JPS582244B2 - Method for manufacturing thick-walled non-tempered high-strength steel with excellent toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the creation of a method for producing thick-walled non-tempered high-strength steel with excellent toughness, and the present invention relates to a method for producing thick-walled non-tempered high-strength steel with excellent toughness. It is an object of the present invention to provide a preferable method for producing non-thermal high tensile strength steel having a thickness of 25 g or more, which solves the problem of deterioration, has a tensile strength of 50 kg/mm 2 or more, and has excellent toughness.

In recent years, line pipes that transport oil, natural gas, and other energy sources buried in cold regions or deep seas have become increasingly larger in diameter.
High strength and toughness are required, and at the same time there is a trend toward thicker walls.

However, controlled rolling technology is the main manufacturing method for mill blanks used to obtain this type of line pipe, but in this controlled rolling technology, the development of new technology is required as the plate thickness increases as described above.

In other words, in conventional controlled rolling technology, the use of elements such as Nb and V is premised on rolling in the low temperature region of the austenite region in an attempt to improve toughness mainly based on the mechanisms of grain refinement and precipitation hardening. .

However, in this case, the basic problem with controlled rolling technology is that, similar to the variation in hardenability as the plate thickness increases due to the mass effect during quenching in tempered high-strength steel, the strength increases as the plate thickness increases. , deterioration of toughness is observed, particularly when the plate thickness becomes 25 mm or more, the toughness deteriorates significantly, and due to constraints on the capacity of the rolling mill, it cannot be used in the same low-temperature range as steel plates of 25 mm or less. It becomes difficult to adopt a high reduction rate.

The present invention was created after repeated studies and speculations in accordance with the above-mentioned actual situation, and it aims to elucidate the cause of the deterioration of toughness in such thick-walled control-rolled materials, and to develop a completely new chemical composition. The company has developed a steel that has the following characteristics, and has made it possible to provide a non-thermal steel with excellent low-temperature toughness and high tensile strength in thick-walled materials with a plate thickness of 25 mm or more, with a fracture surface transition temperature of -100°C or less and excellent low-temperature toughness.

That is, as a result of systematic research into the causes of the deterioration of toughness and strength as the plate thickness increases, the inventors of the present invention have found that the deterioration of toughness occurs even when very strong controlled rolling is performed in the conventional technology as described above. It was found that this is due to the fact that the degree of mixing of ferrite grains in the transformed structure increases markedly as the plate thickness increases.

The mixed grain phenomenon of the transformed structure of the cap is a phenomenon peculiar to control-rolled materials containing Nb, but it has become clear that it cannot be removed simply by controlling the rolling conditions in the low-temperature region of so-called austenite.
It was found that this depended on changes in the austenite structure during rolling at higher temperatures.

However, after the austenite structure during rolling in the high temperature range is sized and refined by adjusting the steel composition, the toughness of the thick controlled rolled material can be significantly improved by applying a cumulative reduction rate of a certain level or higher in the low temperature range. This is the gist of the present invention.

To further explain this, the present invention is comprised of the following three items, and if any one of them is lacking, the objective of the present invention, which is the toughness at a plate thickness of 25 mm or more, will be reduced to a fracture surface transition temperature of -100°C or less. It is clear from the examples described later that it is not possible to obtain an excellent result such as (vTrS).

(1) Limit the C content to 0.10% or less.

(2) A specific amount of Ni is added to increase the tensile strength without impairing toughness or weldability.

(3) By adding a trace amount of Ti in an amount of 0.005 to 0.030%, the precipitated TiN makes the austenite structure fine and grain-sized during high-temperature rolling, and reduces the degree of mixed grains in the transformed structure.

Furthermore, the present invention is a high-toughness technology by controlled rolling based on the addition of Nb, and the strength level is 50 kg/mm2 (tensile strength) or more for steel plates with a thickness of 25 mm or more, and the strength level is In order to ensure a fracture surface transition temperature of -100℃ or less, in addition to the chemical composition limits mentioned above, at least 40% of the final plate thickness in the non-recrystallization temperature range (900℃ or less)
This gives the above cumulative rolling reduction rate.

The above (1) and (2) are necessary conditions for improving toughness, and the decrease in strength due to (1) above can be compensated for by adding Ni to achieve the specified strength while maintaining high toughness regardless of the plate thickness. It is something to tighten.

In addition, in the present invention, as the amount of Ni added increases, the transformation structure changes from ferrite to pearlite to acicular ferri.
However, the effect of adding a small amount of Ni as described above is effective regardless of the structure of the fine TiN.
This effect can be fully exhibited by significantly reducing the precipitation of austenite recrystallized grains during high-temperature rolling and by preventing austenite grain growth between rolling baths.

The reasons for limiting the chemical composition of the product according to the present invention are as follows.

