CN1037700C - Hot-rolled steel sheet with superior uniform elongation after cold working and production process thereof - Google Patents

Abstract

A hot-rolled steel plate with a tensile strength of ^{333.2-607.bN/mm2} and a good elongation after cold working and a production process thereof without reducing productivity. The hot-rolled steel sheet contains 0.04%-0.25% C, 0.0050%-0.0150% N, and 0.003%-0.050% Ti, and also contains 0.0008%-0.015% Ti with an average particle size of 1 μm to 3 μm and dispersed in its matrix TiN in TiN, and 0.10% to 0.45% carbon equivalent (WES). The process consists of heating the above steel slab to 1000 to 1300°C, rolling, finish rolling at least at the transformation point Ar ₃ , air cooling from at least 500°C or coiling and air cooling at at least 500°C, the resulting steel structure has 5% to 20% pearlite phase.

Description

Hot-rolled steel sheet with superior uniform elongation after cold working and production process thereof

This invention relates to hot-rolled steel plates and sheets for general structural and welded structures having excellent uniform elongation and high tensile strength after the cold working process, and to a process for producing such products.

In recent years, with the improvement of the quality of hot-rolled steel plates and thin plates for structural use and major advances in production technology, there has been an increasing demand for steel products with excellent plastic deformation capabilities, especially in the fields of construction and civil engineering from seismic design to This is even more so from a point of view, and the steel plate is required to have high strength, low yield ratio and high uniform elongation.

To meet this requirement, for example, Kokai (Japanese Unexamined Patent Publication No. 57-16118) discloses a process for producing low-yield ratio electric welded pipes for oil wells, wherein the carbon content of the pipes is increased to 0.26% to 0.48%, and Kokai (Japanese Unexamined Patent Publication) No. 57-16119 discloses a process for producing high tensile strength, low yield ratio electric welded pipes with a carbon content of 0.10% to 0.20%. In this process, the manufacture of electric welded pipes that do not require heat treatment needs to be accomplished by producing a hot-rolled steel plate or sheet with a low yield ratio, and cold working the above-mentioned steel product while the amount of strain is limited, whereby the amount of work hardening will not In addition, Kokai (Japanese unexamined patent publication) No.4-176818 proposes a kind of technology that is used to produce the round steel pipe or the square steel pipe of seismic performance good, and this method is by unstrained ferrite-pearlite The bulk duplex structure is thermally processed, and its cooling rate is controlled and heat treated after thermal processing. However, all the above-mentioned processes greatly reduce productivity, and the former process also significantly reduces weldability. Therefore, these processes It is inevitable that the requirements of the industrial field cannot be met at present.

In addition to the above publications, Kokai (Japanese Unexamined Patent Publication) No. 4-48048 and KoKai (Japanese Unexamined Patent Publication) No. 4-99248 disclose methods for improving the thermal influence of welding by dispersing oxide inclusions in the steel matrix. technology for regional resilience. The oxide inclusions in the former patent publication have a grain size of 0.5 μm or less and have a (Ti, Nb)(Q, N) composite crystal phase. The particle size of the oxide inclusions in the latter patent publication is 1 μm or less and has a Ti(O,N) composite crystal phase. The techniques disclosed in these patents are essentially different from the present invention in terms of dispersing the external phase and purpose.

In general, higher strength steels exhibit higher yield ratios and lower ductility, thereby reducing their uniform elongation. Especially when the steel is cold processed into round steel pipes, square steel pipes, section steel, sheet piles, etc., due to the influence of work hardening caused by processing strain, its uniform elongation will be significantly reduced.

The present invention has successfully solved the above-mentioned problems, and it is an object of the present invention to provide excellent uniform elongation even after it is cold-worked into round pipes, square pipes, section steel, sheet piles, etc. to such an ordinary degree that productivity does not decrease. Hot-rolled steel sheets and sheets with high tensile strength (at least 333.2N/mm ² ).

