JPH02217418A - Production of non-heattreated high tensile steel sheet excellent in dwtt characteristic - Google Patents

Abstract

PURPOSE:To mix moderate coarse ferrite grains into fine ferrite and to improve the DWTT characteristic of a steel sheet by heating a slab with a specific composition twice at proper temps., applying proper rolling reductions to the above slab in a recrystallization gamma-region and a nonrecrystallization gamma-region, respectively, and also specifying the initial temp. of cooling. CONSTITUTION:A slab having a composition consisting of, by weight, 0.005-0.15% C, 0.05-1.0% Si, 0.6-2.5% Mn, 0.005-0.08% Al, 0.005-0.1% Nb, and the balance iron with inevitable impurities is prepared. This slab is heated up to 1200-1300 deg.C, cooled, and reheated up to 1050-1250 deg.C, and then, this slab is subjected to rolling reduction at >=40% in a recrystallization gamma-region of >=(Ar3+150 deg.C) and successively to rolling reduction at 65-90% draft in a nonrecrystallization gamma-region between <(Ar3+150 deg.C) and Ar3, followed by air cooling. Subsequently, accelerated cooling is carried out from a temp. in the range of (Ar3-20 deg.C) to (Ar3-70 deg.C) down to 650-400 deg.C at 3-30 deg.C/sec cooling rate.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is a non-woven fabric with excellent DWTT characteristics used in thick-walled UOE steel pipes used in pipelines for transporting crude oil, natural gas, etc. 1! This relates to high-quality high-tensile steel plates.

<Conventional technology> Recently, pipelines for transporting crude oil, natural gas, etc. are being operated at high pressure to improve transport efficiency, and steel plates for UOE steel pipes with high strength (and thick plate thickness) are required. In order to increase the safety of these pipelines against brittle fracture, it is necessary to improve not only the characteristics of brittle fracture occurrence but also the ability to stop the brittle cracks that have occurred.The former is based on the Charpy impact test. The latter is evaluated using the fracture surface transition temperature and CTOD test, whereas the latter is evaluated using the DWTT
) is evaluated by the fracture surface transition temperature.

In order to lower the fracture surface transition temperature (85%5ATT) of conventional DWTT and improve the propagation arresting characteristics of brittle cracks, Charpy impact! Based on the idea that there is a correlation between the transition temperature of the Jl test and the fracture surface transition temperature of DWTT, it is important to achieve grain refinement, and therefore grain refinement techniques such as controlled rolling have been developed. This is well known.

However, if the plate thickness exceeds 20mm,
Although the transition temperature of Charpy shifts to the lower temperature side when grain refinement is performed, the transition temperature of DWTT does not necessarily shift to the lower temperature side, and therefore, there have often been cases where required characteristics cannot be satisfied.

In addition, by carrying out rolling in the (α+γ) two-phase region, a (100) texture is developed in the steel sheet, which reduces the cleavage strength in the Z direction and causes separation on the fracture surface of the DWTT piece. A method of improving the toughness in the T direction by relaxing the triaxial stress at the crack tip is also often used.

However, this method places an excessive load on the rolling mill and
Moreover, it is not preferable because it reduces rolling efficiency.

<Problems to be Solved by the Invention> An object of the present invention is to provide a manufacturing method that improves the DWTT characteristics of such thick-walled materials.

Means for Solving the Problems> The present invention provides C: 0.005 to 0.15% by weight.
.

Si: 0.05-1.0%, Mn: 0.6
~2.5%, AZ: 0.005~0.08%,
Contains Nb: o, oos ~ 0.1%, and if necessary V! 0.01-0.10%, Cu: 1.
Less than 0%.

Ni: 1.0% or less, Cr: 0.5% or less, Mo
: 0.5% or less, Ti: 0.005-0.1
%,B! 0.0003-0.0020%.

