JP3325148B2 - Method for producing thick steel sheet with excellent brittle crack arrestability and low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a structural steel plate which does not require the addition of an expensive alloy element and has a remarkably improved excellent brittle crack propagation stopping characteristic.

[0002]

2. Description of the Related Art As a metallurgical method for improving the brittle crack propagation arresting property of a thick steel plate, the main method is to refine crystal grains and increase the amount of Ni. Increasing the Ni content is a method that can improve brittle crack propagation arrest characteristics regardless of the microstructure, but naturally causes an increase in cost. Therefore, it is preferable to refine the crystal grains by devising a manufacturing method. From the brittle crack propagation and termination behavior of the entire thick steel plate, what actually contributes to the improvement of the brittle crack propagation stop characteristics is the plastic deformation region called the shear lip generated on the surface layer of the thick steel plate when the brittle crack propagates. Is formed, the ability to absorb the propagation energy of the brittle crack increases, and the brittle crack propagation arresting property is dramatically improved. Shear lip formation is also achieved by the refinement of crystal grains.

[0003] Therefore, various attempts have conventionally been made to measure the improvement of the brittle fracture propagation arrestability by refining crystal grains. In general, a method of strengthening controlled rolling in hot rolling or adding Nb to facilitate controlled rolling has been adopted. However, strengthening of controlled rolling causes a decrease in productivity, The addition of Nb tends to cause the toughness of the weld to deteriorate, and these methods cannot be expected to significantly reduce the grain size.
The improvement in brittle crack propagation arrestability obtained is small.

Recently, for example, Japanese Patent Application Laid-Open No. 61-2355
In No. 34, cooling to an Ar ₃ transformation point or less over a distance of 1/8 or more of the plate thickness from the slab surface to the center, rolling is started with a temperature difference in the thickness direction of the slab, By reheating the entire slab thickness to or above the Ac ₃ transformation point during or after rolling, -20 according to the ESSO test was achieved.
Kca representing brittle crack propagation arresting property at ℃ is 460
The ~960kgf · mm ^-3/2 about steel plate has proposed a method of manufacturing.

[0005] However, with the recent tendency for the use environment of structures to be severer, steel materials are required to have a higher specification of stopping brittle fracture propagation, and the characteristics achieved by the above method are not sufficient. Is not the case.
In the method disclosed in JP-A-61-235534, the entire slab is simply reheated to the Ac ₃ transformation point or more, and the α particle size finally obtained by the γ-α transformation is at most 5 μm or less. Therefore, in order to further improve the brittle crack propagation stopping characteristics, a new technology is required.

[0006] More recently, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent No. 41517, there has been proposed a method of remarkably refining the crystal grains in the surface layer by rolling during recuperation after cooling the surface layer, thereby improving the brittle crack propagation stopping characteristics. According to this method, on the average, the surface layer portion becomes ultrafine, and excellent brittle crack propagation stopping characteristics can be obtained even at −50 ° C. due to the formation of a shear lip in the surface layer portion. However, since the ferrite is made ultra-fine by processing and recrystallization of ferrite mainly during recuperation, there is a new problem that the structure and the material are likely to be non-uniform due to minute fluctuations in the heat history.

[0007]

SUMMARY OF THE INVENTION The present invention provides a manufacturing method for obtaining a good brittle crack propagation arresting property as a whole steel sheet by forming a structure having fine crystal grains in a surface layer. In addition, the structure of the surface layer portion can be stably refined, brittle crack propagation arrest characteristics are less changed, and more excellent characteristics can be obtained.

[0008]

According to the present invention, of the prior art, the crystal grains in the surface layer portion are remarkably formed by rolling during cooling while cooling the surface layer portion as disclosed in JP-A-4-141517. In the method of improving the arrestability of brittle crack propagation by finer graining, it is thought that more excellent brittle crack arrestability can be obtained by making the ultrafine grain structure of the surface layer more uniform and finer. As a result, it was found that by making the microstructure before reheating uniform and fine, the ultrafine grain structure of the surface layer finally obtained becomes uniform and fine, and the present invention was completed.

