JPS58100625A - Production of high toughness high tensile steel plate having excellent weldability - Google Patents

Abstract

PURPOSE:To obtain a high tensile steel plate having excellent toughness and weldability by heating a steel slab of specific compsns. to form a solid solution of Nb in the compsn. and rolling the same under control of temp. and draft at specific values. CONSTITUTION:The steel slab consisting basically of 0.04-0.10% C, 0.05-0.30% Si, 1.25-2.50% Mn, 0.010-0.100% Nb, 0.0005-0.0030% B, <=0.0080% N, 0.005- 0.040% Ti, 0.005-0.050% Al, >=0.10 Cu, >=0.10% Ni (0.20-1.00% Cu+Ni) is rolled at >=(Ar3 point+150 deg.C) and >=50% cumulative draft. In succession, the slab is rolled at <=(Ar3 point+150 deg.C), and in an unrecrystallization austenite region higher than the Ar3 point at >=50%. Further the slab is rolled in a temp. region of two phases according to need and is then air-cooled. Thus the structure consisting essentially of ferrite and fine bainite as well as island-like martensite is formed in the hot-rolled steel plate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-toughness, high-tensile steel plate, and in particular, the present invention relates to a method for manufacturing a high-toughness, high-strength steel plate, and in particular, the present invention relates to a method for manufacturing a high-toughness, high-strength steel plate.
The present invention relates to a method for manufacturing a second class non-temperature steel plate for low temperature use.

The steel sheet produced according to the present invention can be used as a steel sheet for high-grade, large-diameter line pipes mainly used for natural gas transportation in cold regions, or as a steel sheet that can replace conventional QT heat-treated steel sheets.

As demand for energy has increased in recent years, there has been a desire to transport large quantities of natural gas, and the operating pressure of line pipes has been increasing from the conventional 75 atm to 100 and 120 atm. Along with this, the materials used are required to have higher tensile strength and thicker walls.

At the same time, it is desired that these line pipes have improved weldability in terms of the efficiency of circumferential welding on site, and lower carbon equivalents are beginning to be required. For example, the strength is 60kgf/mm
Grade 2: 0.35% or less, and strength approximately 70kgf/mm2
There is a need for a steel plate for line pipes that requires a very low carbon equivalent of 0.42% or less in grade and has excellent low-temperature toughness.

In control-rolled steel sheets, it is easy to obtain good values for the fracture surface transition temperature of low-temperature toughness due to the effect of separation, but on the other hand, it has the disadvantage that impact absorption energy is low. It also has a strength of 60, 70,
The higher the tension is, 80 kgf/mm2, the more disadvantageous it becomes. Unlike steel plates for petroleum line pipes, which are the object of the present invention, steel plates for gas line pipes are generally required to have high impact absorption energy from the viewpoint of preventing unstable ductile fracture. In addition, the conventional QT heat-treated steel, which is another steel subject to the present invention, is also characterized by high impact absorption energy, so if the purpose is to replace QT steel, it is recommended to use the conventional QT heat treatment steel as it is rolled. We must devise ways to increase this.

Against this background, the present inventors developed a ferrite steel instead of the generally well-known ferrite-pearlite steel, in order to make a non-tempered steel with high tensile strength and high toughness with a low carbon equivalent. - Paynite - We have been conducting research focusing on island-like martensitic steel. As a result, it was found that a ferrite structure having fine grains of about 6 μm or less of bainite and island-shaped martinsite as a second phase structure is excellent in increasing tensile strength and toughness. As a manufacturing method, it is effective to carry out a special controlled rolling process using Nb-containing steel to which B element is added that increases hardenability without increasing the carbon equivalent. The purpose is to generate island-like martensite, but in order to make the effect of B noticeable, it is important to bring B into a solid solution state, and to do so, it is necessary to prevent N from becoming BN precipitates. It was found that it is important to fix T1 to the N precipitate of Al. However, even under such conditions, the strength of component B does not increase proportionally when the amount added is increased;
In the range of ~30 ppm, it is only effective in increasing the strength by 7~10 kgf/mm2. On the other hand, although it was mentioned above that controlled rolling treatment is disadvantageous in obtaining high impact absorption energy, it has been found that high impact absorption energy can be obtained by lowering the C and S contents in the steel. However, 60kgf/mm2 thick steel and 70kgf/mm2
It has become clear that when targeting high-strength steel, the low C-Nb-B component system alone has excellent toughness but insufficient strength.

