JPH04187713A - Production of high tensile strength steel structure excellent in weldability - Google Patents

Abstract

PURPOSE:To produce a high tensile strength steel structure excellent in weldability by successively applying heating, controlled rolling, water cooling, heating, air cooling, cold working, and ageing heat treatment to a steel bloom, in which composition and critical hardening diameter are respectively specified, under respectively specified conditions. CONSTITUTION:A steel bloom which has a composition consisting of, by weight, 0.04-0.07% C, 0.05-0.40% Si, 0.8-1.5% Mn, 0.5-1.8% Ni, 0.8-1.7% Cu, <=0.2% Mo, 0.05-0.05% Al, 0.005-0.015% Nb, 0.005-0.02% Ti, <=0.0050% N, and the balance Fe and in which critical hardening diameter Di(cal) represented by equation is regulated to 35-65(mm) is heated to 950-1200 deg.C, subjected to controlled rolling at 700-750 deg.C to >=80mm plate thickness, and water-washed. Further, heating is applied to the region between the surface and the position at a depth of 10mm from the surface at a temp. between Ac3 and 1200 deg.C, followed by air cooling. Then, after cold working is exerted at >40-<50% draft, ageing heat treatment is performed at 600-700 deg.C heating temp. By this method, the 80kg/mm<2> class high tensile strength steel structure improved in toughness in a welded bond zone can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing high-strength steel with excellent weldability.
Especially when used as a welded structure, it has excellent weld toughness and can achieve cold working of more than 40% and less than 50%.
The present invention relates to a method for manufacturing a kg/mm class 2 high tensile strength steel structure.

[Prior Art] Generally speaking, high-strength steel is used more and more frequently in recent years for the purpose of reducing the weight of structures, etc., because it is generally used to reduce the thickness of steel structures when constructing steel structures due to its high strength. In particular, high tensile strength steel of 80 kgf/m+n2 class is increasingly being used for large welded structures on the sea and on land.

For example, in recent years, construction of structures for oil test drilling at depths of 100 m or more has been underway, but such structures are subject to harsh environmental conditions such as sea conditions and weather, and are exposed to waves up to 30 m in height. Since it is necessary to withstand the environment in which it is used, it is desired to develop steel materials that can meet such demands. As such a steel material, it is considered advantageous to use a particularly welded steel pipe with a strength of 80 kgf/mm2 class in terms of structural design. For such steel pipes, as can be seen in the report on pages 22-33 of the April 1985 issue of the Journal of the Welding Society, conventional 80 kgf/mm" edge high-strength steel plates with many alloying elements are now semicircular by hot oiling. After forming the two steel plates into a tubular shape, welding the joined parts together to form a pipe, and then quenching and tempering to make a high tensile strength welded steel pipe of 80 kgf/mm2.However, the conventional 80 kgf/mm mm
Z High tensile strength steel cannot be said to have sufficient weld bond toughness during welding due to its high C content and large number of alloying elements.

On the other hand, the present invention 8 etc. has been developed as a result of studying the manufacturing means of high tensile strength steel as shown in JP-A No. 62-142723.
In addition, it is a composition system with a quenching critical diameter Di of 35 to 65 (mm), which is an indicator of hardenability, and the work hardening that occurs when performing cold bending and the subsequent aging heat treatment. 80kgf/ by utilizing the age hardening that occurs.
It is possible to secure a strength of 2 mm2 or more, and as a result, the toughness of the welded part is significantly improved compared to the conventional 80 kgf/mm2 steel pipe.

However, recently, welded steel pipes have become extremely thick and have a smaller diameter, requiring more than 40% cold working. However, in the above-mentioned invention, cold working exceeding 40% causes surface cracks, so that the working cannot be achieved, and the toughness of the base material in the C direction decreases. Therefore, improvement is necessary.

[Problem to be solved by the invention] The present invention provides a novel high-strength steel structure that is particularly used as a welded structural member formed by cold working of more than 40% and less than 50%, and has excellent weld bond toughness during welding. The purpose of this invention is to provide a method for manufacturing.

