JPS62240747A - Thermo mechanical precipitation hardened high tensile steel superior in cold workability and weldability and manufacture thereof - Google Patents

Abstract

PURPOSE:To manufacture the titled steel stock, by specifying contents of Cu, Nb in steel stock having a specified compsn. CONSTITUTION:To Fe, by weight % 0.02-0.30 C, 0.05-1.0 Si, 0.5-2.5 Mn, 0.3-2.0 Ni, 0.005-0.20 Ti, 0.003-0.050 Nb, 0.5-1.5 Cu, 0.005-0.1 Al, <=0.0070 N, <=0.0030 O. The steel is heated to about 950-1,100 deg.C, then rolled at about <=850 deg.C by >=about 50% accumulative draft, then immediately water cooled to <=about 250 deg.C by >=about 0.5m<2>/m<2>.min water quantity density. Cu improves strength without deteriorating HAZ toughness and raises strength due to ppt. hardening in normalizing process after pipe making, hence, >=0.5% Cu is required, but toughness is lowered with the rise thereof. Since Nb steeply deteriorates HAZ toughness due to ppt. hardening, it is regulated to the min quantity required to recrystallization suppressing effect. The steel is easily cold bent at time of use, work hardened by bending while increasing strength and attaining to the aimed strength by successive tempering, further, superior in weldability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a process/precipitation hardening type high tensile strength steel material with excellent cold workability and weldability, and a method for producing the same.

(Conventional technology) In recent years, energy development has been carried out in cold regions and polar regions, and
The sea is getting deeper. As a result, offshore structures have become extremely large, and in deep water, conventional semi-subrigs have been replaced by
Improvements in safety and operational efficiency are being considered with bottom-mounted jack-up rigs. Furthermore, in sea areas with severe sea and weather conditions, it is necessary to withstand high waves. To this end, a new jack-up rig is being considered.In order to reduce the influence of waves, the projected area must be made small, and thick-walled, small-diameter high-tensile steel pipes are used as design considerations for this jack-up rig. is required. Conventionally, this type of steel pipe has been manufactured by hot working HT80 steel. In this case, since HTBe steel is usually manufactured by quenching and tempering, there is a problem in that the strength decreases due to hot working during pipe making, and these conventional 80kg
/me” high tensile strength steel does not have sufficient joint toughness due to its high C!t and many added alloying elements.On the other hand, steel with lower strength by suppressing these alloying elements to a lower level A method of increasing the strength of the raw material through precipitation hardening heat treatment after hot working is described by R., G.I., Gisseman and G.
Murphy, J.A., “Mechanical Properties and Precipitation Hardening Response.
In ASTM A710 Clade 8 And A73
670 Steel Plates” in Session V on the International Conference on Technology and Applications
On II SLA Steels OCT, 3
-6.1983 in Philadelphia (R, J, J
esseman and G.J.Murphy' Mec.
hanical Properties and Pr
cipitationllardening Re5
ponse in ASTM A710 Grade
A and 87 to 6 A11oy 5teel
Plates' in 5ession V
of the International Co.
nference on Technology
andApplications of ll5LA
5teels Oct, 3-6.19831nPhi
However, the joint toughness obtained in this way cannot be said to be sufficient in terms of weldability.

(Problems to be Solved by the Invention) However, these conventional steel materials can only be processed by hot working due to the capacity of the pipe making machine, and there is an upper limit to the thickness that can be made into pipes, and they cannot be processed unless they are relatively thin. The problem was that I couldn't do it.

Therefore, in order to be able to produce thick steel plates through cold working, there is a need for a steel plate that has low strength and is easy to form into pipes at the steel plate stage, and that can obtain a specified strength after pipe forming. .

(Means for Solving the Problems) The present invention has been made to solve the above problems, and the gist thereof is as follows.

(11 Steel material manufactured through a quenching process after hot rolling, in weight ratio: C: 0.02 to 0.30%, Si: 0.0
5-1.0%.

Mn: 0.5-2.5%, Ni: 0.3
~2.0%.

Ti: 0.005-0.20%, Nb: 0.
003-0.050%.

Cu: 0.5-1.5%, p, e: 0
．． 005-0.1%.

N: 0.0070% or less, O: 0.00
Less than 30%.

A precipitation hardening type high tensile strength steel material having excellent cold workability and weldability, characterized in that the remainder is Fe and unavoidable impurities.

(2) Weight ratio: C: 0.02-0.30%, Si: 0.
05-1.0%.