Therefore, as a result of examining alloy components that increase the strength of low C-Nb-B steel, as shown in the graph of the figure, Mm, C
Addition of r, Mo increases TS but degrades vTrs,
On the other hand, if the sum of Cu and Ni is added at 0.20% or more, T
We have newly discovered that it brings about an increase in S and an improvement in vTrs at the same time. The main reason for this is thought to be that the addition of Cu and Ni simultaneously makes the bainite grains finer and the lath packets within the grains finer.

In the present invention, based on the above findings, in order to improve the impact absorption energy, the C and S contents of the steel material are lowered, and in order to increase the fine bainite grains and island martensite in the ferrite structure, along with a predetermined content of Nb, B, Cu, and Ni are added simultaneously in predetermined amounts. This point is a feature of the composition system of the steel material in the present invention. That is, the composition of the slab in the present invention is as follows.

C: 0.04-0.10%, Si: 0.05-0.03
%, Mn: 1.20-2.50%, Nb: 0.010-
0.100%, B: 0.0005-0.0030%, N
:≦0.0080%, Ti:0.005-0.040%
, Al: 0.005-0.050% and Cu: ≧0.10
% and Ni: ≧0.10% (however, Cu+Ni: 0.2
0 to 1.00%), and if necessary further Mo:≦0.
50%, V:≦0.10%, Cr:≦0.50%, Ca
:0.002~0.010%, REM:0.002~0
．． 0.010%, and the remainder contains unavoidable impurities (of which S is 0.0%).
08% or less. ) and Fe.

In the rolling method, the steel slab is made with at least 0.01% Nb in order to refine the ferrite grains and bainite grains.
After heating to a temperature at which solid solution occurs, rolling is performed at a temperature exceeding Ar3 point +150°C so that the cumulative reduction ratio is at least 50%, and then rolling is performed at a temperature exceeding Ar3 point +150°C and at Ar3 point +150°C or less.
Rolling is carried out so that the cumulative reduction rate is at least 50% within the temperature range of unrecrystallized austenite at or above the Ar point, and if necessary, then the two-phase region of austenite and ferrite at an Ar point of less than 3 points and Ar3-80°C or higher. A feature of the rolling method of the present invention is that rolling is carried out so that the cumulative reduction ratio is at least 10% within the temperature range of , and then air cooling is performed.

And the present invention provides a carbon equivalent of 60 kg with a carbon equivalent of 0.35% or less.
f/mm2 or more, and has a strength of 70kgf/mm2 or more with a carbon equivalent of 0.42% or less, and -90°C
The object is to provide a method for producing a high tensile strength steel plate having the following vTrs and vE-25 of 15 kgf·m or more.

The present invention will be explained in detail below.

As constituent elements of the present invention, the reasons for limiting the conditions for rolling and heat treatment of the slab as described above will be explained.

The purpose of the present invention is to produce a steel having fine ferrite as the main structure and containing fine bainite grains and island martensite as the second phase structure, that is, a ferrite structure in which the volume fraction of the second phase structure is less than about 40%. Therefore, it is necessary to contain Nb to generate these fine grains, and Nb
The steel slab must first be heated so that 0.01% or more of b is dissolved in solid solution. The reason is that when Nb is not in solid solution, the unrecrystallized austenite region is at the Ar3 point +50°C, but when 0.01% or more is dissolved in solid solution, the unrecrystallized austenite region increases to the Ar3 point +150°C. This is to expand this unrecrystallized austenite region so that rolling of 30% or more is possible in this unrecrystallized austenite region.

The 50% cumulative reduction rate in the high temperature recrystallized austenite region exceeding Ar3+150°C is the lower limit of the reduction rate necessary to produce fine austenite grains of about 20 μm or less.

