JPH0768601B2 - High-strength hot-rolled steel sheet and its manufacturing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet having excellent workability and a method for producing the same.

[0002]

2. Description of the Related Art Recently, in order to reduce the weight of various structures,
In particular, it has been required to use a high-strength hot-rolled steel sheet having an increased tensile strength, and various researches and developments for that purpose have been studied. For example, as shown in JP-A-57-101649, Nb is 0.01 to 0.08%.
(Hereinafter, unless otherwise specified, it represents% by weight) and by appropriately controlling the area ratio of ferrite and bainite in steel containing 0.01 to 0.08% of Ti, the tensile strength is 50 kgf / mm ² or more, 70 kgf / It is said that a high-strength hot-rolled steel sheet having a workability of less than mm ² can be obtained.

However, recently, it has been desired to develop a high-strength hot-rolled steel sheet having a higher tensile strength, that is, a tensile strength of 70 kgf / mm ² or more and good ductility and other workability. Therefore, the conventional high-strength hot-rolled steel sheet cannot be used. For example, the above N
b: 0.01 to 0.08% and Ti: 0.01 to 0.0
In the case of a steel containing 8%, it is difficult to control the bainite area ratio within the range of Nb and Ti, so that there is a drawback that ductility and other workability deteriorate significantly as the strength increases.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art and maintains a high workability of 70 k.
It is an object of the present invention to provide a hot-rolled steel sheet having a tensile strength of gf / mm ² or more, and a method for easily producing such a hot-rolled steel sheet.

[0005]

In order to achieve the above object, the inventors of the present invention manufactured the high-strength hot-rolled steel sheet made of Nb and Ti-containing steel as exemplified above in order to improve workability in accordance with higher strength. As a result of efforts to investigate the cause of deterioration and various investigations on new measures, in particular, the amount of Nb and Ti added was more strictly regulated, and the obtained structure was made into a two-phase type of polygonal ferrite + bainite. It was found that a high-strength hot-rolled steel sheet having a tensile strength of 70 kgf / mm ² or more with excellent workability can be obtained by appropriately controlling the area ratio of the phases, and suitable manufacturing conditions therefor have been found.

That is, in general, when Nb is added to the steel as described above for precipitation strengthening, the ductility rapidly deteriorates. However, when this behavior was investigated in detail, as shown in FIG. 1, Nb: The best T.I. is in the range of 0.02 to 0.04%. S. X El. It was found to have balance (however, TS: tensile strength (kgf / mm ² ), El: elongation (%)). This is because when Nb <0.02%, the effect of adding Nb cannot be sufficiently brought out, so the strength becomes low. On the contrary, when Nb> 0.04%, the bainite amount can be controlled to 60% or less in area ratio. Because it became difficult,
It is considered that the ductility deteriorated.

Therefore, the amount of Nb added is 0.02 to 0.04.
% Within the range of T, and the insufficient strength is reduced to T with less deterioration of ductility.
i. And a technology for obtaining a high-strength hot-rolled steel sheet having a tensile strength of 70 kgf / mm ² or more, which is excellent in workability such as stretch flangeability and the like. A composite strengthened steel by precipitation strengthening by,
70 kgf / mm ² while maintaining excellent workability by properly and strictly adjusting the amounts of Nb and Ti added.
It succeeded in obtaining the above high strength. Of course,
At the same time, in order to obtain such a high-strength hot-rolled steel sheet, other components, their ranges, production conditions, etc. were examined in detail and the requirements to be regulated were found.

That is, the gist of the present invention is to
C: 0.05 to 0.2%, Si: 0.01 to 0.5%,
Mn: 0.8 to 1.8%, S: 0.01% or less, P:
0.03% or less, Al: 0.01 to 0.08%, Nb:
0.02 to 0.04%, Ti: 0.1 to 0.15%,
N: 0.01% or less, further Ca: 0.0005
0.01% and REM: PEG: at least one selected from the group consisting of 0.005 to 0.1%, the balance consisting of iron and unavoidable impurities, and a polygonal ferrite having an area ratio of 40 to 95%. It has a mixed structure with 5 to 60% bainite and has a tensile strength of 70 kgf / mm ² or more,
(Tensile strength) x (Elongation) ≥ 1700 kgf / mm ² ·%
It is a high-strength hot-rolled steel sheet having excellent workability as described above.

The manufacturing method is also characterized in that the steel having the above chemical composition is heated to 1200 ° C. or higher and 1300 ° C. or lower, hot-rolled, and then wound at 575 ° C. or higher and 400 ° C. or higher.

