JPH06158160A - Production of high tensile strength heat treated steel excellent in cost effectiveness - Google Patents

Abstract

PURPOSE:To provide the method for producing high tensile strength heat treated steel provided with cost effectiveness as well as toughness and weldability. CONSTITUTION:Steel contg. 0.10 to 0.18% C, 0.05 to 0.15% Si, 0.60 to 1.00% Mn, 0.40 to 1.00% Cr, 0.50 to 1.00% Ni, 0.40 to 0.80% Mo, 0.010 to 0.030% Ti, 0.010 to 0.030% Nb, 0.0004 to 0.0015% B and <=0.50% Cu and in which Cr/Mo is regulated to <=2.0 and Nb+Cr+Mo is regulated to <=2.50% and, furthermore, the weld crack sensitivity index Pcm is regulated to <=0.29 and the carbon equivalent Ceq. is regulated to >=0.53 is heated to 1150 to 1200 deg.C, is subjected to hot rolling at 650 to 900 deg.C at >=30% draft, is thereafter reheated to the Ac3 point or above, is hardened and is then tempered at the Ac1 point or below. In this way, the high tensile strength heat treated steel small in the content of expensive Ni, provided with high strength and high toughness of >=950N/mm<2> tensile strength and furthermore excellent in weldability can be produced.

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 heat-treated steel for structures such as penstocks and bridges, which are required to have high strength and good toughness.

[0002]

2. Description of the Related Art In recent years, with the increase in size of welded structures, there is an increasing tendency to use high-strength steel in order to reduce the weight of the structure. This maximizes the benefits of weight reduction in design as well as the associated improvements in transportation and assembly workability on the processing and construction side, as well as welding workability due to the thinning of various structural members. It depends on what you can do.

[0003] For example, in recent years, there has been a marked increase in the trend toward larger sizes in welded structures such as penstocks for pumped storage power plants, pressure vessels, bridges, and offshore structures. The performance has been improved. Recently, an HT 980 (tensile strength 980 N / mm ² ) class material has also been developed, and the applicant of the present invention also discloses in KOKOKU Publication No. 1-20210 that the HT 980 class has good weldability and high strength and high toughness. The method of manufacturing steel is shown. This is a characteristic of the component system, C: 0.07 to 0.15%, Si: 0.15% or less, Mn: 0.40 to 1.0
0%, Cr: 0.40 to 1.20%, Ni: 2.0 to 4.2%, Mo: 0.40 to
0.80%, V: 0.01 to 0.06%, B: 0.004 to 0.0015% and
Cu: 0.50% or less, and the weld crack susceptibility index P _CM (
Hereinafter simply after heating the steel sheet having a thickness of 40~150mm that of P _CM) was prepared from 0.31 following steel Ac ₃ point to 1000 ° C., and quenched and then reheated after tempering from 560 to 630 ° C., Yield point is 980N / mm ² or more, tensile strength is 950N / mm ² or more, vT
It is a method to obtain steel with rs (impact fracture transition temperature) of -60 ° C or less.

Similarly, Japanese Patent Publication No. 1-25371 of the applicant.
In the publication, as a characteristic of the component system, C: 0.07 to 0.15%,
Si: 0.15% or less, Mn: 0.40 to 1.20%, Cr: 0.40 to 1.20
%, Ni: 1.00 to 3.50%, Mo: 0.40 to 0.80%, V: 0.01 to
0.06%, Nb: 0.005-0.030% and B: 0.004-0.0015
%, And if necessary, at least one of Cu: 0.50% or less, Ca: 0.005% or less and W: 1.00% or less is contained, and P _CM is 0.
Steel of 28 or less is heated to 1050 ° C or higher, then hot-rolled, subsequently reheated to an Ac of ₃ points to 1050 ° C and then quenched, then tempered at an Ac of ₁ or less and water-cooled to give a yield point of 880N / mm ² or more,
A manufacturing method for obtaining a tempered high strength thick steel material having a tensile strength of 950 N / mm ² or more and a vTrs of -60 ° C or less was shown.

