JP3367388B2 - High ductility and high toughness steel sheet and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a steel structure requiring a high degree of safety, such as a polar offshore structure, a tank,
The present invention relates to a high-ductility and high-toughness steel sheet suitable for use in low-temperature pressure vessels, bridges, building materials, etc., and a method for producing the same.

[0002]

2. Description of the Related Art It has been known that various properties of steel materials such as low temperature toughness, ductility, yield strength, and corrosion resistance are improved as the metal structure becomes finer. Therefore, a thermomechanical treatment (TMC) for the purpose of refining the metal structure is performed.
P: Thermo-Mechanical Control Process) technology and steel material manufacturing technology such as chemical composition adjustment technology that maximizes its effects have been developed, and have produced great effects on the production and supply of high-quality steel materials. However, it is extremely difficult to obtain a uniform microstructure with an average ferrite grain size of 10 μm or less, even with the TMCP technology, which has a great effect on the refinement of the microstructure of steel, let alone a uniform grain size of 3 μm or less. It was virtually impossible to obtain a fine microstructure.

On the other hand, a steel sheet cooled to below the Ar ₃ point and stopped during cooling is heated by an external heat source, working heat or the like to start rolling, and within the range of Ac ₃ point −50 ° C. to Ac ₃ point temperature. A manufacturing method has been proposed in which rolling is completed and uniform fine crystal grains are formed (JP-A-4-304312, JP-A-4-304313, JP-A-5-202444, etc.). In the present invention, "the necessary amount of processing is applied to ferrite during heating,
Moreover, if the reverse transformation to austenite is prevented, the dislocations introduced into the worked ferrite are recovered and rearranged to achieve the refinement of ferrite. "
In the following description, ferrite may be abbreviated as α and austenite may be abbreviated as γ.

However, in this method, since rolling is performed while the temperature of the (α + γ) two-phase region is being increased to the high temperature side, the refinement is insufficient, and the ductility is reduced due to the refinement of the structure, and thus the ductile fracture energy is reduced. Is not taken into consideration.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a high-tensile steel sheet having a high ductility as well as a fine grained ferrite grain to a degree never achieved before and a high toughness. With the goal. Specifically, the object is to provide a high-strength steel sheet having the following metallographic structure and performance and a method for producing the same.

Average ferrite grain size ≤ 3 µm or less Elongation ≥ 50% vTs of base metal ≤ -110 ° C 3.5kJ / cm heat input submarine arc welding joint bond vE-20 ≥
100J

[0007]

Means for Solving the Problems The inventors of the present invention have made extensive studies on a high ductility, high toughness, high strength steel sheet having a metal structure containing fine ferrite grains and a method for producing the steel sheet, and as a result, have confirmed the following matters. I was able to.

(A) The volume ratio of the transformation phase below the Ar ₃ point in the first cooling (hereinafter referred to as “transformation rate”) is set to 70% or more, and then from Ac ₁ point to “Ac ₁ point and Ac ₃ point”. Intermediate temperature ”
When the heat is reheated to a temperature range below (hereinafter simply referred to as “intermediate temperature”), the ferrite grain size finally obtained is made finer than when heating is performed above the intermediate temperature.

(B) Processing is added as necessary during the above-described first cooling, subsequent reheat, and final cooling, but if the simple sum of the rolling reductions of each processing is 50% or more, The ferrite grain size finally obtained is greatly reduced.

(C) During recuperation and during final cooling, the volume ratio of ferrite recrystallized from worked ferrite is 40% or more of the whole and the volume ratio of ferrite transformed from worked γ is 40% or more. Thereby, the ductility can be greatly improved.

(D) The effect of the steel sheet can be recognized not only in the entire thickness but also in the surface layer area as the area subjected to the three stages of work heat treatment of the above-mentioned first cooling, reheating, and final cooling.

The present invention was completed through a combination of the above items, repeated experiments in a thermomechanical processing experimental apparatus in a laboratory, and a trial test at a manufacturing site. The gist thereof is the following high-tensile steel plate and its production. On the way.

