JPS63162838A - Cu-precipitation strengthening-type extreme thick steel material excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To make a steel material thick-walled and also to improve the weldability and toughness at low temp. of a steel material, by specifying respective contents of C, Si, Mn, Ti, Nb, B, Cu, Al, and N and also by precipitating fine Ti in a steel so that it comprises a proportion in a specific range. CONSTITUTION:An extreme thick steel material has a composition consisting of, by weight ratio, 0.005-0.10% C, 0.05-0.60% Si, 0.5-2.0% Mn, 0.004-0.015% Ti, 0.005-0.10% Nb, 0.0005-0.0020% B, 0.7-2.0% Cu, 0.010-0.10% Al, <=0.0050% Ni, and the balance Fe with inevitable impurities. In the above composition, at least a part of Ti is regulated so that it comprises 0.004-0.015% of the steel in the form of a fine precipitate of 0.01-0.06mu. If necessary, <=1.5% Ni, <=1.0% each of Mo and Cr, and <=0.05% REM are incorporated to the above composition. In this way, a Cu precipitation strengthening-type thick steel material having superior weldability can be made high-strength and high- toughness.

Description

Detailed Description of the Invention (Industrial Application Field) A weldable material used for heavy equipment members that require a plate thickness of 70 to 150 mm in offshore structures used in low-temperature environments such as northbound and icy areas. This paper aims to propose a new Cu precipitation-strengthened extra-thick steel material that has been improved to be suitable as a high-strength, high-strength steel with good properties.

(Prior art) Steel materials aimed at precipitation strengthening by Cu have good weldability, and are disclosed in US Pat. As mentioned above, development research has been progressing, and particularly in recent years, there has been a demand for higher strength and thicker steel materials for use in low-temperature environments, as well as stricter demands for low-temperature toughness.

In general, for high-strength steel with a tensile strength of 6Q kgf/mm2 or more, reheating and partial reconditioning or direct quenching and partial reconditioning are applied to ensure strength and toughness.This process changes the structure to lower bainite or marten sand. ,
This is to improve toughness.

However, when the plate thickness exceeds 70 mm, the hardenability decreases, especially in the center, and the structure becomes coarse upper bainite, resulting in a decrease in toughness.

Therefore, the conventional technology cannot meet the severe demands for low-temperature toughness such as good weldability and ensuring Charpy absorbed energy at -80°C.

(Problems to be solved by the invention) Thickening of Cu precipitated steel material with good weldability (70 to 1
In order to satisfy the requirements of 50mm) and to ensure toughness of E-80≧15kgf-m especially at the center of the plate thickness, the structure is made finer by using fine precipitates of Ti, such as TiN. The purpose of this invention is to propose novel development results.

(Means for Solving the Problems) The above-mentioned objects of the invention can be advantageously satisfied by a configuration based on the following matters.

C: 0.005-0. 10wt% Si: 0.05-0.60wt% Mn: 0.5-2.0wt% Ti: 0.004-0.015wt% Ntl: 0.005-0. I0wt%B: 0
．． 0005~0.0020wt%Cu: 0.7~2
．． 0wt% Al: 0.010 to 0. 10wt%N: 0.0
050 wt% or less, with the balance consisting of Fe and unavoidable impurities, and the above T
At least a part of i is 0.01 μm or more, 0.06 μm
0.004 to 0.0 m as fine Ti precipitates
An extra-thick Cu precipitation-strengthened steel material (first invention) having excellent low-temperature toughness, characterized in that Cu occupies a range of 15 wt% in the steel.

C: 0.005-0. lOν+1% Si: 0.05-0.60wt%), 10: 0.5-2. QvJt%Ti: 0.0
04-0.015wt% Nb: 0.005-0.10wt%B: 0.000
5-0.0020wt%Cu: 0.7-2. 0w
t% Al: 0.010~Q, 10wt% N: 0.0050wt% or less, lJi of 1.5vJt% or less, l, 0, respectively
wt, less than % Mo, Cr and 0. The composition consists of one or more selected from the group consisting of REM of Q5wt% or less, the balance Fe and unavoidable impurities, and at least a part of the above Ti is 0.01 μm or more, 0.06
As fine Ti precipitates with a size of 0.004 to 0.00 μm or less,
An extra-thick Cu precipitation-strengthened steel material (second invention) with excellent low-temperature toughness, characterized in that the content of Cu in the steel is within the range of 0.15 wt%.

(Function) The reason why the composition range of the Cu precipitation hardening type extra-thick steel material is limited as described above according to each invention will be explained below.