Subsequently, rolling in the temperature range of Ar3 point + 150 ° C or lower and Ar3 point or higher for unrecrystallized austenite is to generate a deformation zone in the unrecrystallized austenite grains and generate many ferrite nuclei. When the amount is less than 50%, the generation of ferrite nuclei is insufficient. Therefore, the lower limit pressure reduction amount was limited to 50%. One of the reasons for incorporating Nb as a solid solution has been mentioned above, but another reason is to increase the hardenability of the steel and facilitate the formation of bainite and island martensite.

Rolling in the (γ+α) two-phase region at an Ar point of less than 3 and an Ar of 3-80°C or more, which is performed as necessary, is effective in converting ungrown fine-grained ferrite produced by transformation from unrecrystallized austenite into "micro-processed ferrite grains". ” and on the other hand, it also effectively accumulates strain in the remaining unrecrystallized austenite, so it is effective in refining ferrite grains and bainite grains.

However, if rolling is performed in a temperature range lower than Ar3-80°C, large ferrite grains will be processed and vTrs will deteriorate. Rolling in the two-phase range from less than Ar3 to Ar3-80°C is more effective because it produces "micromachined ferrite grains" that increase strength and do not deteriorate toughness, and at the same time is effective for refining other ferrite grains. Suitable for further lowering the carbon equivalent. However, rolling in this temperature range lowers the rolling efficiency and also causes an increase in separation on the impact fracture surface of the rolled material, so it should be used appropriately depending on specification requirements. Therefore, according to the present invention, Ar3
-80°C is set as the lower limit temperature for rolling in the two-phase region, and if the rolling reduction in the two-phase region is less than 10%, there is no effect of increasing TS, so the rolling reduction needs to be 10% or more. be.

Next, the reason for limiting the component composition of the slab in the present invention will be explained.

If C is less than 0.04%, the strength of the steel plate will decrease, the weld heat affected zone (hereinafter abbreviated as HAZ) will be greatly softened, and the manufacturing cost will increase significantly.
The lower limit of C content is O. It was set to O4%.

Further, if C exceeds 0.10%, the toughness of the base metal deteriorates, hardening of the welded part, and deterioration of crack resistance are significant, so the upper limit was set at 0.10%.

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 deteriorates, so the lower limit was set to 0.05%. On the other hand, the Si content is 0.30%
If the value is exceeded, the promotion of bainide decreases, so the upper limit is set to 0.
It was set at 30%.

The lower limit of Mn was set at 1.20% because if it is less than 1.20%, the strength and toughness of the steel plate will decrease and the HAZ will become more softened. On the other hand, if there is too much Mn, HA
Since the toughness of Z deteriorates, the upper limit was set at 2.50%.

For deoxidizing steel, it is necessary to add Al so that at least 0.005% of Al becomes a solid solution, so the lower limit of Al is set to 0.
．． 005%. On the other hand, when solid solution Al is 0.050% or more, not only the toughness of the HAZ but also the toughness of the weld metal deteriorates. Therefore, the upper limit of Al was set at 0.050%.

If S is not 0.008% or less, vTrm in the C direction will be -
The temperature does not drop below 70°C, and the absorbed energy is significantly lower. Therefore, the upper limit of S was set to 0.008%.

Nb is 0 to avoid deterioration of the weld metal toughness of the welded part.
．． Since it must be 100% or less, the upper limit of Nb was set at 0.100%. On the other hand, if the Nb content is less than 0.010%, the fine grain effect that improves the transition temperature cannot be obtained,
From this, the lower limit of the amount of Nb was set to 0.010%.

When B is less than 0.0005%, it is not effective in promoting bainite formation, while when it exceeds 0.0030%, it is difficult to promote bainite formation.
Since the hardening of Z is large, the upper limit of B was set to 0.0030%.

Ti improves toughness through the fine effect of γ grains, and reduces the amount of solid solution N in unrecrystallized austenite grains by forming Ti carbonitrides, thereby preventing the formation of B nitrides. Add as a purpose. However, if the amount of Ti is less than 0.005%, there is no effect, and if the amount of Ti is less than 0.005%, there is no effect.
If it exceeds 0.040%, the toughness will deteriorate, so the lower limit of Ti should be set to 0.040%.
．． 003%, with an upper limit of 0.040%.