Ca: 0.001-0.010%, REM:
A slab containing 0.001 to 0.010% of one or more of them, with the remainder consisting of Fe and unavoidable impurities, is heated to a temperature range of 1200 to 1300°C, then cooled, and then 1050 to 250 40% in the recrystallization γ region after reheating to a temperature range of ℃ (Ars + 150 ℃) or higher
Apply a pressure of more than
) A pressure reduction of 65 to 90% is applied in the unrecrystallized γ range of 1 or more, followed by air cooling, and then (Δr3 20°C) to (A
Production of non-Pl quality high tensile strength steel sheet with excellent DWTT characteristics characterized by accelerated cooling from a temperature range of rs-70℃ to a temperature range of 650 to 400℃ at a cooling rate of 3 to 30"C/s It's a method.

<Function> The present inventors investigated the microstructure for improving the DWTT characteristics of thick-walled materials, and found that the DWTT characteristics were improved by incorporating somewhat coarse borigonal ferrite, rather than by simply making the ferrite crystal grains finer. I found that it can be improved.

That is, by including 5 to 40% of 10 to 25-borigonal ferrite in the structure, the DWTT characteristics are improved, while the Charby characteristics are not deteriorated.

The present invention relates to a manufacturing method for industrially realizing the above structure.

First, the experiments that formed the basis of the present invention will be explained.

0.06%C-0, 20%5i-1, 15%Mn-0
,03%Nb-0,05%V-0.2%Cu-0.2%
Using Nj steel, first set the first heating temperature to 1250.
After heating the steel at a temperature of 1150° C. in the subsequent second heating, (Ar.

Applying a pressure reduction of 0 to 90% in the recrystallized γ range above (+150°C), the unrecrystallized γ of (Ars +150°C) ~^r,
Apply a pressure reduction of 70% at 540°C with air cooling.
) to 500° C. at a cooling rate of IO° C./S. Changes in Charpy characteristics and DWTT characteristics are shown in FIG. 1.

In the figure, a steel with the same composition is used as a comparative steel, all conditions after the second heating are the same, and changes in Charpy properties and DWTT properties when the first heating is not performed are also shown. The finished thickness at this time was 25 m.

The results shown in this figure show that in the case of the comparative steel, even if the rolling reduction in the recrystallization γ region is increased to 40% or more, the Charpy properties and DW
It can be seen that while the TT properties do not improve significantly, in the case of the steel of the present invention, when the rolling reduction in the recrystallization γ region becomes 40% or more, the Charpy properties do not change, but the DWTT properties significantly improve.

In the case of the steel of the present invention, each time the first heating step is carried out,
The γ grains during the second heating are in a moderate mixed grain state. This mixed particle has a crystallization rate of 40% in the recrystallization γ range above (Ar = + 150°C).
Apply a pressure of more than (Ars + 150℃) and
After applying a reduction of 65% or more in the unrecrystallized γ range of Ar3 or more, air cooling to below (Ars-20°C) and then accelerated cooling tJI is performed to obtain particles with an average grain size of 6- or less before accelerated cooling. Coarse ferrite grains of 10 to 25 tns with little strain are produced.

If such coarse ferrite with low distortion exists,
It became clear that the DWTT characteristics were significantly improved.

The reason for this is thought to be that the plastic region that forms at the tip of a brittle crack that progresses spreads more easily due to the presence of the less strained borigonal ferrite, which relieves the stress at the tip of the crack, but the details are not clear.

Incidentally, conventionally, the cooling start temperature after rolling is measured, and examples of setting it below are, for example, JP-A No. 52-123921 and JP-A No. 5
No. 8-77529, but none of them discloses that the DWTT characteristics are improved by appropriately mixing ferrite grains by heating twice as in the present invention.

Next, the reasons for limiting the composition of the materials used in the present invention will be explained.

C: If C is less than o or oos%, the strength of the steel plate will be insufficient and the weld heat affected zone (hereinafter referred to as HAz) will soften, while if it exceeds 0.15%, the toughness of the base metal will deteriorate. At the same time, in addition to hardening of the welded area, the cracking resistance deteriorates significantly.
C needs to be within the range of o,oos to 0.15%.

Si: Si is an element that is inevitably included for deoxidation during steel refining, but if it is less than 0.05%, the toughness of the base material will be insufficient.
If it exceeds 0%, the cleanliness of the steel will deteriorate and cause a decrease in the VI property, so Si needs to be within the range of 0.05 to 1.0%.

