JPH07224351A - Hot rolled high strength steel plate excellent in uniform elongation after cold working and its production - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel plate excellent in uniform elongation and high in tensile strength even after ordinary cold forming by which productivity is not deteriorated as for round and square steel pipes, shape steels, sheet piles or the like and to provide a method for producing the same. CONSTITUTION:This is a steel plate having a compsn. contg. 0.04 to 0.25% C, 0.0050 to 0.0150% N and 0.003 to 0.050% Ti, and in which TiN having 1 to 30mum grain size is dispersed into a base by the ratio of 0.0008 to 0.015% and Ceq (WES) is regulated to 0.10 to 0.45%. This steel plate is produced by heating the slab contg. the above components to 1000 to 1300 deg.C, hot rolling, and air cooling from a temp. of >=500 deg.C, or coiling at >=500 deg.C and then air- cooling to regulate the area fractional rate of a pearlitic phase in the steel structure to 5 to 20%. The hot rolling is finished at the temp. of Ar3 transformation point or above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for general and welded structures having excellent uniform elongation after cold working and high tensile strength, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the quality and manufacturing technology of structural hot rolled steel sheets have been remarkably advanced, and in particular in the fields of construction and civil engineering, the demand for steel materials having excellent plastic deformability has increased from the viewpoint of seismic design. Therefore, high strength, low yield ratio and high uniform elongation are required.

On the other hand, for example, Japanese Patent Laid-Open No. 57-161.
No. 18 discloses a method for producing an electric resistance welded steel pipe for a low yield ratio oil well in which the amount of C is increased to 0.26 to 0.48%, and Japanese Patent Laid-Open No. 57-16.
Japanese Patent No. 119 discloses a method for producing a low-yield ratio high-strength electric resistance welded steel pipe having a C content of 0.10 to 0.20%.
These are manufacturing methods of electric resistance welded steel pipes that produce hot-rolled steel sheets with a low yield ratio and then process by limiting the strain amount so that the work hardening amount does not increase in cold forming, and Japanese Patent Laid-Open No. 4-176818 proposes a method for producing a steel pipe or a square pipe having excellent seismic resistance obtained by subjecting a strain-free ferrite and pearlite two-phase structure to cooling rate regulation and heat treatment after hot working. There is. However, in the present circumstances, it cannot be said that the former does not necessarily meet the demands of the industry, such as the productivity being remarkably lowered and the weldability being significantly impaired.

In addition, in Japanese Patent Laid-Open No. 4-48048, (Ti, Nb) (O,
N) An oxide-based inclusion having a composite crystal phase is disclosed in
Japanese Patent No. 99248 discloses that Ti of 1 μm or less is contained in the base metal of steel.
A technique for improving the toughness of a weld heat affected zone by dispersing oxide-based inclusions having a (O, N) composite crystal phase, respectively, is disclosed. Are essentially different technologies.

[0005]

Generally, the higher the strength of the steel, the higher the yield ratio and the lower the ductility, and the lower the uniform elongation. In particular, after cold forming into round and square steel pipes, shaped steels, sheet piles, etc., the uniform elongation is remarkably reduced due to the effect of work hardening due to work strain. The present invention is to solve such problems, round and square steel pipe, even after ordinary cold forming does not reduce productivity into shaped steel or sheet pile, uniform elongation is excellent, and An object of the present invention is to provide a hot-rolled steel sheet having high tensile strength (34 kgf / mm ² or more) and a method for manufacturing the same.

[0006]

In order to achieve the above-mentioned object, the present inventors have studied the relationship between the chemical composition and crystal structure of steel and the mechanical properties obtained, and the mechanical properties and materials after cold forming. I investigated and studied the relationship with that in detail. as a result,
Correlation between tensile strength as hot rolled and uniform elongation in hot-rolled steel sheet with tensile strength of 34 to 62 kgf / mm ² which is most commonly used for general and welded structural steel, especially for construction civil engineering. (The uniform elongation decreases as the tensile strength increases) and their correlations after cold forming are almost the same and can be approximated by the same curve. As N in steel increases, the material also becomes cold-worked. It was found that the strength of the material of No. 1 also increases and the uniform elongation decreases, but when Ti is further added, the uniform elongation recovers and deviates from the above correlation, and high uniform elongation can be obtained even with high strength. .