C is 0.1 in order to improve the toughness of thick-walled materials, which is the objective of the present invention.
It is essential to keep the content below 0%, and by keeping it below this upper limit, the volume fraction of pearlite and cementite in the transformed structure is reduced and toughness and ductility are significantly improved.

However, with a C content of 0.02% or less, it becomes difficult to ensure strength.

Furthermore, Mn is extremely effective in increasing the tensile strength and needs to be added in an amount of 0.60% or more, but adding 1.8% or more is effective for improving the toughness of thick-walled materials, which is the main objective of the present invention. Since this is not the case, this is the upper limit.

It is necessary to add Si in an amount of 0.10% or more for deoxidation, but if it exceeds 0.50%, the toughness deteriorates, so this is set as the upper limit.

S must have high ductility in the direction perpendicular to the rolling direction in order to be applied to pipeline materials under cold conditions, which is the main use of the steel of the present invention, and it should be kept at 0.010% or less. This reduction in S content also improves toughness at extremely low temperatures.

However, Ni is one of the essential elements for the present invention, and it is necessary to add at least 0.2% or more, and it is well known that low temperature toughness is improved by adding Ni, but in the case of the present invention In particular, in order to improve the strength of thick high-tensile steel with a plate thickness of 25 mm or more by mackerel rolling, reduction of carbon content,
As will be clarified in the Examples described later, the overall effect of Ni addition with a trace amount of Ni, which will be described later, is to increase the strength and toughness of Ni.

However, if this Ni content exceeds 4.0%, the strength (tensile strength) increases significantly, but the ductility decreases rapidly, so this is not compatible with the present invention, which is intended to be used as rolled. (In other words, if the amount of Ni added is 4% or more, tempering treatment or aging treatment is required to balance strength and ductility).

Nb is an indispensable element for the present invention, which is based on controlled rolling.
．． It is necessary to add 0.1% or more, but the effect is saturated if it is 0.15% or more.

sol. The amount of Al needs to be added at least 0.006% or more for deoxidation, but if it exceeds 0.08%, the toughness deteriorates, so this is the upper limit.

Furthermore, when Ti is added in an amount of 0.030% or less, N in the steel is fixed in the form of 100% TiN.

T is stable and finely precipitated even in the high temperature range of austenite.
Under the Ni and Nb addition conditions of the present invention, iN can significantly refine the austenite structure and prevent the development of a significant mixed grain-transformed structure, which is the main cause of toughness deterioration in thick-walled materials.

There is already some knowledge of high-strength steel using TiN with the addition of a small amount of Ti, but it is used for the purpose of solving problems when manufacturing high-toughness steel with a thickness of 25 mm or more by controlled rolling. Conventionally, T
The effect of adding i can be effectively obtained under such conditions.

However, in order to ensure the amount of fine TiN precipitated, it is necessary to add at least 0.005% or more, and 0.030%.
If the above amount is added, TiC will precipitate and the toughness will deteriorate.

Since N is contained in steel from 0.003% to a maximum of 0.0150% depending on the normal steelmaking process, these were set as the upper and lower limits.

The amount of N necessary for forming TiN is sufficient as long as it is within the above N content range, and there is no need for a special nitriding treatment.

Note that the above basic composition according to the present invention may be 0.30 as necessary.
% or less Mo. 0. V below 15% and 0.50%
Even by adding Cu alone or in combination as described below, the effects of the present invention are not lost, and the strength of the steel can be further increased.

Specific examples and comparative examples of the present invention will be shown together, and the effects will be explained in more detail. The chemical composition of some of the steels of the present invention is shown.

That is, steel numbers 1 to 5 are conventional steels, and steel numbers 6 to 18 are target steels in the present invention.

However, along with the hot rolling conditions applied to the test steel as shown in Table 1, namely the slab heating temperature, the cumulative reduction rate below 900°C (non-recrystallization temperature range), and the final plate thickness, The results of the tensile test and impact test on the steel obtained under these rolling conditions are shown in Tables 2 to 4 below.

All of these test pieces were taken at the center of the plate thickness in the direction perpendicular to the rolling direction, and the tensile test pieces were made from a 9φ round bar with a gage length (GL) of 32 mm. The pieces are 2mm V notch full size (10
x 10 mm) Charpy test piece.

To further explain the comparative examples and examples mentioned above, Steel 1 in Table 1 is a conventional ferrite. It is pearlitic steel, and steel 2 is aci-cular fer
Steels 3, 4, and 5 are rite (acicular ferrite) steels in which the amount of Ni added is approximately 0.2%, 1%, and 2%, respectively.

However, for these steels 1 to 5, a 190 mm thick slab was heated to 1200°C and then continuously rolled down to 9.
Table 2 shows the results of applying a cumulative reduction rate of 70% to the final plate thickness at 00°C or lower and finishing at 790°C to obtain a steel plate of 20 to 32 mm.

