JPS63105921A - Manufacture of thick high tension steel of more than 90kgf/mm2 class having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To obtain the titled high tension steel having high strength at any position in the thickness direction, superior toughness at low temp. and performance to stop the progress of brittle fracture by hot rolling and heat treating a steel slab contg. restricted components under specified conditions. CONSTITUTION:A steel slab consisting of, by weight, 0.07-0.15% C, <=0.5% Si, 0.5-2.0% Mn, 3.0-5.0% Ni, 0.1-1.0% Cr, 0.1-1.0% Mo, 0.01-0.15% V, 0.03-0.10% sol.Al, 0.0005-0.0020% B, <=0.0060% N and the balance Fe with inevitable impurities is heated to 900-1,100 deg.C and hot rolled at 800-900 deg.C finish rolling starting temp. and >=40% total draft. The rolled product is hardened by water cooling from the Ar3 point or above. The product is provided with austenite grain size No.>=7.5 and a martensite structure in the lower part of the surface layer in the thickness direction and a mixed structure consisting of martensite and lower bainite in the central part in the thickness direction. The rolled product is then tempered by heating to the Ac1 point or below to manufacture the titled high tension steel.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a plate with a thickness of 1001 or more, excellent low-temperature toughness, and a tensile strength of 90 Kff/Mj with brittle fracture propagation stopping performance.
The present invention relates to a method for manufacturing the above-mentioned high-strength steel.

(Prior Art) In recent years, energy demand has been showing an increasing trend. In order to ensure a stable supply of these resources, interest in developing seabed resources is turning to deeper waters and colder regions, and the construction of offshore structures that lead to this seabed development, such as the construction of rig materials for oil drilling, are becoming larger and indoors. Therefore, it is desired to develop a material for use in these materials that has high structural strength and excellent low-temperature toughness.

In order to meet the demand for safer and more reliable materials, various thick-walled high-strength steels have been developed and their quality improved. Traditionally, reheating, quenching, and tempering have been the mainstream method for high-strength materials, but especially for thick-walled materials, it is necessary to properly quench the center of the plate to achieve good strength and strength.
Obtaining toughness was a national challenge.

Therefore, to date, in order to improve hardenability with a small amount of alloying elements, many methods have been applied that utilize the hardenability improving effect of B.

For example, as in Japanese Patent Publication No. 60-20461, 5otA
A tensile strength of 70 to 80 f/ was created by creating an L-total B content diagram, limiting the effective B range, and limiting the carbon equivalent (Ceq) and weld cracking susceptibility index (Pcm) from the viewpoint of weldability.
There is a method for manufacturing − grade thick-walled high-strength steel.

Also, as in JP-A-59-170221, strength,
Adding elements to steel that are effective for toughness and resistance to temper softening,
Tensile strength 95 to 9f/ after hot rolling and quenching and tempering
There is a manufacturing method for 9f/- edge high tensile strength steel of - grade 95 to 95mm.

All of these relate to a method for manufacturing high-strength steel using a reheating, quenching and tempering mold, and utilize the hardenability improving effect of B to improve the hardenability at the center of the plate thickness.

(Problem to be solved by the invention) However, in order to ensure safety under harsh usage conditions such as the use environment described above (deep sea and cold regions), these structures must not be deformed or destroyed. Not,
Particularly for the base material, requirements for stopping brittle fracture propagation at low temperatures must also be taken into account.

Regarding the steel obtained by the above-mentioned manufacturing method, such considerations are not sufficiently taken into consideration, and the characteristic values are also low. Therefore, in the case of thick-walled materials, the reheating, quenching and tempering method cannot be said to be sufficiently safe for use.

(Means for Solving the Problems) The present inventors aimed to develop high-strength steel for thick-walled materials that has high strength, uniform low-temperature toughness, and brittle fracture propagation stopping performance at all positions in the thickness direction. The lower part of the surface layer of the plate has an austenite grain size of 7.5 or more and has a martensite structure, and the center part of the plate has a mixed structure of martensite and lower bainite, resulting in high strength and high strength at all positions in the thickness direction. It has been found that in order to obtain toughness and to obtain such a quenched structure, the desired steel can be manufactured by combining an appropriate amount of Ni and processing heat treatment.

