JP2671732B2 - Manufacturing method of high strength steel with excellent weldability - Google Patents

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 high-strength steel for welding structures such as penstocks, bridges and pressure vessels, which require high strength and good weldability.

[0002]

[Prior Art] Generally, tensile strength of 490 N / mm ² or more, yield point 29
A structural steel having good weldability, notch toughness and the like at 0 N / mm ² or more is referred to as high-strength steel. Although the strength of steel increases with an increase in the C content, on the other hand, the weldability decreases, so instead of reducing the C content, Mn, Si, and N other than C are reduced.
In order to increase the strength without deteriorating the weldability and the notch toughness by adding a small amount of alloying elements such as i, Cr, Mo, V, Ti, Nb, B, etc.

[0003] For example, a rolled steel material (S
In M58), strength and toughness are secured by tempering (quenching and tempering) Si-Mn based steel with a C content of 0.18% or less or a small amount of Cr, Ni, Mo, Cu, V, etc. And, on the other hand,
By suppressing the carbon equivalent (Ceq.) To 0.44% or less, the hardness of the weld heat affected zone (hereinafter referred to as HAZ) is reduced and the crack sensitivity during welding is improved. Ceq. Is generally defined by the following equation.

Ceq. = C% + (Mn% / 6) + (Si% / 24) + (Ni% / 40) + (Cr% / 5) + (Mo% / 4) + (V% / 14) Furthermore, in the high-strength 690 to 980 N / mm ² class high-strength steel, Mn, Ni, which improves the hardenability by suppressing the C content to 0.18% or less,
Steel with a high Ceq. Is tempered by containing a small amount of alloying elements such as Cr, Mo, B, and Mo, V, which increase the resistance to temper softening, to obtain high strength and high toughness. However, Ceq.
The higher the value, the higher the susceptibility to cracking during welding and the tensile strength.
When welding high-strength steel of 780 N / mm ² grade or higher, it is necessary to preheat the steel material to 100 to 150 ° C.

As a measure for solving this problem, for example, in Japanese Patent Laid-Open No. 56-256, there is a method for reducing the C content and optimally balancing the contents of Al and N to effectively utilize the action and effect of B. A high-strength steel having excellent susceptibility to welding cracks has been proposed. Japanese Examined Patent Publication No. 27407/1990 discloses that Nb-Mo-B-N component type steel is online quenched and then tempered to utilize Nb-Mo type precipitates for precipitation hardening. A method of making steel is disclosed.

[0006] However, there is a limit to lowering P _CM described later while securing strength and toughness at the same time only by the measures disclosed in JP-A-56-256, and vTrs (brittle fracture transition temperature in the Charpy test is limited. It is difficult to keep) below -90 ° C. As in the method disclosed in Japanese Examined Patent Publication No. 27407/1990, if direct quenching is performed without sufficient recrystallization after controlled rolling, the anisotropy of strength and toughness in the rolling direction is particularly high in Nb-containing steel. May occur.

[0007]

An object of the present invention is to provide a, a tensile strength of 780N / mm ² class welded structures for high tensile steel, 50 ° C. below the preheating temperature before welding from conventional 100 to 150 ° C. Steel with excellent weldability that can be reduced to
It is another object of the present invention to provide a method for producing a film without causing anisotropy in mechanical properties.

[0008]

The gist of the present invention is as follows.
(1) A method for producing a high-strength steel, comprising treating a steel having the chemical composition of (1) or (2) with the process of (2) below.

(1) Chemical composition of material steel C: 0.06 to 0.12%, Si: 0.5% or less, Mn: 0.5% by weight
~ 1.5%, Mo: 0.1-0.5%, V: 0.03-0.1%, Nb: 0.0
05-0.05%, sol.Al: 0.005-0.1%, N: 0.0060% or less, B: 0.0003-0.0015% and Ti: 0.005-0.025%,
And Cr: 1.0% or less , the balance Fe and unavoidable impurities, and the weld crack susceptibility composition P _CM represented by the following formula (I) is 0.23 or less.

P _CM = C% + {(Mn% + Cr% + Cu%) / 20} + (Si% / 30) + (Ni% / 60) + (Mo% / 15) + (V% / 10) + 5B % (I) In addition to the above components, Ni of less than 1.0% by weight ,
It contains one or more of 0.5% or less of Cu and from 0.0005 to 0.0050% of Ca, the P _CM is 0.23 or less.

(2) Process Steel having the above chemical composition or the above (steel continuously cast or slab rolled from ingot) is heated to 1000 to 1250 ° C. and hot-rolled, and then once heated to 400 ° C. Cool below,
Then, it is reheated to a temperature of Ac ₃ points or higher to quench it, and tempered at a temperature of Ac ₁ point or lower.