Al, which is limited to containing N exceeding 0.008%;
The amount of Ti is insufficient to fix Ti nitride and Al nitride, and as a result, B forms B nitride, which worsens the hardenability of B. Therefore, the upper limit of the N content is set to 0.
008%.

Cu and Ni should not be added in an amount of 0.10% or more each at the same time; if it is less than 0.10%, it is not possible to expect a strength increase of more than 2 kgf/mm2, and if they exceed 1.00% in total, the ferritic structure is better. Since the area ratio is smaller than that of the bainite structure and the impact absorption energy is deteriorated, the lower limit of each is set to 0.10%, and the total is set to 0.20 to 1.00%.

The above are the basic components of the steel slab used in the present invention, and if necessary, Mo, V, Cr, Ca, REM
At least one selected from these elements can be added and contained, and proper inclusion of each element adds a unique effect as described below.

Mo improves strength and toughness by regulating the γ grains during rolling and producing fine bainite, but it is not necessary to add more than 0.50% to achieve the purpose of this invention. If it exceeds this, the manufacturing cost will increase, so the upper limit was set at 0.50%.

(2) is added to improve the strength and toughness of the base material of the steel plate according to the present invention and to ensure the strength of the joint. However, if the added amount is too large, the toughness of the base material and HAZ will deteriorate significantly, so the upper limit was set at 0.10%.

If Ca is less than 0.002%, it is insufficient to control the morphology of MnS and has no effect on improving the toughness in the C direction, so the lower limit of Ca is set to 0.002%. On the other hand, if Ca exceeds 0.010%, the cleanliness of the steel deteriorates and causes internal defects, so the upper limit of Ca was set at 0.010%.

If REM is less than 0.002%, it is insufficient to control the morphology of MnS and is not effective in improving the toughness of the steel sheet in the C direction, so the lower limit of REM is set to 0.002%. On the other hand, R.E.M.
If REM exceeds 0.010%, the cleanliness of the steel deteriorates and is also disadvantageous for arc welding, so the upper limit of REM is set to 0.010%.
010%.

Examples of the present invention will be described below.

Example 1 Table 1 shows the steel types of the slab materials used in the examples and comparative examples of the present invention. Steel 1A is related to an example of the present invention, steels 1B and 2B are related to comparative examples, and N
Steel 2B is the conventionally used 60k without containing b.
This is gf/mm2 grade QT treatment steel.

Steel plate manufacturing tests were conducted using slabs of steel 1A, 1B, and 2B under the rolling conditions shown in Table 2, respectively. The mechanical properties of the steel plates produced by each side are also shown in Table 2.

Test example no. 1.No. No. 2 has a solid solution Nb amount lower than 0.01%. No. 5 had a reduction amount of less than 50% in the austenite recrystallization temperature range, and No. 7 and no. 8
The reduction amount from Ar3 point +150℃ to Ar3 point is 50
% and no. 11 has a finishing temperature of Ar3
-80℃ (660℃), neither of which satisfy the manufacturing conditions of the present invention, so the target -90℃
The following vTrs cannot be obtained. Steel 1B, which does not contain Nb in the specified amount according to the present invention, is No. 12 No. As can be seen from No. 13, vTr below -90°C under any rolling condition.
g cannot be obtained.

However, Example No. 1 satisfies the component range and rolling conditions of the present invention. 3.No. 4.No. 6, No. 9
, No. No. 10 achieved a TS of 60 kgf/mm2 or more and a vTrs of -90°C or less despite the low carbon equivalent of 0.33%. The steel plates according to these examples were No. 14 carbon equivalent, T than conventional QT heat-treated steel sheet
All of the characteristics of S, vTrs, and vE-25 are excellent.

Example 2 Table 3 shows the steel types of the slab materials used in the examples and comparative examples of the present invention. Steel 1M~11M, 12M, 13
M is a comparative example, and steels 1P to 10P are examples of the present invention.