In these methods, C and Si are both reduced to 0.
Cr, Ni, Mo, Cu, V,
Nb and B, such as steel temper was added (quenching, tempering) to, while securing the strength and toughness, improve the cracking susceptibility during welding to reduce the hardness of the welding heat affected zone is limited to P _CM is doing. However, all of them contain a large amount of Ni in order to achieve both high strength and toughness, and cannot be said to be a method for producing a high-strength tempered steel that is also economical.

[0006]

The object of the present invention is to achieve a tensile strength of 950 N / mm ² which is economical with toughness and weldability.
An object of the present invention is to provide a method for manufacturing the above high tensile tempered steel.

[0007]

The gist of the present invention resides in the following method for producing a high tensile tempered steel.

% By weight, C: 0.10 to 0.18%, Si: 0.05 to
0.15%, Mn: 0.60 to 1.00%, Cr: 0.40 to 1.00%, Ni: 0.
50 to 1.00%, Mo: 0.40 to 0.80%, Ti: 0.010 to 0.030%,
Nb: 0.010 to 0.030%, B: 0.0004 to 0.0015% and Cu:
Contains 0.50% or less, the balance Fe and unavoidable impurities, Cr / Mo 2.0 or less, Mn + Cr + Mo 2.50
% Or less, and a steel having a weld crack susceptibility index P _CM represented by the following formula (I) of 0.29 or less and a carbon equivalent Ceq. Of 0.53 or more is heated to 1150 to 1200 ° C. to 650 to 900 ° C. After hot rolling with a rolling reduction of 30% or more in the temperature range, Ac ₃
A method for producing a high-strength tempered steel, which comprises reheating to a temperature range of at least a point, quenching, and then tempering at a temperature range of less than Ac ₁ .

P _CM = C% + {(Mn% + Cr% + Cu%) / 2
0} + (Si% / 30) + (Ni% / 60) + (Mo% / 15) +
(V% / 10) + 5B% (I) Note that Ceq. Is based on the following formula that is generally defined.

Ceq. = C% + (Mn% / 6) + (Si% / 2
4) + (Ni% / 40) + (Cr% / 5) + (Mo% / 4) + (V
%/14)

[0011]

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 in (a) to (f) below.

(B) The Ni content is reduced to ensure economic efficiency.

(B) Nb compensates the decrease in strength and toughness of the base material due to the decrease in Ni content. That is, by containing an appropriate amount of Nb, the precipitation effect of the Nb-containing precipitate is further enhanced, the crystal grains are made finer, and the strength and toughness of the base material are secured.

(C) By containing Ti, TiN is precipitated to prevent coarsening of crystal grains during welding and to improve joint performance.

(D) Cr / Mo 2.0 or less, Mn + Cr + Mo 2.50
% Or less, P _CM is 0.29 or less, and Ceq. Is 0.53.
As described above, the alloying element content is limited to suppress the weld cracking susceptibility.

(E) When Nb and Ti are contained in combination, the solid solution of Nb is hindered, and when the heating temperature before rolling is less than 1150 ° C., the solid solution hardly occurs, so that the precipitation effect of Nb is not exhibited. If the temperature exceeds 1200 ° C, the Nb precipitation effect is saturated and surface cracking of the slab occurs, so heating to 1150 to 1200 ° C and hot rolling are performed.

(F) In order to promote the solid solution of Nb, the rolling heating temperature is set in the above range of 1150 to 1200 ° C., and further 650 to
Hot rolling is performed in the temperature range of 900 ° C with a rolling reduction of 30% or more to refine the γ grains.

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

C: 0.10 to 0.18% C is a component necessary for improving the hardenability of steel and ensuring strength. If it is less than 0.10%, the target tensile strength of 950 N / mm ² or more cannot be obtained. on the other hand,
If it exceeds 0.18%, the weldability and low temperature toughness deteriorate.
Therefore, the range of the C content is limited to 0.10 to 0.18%.

Si: 0.05 to 0.15% A component effective in deoxidizing and securing strength, and for that purpose, a content of 0.05% or more is necessary. On the other hand, if it exceeds 0.15%, the toughness of the welded joint deteriorates. Therefore, the range of the Si content is set to 0.05 to 0.15%.

Mn: 0.60-1.00% This is an important component for ensuring the hardenability of steel, but if it is less than 0.60%, this effect is not obtained. On the other hand, if it exceeds 1.00%, both the weldability and the toughness of the base material are deteriorated. Therefore, the range of Mn content is 0.60 to 1.00%
Limited to.

