JPH07138639A - Production of low yield ratio steel tube excellent in low temp. toughness - Google Patents

Abstract

PURPOSE:To manufacture a high-tensile steel tube having a low yield ratio which is suitable to use as the building material for structural steel-work by specifying the chemical composition of steel and further executing special two steps quenching including two phases temp. region quenching. CONSTITUTION:After hot rolling a slab specifying the contents of C, Si, Mn, Cu, Ni, Nb and V, by directly energizing a steel sheet, the temp. of the center of the steel sheet is raised at the rate of 1-25 deg.C/sec, simultaneously the surface and back faces are cooled with water, quenching is executed from the temp. region where the temp. of the center of the steel sheet is higher than the point Ac3 and the temps. of the surface and back faces are lower than the point Ac3, next heating and quenching are executed in a two phases temp. are higher than the point Ac1 and lower than point Ac3 and further the tube making using this heat treated steel sheet is executed. In this way, the low yield ratio/high- tensile steel tube of 0.2% YS>=440N/mm<2>, TS=590-740N/mm<2> and YR >=80% excellent in ductility and toughness is manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low yield ratio high tensile steel pipe suitable for use as a building material for steel structures such as buildings, bridges and tanks.

[0002]

2. Description of the Related Art Conventionally, steel structures such as buildings, bridges, and tanks have been gradually increased in size.
Higher strength and thicker wall are required. The required strength level is a tensile strength of 590 N / mm ² or more.

Further, in order to prevent brittle fracture occurring in a steel structure, the above steel materials are required to have a low yield ratio, that is, a low ratio of yield strength (yield point or proof stress) to tensile strength. Has been done. The required yield ratio level is 80% or less. Various methods for producing such a low-yield-ratio high-strength steel material have been conventionally proposed.

Japanese Unexamined Patent Publication No. 2-205626 discloses C: 0.02 to
0.20%, Si: 0.02-0.50%, Mn: 0.20-2.00% and A
l: 0.010 to 0.100%, Cu: 0.10 to 2.00%, N
b: 0.005 to 0.10%, V: 0.005 to 0.10%, Ti: 0.005
~ 0.10% of 1 or 2 or more, if necessary, Ni: 0.10 ~ 2.00%, Cr: 0.10 ~ 1.00%, Mo: 0.05
To 0.50% and B: 0.0003 to 0.0030%, one or more of them are expressed by the following formula D _I = (17−0.1Nr) × 8√C × (0.7Si + 1) × (3.3Mn + 1) × (0.4Cu + 1) ) × (0.4Ni +1) × (2.2Cr + 1) × (3.0Mo + 1) × (1.8V + 1) × (200B + 1) ≦ ex p (0.015t + 2.0) where Nr: ASTM austenite grain size number during quenching t: Plate thickness (mm) is contained in the steel, and the steel slab consisting of the balance Fe and unavoidable impurities is hot-rolled, and then subjected to quenching and tempering treatment or direct quenching and tempering treatment. There has been proposed a method of producing a high yield steel plate having a low yield ratio suitable for a material for materials.

In short, this method suppresses the hardenability represented by D _I to a specific level, while at the same time Cu,
QT or D by adding precipitation strengthening elements such as Nb, V, Ti
Q-T (however, Q: austenite quenching, DQ:
By performing direct quenching in the austenite region and T: tempering, it has a low yield ratio and a specified high strength (tensile strength: 588N).
/ mm ² (60 kgf / mm ² ) grade or higher) and a low yield ratio high tensile steel sheet with excellent weldability.

Further, in Japanese Patent Laid-Open No. 3-219012, C:
0.03 to 0.10%, Si: 0.05 to 0.60%, Mn: 0.60 to 2.00%,
Mo: 0.10 to 0.50%, P: 0.030% or less, S: 0.020% or less, Ni: 1.00% or less, Cr: 0.70% or less, C
u: 0.70% or less, V: 0.06% or less, Nb: 0.05% or less and B: 0.0050% or less, if necessary, Ti: 0.003 to 0.05%, balance Fe and unavoidable impurities And weld crack susceptibility index P _CM (%) = C
＋ Si / 30 ＋ Mn / 20 ＋ Ni / 60 ＋ Cr / 20 ＋ Cu / 20 ＋ Mo / 15 ＋
Steel with V / 10 + 5B of 0.16 to 0.21% is hot-rolled and then immediately quenched to 300 to 550 ° C or air-cooled to Ac ₃
After being reheated to a temperature above the point, it is rapidly cooled to 300-550 ° C, then air-cooled to room temperature, and further between the Ac ₁ -Ac ₃ transformation points.
After heating and holding at the phase region temperature, quenching is performed at a cooling rate of air cooling or higher, and then tempering is performed at a temperature of 450 to 600 ° C, which reduces weld cracking susceptibility, requires no preheating, and has a yield ratio of 80% or less. A method for producing a high-strength steel sheet having a low yield ratio and a tensile strength of 588 N / mm ² (60 kgf / mm ² ) or more has been proposed.