The above findings will be further described with reference to FIG. FIG. 2 shows the steel types S-1 (comparative example) and S-2 shown in Table 1.
(Comparative Example) and T-1, T-2 (Invention Example),
Steel types S-1 and T-1, T-2 are the manufacturing steps B and S-2 shown in Table 2, and TS is the as-hot-rolled steel material manufactured by the manufacturing step C and the steel material TS (kgf / mm ² )
It is the figure which showed the relationship between (tensile strength) and Elu (%) (uniform elongation). In the steel type S-1, both Ti and N are less than the lower limit of the present invention, and in the steel type S-2, N is within the range of the present invention, but Ti is low and less than the lower limit of the present invention. Manufacturing process C is an example in which the rolling end temperature is a low temperature below the Ar ₃ transformation point.

In FIG. 2, the relationship between TS and Elu in the case of the steel type S-1 shows high TS and Elu in the case of the as-hot-rolled steel sheet, but in the case of the rectangular steel pipe, Elu increases as TS increases. It is dropping sharply. In the case of the steel type S-2, it is more remarkable, and in the case of as-hot-rolled steel sheet, there are some cases where Elu is high when TS is low, but Elu is 1 when TS is high.
It fell to 0% or less, and when it was cold-worked into a square steel pipe, most of it fell to 10% or less, and as TS increased, E
lu is further reduced.

That is, in the case of the steel types S-1 and S-2, Elu after cold working tends to decrease sharply as TS increases. On the other hand, in the case of the steel types T-1 and T-2, the Elu of the as-hot-rolled steel sheet hardly decreases even when the TS becomes high, and even if cold working is performed on the Elu, the Elu slightly decreases. Is almost unaffected by the increase in TS.

That is, in the steel of the present invention to which N and Ti are added in appropriate amounts, the uniform elongation hardly decreases even after the cold working, even if the tensile strength is increased. Especially TS is about 47 kgf
When it is / mm ² or more, the effect of the present invention can be remarkably exhibited. Thus, the steel of the present invention has excellent properties as general and welded structural steel. The present invention is constructed based on these findings, and the gist thereof is C:
0.040-0.25%, N: 0.0050-0.01
50%, Ti: 0.003 to 0.050%, and TiN having a particle size of 1 to 30 μm in the matrix is 0.0008 to
While being dispersed at a rate of 0.015%, Ceq. (WE
S) was set to 0.10 to 0.45%, and a billet containing the above components was heated to 1000 to 1300 ° C. for hot rolling and rolled, and the rolling was completed at a temperature of Ar ₃ transformation point or higher. Cooling by air cooling from a temperature of 500 ° C. or higher, or by winding at a winding temperature of 500 ° C. or higher and then air cooling so that the pearlite phase in the steel structure has an area fraction of 5 to 20%. Excellent tensile strength with uniform elongation after cold forming 34 ~
A high-strength hot-rolled steel sheet of 62 kgf / mm ² and a method for manufacturing the same.

[0011]

The present invention will be described in detail below. In the present invention, first, C: 0 is applied by an ordinary billet manufacturing process in which molten steel manufactured by a melting furnace such as a converter or an electric furnace is manufactured into a steel piece through continuous casting or an ingot / slumping step. .040-
0.25%, N: 0.0050 to 0.0150%, T
i: 0.003 to 0.050% is included, the carbon equivalent (Ceq.) is in the range of 0.10 to 0.45%, and a low alloy steel slab composed of the balance Fe and inevitable impurities is produced.