In order to achieve the above objects, the inventors of the present invention have carefully studied the relationship between the chemical composition and crystal structure of steel and the mechanical properties obtained therefrom, the mechanical properties of cold-worked steel and the mechanical properties of steel in the rolled state. relationship, and so on. Thus, the inventors of the present invention have obtained the following recognition: with regard to steels for ordinary structures and welded structures, especially steels with a tensile strength of 333.2 to 607.6 N/mm ² which are most widely used in the field of construction and civil engineering For rolled steel plate or thin plate, the relationship between tensile strength and uniform elongation of hot-rolled products (whose uniform elongation decreases with the increase of tensile strength) is roughly the same as their relationship after cold working, and the two The relationship in this case can be approximated by the same curve; although both the hot-rolled steel and the cold-worked steel show the characteristic that the strength increases and the uniform elongation decreases with the increase of N in the steel, but by further increasing Ti, even When the steel has a high strength, the uniform elongation can also be restored, and a high uniform elongation can be obtained, and the above-mentioned relationship is no longer maintained at this time.

This realization can be further explained with reference to FIG. 2 as follows.

Fig. 2 is a graph showing the relationship between TS (tensile strength, N/mm ² ) and Elu (uniform elongation, %) obtained from steel products in the hot-rolled state and cold-worked steel products (square steel pipes). relationship, the steel used is listed in Table 1 as S-1 (comparative example), S-2 (comparative example), T-1 (example) and T-2 (example), S-1, T-1 and T-2 is produced according to production process B shown in Table 2, and S-2 is produced according to production process C. The contents of Ti and N in S-1 were both lower than the lower limit of the present invention. Although the N content in S-2 is within the range of the present invention, the Ti content is low and below the lower limit of the present invention. In production process C, the finish rolling temperature is lower and lower than the transformation point Ar ₃ .

In FIG. 2 , in terms of the relationship between TS and Elu, the hot-rolled steel sheet or thin plate of S-1 showed high TS and Elu. However, the square steel tubes of S-1 showed a sharp decrease of Elu with the increase of TS.

In the case of S-2, the above relationship is even more pronounced. Steel plates or sheets in the hot-rolled state show Elu of 10% or less when TS is high, although they can show high Elu when TS is low. Square tubes made by cold working showed Elu of 10% or less in most cases, and further decreased Elu with increasing TS.

That is to say, in the case of S-1 and S-2, the cold-worked steel products showed a trend of a sharp decrease of Elu with the increase of TS.

In contrast, in the cases of T-1 and T-2, the steel sheet or thin plate in the hot-rolled state exhibited a characteristic that Elu hardly decreased even when TS was increased. The resulting cold worked product showed a small decrease in Elu and was hardly affected by the increase in TS.

That is to say, the steel of the present invention containing appropriate amounts of N and Ti added exhibits a characteristic that the uniform elongation hardly decreases as the tensile strength increases even after cold working. In particular, a steel of the present invention having a TS of at least 460.6 N/mm ² can exhibit the effect of the present invention. As described above, the steel of the present invention has excellent properties as a steel for general structural use and welded structural use.

Based on the above facts, the present invention has achieved the object of the present invention, and the subject of the present invention is a high-strength hot-rolled steel plate and sheet having a tensile strength of 333.2 to 607.6/ ^mm2 and a superior uniform elongation after cold forming, The above-mentioned steel plates and thin plates contain 0.040% to 0.25% of C, 0.0050% to 0.0150% of N and 0.003% to 0.050% of Ti, and at the same time contain 0.0008% to 0.015% of the average size of 1 μm to 30 μm and dispersed in the steel matrix TiN, and contains 0.10% to 0.45% carbon equivalent (WES). When preparing steel plates and thin plates, the slabs containing the above components are heated to a hot rolling temperature of 1000 to 1300 ° C, and the slabs are rolled. Finish rolling at a temperature of point Ar ₃ , followed by air cooling of the rolled product to a temperature of at least 500°C or coiling of the rolled product at a temperature of at least 500°C and air cooling of the coiled product to form its overall The amount is converted into the pearlite phase with an area percentage of 5% to 20%. The subject of the present invention is also a production process for producing the above-mentioned products.

Figure 1(A) shows a photomicrograph (400X magnification) illustrating a square tube made from the steel of the present invention [T-2 (middle part) steel in Table 4, containing 15.2% pearlite phase] The metallographic structure of the planar part.