C: 0.005 ~ Q, 60 wj% C must be regulated because it affects weldability and low temperature toughness of the welded part, and 0.005 to Q, 60wj% If it exceeds 10wt%, toughness will be impaired, while if it is less than 0.005wj%, the crystal grains will become coarse, impairing strength and toughness, so 0.005 to 0.0wt%.
Limited to 10wt%.

Si: 0.05 to 0.06wt% Si increases the strength of steel materials, and for that purpose 0.05wt% is required.
The above is necessary. However, if Q exceeds 6 wj%, weldability and weld toughness will be impaired. Therefore 0.05~0.
Limited to 60wt%.

1, ln: 0.5-2. Owt%Mn is useful for making steel materials high in strength and toughness, and for that purpose Q,
It is necessary to use 5 wt% or more, but if it exceeds 2.0 wt%, weldability is impaired, so it is limited to 0.5 to 2.9 wt%.

T+: 0.004 to 0.015 wt% Ti is a characteristic component of this invention, and should be 0.004 to 0.015 wt%, and the size of the Ti1l product is 0, and the size of the Ti1l product is 0 and the size of the former μm or more and 0.06 μm or less. 0.004~0.0 as fine precipitates.
It is necessary to set the content to 0.015 wt%.

The addition amount of ゛r1 is determined from the relationship between the required amount of fine Ti precipitates, and the amount of Ti fine precipitates is 0.00.
If it is less than 4vn%, the base metal structure cannot be refined;
If it exceeds 0.015 wt%, the toughness will be rather impaired.

Note that even if the total amount of Ti is, for example, 0.015 wt%, of which 0°004 wt% becomes fine precipitates, the effects of the present invention do not change.

Nb: Q, 005~O, 1Ov+t% Nb is an element effective in refining austonite grains during rolling, and for this purpose, it is required in an amount of 0.005 wt% or more. However, if it exceeds 10 wt%, the toughness of the weld will be impaired.

B: 0.0O05 to 0.0020wt%B is also a characteristic component of this invention, and the grain refinement by TiN described above causes a decrease in hardenability in thick steel plates, resulting in a decrease in strength and toughness. Therefore, in order to improve this o, 0005
It is necessary to improve the hardenability by adding B in an amount of % by weight or more, thereby changing the structure to a finer martensite or inferior bainite structure. In addition, 0.
Addition of more than 0.020 wt% actually reduces toughness.

Cu: 0.7 to 2.0wt% Cu is a component that basically characterizes this invention, and by using Cu precipitation strengthening, Q, 7wj is used to achieve high strength without impairing weldability. % or more is required,
On the other hand 2. If it exceeds Qw't%, low temperature toughness will be impaired.

Al: O, OLO ~ 0.10 νit% AI is effective in deoxidizing and inhibiting austenite grain coarsening, and for this purpose, O, 010 wt% or more is required. However, 0. If it exceeds 10 wt%, the cleanliness in the steel will be impaired and the toughness will decrease.

N: 0.0050wt% or less Reduction of Nfi is also one of the features of this invention, and by reducing Ni to 0.0050wt% or less, TA
N is refined. Furthermore, the composition of the amount of N improves the toughness of the weld zone.

In addition to the essential components mentioned above, depending on the required strength level, Ni, MO, (:ri, and REM) can be added. All of these elements increase the hardenability of the steel, making it highly It is an equivalent component in terms of the effect of increasing strength.

Here, N+≧Q, 3wt%, Mo≧0.11%,
Cr≧Q, 1wt% and REM≧0.005wt%, while Ni: l, 5w as an upper limit.
t%, Mo: 1.9wt%, Cr: 1.9
wt% and REM: Q, limited to 95wt%,
This is because adding more than 1.5 wt % of Ni has only a destructive effect, and adding more than the upper limit of Ni elements impairs toughness.

1st. In each of the second inventions, the amount of Ti precipitates as fine precipitates of 0.01 μm or more and 0.06 μm or less is 0.0
If it is not contained in an amount of 0.04 to 0.015 wt%, it will not be possible to refine the crystal grains, especially in the center of the plate thickness. The condition is that Ti is 0
．． If the content exceeds 0.015 wt%, the formation of fine precipitates will be limited due to the rotational speed. There are no particular restrictions on the method for producing such fine TiN, but for example, a solution treatment such as heating at 1300° C. for 5 hours or more and cooling with water may be used before the rolling process.