Mn: 0.5-2.5%, Ni: Q, 3
~2.0%.

Ti: 0.005-0.20%, Nb: 0
．． 003-0.050%.

Cu: 0.5-1.5%, A1: 0.
005-0.1%.

N: 0.0070% or less, O: 0.00
Less than 30%.

9. A steel containing
After heating to 50 to 1100°C, the rolling temperature is 850°C or lower, the cumulative rolling reduction is 50% or more, and the water density is 0.
A method for manufacturing a precipitation-hardening high-strength steel material with excellent cold workability, characterized by water cooling to 250° C. or lower at a 5d/d split.

That is, based on the knowledge obtained as a result of numerous experiments, the present inventors have developed a method for producing cold-worked, thick-walled, small-diameter high-tensile steel pipes using a pipe-making machine with limited bending capacity. It was confirmed that the strength of the steel plate before pipe making must be below a certain value. However, after the steel plate is made into pipes,
In order to achieve the desired strength, the present inventors conducted various studies and found that the selection of an appropriate composition of this type of high-strength steel, work hardening in cold working, and normalizing heat treatment after pipe making. The inventors have discovered that the desired strength can be obtained after pipe forming by increasing the strength using precipitation hardening, and that the steel sheets obtained in this way have excellent weldability.

That is, the precipitation caused by the normalizing is caused by precipitation hardening due to Cu. Generally, N is a precipitation hardening element.
B and V are commonly used, but both significantly deteriorate HAZ toughness. On the other hand, it was discovered that Cu increases the strength without substantially deteriorating the HAZ toughness.

Therefore, N is an essential element for improving the toughness of thick steel plates.
Regarding b, conventionally this element had both a recrystallization suppressing effect and a precipitation effect, but in the present invention, it was discovered that precipitation hardening by Nb significantly deteriorates HAZ toughness, and Nb The minimum amount necessary for the crystal suppressing effect is added.

The present invention has been constructed based on the above findings.

Next, the reason for limiting the components in the present invention will be described.

C is an element that increases strength at a low cost, and is set at 0.0% to ensure strength.
It is necessary to add 0.02% or more, but since adding too much will impair the toughness and weldability of the steel, the upper limit was set at 0.30%.

Si needs to be added in an amount of at least 0.05% to deoxidize the steel, but if the amount increases, weldability will be impaired, so the upper limit is set at 0.1%.

It is necessary to add Mn in an amount of 0.5% or more to ensure strength, but if the amount increases, it will cause deterioration of weldability and toughness, so the upper limit should be set at 2.
．． 5%.

AI is required to be present in an amount of 0.005% or more for deoxidation, but if the amount increases, the toughness deteriorates, so the upper limit is set at 0.1%.

Cu can increase the strength without impairing the HAZ toughness and is the main element of the present invention, but since it increases the strength due to the precipitation effect in the normalizing process after pipe making, 0.
5% or more is necessary, but if it increases, the toughness will decrease, so the upper limit is set at 1.5%.

Ni is required to be at least 0.3% in order to improve toughness, but since it is an expensive element, the upper limit is set at 2.0%.

Since Nb is an essential element for improving toughness, especially the toughness in the center of the plate thickness, it should be contained in an amount of at least 0.003%, which is necessary to suppress recrystallization. do.

Ti prevents coarsening of austenite grains and is an essential element from the viewpoint of HAZ toughness, so O,005% or more is required.
．． The upper limit is 20%.

If N increases, it will deteriorate HAZ toughness, so the upper limit is set to 0.
．． It is set at 0070%.

The value of 0 is 0.003 because the higher the value, the more alumina-based inclusions will be present, which will cause bending cracks during pipe forming by cold working.
The upper limit is 0%.

Next, the manufacturing conditions other than the ingredients will be described.

The steel material of the present invention is quenched during the steel manufacturing stage, and when used, is subjected to tempering heat treatment after pipe forming (cold bending).It is easy to form during processing, and has high strength due to work hardening due to processing and precipitation hardening due to heat treatment. , high toughness is obtained, and this is basically achieved by the above-mentioned component composition.

The amount of cold bending when used here is preferably 10% or more, and if it is less than this, the amount of work hardening is insufficient, and the tempering heat treatment is performed at a temperature of 500 to 500, which sufficiently causes precipitation hardening due to Cu. The temperature is preferably 600°C.

Next, the heating, rolling and cooling conditions of the second invention will be described.