[Means for Solving the Problems] As a result of studying methods for manufacturing high-strength steel, the present inventors found that in order to increase cold working to over 40%, it is sufficient to reduce the hardness of the plate surface layer. , Also, regarding strength, if the strength decreases only in the surface layer of the plate, the effect on the strength decrease across the entire plate thickness is small;
Furthermore, it was confirmed that the decrease in toughness of the base material in the C direction due to cold working of more than 40% can be improved by increasing the tempering temperature, and that the resulting decrease in strength can be offset by increasing the cold working.

Therefore, the present invention heats the surface layer of high-strength steel obtained by alloying elements and controlled rolling to achieve cold working of more than 40% and less than 50%, and then performs a tempering treatment to improve the weld bond toughness during welding. We have established a new manufacturing method for high-strength steel structures with excellent performance.

That is, weight (%') TeC: 0.04-0.07
%, Si2 0.05-0.40%, Mn
: 0.8 to 1.5%, Ni: 0.5 to 1.5%
1.8%, Cu: 0.8-1.7%, Mo: 0.2
% or less, Al: 0.005-0.05%, Nb
: 0.005-0.015%, Ti: 0.005-0
．． 0.02%, more than 0.0050% of N, the balance is Fe, and the critical diameter Di (cat) for quenching is 35.
A steel slab with a thickness of ~65 (mm) is heated to 950°C to 120°C, then controlled rolling is performed at 700 to 850°C to a thickness of 80 mm or more, water-cooled, and further heated to an AC of 3 to 1200°C.
It is characterized by heating the area from the surface to 10 mm below the surface at a temperature of 10°C, then air cooling, performing cold working of more than 40% but less than 50%, and then subjecting it to aging heat treatment at a heating temperature of 600 to 700°C. A method for manufacturing high-strength steel structures.

However, Di (cal) = 9.175 ko (1 + 0.7Sj)
(1+3.33Mn) (100, 35Cu) (++0
J6Ni) (1+2.]6Cr)x (1+3.0Mo
) (]+1.75V) (1+1.77AI) (mm
) [Function] The present invention will be explained in detail below.

First, in the present invention, cold working refers to a process of cold forming a target welded structural member into a desired shape,
For example, those that bend steel plates into fan, semicircular, or circular shapes;
Alternatively, the material may be appropriately selected depending on the shape of the steel structural member, such as a steel plate partially bent into a V-shape or a U-shape, or a steel plate punched into a convex or concave shape.

Next, in the present invention, the strength before cold working is lowered,
In order to improve workability and fully demonstrate work hardening by cold working and age hardening by aging heat treatment,
Steel component composition: C: 0.04-0.07%, Si
: 0.05-0.40%, Mn: 0.8-1.
5%, Ni: 0.5-1.8%, Cu: 0.8-1.7
%.

MO: 0.2% or less, Al: 0.005-0.0
5%, Nb: 0.005-0.015%, Ti
: Contains 0.005-0.02%, N: 0.0
050% or less, the remainder consists of Fe, and the quenching critical diameter D
The object is a steel billet whose i (cat) is 35 to 65 (mm).

However, Di (cal) = 9.175 J7r (+ +0.
7Si) (1+3.33Mn) (1+0.35Cu)
(1+0.36Ni) (++2.16Cr)
1+3.0Mo) (1+1.75V)(1+1.77
AI) (mm) The reason why the chemical components are limited in this way in the present invention is as follows.

First of all, C is necessary to obtain strength. If it exceeds 0.07%, it will not be possible to obtain the same level of toughness in the welded area as conventional 80kg class high-strength steel, and for sufficient improvement, 0.07% The following shall apply. If it is less than 0.04%, the hardenability will be extremely reduced, so the lower limit is set at 0.04%.

Sl is essential as a deoxidizing element during steel manufacturing, and if it is less than 0.005%, it is ineffective, and if it exceeds 0.4%, the toughness decreases, so it is set at 0.05 to 0.4%.