In the present invention, the components in the steel are specified as described above for the following reasons. C is an important component in determining the strength of steel and the amount of pearlite phase in the steel structure. In a hot rolled steel sheet having a tensile strength of 34 kgf / mm ² or more, if the pearlite phase in the steel structure is less than 5% in area fraction, the uniform elongation after cold forming remarkably decreases. This is because pearlite bears the strength, prevents the dislocation density of ferrite from increasing, and maintains its plastic deformability. If the area fraction of the pearlite phase exceeds 20%, the hardenability of steel increases and the plastic deformability itself decreases, so the upper limit is 20%.
%. In order to obtain such a steel structure, the amount of C is set to 0.
It is necessary to make it 04% or more. However, if it exceeds 0.25%, the weldability is impaired, so the upper limit was made 0.25%.

N is added to steel and forms a solid solution in the ferrite ground to increase the strength of the steel and reduce the plastic deformability.
When added together with Ti, TiN is formed and solid solution N in steel is formed.
Is an important element that not only reduces the plasticity and recovers the plastic deformability but also contributes to dispersion strengthening and imparts to the steel the characteristics of high strength and large uniform elongation. The average particle size is 1 to 30 μm.
TiN in the range of 0.0008 to 0.015 by weight
It is necessary to disperse it in the matrix at a ratio of%. Ti
If the average particle size of N is less than 1 μm, the dispersion strengthening is not sufficiently performed, and if it exceeds 30 μm, a divergence occurs between the coarse TiN and the matrix during processing, which is microscopic as plastic deformation progresses. The upper limit of TiN is set to 30 μm because it becomes a crack and reduces the plastic deformability of the entire steel. Such Ti
To obtain N, the amount of Ti is 0.003 to 0.050%
The range is valid.

N is required to be at least 0.0050%, preferably 0.0080% or more, but 0.01
If it exceeds 50%, the strengthening will be excessive and the uniform elongation will be rather lowered, so the upper limit was made 0.0150%. The above Ti
In order to effectively form N in steel, it is preferable to add Al in advance and deoxidize it before adding Ti.

Although Ti is added to the steel of the present invention for the above reason, the preferable range is 0.01 to 0.03%. The carbon equivalent (Ceq.) Is calculated by the following formula (based on the WES formula). Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 +
V / 14 This Ceq. The amount is specified in relation to the strength and weldability. If it is less than 0.10%, the strength cannot be secured, and
If it exceeds 45%, high strength is obtained but the weldability is impaired.
Therefore, Ceq. Is limited to the range of 0.10 to 0.45%.

As effective components for improving strength and toughness, Si: 0.01 to 0.7%, Mn: 0.1 to 2.0.
%, Ni: 0.05 to 1.0%, Cr: 0.05 to 1.
0%, Mo: 0.02-0.5% and V: 0.005
It can contain at least one selected from the group of 0.2%. In addition, since P and S contained in the steel slab are harmful impurity components that reduce toughness, weldability, etc., respectively, 0.025% or less and P + S ≦ 0.
04%.

Further, as effective components for improving strength and toughness, Cu: 0.05 to 1.0%, Nb: 0.00
5 to 0.05%, Al: 0.001 to 0.1%, B:
0.0005 to 0.0020%, Ca: 0.0005 to
0.0070%, REM (lanthanide series rare earth containing Y): At least one selected from the group of 0.001 to 0.050% can be contained.

The billet of the low alloy steel adjusted to the above composition range is heated to 1000 to 1300 ° C. for hot rolling and rolled, and the rolling is finished at a temperature of Ar ₃ transformation point or higher,
A thick plate is obtained by air-cooling from a temperature of 500 ° C or higher, or a hot-rolled steel strip is obtained by wind-cooling at a winding temperature of 500 ° C or higher. The lower limit of the heating temperature for hot rolling is set to 1000 ° C. because the rolling end temperature is Ar ₃ depending on the plate thickness of the steel sheet.
This is to prevent the ferrite from being severely worked below the transformation point, increasing the dislocation density in the matrix and increasing the strength to impair the plastic deformability. The product yield will decrease significantly due to
The upper limit is 00 ° C. The reason why the rolling end temperature is set to the Ar ₃ transformation point or higher is also for the above reason. Further, the start temperature of air cooling after rolling and the winding temperature are also set to a high temperature of 500 ° C. or higher in order to avoid unnecessary increase in the strength of the steel sheet.

