JPH0920921A - Production of high toughness steel plate by means of separation - Google Patents

Abstract

PURPOSE: To produce a steel plate excellent in brittle fracture arrest characteristic and increased in Charpy impact absorbed energy. CONSTITUTION: A steel slab, having a composition consisting of 0.04-0.18% C, 0.05-0.50% Si, C1.70-2.0% Mn, <=0.020% P, <=0.010% S, 0.010-0.080% Al, 0.005-0.030% Ti, 0.0015-0.0080% N, and the balance Fe with inevitable impurities, is heated to >=(Ac3 transformation point +100 deg.C). Then, the steel slab is subjected to primary rolling where the steel slab is rolled to a thickness larger than finishing plate thickness by 0.5 to 5mm while regulating rolling completion temp. to a value in the temp. range from the Ar3 transformation point to (Ar3 transformation point +100 deg.C). Subsequently, air cooling or forced cooling is performed down to a temp. in the range from (Ar3 transformation point -50 deg.C) to 550 deg.C, and rolling is started again at this temp. and finish rolling is completed, followed by air cooling. Then, the resultant rolled plate is reheated to a temp. in the range between (Ac3 transformation point +50 deg.C) and the Ac1 transformation point and cooled at <=18 deg.C/s cooling rate. By this procedure, the maximum value (SI max) of the separation index allowed to appear in the fracture surface of a test piece at the Charpy impact test is regulated to 0.08-0.35.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a high toughness steel sheet using separation.

[0002]

2. Description of the Related Art In recent years, brittle fracture propagation stopping characteristics have been emphasized in steel materials used for hull materials, LPG tank members, and offshore structures in cold regions in order to ensure the safety of the structures. There is.

Since brittle fracture occurs from fatigue cracks in welds or structural members, it is necessary to prevent fracture of steel structures.
It is desirable to use a steel material having high resistance to the occurrence of brittle fracture and a steel material having the property of preventing the propagation of the fracture that has occurred.

For improving the brittle fracture propagation stopping property, for example, a method of adding a large amount of Ni to steel is easy and widely used. However, this method is expensive because Ni is a scarce resource, and there is a problem that the manufacturing cost is significantly increased.

To solve this problem, a method has been proposed in which the amount of Ni added is reduced and a combination of rolling method and heat treatment is used. That is, as a method for improving the brittle fracture propagation stopping property by refining the crystal grains of steel, a method of performing normalizing treatment after controlled rolling (Japanese Patent Laid-Open No. 60-155620)
Japanese Patent Laid-Open No. 62-77419) or a method of performing tempering treatment by water cooling followed by reheating after controlled rolling (Japanese Patent Laid-Open No. 62-77419).

Furthermore, as a manufacturing method for improving the brittle fracture propagation stopping property by rolling, there is a so-called γ-α two-phase rolling method in which rolling is performed in a temperature range of Ar ₃ transformation point to Ar ₁ transformation point.

[0007]

[Problems to be Solved by the Invention] However, JP-A-60-155
There is a limit to improving the brittle fracture propagation stopping property only by refining the crystal grains proposed in JP 620 or JP-A-62-77419, and after rolling, tempering is simply performed to improve toughness. Even if the treatment is performed, there is a problem that the effect of improving the brittle fracture propagation stopping property is not necessarily large.

In the γ-α two-phase rolling method, depending on the manufacturing method, the Charpy impact fracture surface transition temperature (vTrs) is dramatically improved, the Charpy impact absorbed energy (vE) is lowered, and the strength in the plate thickness direction is increased. There are problems such as a decrease.

[0009]

In order to solve the above problems, the present inventor has found that a steel sheet having excellent brittle fracture propagation stopping properties, high Charpy impact absorption energy, and excellent brittle fracture resistance properties. As a result of intensive studies on the manufacturing method, a method for effectively utilizing separation was found as a method for preventing brittle fracture. The method controls the rolling conditions more effectively, along with the refinement of the ferrite crystal grain size after rolling, the γ-α two-phase region rolling is strongly carried out in an extremely low temperature range, and the elongation and strength of the crystal grains are increased. Of steel sheet having a rise in temperature and significant separation is reheated to near the γ-α two-phase region and cooled at an arbitrary cooling rate to obtain a structure with good toughness of bainite, ferrite, and pearlite mixed, Adjusts the amount of internal separation generated, and has excellent brittle fracture propagation stopping properties.
Moreover, the present invention has been achieved based on the knowledge that it is possible to manufacture a steel sheet having a large Charpy impact absorbed energy.

