JPH0747798B2 - Hot-rolled steel sheet excellent in burring property and ductility and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is mainly applied to automobile undercarriage parts and the like to be pressed, and has a plate thickness of about 1.4 to 6.0 mm and 38 kgf / mm ²
The present invention relates to a hot-rolled steel sheet having the above-mentioned tensile strength and excellent in burring property and ductility, and a manufacturing method thereof.

(Prior Art) With the development of petroleum equipment in 1970, increasing the tensile strength of steel sheets for automobiles has been studied for underbody parts represented by wheels and members in hot-rolled steel sheets. The materials used for these are, for example, the hot rolled steel sheet described in Japanese Patent Laid-Open No. 58-11734, which uses a component system in consideration of economic efficiency and spot weldability, and effectively utilizes the hot rolling process. By doing so, the structure is controlled, and the hot rolled steel sheet is characterized in that it does not deteriorate workability while increasing strength. However, most of the steel sheets that have been studied have a tensile strength of 50 kgf / mm ² or more.

On the other hand, as a manufacturing technique relating to burring property and ductility of a hot rolled steel sheet having a tensile strength of 35 kgf / mm ² or more and less than 50 kgf / mm ² , there is a method described in Japanese Patent Publication No. 1-32293. This is C
-Si-Mn-based components are used at a coiling temperature of 600 ° C or higher, which is a manufacturing technology for hot-rolled steel sheets with excellent stretch flange formability and vertical crack resistance. However, in this manufacturing technique, since the winding temperature is set to 600 ° C. or higher, the presence of pearlite or coarse cementite is obvious, and the stretch flangeability is deteriorated. Also, ductility, which is important for workability, is only at a normal level.

(Problems to be Solved by the Invention) As described above, in the actual situation, the technology for maximizing the burring property in the hot rolled steel sheet having the tensile strength of 38 kgf / mm ² or more has not been developed so far. In addition, the present invention is the result of thorough research for developing the technology.

(Means for Solving the Problems) The gist of the present invention is as follows.

(1) C: 0.02 to less than 0.07% by weight Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10%, balance Fe and unavoidable impurities The cementtite with a circle equivalent radius of 0.1 μm or more
A hot-rolled steel sheet excellent in burring property and ductility, characterized by having a martensite structure ratio of 0.1% or less and 1% or less.

(2) C: 0.02 to less than 0.07% by weight Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10% Ca: 0.0005 to 0.0050% The structure ratio of cementite composed of the balance Fe and unavoidable impurities and having a circle equivalent radius of 0.1 μm or more
A hot-rolled steel sheet excellent in burring property and ductility, characterized by having a martensite structure ratio of 0.1% or less and 1% or less.

(3) C: 0.02 to less than 0.07% by weight Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10%, balance Fe and unavoidable After making steel made of impurities into a slab, it is heated to 1000-1200 ℃ and hot-rolled (Ar
₃ transformation point +50) Finishing rolling is completed at a temperature of 950 ° C or more and 950 ° C or less, cooling is done at 50 ° C / s or more within 1 second after finishing rolling, and winding is performed at 350 to 500 ° C. Equivalent radius
A method for producing a hot-rolled steel sheet, wherein the microstructure of cementite having a size of 0.1 μm or more is 0.1% or less and the microstructure of martensite is 1% or less, and which is excellent in burring property and ductility.

(4) C: 0.02 to less than 0.07% by weight Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10% Ca: 0.0005 to 0.0050% , Slab made of steel consisting of balance Fe and unavoidable impurities, heated to 1000-1200 ℃ and hot-rolled (Ar
₃ transformation point +50) Finishing rolling is completed at a temperature of 950 ° C or more and 950 ° C or less, cooling is done at 50 ° C / s or more within 1 second after finishing rolling, and winding is performed at 350 to 500 ° C. Equivalent radius
A method for producing a hot-rolled steel sheet, wherein the microstructure of cementite having a size of 0.1 μm or more is 0.1% or less and the microstructure of martensite is 1% or less, and which is excellent in burring property and ductility.

(Operation) Next, the reasons for limiting each constituent element of the present invention will be described in detail.

C is an element for ensuring strength. For that purpose, at least 0.02% or more is required. However, a large amount of C content not only raises the strength too much but also leads to the formation of extra carbides (cementite or pearlite), so the maximum C content is limited to less than 0.07%.

Si is an important element in the present invention. That is, Si
Is utilized in the present invention to the maximum extent that it is an element that delays the formation of carbide due to transformation from austenite and an element that improves the ductility of the hot-rolled steel sheet. The present inventors spared no effort in pursuing a mechanism for improving the burring property. The burring property was evaluated by a punching hole expansion test.
Naturally, the respective behaviors during punching and hole expansion have a great influence. The former is already known about the effect of clearance during punching and the effect of non-metallic inclusions, and the latter is already known about the effect of the structure (ferrite, pearlite, bainite, martensite) on the hole expandability. Is. However, most of these studies so far have involved methods such as simply changing the C content or simply changing the coiling temperature to give a typical microstructural change. Neither the size of carbides in the studied bainite, etc., nor the origin of hole expansion cracks during punching have been investigated in detail.