[0012]

The present invention achieves the above object as a comprehensive effect of appropriately setting the chemical composition of the raw material steel and the conditions from hot rolling to tempering. First, the method of the present invention is achieved. The main features of are listed below.

(A) By ensuring the hardenability with an appropriate amount of C, and by incorporating Mo and Nb in combination, the precipitation hardening of Nb is further enhanced, and the contents of expensive Ni and Mo are reduced. To do.

(B) The content of B, which is the HAZ hardening element, is suppressed, and the weld crack susceptibility composition (hereinafter referred to as P _CM ) is 0.23.
Weldability is improved by limiting the content of alloying elements so that the content is not more than%.

(C) Limitation of heating temperature for hot rolling and trace amount of Ti
Is contained to prevent coarsening of austenite grains and enhance toughness, and by performing heat treatment such as quenching and tempering by off-line treatment, anisotropy of strength and toughness due to rolling is prevented.

Next, the reason why the chemical composition of the raw steel is determined as described above will be explained. Hereinafter,% of the component content means% by weight.

C: It has the effect of improving the hardenability of steel and increasing the strength. In the present invention, in order to improve weldability or economic efficiency, B, Ni, which improves hardenability to increase strength,
Since the content of Mo is kept low and it is necessary to compensate for the strength decrease due to this, 0.06% or more of C is contained. On the other hand, if it exceeds 0.12%, the susceptibility to welding cracks increases. Therefore, the C content is limited to the range of 0.06 to 0.12%.

Si: an element effective for deoxidation and strength increase, but if contained in a large amount, the formation of island martensite is promoted and the toughness deteriorates, so 0.1% or less is preferable, but strength is secured. From this viewpoint, the upper limit was made 0.5%.

Mn: 0.5% or more is contained because it is an important element for improving the hardenability of steel and ensuring strength and toughness. On the other hand, if it exceeds 1.5%, the temper brittleness increases,
This causes problems such as deterioration of weldability. Therefore, the Mn content is limited to the range of 0.5 to 1.5%.

Mo: An element effective in improving hardenability and ensuring strength. For that purpose, the content of 0.1% or more is required. On the other hand, a large content of more than 0.5% impairs weldability and is an expensive element.
The content was in the range of 0.1 to 0.5%.

V: 0.03% or more is required to secure the strength, but if it exceeds 0.1%, the weldability is deteriorated. Therefore, the V content is set to the range of 0.03 to 0.1%.

Nb: an element effective for improving strength and toughness by precipitation hardening during tempering. Especially Mo
0.005% because the effect of increasing strength is remarkable due to the combined inclusion of
Contain at least 5%. However, if contained in a large amount, HA
Since it causes the hardening of Z, it is set to 0.05% or less, but the preferable one is
0.02% or less.

Sol.Al: For deoxidation and grain refinement of steel,
0.005% or more is required as sol.Al, but if it exceeds 0.1%, the cleanliness of steel is impaired.

N: When present in a large amount in steel, the HAZ toughness is impaired, and in combination with B, it is effective for improving hardenability.
The upper limit of N was set to 0.0060% in order to reduce l.B.

B: Although it is an element that significantly improves the hardenability of steel and greatly contributes to the strength increase in a very small amount, it also causes the hardening of HAZ, so the B content was made 0.0003 to 0.0015%.

Ti: Prevention of coarsening of austenite grains and HA
Since it is effective for improving the toughness of Z, it is necessary to contain 0.005% or more, but excessive Ti causes toughness deterioration.
It was set to 0.025% or less.

In addition to the above components, 1.0% or less of Cr is further contained.
To have. Cr improves the hardenability of steel, but at the same time welds
Since it also causes deterioration of the property, it should be 1.0% or less .

Ni, Cu, and Ca may be used in one or more types, if necessary.
Can be added. Each effect and content
The reason for the limitation is as follows .

Ni: Effective in improving low temperature toughness and hardenability, but expensive
However, even if added, the upper limit of Ni content is
It was less than 1.0% .

Cu: An element effective for improving the strength, but if it is contained in a large amount exceeding 0.5%, the weldability is impaired, and there is a concern of hot cracking due to Cu checking. The upper limit of the Cu content was 0.5%.

Ca: An element effective for improving toughness and reducing anisotropy by spheroidizing non-metallic inclusions, but for that purpose, a content of at least 0.0005% is necessary. on the other hand,
When the content exceeds 0.0050%, the toughness deteriorates due to the increase of inclusions, so the Ca content was limited to the range of 0.0005 to 0.0050%.