Hn: If Mn is less than 0.06%, the strength and toughness of the steel plate will be insufficient, and the softening of IIAZ will become severe;
If it exceeds Mn%, the toughness of 11AZ will deteriorate.
must be within the range of 0.06 to 2.5%.

8l = It is necessary to contain at least o, oos% of Al as a solid solution in order to deoxidize the steel.On the other hand, if it exceeds 0.08%, not only the UJ property of 11 A Z but also the toughness of the weld metal It also deteriorates significantly, so 8! must be within the range of 0.005 to 0.08%.

Nb: Nb is effective in making ferrite grains finer, but 0.005
If it is less than 0.1%, the effect will not be expressed, while if it exceeds 0.1%, it will diffuse into the weld metal during welding and reduce the toughness of the weld metal, so Nb should be in the range of 0.005 to 0.10%. limited to.

In the above component composition, the desired effect by the method of the present invention is achieved, but even if other components listed below are included for the purpose of their addition, the achievement of the effect by the present invention may be hindered. There isn't.

Ni: Formerly known as I (not having a negative effect on the hardenability and toughness of AZ), it is useful for improving the strength and toughness of the base material, but it is important to include it in excess of 1.0% during manufacturing. Since this will lead to an increase in cost, it should be set at 1.0% or less.

Cu: Cu not only has almost the same effect as N1 described below, but also
It also contributes to improving corrosion resistance, but if it exceeds 1.0%, cracks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates, so it is necessary to keep it below 1.0%.

Mo: Mo is useful for improving strength and toughness because it makes 7 grains uniform during rolling and also produces fine bainite, but it does not need to exceed 0.5%, and on the contrary, it increases manufacturing costs. Mo content is limited to 0.5% or less since it causes disadvantages.

■= ■In order to improve the strength and toughness of the base material of the steel plate and ensure the strength of the joint, it is possible to contain 0.01% or more, but 0.10% or more is allowed.
If it exceeds %, the toughness of the base material and HAZ will be significantly deteriorated, so ▪ is limited to 0.10% or less.

C: Cr can be included to ensure the strength of the base metal of the steel plate and the strength of the joint, but if it exceeds 0.5%, it will adversely affect not only the toughness of the base metal but also the toughness of the weld, so it should be 0.5% or less. It is necessary to

Ti: Ti is added for the purpose of improving toughness and increasing the strength of Ti carbonitride through the effect of making each grain finer. However, if Ti1l is less than 0.005%, there is no effect, and if Ti1l is less than 0.10%,
%, the toughness deteriorates, so the range of Ti and l is o,
oos to 0.10%.

B is added for the purpose of improving hardenability and increasing strength by increasing the bainite volume fraction. However, BJJ is 0.
If it is less than 0.003%, there is no strength increasing effect, and if it is less than 0.03%, there is no strength increasing effect.
If it exceeds 0.020%, martensite will occur and the toughness will deteriorate, so the Blt range is 0.0003 to 0.0020%.
shall be.

Ca: Ca has an effect on controlling the morphology of MnS in a trace amount of about 0.001%, and is effective in improving the toughness of the steel plate in the direction perpendicular to rolling, but when it exceeds 0.010%, the cleanliness of the steel deteriorates. 0.001 to 0.01 as this may cause internal defects.
It was limited to a range of 0%.

REM: REM (La, Ce, Pr, Nd, TI,
Sm+Eu, Gd+Tb+Dy, llo, O
Rare earth elements (r, Tu, Yb, I, u, Sc, Yt) are also present in MnS in trace amounts of about 0.003%.
It is effective in controlling the shape of the steel plate and improving the toughness in the direction perpendicular to rolling, but if it exceeds 0.010%, the cleanliness of the steel will deteriorate and it will also be disadvantageous in terms of arc welding. It was limited to a range of .001 to 0.010%.

Next, the reason for limiting the heating-rolling-cooling conditions, which is the second component of the present invention, will be explained.

