JPH07100814B2 - Method for producing steel sheet with excellent brittle crack propagation arresting properties and low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is 400 kg when used at a level of -50 ° C or higher.
The present invention relates to a method for manufacturing a structural steel sheet having excellent brittle crack propagation arresting characteristics of f · mm ^−3/2 or more and a toughness of which the central portion exceeds −100 ° C.

(Prior Art) In a brittle crack that occurs in a steel plate, usually, a main crack is connected to a preceding crack and propagates.

This connection is formed by a ductile fracture called a tear at the grain boundary, at which time the propagation energy of the crack is absorbed by the ductile fracture of the tear.

This absorptivity improves the crack deceleration effect by increasing the chances of ductile fracture due to the refinement of the crystal grain size, and improves the brittle crack propagation stopping property.

In fact, it is the plastic deformation region called shear lip that occurs in the steel sheet surface layer during brittle crack propagation that greatly contributes to the improvement of the brittle crack propagation arresting characteristic. The ability to absorb propagating energy is increased.

Therefore, various attempts have been made to effectively reduce the grain size.

In order to realize this, for example, in Japanese Patent Laid-Open No. 61-235534, as shown in FIGS. 1 (a) and 1 (d), the temperature is _1/3 of the plate thickness from the surface of the slab at the Ac ₃ point or more to the center. Cool to Ar ₃ points or less over a distance of 8 or more, start rolling while keeping the temperature difference in the thickness direction of the slab, and reheat the entire area of the slab thickness to Ac ₃ points or more during or after the rolling. K, which indicates the brittle crack propagation arresting property at -20 ° C by the ESSO test.
ca has proposed a method of manufacturing a steel plate of about 460~960kg f · mm ^-3/2.

However, the Kca value in the above-mentioned JP-A-61-235534 is a value at -20 ° C, and the Kca value at -50 ° C with a constant plate thickness is set as a condition that is not affected by the plate thickness effect. When converted,
About 1 / 2.5 to 1 / 2.7 next to the Kca value of the Kca value -20 ° C. of -50 ° C., Kca value of -50 ° C. is generally becomes 170~380kg f · mm approximately ^-3/2, is desired in the art Kca value of 400kg f ・ mm
^The actual situation is less than ^−3/2 or more.

Further, the proposal of the above-mentioned JP-A-61-235534 discloses that the entire area of the slab is
Since the recuperation time is increased to the Ac ₃ point or higher, the recuperation time becomes long and the productivity decreases, and the thickness range cooled to the Ar ₃ point or lower is small, and therefore the reverse transformation from austenite to ferrite and recrystallization can be used. The region is narrow, the grain refinement and the structure refinement are insufficient, and the shear lip effect is not sufficiently exhibited, which hinders practical use.

Further, as a method for producing a thick steel plate having excellent weldability and having good strength and toughness, there is, for example, a proposal described in Japanese Patent Publication No. Sho 49-7291.

The proposal is a method of refining crystal grains by increasing the number of transformations until reaching the final structure by repeating cooling and heating, but it is limited to refining crystal grains only by simply increasing the number of transformations. However, a good brittle crack propagation arresting property and a good toughness cannot be obtained, and a manufacturing method using a process in which only such temperature control is repeated is poor in both economical efficiency and productivity.

(Problems to be Solved by the Invention) The present invention forms an excellent brittle crack propagation stopping property over the entire steel sheet without the above-mentioned problems of the conventional art,
It is an object of the present invention to provide a method for economically producing structural thick steel sheets having excellent toughness not only in the upper surface layer portion but also in the plate thickness center portion, with high productivity.

(Means for Solving the Problems) In order to achieve the above-mentioned objects, the present invention provides C: 0.01-
0.30%, Si: ≤0.5%, Mn: ≤2.0%, Al: ≤0.1%,
N: 0.001 to 0.01% Further, if necessary, Cr: ≤ 0.5%, Ti: ≤
0.1%, Ni: ≤1.0%, Nb: 0.05%, Mo: ≤0.5%, B:
≤ 0.0015%, V: ≤ 0.1%, Cu: ≤ 0.9%, containing 1 or 2 or more kinds, and other structural steel consisting of Fe and inevitable components corresponding to 2 to 33% of the steel thickness of the steel. At least once, start cooling the upper and lower surface layer regions from a temperature of Ar ₃ points or more at a cooling rate of 2 ° C./sec or more, cool to less than Ar ₃ points, stop the cooling, and reheat. In the course of performing the finishing rolling, finish rolling is performed after cooling the surface layer portion of the steel material until the recuperation is completed, and the upper and lower surface layer portions of the steel sheet after the finish rolling are subjected to Ac
Or Ac ₃ point or more than ₃ points or Ac ₃ point and on the upper and lower temperature range in the manufacturing method of the steel sheet excellent in recovery heat brittle crack propagation stopping characteristics and low-temperature toughness.

