JPH075969B2 - 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 kgf when used at a temperature of -50 ° C or higher.
The present invention relates to a method for producing a structural steel sheet having excellent brittle crack propagation arresting characteristics of 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 sheet, a main crack is usually connected to a preceding crack and propagates.

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

This absorptive capacity is increased by increasing the chances of the ductile fracture due to the refinement of the crystal grain size, improving the crack deceleration effect, and improving the brittle crack propagation stopping property.

In fact, the major contribution to the improvement of the brittle crack propagation arresting property is the plastic deformation that occurs in the surface layer of the steel sheet during brittle crack propagation and the plastic deformation that occurs. 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), a slab having a temperature of Ac ₃ point or more, the thickness of the slab from the surface to the center is measured. Cooled to less than Ar ₃ points over a distance of 1/8 or more, and started free rolling with a temperature difference in the thickness direction of the slab, and during the rolling or after rolling, the entire slab thickness was Ac ₃ points or more. Kc, which represents the brittle crack propagation arresting property at -20 ° C by ESSO test
a is proposed a method of manufacturing a steel plate of about 460~960kgf · 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 170~380kgf · mm approximately ^-3/2, is a need in the art do not meet the Kca value 400kgf · mm ^-3/2 more than you are is the actual situation.

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 longer than Ac ₃ point, the recuperation time is long and the productivity is low, and the range cooled to Ar ₃ point or less is small, and therefore the region where the reverse transformation from austenite to ferrite and recrystallization can be used. Is narrow, the crystal grains are finer,
The micronization of the structure is insufficient, and the shear lip effect is not sufficiently exerted, which is an obstacle to practical use.

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

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

<Problems to be Solved by the Invention> The present invention forms excellent brittle crack propagation stopping characteristics over the entire steel sheet without the above-mentioned problems of the conventional art, and the toughness of the central portion of the upper sheet thickness is particularly high. An object of the present invention is to provide a method for manufacturing an excellent structural thick steel plate with good productivity and economically.

