JP2002020835A - Steel excellent in brittle crack propagation stopping characteristics and rupture characteristics in sheet thickness and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel improved in arrest performance while securing rupture characteristics in the sheet thickness direction by performing the modification of the structure at least in the surface layer of the steel and to provide its production method. SOLUTION: In this steel excellent in brittle crack propagation stopping characteristics and rupture characteristics in the sheet thickness direction, as to steel having prescribed components, in a region of at least >=5% of the sheet thickness from the surface layer, a texture colony with the average minor axis grain size of 2 to 5 μm containing equi-axed ferritic grains with the average grain size of <=3 μm in >=20% by area ratio and further consisting essentially of elongated ferritic grains is present, and also, the (100) face intensity ratio is controlled to 1.5 to 4.0. In its production method, a slab heated at Ac3 to 1,200 deg.C is cooled at a cooling rate of >=2 deg.C/s till the temperature of the surface layer region of at least >=5% of the slab thickness in the central direction of the slab thickness from the surface reaches <=Ar3, and after that, in a process in which the surface layer region is recuperated, rolling in which the cumulative draft in the temperature region of Ac1 to Ac1+70 deg.C is controlled to 30 to 70%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel material having excellent crack propagation stopping characteristics when brittle fracture occurs, and a method for producing the same.

[0002]

2. Description of the Related Art With the recent increase in the size of steel structures, requirements for steel materials to be used have become more severe. For this reason, it is strongly desired to have brittle crack propagation arrest characteristics together with strength and toughness, and various technologies have been proposed, but in reality, they have not always met expectations. As means for improving the arrest property, as described in JP-A-59-47323, etc., a production method in which the material is sufficiently reduced in an unrecrystallized region, or a second-phase particle which is apt to cause brittle fracture positively Disperse finely and generate many micro cracks at the brittle crack tip,
A method has been proposed in which the state of stress at the tip of a crack is alleviated, and the termination of the crack is facilitated by ductile fracture that occurs when the microcrack and the main crack are united.

This technique is to secure arrest characteristics by modifying the structure over the entire thickness. However, although there is no description about the applicable plate thickness, since the structure is made of fine ferrite (α) and martensite (M),
It is presumed that considerable low-temperature strong-pressure and strong water-cooling is necessary, and there is naturally an upper limit of the sheet thickness. That is, it is difficult to secure the fine M while minimizing [alpha] at the center of the thickness of the thick material with the current equipment, and there is a limit to the improvement of the arrest performance.

Japanese Patent Application Laid-Open No. 06-88161 discloses that
Improvement of arrestability due to development of texture is described, in which the average minor axis diameter of the texture colonies is 5 μm or less, and the (100) plane strength ratio of the texture parallel to the rolling surface is 1.5 or more. I have. According to this method, while exhibiting good arrest performance against brittle cracks in the direction perpendicular to the rolling direction (width direction), the thickness direction is the same as in the beam-column joints of building materials. Arrest performance when brittle cracks parallel to the rolling surface are generated by applying a large force to the rolling surface. Therefore, the application site is naturally limited.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to improve the arrest performance while ensuring the fracture characteristics in the thickness direction by modifying the structure of at least the surface layer region of the steel material.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a steel material excellent in brittle crack propagation arresting property and breaking property in the thickness direction, which can advantageously eliminate the above-mentioned conventional disadvantages.
The summary is as follows. (1) In mass%, C: 0.02-0.2%, Si: 0.01-1.0%, Mn: 0.30-2.0%, Al: 0.002-0.10% , N: 0.001 to 0.01%, P: 0.01% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities. % Or more, equiaxed ferrite grains having an average grain size of 3 μm or less have an area ratio of 20% or more, and further, textured colonies mainly composed of elongated ferrite grains and having an average short axis grain size of 2 to 5 μm. And a steel material having a (100) plane strength ratio of 1.5 to 4.0 and excellent in brittle crack propagation arrestability and thickness direction fracture characteristics.