(1) By weight ratio, C: 0.02 to 0.2
%, Si: 0.02-0.5%, Mn: 0.4-2.5
%, Sol.Al: 0.08% or less, N: 0.008% or less, Cu: 0 to 0.6%, Ni: 0 to 2%, Cr: 0 to
0.8%, Mo: 0-0.8%, Nb: 0-0.05
%, V: 0 to 0.08%, B: 0 to 0.005%, and Ti: 0 to 0.05%, with the balance being Fe and inevitable impurities, and transformed from unrecrystallized austenite at least in the surface layer region. 40% by volume of ferrite and ferrite recrystallized from processed ferrite
A high ductility and high toughness steel sheet (referred to as [invention 1]) characterized by occupying the above and having an average grain size of 3 μm or less for both ferrites.

(2) A steel piece having the chemical composition described in [Invention 1] is heated to a point of Ac ₃ or more, and then subjected to three thermomechanical treatments in which at least the surface layer region satisfies the following conditions, A method for producing a high ductility and high toughness steel sheet according to [Invention 1], which has a sum of rolling reductions of 50% or more.

The volume ratio of the transformation phase below the Ar ₃ point is 70%.
Cooling is performed as described above, and cooling is stopped midway. If necessary, processing is added during this cooling.

After the above cooling is stopped, Ac _{1 is} exceeded by ₁ or more points.
Reheat to a temperature below the midpoint between the point and the Ac ₃ point. If necessary, processing is performed during this recuperation and during cooling after recuperation.

The steel sheet is cooled. Ac ₁ point during this cooling
Processing is performed as necessary in the temperature range up to 100 ° C.

In the above, the high-strength steel plate refers to a thick steel plate and a hot-rolled steel plate. In the above [Invention 1] and [Invention 2], the metallographic conditions described in the respective inventions may be satisfied at all positions in the plate thickness direction, or the "plate thickness 5
%, The above condition of the metallographic structure may be satisfied only in the surface area corresponding to 5% or more of the surface area.
When it corresponds to 0%, all the plate thickness portions correspond to the surface layer region.

Temperatures such as Ar ₃ point, Ac ₁ point, and intermediate temperature in [Invention 2] are measured values on the surface. When processing is performed during reheating, the surface temperature at the time of processing should be below the intermediate temperature, the internal temperature becomes above the intermediate temperature due to processing, and as a result, the surface temperature after processing becomes above the intermediate temperature. Good.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the composition of steel will be described. In the following description, "%" of the alloying element shall indicate "% by weight".

1. Chemical composition of steel C: 0.02-0.2% C is an element effective for increasing strength, and 0.02% or more is necessary to obtain desired strength, but exceeding 0.2% If it is contained excessively, the toughness of the steel is deteriorated, and in the steel of the present invention to be welded, the toughness of the heat-affected zone of welding is significantly deteriorated, so the upper limit is made 0.2%.

Si: 0.02-0.5% Si is an element effective for deoxidation, but if it exceeds 0.5%, the toughness of the heat-affected zone of the weld is reduced, so the upper limit is made 0.5%. . If the Si content is less than 0.02%, the loss of Al increases during deoxidation, so the lower limit of Si is made 0.02%.

Mn: 0.4 to 2.5% Mn is an element effective in increasing strength, and for that purpose,
0.4% or more is required. However, if it exceeds 2.5%, the toughness of the heat-affected zone of the weld deteriorates and Ac ₃ point and Ac ₁
The points are remarkably lowered, and therefore, in the temperature range below the intermediate temperature, recrystallization of the worked ferrite by rolling during recuperation cannot be sufficiently achieved. Therefore, the upper limit of Mn is 2.5%.