The present invention was constructed based on such knowledge, and the gist thereof is: C: 0.07 to 0.15%, Si: 0.5
Below, Mn: 0.5-2.0%, Ni: 3.
0-5.0%, Cr: 0.1-1.0%, Mo:
0.1-1.0%, V: 0.01-0.15%, so
lAl: 0.03-0.10%, B: O, 0O05-
A steel piece containing 0.0020%, N: 0.0015-0.0060%, with the balance consisting of Fe and inevitable impurities, or further Cu: 0.05-1.0%, Nb
: 0.005-0.050%, Ti: 0.005
~0.020% 1 and Ca: 0.0050% or less 1
A steel billet containing the seed or two or more types is heated to 900 to 1100°C, and then hot rolled to a finish biting temperature of 800 to 9.
The plate is rolled at 00°C with a cumulative reduction rate of 40% or more of the finished plate thickness, and after the completion of rolling, a quenching treatment is performed by water cooling from a temperature above the Ars point, and the lower part of the plate thickness surface has an austenite grain size of No. 7.5 or more. The center of the plate thickness is a mixed structure of martensite and lower bainite.
This is a method for producing a thick edge high tensile strength steel of 90 hf/- or more and excellent low temperature toughness, which is characterized by subsequently performing a tempering treatment by heating to a temperature below the AcI point.

The present invention will be explained in detail below.

First, the reason why the present invention is limited to the above-mentioned steel components will be described.

C: C is an effective element for improving hardenability and easily increasing strength. However, if it is less than 0.07%, the strength is insufficient, and if it exceeds 0.15%, low temperature toughness and weldability deteriorate. Therefore, the C content range is from 0.07 to
It shall be 0.15%.

Si: Si is effective in improving strength, but if it exceeds 0.5%, tempering brittleness increases and low-temperature toughness deteriorates. Therefore, the upper limit is set at 0.5% in order to ensure a certain degree of strength and not to deteriorate the notch toughness.

Mn: Hln improves hardenability and is effective in ensuring strength and toughness, but if it is less than 0.5%, strength and toughness decrease, and if it exceeds 2.0%, notch toughness and solution properties deteriorate. let Therefore, the content of seeds is set to 0.5 to 2.0%.

Ni: Ni is the most effective element for improving strength and toughness. In particular, in the present invention, a minimum content of 3.0% is required in order to stably obtain a mixed structure of martensite and lower bainite at the center of the plate thickness through the quenching process.

On the other hand, if it exceeds 5.0%, the effect of improving toughness will be small compared to the strength, and it will be economically disadvantageous. Therefore, the Ni content is set to 3.0 to 5.0%.

Cr: Cr is effective in improving hardenability and ensuring strength, and if it is less than 0.1%, it has no effect (and if it exceeds 1.0%, notch toughness and weldability deteriorate. Therefore, Cr
The content of is set to 0.1 to 1.0%.

Mo: Like Cr, Mo is also an element that increases hardenability and is effective in ensuring strength. However, if it is less than 0.1%, the target strength cannot be obtained, and if it exceeds 1.0%, notch toughness and weldability deteriorate. Therefore, the content of MO is 0.
．． 1 to 1.0%.

v: v forms carbonitrides during tempering treatment and is effective in securing strength through precipitation strengthening. In particular, the processing heat treatment method of the present invention can achieve effective precipitation strengthening by sufficiently converting carbonitrides into a solid solution before the quenching treatment.

However, if it is less than 0.01%, the target strength cannot be obtained, and if it exceeds 0.15%, notch toughness deteriorates.

Therefore, the content of ■ is set to 0.01 to 0.15%.

5oLkL: At is combined with N in the steel during heating of the steel billet, and must be added in an amount of 0.03% or more to ensure that it is effective for hardenability in the subsequent hardening treatment.

Furthermore, the fine precipitates of AtN are effective in refining austenite grains. However, if it exceeds 0.10%, Al
Inclusions such as 1 Os increase and impede toughness.

Therefore, the content of so tAt should be 0.03 to 0.10%.
shall be.

B: B is an effective element for improving hardenability, and in consideration of the above-mentioned hardenability elements of C, Mn, Ni, Or, and Mo, it is possible to improve hardenability by adding these elements in smaller amounts. I can demonstrate it.

Therefore, the quenched structure at the center of the plate thickness in the present invention can be stably obtained. If it is less than o,oos%, there is no effect, and if it exceeds 0.0020%, the effect is saturated,
On the contrary, it reduces the toughness. Therefore, the content of B is 0.
0005 to 0.00020%.

N: In order to ensure the effect of improving hardenability due to B, it is preferable to reduce N as much as possible. However, 0.0015%
If it is less than 1, the austenite grains in the lower surface layer of the thick-walled material become coarse and the low-temperature toughness deteriorates.

Moreover, if it exceeds 0.0060%, solid solution N increases in the weld heat affected zone, particularly in the coarse grain region, at high temperatures, deteriorating the weld toughness. Therefore, the N content should be 0.0015~
It shall be 0.0060%.