First, the heating, cooling conditions, etc. in the first step of the present invention will be explained. The first step of heating the steel to a temperature range of 1,200 to 1,300°C and then cooling it is the most important step of the present invention, and by including this step, the mixed grains of the austenitic level during reheating in the subsequent second step are formed. This is because the precipitated coarse Nb (C, N) is completely dissolved in solid solution during heating, then finely precipitated during cooling, and then the Nb is solidified during heating in the second step. The purpose is to facilitate melting. However, if the heating temperature is less than 1200°C, coarse precipitated Nb
Since it is not completely dissolved and remains, it is necessary to heat it to a temperature of 1200° C. or higher by 7JI+. Furthermore, if the austenite grain size is heated to a temperature exceeding 1300° C., the austenite grain size becomes significantly coarsened, so even if the subsequent second step is carried out, the crystal grains cannot be refined. Therefore, the heating temperature in the first step is 120
The temperature range is 0 to 1300°C.

Further, regarding the cooling conditions, the rate is not particularly limited, but it is preferably air cooling that is normally performed or a faster cooling rate, and the cooling stop temperature is preferably Ar or less.

Next, the conditions for heating, rolling, cooling, etc. in the second step will be explained.

In the heating in the second step, the austenite grains are mixed and the 0.
It is a necessary condition that 0.1% or more of Nb is dissolved in solid solution.

If the heating temperature is lower than 1050° C., austenite grains will be mixed, but the solid solution amount of Nb will be less than 0.01%, so high strength cannot be achieved. Furthermore, when heated above 1250° C., the austenite grains become significantly coarsened, and even if subsequent rolling is performed in the recrystallized γ region and the non-recrystallized γ region, the austenite grains are insufficiently refined and the toughness deteriorates. Therefore, the heating temperature is in the range of 1050 to 1250°C.

” The steel heated under the conditions described in (Ars + 150
It is necessary to perform a reduction of 40% or more in the recrystallization γ range of 40% or more. Coarse austenite grains are refined by repeated rolling and recrystallization, but the rolling temperature is (Ars + 150°C).
), recrystallization does not occur below (Ars+150
℃) or higher. Further, if the rolling reduction is less than 40%, the austenite grains will not be sufficiently refined and the toughness will not improve, so the rolling reduction must be 40% or more.

Subsequently, it is necessary to perform a reduction of 65 to 90% in the unrecrystallized γ range of less than (Ar3 + 150°C) and more than Ars.

If rolling within this range is inappropriate, coarse baini] and martensite will be mixed into the &Il fiber after accelerated cooling, resulting in a significant deterioration of toughness. If the rolling reduction in this temperature range is less than 50%, the above-mentioned coarse bainite and martensite will be mixed in and the toughness will deteriorate. Moreover, when the reduction ratio exceeds 90%, the amount of borigonal ferrite increases and strength decreases.

Subsequently, air cooling is performed to a temperature range of (Ars 20°C) to (Ars 70°C), which is the main purpose of the present invention.
The end temperature of air cooling, which is performed to generate coarse polygonal ferrite to improve TT characteristics, is (Ars2
0℃), the coarse polygonal ferrite grain size does not exceed IOμ, and its volume fraction does not exceed 5%. On the other hand, when the end temperature of air cooling is lower than (Ars 70℃), the coarse polygonal ferrite grains do not exceed IOμ. The diameter becomes 25p1 or more, and the volume fraction becomes 40% or more, both of which deteriorate the DWTT characteristics. Therefore, the temperature range of air cooling is (A
r.

-20°C) to (Ar20°C).

Accelerated cooling is performed after air cooling to the above range, but the cooling rate is 3℃
/S, there is no effect of accelerated cooling; on the other hand, 30℃/
If the temperature exceeds S, a hardened structure will be formed and a tempering step will be required, so the cooling rate was limited to a range of 3 to 30° C./s. The accelerated cooling continues from 650°C to 400°C, but if cooling is stopped at a temperature exceeding 650°C, the effect of accelerated cooling will not occur, and if cooling is stopped below 400°C, the M plate thickness Since distortion is likely to occur, the stopping temperature of accelerated cooling is set in the range of 650 to 400°C.

<Example> Regarding the test steel whose composition is shown in Table 1, changes in the mechanical properties and ferrite structure of the steel plate were investigated under the heating-rolling-cooling conditions shown in Table 2, and the results were They are summarized in Table 2.