Fig. 1(B) shows a photomicrograph (400 times magnification) illustrating the steel produced by comparative steel [S-2 steel in Table 4 (thickness (t) = 3.2 mm), containing 4% pearlite phase]. Take the metallographic structure of the flat part of the square tube.

FIG. 2 shows the relationship between the tensile strength and uniform elongation of various hot-rolled steel sheets in Table 4 and square tubes.

The present invention will be described in detail below.

In the present invention, a kind of by 0.040% to -0.25% of C, 0.0050% to 0.0150% of N, 0.003% to 0.050% of Ti is made by traditional production steps at first, and the range of its carbon equivalent (Ceq) is 0.10% to 0.45%, the rest is Fe and unavoidable impurities low-alloy steel slabs, which are made by continuous casting of molten steel produced in a melting furnace such as a converter or electric furnace or by making molten steel into ingots and passing the ingots through Initially rolled.

In the present invention, the components in the steel are specified as described above, and the reason for this is explained below.

C is an important component that determines the strength of steel and the total amount of pearlite phase in the steel structure. When a hot-rolled steel sheet or sheet having a tensile strength of at least 333.2 N/ ^mm2 contains a pearlite phase occupying an area ratio of less than 5% in the steel structure, its uniform elongation after cold forming is significantly reduced. This is because pearlite secures the strength of steel, prevents an increase in dislocation density, and maintains plastic deformation ability.

In addition, when the hot-rolled steel sheet contains a pearlite phase occupying an area ratio of 20% or more, the hardness of the steel increases and the plastic deformability decreases. In order to obtain such a steel structure having a pearlite phase whose total area percentage is 5% to 20%, it is required that the steel contains at least 0.04% of C. However, when the C content exceeds 0.25%, the weldability of steel may be impaired, so the upper limit of the C content is limited to 0.25%.

The N added to the steel dissolves in the ferrite matrix, which increases the strength of the steel and reduces the plastic deformation capacity. However, when N is added together with Ti, TiN is formed. This formation not only reduces the dissolved N in the steel and improves the plastic deformation ability, but also strengthens the steel due to its dispersion effect. Therefore N is an important element which endows steel with high strength and large uniform elongation.

In order to impart such properties to steel, it is necessary to disperse TiN having an average particle size of 1 μm to 30 μm in the matrix in a total amount of 0.0008% to 0.015% by weight. In other words, when the average particle size of TiN is smaller than 1 µm, the dispersion enhancing effect is not very effective.

Of course, when the average particle size of TiN exceeds 30 μm, there will be an isolation effect between the coarse TiN particles and the matrix during cold working. Corresponding to the increase in plastic deformation, microcracks appear and thereby reduce the plastic deformation capacity of the steel.

In order to obtain the above-mentioned TiN, the content of Ti is preferably in the range of 0.003% to 0.050%.

Furthermore, although the necessary amount of N is at least 0.0050%, preferably at least 0.0080%, when the N content exceeds 0.0150%, the reinforcing effect is too large and the uniform elongation decreases. Therefore, the upper limit of the N content is limited to 0.0150%. In addition, in order to efficiently form the above-mentioned TiN in steel, it is preferable to add Al to the steel to deoxidize it before adding Ti.

The reason for adding Ti to the steel of the present invention is as described above, and its content is preferably 0.01% to 0.03%.

Carbon equivalent (Ceq) is obtained by the following formula (according to WES):

Ceq=C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14.

The determination of the total amount of Ceq is related to strength and weldability. When the total amount is less than 0.10%, sufficient strength cannot be guaranteed. When the total amount exceeds 0.45%, although high strength can be obtained, weldability is impaired. Ceq. is therefore limited to a range of 0.10% to 0.45%.

Steel can contain effective ingredients for improving strength and toughness, at least from 0.01% to 0.7% of Si, 0.1% to 2.0% of Mn, 0.05% to 1.0% of Ni, 0.05% to 1.0% of Cr, 0.02 % to 0.5% of Mo and 0.005% to 0.2% of V to select one element.

In addition, P and S contained in the steel slab of the present invention are harmful impurities that lower the toughness, weldability, and the like of the steel. Therefore, the content of P and the content of S are limited to 0.025% or less, respectively, and the content of P+S is limited to 0.04% or less.