Sol.Al: 0.08% or less sol.Al may be substantially zero. However, since the plate thickness is thick, the total rolling reduction ratio [{(slab thickness-product plate thickness) / slab thickness} × 100 (%)] cannot be 50% or more, and the pinholes are not crimped during solidification. If it cannot be expected, it is desirable to set sol.Al to 0.001% or more in order to suppress the generation of pinholes. If sol.Al is less than 0.001%, pinholes are likely to occur in the slab after solidification. On the other hand, sol.Al
Is more than 0.08%, the toughness such as brittle fracture propagation stopping property deteriorates, so it is set to 0.08%.

N: 0.008% or less N forms a nitride together with Al and is effective for refining the crystal grains, but if N is too high, the toughness of the welded portion is impaired, so the upper limit is 0.008. %. On the other hand, a proper amount of N has the effect of improving weldability. To obtain this effect, it is desirable to contain 0.0015% or more.

Cu: 0 to 0.6% Cu may not be contained. Since Cu is effective in increasing the strength, it may be added particularly for adjusting the strength. 0.15
If the content is less than%, the effect is not clear.
It is desirable to be 15% or more. However, if it exceeds 0.6%, the toughness of the base material and the weld will deteriorate, so 0.6%.
Below. In order to secure a good balance between toughness and strength, it is desirable that the content be 0.3% or less.

Ni: 0 to 2% Ni may not be contained. Since Ni improves the toughness of the matrix, 0.15% is added to obtain the effect clearly, especially when improving the brittle crack propagation stopping property.
It is desirable to include the above. On the other hand, Ni becomes higher than necessary hardenability exceeds 2%, not only the transformation rate below Ar ₃ point can not be obtained sufficiently high during cooling, Ac ₃ point or Ac ₁
Since the point is remarkably lowered, recrystallization of the worked ferrite is not achieved during rolling during recuperation. Therefore, the upper limit of Ni is 2%.

Cr: 0 to 0.8% Cr may not be contained. Since Cr is effective in increasing the strength, it is preferable to contain 0.2% or more in order to reliably obtain the effect, especially for adjusting the strength. On the other hand, if it exceeds 0.8%, the toughness of the base material and the weld heat affected zone deteriorates, so the upper limit is made 0.8%. In order to obtain a better balance between strength and toughness, it is desirable to set it to 0.4% or less.

Mo: 0 to 0.8% Mo may not be included. Mo is effective in increasing the strength, so is added especially when improving the strength. If the content is less than 0.05%, the effect of improving the strength is not clear, so when it is included, it is desirable to set it to 0.05% or more. On the other hand, 0.
If the content exceeds 8%, the toughness of the base material and the weld heat affected zone deteriorates, so the upper limit is made 0.8%.

Nb: 0 to 0.05% Nb may not be included. Nb combines with C and N to form fine precipitates, suppresses γ grain refinement during heating and recrystallization of processed γ, and is effective for ferrite grain refinement. Add to. If it is less than 0.01%, this effect cannot be obtained reliably, so when it is included, it is desirable to set it to 0.01% or more. On the other hand, 0.0
If it exceeds 5%, the toughness will deteriorate, so the upper limit is 0.05%.
And

V: 0 to 0.08% V may not be contained. However, since V is effective in increasing the strength, it is desirable to contain 0.02% or more in order to surely obtain the effect, particularly when the strength is adjusted. But,
If it exceeds 0.08%, the toughness of the base material and the weld heat affected zone deteriorates, so the upper limit is made 0.08%. In order to balance and secure good toughness and strength, it is desirable that the content be 0.05% or less.

B: 0 to 0.005% B may be added without addition. But B is M ₂₃ (CB) ₆ or BN
If it is segregated to the γ grain boundary in the solid solution state without forming the above, a very small amount will exert a remarkable effect of improving the hardenability, so that it is added particularly when the strength is adjusted. If it is less than 0.0003%, a clear effect may not be obtained depending on the thermomechanical treatment conditions, so when it is included, it is preferably 0.0003% or more. On the other hand, if it exceeds 0.005%, the toughness of the base material and the heat-affected zone deteriorates, so the upper limit is made 0.005%.

