preferably 20 ° C./sec or more. 20 ° C / sec
This is because if it is less than 1, harmful pearlite is easily generated. And the stop temperature of this rapid cooling, that is, the winding temperature is 350
It is desirable to set it to ~ 600 ° C. This is because if the coiling temperature is lower than 350 ° C, hard martensite, which is harmful to the hole expandability, is generated, while if it exceeds 600 ° C, pearlite and cementite, which are harmful to press molding, are easily generated.

By combining the above chemical components and rolling conditions, a high-strength hot-rolled steel sheet having a strength of 780 N / mm ² or more and excellent in press formability and punching workability can be manufactured. Even if the surface of the high-strength hot-rolled steel sheet of the present invention is subjected to a surface treatment (for example, zinc plating), the effect of the present invention can be obtained and is within the technical scope of the present invention.

[0031]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted in a converter and made into slabs by continuous casting.
The slab is heated under the heating conditions shown in and hot-rolled.
High-strength hot-rolled steel sheets of 6 to 3.2 mm were manufactured. The hot rolling finish temperature was set to 895 to 930 ℃ and hot rolling was performed.
Cool to 675 to 750 ° C at a cooling rate of ~ 85 ° C / sec, air cool for 3 to 10 seconds, then cool to 400 to 550 ° C at a cooling rate of 52 to 85 ° C / sec, wind, and cool. A strong hot rolled steel sheet was manufactured.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

The hot-rolled steel sheet thus obtained was subjected to structure observation, tensile test of as-rolled material by JIS No. 5 test piece, hole expansion test for evaluating press formability, and punching test. The steel structure was observed with an optical microscope after corrosion with Nital. In addition, when measuring Ti-based nitrides, up to alumina polishing was carried out in the same manner as for observing the structure of steel, and the total visual field range was 1 m at the magnification of an optical microscope × 1500 without corroding nital.
The Ti-based nitride was measured until it reached m ^2, and the equivalent circle diameter was determined by image analysis. The hole expansion test is the initial hole diameter (d ₀ : 10 mm)
The punched hole of is expanded with a 60 ° conical punch, and the hole expansion value (λ value) is calculated from the hole diameter (d) when the crack penetrates the plate thickness.
= (Dd ₀ ) / d ₀ × 100 was obtained and evaluated. In the punching test, using a 12mmφ punch, the clearance is 20%.
Each of the three pieces was punched out under the conditions (total length 113 mm), and the length of cracks occurring in the fracture surface perpendicular to the sheet thickness direction was measured. When the lengths of cracks exceeding 2 mm were summed up, those exceeding 40% with respect to the entire circumference were evaluated as x, and those less than this were evaluated as o. The results are also shown in Tables 2 and 3.

Among the steels shown in Table 1, steel a has Mn higher than the range of the present invention, and steel b has higher C than the range of the present invention and steel c
Is higher than the range of the present invention, steel d is higher than the range of the present invention in Ti, and all other steels have chemical composition within the range of the present invention.

In the test results shown in Tables 2 and 3, the test No. 3, 7, 8, 16, 22, 25, 32, 37,
In all of the 40 types, the holding time at 1100 ° C or higher was longer than 300 minutes, and therefore the number of Ti-based nitrides with a size of more than 2 μm was more than 160 / mm ² , so there were many punching processes. It was inferior in sex. In addition, steel a, b, c,
Test No. In the case of Nos. 47, 48, 49, and 50, the chemical components are out of the range of the present invention as described above, so that either or both of the elongation and the hole expansion value are inferior. Other test Nos. 1
~ 2, 4-6, 9-15, 17-21, 23-24, 2
6-31, 33-36, 38-39, 41-46, all of the chemical composition and the heating conditions of the steel are within the scope of the present invention, and the steel is composed of 80% or more of ferrite and bainite. The obtained ferrite / bainite structure had sufficiently high strength and elongation, and had a high hole expansion value and good punching workability.

[0038]