(2) In mass%, Cu: 0.01 to 2.0%, Ni: 0.01 to 10.0%, Cr: 0.01 to 10.0%, Mo: 0.01 to The steel material according to the above (1), which has one or more of 3.0% and B: 0.0002 to 0.0030%, and has excellent brittle crack propagation arrestability and thickness direction fracture characteristics. (3) In mass%, one or more of Ti: 0.002 to 0.10%, Nb: 0.002 to 0.05%, V: 0.005 to 0.20% The steel material according to the above (1) or (2), which has excellent brittle crack propagation arrestability and thickness direction fracture characteristics. (4) Mg: 0.0004-0.0% by mass.
The steel material according to any one of the above (1) to (3), which contains 1% and has excellent brittle crack propagation arrestability and thickness direction fracture characteristics. (5) Rem: 0.002 to 0.1% by mass.
The brittle crack propagation arresting property and the thickness direction fracture property according to any one of the above (1) to (4), containing one or two of 0% and Ca: 0.0002 to 0.0030%. Excellent steel material.

(6) Any one of the above (1) to (5)
After heating the steel slab comprising the components described in the above section to Ac3 to 1200 DEG C., the surface area of at least 5% or more of the steel slab thickness from the surface to the center of the steel slab thickness becomes 2 DEG C./sec.
After cooling at a cooling rate of s or more, in the process of reheating the surface layer region, the brittle crack propagation arresting property of performing rolling at which the cumulative draft in the temperature range of Ac1 to Ac1 + 70 ° C. becomes 30 to 70% is obtained. A method for producing steel with excellent fracture characteristics in the thickness direction. (7) The method for producing a steel material having excellent brittle crack propagation arrestability and thickness direction fracture characteristics according to (6), wherein after the rolling is completed, accelerated cooling is performed at a cooling rate of 2 ° C./s or more on average in the thickness of the sheet. . (8) The method for producing a steel material having excellent brittle crack propagation arrestability and thickness direction fracture characteristics according to the above (7), wherein tempering is performed at a temperature of Ac1 or less after the completion of the accelerated cooling.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. As shown in the above-mentioned Japanese Patent Application Laid-Open No. 06-88161, in a steel material with developed texture, a crack called separation is generated parallel to the rolled surface, so that the stress state at the tip of the brittle crack is alleviated, and the propagation of the crack is reduced. Can be suppressed. This separation consists of (100) and (11)
1) When stress is applied to a steel with a developed texture, the corresponding strain varies depending on the crystal orientation.
It is known that a shift occurs at the interface between the (100) and (111) textured colonies, and is generated as a result of the occurrence of crack buds.

[0010] However, if the texture is excessively developed, the propagation resistance can hardly be obtained for a crack parallel to the rolling surface, such as when stress is applied in the thickness direction. As described above, the arrest performance and the fracture property in the thickness direction were incompatible with each other, so that the use of the steel material having the high arrest performance was extremely limited.

In order to solve this problem, the inventors of the present invention have intensively studied through experiments the necessary structure for achieving both the arrest performance and the breaking property in the thickness direction. As a result, it was clarified that the mixed grain structure of the steel surface layer was effective. In other words, the arrest performance is ensured by elongated α grains (a colony having essentially the same crystal orientation) with a moderately developed texture, and a certain amount or more of equiaxed fine α grains with random crystal orientations are mixed. This significantly improves the breaking characteristics when a force is applied in the thickness direction.

Here, the arrest performance can be evaluated by a conventional ESSO test, but there is no appropriate evaluation method for the fracture characteristics in the thickness direction. Therefore, the present inventors have devised a wide-width tensile test simulating a beam-column joint of a building column material as shown in FIG. That is, a test steel 1 serving as a column material is erected at the center, a backing metal 3 is placed on both sides thereof, and a plate 2 corresponding to a beam flange is welded. Reference numeral 4 denotes a weld. The backing metal 3 is entirely fillet welded to the beam flange 2. 5 is the weld. As the beam flange 2, a plate having the same strength as the test steel 1 is used.