The summary is as follows: C: 0.04 to 0.18%, Si: 0.05 to
0.50%, Mn: 0.70 to 2.0%, P: 0.020% or less, S: 0.010
% Or less, Al: 0.010 to 0.080%, Ti: 0.005 to 0.030%, N:
A steel slab containing 0.0015 to 0.0080% and the balance Fe and unavoidable impurities is heated to the Ac ₃ transformation point + 100 ° C or higher,
Primary rolling is carried out by rolling to a thickness of 0.5 to 5 mm thicker than the finish plate thickness, which has a temperature range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C as the rolling completion temperature, and then (Ar ₃ transformation point -50 ° C).
Air-cooled or forcedly cooled to a temperature range of up to 550 ℃, start rolling again at this temperature and finish rolling, then allow to cool, and then (Ac ₃ transformation point + 50 ℃) to Ac ₁ transformation point temperature range Reheat to 18 ℃ / s or less and perform reheat treatment to cool at a cooling rate of 18 ° C / s or less, and the maximum value (SImax) of the separation index to appear on the fracture surface of the test piece in the Charpy impact test for each manufactured steel sheet It is a method of manufacturing a high toughness steel sheet using the separation adjusted to. Here, the Charpy impact test piece is JIS Z 2202 No. 4 test piece, and the separation index (SI) is SI = (total length of separation of fracture surface of test piece (mm)) / (test piece cross-sectional area (80 mm ² )) is there. SImax is the maximum value of the separation index (SI) for each manufactured steel sheet. Use separation to adjust the maximum value of separation index (SI) to 0.08 to 0.35

Further, Cu: 0.90% or less as a chemical component,
Ni: 1.20% or less, Mo: 0.50% or less, V: 0.080% or less, Nb:
0.005 to 0.050%, B: 0.0004 to 0.0030%, Ca: 0.0005 to
It is a method of manufacturing a high toughness steel sheet using the above-mentioned separation containing one or more selected from 0.0050%.

[0012]

The operation of the present invention will be described in detail below. First, the reasons for limiting the chemical components in the present invention will be described. C is an element that is necessary and effective for ensuring the strength of the steel sheet, and for this purpose 0.04% or more addition is necessary. However, if the addition amount exceeds 0.18%, the toughness of the steel sheet deteriorates significantly and the weldability also deteriorates. Therefore, the content of C is set to the range of 0.04 to 0.18%.

Si is an element necessary for deoxidation, and it is necessary to add at least 0.05%, but if added over 0.50%, toughness deteriorates. Therefore, the Si content is 0.
The range is from 05 to 0.50%.

Mn is an element necessary for ensuring the strength and toughness of the steel sheet and preventing softening of the weld heat affected zone, and for this purpose, 0.70% or more must be added. However, if the addition amount exceeds 2.0%, the weldability and the toughness of the weld heat affected zone deteriorate sharply. Therefore, the content of Mn is 0.70-2.
The range is 0%.

P and S are present in the steel as impurities, but if they are present in large amounts, they deteriorate the toughness of the base metal and the weld heat affected zone, and are not preferred elements. Therefore, in the present invention, in order to increase the toughness and prevent the deterioration of the brittle fracture propagation stopping property and the brittle fracture generation preventing property, the respective limits are set to 0.020% and 0.010% or less, respectively.

Al is an element necessary for improving toughness by deoxidizing steel and refining crystal grains, and for this purpose, 0.010
% Or more is required. However, excessive addition of Al
Since oxide-based nonmetallic inclusions are generated and the toughness is deteriorated,
The upper limit of the amount added is 0.080%. Therefore, the Al content is in the range of 0.010 to 0.080%.

Ti is combined with N to form TiN, which is effective in refining the austenite crystal grains during the heating of the billet and improving the toughness of the base metal and the toughness of the weld heat affected zone. However, if added in excess, it forms coarse TiN and deteriorates toughness. Therefore, the Ti content should be in the range of 0.005 to 0.030%.

N is a kind of impurity, but when contained in an appropriate amount, it forms TiN together with Ti to improve the toughness of the base metal and the weld heat affected zone. It contains 0.0015 to 0.0080%. The lower limit of the content is limited to 0.0015% because this is the lower limit amount that can secure the toughness of the weld heat affected zone.
If the content exceeds 80%, cracks may occur during the production of the steel slab, which will be an obstacle to the subsequent process. Therefore, the content of N is 0.
0015 to 0.0080%.