Ni: 0.50 to 1.00% Ni is an effective component for securing the low temperature toughness and strength of the base material and the welded joint. For that purpose, the content of 0.50% or more is required. On the other hand, considering economy, the upper limit was set to 1.00%.

Cr: 0.40-1.00% This is an important component for ensuring the hardenability of steel, but if it is less than 0.40%, this effect is not obtained. On the other hand, if it exceeds 1.00%, both the weldability and the toughness of the base material are deteriorated. Therefore, the range of Cr content is 0.40 to 1.00%
Limited to.

Mo: 0.40 to 0.80% Mo has the effects of improving the hardenability and increasing the resistance to temper softening to increase the strength. If it is less than 0.40%, the effect is not exerted, while if it exceeds 0.80%, the effect is saturated and the economy is impaired because it is an expensive element. Therefore, the range of Mo content is 0.40 to 0.80%
And

Cr / Mo: 2.0 or less, Mn + Cr + Mo: 2.50% or less Mn, Cr and Mo all improve the hardenability or strength as described above, but if Cr / Mo exceeds 2.0, the toughness of the base material is Although improved, the desired strength cannot be secured because the Mo precipitation effect decreases. On the other hand, Mn + Cr + Mo is 2.50%
If it exceeds, the strength improving effect is saturated, and the toughness of the base material and the joint portion decreases. Therefore, further, while satisfying the above range of the content of the single component, Cr / Mo is 2.0 or less,
In addition, Mn + Cr + Mo was set to 2.50% or less.

Ti: 0.010 to 0.030% Since it is effective in improving the toughness of the welded joint, it is necessary to add 0.010% or more. However, since excessive Ti causes deterioration of the toughness of the base metal, it is 0.030%.
Below.

Nb: 0.010 to 0.030% This is an effective component for ensuring the strength and toughness of the base material by refining the crystal grains.
If it is less than 0.010%, its effect cannot be expected. On the other hand, 0.
If it is contained in a large amount exceeding 030%, the toughness of the welded joint is deteriorated. Therefore, the range of Nb content is 0.010 to 0.030.
%.

B: 0.0004 to 0.0015% A small amount of the component greatly improves the hardenability and further improves the toughness of the base material. For that purpose, it is necessary to contain at least 0.004% or more. On the other hand, if it exceeds 0.0015%, the effect is saturated and, conversely, the toughness of the base material is deteriorated. Therefore, the range of B content is 0.0004 to 0.0015%.

Cu: 0.5% or less Cu is a component necessary for improving strength and corrosion resistance, but if it exceeds 0.5%, hot working brittleness, toughness and weldability of the base material deteriorate.

P _CM : 0.29% or less It has been conventionally used as an index showing the weldability of steel materials, and it is desirable that it is small to have weldability. If P _CM is suppressed to 0.29% or less in the material steel of the method of the present invention, the preheating temperature necessary for stopping cracking in the Y-groove constrained cracking test can be set to 150 ° C. or less as shown in the examples below. it can.

Ceq .: 0.53 or more Ceq. Is one of the indexes showing the hardenability of steel. In the present invention, the above-mentioned commonly used formula is used. If this Ceq. Is less than 0.53, the hardenability is insufficient and the target strength cannot be obtained.

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

The hot rolling of the material steel of the present invention is not so-called controlled rolling but ordinary hot rolling in which the rolling is finished in the high temperature region of the austenite single phase. However, the heating temperature range of the steel at that time is 1150 to 1200 ° C, and the rolling temperature range is 650.
~ 900 ℃, reduction rate of 30% or more. Heating temperature is 1200
℃ and rolling temperature are over 900 ℃, the austenite grains are coarsened and the base metal toughness deteriorates even in steel containing Nb and Ti for grain refinement. The temperature was 1200 ° C and the rolling temperature was 900 ° C.