The test temperature is set at 0 ° C., and a tensile stress is applied in the direction of the thick arrow shown in FIG. Since there is a notch between the backing metal 3 and the test steel 1, stress concentrates there and a crack occurs. The stress (σ) was calculated from the load at the time of fracture and the beam cross-sectional area, and the value (σ) obtained by dividing by the tensile strength TS obtained from a normal tensile test (test piece taken at right angles to the rolling direction) of the test steel
/ TS) to evaluate the fracture characteristics in the thickness direction. If σ / TS is close to 1, there is no problem because the yield of the beam occurs first, but if σ / TS is 0.7 or less, the column will break before the beam yields, and Cannot be applied.

Producing steel materials having various surface layer structures,
A wide tensile test and a temperature gradient type ESSO test were performed. E
The result of the SSO test was Kca at 0 ° C (Kca (0 ° C)
). The test results will be described with reference to FIGS. FIG. 1 shows the (100) plane intensity ratio and σ / TS, Kca (0
° C). If the (100) plane strength ratio is less than 1.5, Kca (0 ° C.) is low and sufficient arrestability cannot be secured. On the other hand, when the (100) plane strength ratio exceeds 4.0, extremely low stress (σ / TS is in the range of 0.
4) and brittle fracture. Therefore, in order to achieve both arrestability and fracture characteristics in the thickness direction, the (100) plane strength ratio needs to be 1.5 to 4.0.

FIG. 2 shows the minor axis diameter and σ /
The relationship between TS and Kca (0 ° C.) is also required. In order to achieve both arrestability and fracture properties in the thickness direction, the minor axis diameter must be 2 to 5 μm.
It is necessary to Generally, the minor axis diameter of a textured colony has some correlation with the degree of texture development (surface intensity measured by X-ray diffraction) and does not independently change. However, when the minor axis diameter is less than 2 μm, the plastic deformation ability of α is remarkably reduced by dislocations introduced during the texture formation process, so that separation caused by stress in the thickness direction easily causes brittle cracks. And the fracture characteristics in the thickness direction are reduced. On the other hand, the minor axis diameter is 5μ.
If it exceeds m, a texture is not substantially formed, and the basic unit of brittle fracture is increased, so that resistance to cracks is reduced and arrestability is deteriorated.

3 and 4 show the equiaxed α particle size, the equiaxed α area ratio and the σ
/ TS, it is necessary that the grain size is 3 μm or less and the area ratio is 20% or more in order to satisfy the breaking characteristics in the thickness direction. This is because the equiaxed fine α grains having various crystal orientations existing between the (100) and (111) texture colonies alleviate local plastic deformation when a force is applied in the thickness direction. This is presumably because the generation of brittle crack buds caused by the plastic anisotropy of the texture is suppressed.

As described above, the (100) plane strength ratio is set to 1.5 to
4.0, the average minor axis diameter of the textured colonies is 2 to 5 μm,
In addition, it has been found that the equiaxed α area ratio having an average particle diameter of 3 μm or less needs to be 20% or more.

Next, the required thickness of the surface layer structure will be described. What actually has a great influence on the brittle crack arrest is the structure state of the surface layer portion of the sheet thickness. This is because the surface layer of the plate shows plastic deformation called shear lip during the propagation of a brittle crack, and absorbs the kinetic energy of the brittle crack that propagates. Shows excellent arrestability due to the formation of Therefore, as a result of investigating the shear lip formation behavior by changing the thickness of the surface layer structure, it was found that a shear lip was formed if the thickness was 5% or more of the plate thickness. It is not necessary to particularly define the upper limit of the thickness of the surface layer structure.

The manufacturing method and principle of the steel of the present invention are as follows. In the process of cooling the surface layer portion of the billet in the temperature range of Ac3 to 1200 ° C. until it becomes Ar3 or less and reheating the surface layer region, the cumulative rolling reduction in the temperature range of Ac1 to Ac1 + 70 ° C. is 30 to 70%. Is applied. This stabilizes α even at high temperatures compared to the cooling process. In addition, since it is processed in the presence of a small amount of austenite, it is moderately recovered and recrystallized by strain concentration on α and an increase in the γ / α interfacial area. Progresses. As a result, the predetermined size,
A mixed grain structure of the fractional extension α and the equiaxed α is achieved, and a predetermined texture is obtained.