The above are the basic components of the steel in the present invention, but in the present invention, the following components are selectively added in order to further improve the strength and toughness.

Cu: Cu does not deteriorate toughness, increases strength, and is also effective in improving corrosion resistance. If it is less than 0.05%, it has no effect, and if it exceeds 1.0%, it actually deteriorates hot workability and toughness. Therefore, the Cu content is set to 0.05 to 1.0%.

Nb: Nb forms carbonitrides during tempering treatment, and is effective in securing strength through precipitation strengthening. In addition, in order to expand the non-recrystallization temperature range during heating and rolling in the present invention, it makes the austenite grains in the lower surface layer finer. hardness and improves toughness.

Therefore, it is effective at o, oos% or more, but o, os
.

If the temperature exceeds this threshold, the toughness and weld toughness will deteriorate. Therefore,
Let the content in the corner be o, oos to o, oso%.

Ti: Ti prevents coarsening of the heat-affected zone of the bath, and is therefore effective in improving the toughness of that area. Therefore, 0.0
0.05% or more, but if it exceeds 0.020%, even the toughness of the base material will deteriorate. Therefore, the Ti content is reduced to 0.
．． 005 to 0.020%.

The above components are elements added to improve strength, toughness, and weldability in the present invention, and Ca is selectively added to improve anisotropy and lamellar tear resistance.

Ca: Ca is extremely effective in spheroidizing nonmetallic inclusions, and has the effect of improving toughness and reducing anisotropy of toughness.

However, when it exceeds 0.0050%, inclusions increase and the toughness deteriorates. Therefore, its content is set to 010050% or less.

In addition to the above-mentioned components, unavoidable impurities such as P and S are harmful elements that deteriorate the toughness, which is a characteristic of the present invention, so the smaller the amount, the better. I like it.

Alternatively, P is adjusted to 0.015% or less, and S is adjusted to 0.005% or less.

Furthermore, in the present invention, after quenching, the lower part of the surface layer of the plate has an austenite grain size of 7.5 or more and martensite structure, and the center of the plate thickness has a mixed structure of martensite and lower bainite, which is an important crystal structure of the invention. I will explain the reason.

In general, the quenched structure is martensite → lower bainite → upper bainite in order of cooling rate. Even in thick-walled materials, the cooling rate decreases from the surface layer to the center of the thickness, and these structures change. are generated sequentially. Here, the upper bainite structure is a structure obtained by insufficient quenching, and the strength and toughness are reduced.

On the other hand, the martensitic structure is the most effective structure for increasing strength, but the toughness is highly dependent on the austenite grain size, and coarse grains result in significantly low toughness.

In addition, it is not preferable to form a martensitic structure up to the center of the plate thickness in a thick-walled material because of the deterioration of weldability due to an increase in alloying elements and economic efficiency.

The lower bainite structure, which provides the highest strength next to the martensitic structure, has the least grain size dependence and is the structure that provides the highest toughness. A bainite structure is widely used.

However, even if the central part of the plate thickness has a lower bainitic structure, the lower part of the surface layer of the plate inevitably has a martensitic structure, and if it is coarse grained, this will lead to deterioration of toughness.

In addition, the lower bainite structure is effective in improving the strength and toughness of high tensile strength steels with a tensile strength of 90Ktf/- or higher, as in the present invention.
It cannot be said that the quenched structure is sufficient for achieving high strength and high toughness.

In other words, a mixed structure of martensite and lower bainite is the structure in which the strength and toughness that are the objectives of the present invention can be obtained most simultaneously. By making the grains finer than 5 Homare, the toughness at all positions in the plate thickness direction becomes high.

Figure 1 shows the lower surface layer and plate of @D's 3.5% Ni-based 130-gourd material, the re-cut heat quenched and tempered material, and the direct quenched and tempered material with different heating and rolling conditions. This figure shows the relationship between the austenite grain size in the thick center and toughness.

The central part of the plate thickness is a mixture of martensite and lower bainite, showing low grain size dependence and high toughness, and the lower surface layer has a martensite structure with remarkable grain size dependence, with an austenite grain size of 7.5 mm. With the above, high low temperature toughness can be obtained,
Uniformly high toughness is achieved at all locations in the plate thickness direction.

Furthermore, the reason for limiting the manufacturing conditions to obtain such a quenched structure and austenite grain size will be explained below. First, molten Ni-containing low-alloy steel that has been melted into the above-mentioned composition is processed by continuous casting or ingot making. After forming a steel billet by the blooming method, it is subjected to a pretreatment of repeating heating and cooling, either directly or if necessary, for the purpose of diffusing the segregated components, and then heated to a temperature of 900 to 1100° C. and hot rolled.