The bainite structure referred to in the present invention is
In addition to the so-called bainite phase, it is a general term for structures such as a structure called acicular ferrite that has no clear metallurgical distinction from bainite and can be regarded as a structure that is substantially the same as bainite. The present invention will be described in more detail below.

[0011]

First, the reasons for limiting the chemical composition of the steel according to the present invention will be described below. C is added to enhance the strengthening and hardenability of steel, and it is necessary to add at least 0.05% in order to effectively exert this effect. However, if added too much, the ductility and stretch flangeability deteriorate, so the upper limit of the addition is made 0.2%.

Si is an element effective for promoting the formation of polygonal ferrite and obtaining a mixed structure according to the present invention, and is also an element suitable for enhancing strength and ductility. Therefore, addition of 0.01% or more is required. However, if excessively added, the surface quality is deteriorated due to scale flaws, so in the present invention, the addition amount of Si is 0.0
It is 1% or more and 0.5% or less.

[0013] Mn is necessary for compensating the decrease in strength due to the lowering of C, and for enhancing the hardenability of steel and transforming untransformed austenite into bainite, and is at least 0.8.
% Addition is required. However, adding a large amount exceeding 1.8% produces a band structure and deteriorates ductility in the direction perpendicular to the rolling direction. Therefore, in the present invention,
Mn is added in the range of 0.8 to 1.8%.

When P is contained in a large amount in excess of 0.03%, the transition temperature after drawing increases, so that
The range is 0.03% or less. S is 0.01%
If it is contained in a large amount exceeding 0.1%, the stretch flangeability is deteriorated, so the content is made 0.01% or less. Al is added as a deoxidizer during the melting of steel, and the range is 0.01 to
It is 0.08%.

Nb affects the transformation behavior after hot rolling and is effective in producing a bainite structure. Further, it also has the effect of precipitation strengthening, and is effective for increasing the strength of steel. Therefore, it is necessary to add 0.02% or more. However, if too much is added, the ductility deteriorates rapidly, so the upper limit of the addition is made 0.04%. If Nb is regulated within this range (0.02 to 0.04%), T. S. × E
l. The balance is the best, the Ti addition effect described below can be sufficiently exerted, and a synergistic effect of Nb and Ti is brought about.

Ti has the effect of precipitation strengthening and is effective in increasing the strength of steel. To obtain the required strength, Ti is
It is necessary to add 0.1% or more under Nb of 0.02 to 0.04%. However, if added too much, the ductility deteriorates and the above effect is saturated, which is economically disadvantageous. Therefore, the upper limit of the addition amount is 0.15%. When N is contained in a large amount, workability deteriorates, so N is set to 0.01% or less.

Further, in the present invention, Ca and REM
At least one element selected from the group consisting of (rare earth elements) is added. These elements have the effect of controlling the form of the sulfide, rendering the inclusions harmless, and enhancing the formability. In order to effectively exhibit such effects, it is necessary to add at least 0.0005% for Ca and at least 0.005% for REM. The upper limit of the amount added is usually 0.01% for Ca, RE
M is 0.1%.

Next, in producing a steel having the above chemical composition, 1200 ° C. is used for solutionizing Nb and Ti.
After heating to the above temperature and hot rolling according to a normal method, it is necessary to cool to a temperature of 575 ° C. or lower and wind it up in order to obtain the required bainite area ratio (5 to 60%).

It should be noted that heating to a temperature higher than 1300 ° C. is not preferable because there is a problem that the fuel cost in the heating furnace increases, the load of the refractory material increases, and the crystal grains become coarse and the strength decreases. Further, when the winding temperature is 400 ° C. or lower, the elongation is lowered, and the T. S. X El. It is not preferable because the balance is poor.

According to the method of the present invention, the bainite area ratio in the two-phase mixed structure of polygonal ferrite + bainite is controlled to 5 to 60%. S. X El. The best balance (TS × El. ≧ 170
0) and required high strength (TS ≧ 70 kgf / m
m ² ) and excellent workability (ductility, stretch flangeability, etc.). Next, examples of the present invention will be described.

[0021]

EXAMPLE Steels having the chemical composition shown in Table 1 were melted, hot-rolled under the hot-rolling conditions shown in Tables 2 and 3, and wound to produce hot-rolled steel sheets. In Table 1, the sample steel No.
Nos. 1 and 4 and 7 are steels of the present invention, and all the other test steels are out of the chemical composition range of the present invention.
The amount of Nb is outside the range of the present invention, and the amounts of Ti in Nos. 5 and 6 are outside the scope of the present invention, but the amount of N is 0.01% or less in all the sample steels. The hot rolling conditions shown in Table 2 are within the scope of the present invention, but the hot rolling conditions shown in Table 3 are outside the scope of the present invention.