In addition, the steel of the present invention may contain an active ingredient for improving strength and toughness, at least from 0.05% to 1.0% of Cu, 0.005% to 0.05% of Nb, 0.001% to 0.1% of Al, 0.0005% One of the components consisting of B to 0.0020%, Ca from 0.0005% to 0.0070%, and REM (rare earth metals in lanthanides including Y) from 0.001% to 0.050% is selected.

The low alloy steel slab whose composition is adjusted within the above range is heated to 1000 to 1300° C. for hot rolling and rolled, and then finish rolled when the temperature of the steel is at least the transformation point Ar ₃ . The resulting steel is air-cooled to a temperature of at least 500°C to obtain a steel plate, or the resulting steel is coiled at a coiling temperature of at least 500°C and air-cooled to obtain a hot-rolled strip.

The lower limit of the heating temperature used for hot rolling is limited to 1000°C in order to prevent the increase in strength and decrease in plastic deformation ability caused by the following: depending on the thickness of the steel, the finish rolling temperature of the steel can be lower than the transformation point Ar ₃ ; as a result The ferrite in it will be forced to be processed, so the dislocation density in the matrix will increase. When the slab temperature of the steel exceeds 1300° C., the productivity of the product will drop significantly due to the oxidation of the product. Therefore, the upper limit of the temperature is limited to 1300°C. The finishing temperature is limited to at least the transformation point _Ar₃ for the reasons mentioned above. Furthermore, in order to avoid unnecessary increase in the strength of the steel sheet and sheet of the present invention, the air cooling start temperature after rolling and the coiling temperature are limited to at least 500°C.

In the steel sheet produced according to the present invention, TiN having an average particle size of 1 μm to 30 μm is well dispersed and precipitated in the matrix at a ratio of 0.0008% to 0.015%. As a result, the steel exhibits a fine-grained ferrite-pearlite structure (partially containing a bainite structure) containing a content of 5% to 20% pearlite phase. Since the steel plates and thin plates of the present invention have such a steel structure, they have excellent uniform elongation after cold working and have a high strength of 333.2 to 607.6 N/mm ² .

Embodiments of the present invention are described below.

TiN-containing steel slabs and comparative steels having the chemical composition shown in Table 1 were hot-rolled into steel plates and thin plates with a thickness of 3.0 to 22.2 mm, and the mechanical properties of the finished steel plates were studied. Its production process is shown in Table 2. The properties of the steel product as hot rolled or processed to a 10% strain are shown in Table 3.

Table 4 and Table 5 show the performance research results of various parts on the rolled steel and the square tube made of it. Fig. 1 (A) has shown the micrograph (magnification 400 times) of the metallographic structure of the plane part (middle part) of the square pipe made of steel T-2 of the present invention, and Fig. 1 (B) has shown with steel S for comparison -2 Micrographs of the metallographic structure of the same part of the manufactured square tube. In the steel of the present invention in Fig. 1 (A), the content of the pearlite phase converted by area percentage is about 15.2%, while in the comparative steel in Fig. 1 (B), only about 4% of the pearlite phase is contained. A very small amount of pearlite phase. FIG. 2 shows the relationship between tensile strength and uniform elongation of steels of the present invention and comparative steels, mainly using the results in Table 4. FIG.

According to these results, the steels of the present invention (C-4, C-6, T-1, T-2, T-3, T-4) were somewhat higher in strength than the respective comparison steels. High uniform elongation is maintained, especially after cold working. These results can be well understood from Fig. 2, which shows the relationship between uniform elongation and strength of hot-rolled steel plates and sheets of the steels of the present invention and comparative steels, and the above-mentioned The relationship between the uniform elongation and the strength of the square tube obtained from the steel plate.

As described above, in the present invention, a high-strength hot-rolled steel sheet having a tensile strength of 333.2 to 607.6/ ^mm2 and an extremely superior uniform elongation even when cold-formed to such an extent that ordinary productivity is not lowered and Thin plates can be produced by limiting the composition of the steel to form larger TiN particles with dispersion enhancing capabilities and by forming an effective pearlite phase therein. High-strength hot-rolled steel plates and thin plates are extremely useful as steel products for general structural use and welded structural use and as materials for round and square pipes, section steel, sheet piles, etc.