The steel sheet produced according to the present invention contains 0.0008 to 0.00 of TiN having an average grain size of more than 1 μm.
Fine-grained ferrite-pearlite containing a pearlite phase in an area fraction of 5 to 20% finely dispersed and precipitated in the matrix at a ratio of 15% (partially including paynite) as shown in FIG. 1 (A).
Present an organization. Because of having such a steel structure, the steel sheet of the present invention is excellent in uniform elongation after cold working and can have high tensile strength of 34 to 62 kgf / mm ² .

[0020]

EXAMPLES Examples of the present invention will be described below. Ti-N-containing steel slabs having the chemical composition shown in Table 1 were hot-rolled together with comparative steel to a plate thickness of 3.0 mm to 22.2 mm, and the mechanical properties of the steel plates were investigated. Table 2 shows the manufacturing process, Table 3 shows as hot rolled and 10
The respective characteristics after the% strain processing are shown in Tables 4 and 5, and the results of examining the characteristics in the respective portions after the hot rolling and after the rectangular steel pipe forming are shown. Further, FIG. 1 (A) shows the steel T- of the present invention.
2 shows an optical micrograph (400 times) of the metal structure of the rectangular steel pipe flat portion (MID), and FIG. 1 (B) shows the metal structure of the comparative steel S-2. In the steel of the present invention shown in FIG. 1 (A), the pearlite phase is approximately 15.2% (area ratio), while in FIG.
It can be seen that the comparative steel of (B) is extremely small, about 4%. FIG. 2 mainly shows the results of Table 4 to compare the relationship between the tensile strength and the uniform elongation of the steel of the present invention and the comparative steel.

As is clear from these results, the steels of the present invention (C-4, C-6, T-1, T-2, T-3, T-4)
Despite having a high strength, each of the comparative steels retains a large uniform elongation, especially after cold working. This is well understood by referring to FIG. 2 which shows the relationship between the uniform elongation and the strength after cold forming a rectangular steel pipe in an actual production line using the hot rolled steel sheets of the present invention steel and the comparative steel and the steel sheet as a raw material. It

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

As described above, according to the present invention, the components in the steel are specified, a relatively large TiN is formed to have the dispersion strengthening ability, and an effective pearlite phase is generated in the steel. Even after cold-forming without lowering the productivity of, the uniform elongation is extremely excellent and the tensile strength is 34 to 62.
A high strength hot rolled steel sheet having a kgf / mm ² can be manufactured. This high-strength hot-rolled steel sheet, as a general and welded structural steel material,
In particular, it is extremely useful as a material for round and square steel pipes, shaped steel or sheet pile for civil engineering and construction.

[Brief description of drawings]

FIG. 1 (A) shows the steel of the present invention (Table 4 No. T-2 (MI
It is a 400 times magnification micrograph which shows the metallographic structure of the square steel tube plane part of the steel of D part) containing 15.2% of a pearlite phase). FIG. 1 (B) shows a comparative steel (Table 4 No. S-2 (thickness (t)
= 3.2 mm) steel containing 4% of pearlite phase)), and is a 400 times magnified micrograph showing the metallographic structure of the flat portion of the rectangular steel tube.