<Means for Solving the Problems> In order to achieve the above-mentioned problems, the present invention provides (1) in% by weight, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 ~ 0.01% Other cast iron obtained by continuous casting of structural rolled steel consisting of Fe and unavoidable components, from the temperature of Ac ₃ points or more, cooling rate 2 ℃ /
Start cooling in sec or more, cool the surface layer portion from the front and back surfaces of the slab to about 1/8 of the thickness to Ar ₃ points or less, and stop the cooling,
Finishing rolling is started until the reheating of the slab is completed after the cooling and recuperating heat is passed twice or more, and after the finishing rolling is finished, from the front and back surfaces of the slab to about 1/8 of the thickness. As a first method, a steel sheet having excellent brittle crack propagation arresting characteristics and low-temperature toughness, which is characterized by recovering heat to less than Ac ₃ point, is used. (2) C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Furthermore, Cr: ≤ 0.5% Ti: ≤ 0.1% Ni: ≤ 1.0% Nb: ≤ 0.05% Mo: ≤ 0.5% B: ≤ 0.0015 % V: ≤ 0.1% Cu: ≤ 0.9% One or more kinds, and a slab obtained by continuous casting of structural rolled steel composed of other Fe and unavoidable components from a temperature of Ac ₃ points or higher. Start cooling at a cooling rate of 2 ° C / sec or more,
The surface layer from the front and back surfaces of the slab to about ⅛ of the thickness is cooled to Ar ₃ point or less, the cooling is stopped, and the reheating of the slab is recovered by passing the cooling and the recuperation twice or more. Finish rolling is started until it is finished, and after finishing rolling, the thickness of
The second method is to manufacture a steel sheet with excellent brittle crack propagation arresting characteristics and low temperature toughness, which is characterized by recovering heat up to about 1/8 to less than the Ac ₃ point. (3) C: 0.01% by weight ~ 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Other than Fe, the slab obtained by continuous casting of structural rolled steel consisting of inevitable components has Ac ₃ points or more. Cooling rate 2 ℃ /
Start cooling in sec or more, cool the surface layer portion from the front and back surfaces of the slab to about 1/8 of the thickness to Ar ₃ points or less, and stop the cooling,
Finishing rolling is started until the reheating of the slab is completed after the cooling and recuperating heat is passed twice or more, and after the finishing rolling is finished, from the front and back surfaces of the slab to about 1/8 of the thickness. The third method is the method for producing a steel sheet excellent in brittle crack propagation arresting characteristics and low temperature toughness, which is characterized by recovering heat of Ac to ₃ points or more, and (4)% by weight, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Furthermore, Cr: ≤ 0.5% Ti: ≤ 0.1% Ni: ≤ 1.0% Nb: ≤ 0.05% Mo: ≤ 0.5% B: ≤ 0.0015 % V: ≤ 0.1% Cu: ≤ 0.9% One or more kinds, and a slab obtained by continuous casting of structural rolled steel composed of other Fe and unavoidable components from a temperature of Ac ₃ points or higher. Start cooling at a cooling rate of 2 ° C / sec or more,
The surface layer from the front and back surfaces of the slab to about ⅛ of the thickness is cooled to Ar ₃ point or less, the cooling is stopped, and the reheating of the slab is recovered by passing the cooling and the recuperation twice or more. Finish rolling is started until it is finished, and after finishing rolling, the thickness of
The method for producing a steel sheet excellent in brittle crack propagation arresting characteristics and low temperature toughness, which is characterized by recovering the heat up to about 1/8 to Ac ₃ point or more, is the fourth means, and (5) described in the above means 1 to 4. After the completion of rolling during the recuperation process, the steel sheet temperature is maintained for 5 to 300 seconds, and a method for producing a steel sheet excellent in brittle crack propagation arresting characteristics and low temperature toughness is defined as fifth to eighth means, 6) The steel plate after the recuperation described in the above 1 to 8 is 40 ° C.
A ninth to sixteenth means is a method for producing a steel sheet excellent in brittle crack propagation arresting characteristics and low temperature toughness, which is characterized by cooling to 650 ° C. or less at a cooling rate of not more than / sec.

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 an upper limit is set to prevent weld cracking.

N is added together with Al for the purpose of refining 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 all added for the purpose of improving hardenability and effectively increasing strength, suppressing the formation of low-temperature transformation products, and preventing the ferrite area ratio from decreasing. It has established.

Ti and Nb are added for the purpose of refining the crystal grains, and the upper limit is set in order to prevent the toughness reduction of the weld.

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

Further, the structural rolled steel slab targeted by the present invention is a continuous high temperature high temperature steel slab, and a steel slab that has been once cooled and then reheated, and further a continuous casting sill shaped steel slab, and a continuous cast. It refers to a steel slab or the like that has undergone rolling such as post-shape adjustment rolling.

Further, the cooling in the present invention can be sufficiently performed by using a water cooling means having a cooling rate of 2 ° C./sec or more which is usually used in continuous casting.

<Function> In order to achieve the above-mentioned object, the present inventors have achieved, in weight%, C: 0.01 to 0.30% Si: ≦ 0.5% Mn: ≦ 2.0% Al: ≦ 0.1% N: 0.001 to 0.01% and 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 1 or 2 or more Fe And various experimental investigations were repeated using a slab obtained by continuous casting of structural rolled steel composed of unavoidable components.

As a result, as shown in FIGS. 1 (a) and 1 (b), the steel sheet has a thickness of 1/8 of the thickness t from the front and back surfaces of the slab (hereinafter referred to as 1/8).
(Noted as t) is cooled at a cooling rate of 2 ° C./sec or more to ₃ points or less of Ar, and the surface finish and the center of the slab are subjected to a temperature finish rolling, and the finish rolling is finished. After that, when the surface layer portion from the front and back surfaces of the slab to about 1/8 t is reheated to less than the Ac ₃ point, the rolling is performed in the temperature rising process, so that the ferrite is sufficiently recrystallized to form crystal grains. We have found that the grain size is reduced and the brittle crack propagation arresting property is improved.