In the present invention, each of the above components is
If necessary, one or more selected from the following elements Cu, Ni, Mo, V 2, Nb, B and Ca may be contained.

Cu, Ni, and Mo are elements that improve the hardenability, and when used in the present invention, they are extremely effective in increasing the strength of the steel sheet. However, if added in excess, low temperature products such as bainite will be produced after rolling is completed. Occurs more than necessary and hinders the formation of ferrite, which causes harmful effects due to an excessive increase in strength. Therefore, the content of each is 0.90% for Cu.
Below, Ni is 1.20% or less and Mo is 0.50% or less.

V is an element effective in securing strength and toughness due to the refinement of grain structure and precipitation hardening after rolling, but since it is an expensive element, its content is 0.080% or less from the economical viewpoint. To do.

Nb also secures strength and toughness by fine-graining and precipitation hardening of the structure after rolling, and fine-grain ferrite-fine grain ferrite containing a pearlite structure or a small amount of bainite-
It is an effective element for obtaining a pearlite structure and manufacturing a steel sheet with excellent brittle fracture propagation stopping properties.The lower limit of the content in which the effect is remarkable is 0.005%, and the content exceeding 0.050% is the welding heat. The upper limit is set to 0.050% to deteriorate the toughness of the affected area. Therefore, the Nb content is
The range is 0.005 to 0.050%.

B is an element effective for securing the base metal strength or the toughness of the weld heat affected zone, but addition of less than 0.0004% has no effect of increasing the strength, and if it exceeds 0.0030%, it increases the toughness of the weld heat affected zone. To be harmful. Therefore, the content of B is set to the range of 0.0004 to 0.0030%.

Ca is effective in controlling the morphology of sulfides and is effective in improving the Charpy impact absorption energy in the direction perpendicular to the rolling direction. To obtain this effect effectively, Ca
It is necessary to add 0.0005 to 0.0050%. Therefore, Ca
Content of 0.0005 to 0.0050%.

Further, the reasons for limiting the manufacturing conditions of the high toughness steel sheet of the present invention will be described. Steel billet, Ac ₃ transformation point +100
0.5 to 5 mm from the finish plate thickness, which is heated to ℃ or more and the temperature range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ℃ is the rolling completion temperature.
The primary rolling for rolling to a large thickness is necessary for refining the coarse crystal grains during solidification in the recrystallization region and for refining the crystal grains that control the properties of the steel sheet in the non-recrystallization region. If the completed thickness of the primary rolling is too large, the number of rolling passes that will cause large deformation resistance at extremely low temperatures during finish rolling will increase, making it difficult to secure the shape of the steel sheet.

The heating temperature of the steel slab is such that the steel slab is uniformly austenitized and the coarse crystal grains remaining during solidification are prevented, and when Nb is added, the solid solution of Nb is sufficiently carried out to form austenite grains. Temperature required to prevent coarsening, refine grains during rolling, and especially to expand the unrecrystallized region,
If the Ac ₃ transformation point is lower than + 100 ° C, coarse crystal grains remain during solidification, the solid solution of Nb is not sufficient, and the crystal grains are not sufficiently refined even after rolling. Cannot be secured. Therefore, the heating temperature of the billet is A
c ₃ Transformation point + 100 ° C or higher.

Since the propagation of brittle fracture is absorbed by propagation energy due to plastic deformation of the surface layer of the steel sheet and stops, in order to obtain a steel sheet having excellent brittle fracture propagation stopping characteristics, the surface layer portion is finer than the central portion, And rolled into elongated crystal grains,
The temperature sensitivity of shock absorption energy is low, and the fracture surface transition temperature (v
It is necessary to keep excellent Trs). On the contrary, it is more important that the shock absorption energy in the center is high,
If the amount of reduction is more than necessary in the finish rolling, the impact absorption energy of the central portion will be deteriorated. From these things, the upper limit of the completed thickness of the primary rolling is the finish thickness of the steel plate +
Set to 5 mm. Further, if there is no reduction amount at the time of finish rolling, elongation of crystal grains, increase in tensile strength, occurrence of separation is reduced, and it becomes difficult to secure material properties after reheating treatment. The lower limit of the finished thickness is the finished thickness of the steel plate + 0.5 mm.