The present invention has been obtained by these studies by the present inventors, one of which is that Si plays an important role.
First, Si is an important element for improving the burring property. This is because the present inventors have revealed for the first time that Si has the effect of making the carbide size in bainite extremely fine. This has an extremely effective effect on the formation of the starting point of subsequent hole expansion cracks when forming a fracture surface during punching.
As a result, the burring property is dramatically improved. In addition, Si has the additional effect of increasing the ductility of the steel sheet. In order to exert these effects, it is necessary to contain Si at a minimum of 0.4%. The upper limit does not have to be specified originally, but was set to 1.5% in consideration of economy and spot weldability.

Mn is an element necessary for securing strength, and is at least 0.5%.
Must be included. The upper limit was set to 1.5% in consideration of economy, spot weldability, and strength stability.

Since P is an element that deteriorates the spot weldability and raises the Ar ₃ transformation point temperature, it is necessary to thoroughly reduce the content thereof, and the content was made 0.02% or less. Preferably, it is better to lower it to 0.01% or less.

From the viewpoints of spot weldability and burring property, S must be thoroughly reduced, and it must be 0.005% or less. Preferably, it should be reduced to 0.002% or less.

Further, Ca is added as needed to control the morphology of sulfide inclusions. The addition of less than 0.0005% has no morphological control effect, and the addition of more than 0.005% adversely affects the burring property and ductility due to the increase of Ca-based inclusions. Therefore, the upper limit was set here.

Al is necessary as a deoxidizer. If it is less than 0.01%, there is no effect, and if it exceeds 0.10%, alumina-based inclusions increase, and the burring property and ductility deteriorate.

Next, the hot rolling conditions will be described in detail.

The heating temperature of the slab must be 1200 ° C or lower. In the present invention, phytoalite, which is a compound of Si oxide and iron oxide, is added in the heating furnace by adding Si and becomes red scale after winding or after pickling. Cloud-shaped patterns remain on the surface of the steel sheet, resulting in poor appearance. The upper limit is regulated to avoid this. It is preferably 1150 ° C or lower. The lower limit of heating temperature is 1000 ℃. If a temperature lower than this is adopted, the rolling will be overloaded and it will be difficult to secure the rolling temperature.

The finishing temperature is specified to be (Ar ₃ transformation point + 50) ° C or higher. Below this temperature, the carbides in bainite do not become fine to the size intended by the invention. The upper limit was 950 ° C. The reason for this determination is that if the temperature is exceeded, not only the effect of improving the burring property is saturated, but also the structure becomes rough and the ductility deteriorates.

It is necessary to cool immediately after finishing rolling. this is,
This is essential for obtaining the carbide size intended by the present invention, and at the latest, it is necessary to perform rapid cooling within 1 second after finishing rolling. Air cooling for more than 1 second is disadvantageous for improving the burring property.

Further, the cooling rate needs to be 50 ° C./sec or more. this is,
This is necessary in order to fully exert the effect of suppressing carbide formation by rapid cooling. The upper limit is not particularly limited as long as the cooling end point temperature can be accurately controlled by the development of the operation technology, but it is preferably 150 ° C./sec or less under the present circumstances.

If the coiling temperature is less than 350 ° C, the martensite structure ratio exceeds 1%, and if the coiling temperature is more than 500 ° C, the cementtite agglomerates, and in some cases forms pearlite to form burring property. Is reduced to 350-500
℃ was specified.

The structures obtained by combining these components and hot rolling conditions are so-called ferrite and bainite, but the triple points of ferrite grain boundaries and the size of carbides in bainite are extremely fine. However, if these carbides have an equivalent circle radius of 0.1 μm or more and a structure ratio of more than 0.1% is present due to an accidental factor, they are not treated as the hot-rolled steel sheet obtained by the present invention.

The definition of the tissue ratio is (total area of cementite having an equivalent circle diameter of 0.1 μm or more obtained by image analysis of a transmission electron micrograph) / (total area) × 100 (%).

(Example) Steels having the components shown in Table 1 were melted in a converter and continuously cast into slabs. The table also shows the Ar ₃ transformation point of each steel.

In Table 1, B steel is Si, C steel is Mn, D steel is P, S, E.
Steel C and F steel have Si outside the scope of the present invention (chemical components outside the scope of the present invention are underlined).

Table 2 shows the hot rolling conditions in the examples. In Table 2, cooling was carried out at 60 ° C./second 0.8 seconds after finishing rolling. After winding, it was subjected to 0.8% temper rolling to obtain a product with a plate thickness of 2.9 mm, which was subjected to a material test.

JIS Z 2201,5 test piece was used for the tensile test. The tissue ratio was judged from the transmission electron micrograph, and the circle equivalent radius of the carbide used the image analysis result of the transmission electron micrograph.