The present invention solves the above-mentioned problems, and the gist of the invention is that C: 0.04 to 100% by weight.
0.30%, Si: ≦ 0.5%, Mn: ≦ 2.0%, A
l: ≦ 0.1%, Ti: 0.003-0.10%, N:
Containing 0.001% to 0.01%, the steel strip and the balance being Fe and inevitable impurities Ac ₃ transformation point or higher, 1100
° C. is performed 10 to 50% of the rolling cumulative rolling reduction at 950 ° C. or less and heated to a temperature, 2 to 33 of plate thickness at that time
% The area of the upper and lower surface portion corresponding to the cooling at 2 ° C. / s or more cooling rate from Ar ₃ transformation point or more of the temperature, to stop the cooling below Ar ₃ transformation point, then further 30% or more
While rolling, the upper and lower surface areas are transformed into Ac ₃
Reheat to more than the point and more than 30%
Reverse-transformed or untransformed austenite during rolling
Is less than 50%, and is a method for producing a thick steel sheet having excellent brittle crack propagation stopping characteristics and low-temperature toughness.

In the present invention, the slab of the above-mentioned composition is heated to a temperature of not less than the Ac ₃ transformation point and not more than 1100 ° C., and is rolled at 950 ° C. or less with a cumulative draft of 10 to 50%.
Upper and lower regions of the surface layer portion is cooled at 2 ° C. / s or more cooling rate from Ar ₃ transformation point or more of the temperature, to stop the cooling below Ar ₃ transformation point corresponding to 2-33% of the plate thickness can ,afterwards
Further re-heats the upper and lower surface areas while rolling.
Then, the upper and lower surface layer areas are cooled.
At least once, re-heat while rolling.
And the rolling reduction after the further rolling is 30% or less.
The reverse or untransformed
A method for producing a thick steel sheet having excellent brittle crack propagation arrestability and low-temperature toughness, characterized in that the fraction of stainite is less than 50% .

[0011] Here, the billet further comprises, by weight%, C
r: ≦ 0.5%, Ni: ≦ 1.0%, Mo: ≦ 0.5
%, V: ≦ 0.1%, Nb: ≦ 0.05%, B: ≦ 0.
0015%, Cu: 1 of ≦ 1.5%, or two or more of
It is also characterized by containing. Continue with the above steps
To a temperature of 650 ° C or less at a cooling rate of 60 ° C / s or less.
Or at a cooling rate of 60 ° C / s or less.
Cool to a temperature of 650 ° C. or lower and then Ac ₃ transformation point
It is also characterized by tempering below.

[0012]

[Function] During hot rolling of a steel slab, during hot rolling or hot rolling, a region of an appropriate thickness of the surface layer is once cooled to a temperature lower than the Ar ₃ transformation point by means of water cooling or the like. After applying a temperature difference from the inside, if hot rolling is further performed from the state where the temperature difference is kept, the surface layer having ferrite-based structure will be processed while being recuperated by the internal sensible heat. However, by optimizing the processing conditions during this reheating, the ferrite crystal grains in the surface layer part are remarkably fine-grained, and the surface layer part is rolled at a lower temperature than the inside, so that the inner part is smaller than the surface part. Compared to rolling a steel slab having a low deformation resistance and a uniform temperature distribution, the working effect is effectively transmitted to the inside, so that the internal transformed microstructure is also refined. As a result, it was found that the low-temperature toughness of the central portion was significantly improved as well as the brittle crack propagation stopping characteristics.

From the above facts, the present inventors have analyzed in detail the relationship between the microstructural characteristics of the very fine ferrite microstructure layer formed in the surface layer portion and the brittle crack propagation arresting characteristics, and have found that In order to stably form a shear lip without brittle fracture of the surface layer at the early stage of brittle crack propagation and to provide good brittle crack propagation arresting properties, the thickness is 2 to 33% of the sheet thickness after reheat processing. The ferrite structure in the upper and lower surface layer regions corresponding to the above needs to be uniform ultra-fine grains over the entire surface, and for this purpose, the heating and rolling conditions before cooling the surface layer to the Ar ₃ transformation point or below are optimized It is necessary to do.

