JPH06104861B2 - Manufacturing method of V added high toughness high strength steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a method for producing a tensile strength 60 kgf / mm ² grade V-added high toughness high strength steel sheet having excellent weldability, and a steel sheet having a thickness of 20 mm or less. Target.

The present invention is most preferably applied to thick plate mills, but can also be applied to hot coils, shaped steel and the like. The thick steel plate produced by this method can be used for structural steel pipes such as line pipes and steel towers, and is particularly suitable for hot-dip galvanized steel pipes.

[Prior Art] In order to improve the strength, toughness and weldability of the base metal of steel and the heat affected zone (HAZ), low C (low carbon equivalent), small amount of Ti
Addition (use of nitride and oxide of Ti) is effective (for example, JP-A-52-128821 and JP-A-55-131125).
However, all of them lead to a reduction in strength. In particular
When precipitation hardening elements such as Nb and V are used, the decrease in the amount of C and N causes a remarkable decrease in strength (when Ti is added, N is strongly fixed as TiN and the free N decreases). For this reason, in the production of steel sheets, it is general to increase the strength by controlled rolling or controlled rolling + accelerated cooling. However, when the plate thickness becomes thin, accelerated cooling has problems such as a decrease in temperature before the start of water cooling and a defective shape after cooling. Further, in order to obtain high strength of tensile strength 60 kgf / mm ² class (hereinafter referred to as HT60) by controlled rolling, it is necessary to further increase the amount of alloying elements, and high strength and excellent low temperature toughness and weldability (hereinafter, welding) It was impossible to simultaneously obtain the toughness).

[Problems to be Solved by the Invention] The present invention provides an inexpensive manufacturing technique of HT60 excellent in low temperature toughness and weldability. HT manufactured according to the present invention
60 alloy cost is low, and low temperature toughness and weldability are good, so preheating can be reduced or omitted in welding work.
It has advantages such as improved safety of the welded structure.

[Means for Solving the Problems] The gist of the present invention is that C: 0.03 to 0.09%, Si: 0.5
% Or less, Mn: 1.4 to 1.9%, P: 0.02% or less, S: 0.003% or less, N
b: 0.005-0.03%, V: 0.03-0.08%, Ti: 0.005-0.02%, A
l: 0.06% or less, N: 0.001-0.003%, O: 0.002-0.004%,
As necessary Ni: 0.05-1.0%, Cu: 0.05-0.6%, Ca: 0.001
~ 0.005% of one or more kinds of steel, with the balance being iron and unavoidable impurities, the steel is heated to a temperature range of 1100 to 1250 ° C, and the cumulative reduction amount of 950 ° C or less and the Ar ₃ transformation point or more is obtained.
Tensile strength of 60kgf / mm ² grade, characterized by rolling at 40% or more, cumulative reduction below Ar ₃ transformation point of 20% or more, and rolling end temperature of 690 ° C or less, 630 ° C or more It is in a method for manufacturing a V-added high toughness high strength steel sheet.

[Operation] Hereinafter, the present invention will be described in detail.

In order to dramatically improve the low temperature toughness and weldability of HT60, it is essential to limit the constituent elements. Therefore, the amount of C was reduced, and the amount of Nb added, which had a particularly large precipitation hardening ability and was detrimental to weldability, was reduced. The addition of a small amount of Ti was to improve the HAZ toughness due to the oxides mainly composed of TiN and Ti ₂ O ₃ , and the N and O contents were limited to obtain the effect sufficiently. In order to secure the high strength of HT60 without deteriorating the weldability, V
An addition was made. However, the effect is not exhibited by simply adding V. This is because the precipitation hardening due to V ₄ C ₃ and VN cannot be sufficiently obtained due to the decrease of the C amount and the decrease of the free N amount by the addition of Ti.