The burring property was evaluated by a punching hole expansion test. The punching was performed using a punch having a diameter of 20 mm and a die having a clearance of 20% of the plate thickness (= d ₀ = [20.0 + plate thickness × 0.2] mm die). The hole was expanded by punching the punched cutting hole with a 30 ° conical punch from the plate surface without burrs (on the side opposite to the burr) by punching (at this time, use a flange 60 to prevent material from flowing into the hole expansion part). I applied the tongue wrinkles). Burring properties, cracking showed hole diameter (d) when stopping a punch when passing through the plate thickness ratio obtained by dividing the _{d 0 (d / d 0)} .

In the spot welding test, single spot welding was carried out under the current condition immediately before the occurrence of the dispersion, which was peeled off and the fracture surface of which was not applied to the original joint surface was indicated by ◯, and the fractured surface was indicated by ×.

Regarding the comparative steels, the conditions for being comparative steels are indicated by *, and the reason is underlined. The present invention steel No. 1,6,7,13,
14 and 15, and comparative steels are No. 2, 3, 4, 5, 8, 9, 10, and 12.

In No. 2, red scale was generated because the heating temperature was too high. In No. 3, the finishing temperature was lower than the range of the present invention and the burring property was deteriorated. In No. 4, the burring property was deteriorated because the winding temperature was higher than the range of the present invention. In No. 5, the burring property was deteriorated because the winding temperature was lower than the range of the present invention. In No. 8, the desired burring property and ductility were not obtained because the Si content was lower than the range of the present invention. In No. 9, since the Mn content was lower than the range of the present invention, the desired strength was not obtained and the burring property was poor. In No. 10, the burring property and spot weldability were deteriorated because the P and S contents were higher than the range of the present invention. In No. 11, since the C content was higher than the range of the present invention, extra carbide was generated and the burring property was deteriorated. In No. 12, since the Si content was too high, the Ar ₃ transformation point increased and it was difficult to secure the rollability.

On the other hand, No. 1,6,7,13,14,15 which is the steel of the present invention,
All of them showed high burring property and high ductility, and had a beautiful surface without causing any problems in spot weldability.

Table 3 is an example of quenching immediately after finishing. The sample steel is code A, heating temperature: 1120 ℃, finishing temperature: 910 ℃
And

No. 16 had a cooling rate outside the range of the present invention, and caused a decrease in burring property due to coarse carbide precipitation.

In No. 17, the time until the start of quenching was out of the range of the present invention, and the carbides in bainite became large and the burring property deteriorated.

(Effects of the Invention) As described above, according to the present invention, it is possible to provide a hot-rolled steel sheet having a burring property and a ductility of 38 kgf / mm ² or more, which does not have a red scale or cloud pattern. The steel strip according to the present invention may be used as it is on a black leather or may be pickled and used. Alternatively, the shear line may be a cut plate. The shape may be adjusted by a leveler or temper rolling, and the curl may be corrected.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masanori Nishimoto Inventor, 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Stock Co., Ltd. Inside the Kimitsu Works, Kaisha (56) Reference JP-A-58-11734 (JP, A) JP-A-SHO 62-180021 (JP, A)

Claims (4)

[Claims] 1. C .: 0.02 to less than 0.07% by weight Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10%, balance Fe and unavoidable Of cementite with a circular equivalent radius of 0.1 μm or more
A hot-rolled steel sheet excellent in burring property and ductility, characterized by having a martensite structure ratio of 0.1% or less and 1% or less. 2. By weight%, C: 0.02 to less than 0.07% Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10% Ca: 0.0005 to 0.0050% Including the balance of Fe and unavoidable impurities, the structure ratio of cementite with a circle equivalent radius of 0.1 μm or more
A hot-rolled steel sheet excellent in burring property and ductility, characterized by having a martensite structure ratio of 0.1% or less and 1% or less. 3. By weight%, C: 0.02 to less than 0.07% Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10% with the balance Fe and Steel made of unavoidable impurities is made into a slab, which is then heated to 1000-1200 ℃ and hot rolled (Ar
₃ transformation point +50) Finishing rolling is completed at a temperature of 950 ° C or more and 950 ° C or less, cooling is done at 50 ° C / s or more within 1 second after finishing rolling, and winding is performed at 350 to 500 ° C. Equivalent radius
A method for producing a hot-rolled steel sheet, wherein the microstructure of cementite having a size of 0.1 μm or more is 0.1% or less and the microstructure of martensite is 1% or less, and which is excellent in burring property and ductility. 4. By weight%, C: 0.02 to less than 0.07% Si: 0.4 to 1.5% Mn: 0.5 to 1.5% P: 0.02% or less S: 0.005% or less Al: 0.01 to 0.10% Ca: 0.0005 to 0.0050% After slabs made of steel containing the balance Fe and unavoidable impurities, the steel is heated to 1000 to 1200 ° C and hot rolled (Ar
₃ transformation point +50) Finishing rolling is completed at a temperature of 950 ° C or more and 950 ° C or less, cooling is done at 50 ° C / s or more within 1 second after finishing rolling, and winding is performed at 350 to 500 ° C. Equivalent radius
A method for producing a hot-rolled steel sheet, wherein the microstructure of cementite having a size of 0.1 μm or more is 0.1% or less and the microstructure of martensite is 1% or less, and which is excellent in burring property and ductility.