The completion temperature of the primary rolling is an important item for refining the crystal grains. If the temperature is too high, the cooling before the finish rolling is started in the state where the crystal grains are not refined. Grain growth occurs and finish rolling is performed in the state of coarse crystal grains, and the crystal grains after rolling become large, resulting in reduction in tensile strength and deterioration in toughness. Further, if the completion temperature of the primary rolling is too low, the γ-α two-phase region rolling is performed in a state where the grains are not refined in the unrecrystallized region. Cannot shift to two-phase rolling. Therefore, the completion temperature of the primary rolling depends on the Ar ₃ transformation point.
Limited to the temperature range of Ar ₃ transformation point + 100 ° C.

After the completion of the primary rolling, the billet (material being rolled) is air-cooled or forcedly cooled to a temperature range of (Ar ₃ transformation point −50 ° C.) to 550 ° C., and rolling is restarted at this temperature. And finish rolling. This cooling is finish rolling, rolling at extremely low temperature to elongate crystal grains, increase tensile strength,
It is necessary to promote the occurrence of separation. In finish rolling, it is indispensable to finish a steel sheet into a predetermined shape and size, and to internally incorporate sufficient tensile strength and separation generation so as to adjust the structure and the generation amount of separation in the subsequent reheating treatment. When the finish rolling end temperature is higher than Ar ₃ transformation point −50 ° C., the above-mentioned essential items cannot be satisfied, and the tensile strength becomes insufficient and the inherent separation disappears after the reheating treatment. On the other hand, if the cooling temperature of the billet and the finish rolling finish temperature are lower than 550 ° C, it will be difficult to secure the shape and dimensions of the steel sheet during finish rolling. Therefore, the cooling temperature, finish rolling start temperature and finish temperature of the steel slab after the completion of the primary rolling are set to a temperature range of (Ar ₃ transformation point −50 ° C.) to 550 ° C.

After finishing rolling, the steel sheet is left to cool to room temperature, but accelerated cooling does not cause deterioration of the material. In this case, the structure is mostly ferrite,
Has a ferrite structure containing a small amount of bainite,
The accelerated cooling causes little change in the structure.

The steel sheet after finish rolling has high tensile strength, separation occurs densely and inferior toughness, and it cannot be said that it has sufficient steel sheet characteristics. Therefore, reheating is performed, and the amount of microstructure and the amount of separation generated is adjusted by air cooling or forced cooling to perform reheating treatment for obtaining the desired characteristics of the steel sheet. The reheating temperature is A
When the temperature exceeds the c ₃ transformation point + 50 ° C, a large amount of bainite structure or a structure in which martensite is mixed appears when the cooling rate is high, which causes deterioration of toughness. On the other hand, if the reheating temperature is lower than the Ac ₁ transformation point, the change in the structure is small and only the ferrite structure is softened, and only the tensile strength is lowered, and the desired steel sheet properties cannot be obtained. However, by adjusting the reheating temperature and cooling at a specified cooling rate, the structure changes to bainite + ferrite + pearlite structure, and the specified tensile strength and toughness are secured, and the maximum separation index (S
Imax) can be adjusted. The cooling rate is
When it exceeds 18 ℃ / s, only bainite structure is produced in large quantity,
The desired bainite + ferrite + pearlite structure cannot be obtained, and the balance between strength and toughness deteriorates. Therefore, the heating temperature of the reheating treatment performed after the finish rolling is (Ac ₃ transformation point + 50 ° C) to Ac ₁ transformation point, and the cooling rate is 18
Limited to ℃ / s or less.

The maximum value (SImax) of the separation index is adjusted by the reheating treatment, but as shown in FIG.
When x is less than 0.08, the impact absorption energy is large and the brittle fracture resistance property is excellent, but the brittle fracture propagation stopping property is not excellent. On the other hand, when the SI exceeds 0.35, the brittle fracture propagation arresting property becomes excellent, but the brittle fracture initiation property deteriorates. Therefore, the maximum value (SImax) of the separation index is limited to the range of 0.08 to 0.35. Note that the reheating treatment can be repeated to change the balance between strength and toughness, and a high toughness steel plate can be manufactured according to the application.