Hereinafter, the reasons for limiting the manufacturing conditions will be described in detail. In the present invention, the initial temperature of the billet or steel plate is set to Ac3 to
The temperature was set to 1200 ° C. This is because when the slab is heated, if the temperature is lower than Ac3, the solution is not sufficiently formed, and
If the temperature exceeds 0 ° C., the heated γ particle size becomes extremely coarse, and it may be difficult to reduce α in the subsequent rolling. This billet may be as cast as long as it is in a γ single phase state. A heated slab or a hot slab may be subjected to recrystallization zone rolling in a γ high temperature range for the purpose of refining γ, but may be left unrolled.

The subsequent rolling from the cooling to the recuperation stage is the most important part of the present invention. That is, after cooling at a cooling rate of 2 ° C./s or more until a region of at least 5% or more of the slab thickness from the slab surface becomes Ar 3 or less,
In the recuperation process, rolling is performed so that the cumulative draft in the temperature range of Ac1 to Ac1 +70 DEG C. becomes 30 to 70%.

When the cooling stop temperature is higher than Ar3, ferrite is not generated even in the surface layer, so that a normal two-phase region rolling structure is obtained, and a fraction of equiaxed ferrite of 20% or more cannot be ensured. The destruction characteristics deteriorate. If the cooling rate is less than 2 ° C./s, the temperature difference between the surface layer and the inside becomes small, and it becomes impossible to recover the temperature to the temperature range of Ac1 or more necessary for α recovery and recrystallization. In addition to the possibility of securing 20% or more, the short axis diameter of the textured colony is less than 2 μm or the (100) plane strength ratio exceeds 4.0, and the fracture characteristics in the thickness direction deteriorate. I will.

The reason that the surface cooling area is defined as 5% or more is as follows.
This is because the portion that has become Ar3 during cooling finally undergoes the recuperation / rolling process to finally reach the microstructure specified by the present invention.
In rolling in the recuperation process, reduction of Ac1 to Ac1 + 70 ° C. is essentially important. If the reduction is less than 30%, the minor axis diameter of the textured colonies exceeds 5 μm or the (100) plane strength ratio is less than 1.5. And the arrest characteristics are not improved. On the other hand, when the rolling reduction exceeds 70%, the short axis diameter becomes less than 2 μm or the (100) plane strength ratio becomes more than 4.0 due to excessive texture development, and the fracture properties in the thickness direction deteriorate.
Even when rolling at a rolling reduction of 30 to 70%, the rolling temperature is Ac1
If it is less than 10%, austenite does not exist.
Insufficient recrystallization causes the destruction characteristics in the thickness direction to deteriorate as described above.

When the rolling temperature is higher than Ac 1 + 70 ° C., the equiaxed ferrite grains are coarsened to more than 3 μm and simultaneously (1
00) Since it is difficult to secure a surface strength ratio of 1.5 or more, both the arrest characteristics and the thickness direction fracture characteristics are deteriorated.
Although the rolling start temperature may be lower than Ac1,
In order to obtain a cumulative rolling reduction of 30 to 70% at .about.Ac1 + 70.degree.

After the finish rolling is completed, air cooling may be performed.
Water cooling may be performed at a cooling rate of not less than ° C / s. After water cooling, tempering may be performed to a temperature equal to or lower than Ac1. According to the above-described manufacturing method, it is possible to efficiently and reliably provide a steel material having excellent arrestability and thickness-direction fracture characteristics without a complicated step of externally heating during rolling.

Next, the reasons for limiting the components of the present invention will be described. C is inexpensive as an effective component for improving strength.
When added in an amount of at least 0.20%, the toughness of the welded portion is impaired.

Since Si is an inexpensive deoxidizing element and is an effective element for improving the strength, it is added in an amount of 0.01% or more. Impair the properties.

Although Mn is effective as an element for improving the strength and toughness of the base material, a sufficient effect cannot be obtained with a content of less than 0.30%. On the other hand, if added in excess of 2.0%, there is a possibility that weld cracking properties may be promoted.