This heating fixes N with AA, aims at fine precipitation of AtN, increases solid solution B before quenching treatment, and improves hardenability. It also refines the heated austenite grains and further tempers them. In order to utilize the strengthening caused by the precipitation of fine carbonitrides such as Mo and V during the treatment, it is also a process in which the carbonitrides such as Mo and V present in the steel slab are sufficiently converted into a solid solution.

Therefore, at a low temperature below 900°C, this isosolubilization effect becomes insufficient. On the other hand, at temperatures exceeding 1100°C, M
Although carbonitrides such as O and V are sufficiently dissolved in solid solution, the fine precipitates of AtH are also re-decomposed, and this solid solution N is combined with B during rolling and reprecipitated as BN, so the solid solution B during quenching is hardenability decreases.

In addition, the heated austenite grains become coarser, and during subsequent rolling, the austenite grains in the lower part of the surface layer are difficult to become finer (especially in the lower part of the surface layer), which causes a decrease in the toughness of the lower part of the surface layer. heating temperature of 900-110
The temperature was 0°C.

The steel billet heated to such a high temperature is hot-rolled at a finish bite temperature of 800 to 900°C and a cumulative reduction rate of 40% or more relative to the finished plate thickness, and after the completion of this rolling, Ar 3 or more points are applied. Water cooling treatment is performed from a temperature of .

Here, the finishing biting temperature in hot rolling is 800 to 9
The reason for limiting the temperature to 00℃ is to refine the austenite grains at all positions in the sheet thickness direction and to ensure hardenability in the center of the sheet thickness. As a result, the austenite grains become insufficiently refined.

In addition, at low temperatures below 800℃, the
The formation of deformation bands within the stenite grains increases, and the solid solution B instead sticks to these, reducing the amount of B segregated to the austenite grain boundaries, reducing hardenability, and thus making it easier to form an upper bainite structure. Because it will be.

Next, the reason for rolling at a cumulative reduction rate of 40% or more with respect to the finished thickness is to refine the austenite grain size in the lower surface layer to No. 8 or more. In addition, the reason why water cooling is performed from a temperature above the Ars point after the rolling is completed is that quenching is performed from a temperature in the complete austenite region at all positions in the thickness direction, and the lower part of the surface layer has a martensitic structure, and the center of the thickness has a martensitic structure. is intended to obtain a mixed structure of martensite and lower bainite, and if the quenching temperature is below the Ars point, an upper bainite structure will be formed significantly and the strength and toughness will deteriorate.

After such hot rolling, the water-cooled steel has an austenite grain size of 7.5 or more and a martensite structure in the lower part of the surface layer of the plate, and a mixed structure of martensite and lower bainite in the center of the plate thickness.

However, as it is, the yield strength and toughness are insufficient, so tempering treatment is performed at a temperature below Ac1 point, V.
It is necessary to sufficiently precipitation strengthen carbonitrides such as Mo to obtain strength and toughness.

The steel obtained through such a manufacturing process has high strength and high toughness at all positions in the plate thickness direction, and also has significantly improved brittle fracture propagation stopping performance.

(Example) A steel slab obtained by melting steel having the composition shown in Table 1 was melted to a plate thickness of 100 to 200I based on the manufacturing conditions of the present invention method and the comparative method shown in Table 2. Manufactured from steel plate.

Regarding these, the mechanical properties of the base material and the temperature gradient WESS
Table 3 shows the Kca test results for brittle fracture propagation arresting performance by the O test.

In addition, the chemical components mI and J in Table 1 are examples of components that deviate from the chemical component range specified in the four inventions.

As is clear from the results in Table 3 above, the mechanical properties of the steel plates obtained according to the present invention are higher in strength and toughness than those obtained by the comparative method, especially in the lower part of the surface layer of the plate. The toughness is significantly improved and the austenite grain size is also significantly improved compared to the steel plate made by the comparative method.

It is fine grained. Furthermore, the brittle fracture propagation arresting performance Kca value obtained by the temperature gradient type ESSO test is also superior to that of the steel plate obtained by the comparative method.