That is, this method has a C content of 0.10%.
This is a method of performing quenching and tempering treatment on the steel having the above-described composition with the alloying elements reduced and having the above-described composition, with the structure before two-phase quenching being made a slightly rough structure.

Further, in Japanese Patent Laid-Open No. 61-288018, a steel pipe that has been hot-rolled or reheated to a high temperature is rapidly cooled from a high temperature, and then locally tempered to a surface hardened layer of the steel pipe by a high frequency heating method. Steel pipe with high strength and low yield ratio (tensile strength: 634 N / m
m ² or more, a yield ratio: 83.0% or less) A method for producing have been proposed.

JP-A-2-205626 or JP-A-3-205626
According to the method proposed by Japanese Patent No. 219012, it is possible to achieve the target tensile strength and yield ratio for a steel sheet. However, when steel pipes are manufactured, the yield ratio of steel pipes exceeds 80% due to pipe manufacturing, and the yield ratio is 80% or less.
It is not possible to manufacture high-strength steel pipes with a low yield ratio of 8 N / mm ² or more.

On the other hand, according to the method proposed in Japanese Patent Laid-Open No. 61-288018, although it is possible to surely reduce the yield ratio, it is necessary to perform quenching and tempering on the steel pipe after pipe forming.
In order to carry out heat treatment over a period of time, it is necessary to newly install heating and quenching equipment for steel pipes and secure schedules for that, which is a problem in terms of cost and delivery time of steel pipes.

[0011]

Based on the above, the present inventors have previously proposed in Japanese Patent Application No. 5-130682, Ceq.
(%) = C + Si / 24 + Mn / 6 + Ni / 40 + Mo / 4 + V / 14 + Cr /
The carbon equivalent Ceq shown in 5 is specified to 0.38 to 0.45%, and the hot rolling conditions are also specified to a predetermined range to control the hardness of the steel sheet surface, and to increase the proof stress due to the pipe forming process in advance. Yield strength of hot rolled steel sheet after hot rolling,
We proposed a method to suppress the tensile strength, yield ratio, etc. near the lower limit of the target values and satisfy the target values after pipe manufacturing.

However, even with this method, hardening of the surface after pipe forming is unavoidable, and the toughness at the position 1/4 of the thickness (t / 4 position) from the surface of the steel pipe is the Charpy absorbed energy at 0 ° C. It only guarantees vE ₀ ≧ 27J.

Considering future use in cold regions and improvement in safety against fracture, further improvement in low temperature toughness is required. Controlling Ceq to 0.38 to 0.45% is also a problem in terms of steps and manufacturing costs in the current manufacturing method.

Here, an object of the present invention is to construct a building, a bridge,
It is to provide a method for producing a high-strength steel pipe having a low yield ratio and a yield ratio of 80% or less and a tensile strength of 590 N / mm ² or more, which is suitable for use as a building material for a steel structure such as a tank.

More specifically, the object of the present invention is to
(0.2% YS) ≧ 440N / mm ² , Tensile strength (TS): 590〜740N / m
m ^2, the yield ratio (YR) ≦ 80%, elongation (El) ≧ 20%, - 20 ℃ low yield ratio high-strength steel pipe excellent in low temperature toughness having the mechanical properties of the Charpy absorbed energy vE _-20 ≧ 27 J of It is to provide a manufacturing method of.

Still another object of the present invention is to meet the above-mentioned object at a low cost without the restrictions on the Ceq and hot rolling conditions required for the method proposed in the above-mentioned Japanese Patent Application No. 5-130682. Manufactures steel pipes, and 1 / thick of the thickness from the surface of the steel pipe
It is an object of the present invention to provide a method for manufacturing a low yield ratio high strength steel pipe that improves the toughness at position 4.

