JPH0320408A - Production of high tensile steel stock excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To produce a high tensile steel stock improved in toughness at low temp by heating a steel incorporating Nb having a specific composition, completing rolling at a temp. in an austenite single phase region, and carrying out cooling and heat treatment under specified conditions, respectively. CONSTITUTION:A steel stock having a composition consisting of, by weight, 0.05-0.20% C, 0.01-0.60% Si, 0.50-2.00% Mn, <=0.020% P, <=0.010% S, 0.005-0.030% Nb, 0.0005-0.0030% B, 0.01-0.10% solAl, 0.001-0.010% N, and the balance Fe with inevitable impurities is heated up to 950-1200 deg.C, and rolling is completed at a temp. in an austenite single phase region. Successively, accelerated cooling is performed from a temp. of the Ar3 point or above to <=400 deg.C at >=2 deg.C/sec cooling rate in the central part of the steel stock. Further, an operation of both the reheating up to a temp. range between no lower than Ac3 point and no higher than (Ac3+200 deg.C) and hardening is repeated one or more times. Then tempering treatment is exerted at the temp. of the Ac1 point or below. Further, proper amounts of one or more elements among NI, Cr, Mo, Cu, V, and Ti are incorporated to the above-mentioned steel stock, as necessary.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing high-tensile steel materials with excellent low-temperature toughness. (Prior Art) For example, in recent years, hydroelectric power plants have tended to become larger in order to improve power generation efficiency as the demand for electricity increases significantly. Along with this, penstocks (penstocks) at hydroelectric power plants
The strength required for steel materials used in automobiles is increasing, and in response to this, the steel materials used are also becoming thicker and stronger. However, penstocks, which are steel structures, are not only required to have simple strength, but also to have toughness, especially low-temperature toughness, since they are used under harsh environmental conditions. In other words, as structures have become larger in recent years, there has been an increasing demand for steel materials that have good weldability, are inexpensive, and have high strength and toughness. Therefore, various methods have been proposed to improve the low-temperature toughness of extra-thick, high-strength steel sheets used for the above-mentioned purposes. For example, by maximizing the hardenability improvement effect of B. As a thing,
Various methods have been proposed to increase the tensile strength of steel by incorporating B into it. Specifically, (i) JP-A-59-166620;
Japanese Patent Application Laid-open No. 59-166621 discloses that AQ-B one low N
As a manufacturing method for high-toughness high-tensile steel, a method that combines direct quenching and quenching and tempering after heating the steel material to high temperature is used.
(ii) Japanese Patent Application Laid-Open No. 48-28316 has rA
．． (iii) Japanese Patent No. 1,151,095 further describes a method for imparting strength and toughness to an extra-thick steel plate by repeating quenching at three or more temperatures two or more times and then tempering. , No. 1,333,560 describes a method for improving the low-temperature toughness of heat-treated high-strength steel sheets, ``a method in which a small amount of Nb is added to a steel billet and the hot rolling conditions are controlled under certain conditions.'' J is disclosed respectively. (Problem to be solved by the invention) However, depending on these methods, extremely thick and high tensile strength!
! 4. It is not possible to reliably improve the strength and low-temperature toughness of the plate to the desired values. For example, the method disclosed in Japanese Patent Application Laid-Open No. 48-28316 does not mention the respective heat treatment temperatures, and merely specifies a preferable heat treatment temperature range of 880 to 980.
This is just an example of ℃. Therefore, for example 95
If quenching is repeated twice at a high temperature of 0°C, coarsening of crystal grains will occur during the second quenching.
It was not possible to achieve the desired improvement in low temperature toughness, while AC3
If quenching is repeated twice at a low temperature just above the point, a mixed grain structure of austenite crystal grains and ferrite crystal grains tends to occur, and the desired low-temperature toughness cannot be improved. Furthermore, the effect of Nb addition is not sufficiently mentioned. In other words, the method disclosed in Japanese Patent Application Laid-Open No. 48-28316 lacks the effect of improving toughness depending on the conditions such as the assembly regulations of the steel material and the quenching temperature, and sufficient low-temperature toughness cannot be obtained. Also, Japanese Patent No. 1,151,095, Japanese Patent No. 1,33
Although the method disclosed in No. 3.560 can be expected to be effective for thin plate materials, it cannot be expected to be effective in controlling rolling for extremely thick steel plates. As described above, it was not possible to improve the low-temperature toughness of ultra-thick, high-strength steel plates using any of the methods. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing high-strength steel materials with excellent low-temperature toughness, such as extremely thick high-strength steel. (Means for Solving the Problems) The present inventors have solved the above problems and have conducted various studies to produce steel materials with even higher strength and toughness. We learned that by heat-treating a steel material containing Nb, which has the effect of improving hardenability, under certain specified heat treatment conditions, the strength and toughness of the resulting steel material can be significantly improved. Completed the invention. Here, the gist of the present invention is as follows: C: 0.05-0.20%, Sl: 0.01 in weight%
~0.60%, Mn: 0.50~2.00%, P
+ 0.020% or less, S: 0.010% or less,
Nb: 0.005-0.030%, B: 0
．． 0005 to 0.0030%, sol. Eighth Q: 0.
01 ~o. to%, N: 0.0G1~o. oio
% A steel material having steel &IIIi consisting of the balance Fe and unavoidable impurities is heated to a temperature of 950°C or higher and 1200°C or lower, then completed rolling at a temperature in the austenite single phase range, and then rolled from a temperature of Ars point or higher to 400°C. Accelerated cooling is performed at a cooling rate of 2°C/sec or more at the center of the sea bream material to a temperature below ℃, and then quenching is performed once after heating to a temperature range above AC (Acz point + 200℃). or repeat it two or more times,
After that, Ac. This is a method for producing high-strength steel materials with excellent low-temperature toughness, which is characterized by performing tempering treatment at temperatures below 100%. In addition, in the above-mentioned present invention, the cavity material further contains Ni: 0.1 to 5.0% and Cr: 0.1 to 1%.
．． 5%, Mo: 0.1 to 0.9% Cu: 0.
1 ~ 0.5%, V: 0.01 ~ 0.10%, T!
: o. It may contain one or more alloying elements selected from the group consisting of ~0.030%. In the present invention, the steel material includes, for example, an extremely thick steel plate that can be used as a material for penstocks. Preferably, it is a high-tensile steel plate with a thickness of 100 am or more. (Function) Hereinafter, the present invention will be explained in detail together with the function and effect. In this specification, unless otherwise specified, "%" means "% by weight".
”. The present invention is characterized in that a steel material containing Nb and having a specific steel composition is heated and rolled in a specific temperature range, quenched online, and then subjected to quenching and tempering treatment. This is a method for producing high-strength steel with excellent low-temperature toughness.