FIG. 2 is a diagram showing the relationship between tensile strength and uniform elongation in various hot-rolled steel sheets and rectangular steel tubes shown in Table 4.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Harabuchi 46-59 Nakahara-Sonohama, Tobata-ku, Kitakyushu City, Fukuoka Prefecture, Japan Nihon Cast Forging Steel Co., Ltd. (72) Inventor Yoshikazu Nakano, Tobata-ku, Kitakyushu, Fukuoka Prefecture Harajinohama 46-59 In Japan Cast and Forged Steel Co., Ltd. (72) Inventor Hiroshi Mochiki 1-1, Toibata-cho, Tobata-ku, Kitakyushu, Kitakyushu, Fukuoka In-house of Nippon Steel Co., Ltd. Yawata Works (72) Inventor Nagata Kimio Fukuoka 1-1 Hibata-cho, Tobata-ku, Kitakyushu, Japan Inside the Yawata Works, Nippon Steel Corporation

Claims (10)

[Claims] 1. C: 0.04 to 0.25% by weight, N:
0.0050 to 0.0150% and Ti: 0.003
To 0.050%, and the carbon equivalent (Ceq.) Calculated by the following formula is 0.10 to 0.45%, and the pearlite phase is in the range of 5 to 20% in area fraction. And TiN with an average grain size of 1 to 30 μm in steel.
Is dispersed at a rate of 0.0008 to 0.015% by weight, and a high strength hot rolled steel sheet having excellent uniform elongation after cold working. Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 +
V / 14 2. The high-strength hot-rolled steel sheet according to claim 1, having a tensile strength in the range of 34 to 62 kgf / mm ² . 3. Further, Si: 0.01 to 0.7 by weight.
%, Mn: 0.1 to 2.0%, Ni: 0.05 to 1.0
%, Cr: 0.05 to 1.0%, Mo: 0.02 to 0.
The high-strength hot-rolled steel sheet according to claim 1, containing at least one component selected from the group consisting of 5% and V: 0.005-0.2%. 4. By weight, Cu: 0.05 to 1.0
%, Nb: 0.005 to 0.05%, Al: 0.001
~ 0.1%, B: 0.0005 to 0.0020%, C
a: 0.0005 to 0.0070% and REM: 0.
The high-strength hot-rolled steel sheet according to claim 1 or 3, containing at least one component selected from the group of 001 to 0.050%. 5. P and S in the steel sheet are each 0.02
The high-strength hot-rolled steel sheet according to claim 1, wherein the content is regulated to 5% by weight or less and P + S ≦ 0.04%. 6. C: 0.04 to 0.25% by weight, N:
0.0050 to 0.0150 and Ti: 0.003 to
A steel slab containing 0.050% and having a carbon equivalent (Ceq.) Determined by the following formula in the range of 0.10 to 0.45% is heated to a temperature range of 1000 to 1300 ° C. Manufacture of a high-strength hot-rolled steel sheet excellent in uniform elongation after cold working, which comprises rolling a piece, finishing rolling at a temperature of Ar ₃ transformation point or higher, and then air cooling from a temperature of 500 ° C. or higher. Method. Ceq. = C + Si / 24 + Mn / 6 + Ni / 40 + Cr / 5 + Mo / 4 +
V / 14 7. The manufacturing method according to claim 6, wherein the thick plate is manufactured by air cooling from a temperature of 500 ° C. or higher. 8. The production method according to claim 6, wherein the steel strip is produced by winding at a temperature of 500 ° C. or higher and air cooling. 9. Further, Si: 0.01 to 0.7 by weight.
%, Mn: 0.1 to 2.0%, Ni: 0.05 to 1.0
%, Cr: 0.05 to 1.0%, Mo: 0.02 to 0.
7. The method according to claim 6, which contains at least one component selected from the group consisting of 5% and V: 0.005-0.2%. 10. Further, by weight, Cu: 0.05-1.
0%, Nb: 0.005 to 0.05%, Al: 0.00
1 to 0.1%, B: 0.0005 to 0.0020%, C
a: 0.0005 to 0.0070% and REM: 0.
The production method according to claim 6 or 9, containing at least one component selected from the group consisting of 001 to 0.050%.