(Effects of the Invention) As is clear from the above examples, according to the present invention, a thick-walled high-strength material has excellent low-temperature toughness at all positions in the plate thickness direction, and has the ability to stop brittle and structural fracture propagation. 9f
It has become possible to produce high tensile strength steel of /- or higher grade, and the industrial effect is extremely significant.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the relationship between austenite grain size and toughness at all positions in the thickness direction of steel plates manufactured by the method of the present invention and the comparative method using steel having the composition of the present invention. Figure 1

Claims (4)

[Claims] (1) C: 0.07 to 0.15% by weight, Si: 0.5% or less, Mn: 0.5 to 2.0%, Ni: 3.0 to 5.0%, Cr: 0 .1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.15%, solAl: 0.03-0.10%, B: 0.0005-0.0020 %, N: 0.0060% or less, with the remainder consisting of Fe and unavoidable impurities, is heated to 900 to 1100°C, and hot-rolled at a finish biting temperature of 800 to 900°C to reduce the finished plate thickness. The plate is rolled at a cumulative reduction rate of 40% or more, and after the completion of rolling, a quenching treatment is performed by water cooling from a temperature of Ar_3 or higher, and the lower part of the surface layer of the plate has an austenite grain size of No. 7.5 or higher and a martensitic structure, The center of the plate thickness has a mixed structure of martensite and lower bainite, followed by Ac
A method for producing thick-walled 90Kgf/mm^2 or higher grade high-strength steel with excellent low-temperature toughness, characterized by performing a tempering treatment by heating to a temperature of _1 or less. (2) In weight%, C: 0.07-0.15%, Si: 0.5% or less, Mn: 0.5-2.0%, Ni: 3.0-5.0%, Cr: 0 .1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.15%, solAl: 0.03-0.10%, B: 0.0005-0.0020 %, N: 0.0060% or less, Cu: 0.05-1.0%, Nb: 0.005-0.0
A steel slab containing 50% Ti, 0.005% to 0.0020% Ti, and the balance consisting of Fe and unavoidable impurities is heated to 900 to 1100°C, and then finished by hot rolling. Rolling is carried out at a rolling temperature of 800 to 900°C and a cumulative reduction rate of 40% or more relative to the finished plate thickness. After the rolling is completed, a quenching process is performed by water cooling from a temperature of Ar_3 or higher, and the lower part of the surface layer of the plate has an austenite grain size of 7. .5 or higher and has a martensitic structure, the center of the plate thickness has a mixed structure of martensite and lower bainite, and is then tempered by heating to a temperature of Ac_1 point or less. Excellent low-temperature toughness. A method for manufacturing high-strength steel with a thickness of 90Kgf/mm^2 or higher. (3) C: 0.07 to 0.15% by weight, Si: 0.5% or less, Mn: 0.5 to 2.0%, Ni: 3.0 to 5.0%, Cr: 0 .1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.15%, solAl: 0.03-0.10%, B: 0.0005-0.0020 %, N: 0.0060% or less, Ca: 0.0050% or less, with the balance consisting of Fe and unavoidable impurities, is heated to 900 to 1100°C, and hot rolled to a finish biting temperature of 800°C. Rolling is performed at ~900°C with a cumulative reduction rate of 40% or more relative to the finished plate thickness, and after completion of rolling, a quenching process is performed by water cooling from a temperature of Ar_3 or higher, and the lower part of the surface layer of the plate has an austenite grain size of No. 7.5. Above, the martensitic structure is formed, and the central part of the plate thickness is a mixed structure of martensite and lower bainite, followed by Ac
A method for producing thick-walled 90Kgf/mm^2 or higher grade high-strength steel with excellent low-temperature toughness, characterized by performing a tempering treatment by heating to a temperature of _1 or less. (4) C: 0.07 to 0.15% by weight, Si: 0.5% or less, Mn: 0.5 to 2.0%, Ni: 3.0 to 5.0%, Cr: 0 .1-1.0%, Mo: 0.1-1.0%, V: 0.01-0.15%, solAl: 0.03-0.10%, B: 0.0005-0.0020 %, N: 0.0060% or less, Cu: 0.05 to 1.0%, Nb: 0
．． 005-0.050%, Ti: 0.005-0.02
A steel piece containing one or more of 0% and 0.0050% or less of Ca, with the balance consisting of Fe and unavoidable impurities is heated to 900 to 1100°C, and the finish biting temperature is reached during hot rolling. Rolling at 800 to 900°C with a cumulative reduction rate of 40% or more relative to the finished plate thickness, and after completing this rolling, perform a quenching treatment by water cooling from a temperature of Ar_3 or higher,
The lower part of the surface layer of the plate has an austenite grain size of 7.5 or more and has a martensite structure, and the center part of the plate has a mixed structure of martensite and lower bainite, and then is tempered by heating to a temperature of Ac_1 point or less. A method for manufacturing thick-walled, 90Kgf/mm^2 or higher grade high-tensile steel steel with excellent low-temperature toughness.