First, the reason why the strength of the steel assembly is limited as described above in the present invention will be explained.

If C is less than 0.05%, it is difficult to obtain sufficient hardening strength, so it should be 0.05% or more, and if it exceeds 0.20%, toughness and weldability will be impaired, so it should be limited to 0.20% or less. do. St is 0 in steel manufacturing from the viewpoint of ensuring the strength of steel materials.
．． This element is required in an amount of 0.1% or more, but if it exceeds 0.60%, the toughness of the steel material will be significantly reduced, so it should be kept at 0.60% or less.

Mn is 0.5 to ensure both hardenability and toughness.
It is necessary to add 0% or more, but if it is added in a large amount exceeding 2.00%, it may cause micro-segregation of Mn or extended Mn.
is, etc., the anisotropy of mechanical properties becomes extremely large, making it unsuitable as a high-tensile steel material. Therefore, the amount of Mn added is limited to 0.50% or more and 2.00% or less. In order to ensure toughness, P and S are harmful elements, and it is desirable to reduce them to zero, but it is difficult to reduce them to zero due to current steelmaking technology, so the upper limit is set at 0.020%, respectively.
Limit it to 0.010%. Nb is an important element in the present invention. By adding 0.005% or more of Nb, crystal grains can be made finer and therefore effective in improving toughness. Furthermore, when B coexists with B, it is easy to keep B segregated at grain boundaries due to its recrystallization suppressing effect, so that the hardenability improving effect of B can be promoted.