In Table 2, the test Nal~10 was obtained by applying various heating-rolling-cooling conditions to slabs of A steel in Table 1 having a composition within the range defined by the present invention, and in each case, the plate thickness was 22 m + a.
It is a product of First, test NαI did not undergo the first heat treatment, and test Nα2 did not undergo the first heat treatment.
Since the second heating temperature was as low as 1050°C (less than 1200°C), the DWTT characteristics were poor.

In test Nα3, the first heating temperature was 1350”C (13
Charpy characteristics and DWTT
The characteristics are poor. In test Nα4, the second slab heating temperature is as low as 1000°C (less than 1050°C), so the TS is low. In test Nα5.6, the rolling reduction rate in the recrystallization γ region is 20% (4
The Charpy and DWTT properties are poor because the rolling reduction in the non-recrystallized γ region is as low as 20% (less than 65%). Test Nα7 has a cooling start temperature of 8
00℃ (over (Ar5-20℃)), DWTT
Test Nα8, which has poor characteristics, has a cooling start temperature of 660°C ((
(Art below 70℃), the TS is low and the DWT
In contrast, test Nct9 and 10 were manufactured according to the constituent requirements of this invention, so they had high strength, Charpy properties (νTrs of 1100°C or less), and DWT
It can be seen that it has a T characteristic (85% FA T T is -20°C or less).

Next, tests kll to I4 were conducted on slabs B to Em having the component compositions according to the present invention, and plate 1! which was manufactured satisfying all the constituent requirements of the present invention! The mechanical properties of the steel plates of J20-30 heels and changes in the micro All weave are shown.

It can be seen that all of these steels have high strength and excellent DWTT characteristics.

<Effects of the Invention> As described above, according to the present invention, the slab is heated to an appropriate temperature for 2
Heating to a certain degree, applying appropriate pressure in the γ region of recrystallized and non-recrystallized regions, and measuring the temperature at which cooling begins.

By setting it as follows, it is possible to mix appropriate coarse ferrite grains into fine ferrite and improve the DWTT characteristics.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the reduction rate and the fracture surface transition temperature in the recrystallization γ region.

Claims (1)

[Claims] 1. Weight ratio: C: 0.005 to 0.15%, Si:
0.05-1.0%, Mn: 0.6-2.5%, Al:
0.005-0.08%, Nb: 0.005-0.1%
A slab containing iron with the balance consisting of Fe and unavoidable impurities is heated to a temperature range of 1200 to 1300 °C, cooled, and then reheated to a temperature range of 1050 to 1250 °C (Ar
A reduction of 40% or more is applied in the recrystallization γ range above (Ar_3 + 150°C), and then A less than (Ar_3 + 150°C)
A pressure reduction of 65 to 90% is applied in the unrecrystallized γ region of r_3 or more, followed by air cooling, and then (Ar_3-20°C) to (Ar
A method for producing a non-tempered high tensile strength steel sheet with excellent DWTT properties, characterized by accelerated cooling from a temperature range of _3-70°C to a temperature range of 650-400°C at a cooling rate of 3-30°C/s. 2. Weight ratio: C: 0.005-0.15%, Si:
0.05-1.0%, Mn: 0.6-2.5%, Al:
0.005-0.08%, Nb: 0.005-0.1%
Further, V: 0.01 to 0.10%, Cu: 1.
0% or less, Ni: 1.0% or less, Cr: 0.5% or less,
Mo: 0.5% or less, Ti: 0.005 to 0.1%, B
:0.0003~0.0020%, Ca:0.001~
A slab containing one or more of 0.010% and REM: 0.001 to 0.010%, with the balance consisting of Fe and unavoidable impurities is heated to a temperature range of 1200 to 1300°C and then cooled. Then, after reheating to a temperature range of 1050 to 1250°C, a pressure reduction of 40% or more is applied in the recrystallization γ region above (Ar_3 + 150°C), and then (A
A reduction of 65 to 90% is applied in the unrecrystallized γ region of Ar_3 or higher (less than r_3 + 150 °C), then air-cooled, and then (A
Non-thermal treatment with excellent DWTT characteristics characterized by accelerated cooling from the temperature range of r_3-20℃) to (Ar_3-70℃) to the temperature range of 650-400℃ at a cooling rate of 3-30℃/s A method for manufacturing high-strength steel plates.