The steel plate is manufactured using a controlled rolling-controlled cooling method. First, the upper limit of the heating temperature is set to 1100 to make the austenite grains finer.
℃. In order to form a solid solution of Nb during heating, the lower limit temperature needs to be 950°C.

Next, hot rolling is performed at a temperature of 850° C. or lower and a reduction of 50% or more. This is because grain refinement is not achieved above this temperature and below this rolling reduction.

Next, in order to obtain a sufficient increase in strength during accelerated cooling after rolling, the lower limit of water density is 0.5n? /rrr-m
It is set as in. Further, the water cooling stop temperature is set to 250° C. or less at the surface temperature of the steel plate in order to obtain uniformity in the thickness direction.

Therefore, steel may be melted in either a converter or an electric furnace, and after melting, it is formed into a slab by an ingot-forming method or a continuous casting method, and then hot-rolled to a predetermined size.

(Example) Example 1 Using test steel having the chemical composition shown in Table 1, 1250
°C, hot rolled at a finishing temperature of 900 °C, once cooled, quenched, cold bent, and tempered. Manufacturing conditions at this time, mechanical properties at each stage after quenching, cold bending, and final tempering, low temperature toughness after final tempering, welding heat input 45 kJ /
Table 2 shows the joint toughness values of Fusion Line in AM submerged arc welding.

It can be seen that the thick steel plates AI, Bl, and CI obtained by applying the present invention are all excellent in tensile strength, low-temperature toughness value, and welded joint toughness.

On the other hand, in Dl of the comparative example, the Cu content was as high as 2.10%, making cold bending impossible. El is 0.060% Nb
Fl is the current 80 kg steel, which has a high joint toughness value, but it has Mo, Cr, and V added, and cannot be cold bent.

Example 2 Steel A shown in Table 1 was hot-rolled, immediately cooled with water, cold bent, and tempered. Table 3 shows the manufacturing conditions, mechanical properties at each stage after water cooling, cold bending, and final tempering, low temperature toughness after final tempering, and latent arc welding with a welding heat input of 45 kJ/am. Fus of
ion Line joint toughness values are shown.

It can be seen that the thick steel plates A2 to A5 obtained by applying the method of the present invention are all excellent in tensile strength, low-temperature toughness value, and welded joint toughness.

On the other hand, the comparative example A6 has a heating temperature as high as 1150℃ (
, A7 has a low heating temperature of 900°C, and A8 has a high rolling hot water temperature of 870°C and has a low low temperature toughness value.

A9 has a water density of 0. arrr/rrr-min, AIO has a low cold bending rate of 5%, All has a low tempering temperature of 450℃, and A13 has a low tempering temperature of 6%.
The temperature is as high as 50°C, and the tensile strength is low.

(Effects of the Invention) As described above, the present invention provides a steel material that is easy to cold bend during use, improves strength through work hardening through bending, and reaches the target strength through subsequent tempering. Therefore, a high-strength steel pipe with a small wall thickness and small diameter that can be formed by cold working, which was previously impossible, can be easily obtained, and it also has excellent weldability by selecting an appropriate component composition and regulating the upper limit of the content.

In addition, during manufacturing, the reduction in alloying elements results in a significant effect of low cost.

Claims (2)

[Claims] (1) A steel material manufactured through a quenching process after hot rolling, with a weight ratio of C: 0.02 to 0.30%, Si: 0.05 to 1.0%
, Mn: 0.5-2.5%, Ni: 0.3-2.0%,
Ti: 0.005-0.20%, Nb: 0.003-0
．． 050%, Cu: 0.5-1.5%, Al: 0.00
5-0.1%, N: 0.0070% or less, O: 0.00
30% or less, with the balance consisting of Fe and unavoidable impurities. (2) Weight ratio: C: 0.02-0.30%, Si: 0.05-1.0%
, Mn: 0.5-2.5%, Ni: 0.3-2.0%,
Ti: 0.005-0.20%, Nb: 0.003-0
．． 050%Cu: 0.5-1.5%, Al: 0.005
~0.1%, N: 0.0070% or less, O: 0.003
0% or less, with the balance consisting of Fe and unavoidable impurities.
℃ or less, the cumulative reduction rate is 50% or more, and immediately after rolling, water cooling is performed to 250℃ or less at a water flow density of 0.5 m^3/m^2 min or more, which has excellent cold workability, A method for producing precipitation hardening high tensile strength steel.