[0017]

[Means for solving the problems] As quenching
As is conventionally known, there is a certain correlation between the austenite grain size on the surface of d) and the surface hardness. On the front and back of the steel sheet, the austenite crystal grains become coarse by heating, but if they are just quenched, such coarse crystal grains make it easy to quench, and the hardness of the front and back surfaces increases significantly. The high hardness due to as-quenching remains unresolved even by (two-phase region quenching and tempering), and the hardness of the front and back surfaces is significantly increased due to work hardening during pipe manufacturing, resulting in an increase in the yield ratio (YR). I will end up. Therefore, in order to suppress the yield ratio of the steel pipe to 80% or less, coarsening of the austenite crystal grains on the front and back surfaces and the Vickers hardness (Hv 10kgf) on the surface of the steel pipe are prevented.
It should be kept below 200.

As a result of further diligent studies, the present inventors have found that the first quenching, which is carried out prior to the two-phase region quenching and tempering, is performed to prevent the austenite crystal grains on the front and back surfaces from coarsening by changing the heating temperature of the front and back surfaces. It was found that it is effective to set the Ac to less than ₃ points and to set only the central part to Ac ₃ or more. Such surface and center heating with a temperature difference is not possible by a normal method, but the present inventors have used an electric heating device to maintain the surface at a high temperature (Ac ₃ or more) while maintaining the center at a high temperature. We found that it is possible by forced cooling with water.

In the present invention, C: 0.02% by weight is used.
0.20%, Si: 0.02-0.50%, Mn: 0.50-2.00%, Cu: 0.10-1.5%, Ni: 0.10-0.50%, Nb:
A steel slab having a steel composition consisting of 0.005 to 0.10% and V: 0.005 to 0.10%, one or more selected from the group consisting of the balance Fe and unavoidable impurities is directly applied to a steel sheet after hot rolling. By energizing, the temperature of the steel sheet center is raised at a rate of 1 to 25 ° C / sec, and at the same time the front and back surfaces are forcibly cooled, and quenching is performed from a temperature range where the steel sheet center is at least Ac ₃ points and the front and back surfaces are less than Ac ₃ points. Low yield ratio excellent in low temperature toughness, characterized by performing reheating to a two-phase temperature range of Ac ₁ point or more and Ac ₃ point or less, quenching, and then tempering before pipe forming. It is a method of manufacturing steel pipes.

[0020]

A major feature of the manufacturing method according to the present invention is that the center of the steel sheet is heated to the Ac ₃ point or more and the front and back surfaces are heated to the Ac ₃ point or less. Specifically, in the first heating and quenching That is, the direct current heating method in which heating is performed by resistance heating of the steel sheet itself by directly passing an electric current through the steel sheet is adopted. By utilizing such direct electric heating, quenching can be performed in a state in which the temperature of the front and back surfaces of the steel sheet is lowered only by forcedly cooling the steel sheet surface as described above, for example, water cooling.

Further, as a function of the direct current heating method, it becomes possible to control the rate of temperature rise near the set temperature, which cannot be realized by the conventional furnace, and heating can be performed up to the set temperature at a constant rate, thus shortening the heating time and the temperature. It is possible to manage easily. The operation itself of direct electric heating is considered to be obvious to those skilled in the art from the above description, and therefore further description is omitted.

Among the steel sheets thus heated by direct energization, the front and back surfaces of the steel sheet are subjected to quenching treatment by adjusting the quenching temperature by forced cooling such as water cooling to suppress the coarsening of γ grains on the surface. Low surface hardness on steel plate (Hv =
It will be possible to control (below 150). As a result, the Charpy absorbed energy vE _-20 ≧ 27J at −20 ° C. can be stably guaranteed even if the surface is hardened by work hardening by pipe manufacturing, and since the rapid heating is performed, the growth of austenite grains in the central part is also small, It is also possible to control the yield ratio (YR) after pipe production without restriction of the carbon equivalent Ceq. This makes it possible to manufacture a low yield ratio steel pipe excellent in low temperature toughness.

FIG. 1 is a graph showing changes in the steel plate temperature at this time. In the heating of the central portion by electric heating shown as the upper line in the figure, the rate of temperature rise becomes a problem. Heating rate is 1 ℃ /
If it is less than a second, dislocations etc. obtained by hot rolling disappear,
If the heating rate is higher than 25 ° C / sec, the heating of the steel sheet becomes uneven, so the heating rate at the center of the steel sheet was limited to 1 to 25 ° C / sec. This heating causes the additional element to form a solid solution. In this way, the central part of the plate is heated to the Ac ₃ point or higher, and the front and back surfaces are shown as the lower line in the figure, but are heated to less than the Ac ₃ point by, for example, water cooling. It may be performed by adjusting the degree of forced cooling such as water amount adjustment. The subsequent heat treatment will be described later. Next, the reasons for limiting the steel composition and heat treatment conditions in the present invention as described above will be explained.