For each of the hot-rolled steel sheets obtained, the bainite area ratio was investigated, the tensile properties and the hole expansion ratio λ were determined, and the strength-elongation balance (TS × El.) Was determined. These are also shown in Table 2 and Table 3.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

As is apparent from Table 2, the steels of the present invention obtained under the hot rolling conditions within the scope of the present invention (Test Examples No. 1, 4, 7).
Has an area ratio of bainite within the range of the present invention and is excellent in the strength-elongation balance (T.S. × El.) Of 1700 or more, while the comparative steel (Test Example No. 2, 3,
5 and 6), even under the hot rolling conditions within the scope of the present invention, the strength-elongation balance (T.S. × El.) Was 160 in both cases.
It remains at a low level of around 0 or less.

Also, with respect to the hole expandability, the steel of the present invention is superior to the comparative steel. This means that the stretch flangeability cannot be improved simply by bainite.
It can be seen that it is affected by the amounts of i and Nb. Sample steel
Since Nos. 2 and 5 are below the lower limit of the appropriate range of Nb and Ti, the tensile strength T. S. Is 70 kgf / mm ² or less.

Further, as is apparent from Table 3, when low-temperature heating was performed where the heating temperature was below the lower limit of the range of the present invention, test examples
As can be seen from Nos. 11 to 12, even with the steel of the present invention, T. S. Is as low as 70 kgf / mm ² or less. It is considered that this was because the solution treatment of Nb and Ti was sufficient.

When the winding temperature exceeds the upper limit of the range of the present invention,
As can be seen from Test Examples Nos. 13 to 14, the bainite ratio was 0%, that is, a ferrite + pearlite structure was obtained. S. X El. The balance is good, but the stretch flangeability is deteriorated.

On the other hand, if the bainite area ratio exceeds the range of the present invention because the coiling temperature is excessively low and it is rapidly cooled,
As shown in Test Examples Nos. 15 to 16, although the strength is further increased, T.I. S. X El. The balance is deteriorated and stretch flangeability is low.

[0031]

As is clear from the above description, the polygonal ferrite + bainite dual phase steel according to the present invention containing Nb and Ti in combination has a strength-ductility balance of T.38. S.
X El. Since it has a tensile strength of 70 kgf / mm ² or more while maintaining excellent workability, it can be applied not only to ordinary cold working but also to press applications involving high working. Also, because of its low carbon equivalent, spot welding, flash pad welding,
It is an industrially excellent steel sheet with excellent weldability such as arc welding. Furthermore, since a high-strength hot-rolled steel sheet having such excellent properties can be easily manufactured, it is possible to sufficiently meet the demand for a hot-rolled steel sheet having high strength and good workability.

[Brief description of drawings]

FIG. 1 shows that the amount of Nb added is T. S. X El. 6 is a graph showing the effect on balance.

Claims (2)

[Claims] 1. C: 0.05-0.2%, S by weight%
i: 0.01 to 0.5%, Mn: 0.8 to 1.8%,
S: 0.01% or less, P: 0.03% or less, Al: 0.
01-0.08%, Nb: 0.02-0.04%, T
i: 0.1 to 0.15%, N: 0.01% or less,
Further, Ca: 0.0005 to 0.01% and REM: 0.
At least 1 selected from the group consisting of 005 to 0.1%
Includes seeds, balance iron and unavoidable impurities, area ratio 40-95% polygonal ferrite and 5-60%
It has a mixed structure with bainite and has a tensile strength of 7
0 kgf / mm ² or more, (tensile strength) × (elongation) ≧ 17
A high-strength hot-rolled steel sheet with excellent workability, which is characterized by exhibiting at least 00 kgf / mm ² ·%. 2. C: 0.05 to 0.2% by weight, S
i: 0.01 to 0.5%, Mn: 0.8 to 1.8%,
S: 0.01% or less, P: 0.03% or less, Al: 0.
01-0.08%, Nb: 0.02-0.04%, T
i: 0.1 to 0.15%, N: 0.01% or less,
Further, Ca: 0.0005 to 0.01% and REM: 0.
At least 1 selected from the group consisting of 005 to 0.1%
Steel containing seeds and the balance iron and inevitable impurities,
After hot-rolling by heating to 1200 ° C or higher and 1300 ° C or lower, it is wound up at 575 ° C or lower and higher than 400 ° C to obtain 40 to 95% in area ratio of polygonal ferrite and 5
A steel sheet having a mixed structure with bainite of up to 60% and having a tensile strength of 70 kgf / mm ² or more and (tensile strength) × (elongation) ≧ 1700 kgf / mm ² ·% or more is manufactured. Manufacturing method of high strength hot rolled steel sheet with excellent workability.