Mn is an effective element for ensuring hardenability, and it also has the property of shortening the age hardening time of Cu, so it is effective in steels that utilize age hardening, and it is effective when added at 0.8% or more. It is true. However, addition of more than 1.5% increases the rolling anisotropy of ductility and toughness and deteriorates the toughness and ductility in the direction perpendicular to rolling and in the thickness direction, so the content is set at 0.8 to 1.5%.

Ni is effective in improving the toughness of the base metal and the weld core, but if it is less than 0.5%, the effect is small, and on the other hand, even if it contains 1.8% super gold, the effect is saturated. Therefore, the upper limit is set at 1.8%.

Cu is an element that undergoes significant age hardening and is effective in steels that utilize age hardening, and is most effective when added in an amount of 0.8 to 1.7%. If it is less than 0.8, age hardening is small, and even if it exceeds 1.7%, it is also small, so the amount is set to 0.8 to 1.7%.

Further, although Mo is effective in increasing resistance to temper softening and increasing strength, addition of more than 0.2% reduces age hardening of Cu. Therefore, the amount was set to 0.2% or less.

Furthermore, since Al is an effective alloying element that is not only effective in deoxidizing, but also fixes N and becomes AIN to refine the crystal grains, the lower limit is set at 0.005%.
．． If it exceeds 0.05%, Al2O3 generated during deoxidation will cause surface cracks during cold bending, so the upper limit is set at 0.05%.
05%.

Nb, like Cu, is an element that exhibits remarkable age hardening, and 0.0
An effect is seen when the addition amount is 0.05% or more, but if it exceeds 0.015%, the weldability (pound toughness of the welded part) decreases, so the amount is set to 0.005 to 0.015%.

Ti is an element that fixes N and contributes to improving the toughness of welded parts, and is effective when added at 0.005% or more.
If it exceeds 0.02%, TiC will be generated and cause a decrease in the toughness of the base material, so the amount is set to 0.005 to 0.02%.

Furthermore, if there is too much N, it causes tempering brittleness and reduces ductility and brittleness.

Are the above basic components of the steel targeted by the present invention? Furthermore, in the present invention, the quenching critical diameter Di due to these components
(cal) or 35 to 65 (mm). Di (cal) means that when a round bar is cooled in water as quickly as possible, the center is fried (center part 50
% martensite) represents the component regression calculation formula for the maximum diameter, expressed in (mm). In this case Di(ca
If l) is less than 35, the strength before cold working T and aging heat treatment is too low, making it difficult to manufacture 80 kgf/mm2 class high tensile strength steel. In addition, if the temperature exceeds 65, even if the surface layer is heat-treated, the strength before cold working will be high, making it difficult to cold bend over 40%, so D i (cal) should be set at 35 to 65 (
mm). In this case, Di(cal)=9.1
75 Ko(1+0.7si)(]+3.33Mn>(1
+0.35Cu)(++0.36Ni)(]+2.16
Cr)X (++3.OMo)(1+1.75V)(
]-], 77Al) (1m), and this formula was written by Grossman in 1979.
34 of ``Harenability'' published by Nikkan Kogyo Shimbun, first edition, on 25th April.
This was derived from the formula proposed in line 5 of page 5, and the influence of various added elements is calculated based on the Di value determined from 6% and the crystal grain size (in this case, Nγ-8), and the element amount is calculated as a multiple of each element. It was calculated by multiplying

Next, the manufacturing conditions according to the present invention will be described. Steel having the above components is heated to 950°C to 1200°C,
Controlled rolling is performed at ℃ to a plate thickness of 80 mm or more, water-cooled, and then the area from the surface to 10 mm below the surface is heated at a temperature of AC 3 to 1200 ℃, and then air-cooled to achieve a cooling rate of more than 40% and less than 50%. Heating temperature 600~ after performing temporary processing
Aging heat treatment is performed at 700°C.

1) The reason why the heating temperature during hot rolling should be 1200℃ or lower is that if it exceeds 1200℃, the γ grains will become coarser and it will be difficult to make them finer by later controlled rolling, which will reduce the toughness of the base material. This is to reduce the In addition, the lower limit was set at 950°C for the purpose of solutionizing precipitated elements such as U and Nb. I can't hope enough.