C: 0.02 to 0.20% C needs to be added in an amount of 0.02% or more to secure the strength, but if added in excess of 0.20%, the weldability is impaired.
Therefore, in the present invention, the C content is limited to 0.02% or more and 0.20% or less.

Si: 0.02 to 0.50% Si is added mainly for deoxidizing steel, and the addition amount is usually 0.02% or more, and it is also effective for reducing the yield ratio. However, if added in excess of 0.50%, the toughness deteriorates, which is not preferable. Therefore, in the present invention, the Si content is limited to 0.02% or more and 0.50% or less.

Mn: 0.50 to 2.00% Mn needs to be added in an amount of 0.50% or more to secure the strength, but if added in excess of 2.00%, the weldability is impaired.
Therefore, in the present invention, the Mn content is limited to 0.50% or more and 2.00% or less.

Further, in the present invention, in order to increase the strength, Cu: 0.10 to 1.5%, Ni: 0.10 to 0.50%, Nb: 0.005.
.About.0.10% and V: 0.005 to 0.10%, and one or more kinds selected from the group consisting of. The reasons for limiting the composition of these elements will be described below.

Cu: 0.10 to 1.5% Cu is effective in increasing the strength by adding 0.10% or more, but if added in excess of 1.5%, the weldability is impaired and hot cracking occurs. Therefore, when Cu is added, its content is preferably limited to 0.10% or more and 1.5% or less.

Ni: 0.10 to 0.50% Ni is effective in increasing the strength by adding 0.10% or more, but if added excessively, both weldability and low temperature toughness deteriorate. In particular, Ni is an expensive element, and if added in excess of 0.50%, there is a problem in terms of economy. Therefore, when Ni is added, its content is preferably limited to 0.10% or more and 0.50% or less.

Nb: 0.005 to 0.10% Nb is effective for increasing the strength particularly in the direct quenching and tempering method by adding 0.005% or more, but if added in excess of 0.10%, both toughness and weldability deteriorate. . Therefore, when Nb is added, its content is preferably limited to 0.005% or more and 0.10% or less.

V: 0.005 to 0.10% V increases the strength of steel by adding 0.005% or more, but 0.10% to 0.10%
If it is added excessively in excess of%, not only the effect is saturated, but also the weldability is deteriorated. In the present invention, when V is added, its content is preferably limited to 0.005% or more and 0.10% or less. Compositions other than the above are Fe and inevitable impurities.

According to the present invention, the steel slab having the above-mentioned steel composition is subjected to hot working, but in the case of the present invention, hot rolling conditions are not particularly limited. Hot rolling start temperature is 1100 ~
For example, about 1000 ° C, the finishing temperature is about 800 to 700 ° C, and the total rolling reduction during hot rolling is about 66 to 96%.

Quenching from the center Ac ₃ points or more, Ac ₁ point
Quenching from 2 phase region to Ac ₃ point, tempering in the present invention, in order to ensure the strength of the hot rolled steel sheet finished hot-rolled steel sheet center performs quenching from a temperature range of not lower than ₃ points Ac. If the quenching temperature of the center is less than Ac ₃ points,
This is because the desired strength cannot be secured. At this time, the front and back surfaces of the steel sheet are rapidly cooled from less than Ac ₃ point, as described above. The mode of quenching is not particularly limited. Water quenching, oil quenching, etc. may be appropriately performed.

Next, in the present invention, quenching is carried out from the two-phase region of Ac ₁ point to Ac ₃ point to precipitate the soft ferrite phase and the hard bainite phase to achieve a low yield ratio, and the soft ferrite phase and Tempering is performed to prevent deterioration of toughness due to precipitation of hard bainite phase. Tempering may be performed at a normal temperature (500 to 600 ° C).

In the present invention, the hot-rolled steel sheet thus heat-treated is used as a raw material for pipe manufacturing to manufacture a steel pipe. The pipe manufacturing method is not limited to a particular means. For example, a method for producing a welded steel pipe such as an electric resistance welded pipe, a forged steel pipe, and a UOE steel pipe can be applied.

Thus, according to the present invention, 0.2% YS
≧ 440N / mm ² , TS: 590 to 740N / mm ² , YR ≦ 80%, El ≧ 20
%, VE _-20 ≧ 27J mechanical properties, structures, bridges,
It has become possible to manufacture low yield ratio high tensile steel pipes suitable for use as building materials for steel structures such as tanks.
Further, the present invention will be described in detail with reference to examples, but this is an example of the present invention and the present invention is not limited thereto.