Al is added as it is effective as a deoxidizing element and is also effective in refining γ grains. If the content is less than 0.002%, there is no effect, and if it exceeds 0.10%, inclusions harmful to the material are generated.

N forms a nitride together with Al and is effective in refining the γ grains. However, if added in an excessive amount, the toughness of the welded portion is impaired, so the content is limited to 0.001 to 0.01%.

Both P and S are set to 0.01% or less in order to secure the toughness of the base material and the fracture characteristics in the thickness direction.

Cu, Ni, Cr, M selectively added
o, B, Ti, Nb, V, Mg, Rem, and Ca are preferably added for the following reasons. Cu, Ni, C
Both r and Mo are known as elements that improve hardenability and can increase the strength of steel.
Excessive addition not only saturates the effect and leads to an increase in cost, but also impairs the weldability.
And Cr are limited to 10.0% or less, and Mo is limited to 3.0% or less. Further, if the addition amount is too small, there is no effect, so the lower limit of the addition amount is set to 0.01% for any element.

B is an element that improves the hardenability and is effective in increasing the strength of steel by its addition. However, excessive addition increases the precipitation of B and impairs the toughness of the steel.
The upper limit of the content is 0.0030%. If the amount is too small, there is no effect, so the lower limit is 0.0002%.
And

Both Ti and Nb function effectively in terms of suppressing coarsening of reheated γ grains and strengthening precipitation during rolling and cooling when added in trace amounts. However, if added in an excessive amount, the toughness of the weld zone deteriorates, so that Ti is 0.10% or less and Nb is 0.0% or less.
5% or less. Also, in both cases, if the added amount is too small, there is no effect, so the lower limit is made 0.002%.

Since V is effective in increasing the strength by precipitation strengthening, V is added in an amount of 0.005% or more, but if added in an amount exceeding 0.20%, the toughness of the welded portion is impaired.

Mg forms a fine oxide in the molten steel stage,
In order to improve the HAZ toughness during large heat input welding, 0.00
Add at least 04%. On the other hand, excessive addition impairs weldability and low-temperature toughness, and is therefore limited to 0.01% or less.

Rem and Ca are effective in detoxifying S. However, if the added amount is small, S remains harmful, and excessive addition impairs toughness.
%, Ca: 0.0002 to 0.0030%.

[0038]

EXAMPLES Table 1 shows the chemical composition of the test steel used in the examples, Table 2 shows the production conditions, and Table 3 shows the surface layer structure and mechanical properties of the test steel. The average particle diameter and area ratio of the equiaxed α were measured using a SEM photograph and an optical micrograph of the cross section of the steel sheet, and the average minor axis diameter of the textured colonies was obtained from the optical micrograph of the structure revealed by the temper color method. It was measured. The (100) plane strength of the texture was determined by an X-ray positive pole figure and an inverse pole figure using a sample taken from the surface layer in parallel with the rolled surface. The fracture characteristic in the thickness direction is a value obtained by dividing the fracture stress in the wide tensile test by the tensile strength of the test steel (σ /
TS), and the arrest performance was represented by a value (Kca (0 ° C.)) at 0 ° C. in a temperature gradient type ESSO test.

Examples Nos. 1 to 8 are examples of the present invention, and exhibit excellent arrest performance and sheet thickness direction fracture characteristics because the components, manufacturing conditions and surface layer structure satisfy predetermined conditions.

On the other hand, Nos. 9 to 16 are comparative examples. Since the underlined portions in the table do not satisfy the range of the present invention, at least one of the arrest performance and the breaking property in the thickness direction is inferior. That is, in Comparative Examples 9 and 15, since the rolling temperature is low or the rolling reduction is excessive, the texture is excessively developed, and the fracture properties in the thickness direction deteriorate. In Comparative Example 10, the arrest performance was inferior because the thickness of the surface layer structure was insufficient. Comparative Example 11
Because the cooling stop temperature is high and the temperature difference in the thickness direction is small,
As in ordinary two-phase zone rolling, the equiaxed α area ratio is small, and the properties in the thickness direction are inferior. In Comparative Examples 12, 13, and 14, the rolling temperature is high, or the rolling reduction in a predetermined temperature range is small, so that the equiaxed α-particle size is large, or the texture is insufficiently developed, and the arrest performance is poor. Inferior. Comparative Example 16 is inferior in both properties because C is out of the range of the present invention.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