Next, the plate thickness is 80t + ua or more in the range of 700 to 85
The purpose of performing controlled rolling at 0°C and water cooling is to refine the γ grains and improve the toughness of the base material. The granulation effect becomes small, and if the finishing temperature is less than 700°C, the deformation resistance during rolling increases, making rolling difficult. Furthermore, if the steel is not water-cooled after controlled rolling, it will become a mixed structure of large-grained ferrite and upper heinite, resulting in a decrease in strength. This increases the strength before cold working to 60 to 70 kgf/n+m.
It can be kept to a low level of about 2, and then achieved by cold working of 40% or less.

Furthermore, in order to increase the cold working to more than 40%, which is the objective of the present invention, a treatment aimed at reducing the hardness of the plate surface layer is required. That is, from the surface to the subsurface temperature at AC3~1200°C.
This is because the surface hardness is reduced by heating the region to 0[DIll and then air cooling to form a ferrite and upper bainite structure.

Here, if the heating temperature is AC3 or lower, no reduction in hardness due to ferrite transformation can be expected, and if it exceeds 1200°C, the ferrite grains will also become coarse due to the coarsening of the γ grains and the toughness will deteriorate.

In addition, if air cooling is not used, ferrite cannot be formed with the component system developed in this study, and furnace cooling causes excessive softening, making it difficult to maintain strength, so air cooling is used.

The depth of heating is below the surface] If it is shallower than 40%5
Cold working of less than 0% can be achieved, but the deeper the plate thickness, the greater the rate of strength reduction relative to the plate thickness.
10 mI below the surface.
Specify up to I+. In that case, a plate thickness of 80 mm or more is required.

In that case, the heating means from the surface to 10 mm below the surface may be a high-frequency surface heating method that can control the heating depth of the surface. In addition, it is difficult to heat only the surface part of the furnace heating method, but since there is a difference in temperature rise between the surface part and the center of the plate thickness, it can be applied as long as extraction is carried out as soon as the temperature reaches AC3~1200°C from the surface to the subsurface 10111111. It is possible.

This heat treatment improves the cold working characteristics by reducing the hardness of the plate surface layer, and is most effective in cold bending, where the degree of cold working on the surface is highest.

Next, the strength is made to be 78 to 86 kgf/'mm2 by performing cold working of more than 40% and less than 50%. As mentioned earlier, cold working refers to cold working a steel plate into the desired shape of the target welded structural material, and together with the subsequent aging heat treatment, it is one of the constituent requirements of the method of the present invention. , is the biggest feature. In other words, the steel with the above components has a cold working strength of 15 kgf/ma+
It becomes possible to set the rising sassage rate of 2 or more to 80 kgf/mm2 or more. For that purpose, more than 40% is sufficient,
Cold working of 50% or more will reduce the toughness in the C direction.
The amount was set to more than 40% and less than 50%.

Next, by performing aging heat treatment at a heating temperature of 600 to 700°C, the strength becomes 82 to 88 kgf/mm2, making it possible to manufacture 80 kgf/mm2 class high tensile strength steel.

Here, the increase in strength due to aging heat treatment is [;u, N
Due to precipitation hardening according to b, heating temperature 500-55
0°C is most effective. However, in the present invention, 40
% or more is required, and in order to improve the decrease in toughness caused by cold working, a temperature of 600°C or higher is required. However, since 80 kgf/mm2 cannot be ensured at 700°C or higher, the temperature is set at 600 to 700°C.

In that case, if the heating time is not longer than 10 minutes, the Cu
Since precipitation hardening of Nb is small and heating for more than 600 minutes will result in overaging, heating for 10 minutes to 600 minutes is effective.

Here, for those with circular or semicircular bending or local bending into V-shapes or U-shapes, the degree of processing is highest at the surface and lowest at the neutral axis at the center of the plate thickness, so the surface area has a weight of 80 kgf.
/mm2 or less, it is less than 80 kgf/mm2 at the center of the plate thickness. However, from a design standpoint, it is common to guarantee the strength of the entire thickness, and it is sufficient if 80 kgf/[l]m2 can be secured for the entire thickness.

