JPS5827327B2 - Manufacturing method of controlled rolled high strength steel without separation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing controlled-rolled high-strength steel that does not cause separation, and is intended to prevent separation, which causes deterioration of toughness in the thickness direction, deterioration of stress corrosion cracking resistance, unstable ductile fracture, etc. It is an object of the present invention to provide a method capable of appropriately manufacturing high-strength steel by controlled rolling that does not occur.

Pipelines, etc. that transport oil, natural gas, and other energy sources buried in cold regions or deep seas have gradually become larger in diameter in recent years, and are required to have high strength and high toughness. The main manufacturing method for obtaining the original plate for producing line pipes, etc. is ferrule rolling.

In other words, the controlled rolling method refines the structure by rolling in the γ range or γ+α range at relatively low temperatures, and simultaneously increases strength and improves toughness, with the latter being particularly remarkable. In particular, a product suitable for the above-mentioned uses can be obtained.

By the way, in order to refine the structure and improve toughness by cumulative reduction in a low temperature region according to such controlled rolling technology, it is necessary to apply rolling at a cumulative reduction rate of 30% or more in the non-recrystallized temperature region of austenite. At some point, as the cumulative reduction rate in the low-temperature region increases and the toughness is improved, a texture is generated, resulting in so-called separation.

The separation in the cap is unique to controlled rolled materials, and is used in Charpy tests and DWT on thick steel plates with high toughness.
T test (large brittle fracture test, Drop W
This is a crack that occurs parallel to the rolling surface as the test temperature decreases when testing (abbreviation for eipht tear Te5t), and tends to occur frequently at temperatures just above the point at which a brittle fracture surface begins to appear. The occurrence of such separation leads to deterioration of toughness in the thickness direction and deterioration of stress corrosion cracking resistance.

Figure 1 shows the test temperature of 1150°C using test steel A as described later.
The changes in toughness and separation when the cumulative reduction rate is changed in the austenite non-recrystallization temperature range after heating are shown, and the separation index TSma in this case is
x is the total length of separations of one length or more on the fracture surface divided by the fracture surface area in the standard 211t1rLv Schalpou test, which is 0, and such a separation index ■Sm8X is the rolling reduction ratio is 30 or more, and the toughness vTs is greatly improved and gradually increases.

That is, when the rolling reduction ratio is as low as 30 parts or less, there is almost no separation, but when the reduction ratio is 30 parts or more, the occurrence of separation becomes noticeable.

However, the causes of such separation may be band structures, mixed grain structures, textures, inclusions, etc., but the medium band structures are caused by micro-segregation, and the mixed grain structures are caused by deformed γ grain boundaries or deformation zones. This is due to the precipitation of pro-eutectoid ferrite from the sulfide silicate, and the texture and inclusions are related to the reduction in the low-temperature region.In particular, the reduction in the γ+α two-phase region produces a strong texture, and the sulfide silicate The degree of deformation of inclusions such as salt is also large.

When rolling is completed at a temperature above the transformation point, the texture is weak and the deformation of these inclusions is small.

Figure 2 shows saturated H2S for sample steel A as described above.
Sulfide stress corrosion cracking test (generally N
The results of the ACE test) are expressed as the separation index Ismax and the critical stress ratio σcrit/
It is shown in relation to ay (this σerit is the critical stress at which cracking does not occur in a stress corrosion cracking test (test time is approximately several weeks), and σy is the yield stress of the material), but if the separation index is 01 or more, Stress corrosion cracking characteristics (commonly referred to as SCC) are significantly thickened.

In this way, separation impairs stress corrosion cracking resistance and is a problem, especially in line pipes for sour gas, and it also causes deterioration of low-temperature toughness in the plate thickness direction and double alignment in unstable ductile fracture. It also becomes a factor such as deterioration of propagation stopping characteristics.

Therefore, there is a need to obtain non-tempered, high-strength steel that does not cause separation as described above in controlled-rolled materials, but no technology has hitherto been able to provide a preferable solution to this problem. Not yet.

The present invention was created after repeated studies in view of the above-mentioned circumstances.

That is, as mentioned above, the causes of separation in controlled rolled materials include the pearlite band structure, mixed grain structure, and inclusions. I learned that it can be improved by subsequent accelerated cooling.

Accelerated cooling after rolling with a cap increases the driving force for transformation, and ferrite nucleates uniformly both at the grain boundaries and within the grains of austenite, so the band structure disappears and grain size is regularized. Pearlite is in a finely dispersed state.

This accelerated cooling is preferably performed while the steel material is being fed in order to shorten the time required for cooling.