Ti: 0 to 0.05% Ti may not be added. Ti has the effect of refining the crystal grains of γ by fixing N as TiN in a trace amount, and at the same time, in the case of Nb-containing steel, it is effective in suppressing the cracking of the surface of the continuously cast billet promoted by Nb. Therefore, it is desirable to add it. If it is less than 0.005%, such an effect is small. Therefore, if it is included, it is preferably 0.005% or more. If it exceeds 0.05%, the toughness of the base material and the welded portion deteriorates, so the upper limit is made 0.05%.

Inevitable impurities: Of the inevitable impurities, P is preferably 0.01% or less. If it exceeds 0.01%, not only P but also C, Mn, S, and the like are concentrated in the segregated portion formed during solidification to increase hardness and deteriorate toughness and weldability. S is preferably 0.007% or less. If it exceeds 0.007%, coarse M in the segregation part
It produces nS and becomes the starting point of welding cold cracking and the starting point of hydrogen defects. Other impurities are reduced to the levels obtained by normal refining.

2. Production method Next, the manufacturing method will be described.

First, the steel slab is heated to Ac ₃ point or more. This is to make the coarse metallographic structure formed during solidification into uniform γ grains by heating to the Ac ₃ point or higher. The lower the heating temperature is, the more effective it is in the refinement of γ grains, but when Nb or the like is added, it is necessary to form a solid solution of carbonitrides of Nb.
It is desirable to heat to about 00 ° C. Then the steel plate A
Cool to r ₃ or less to transform γ into ferrite and other structures. Cooling at this time can be performed by a normal cooling facility provided on the rolling line. At this time, it is necessary to appropriately control the cooling stop temperature so that the volume ratio of the transformation phase below the Ar ₃ point (hereinafter referred to as “transformation rate”) becomes 70% or more. The transformation rate is determined as the temperature at which the transformation rate becomes 70% by measuring the thermal expansion when creating a working continuous cooling curve (working CCT diagram), so the plate thickness depends on the depth of the surface layer region that has a fine structure. It can be controlled by the surface temperature so as to reach the temperature in the central portion or the surface layer region. If the surface temperature is known, the temperature at any position in the plate thickness direction can be known by calculation, and thus the depth of the surface layer region can be known.

On the other hand, although the upper limit of the transformation rate is not specified,
It is desirable to be 90% or less. If cooled to more than 90%, it may take time to reheat and the rolling efficiency may decrease.

The transformation phase preferably has a transformation rate of 50% or more at 575 ° C. or higher. The transformation phase at 575 ° C. or lower mainly has a lath-like upper bainite structure. This upper bainite lath-like ferrite has a uniform crystallographic orientation, and its length corresponds to a maximum grain size of approximately γ, and unless a very large reduction is applied in the unrecrystallized region of γ, a fine structure cannot be obtained. Have difficulty. In contrast, 575
In the transformation phase transformed at a temperature of ℃ or more, since the ferrite is granular and the crystal orientation is random, γ grains having different orientations are generated in the reheating process, which is also effective in making the final ferrite structure fine. If rolling is applied during this first cooling, the formation of lath-shaped ferrite with a uniform crystal orientation is prevented, the ratio of granular ferrite is increased, and the grain size of this granular ferrite is effectively reduced. When rolling is added in the first cooling, the reduction rate is preferably 30% or more.

Next condensate heat the steel metal structure consisting of the γ and transformation phase to an intermediate temperature below the temperature of the Ac ₁ point and the Ac ₃ point at Ac ₁ point or more. This is because α grains and the like grow when heated above the intermediate temperature. On the other hand, the recuperation temperature is “Ac ₁ point + 40 ° C”
If it is less than γ, the production of γ is not sufficient and α is transformed from processing γ.
Since it may be difficult to make the transformation rate of 40% or more, it is desirable that the lower limit temperature of the recuperation be "Ac ₁ point + 40 ° C".