Table 1

(Weight %) Steel C SI MN P S Cu Ni CR Mo V Al Ti N CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCS CEQCs C-0.05 0.009 0.007-0.11 0.025-0.0027 0.26CS C-2 0.05 0.009 0.017-0.11 0.03 -     0.030     -        0.0034     0.26CS C-3      0.15     0.05    0.44     0.010    0.016    -     -    0.07     0.02    -     0.026     -        0.0071     0.24IS C-4      0.15     0.05    0.45     0.010    0.017    -     -    0.07     0.02    -     0.027    0.015     0.0071     0.24CS C-5      0.08     0.07    0.31     0.012    0.017 0.20  0.59  0.06     0.10   0.01   0.027    0.001     0.0058     0.19IS C-6      0.08     0.08    0.28     0.010    0.016   0.21  0.60  0.05     0.11   0.01   0.012    0.012     0.0092     0.18CS C-7      0.08     0.07    0.30     0.010    0.017   0.20  0.57  0.05     0.09   0.01   0.023    0.011     0.0167     0.18CS S-1      0.14 0.01    0.46     0.013    0.003    -     -     -        -      -     0.032     -        0.0015     0.22CS S-2      0.12     0.09    0.29     0.016    0.022    -     -    0.05      -     0.005  0.038    0.001     0.0074     0.18CS S-3      0.15     0.39    1.40     0.012    0.013    -     -    0.05      -      -     0.033     -        0.0040     0.41CS S-4 0.06 0.04 0.009 0.010-0.03-0.034-0.0110 0.12IS T-0.09 0.27 0.019-0.06 0.039 0.011I 0.0.09 0.015 0.015 0.015 0.015 0.01115 0.01115 0.01115 0.015 0.015 0.015 0.015 0.015 0.015 0.09 0.0 0.09 0.0 0.09 0.0 0.09 0.0 0.09 0.0 0.015 0.09 0.7 0.09 0 0.09 0.0 0. 0.21IS 0.09 0.09 0.0 0. 0.09 0.0 0.09 0.0 0. 0.7 0.0 0.09 0.0-0-. That 0.021 0.0110 0.23is T-3 0.15 0.39 1.39 0.013 0.013-0.06-0.031 0.022 0.0100 0.41IS T-4 0.05 0.39 0.010-0.031 0.027 0.0090 0.13

Note: CS = comparative steel; IS = steel of the present invention

Ceq.(WES)＝C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14

Table 2

(Temperature °C) Production process Steel slab processing Finish rolling temperature Thin steel coil Air cooling

Thermal temperature takes temperature, start temperature IB A 1200 950 680 -ib B 1230 980 630 -CB C 1230 790 490 -ib D 1180 900-700

Note: (1) TS = steel of the present invention; CS = comparative steel

(2) The temperature is lower than the transformation point Ar ₃

table 3

Converted into Area Steel Type Process Thickness YSl ^* TS ^* El ^* ELu ^* TiN Percentage of TiN Pearlescent Note

(mm) (N/mm ² )(N/mm ² ) (%) (%) Weight (%) Average particle size Bulk phase

(Μm) ( %) CS C-1 A 5.7 305.0 42.7 42.0 22.2 0-16.9 hot-rolled state state

5.4 475.6 475.6 28.0 7.2--10 % Stocking CS CS C-2 A 5.7 286.3 428.5 43.5 21.6 0-17.1 The hot rolling state

5.4 483.4 488.4 26.0 5.2--10 % Stock CS CS C-3 A 5.7 306.0 40.5 21.0 0-16.6 The hot rolling state

510.9 517.8 23.0 2.0-10 % strain IS C-4 A 5.7 319.7 451.1 44.0 20.051 3.82 16.8 hot-rolled state

515.5 522.7 31.0 9.0-10 % Stocking CS CS CS C-5 A 8.5 240.3 339.3 47.0 22.8 0-10.7 hot rolling state

6.9 415.8 424.6 21.0 1.2--10 % Stocking IS C-6 A 8.7 247.1 45.5 22.0 0.0041 3.44 10.9 The hot rolling state