That is, in order for the steel sheet as a whole to have excellent brittle crack propagation arresting characteristics, the surface layer must have a uniform and ultrafine ferrite structure. Brittle fracture is likely to occur, and it is difficult to stably exhibit good brittle crack propagation stopping characteristics. All coarse structures are unrecrystallized grains that have not been recrystallized with ferrite being processed or have not been sufficiently recovered. Therefore, in order to obtain a uniform structure, it is necessary to refine the structure of the surface layer before reheat processing so as not to leave an unrecrystallized structure. The grains need to be sufficiently refined.

In the present invention, the pinning effect of austenite grains due to the dispersion of TiN during heating is used by determining the contents of Ti and N, and the heating temperature of the steel slab is set to 11
By limiting the temperature to 00 ° C. or lower, it is possible to reduce the size of austenite grains. The lower limit of the heating temperature is equal to or higher than the Ac ₃ transformation point. When the heating temperature is lower than the Ac ₃ transformation point, the solution is insufficient and the internal sensible heat for recuperation processing is secured. This is because it becomes difficult.

For the same reason, the hot rolling conditions are the same as above. In order to measure the refinement of austenite grains before transformation and the accumulation of strain due to processing, and to refine the transformed structure before reheating, the cumulative rolling at 950 ° C. or lower is performed. The rate was 10 to 50%. The reason why the reduction rate is limited to 950 ° C. or less is that the effect of the grain size of the recrystallized austenite and the strain accumulation in the non-recrystallized austenite grains is 95%.
This is because hot rolling at 0 ° C. or less becomes remarkable. 950 ° C
If the rolling reduction below is less than 10%, the effect of rolling is insufficient, so the lower limit was made 10%. If the rolling reduction is further increased, it is advantageous for refining the structure before recuperation processing.However, if this rolling reduction is too large, sufficient rolling reduction can be ensured for the refining of ferrite in rolling during subsequent reheating. Since the case may disappear, the rolling reduction appropriate for the final refinement of the structure of the surface layer is determined based on the results of a basic experiment, and the value is set to 50%.

After sufficiently austenite grains are refined under the above conditions and rolling is performed in a non-recrystallized region, the upper and lower surface layers of the steel material are cooled by means such as water cooling. The upper and lower surface layer regions corresponding to 2 to 33% of the sheet thickness at the time of hot rolling are cooled to the Ar ₃ transformation point or lower, and a temperature difference is formed between the surface layer portion and the inside. At the time of hot rolling before the water cooling of the steel material, the thickness of the steel plate was 2 to 33.
%, The cooling rate of the upper and lower surface layer regions is 2 ° C./s
It is necessary to do above. When the cooling rate is less than 2 ° C./s, the transformed structure after cooling becomes coarse even if the austenite is refined by hot rolling before cooling, and the uniform ultrafine ferrite structure is obtained by rolling during subsequent reheating. Is difficult to obtain.

The structure fraction and the rolling reduction during rolling are specified for the following reasons. In plate rolling, when the transformation resistance of austenite and ferrite is measured, the transformation resistance of austenite is higher. Then, as a result of conducting a basic experiment in which the fractions of austenite and ferrite were changed at the same temperature, an experimental result was obtained in which the presence of austenite resulted in stable and ultrafine ferrite grains. From this result, the austenite fraction was 50%.
If it is less than the above, ultra-fine graining becomes remarkable, and it has been found that the reduction rate at that time can be stably ultra-fine grained at 30% or more. The austenite at this time may be transformed by cooling before finish rolling, may remain untransformed austenite, or may be austenite reverse transformed after cooling. It is considered that the reason why the deformation resistance of austenite is higher than that of ferrite is concentration of alloying elements and the like.