In other words, the rolling conditions are the same in all cases, which corresponds to extremely severe cases of pedestal rolling, but in these conventional comparison steels, when the plate thickness is 25 mm or more, -100
It is understood that it is extremely difficult to obtain transition temperatures below 0C.

In steels 3, 4, and 5 with capped Ni added, the toughness clearly improves as the amount of Ni added increases, and when the plate thickness is 20 mm, the transition temperature is -100°C like steel plate 4C or 5C.
In the following cases, if the plate thickness is 25g or more, the steel plate 5B
The highest temperature is -92°C.

Figures 1 and 2 summarize these results, with Figure 1 showing the results for 0.2% Ni and Figure 2 showing the results for 2% Ni. In any case,
The toughness (vTrs) deteriorates significantly when the plate thickness is increased from 20 mm to 25 mm, and in the conventional technology shown by the solid line, even with 2% Ni, the transition temperature is around -90°C and below -100°C. The difficulty of obtaining something is very clear.

In contrast to these steels, steels 6, 7 and 8 subject to the present invention have compositions corresponding to the above-mentioned conventional steels 3, 4 and 5, respectively, and are rolled into final plates under the same hot rolling conditions as the conventional steels. Thickness 20~
Steel plates 6A to 6C, 7A, 7B, and 8A to 8C shown in Table 3 are finished to 36 mm, and they are also separately shown by dotted lines in Figures 1 and 2 above. It is clear that the toughness of low-carbon steel containing Ni is significantly improved by adding Ti, and it is also clear that fluctuations in toughness due to increase in plate thickness are significantly reduced.

Steels 9 and below are also steels subject to the present invention, and steel 9 has a Ni content of 1.5.
%, Steels 10 and 11 each contain around 1% Ni and Mo.
and V are respectively added, and steels below 12 are also cases where Ni and other components are varied within the scope of the present invention, and these are obtained under the conditions shown in Table 3 and below. It is rolled.

I would like to add a few comments regarding these results: Steel 7,
The results for Steels 7D, 9D, and 12D clearly show that the toughness of Steels 7D, 9D, and 12D can be further improved by heating the slabs at 1200°C and 1100°C in the latter case.

In addition, in the present invention, as mentioned above, it is necessary to make the cumulative reduction rate in the non-recrystallization temperature range at least 40%, and for example, the 40% line is secured as in steel plates 6F and 8F. As described above, the toughness targeted by the present invention can be secured, but even if the steel has a composition within the scope of the present invention, the steel plate 6H
If the predetermined cumulative reduction ratio is not reached, the target toughness cannot be obtained.

Figures 3 and 4 show the toughness, strength, toughness, and Ni of the 32 mm thick plate based on the above results.
The relationship between the quantities is shown below.VTr of -100℃ or less is achieved by controlled rolling in a large steel plate such as 32mm.
It is understood from FIG. 4 that the addition of Ti is essential to obtain a tough steel plate having a high s value, and the amount of Ni added must be 0.2% or more.

According to the present invention as explained above, steel having a specific composition containing Ni, Nb, and Ti within a specific range can be produced at a cumulative reduction rate of 40% or more with respect to the final plate thickness in a non-recrystallized temperature range. By rolling it at the same time, it is possible to accurately obtain excellent toughness with a vTrs value of -100°C or less while ensuring a strength level of 50 kg/mm2 or more in thick-walled materials with a finished plate thickness of 25 mm or more. This invention has great industrial effects.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention together with a comparative example using a conventional method.
A chart showing changes in toughness and strength in relation to plate thickness when Ni is added. Figure 2 is a chart showing changes similar to Figure 1 in the case of 2% Ni addition. Figure 3 is a graph showing changes in toughness and strength in relation to plate thickness.
Diagram showing the relationship between strength and toughness for items of m, No. 4
The figure is a chart showing the relationship between the amount of Ni and the toughness for a plate having a thickness of 32 mm.

Claims (1)

[Claims] 1 C: 0.02 ~ 0.10%, Mn: 0.6 0 ~
1.80%, Si: 0.10-0.50%, S: 0.0
10% or less, Ni: 0.2-4.0%, Nb: 0. O1
~0. 15%, sol. Al: 0. 006 ~0
．． 0 8%, Ti: 0.005-0.030%, N: 0
．． 003 to 0.015%, and 0.30% if necessary
% or less of Mo, 0.15% or less of V, and 0.50% or less of Cu, and the balance is iron and unavoidable impurities. A method for producing thick-walled, non-tempered high-strength steel with excellent toughness, which is rolled at a cumulative reduction rate of 40% or more relative to the plate thickness and has a finished plate thickness of 25 mm or more.