6.9 426.6 454.0 26.0 6.4--10 % Stock CS CS C-7 C 8.5 478.5 34.0 17.5 0.0038 3.20 4.5 The hot rolling state

6.9 559.0 566.8 20.1 1.4 - - - 10% strain

Note: IS = steel of the present invention; CS = steel for comparison

^* YSI=yield strength; TS=tensile strength; El=elongation;

Elu = uniform elongation

The tensile test piece is prepared according to the test piece specification of JIS Z22015

Table 4 Steel Type Process Thickness YSl ^* TS ^* El ^* ELu ^* TiN Flat converted area of TiN Note

(mm) (N/mm ² ) (N/mm ² ) (%) (%) Weight Average particle size Percentage of pearlescent

(%) Bulk(%)

(Μm) ( %) CS S-B 3.2 306.9 444.2 39.0 19.2 0-9.3 hot-rolled state state

3.3 441.3 468.7 33.2 11.6 - - - - sq.tube F

B 3.2 311.8 450.1 39.2 18.8 - - 9.5 Hot-rolled

3.2 376.6 454.0 36.0 17.3 - - - - sq.tube F

B 6.0 312.8 438.3 40.6 19.7 - - 9.7 Hot-rolled state

6.1 396.2 444.2 37.0 14.5--SQ.Tube FCS S-2 C 3.2 337.4 34.8 16.3 0-4.4 hot-rolled state state

3.2 471.7 505.0 20.4 4.0 - - - - - sq.tube F

C 6.0 390.3 471.7 29.0 9.8 - - 4.2 Hot-rolled

6.0 454.0 498.2 23.6 4.9 - - - - - sq.tube F

C 5.7 310.9 433.4 38.0 18.7 - - 4.3 Hot-rolled

5.8 423.6 477.6 29.0 5.5 - - - - sq.tube F

Table 4 (Continued) Steel Type Process Thickness YSl ^* TS ^* El ^* ELu ^* TiN Flat Converted Area of TiN Note

(mm) (N/mm ² ) (N/mm ² ) (%) (%) Weight Average particle size Percentage of pearlescent

( %) (Μm) body phase ( %) IS T-1 B 3.0 314.8 444.2 39.5 19.5 0.0055 6.50 14.7 hot-rolled state (top)

3.1 330.5 458.9 36.0 16.6 - - - - - sq.tube F(Top)

                                                                                 

3.1 373.6 461.9 36.5 17.0 - - - - - sq.tube F(middle)

B 3.1 337.3 502.1 34.0 17.5 - - - 14.1 Hot rolled state (bottom)

3.1 419.7 508.0 31.0 13.6 - - - - - sq.tube F(bottom)

B 3.1 640.3 705.1 28.0 6.2 - - - - - sq.tube C(bottom)

3.1 597.2 642.3 32.0 7.9--SQ.Tube C (bottom) IS T-2 B 3.0 340.7 479.5 40.0 19.8 0.0072 6.21 15.5 hot rolling state (top)

3.1 327.5 472.7 35.0 16.0 - - - - - sq.tube F(Top)

B 3.0 303.0 463.8 37.0 19.4 - - - 15.3 as rolled (middle)

3.1 380.5 471.7 35.0 16.2 - - - - - sq.tube F(middle)

B 3.1 326.5 518.7 35.0 17.6 - - - 16.0 Hot rolled state (bottom)

3.1 386.4 480.5 35.0 16.0 - - - - - sq.tube F(bottom)

B 3.1 596.2 660.9 33.0 12.0 - - - - - sq.tube C (bottom)

3.1 581.5 652.1 35.0 12.3 - - - - - sq.tube C(bottom)

Note: (1) IS = steel of the present invention; CS = steel for comparison

(2) ^* YSl=yield strength; TS=tensile strength; El=elongation; Elu=uniform elongation

(3) Sq.tube F = plane part of square tube; Sq.tube C = corner of square tube

(4) Tensile specimens are prepared according to the specimen specifications of JIS Z 2201 5, only the corner specimens are rooted

Prepared according to the test piece specification of JIS Z2201 12B.