Note that the high tensile strength steel obtained by the manufacturing method of the present invention is
It can be applied to welded steel pipes obtained by pipe making welding, structural members assembled by welding, such as coating materials with racks, etc. Welding methods include normal submerged arc welding, manual welding, and MIG welding. , electronic beam welding, etc. can be used.

Hereinafter, the effects of the present invention will be illustrated in more detail with reference to Examples.

[Example] Steels 1 to 6 having the chemical composition shown in Table 1 were melted in a 50 ton converter, and then bloomed to a thickness of 200 mm x width + 500 + n.
Slabs with a length of m
Those with and without heat treatment from the surface to 10 mm below the surface at a temperature of 3 to 1200°C were further processed into semicircular shapes with different curvatures by cold targeting, and then the aging heat treatment conditions were changed. This material was manufactured as a test material. The manufacturing conditions are shown in Table 2.

The tensile properties of the semicircular plate material having a thickness of 80 mm manufactured under the above conditions were investigated by a full-thickness tensile test, and the toughness of the 1/4 t base material in the C direction was investigated using a JIS No. 4 full-size square test piece.

Next, the two semicircular materials mentioned above were
A leg member of an offshore structure was manufactured by attaching kgf/mm2 steel rack material by fillet submerged arc welding as instructed. The shape and dimensions of this member are shown in Figure 1 (A) (B
), in which (A) is a slope view and (B) is a plan view, where a is a semicircular material that has undergone cold working,
b is a rack material, C is a rack formed on the rack material, d
is the fillet weld metal, D is the diameter, and the dimensions are expressed in mm. The welding conditions were submerged arc welding with a 10° diamond groove on the semicircular material, the flux was a sintered type flux, the welding wire was a combination of 51-Mn based 70kgf/mm2 wire, and the heat input was 45kJ/cm. There was no line.

The toughness of the welded pound part was investigated using a JIS No. 4 Charpy test piece. The results are shown in Table 3.

As is clear from the table, according to the present invention, the bending process (bending process ('4)-a/(D-
a) X 100, a: plate thickness, D: bending diameter) is 40%
There is no surface cracking due to super cold bending, and the strength of the base material is 80.
kgf/mm2 or more, the weld bond toughness is significantly improved compared to the comparative example, and the base metal toughness is also sufficient.

In the comparative example, surface cracking occurred during cold bending, or the strength of the base metal and the toughness of the weld pound were low.

[Effects of the Invention] As is clear from the above examples, according to the present invention, it is possible to provide a high tensile strength steel whose true part toughness is significantly improved compared to conventional materials. is extremely significant.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the shape and dimensions of a structure manufactured in an example, in which (A) is a perspective view and (B) is a plan view. a: semicircular material, b: rack material, C: rack, d: fillet weld metal, D: diameter.

Claims (1)

[Claims] 1. By weight (%): C: 0.04-0.07%, Si: 0.05-0.40%, Mn: 0.8-1.5%, Ni: 0. 5-1.8%, Cu: 0.8-1.7%, Mo: 0.2% or less, Al: 0.005-0.05%, Nb: 0.005-0.015%, Ti: A steel billet containing N: 0.005% to 0.02%, the balance consisting of Fe at 0.0050% or less, and having a quenching critical diameter Di (cal) of 35 to 65 (mm) was heated at 950°C to 1200°C. Heat to 700-850℃
Controlled rolling is performed at ℃ to a plate thickness of 80 mm or more, water-cooled, and then the area from the surface to 10 mm below the surface is heated at a temperature of Ac_3 to 1200 ℃, and then air-cooled to 40%
Heating temperature 600 after cold working of less than 50%
A method for manufacturing a high-strength steel structure, characterized by subjecting it to aging heat treatment at ~700°C. However, Di(cal)=9.175√C(1+0.7Si)(
1+3.33Mn)(1+0.35Cu)(1+0.3
6Ni) (1+2.16Cr)×(1+3.0Mo)(
1+1.75V) (1+1.77Al) (mm)