For the above reasons, the upper and lower surface regions corresponding to 2 to 33% of the sheet thickness at the time of hot rolling before cooling the steel material are subjected to Ar ₃ transformation at a cooling rate of 2 ° C./s or more. Cooling to below the point, and then performing finish rolling at a predetermined structure fraction and rolling reduction, use the internal sensible heat or use external heating to adjust the upper and lower sides corresponding to 2 to 33% of the sheet thickness. By rolling the surface layer region during the temperature rise, the structure of the region is refined, and the brittle crack propagation stopping characteristics can be improved. In the present invention, the pressure during cooling is 5 kg / cm
Limited to ^two or more. This is from the minimum pressure that can prevent the interference of the header, and the pressure at the time of descaling is set to 150 kg / cm ² or more because a certain pressure or more is required due to the peelability of the scale.

The type of behavior of the formation of the microstructure in the surface layer varies depending on the reheating temperature after the finish rolling, but each contributes to the improvement of the brittle crack propagation stopping characteristics as described below.
That is, when the reheat temperature is lower than the Ac ₃ transformation point, the ferrite-based structure is rolled, and the ferrite is processed at a high temperature, whereby the ferrite is sufficiently recrystallized and the ferrite grains are remarkably refined. As a result, the toughness of the surface layer is remarkably increased, and the brittle crack propagation stopping characteristics of the entire steel sheet are improved.

The upper and lower surface layer regions corresponding to 2 to 33% of the sheet thickness at the time of hot rolling before cooling the steel material are denoted by A.
When recuperated to c ₃ transformation point or higher, for rolling in the rolling reverse transformation process, due to the influence of the processing when the ferrite reverse transformation to austenite, the austenite is fine, then the cooling process after rolling When it is transformed again into ferrite, the ferrite can be remarkably miniaturized. As a result, the toughness of the surface layer is remarkably increased, and the brittle crack propagation stopping characteristics of the entire steel sheet are improved.

The above-described processing steps during cooling to below the Ar ₃ transformation point and during reheating are repeated a plurality of times during finish rolling, so that the effect is superimposed. Therefore, a desired microstructure can be obtained even if performed one or more times. It is possible. If so, recuperation temperature Ac ₃ below transformation temperature need not be constant over one of the conditions, the recuperation process of repetition, to Ac ₃ below transformation point or Ac ₃ transformation point or above in any combination The reheating process does not impair the effect.

The above is the reason for limiting the manufacturing method for remarkably miniaturizing the structure of each of the upper and lower surface portions corresponding to 2 to 33% of the plate thickness. In addition, the internal toughness can be achieved at the same time.
That is, the regions of the upper and lower surface layers corresponding to 2 to 33% of the plate thickness are cooled at a temperature of not less than the Ar ₃ transformation point at a cooling rate of 2 ° C./s or more, and are cooled at a temperature of not more than the Ar ₃ transformation point. When the heating is stopped and the heat is recovered, the deformation resistance is increased due to the lower temperature of the surface layer portion and the finer grain size than the inside. By performing rolling in such a state, the inside having a small deformation resistance receives more strain, the ferrite structure after the transformation is further refined, and the center porosity can be easily pressed. As a result, the internal toughness is significantly improved.

From the crack propagation behavior of brittle fracture, the thickness of the superfine region in the surface layer portion is not more than 2% in each of the upper and lower surface layer portions of the sheet thickness. In addition, the brittle crack propagation stopping characteristics cannot be improved. The greater the thickness of the fine-grained portion of the surface layer, the better the brittle crack propagation arresting property is. However, if the thickness exceeds 33%, the effect saturates. When the steel is cooled under conditions such that the thickness of the fine-grained portion of the surface layer is 33% or more, the sensible heat of the steel itself disappears, so that the temperature at the center of the sheet thickness is too low, and the toughness is deteriorated. Therefore, as long as both the improvement of the brittle crack propagation stopping characteristic of the thick steel plate and the toughness of the central portion of the plate thickness can be satisfied, the thickness of the portion where the surface layer is to be refined is 3 to 33 in each of the upper and lower surface layers.
A range of% is appropriate.