The reasons for limiting the above components will be described below.

C is added as a strengthening component of the steel material, and its upper limit is set to prevent deterioration of the toughness of the welded portion.

Si is added for the purpose of deoxidizing and maintaining strength, and an upper limit is set to prevent deterioration of weldability.

Mn is added for the purpose of improving low temperature toughness, and its upper limit is set to prevent weld cracking.

N is added together with Al for the purpose of refining the crystal grains due to the formation of nitrides, and its upper limit is set to prevent deterioration of the toughness of the welded portion.

Cr, Ni, Mo, B and Cu are added for the purpose of improving the hardenability and effectively increasing the strength, suppress the formation of low temperature transformation products, and prevent the decrease of the ferrite area ratio. It has established.

Ti and Nb are added for the purpose of refining the crystal grains, and their upper limits are set to prevent the reduction of the toughness of the weld.

V is added for the purpose of precipitation strengthening, and its upper limit is set from the economical viewpoint.

The structural steel material targeted by the present invention has a thickness of 50 to 400 mm.
Continuously cast high-temperature steel slab, or a steel slab that is once cooled and then reheated, and further subjected to shape-casting rolling or rough rolling after continuous casting, and a high-temperature steel plate with a thickness of 10 to 360 mm, etc. Refers to.

(Operation) The present inventors, by weight%, C: 0.01 to 0.30%, Si: ≤ 0.5
%, Mn: ≤ 0.2%, Al: ≤ 0.1%, N: 0.001 to 0.01%, and if necessary, Cr: ≤ 0.5%, Ti: ≤ 0.1%, Ni: ≤
1.0%, Nb: ≤0.05%, Mo: ≤0.5%, B: ≤0.0015%,
V: ≤0.1%, Cu: ≤0.9%, including one or more of
Other Fe and unavoidable components with a thickness of 10 to 360 mm
The experimental study of various finishing rollings was repeated using the structural steel materials described above.

As a result, as shown in FIGS. 1 (a) and 1 (b), the regions of the upper and lower surface layer portions corresponding to 2 to 33% of the steel material thickness of the steel material were cooled at a cooling rate of 2 ° C./second or more to Ar ₃ points or less. Then, after finishing rolling with the temperature difference between the surface layer portion and the central portion of the steel material is started and the finishing rolling is finished, the region of each upper and lower surface layer portion corresponding to 2 to 33% of the steel material thickness of the steel material is Ac _When reheated to less than ₃ points, since the rolling is rolling in the temperature rising process, ferrite is sufficiently recrystallized and the crystal grains become finer, and not only toughness but also brittle crack propagation stopping property is improved. I knew that.

In addition, as shown in FIGS. 1 (a) and 1 (c), the steel material has a thickness of 2
When the upper and lower surface layer regions corresponding to ~ 33% are reheated to Ac ₃ points or more, the rolling is performed in the reverse transformation process, so that the crystal grains are further refined by the reverse transformation from ferrite to austenite, It was found that not only toughness but also brittle crack propagation arresting property is improved.

At this time, if the reverse transformation by cooling to the Ar ₃ point or less and recuperating to the Ac ₃ or more is performed once or more times while keeping the cooling conditions per one time, the plate thickness and the heating temperature constant, Figures (a), (b)
It was found that the brittle crack propagation arresting property and vTrs are further improved as shown in FIG.

The rolling at this time follows the pattern shown in FIG. 2 in any case, and the rolled material at that time is, as shown in FIG. 3, upper and lower surface layer portions corresponding to 2 to 33% of the steel thickness of the steel. There is a temperature difference between the region and the central part, and the crystal grain size similarly has a difference between the region and the central part of the upper and lower surface layer portions corresponding to 2 to 33% of the steel thickness of the steel material.

If this is rolled as it is, the rolled material becomes a plate-shaped resistor in which the regions of the upper and lower surface layer portions corresponding to 2 to 33% of the steel thickness of the steel material are formed with large deformation resistance, and the central portion with small deformation resistance is As a result of strong reduction of the plate-like low antibody, it was found that extremely large strain is accumulated in the central part, the ferrite after transformation is further refined, the toughness of the central part is improved, and the center porosity is pressed.

In addition, the thickness range where the surface layer becomes finer has no substantial effect such as 2% or more, and when it is 33% or more, the sensible heat of the steel itself disappears and the temperature at the center of the plate thickness falls too much and the toughness deteriorates. I found that I would do it.

At this time, in the steel sheet not containing Ti and Nb, the ferrite having a grain size of 5 μm or less reaches the area ratio of 50% or more of the surface layer portion, and in the steel sheet containing Ti and Nb, the ferrite having a grain size of 3 μm or less is used. Has reached an area ratio of 50% or more in the surface layer, and both have Kca (-
It was found that a brittle crack propagation arresting property of ≧ 400 kg f · mm ^−3/2 was exhibited at 50 ° C.