On the other hand, if the heating temperature is less than 1150 ° C., it is attempted to form a solid solution of Nb in the γ-grains and then to refine the γ-grains by fine Nb-containing precipitates to improve the strength and toughness. The effect cannot be exerted. If the rolling temperature is lower than 650 ° C, rolling itself becomes difficult. Therefore, the lower limit of each is 1150 ℃ at heating temperature and rolling temperature.
The temperature was 650 ° C. If the reduction ratio at this time is less than 30%, a sufficient fine grain structure cannot be obtained, so the reduction ratio of hot rolling is 30%.
That's it.

After the hot rolling is finished, it is reheated to a temperature range of Ac ₃ point or higher for quenching for the following reason, and then tempered at a temperature of Ac ₁ point or lower. That is, in order to make the steel structure uniform after quenching, the quenching temperature is set to a temperature of Ac ₃ point or higher at which quenching from the austenite single phase region can be performed, and the tempering temperature is set to an Ac ₁ point or lower in order to avoid excessive strength reduction. The temperature.

[0036]

EXAMPLE Continuously cast slabs having the chemical composition shown in Table 1 were hot-rolled under the conditions shown in Table 2, and after hot-rolling, they were further subjected to quenching and tempering heat treatment at the temperature shown in Table 2 to give a sheet thickness of 75 mm. High tensile tempered steel was manufactured.

Table 2 also shows the performance evaluation results of the mechanical properties, toughness and weldability of the steel material thus obtained. The weldability is JIS Z in CO ₂ arc welding with a heat input of 1.7 kJ / cm (welding rod L-80, room temperature 25 ° C, humidity 70%).
The Y-groove restraint cracking test specified in 3121 was performed, and the toughness was the impact fracture surface transition temperature vTrs at the preheating temperature required to stop cracking.
Then, each was evaluated.

As can be seen from Table 2, in the examples of the present invention in which all the conditions are within the range defined by the present invention, good performance exceeding the target value was obtained.

In Comparative Examples 1 to 9 and Comparative Examples 13 to 16, the chemical composition is outside the range defined by the present invention, but even if the other conditions are within the range defined by the present invention, any of the performances Did not reach the goal. Comparative Examples 10, 11 and 12 have the same chemical composition as that of Inventive Example A, but one of the performances did not reach the target because the temperature condition of hot rolling was outside the range defined by the present invention. . That is, in Comparative Example 10, since the rolling heating temperature was below the lower limit, the Nb refinement effect was not obtained, and neither the tensile strength nor the toughness reached the predetermined target values. Comparative Example 11 is an example in which the rolling heating temperature exceeds the upper limit, but although the target performance was achieved, the performance improving effect was clearly saturated, and slab surface roughness occurred. Therefore, it was found that the rolling heating temperature should be within the range defined by the present invention if the same level of performance can be obtained. In Comparative Example 12, the hot rolling finishing temperature exceeded the upper limit, so that only the same results as in Comparative Example 10 were obtained.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

According to the method of the present invention, the high-strength and high-toughness of 950 N / mm ² or more in tensile strength can be obtained by reducing the content of expensive Ni generally used for strengthening. It is possible to manufacture a high-strength tempered steel that is equipped with and has excellent weldability.

Claims (1)

[Claims] 1. By weight%, C: 0.10-0.18%, Si: 0.05-
0.15%, Mn: 0.60 to 1.00%, Cr: 0.40 to 1.00%, Ni: 0.
50 to 1.00%, Mo: 0.40 to 0.80%, Ti: 0.010 to 0.030%,
Nb: 0.010 to 0.030%, B: 0.0004 to 0.0015% and Cu:
Contains 0.50% or less, the balance Fe and unavoidable impurities, Cr / Mo 2.0 or less, Mn + Cr + Mo 2.50
% Or less, and a steel having a weld crack susceptibility index P _CM represented by the following formula (I) of 0.29 or less and a carbon equivalent Ceq. Of 0.53 or more is heated to 1150 to 1200 ° C. to 650 to 900 ° C. After hot rolling with a rolling reduction of 30% or more in the temperature range, Ac ₃
A method for producing a high-strength tempered steel, which comprises reheating to a temperature range of at least a point, quenching, and then tempering at a temperature range of less than Ac ₁ . P _CM = C% + {(Mn% + Cr% + Cu%) / 2
0} + (Si% / 30) + (Ni% / 60) + (Mo% / 15) +
(V% / 10) + 5B% ・ ・ ・ ・ ・ (I)