Regarding the texture and inclusions, by terminating the rolling at or above the transformation point, it is possible to prevent a significant increase in the texture and large deformation of the inclusions due to rolling in the γ+α two-phase region.

Therefore, it has been confirmed that in order to prevent the occurrence of separation in controlled rolled materials, it is essential to finish rolling at a temperature above the transformation point and to perform accelerated cooling. Index■
It is possible to obtain high tensile strength steel with a non-tempered maX of ≦0.06.

The reason for limiting the chemical components in the present invention is as follows.

C is required to be 0.03% or more to ensure strength, and if it is 0.18% or more, weldability deteriorates.

Further, as a deoxidizing element, Si is required to have a coefficient of 0.05 or more, but if it exceeds 0.60%, weldability deteriorates, as in the case of C, and this is the upper limit.

Furthermore, Mn is an effective element for increasing the tensile strength, and it is necessary to have an amount of 60% or more.

But that's 2. When it exceeds oo%, weldability deteriorates.

For optional elements such as Nb, V, Cu, Cr, and Mo, 0.15 for Nb in consideration of high tensile strength, toughness, and weldability.
% or less, ■ is 0.15% or less, Cu, Cr is 0.70%
Hereinafter, 0.30% or less of MO may be contained singly or in combination as necessary.

Ni is an effective element in terms of both strength and toughness, but from economical considerations it is appropriate to set the upper limit to 2.

Specific embodiments of the present invention will be described below.

The composition of the test steel specifically adopted by the inventors is shown in Table 1 below.

Examples carried out by the present inventors using the above-mentioned test steel are as follows.

In these Examples, the austenite non-recrystallized region was set at 900° C. or lower for the Nb-added steel due to the relationship with the Nb content, and was set at 840° C. or lower for the 51-Mn steel (H steel).

Example 1 After heating to 1150°C using test steel A shown in Table 1,
60% controlled rolling at 900℃ or less, finished thickness 20m
The separation index Φ relationship according to finishing temperature is as shown in FIG. 3 for the air-cooled material with m and the accelerated cooling at 9° C./sec between 7808° C. and 600° C. after rolling.

In other words, looking only at the air-cooled material, the separation index significantly decreases from 04 to 0.2 when rolling is finished above the transformation point, and this is because the deformation of inclusions is reduced by finishing rolling above the transformation point. This is because the influence of the texture is also reduced.

However, since a mixed flow state with a band structure still remains, the separation index is about 0.1 to 0.2.

Comparing air-cooled materials and accelerated cooling materials, those that are accelerated and cooled after finishing rolling at or above the transformation point have almost no separation, but those that are accelerated and cooled after finishing rolling at or below the transformation point have 0. A separation of about 2 degrees occurs.

In other words, by performing accelerated cooling, the driving force for transformation is increased, ferrite is nucleated both at the grain boundaries and inside the austenite grains, and separation is reduced by creating a structure that does not produce mixed grains or have a band structure. be understood.

Among the items shown in this Figure 3, the results of measuring the mechanical properties of the main ones marked as perfume 1, 2.3 in the index in Figure 3 are shown in Table 2 below. As shown.

Air-cooled material (A-C) rolled below the transformation point has a strength of 5 kg.
/rIL17 is higher than that, but the separation index is as high as 0.39, as shown in perfume 1, and the product that was acceleratedly cooled after rolling at a temperature above the transformation point (fragrance 3) has a high separation index of 0.39. It has almost the same strength as the one finished rolling below the point, but there is almost no separation.

Furthermore, it is clear from the comparison between steel plates 3 and 1 that the toughness is better than that of steel plates finished rolling below this transformation point.

Therefore, it is clear that performing accelerated cooling after rolling at a temperature above the transformation point improves strength and toughness, and does not cause separation.

Example 2 Using the above-mentioned test steel H, heated at 1150°C and then heated to 840°C.
The air-cooled material was subjected to 60% controlled rolling at a temperature below C to a finished thickness of 25 mm, and after rolling it was rolled at 5°C/s from 750°C to 600°C.
For those acceleratedly cooled by EC, the finishing temperature is 76
The results of testing the mechanical properties at temperatures ranging from 0°C to 710°C are shown in Table 3 below.

That is, no separation occurs in the specimens which were acceleratedly cooled at 5° C./sec after completion of rolling at a temperature above the transformation point.

In other words, because the accelerated cooling rate was low at 5°C/sec, there was little bainite formation, and the increase in strength was low at 1.5 to 2.0 kg/ma compared to air-cooled materials under the same conditions, but the pearlite band structure disappeared and the grain size improved. Therefore, the separation index is zero.