Also, after finishing rolling in the recuperative process, the temperature at that time is 5 to
It was found that holding for 300 seconds randomizes the orientation of the refined crystals and further improves the brittle crack propagation arresting property.

Furthermore, the steel plate that has undergone this finishing rolling and has undergone recuperation is
When forced cooling to 650 ° C or lower at a cooling rate of ℃ / sec or less,
It has been found that the base material strength and toughness are improved.

The present invention has been made to achieve the above-mentioned problems based on these findings.

(Examples) 1. Steel under test: Shown in Table 1.

2. Rolling conditions: shown in Table 2.

3. Cooling conditions: shown in Table 2.

4. Cooling stop condition: shown in Table 2.

5. Reheat condition: shown in Table 2.

6. Controlled cooling conditions after hot rolling: shown in Table 2.

7. Brittle crack propagation stopping properties: shown in Table 3.

8. Toughness and other properties: shown in Table 3.

The brittle crack propagation arresting property is the ESSO test value (Kca), and the toughness is the fracture surface transition temperature (vTrs) in the Charpy test.
The pressure of center porosity was evaluated by the reduction value (R _AZ ) by the Z-direction tensile test.

Steel Nos. A-1 to A-23 of the present invention have a Kca (-40 ° C) of 40.
Far superior to the conventional example and 5~1200kg f · mm ^-3/2,
The center of the steel sheet in the thickness direction, that is, vTrs at the 1 / 2t portion is -100 to-
160 ° C., R _AZ of 1 / 2t showed excellent characteristics as 72-85%.

On the other hand, in the steel numbers B-1 to B-19 of the comparative example, Kca (-50 ° C) is
102~385kg f · mm ^-3/2 and a conventional par, 1 / 2tvTrs is -50~-90 ℃, R _AZ of 1 / 2t is was only 43-53%.

(Effects of the invention) The present invention performs rolling in parallel with ferrite recrystallization during heating after one or more reverse transformations, or recrystallization of ferrite during heating after one or more reverse transformations. Since the rolling is performed during the substantial transformation from ferrite to austenite, the structure is refined over the entire area of the steel sheet, the shear lip effect of the steel sheet surface layer portion is improved, and one of the problems is that the brittle crack propagation arresting property is Kca (-50
℃) ≧ 400 kg f ・ mm ^−3/2, and since the strong reduction effectively acts on the central part of the steel plate thickness direction, the toughness of the central part of the plate thickness, which is another issue, is vTrs ≦ −100. (° C), the productivity and economy of this steel sheet are high, and the effect of producing and using this type of steel sheet is extremely large.

[Brief description of drawings]

FIG. 1 (a) shows the positions of the slab and the steel plate in the thickness direction regulated by the present invention. Heat temperature relationship,
(D) is a diagram showing the relationship between the above-mentioned position and cooling / recuperation temperature in the conventional example (Japanese Patent Laid-Open No. 61-235534), FIG. 2 is the rolling pattern of the present invention, and FIG. 3 is the rolling of the present invention. Fig. 4 (a) is a diagram comparing the temperature relationship between the upper and lower surface layers of the material to be rolled and the central part in comparison with the conventional example. The chart of the relationship between the number of times and the toughness (fracture transition temperature vTrs in the Charpy test) is shown.

Claims (4)

[Claims] 1. By weight%, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Structural steel comprising other Fe and unavoidable components. Area of the upper and lower surface layers corresponding to 2 to 33% of the steel thickness of
Cooling is started at a cooling rate of 2 ° C / sec or higher from a temperature of Ar ₃ points or higher, and is stopped at the Ar ₃ point or lower, and the cooling is stopped and reheated. perform finish rolling until recuperation is completed after, the upper and lower surface areas of the steel sheet after the completion of rolling the partition at or Ac Ac ₃ point or more than ₃ points or Ac ₃ point and on the upper and lower temperature range A method for producing a steel sheet excellent in brittle crack propagation arresting characteristics and low temperature toughness, which is characterized by recovering heat. 2. In wt%, Cr: ≦ 0.5% Ti: ≦ 0.1% Ni: ≦ 1.0% Nb: ≦ 0.05% Mo: ≦ 0.5% B: ≦ 0.0015% V: ≦ 0.1% Cu: ≦ 0.9% The method for producing a steel sheet having excellent brittle crack propagation arresting properties and low temperature toughness according to claim 1, characterized in that the steel sheet contains one or more types. 3. The method for producing a steel sheet having excellent brittle crack propagation arresting properties and low temperature toughness according to claim 1 or 2, wherein the steel sheet temperature is maintained for 5 to 300 seconds after finishing rolling. 4. The steel sheet after completion of recuperation is cooled to 650 ° C. or less at a cooling rate of 40 ° C./second or less.
Alternatively, the method for producing a steel sheet having excellent brittle crack propagation arresting properties and low-temperature toughness as described in 3 above.