Therefore, as a result of earnest research on the manufacturing method, the present inventors have found that Ar in order to increase the strength by adding V in low C and low N steels.
It was confirmed that (γ + α) 2 phase rolling is effective for ₃ points or less. That is, it has been found that fine V precipitation in the ferrite base is remarkably promoted by adding appropriate (γ + α) two-phase region rolling to the above steel, and the strength is significantly improved. In order to obtain such an effect of V addition sufficiently, it is necessary to limit the reheating and rolling conditions of steel (slab) as follows. First, the reheating temperature is limited to the range of 1100 to 1250 ° C. The reheating temperature must be 1100 ° C or higher (desirably 1150 ° C or higher) in order to form a solid solution with precipitates such as Nb and V and to secure high strength. Below this temperature, Nb hardly forms a solid solution and sufficient strength cannot be obtained. However, when the reheating temperature exceeds 1250 ° C., the austenite grains (γ grains) are remarkably coarsened and cannot be completely refined even by rolling, so that excellent low temperature toughness cannot be obtained. Therefore, the reheating temperature must be 1250 ° C or lower. Next, the cumulative reduction of 950 ° C or less and Ar ₃ or more must be 40% or more. This is to refine the γ structure and improve the low temperature toughness.
This is because unless the γ structure is refined and the ferrite grain size is not reduced, the low temperature toughness is significantly deteriorated due to the precipitation hardening of V by the subsequent (γ + α) two-phase rolling. In addition, enough V
In order to obtain the precipitation hardening, the cumulative reduction amount in the two-phase region and the rolling end temperature must be limited to 20% or more and 690 to 630 ° C, respectively. If the reduction amount in the (γ + α) two-phase region is less than 20%, the precipitation of V will be insufficient. On the other hand, if the rolling end temperature exceeds 690 ° C, the size of V precipitates will be large and the precipitation hardening ability will be weak.

As described above, even if the manufacturing method is suitable, if the basic components are not suitable, excellent properties as HT60 cannot be obtained.
Hereinafter, this point will be described.

The lower limit of 0.03% of C is the minimum amount for ensuring the strength of the base material and the welded portion and exerting these effects when Nb, V and the like are added. However, if the C content is too large, the weldability is significantly deteriorated, so the upper limit was made 0.09%.

Addition of a large amount of Si deteriorates weldability and HAZ toughness, so the upper limit was made 0.5%. Deoxidation of steel is sufficient only with Al and Ti, and Si is not necessarily added.

Mn is an essential element for securing strength and toughness, and its lower limit is 1.4%. However, if the amount of Mn is too large, not only hardenability increases and weldability and HAZ toughness deteriorate, but
The upper limit is 1.9% because it promotes center segregation of continuous cast slabs.
And

In the steel of the present invention, P and S which are impurities are 0.02% and 0.
The reason why the content is 003% or less is to further improve the low temperature toughness of the base material and HAZ. Reduction of P prevents grain boundary fracture, and reduction of S content prevents deterioration of toughness due to MnS. In particular, in the steel of the present invention, the reduction of S is indispensable because the separation occurs on the Charpy impact fracture surface due to the (γ + α) two-phase rolling and the reduction of absorbed energy is caused. The preferred amounts of P and S are 0.01% or less and 0.002% or less, respectively.

In the present invention, Nb is an essential element for obtaining the strength and low temperature toughness of the base material, and its lower limit is 0.005%. However, if the addition amount is too large, the HAZ toughness and weldability are significantly impaired, so the upper limit is made 0.03%.

V is an element having almost the same effect as Nb, but its precipitation hardening ability is slightly weaker than that of Nb. However, it is a very valuable element if the precipitation hardening is effectively obtained with little harm to HAZ toughness and weldability. In the present invention, it is possible to obtain sufficient precipitation hardening of V, and V is an essential element. But 0.03
If it is less than%, the effect is small and the upper limit is 0.08%.

When the amount of Al is small (for example, 0.003% or less), Ti combines with O to form an oxide containing Ti ₂ O ₃ as a main component to improve HAZ toughness. Further, it combines with N to form TiN, suppresses γ grain phase enlargement at the time of reheating, and refines the structure after rolling. To obtain these effects, Ti must be at least 0.005%.
However, if too much, TiC is formed and the low temperature toughness and weldability are deteriorated, so the upper limit is made 0.02%.

Al, an element generally contained in deoxidized upper steel, but deoxidized is Si
Alternatively, Ti alone is sufficient, and in the steel of the present invention, the lower limit is not limited. However, if the amount of Al increases, not only the cleanliness of the steel deteriorates but also the toughness of the weld metal deteriorates, so the upper limit was made 0.06%.

N is an element contained in steel as an unavoidable impurity, but it forms TiN and enhances the properties of HT60 as described above.
For this purpose, the minimum amount of N is 0.001%. However, excessive N is extremely harmful to HAZ toughness and weldability, and this effect is more remarkable in high strength steel. In the case of HT60, the upper limit is 0.003%.