Note that the size and amount of TiN can be easily determined by the following analytical method. TiN found in steel can be extracted by a potentiostatic electrolysis method using an electrolyte solution based on (0% acetylacetone-1% tetramethylammonium chloride-methanol) system. 0. The old ~0.06 μm TiN particles are +l2sO4(1+9
) for 20 minutes at 70°C. Using this method, 0. Particles of 0.06 μm can be separated.

The Cu precipitation-strengthened extra-thick steel material according to the present invention can be produced by, for example, hot-rolling a material that has been subjected to solution treatment before the rolling process, and then directly quenching a part of it, or reheating and quenching a part of it. It can be manufactured using the following process. Of course, there are no restrictions on slab heating conditions, rolling conditions, etc. in the rolling process, and they can be the same as in normal processes, but
In order to strengthen the precipitation of Cu during tempering, a suitable heating temperature is 500 to 650°C.

Examples Example (1) Using the steel shown in Table 1, after ingot formation, it was heated for 8 hours at each temperature of 1000, 1100, 1200 and 1300 degrees Celsius! After that, solution treatment was performed by cooling with water.

Fine T+ of 0.01 to 0.06 μm for each solution temperature
The amount of Ti as N was as shown in Table 2.

After that, 1) Slab heating conditions: 1000℃ x lh rolling finishing temperature = 850℃ Cooling after rolling: Air cooling Reheating and quenching temperature: 930℃ Tempering temperature: 630℃ Reheating and quenching partial resetting process, and ■) Slab Heating conditions: 1000℃Xlh rolling finishing temperature: 8
50℃ Cooling near 0mm thick material after rolling is 2.5℃/5130mm
Steel materials of 70 and 130 mm FE were manufactured using a direct quenching and partial tempering process with a thick material of 2°C/S and a tempering temperature of 630°C.

Table 3 shows the strength of the base material and V E-so, and also the Ti world and Vε-g as TAN of 0.01 to 0.06 μm.

The relationships between the two are summarized and shown in Figure 1.

From the above results, it can be seen that the Cu precipitation-strengthened thick steel material according to the present invention has excellent toughness.

Example (2) B-free steel J and test wJK with reduced B addition amount,
For L and M (see Table 4), both were 7 at 1300°C.
Solution treatment for finer TiN is performed for h,
130 mm by subsequent direct quenching and partial restoration process
steel plates were manufactured.

The conditions for this process were almost the same as in Example (1).

The strength and toughness of the base material at this time are shown in Table 5, and it can be seen that particularly high strength and high toughness can be obtained with Bl within the range of the present invention.

Table 5 Strength and toughness of base metal (effects of the invention) The present invention achieves high strength and high toughness of a Cu precipitation-strengthened thick steel material with good weldability.

[Brief explanation of the drawing]

Figures 1 and 2 are 0. It is a graph showing the results of investigating the effect of Til as TiN fine precipitates with a diameter of 0.06 μm on the vB-8o value for a reheated and quenched partially tempered material and a directly quenched and tempered material. Fig. 1 0.0/~θ, Of; 7tpn-IN and L70, pot (〃) Fig. 2

Claims (1)

[Claims] 1. C: 0.005 to 0.10 wt% Si: 0.05 to 0.60 wt% Mn: 0.5 to 2.0 wt% Ti: 0.004 to 0.015 wt% Nb: 0.005 to 0.10 wt% B: 0.0005 to 0.0020 wt% Cu: 0.7 to 2.0 wt% Al: 0.010 to 0.10 wt% N: 0.0050 wt% or less, and the balance is Fe and Due to the composition of unavoidable impurities, the above T
At least a part of i is 0.01 μm or more, 0.06 μm
0.004 to 0.0 m as fine Ti precipitates
An extra-thick Cu precipitation-strengthened steel material with excellent low-temperature toughness characterized by a Cu content in the steel in the range of 15 wt%. 2. C: 0.005-0.10 wt% Si: 0.05-0.60 wt% Nn: 0.5-2.0 wt% Ti: 0.004-0.015 wt% Nb: 0.005-0. 10wt% B: 0.0005-0.0020wt% Cu: 0.7-2.0wt% Al: 0.010-0.10wt% N: 0.0050wt% or less, as well as 1.5wt% or less Ni, 1.0wt each
% or less of Mo, Cr, and 0.05 wt% or less of REM, and the balance is Fe and unavoidable impurities, and the above T
At least a part of i is 0.01 μm or more, 0.06 μm
0.004 to 0.0 m as fine Ti precipitates
An extra-thick Cu precipitation-strengthened steel material with excellent low-temperature toughness characterized by a Cu content in the steel in the range of 15 wt%.