However, if more than 0.030% is added, it will cause deterioration of the toughness of the welded joint.
The following restrictions apply. Note that although Nb has been added to thin sheets in order to refine grains, there is no example of this in the case of extremely thick steel sheets as in the present invention. This is because it is difficult to expect a controlled rolling effect, and therefore, in the present invention, grain refinement is promoted by fine precipitation of solid solution Nb. In the present invention, B is an extremely important element because it significantly improves hardenability even with the addition of a small amount, and o. ooos
It is necessary to add more than 0.0%. If the amount of B added exceeds 0.030%, the toughness will deteriorate significantly, so the amount of B added is 0.03%. It is limited to 0.0005% or more and 0.0030% or less.

sol. AQ is the refinement of T grains, fixation of solid solution N, and B
It is an indispensable element for processing, and in order to exhibit this effect, it is necessary to add it in an amount of 0.01% or more, but if it is added in an amount exceeding 0.10%, it will cause a decrease in toughness. 01% or more 0. Limit to 10% or less. Since N contributes to the refinement of γ grains according to the life rules of AQN, it is necessary to add 0.001% or more, but 0.001% or more is necessary.
If it is added in excess of 10%, solid solution N increases and toughness deteriorates. Therefore, it is limited to 0.001% or more and 0.010% or less.

By heat-treating a steel material having the above-mentioned strength under the conditions described below, it is possible to obtain a core material with excellent low-temperature toughness.
selected from the group consisting of. By adding one type or a combination of two or more types, it is possible to further improve toughness or strength.

Therefore, the amounts of these elements added will be explained below. It is an effective element for improving strength and toughness when added in an amount of 0.1% or more, but since adding a large amount increases cost, it is limited to 5.0% or less. Adding 0.1% or more of Cr increases hardenability and is effective in improving toughness, but adding more than 1.5% significantly deteriorates toughness, so Cr is added in an amount of 0.1% or more and 1.5% or less. . Mo is effective in improving strength when added in an amount of 0.1% or more, but adding more than 0.9% impairs toughness, so it is limited to 0.1% or more and 0.9% or less. ．． Cu is an element that is effective in increasing strength when added in an amount of 0.1% or more, but when added in an amount exceeding 0.5%, the surface quality of the steel material is significantly deteriorated due to scale generation. Therefore, it is limited to 0.1% or more and 0.5% or less. Adding 0.01% or more of (2) is effective in increasing strength, but adding more than 0.10% causes deterioration of toughness. Therefore, it is limited to 0.01% or more and 0.10% or less. Tf is o. Adding more than oos% has the effect of suppressing the length of T grains during heating as T-N, and by fixing N, it has the effect of suppressing the length of T grains during heating. Since it increases the amount of $B, it is effective in improving the hardenability of B, but if it exceeds 0.030%, the toughness will deteriorate, so o. oos% or more 0.030%
Limited to:

For steel materials having the above assembly strength, the present invention provides (1)
Heat to a temperature of 950°C or higher and 1200″C or lower.

(2) Rolling is completed in the austenite single phase region. (3)
Following (2), Ar. accelerated cooling from a temperature above a point to a temperature below 400° C. at a cooling rate of 2”C/see or above at the center of the steel material.

(4) Ac. Repeat the process of reheating to a temperature range above the point (ac point 3 + 200°C) and then quenching it one or more times. (5) After (4), tempering is performed at a temperature below the Acn point. Steel products are manufactured under these conditions.

The above manufacturing conditions are detailed below. First, the heating temperature should be 950°C or higher. This is necessary to form a solid solution of Nb, B, etc. in the steel, but if the temperature exceeds 1200°C, the coarsening of T grains becomes noticeable, so the upper limit should be set at 1200°C.
limited to. Next, in order to make the structure after rolling into a completely quenched structure, rolling is completed at a temperature in the T-phase region, and accelerated cooling is performed in this state to rapidly cool down to 400°C or less where transformation is completed. ．． This allows the T grains produced during subsequent reheating to become finer. In the present invention, even if rolling is performed in the low-temperature T region during online quenching, recrystallization is suppressed by Nb, so B remains segregated at grain boundaries, and B
Therefore, the hardenability improvement effect can be fully utilized. Furthermore, the cooling rate at the center is limited to 2°C/see or higher because a cooling rate smaller than this will not provide a sufficient hardening effect. Furthermore, by setting the upper limit of the T temperature during reheating to (Ac, point 1200)'C, it is possible to prevent the fine T grains generated from becoming coarse. This heating/quenching treatment may be performed once, but by repeating it two or more times, further grain refinement can be achieved.