O, like N, is an element contained in steel as an impurity, but when the amount of Al is small, it combines with Ti to form Ti ₂ O ₃ and improves the HAZ toughness. The minimum amount of O necessary for this is 0.002%. However, if the amount of O is too large, the cleanliness of the steel deteriorates and the toughness is adversely affected.
0.004%.

Next, the reason for adding Ni, Cu and Ca will be explained. The main purpose of adding these elements to the basic components is to improve the properties such as strength and toughness without impairing the excellent characteristics of the steel of the present invention. Therefore, the amount of addition is limited by itself.

It improves strength and toughness without adversely affecting Ni weldability, and is also effective in preventing Cu-cracks. However, if it exceeds 1.0%, the weldability is not preferable, so the upper limit was made 1.0%.

Cu also improves the strength without adversely affecting the weldability and HAZ toughness, and is also effective in improving the corrosion resistance.
However, if it exceeds 0.6%, the weldability is impaired, so the upper limit is 0.6.
%.

The lower limit of the amounts of Ni and Cu is 0.05%, which is the minimum amount at which the effects of these elements can be obtained.

Ca controls the morphology of sulfides, improves low temperature toughness (Charpy absorbed energy), and is also effective in improving hydrogen-induced cracking resistance. However, if the amount of Ca is less than 0.001%, there is no practical effect, and if it is added over 0.005%,
A large amount of aO and CaS are generated and become large inclusions, which impairs the cleanliness of steel and reduces toughness. It also adversely affects the weldability. Therefore, the range of the added amount is limited to 0.001 to 0.005%.

[Examples] Next, examples of the present invention will be described. Converter continuous casting-
In the thick plate process, thin steel plates with various steel components (plate thickness 10 to 20 mm) were manufactured, and their strength and toughness were investigated. Examples are shown in Table 1. The steel sheets produced according to the invention (invention steel) all have good properties. In contrast, the comparative steels not according to the present invention are inferior in strength and low temperature toughness. Steels 10 and 11 do not have Nb or V added and lack strength, and Steel 12 has no Ti added and HAZ.
Inferior toughness. Further, in Steels 13 to 16, the strength or low temperature toughness is not sufficient because the manufacturing conditions are not appropriate.

[Effects of the Invention] The present invention makes it possible to manufacture HT60 having excellent low temperature toughness and weldability, resulting in a reduction in manufacturing cost and a marked improvement in on-site welding work efficiency and safety of welded structures. be able to.

Claims (2)

[Claims] 1. C: 0.03 to 0.09%, Si: 0.5% or less, Mn: 1.4 to 1.
9%, P: 0.02% or less, S: 0.003% or less, Nb: 0.005 to 0.03%, V:
0.03 to 0.08%, Ti: 0.005 to 0.02%, Al: 0.06% or less, N: 0.00
Steel containing 1 to 0.003%, O: 0.002 to 0.004% and the balance iron and unavoidable impurities is heated to a temperature range of 1100 to 1250 ° C and the cumulative reduction amount of 950 ° C or less and the Ar ₃ transformation point or more is 40% or less. %, Less than Ar ₃ transformation point cumulative rolling amount is 20% or more, and rolling is performed such that the rolling end temperature is 690 ° C. or less, 630 ° C. or more, tensile strength 60 kgf / mm ² class V Manufacturing method of added high toughness and high strength steel sheet. 2. C: 0.03 to 0.09%, Si: 0.5% or less, Mn: 1.4 to 1.
9%, P: 0.02% or less, S: 0.003% or less, Nb: 0.005 to 0.03%, V:
0.03 to 0.08%, Ti: 0.005 to 0.02%, Al: 0.06% or less, N: 0.00
1 to 0.003%, O: 0.002 to 0.004%, Ni: 0.05 to 1.0%, Cu: 0.0
Steel containing 1 to 2 or more of 5 to 0.6% and Ca: 0.001 to 0.005% with the balance being iron and unavoidable impurities 1100
To 1250 950 ° C. by heating to a temperature range of ° C. or less, Ar ₃ cumulative reduction ratio of more than the transformation point of 40% or more, Ar ₃ cumulative reduction ratio of less than transformation point is 20% or more and the rolling finishing temperature is 690 ° C. or less Tensile strength of 60%, characterized by rolling to 630 ℃ or higher
kgf / mm ² class V added high toughness high strength steel plate manufacturing method.