In addition, 1 / 8t from the front and back of the slab, as in No. 1 (a) and (c)
When the temperature is reheated up to the Ac ₃ point or higher, since the rolling is performed in the reverse transformation process, the grains are further refined by the reverse transformation from ferrite to austenite, and the brittle crack propagation arresting property is improved. I got it.

At this time, the reverse transformation by cooling to below the Ar ₃ point and recuperating to above Ac ₃ is carried out two or more times under constant cooling conditions, plate thickness and heating temperature. As shown in (a) and (b), it has been found that the brittle crack propagation stopping property and vTrs are further improved.

The rolling at this time follows the pattern shown in FIG. 2 in any case, and the rolled material at that time has a temperature difference between the surface layer portion and the central portion of about 1/8 t from the surface as shown in FIG. Similarly, the crystal grain size also has a difference of about 1/8 t between the surface layer part and the center part,
If this is rolled as it is, the rolled material will be
The surface layer up to about 8t becomes a plate-shaped resistor formed with a large deformation resistance, and as a result of the plate-shaped resistor strongly pressing down the central part with a small deformation resistance, an extremely large strain is accumulated in the central part and It has been found that the ferrite of 1 is further miniaturized to improve the toughness of the central portion and to press the center porosity.

At this time, in the steel sheet containing no Ti and Nb, the ferrite having a crystal grain size of 5 μm or less reached 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 crystal grain size of 3 μm or less. Has reached the surface area ratio of 50% or more, and both have kca (−50
℃) it was found that exhibit stable brittle crack propagation stop characteristics and ≧ 400kgf · mm ^-3/2.

Further, it was found that if the temperature at that time is kept for 5 to 300 seconds after the completion of rolling, the orientation of the refined crystals becomes random and the brittle crack propagation stopping property is further improved.

Furthermore, these steel sheets have a cooling rate of 40 ° C / sec or less and 650 ° C.
It was found that the base material strength and toughness are improved by forced cooling to the following.

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

<Example> 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 stopping property is the ESSO test value (Kca), the toughness is the fracture surface transition temperature (vTrs) in the Charpy test,
The crimping of the 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 (−50 ° C.) of 405.
Far superior to the conventional example and ~660kgf · mm ^-3/2, the thickness direction center of the steel sheet, that is -100 to-160 ° C. is vTrs of 1 / 2t,
1/2 t R _AZ showed excellent characteristics of 72-85%.

On the other hand, in steel numbers B-1 to B-19 of the comparative example, Kca (-50 ° C) is 2
10~385kgf · mm ^-3/2 and is a conventional par, 1 / 2tvTrs -50
~-90 ℃, R _AZ of 1 / 2t is was only 43-53%.

<Effect of the Invention> In the present invention, since rolling is performed in parallel with recrystallization during temperature increase after two or more reverse transformations, or rolling is performed during two or more reverse transformations after reverse transformation, the steel sheet The organization has become finer over the entire area of
Improved shear lip effect of the steel sheet surface layer portion, one of the brittle crack propagation stop characteristics challenges Kca (-50 ℃) ≧ 400kgf · mm -3/2
In addition, 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
Excellent with vTrs ≤ -100 (℃), and the productivity of this steel sheet,
The economy is high, and the effect of producing and using this kind of steel sheet is extremely large.

[Brief description of drawings]

FIG. 1 (a) shows the positions in the thickness direction of the slab and the steel plate regulated by the present invention, and (b) shows the relationship between the position and the cooling / recuperation temperature in the present invention example of claims 1 and 2. Indicates (c)
Shows the relationship between the above-mentioned position and the cooling / recuperation temperature in the examples of the present invention of claims 3 and 4, and (d) shows the conventional example (JP-A-61-23).
The relationship between the above-mentioned position and the cooling / recuperation temperature in 5534 publication) is shown. FIG. 2 shows a rolling pattern of the present invention, FIG. 3 shows a temperature relationship between a surface layer and a central portion of a rolled material of the present invention in comparison with a conventional example, and FIG. 4 (a) shows The relationship between the number of reverse transformations and the brittle crack propagation stopping property is shown, and (b) shows the relationship between the number of reverse transformations and toughness (fracture transition temperature in Charpy test).