[0037]

EXAMPLES Table 1 shows the conditions for producing steel pipes, and Table 2 shows the results of measuring the dimensions and characteristic values of the steel pipes. Both Tables 1 and 2 are the top 1
8 to 8 are examples of the present invention, and lower 9 to 19 are comparative examples. Less than,
An example of the present invention will be described.

The steel compositions shown in Table 1, Ac ₁ point (° C.), and
A steel piece with an Ac ₃ point (° C) is water-cooled on the front and back sides while being electrically heated, and the first quenching (Q) is performed from the temperature range where the steel plate center is at or above the Ac ₃ point and the front and back sides are at or below the Ac ₃ point. went. Next, reheat to a two-phase temperature range of Ac ₁ point or more and Ac ₃ point or less, perform the second quenching (Q '), and further temper (T), and then perform pipe forming processing, and describe in Table 2. A steel pipe with a plate thickness and outer diameter of

[0039]

[Table 1]

[0040]

[Table 2]

From these ERW pipes, in the longitudinal direction
A JIS No. 4 test piece is cut out and the yield strength (0.2% YS), tensile strength (TS), yield ratio (YR) and elongation (El) are measured, and at a position 1/4 of the thickness from the surface of the steel pipe. A JIS No. 4 test piece was cut out in the longitudinal direction from the sample, and the Charpy absorbed energy vE- ₂₀ (J) at -20 ° C was measured. The results are also summarized in Table 2.

Since the steel types No. 1 to No. 8 in Table 2 satisfy all the conditions specified in the present invention, 0.2% YS ≧ 440 N / m
m ² , TS: 590 to 740 N / mm ² , YR ≤ 80%, El ≥ 20%, vE _-20
All of the target values of the present invention of ≧ 27 J are satisfied.

On the other hand, in the steel type No. 9, the C content was below the range of the present invention, so that 0.2% YS was reduced. Steel type No. 10 had a Si content above the upper limit of the range of the present invention, and thus had a decreased vE- ₂₀ .

In steel type No. 11, since the Mn content was below the lower limit of the range of the present invention, TS and vE- ₂₀ were lowered. Steel grade
In No. 12, since the Nb and V contents exceeded the upper limit of the range of the present invention, vE- ₂₀ was lowered.

In steel type No. 13, the Cu and Ni contents were above the upper limit of the range of the present invention, so vE _-20 was lowered. Steel types No. 14 and 15 had vE- ₂₀ lowered because the front / back surface temperature during the first quenching exceeded the Ac ₃ point.

In steel type No. 16, the center temperature of the steel sheet during the first quenching was below the Ac ₃ point, so 0.2% YS and TS decreased. Steel type No. 17 had a higher YR because the quenching temperature from the two-phase region (second quenching, Q ') was above the upper limit of the range of the present invention.

Since the steel types No. 18 and 19 did not conduct electric heating in the first quenching (Q) and used a normal heating furnace, the front surface / back surface temperature exceeded the Ac ₃ point like the steel plate center temperature. Therefore, vE _-20 has decreased.

[0048]

As described above in detail, according to the present invention,
^{2% YS ≧ 440N / mm 2} , TS: 590 ~740N / mm 2, YR ≦ 80%, El
It has mechanical properties of ≧ 20% and vE _-20 ≧ 27J, and it is possible to manufacture high-strength steel pipe with low yield ratio suitable for use as a building material for steel structures such as buildings, bridges and tanks. Became.

[Brief description of drawings]

FIG. 1 is a graph illustrating temperature changes of a central portion of a steel plate and front and back surfaces during the first heating and quenching of the present invention.

Claims (1)

[Claims] 1. By weight%, C: 0.02 to 0.20%, Si: 0.02 to 0.50%, Mn: 0.50 to 2.00.
%, Cu: 0.10 to 1.5%, Ni: 0.10 to 0.50
%, Nb: 0.005 to 0.10%, and V: 0.005 to 0.10%, one or two or more selected from the group consisting of the balance Fe and unavoidable impurities. , By directly energizing the steel sheet, the center of the steel sheet is heated at a rate of 1 to 25 ° C / second, and at the same time, the front and back surfaces are forcibly cooled, and the center of the steel sheet becomes Ac ₃ points or more and the front and back surfaces become less than Ac ₃ points. Like above, quenching from the temperature range at that time, then reheating to a two-phase temperature range of Ac ₁ point or more and Ac ₃ point or less, quenching, and further tempering before pipe making A method for producing a low yield ratio steel pipe excellent in low temperature toughness, which is characterized by the following.