The above is a requirement of the present invention, and a desired structure can be obtained at the stage where rolling and reheating are completed. Cooling after the end of recuperation does not depend on means such as cooling or forced cooling.
Although the desired brittle crack propagation arresting property and toughness can be obtained, the steel sheet after the completion of reheating is cooled to 650 ° C. or less at a cooling rate of 60 ° C./s or less, depending on applications such as improvement of strength. It is also possible to cool to 650 ° C. or lower at a cooling rate of not more than 650 ° C./s and then to temper at an Ac ₁ transformation point or lower. In addition, factors other than crystal grain boundaries also affect brittle crack propagation arresting characteristics and low-temperature toughness, so that chemical components also need to be considered.

The reasons for limiting the chemical components will be described below. C
Is an element effective for securing the strength of the steel material. However, if added excessively, the toughness and weldability are deteriorated.
The range is 0.30%. Si is an element necessary for deoxidation, but if added excessively, it particularly deteriorates the toughness of the welded portion, so the upper limit is made 0.5%. Mn is added for the purpose of improving the strength and toughness, but if added in an excessive amount, weld cracks are likely to occur, so Mn is limited to 2.0% or less. Al is necessary for deoxidation, similar to Si, and by forming AlN, refines crystal grains and contributes to improvement of toughness. However, excessive addition of Al causes toughness degradation or inclusion of inclusions in steel. Since the increase is likely to occur, the addition is made 0.1% or less.

Ti contributes to improving the toughness of the entire steel material through refinement of the heated austenite grain size as TiN, and, as will be described later, the surface layer required to obtain a stable and uniform microstructure of the surface layer. It is an element effective for refining the structure before reheating the part. If the addition is less than 0.003%, the effect of reducing the austenite grain size is small, and if the addition exceeds 0.10%, the effect is saturated and the TiN formed becomes coarse, which may deteriorate the toughness. 0.0
The range of 03 to 0.10% is preferable. N must be contained in an appropriate amount in order to form a nitride with Al or Ti. However, if added excessively, solute N increases and even toughness deteriorates. And

Cr, Ni, Mo, V, Nb, B, and Cu are all effective in increasing the strength of the base material, and one or more of them may be added in appropriate amounts in combination to obtain a desired strength. it can. However, if any of them is added excessively, the toughness of the steel itself is deteriorated, and the weldability and the toughness of the welded portion are also deteriorated.

A slab having a limited chemical composition for the above reasons and consisting of the balance of Fe and unavoidable impurities is heated to a temperature of not less than the Ac ₃ transformation point and not more than 1100 ° C., and the cumulative draft at 950 ° C. or less is reduced. After rolling of 10 to 50%, the upper and lower surface layer regions corresponding to 2 to 33% of the plate thickness at that stage are cooled at a cooling rate of 2 ° C./s or more from a temperature of the Ar ₃ transformation point or higher. A process in which cooling is started, cooling is stopped at or below the Ar ₃ transformation point, and reheating is performed at least once, and reverse transformation or untransformed austenite is completed until reheating after the last cooling is completed. In the organization where the fraction of is less than 50%,
The hot rolling is completed by performing rolling of 30% or more, and the upper and lower surface areas of the steel sheet after the rolling are completed are less than the Ac ₃ transformation point,
Alternatively, by recovering the heat to a temperature higher than or equal to the Ac ₃ transformation point or to the temperature range above and below the Ac ₃ transformation point, it becomes possible to produce a thick steel sheet having excellent brittle crack propagation stopping characteristics and low-temperature toughness.

[0030]

EXAMPLES Using test steels having the chemical compositions shown in Table 1, Table 2
And toughness (fracture surface transition temperature vTrs) by Charpy test of thick steel plates manufactured under the manufacturing conditions shown in Table 3, ESSO
Brittle crack propagation arrest characteristics by test (Kca value is 600k
gf · mm ^−3/2 ) is shown in Table 4.