As described in detail above, the high-strength hot-rolled steel sheet according to the present invention contains C: 0.01 to 0.30%, Si: 0.01 to 2.0%, and Mn:
0.5 to 3.0%, P ≤ 0.03%, S ≤ 0.009%, N ≤ 0.010
%, Al: 0.002 to 0.70%, Ti: 0.03 to 0.40%, the balance being iron and unavoidable impurities, and the Ti-based nitride particles having a particle size of 2.0 μm or more should be 160 or less per square mm. As a result, it is possible to suppress the occurrence of cracks originating from the Ti-based nitride, so that it has both excellent press formability and punching workability. Further, Nb is added to the above hot-rolled steel sheet in an amount of 0.01 to 0.
Even if 10% is contained, a high-strength hot-rolled steel sheet excellent in press formability and punching workability can be obtained. Further, the high-strength hot-rolled steel sheet as described above, Ca: O.OOO5 ~ 0.0100% and REM:
By containing one or both of O.OOO5 to 0.0100%, the form of the sulfide can be controlled to further improve press formability and punching workability. In addition, high strength hot rolled steel sheet, Mo: 0.01 ~ 0.5%, V: 0.01 ~ 0.2%,
Zr: 0.01-0.2%, Cr: 0.01-2.0%, Cu: 0.2-2.0
%, Ni: 0.1 to 1.5% of one or two or more of them are contained to obtain a high-strength hot-rolled steel sheet excellent in press formability and punching workability by optimizing the precipitation amount of steel structure and carbide. Obtainable. Further, when the steel structure is a ferrite bainite structure mainly composed of ferrite, it is possible to obtain a high-strength hot-rolled steel sheet having a strength of 780 N / mm ² or more and excellent in press formability and punching workability. Therefore, the high-strength hot-rolled steel sheet of the present invention can reduce the weight of the vehicle body, integrally mold the parts, and rationalize the working process, and can improve the fuel consumption and reduce the manufacturing cost. Also,
The method for producing a high-strength hot-rolled steel sheet according to the present invention, when producing a high-strength hot-rolled steel sheet as described above, the steel having the above composition has a maximum heating temperature of 1200 ° C. or higher, and a holding time of 1100 ° C. or higher. After heating as 300 minutes or less, it is possible to produce a high-strength hot-rolled steel sheet excellent in press formability and punching workability by suppressing coarsening of Ti-based nitride by hot rolling. In this method, the hot rolling finish temperature is set to Ar
Hot rolling at ₃ transformation points ~ 950 ℃, then 20 ℃ / sec
Cool down to 650-800 ° C at the above cooling rate, and
Air-cool for 15 seconds, and 350-at a cooling rate of 20 ° C / sec or more.
By cooling to 600 ° C and winding, it is possible to obtain an optimum steel structure in which precipitation of excessive carbide is suppressed. Therefore, the method for producing a high-strength hot-rolled steel sheet according to the present invention is of great industrial value because it can economically provide a high-strength hot-rolled steel sheet having excellent press formability and punching workability.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshimitsu Aso             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works (72) Inventor Yuichi Taniguchi             5-3 Tokai-cho, Tokai-shi, Aichi Nippon Steel Corporation             Ceremony Company Nagoya Steel Works F-term (reference) 4K037 EA01 EA05 EA06 EA09 EA11                       EA13 EA15 EA16 EA17 EA18                       EA19 EA20 EA23 EA25 EA27                       EA28 EA31 EA32 EA35 EA36                       EB05 EB07 EB08 EB09 EB11                       FA03 FC00 FD03 FD04 FD05                       FE01 FE02 FE06

Claims (7)

[Claims] 1. C: 0.01-0.30%, Si: 0.01-
2.0%, Mn: 0.5-3.0%, P ≤ 0.03%, S ≤ 0.009%, N
≦ 0.010%, Al: 0.002 to 0.70%, Ti: 0.03 to 0.40%, the balance is iron and inevitable impurities, and the particle size is
A high-strength hot-rolled steel sheet having excellent press formability and punching workability, characterized in that the number of Ti-based nitrides of 2.0 μm or more was 160 or less per 1 mm 2. 2. A high-strength hot-rolled steel sheet containing 0.01 to 0.10% of Nb, which is excellent in press formability and punching workability. 3. Ca: O.OOO5 ~ 0.0100% and REM: O.OOO5 ~
A high-strength hot-rolled steel sheet excellent in press formability and punching workability according to claim 1 or 2, which contains either or both of 0.0100%. 4. Mo: 0.01 to 0.5%, V: 0.01 to 0.2%, Z
r: 0.01 to 0.2%, Cr: 0.01 to 2.0%, Cu: 0.2 to 2.0%,
The high-strength hot-rolled steel sheet having excellent press formability and punching workability according to any one of claims 1 to 3, which contains one or more of Ni: 0.1 to 1.5%. 5. The press formability and punching workability according to any one of claims 1 to 4, wherein the steel structure is a ferrite bainite structure mainly composed of ferrite and has a strength of 780 N / mm ² or more. Excellent high strength hot rolled steel sheet. 6. When producing a high-strength hot-rolled steel sheet excellent in press formability and punching workability according to any one of claims 1 to 5, the steel having the above composition has a maximum heating temperature of 1200 ° C. or higher. A method for producing a high-strength hot-rolled steel sheet excellent in press formability and punching workability, which comprises hot-rolling after heating at a holding time of 1100 ° C. or higher for 300 minutes or less. 7. Hot-rolling is carried out at a hot rolling finish temperature of Ar ₃ transformation point to 950 ° C., followed by 65 at a cooling rate of 20 ° C./sec or more.
The press formability and the punching workability according to claim 6, which are cooled to 0 to 800 ° C., air-cooled for 2 to 15 seconds, and further cooled to 350 to 600 ° C. at a cooling rate of 20 ° C./sec or more and wound up. A method for producing a high-strength hot-rolled steel sheet which is excellent in