(5) The size of each type of steel pipe is 75mm×75mm

Table 5 Steel type Process Thickness 0.2%PS ^* TS ^* El ^* ELu ^* TiN Average converted area of TiN Note

(mm) (N/mm ² ) (N/mm ² ) (%) (%) Weight Particle Size Percentage of Pearlescent

( %) (Μm) Body phase ( %) CS S-3 D 22.2 353.0 538.4 28.0 20.0 0-5.5 hot-rolled state

22.0 373.6 549.1 24.7 16.6 - - - - - sq.tube F

22.1 559.9 649.2 15.0 5.2--SQ.Tube CCS S-4 C 9.0 294.2 421.7 40.0 17.5 0-3.8 The hot rolling state

9.1 374.6 449.1 35.0 9.5 - - - - - sq.tube F

9.0 479.5 530.5 26.0 4.2--SQ.Tube CIS T-3 D 22.1 355.0 540.3 29.0 21.3 0.0076 5.77 5.5 The hot rolling state

22.0 377.5 551.1 27.1 18.7--SQ.Tube CIS T-4 B 8.9 287.3 441.3 38.5 20.5 0.0093 4.06 6.2 The hot rolling state

9.0 335.4 444.2 38.0 19.6 - - - - - sq.tube F

9.0 493.3 554.1 36.0 16.0 - - - - - sq.tube C Note: (1) Prodn＝Product

(2) IS = steel of the present invention; CS = comparative steel

(3) ^* 0.2% PS = 0.2% yield point, TS = tensile strength; El = elongation rate;

Elu = uniform elongation

(4) gq.tube F = plane part of square tube; Sq.tube C = corner of square tube

(5) S-3, T-3: Each square tube has a size of 350mm×350mm, stretched

The test piece is prepared according to the test piece specification of JIS Z2201 1B.

(6) S-4, T-4: Each square tube has a size of 250mm×250mm, stretched

The test piece is prepared according to the test piece specification of JIS Z2201 5.

Claims (5)

1. high tensile hot rolled steel sheet and thin plate that superior uniform elongation is arranged after cold working, contain weight percent and be 0.04% to 0.25% C, 0.0050% to 0.0150% N and 0.003% to 0.050% Ti, described steel plate and thin plate have 0.10% to 0.45% the carbon equivalent of determining by following formula (Ceq.), it also contain have average particle size particle size be 1 μ m and 30 μ m and disperse therein, weight percent is 0.0008% to 0.015% TiN:

Ceq.＝C+Si/24+Mn/6+Ni/40+Cr/5+Mo/4+V/14。

2. high tensile hot rolled steel sheet according to claim 1 and thin plate, it is characterized by, steel plate and thin plate also contain at least a composition of choosing from following combination, this combination is that 0.01% to 0.7% Si, weight percent are that 0.1% to 2.0% Mn, weight percent are that 0.05% to 1.0% Ni, weight percent are that 0.05% to 1.0% Cr, weight percent are that 0.02% to 0.5% Mo and weight percent are that 0.005% to 0.2% V forms by weight percent.

3. according to claim 1 or 3 described high tensile hot rolled steel sheet and thin plates, it is characterized by, also further contain at least a composition of from following combination, choosing in steel plate and the thin plate, this combination is by by weight percentage, is respectively 0.05% to 1.0% Cu, 0.005% to 0.05% Nb, 0.001% to 0.1% Al, 0.0005% to 0.0020% B, 0.0005% to 0.0070% Ca and 0.001% to 0.050% REM and forms.

4. high tensile hot rolled steel sheet according to claim 1 and thin plate is characterized in that, will control to satisfy following condition the content of P and S:

P≤0.025%, weight %,

S≤0.025%, weight %, and

P+S≤0.04%, weight %.

5. technology that is used for after cold working, producing the high tensile hot rolled steel sheet and the thin plate of excellent in uniform elongation, it comprises that with containing weight % be that 0.04% to 0.25% C, weight % are that 0.0050% to 0.0150% N and weight % are that 0.003% to 0.050% Ti and the plate slab with carbon equivalent (Ceq.) 0.10% to 0.45% are heated to 1000 to 1300 ℃, begin rolling through heating hot plate slab and be at least transition point Ar ₃Temperature under carry out finish rolling, begin air cooling from the temperature that is at least 500 ℃ then.

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