P _CM : It has been conventionally used as an index showing the weldability of steel materials, and it is desirable to be as small as possible in order to improve the weldability which is the object of the present invention. However, if Osaere the P _CM in steel as a material of the present invention a method to 0.23 below, as shown in Examples later, there is no risk of even cracking welded without preheating.

Next, the conditions for hot rolling and subsequent heat treatment will be described.

The hot rolling of the present invention is not so-called controlled rolling but ordinary hot rolling in which the rolling is completed in the high temperature region of the austenite single phase. However, the heating temperature range at that time is 1000 to 1250 ° C. When the heating temperature exceeds 1250 ° C, even in the steel containing Ti for grain refinement, the austenite grains become coarse and the toughness of the base material deteriorates, so the upper limit was made 1250 ° C. On the other hand, when heated at a low temperature of less than 1000 ° C., the solid solution of Nb is insufficient, and the effect of improving the strength and toughness due to the subsequent fine Nb-containing precipitates cannot be exhibited,
The lower limit was set to 1000 ° C. The lower limit temperature varies depending on the Nb content, but when it is necessary to contain Nb in an amount of 0.02% or more, it is preferable to set it to 1150 ° C or higher and 1250 ° C or higher in consideration of the solid solution of Nb.

After the hot rolling is finished, the material is once cooled to a temperature range of 400 ° C. or lower and then reheated to quench and temper.
The reason for this is that if reheating is performed without sufficient transformation, a mixed grain structure is likely to occur, leading to deterioration in toughness. The quenching temperature is set to a temperature of Ac ₃ or higher at which quenching from the austenite single-phase region is possible, and the tempering temperature is set to a temperature of Ac ₁ or lower in order to avoid excessive strength reduction.

[0036]

EXAMPLE Continuously cast slabs having the chemical composition shown in Table 1 were hot-rolled under the conditions shown in Table 2, after hot-rolling, once cooled to 400 ° C. or lower, and further quenched at the temperature shown in Table 2. By tempering heat treatment, a high-tensile steel plate having a plate thickness of 25 to 40 mm was manufactured.

The mechanical test results and weldability of these base materials are also shown in Table 2. The weldability of the heat input is 1.7 kJ / mm.
A Y-groove restraint cracking test was conducted by CO ₂ arc welding, and the preheating temperature required to stop the cracking was evaluated, and the toughness was evaluated by the brittle fracture transition temperature by the Charpy impact test.

As is clear from Tables 1 and 2, the examples of the present invention satisfying all the conditions specified in the present invention had good base material strength and toughness, and exhibited excellent weldability.
In the comparative example, it was confirmed that the strength and toughness required for the HT780 class steel were not both satisfied, or even if they were satisfied, the weldability was deteriorated.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

EFFECTS OF THE INVENTION According to the method of the present invention, the content of expensive alloying elements, which are generally used for strengthening, is reduced to achieve high strength, high toughness, and high tensile strength with excellent weldability. Steel can be produced.

Claims (2)

(57) [Claims] 1. By weight%, C: 0.06-0.12%, Si: 0.5% or less, Mn: 0.5-1.5%, Mo: 0.1-0.5%, V: 0.03-0.1.
%, Nb: 0.005-0.05%, sol.Al:0.005-0.1%, N: 0.
0060% or less, B: 0.0003 to 0.0015% and Ti: 0.005 to 0.
A steel containing 025% and Cr: 1.0% or less, consisting of balance Fe and unavoidable impurities, and having a weld crack susceptibility composition P _CM represented by the following formula (I) of 0.23 or less is 1000 to 1250.
After hot rolling by heating to ° C., once cooled to 400 ° C. or less, then quenched by reheating to above Ac ₃ point, Ac ₁
A method for producing a high-strength steel excellent in weldability, which comprises tempering at a temperature below the point. _{P CM = C% + {(} Mn% + Cr% + Cu%) / 20} + (Si% / 30) + (Ni% / 60) + (Mo% / 15) + (V% / 10) + 5B% ··・ (I) 2. In addition to the components of claim 1, further in wt%
Ni less than 1.0%, Cu less than 0.5% and 0.0005 to 0.0050
% Steel, containing at least one of Ca and having a P _CM of 0.23 or less, after heating to 1000 to 1250 ° C. and hot rolling,
A method for producing high-strength steel excellent in weldability, which comprises once cooling to 400 ° C. or lower, then reheating to an Ac ₃ point or higher to quench and tempering at a temperature of ₁ point or lower.