[0031]

[Table 1]

The test symbol A- produced by the method of the present invention using steel types 1 to 12 having the chemical components of the present invention.
Thick steel plates 1 to A-15 have a Kca value of 600 kgfm.
temperature at which the m ^-3/2 is -85-110 ° C., and the it exhibits excellent brittle crack propagation stopping characteristics, the surface layer portion,
The center portion of the sheet shows excellent toughness of -85 ° C or less in vTrs. On the other hand, the test symbols B-1 to B-1 of the comparative examples in which the chemical components are out of the range of the present invention or the production method does not conform to the present invention.
In the steel plate of B-8, compared with the steel plate according to the method of the present invention,
Both brittle crack propagation arrestability and Charpy properties are clearly inferior. Naturally, as with the comparative steels B6 and B7, sufficient properties cannot be obtained only by rolling by ordinary controlled rolling and controlled cooling after rolling, but the comparative steels B-1 to B-5 and B-8
Even if the surface layer is quenched and reheated before finish rolling as described above, if the other conditions do not satisfy the present invention, excellent brittle crack propagation stopping characteristics like the steel of the present invention cannot be obtained. Are clear from the examples in Table 4.

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

According to the present invention, it is possible to stably achieve the improvement of brittle crack propagation arresting property which was conventionally obtained only by adding a large amount of an alloy by an unprecedented manufacturing method. The industrial effect as a method for producing a highly safe thick steel plate without impairing the economy and productivity is extremely large.

Claims (5)

(57) [Claims] C: 0.04 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% Ti: 0.003 to 0.10% by weight% A steel slab containing N: 0.001 to 0.01%, with the balance being Fe and unavoidable impurities, is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1100 ° C. to have a cumulative draft of 10% at 950 ° C. or lower. 50% line the rolling of
There, the area of upper and lower surface portion corresponding to 2-33% of the thickness of the case is cooled at 2 ° C. / s or more cooling rate from Ar ₃ transformation point or more of temperature, the cooling below Ar ₃ transformation point Is stopped, and then rolling is further performed by 30% or more.
Recover the lower surface area to above the Ac ₃ transformation point, and
The reverse transformation during rolling of 30% or more, which is further performed as described above,
Or less than 50% of untransformed austenite
Method for producing a superior steel plate of brittle crack propagation stopping characteristics and low-temperature toughness, characterized in that that. 2. In% by weight, C: 0.04 to 0.30% Si: ≤ 0.5% Mn: ≤ 2.0% Al: ≤ 0.1% Ti: 0.003 to 0.10% A steel slab containing N: 0.001 to 0.01%, with the balance being Fe and unavoidable impurities, is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1100 ° C. to have a cumulative draft of 10% at 950 ° C. or lower. 50% line the rolling of
There, the area of upper and lower surface portion corresponding to 2-33% of the thickness of the case is cooled at 2 ° C. / s or more cooling rate from Ar ₃ transformation point or more of temperature, the cooling below Ar ₃ transformation point Is stopped, and then further rolling is performed on the upper and lower surface portions.
The region is reheated, and then the upper and lower surface layers
To cool the area and then re-roll it while rolling.
Rolling reduction at least once and after the further rolling
Is 30% or more and reverse transformation during rolling or not
A method for producing a thick steel sheet having excellent brittle crack propagation arrestability and low-temperature toughness, characterized in that the fraction of transformed austenite is less than 50% . 3. The steel slab further comprises : Cr: ≦ 0.5% Ni: ≦ 1.0% Mo: ≦ 0.5% V: ≦ 0.1% Nb: ≦ 0.05% B : ≦ 0.0015% Cu: ≦ 1.5% One or more types are contained.
Item 1. Brittle crack propagation arrestability and low temperature toughness according to item 1 or 2.
Manufacturing method of excellent steel plate. 4. A method according to claim 1, further comprising the step of :
The method for producing a thick steel sheet having excellent brittle crack propagation arrestability and low-temperature toughness according to any one of claims 1 to 3, wherein the steel sheet is cooled to a temperature of 650 ° C or less at the following cooling rate. 5. The method according to claim 5, further comprising the step of :
4. The brittle crack propagation arresting property and low-temperature toughness according to any one of claims 1 to 3 , wherein the alloy is cooled to a temperature of 650 ° C. or less at the following cooling rate, and then tempered at an Ac ₃ transformation point or less. Method of manufacturing thick steel plate.