Using slabs made of the steel types shown in Table 3, steel plate manufacturing tests were conducted under the rolling conditions shown in Table 4. The mechanical properties of the steel plates manufactured in each example are also shown in Table 4.

Test example no. 1~No. In Comparative Example 11, the rolling conditions were almost the same, but the chemical composition of the slab was changed.

No. Nos. 1, 2, 5, and 7 were selected because the Ti or B content of the slabs was outside the component range of the present invention. 3 is S
Since the i content exceeds the upper limit of 0.30% of the present invention, 60
TS of kgf/mm2 or more cannot be obtained. Similarly,
No. 8 has a N content exceeding 0.0080%, and No.
．． No. 10 does not contain Cu or Ni. Since No. 11 does not contain Cu, a TS of 60 kgf/mm2 or more cannot be obtained.

No. 4 has a Nb content of less than 0.04%, and Nb content is less than 0.04%.
o. No. 6 has a Ti content of over 0.04%, and No. 6 has a Ti content of more than 0.04%. 9
Since the C content exceeds 0.10% and none of them satisfy the component range of the present invention, vTrs does not exceed -90°C or vE-25 does not exceed 15 kgf·m. No. 2
2 and 23 have vTrs below -90℃ while 60k
gf/mm2 or more, but because it does not contain the essential component B of the slab of the present invention, the carbon equivalent is No. 22 and 0
．． 42%, and No. Since it exceeds 0.35% in No. 23, the obtained steel plate does not have excellent weldability.

On the other hand, Example No. Nos. 12 to 21 were manufactured using slabs of component steel specified in the present invention under rolling conditions specified in the present invention. 12, 13, 14, 15, 16
, 17 and 21 with carbon equivalents below 0.35%.
kgf/mm2 or more, and No. 18, 19,
20, 70kgf/mm with carbon equivalent of 0.42% or less
A TS of 2 or more is obtained, and at the same time -9 in any of the examples.
A vTrs of 0° C. or lower was obtained.

As described above in detail, the present invention is a method for manufacturing high-strength steel sheets using slabs of a specific composition and controlled rolling under specific conditions, and by this method, it can replace conventional QT heat-treated steel sheets and improve toughness. It is possible to produce a high-strength steel sheet that has excellent weldability and has a structure mainly composed of ferrite, fine payinite, and island-like martensite.

[Brief explanation of the drawing]

The drawing is a graph showing changes in TS and vTrs of steel sheets produced by controlled rolling of the present invention using low C-Nb-B steel as a basic component and varying additive components.

Claims (1)

[Claims] 1, C: 0.04-0.10%, Si: 0.05-0.
30%, Mn: 1.20-2.50, Nb: 0.010
~0.100%, B: 0.0005-0.003O%,
N:≦0.0080%, Ti:0.005-0.040
%, Al: 0.005-0.050% and Cu: ≧0.1
0% and Ni: ≧0.10% (however, Cu+Ni: 0.
20 to 1.00%), and if necessary further Mo:≦
0.50%, V:≦0.10%-Cr:≦0.50%,
Ca: 0.002-0.010%-REM: 0.002
Any one or two selected from ~0.010%
species, and the remainder is unavoidable impurities (of which S is 0).
．． 008% or less. ) and Fe to a temperature at which at least 0.01% Nb is dissolved in the steel slab, and then heated so that the cumulative reduction rate is at least 50% at a temperature exceeding Ar3 point + 150 ° C. Rolling is performed, followed by rolling at Ar3 point + 150 ° C or less and within the temperature range of the unrecrystallized austenite region of Ar3 point or more so that the cumulative reduction rate is at least 50%, and if necessary, then below Ar3 point The hot-rolled steel sheet is rolled with a cumulative reduction of at least 10% within the two-phase temperature range of austenite and ferrite at three Ar points of -80°C or higher, and then air-cooled to form mainly ferrite. A method for producing a high-strength steel plate with extremely excellent toughness and weldability, which is characterized by forming a structure consisting of fine bainite and island martensite.