It can be seen that 51-Mn steel has the same tendency as Nb steel with regard to lid separation.

Example 3 Heating at 1100'C using test steels B and C at 1900°C
Controlled rolling of 50 steps is performed below, and 790'
When the cooling rate was changed after rolling was completed at C, the relationship between the separation index and the cooling rate is as shown in FIG.

The finished thickness in this case is 16 mm, and the cooling rate is 78 mm.
Average cooling rate between 0 and 600°C.

In other words, in both test steels B and C, separation can be significantly reduced by accelerated cooling at 3°C/sec or more, and this is because the pearlite band structure is reduced by accelerated cooling at 3°C/sec or more. This is because it disappeared and was rectified.

The cooling stop temperature is preferably 650°C or less from the viewpoint of removing the bunt structure by accelerated cooling and increasing the tensile strength.
If the temperature is below 00°C, the uniformity of the structure and hardness in the thickness direction will be impaired.

That is, immediately after the rolling is completed, the cooling rate is 6° C./sec or more.
It is clear that accelerated cooling to a temperature range of 50 to 500°C is necessary.

Example 4 Test steels A-G were heated to 1150°C, then subjected to 60% controlled rolling at 900°C or lower, and rolled at 7800°C to 60°C under the same conditions as those in which the rolling was completed at 790°C and then air cooled.
The relationship between the separation index for samples cooled between 0° C. and 8° C. is as shown in FIG.

In other words, the air cooling material is 0, which is 01 to 02, while the accelerated cooling material is O.
-0.02, and almost no separation occurred.

In addition, regarding the sample steel E in this case, the structure of the air-cooled material is as shown in the micrograph in Figure 7, and the structure of the accelerated cooling material is as shown in the micrograph in Figure 8. It is clear that the band structure has completely disappeared.

Figure 6 shows all the above results as a diagram.
Papers that are air-cooled after finishing rolling at or below the transformation point have a separation index of 0.25 to 045, and papers that are air-cooled after rolling at or above the transformation point have a separation index of 0.1 to 0.25. The separation index of those subjected to accelerated cooling after completion of rolling is extremely low at 0 to 0.05.

According to the present invention as described above, it is possible to effectively avoid the occurrence of separation that causes deterioration of toughness in the thickness direction, deterioration of stress corrosion cracking resistance, unstable ductile fracture, etc. in controlled rolled high strength steel. It is possible to precisely manufacture steel materials that have high strength and high toughness, and does not have the disadvantages mentioned above, and provides a product that is suitable for use in energy source transportation line pipes and other applications in cold regions and deep seas. This invention is industrially very effective.

[BRIEF DESCRIPTION OF THE DRAWINGS] The drawings show the technical relationship between the present invention and the comparative example, and FIG. 1 is a chart showing the relationship between cumulative rolling reduction, toughness, and separation index at temperatures below 900°C;
Figure 2 is a chart showing the relationship between separation index and critical stress ratio, Figure 3 is a chart showing the relationship between finishing temperature and separation index in the case of Example 1, and Figure 4 is a chart showing the relationship between the separation index and the separation index in the case of Example 3. Figure 5 is a diagram showing the relationship between speed and separation index. Figure 5 is a diagram showing the relationship between separation index and air cooling material and accelerated cooling material for Example 4. Figure 6 is a diagram showing the relationship between manufacturing conditions and separation index for the results of each example. A diagram summarizing the relationship with the index, FIG. 7 is a microscopic photograph showing the structure of the air-cooled material in an example of the present invention, and FIG. 8 is a microscopic photograph showing the structure of the accelerated cooling material.

Claims (1)

[Claims] IC: 0.03-0.18, Si: 0.05-0
．． 60%, Mn: 060 to 2.0 ratio as the basic composition, Nb: 0.15 ratio or less, V: 0.15φ as necessary
Below, Cu: 0.7% or less, (J: 0.7% or less, Mo
: 0.3% or less, Ni: 2.0% or less, A
l: A steel containing one or more types of inclination of 0.1fO or less, with the remainder being iron and unavoidable impurities, is heated and rolled, and the cumulative reduction rate in the non-recrystallized temperature range is 30 factors or more. Rolling is performed at finishing temperatures Ar and PJ, and after this rolling is completed, accelerated cooling is performed at a cooling rate of 3°C/sec or higher to 500 to 65°C.
A method for producing controlled rolling high tensile strength steel without separation, characterized by stopping within a temperature range of 0°C.