According to the present invention, it is possible to efficiently and reliably provide a steel material having excellent arrest characteristics and fracture characteristics in the thickness direction. As a result, it is possible to expand the application of high arrest steel to buildings, bridges, offshore structures, and the like. Further, the present invention can also be applied to a secondary processed product such as a steel pipe or a column using the steel sheet obtained in the present invention, or a shaped steel. Furthermore, industrial effects such as improvement in safety of large steel structures to be applied are extremely large.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a (100) plane strength ratio, an arrest property, and a fracture property in a thickness direction.

FIG. 2 is a graph showing the relationship between the average minor axis diameter of a textured colony, arrest characteristics, and plate thickness direction fracture characteristics.

FIG. 3 is a diagram showing a relationship between an average equiaxed α-particle diameter and a fracture characteristic in a thickness direction.

FIG. 4 is a diagram illustrating a relationship between an equiaxed α area ratio and a fracture characteristic in a thickness direction.

FIG. 5 is a diagram showing the shape and dimensions of a wide tensile test specimen for evaluating the thickness direction fracture characteristics.

Continuing from the front page (72) Inventor Jun Otani Oishi, Oita-shi 1st place in Oita Works Nippon Steel Corporation (72) Inventor Takehiro Inoue 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Division F term (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA12 AA14 AA15 AA16 AA19 AA20 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 AA40 CA01 CA02 CB01 CB02 CC02 CC02 CF01 CF02

Claims (8)

[Claims] C .: 0.02 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.30 to 2.0%, Al: 0.002 to 0. 10%, N: 0.001 to 0.01%, P: 0.01% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities. Textured colonies having at least 5% or more of equiaxed ferrite grains having an average grain size of 3 μm or less in an area ratio of 20% or more, and mainly composed of elongated ferrite grains and having an average short axis grain size of 2 to 5 μm. And a (100) plane strength ratio of 1.5 to 4.0, which is excellent in brittle crack propagation arrestability and thickness direction fracture characteristics. 2. In mass%, Cu: 0.01 to 2.0%, Ni: 0.01 to 10.0%, Cr: 0.01 to 10.0%, Mo: 0.01 to 3% 2. The steel material according to claim 1, wherein the steel material has one or more of B: 0.0002 to 0.0030%. . 3. In mass%, one or more of Ti: 0.002 to 0.10%, Nb: 0.002 to 0.05%, V: 0.005 to 0.20% The steel material according to claim 1 or 2, wherein the steel material has excellent brittle crack propagation arrestability and thickness direction fracture characteristics. 4. The method according to claim 1, further comprising Mg: 0.0004 to 0.01% by mass%.
A steel material excellent in brittle crack propagation arrestability and plate thickness direction fracture characteristics as described in the item. 5. The composition according to claim 1, further comprising one or more of Rem: 0.002 to 0.10% and Ca: 0.0002 to 0.0030% by mass%.
5. A steel material excellent in brittle crack propagation arrestability and thickness direction fracture characteristics according to any one of the above items 4 to 4. 6. A steel slab comprising the components according to claim 1 is heated to Ac3 to 1200 ° C.
After cooling at a cooling rate of 2 ° C./s or more until the surface layer region of at least 5% or more of the slab thickness from the surface to the center of the slab thickness becomes Ar3 or less, in the process of reheating the surface layer region, Ac1 to A method for producing a steel material having excellent brittle crack propagation arrestability and thickness direction fracture characteristics, characterized in that rolling is performed so that the cumulative rolling reduction in the temperature range of Ac1 + 70 ° C is 30 to 70%. 7. After the end of rolling, the average of the thickness is 2 ° C. /
The method according to claim 6, wherein accelerated cooling is performed at a cooling rate of not less than s. 8. The method according to claim 7, wherein tempering is performed at a temperature of Ac1 or less after the completion of the accelerated cooling.