Finally, a tempering process is performed. The tempering treatment does not require any special treatment, and a normal tempering treatment may be performed. That is, the steel material that has undergone the above quenching is
It may be tempered to a temperature below the + point, for example to about 550 to 650''C.

As explained above in detail, by the method according to the present invention,
This makes it possible to reliably obtain high-strength steel materials with excellent low-temperature toughness. Furthermore, Figure 1 shows the thermal cycle of the method for producing high-strength steel with excellent low-temperature toughness according to the present invention. Further, the present invention will be explained in detail using Examples, but these are merely illustrative of the present invention and the present invention is not unduly limited thereby.

Example: A piece of rope having the composition shown in Table 1 was rolled and heat treated under the conditions shown in Table 2 to give a plate thickness of 30 to 100 ms+.
Samples No. 1 to NlAll were obtained.

For these samples, a tensile test was conducted to measure the yield point and tensile strength, and a Charpy test was conducted to measure the transition temperature. The results are shown in Table 2.

(Left below) Samples Nα1 to Nα゜6 are samples obtained by the method according to the present invention, and it can be seen that high-strength steel plates with high tensile strength, low transition temperature, and excellent low-temperature toughness were obtained. ．． On the other hand, samples N (L7 to sample kll) are comparative samples.

First, it can be seen that in sample Na7, since the heating temperature of the steel material exceeds the range of the present invention, coarsening of γ grains cannot be prevented, and toughness cannot be improved. It can be seen that sample Nα8 was not cooled after completion of rolling, so the γ grains were not refined during the subsequent reheating, so the toughness could not be improved. It can be seen that the toughness of sample Na9 could not be improved because it was not reheated.

It can be seen that the toughness of sample No. 10 deteriorated because the reheating temperature was higher than Acs + 200°C. Furthermore, it can be seen that sample NQII has no improvement in toughness because the steel material used does not contain Jib. (Effects of the Invention) As detailed above, the present invention involves heating and rolling a steel material having a specific steel composition in a specific temperature range, quenching it online, and then subjecting it to quenching and tempering. With this method, a method for providing an extra-thick high-strength steel plate having good low-temperature toughness compared to the conventional extra-thick high-tensile steel plate has been obtained.

A manufacturing method for ultra-thick high-strength steel plates with excellent low-temperature toughness that can be used in various large structures, including steel plates for penstocks, where demands for higher tensile strength and thicker steel plates have been increasing year by year in recent years. The significance of the present invention in providing this is remarkable.

[Brief explanation of the drawing]

FIG. 1 is a graph showing an example of a thermal cycle of the method for manufacturing a steel material with excellent low-temperature toughness according to the present invention.

Claims (2)

[Claims] (1) C: 0.05-0.20%, Si: 0.01-0.60 in weight%
%, Mn: 0.50-2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.005-0.0
30%, B: 0.0005-0.0030%, sol.
Al: 0.01-0.10%, N: 0.001-0.0
A steel material having a steel composition consisting of 10% Fe and unavoidable impurities is heated at 950°C or higher at 1200°C.
After heating to a temperature below ℃, rolling is completed at a temperature in the austenite single phase region, followed by
Accelerated cooling is performed at a cooling rate of 2°C/sec or more at the center of the steel material to a temperature of 00°C or lower, and then Ac_3 points or higher (Ac
After heating to a temperature range of _3 points + 200℃ or less, the quenching process is repeated once or twice, and then,
A method for producing high-strength steel with excellent low-temperature toughness, characterized by performing tempering treatment at a temperature of Ac_1 point or lower. (2) The steel material further includes Ni: 0.1 to 5.0%, Cr: 0.1 to 1.5%, M
o: 0.1-0.9%, Cu: 0.1-0.5%, V:
0.01-0.10%, Ti: 0.005-0.030
% of the alloying element selected from the group consisting of %.