Claims (6)

[Claims] 1. C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Continuous structural rolled steel consisting of other Fe and unavoidable components in weight% The slab obtained by casting is cooled at a temperature of Ac ₃ or higher and the cooling rate is 2 ° C /
Start cooling in sec or more, cool the surface layer portion from the front and back surfaces of the slab to about 1/8 of the thickness to Ar ₃ points or less, and stop the cooling,
Finishing rolling is started until the reheating of the slab is completed after the cooling and recuperating heat is passed twice or more, and after the finishing rolling is finished, from the front and back surfaces of the slab to about 1/8 of the thickness. A method for manufacturing a steel sheet having excellent brittle crack propagation arresting characteristics and low-temperature toughness, which is characterized by recovering the heat to less than Ac ₃ point. 2. In% by weight, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Further, Cr: ≤ 0.5% Ti: ≤ 0.1% Ni: ≤1.0% Nb: ≤0.05% Mo: ≤0.5% B: ≤0.0015% V: ≤0.1% Cu: ≤0.9% One or more types of structural rolled steel containing Fe and inevitable components The slab obtained by continuously casting is started to be cooled at a cooling rate of 2 ° C./sec or more from a temperature of Ac ₃ point or more,
The surface layer from the front and back surfaces of the slab to about ⅛ of the thickness is cooled to Ar ₃ point or less, the cooling is stopped, and the reheating of the slab is recovered by passing the cooling and the recuperation twice or more. Finish rolling is started until it is finished, and after finishing rolling, the thickness of
A method for producing a steel sheet having excellent brittle crack propagation arresting characteristics and low temperature toughness, characterized by recovering heat up to about 1/8 to less than Ac ₃ point. 3. By weight%, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% A continuous structural rolled steel composed of other Fe and inevitable components. The slab obtained by casting is cooled at a temperature of Ac ₃ or higher and the cooling rate is 2 ° C /
Start cooling in sec or more, cool the surface layer portion from the front and back surfaces of the slab to about 1/8 of the thickness to Ar ₃ points or less, and stop the cooling,
Finishing rolling is started until the reheating of the slab is completed after the cooling and recuperating heat is passed twice or more, and after the finishing rolling is finished, from the front and back surfaces of the slab to about 1/8 of the thickness. A method for producing a steel sheet having excellent brittle crack propagation arresting characteristics and low-temperature toughness, which is characterized by recovering heat of Ac to ₃ points or more. 4. By weight%, C: 0.01 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% N: 0.001 to 0.01% Further, Cr: ≤ 0.5% Ti: ≤ 0.1% Ni: ≤1.0% Nb: ≤0.05% Mo: ≤0.5% B: ≤0.0015% V: ≤0.1% Cu: ≤0.9% One or more types of structural rolled steel containing Fe and inevitable components The slab obtained by continuously casting is started to be cooled at a cooling rate of 2 ° C./sec or more from a temperature of Ac ₃ point or more,
The surface layer from the front and back surfaces of the slab to about ⅛ of the thickness is cooled to Ar ₃ point or less, the cooling is stopped, and the reheating of the slab is recovered by passing the cooling and the recuperation twice or more. Finish rolling is started until it is finished, and after finishing rolling, the thickness of
A method for producing a steel sheet having excellent brittle crack propagation arresting characteristics and low-temperature toughness, which is characterized by recovering the temperature of about 1/8 to Ac ₃ or higher. 5. A brittle crack propagation arresting property and low temperature toughness, characterized in that the steel sheet temperature is maintained for 5 to 300 seconds after the completion of rolling during the recuperation process according to any one of claims 1 to 4. Excellent steel sheet manufacturing method. 6. A brittle crack propagation characterized by cooling the steel sheet after completion of recuperation according to claim 1 to 650 ° C. or less at a cooling rate of 40 ° C./sec or less. A method for manufacturing a steel sheet having excellent stopping properties and low temperature toughness.