As the heat source for this recuperation, high frequency heating, direct current heating, simple gas heating in a sub line, or the like can be used. When the rolling process is performed during the recuperation, it may be added as long as the surface temperature of the steel sheet is not higher than the intermediate temperature. However, if the surface temperature exceeds the intermediate temperature due to processing heat after rolling, rolling should not be applied. By returning to this temperature range, the ferrite structure becomes coarse due to grain growth, but part of the ferrite grain boundary undergoes reverse transformation to γ, resulting in a fine (α + γ) two-phase structure and mutual grain growth. Restrain each other. Rolling may be added as long as the surface temperature is not higher than the intermediate temperature while the recuperation is completed and the material is allowed to cool. When processing is added as needed during this recuperation or during the period from the end of recuperation to the final cooling during cooling, it is desirable that the reduction rate of the processing be 30% or more.

After completion of the recuperation and cooling, the cooling is continued without passing through the cooling. This cooling is preferably water cooling to 600 to 650 ° C. in order to obtain fine α particles. At this time, the temperature is below the cooling start temperature and "Ac ₁ point-100"
When processing in the temperature range above ℃ ”, the reduction rate is 30%
It is desirable to carry out the above reduction.

The sum of the rolling reductions applied during the first cooling, during the recuperation and during the cooling after the recuperation, and during the final cooling is 50% or more in order to obtain fine ferrite grains. . It is desirable that the total sum of the rolling reductions is high, but it is 15 from the viewpoint of the rolling mill rolling capacity.
It is desirable to set it to 0% or less. By these workings, both γ and α are further refined through the process of recovery and recrystallization, and on the other hand, working strains are also accumulated in γ by these workings, and transformed into fine ferrite during the final cooling.

3. Metal structure Due to the above chemical composition and manufacturing conditions, the volume ratio of α transformed from processing γ is 40% or more, α recrystallized from processing α
And the average particle size over both α can be 3 μm or less. Since C discharged from γ is distributed at a high concentration around α transformed from processing γ, pearlite, bainite and the like are generated. That is,
It has an adjacent structure in which cementite is densely distributed. In steel, some degree of segregation always occurs during solidification, which remains even after rolling. Since Mn and the like are distributed in a high concentration in this portion and are likely to be gamma-ized, the segregation portion is gamma-ized in the temperature range of the two-phase region. The α that transforms from the processing γ is a portion around the portion where the concentration of Mn or the like in the segregation portion is relatively low, and the core portion of the segregation portion transforms into pearlite or the like in which cementite is distributed at a high density as described above. . On the other hand, α recrystallized from the processed α occupies the intermediate portion of the segregation portion of the negative segregation portion. Therefore, both α are identified by the optical micrograph or the like.

Even if only the surface layer region of the plate thickness has such a metal structure, the target mechanical properties can be secured. A desired fine structure can be obtained by such recrystallization, reverse transformation, and suppression of grain growth by forming a two-phase structure.
The final metallographic structure is mainly composed of a mixed structure of ferrite, upper bainite, and pearlite, and some martensite may be added in some cases.

[0045]

EXAMPLES Next, the effects of the present invention will be described with reference to examples.

Tables 1 and 2 are tables showing the chemical compositions of the test steel sheets.

[0047]

[Table 1]

[0048]

[Table 2]

The test steel sheets were obtained by subjecting steel pieces having these chemical compositions obtained by melting and casting by a conventional method to thermomechanical treatment. Final thickness is 10-4
It was set to 0 mm.

Table 3 is a table showing the thermomechanical treatment conditions applied to the test steel sheet.

[0051]

[Table 3]

When observing the metallographic structure, when fine-grained microstructures are obtained over the entire thickness of the steel sheet, test specimens for microstructure observation are taken from the center portions of the steel sheet thicknesses, and only the surface layer region of the steel sheet is finely divided. When the grain structure was obtained, the central part of the surface layer region was used for structure observation. The tensile test is JIS No. 4 (JIS
Z 2201) was used to sample in the rolling width direction (C direction) from the center portion of the plate thickness or the surface layer portion, and a test was conducted in accordance with JIS Z 2241 to measure elongation and the like. As the Charpy impact test piece, a half size JIS 4 (JIS Z 2202) sample was sampled from the center part of the plate thickness or the surface layer region.

The welding test was carried out by submerged arc welding (SAW) using a commercially available welding material at 35 kJ / cm.
Welding was carried out under the heat input condition of No. 2, and a Charpy test piece was sampled from the weld bond portion, and the test was carried out at a temperature of -20 ° C.

Table 4 is a list showing the results of these tests.

[0055]

[Table 4]

Test numbers 9, 10, 13, and 1 which are comparative examples
In Nos. 5, 16, 17, and 20, the average grain size of ferrite became coarse mainly because the transformation rate at the Ar ₃ point or less during the initial cooling was less than 70%, and the toughness of the base material was first set to the target value. Did not reach. Of these, test numbers 13, 15, 16, 1
No. 7 had a low absolute value of elongation, and Nos. 13, 15, and 17 had large anisotropy of elongation. This is because the processed ferrite is insufficiently recrystallized. Exam number 8
In the case of, the reheat temperature exceeded the intermediate temperature, so the average ferrite grain size became coarse and the toughness of the base material was low. In Test No. 11, the toughness of the base metal and the weld heat affected zone deteriorated because the C content was too high. Test numbers 12 and 14
Of Si and Cu were too high, respectively, and the toughness of the bond was significantly deteriorated. Similarly, in Test Nos. 18 and 19, V, Nb, and Ti were too high, and the bond toughness deteriorated.

On the other hand, test number 1 which is an example of the present invention
As for Nos. 7 to 7, the chemical composition and the manufacturing conditions were performed within the limits of the present invention, and as a result, the desired metallographic structure was obtained, and the target performances were able to be satisfied for all of the toughness of the base metal, the elongation and the toughness of the welded part. .

[0058]

Industrial Applicability According to the present invention, a high-ductility, high-toughness, high-tensile steel plate can be produced with high efficiency, and a high-tensile steel plate suitable for polar marine structures, tanks, low temperature pressure vessels, bridges, building materials, etc. can be produced with high efficiency.

Continuation of the front page (56) Reference JP-A-9-41080 (JP, A) JP-A-8-209291 (JP, A) JP-A-8-295982 (JP, A) JP-A-9-176782 (JP , A) JP-A-10-168542 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02

Claims (2)

(57) [Claims] 1. A weight ratio of C: 0.02 to 0.2%, S
i: 0.02-0.5%, Mn: 0.4-2.5%, so
l.Al: 0.08% or less, N: 0.008% or less, C
u: 0-0.6%, Ni: 0-2%, Cr: 0-0.8
%, Mo: 0 to 0.8%, Nb: 0 to 0.05%, V:
0-0.08%, B: 0-0.005% and Ti: 0
.About.0.05% and the balance consisting of Fe and unavoidable impurities, and at least in the surface layer region, ferrite transformed from unrecrystallized austenite and ferrite recrystallized from processed ferrite each account for 40% by volume or more, and Average particle size of ferrite is 3 μm
A high ductility and high toughness steel sheet characterized by being: 2. A manufacturing method in which a steel slab having the chemical composition according to claim 1 is heated to an Ac ₃ point or more and then _three thermo-mechanical treatments in which at least the surface layer region satisfies the following conditions The sum of the rolling reductions in and is 50% or more, and the method for producing a high ductility and high toughness steel sheet according to claim 1, wherein. Cooling is performed so that the volume ratio of the transformation phase at the Ar ₃ point or less becomes 70% or more, and the cooling is stopped midway. If necessary, processing is added during this cooling. After the cooling is stopped, the temperature is reheated to a temperature between the Ac ₁ point and the Ac ₃ point and below the Ac ₁ point or higher. If necessary, processing is performed during this recuperation and during cooling after recuperation. Cool the steel plate. During this cooling, processing is added as necessary in the temperature range of Ac ₁ point to -100 ° C.