FIG. 1 shows that steel types A used in the examples are finished into steel sheets under various conditions.
It shows the relationship between the maximum value of separation index (SImax) at 60 ° C and the impact absorption energy, and the relationship between the maximum value of separation index (SImax) at -40 ° C and the fracture toughness value. Maximum separation index (SIma
x) is the value when the separation index of the fracture surface of the test piece for each test becomes maximum regardless of the Charpy impact test temperature.

The Ac ₃ transformation point, the Ac ₁ transformation point and the Ar ₃ transformation point
The temperature of the transformation point is determined by the following formula. Ac ₃ (℃) = 908-223.7C + 438.5P + 30.5Si + 37.9V-34.4Mn-2
3.0Ni Ac ₁ (℃) = 723 + 22Si-14Mn-14.4Ni + 23.3Cr Ar ₃ (℃) = 910-310C-80Mn-20Cu-15Cr-55Ni-80Mo However, the components are mass%

[0035]

EXAMPLES The constitution of the present invention is as described above, and examples will be described below. The test steel sheet was prepared by melting a low-carbon low-alloy steel containing the chemical components shown in Table 1 by a conventional method, and the obtained billet had a thickness of 25 to 45 according to the production conditions shown in Table 2.
It is rolled into a steel plate of mm. Test pieces were taken from these test steel plates and subjected to tensile test, Charpy impact test, NRL.
Drop weight test, double tensile test and separation index (SI)
Was measured. Table 3 shows the results.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

Experiment Nos. 1, 3, 5, 7 of Tables 2 and 3,
9, 11, 13 are examples of the present invention, Experiment No. 2, 4, 6, 8, 10,
12 and 14 are comparative examples. The examples will be described below in the order of the experiment numbers.

In comparison with Experiment No. 1 of the present invention, Experiment No. 2 of Comparative Example has a Charpy used for evaluation of brittle fracture resistance because the primary rolling completion temperature is lower than the Ar ₃ transformation point. The impact absorption energy (vE- ₄₀ ) of the impact test and the fracture toughness value (Kca) of the double tensile test used as an evaluation of the brittle fracture propagation stopping property are inferior.

In contrast to Experiment No. 3 of the present invention example, Experiment No. 4 of Comparative Example has a high reheating temperature and a high cooling rate, so that the tensile strength is high, and vE- ₄₀ and fracture surface are high. Transition temperature (vT
rs) is inferior and the separation index (SI) is low. Further, the NDT temperature of the NRL drop weight test used for evaluating the brittle fracture propagation stopping property is high, the Kca value is also low, and the brittle fracture propagation stopping property is inferior.

In contrast to Experiment No. 5 of the present invention example, Experiment No. 6 of the comparative example completed the rolling in the primary rolling, did not perform finish rolling, and did not perform reheat treatment, and therefore NDT. High temperature, low Kca value and poor brittle fracture propagation stopping property.

In contrast to Experiment No. 7 of the present invention example, Experiment No. 8 of Comparative Example has a large primary rolling completion thickness and a high finish rolling start temperature, so that the tensile strength is low. Furthermore, SI is also small, N
The DT temperature is high, the Kca value is low, and the brittle fracture propagation stopping property is poor.

In contrast to Experiment No. 9 of the present invention, Experiment No. 10 of Comparative Example has a low billet heating temperature and no reheating treatment, and therefore has low tensile strength and large SI, The Charpy impact property is poor, the NDT temperature is high, the Kca value is low, and the brittle fracture propagation stopping property is poor.

In contrast to the experiment No. 11 of the present invention, the experiment No. 12 of the comparative example has a large primary rolling completion thickness and a low rolling completion temperature, and also has a rolling start temperature and an end temperature in finish rolling. Therefore, after finishing rolling finish thickness to the same plate thickness as the thickness 40 mm of the present invention example, even if reheat treatment,
Low tensile strength and small SI. Therefore, vTrs is inferior,
High NDT temperature, low Kca value and poor brittle fracture propagation stopping property.

Experiments Nos. 13 and 14 are reheat treatments performed twice. In contrast to Experiment No. 13 of the example of the present invention, Experiment No. 14 of the comparative example has a primary rolling completion thickness. Is thick, the completion temperature is high, and since the finish rolling start temperature and end temperature are high, after finishing the finish rolling finish thickness to the same plate thickness of 45 mm of the present invention example, even if reheat treatment is performed, it is stretched. Low strength, small SI, poor vTrs, high NDT temperature,
The Kca value is also low and the brittle fracture propagation stopping property is inferior.

[0047]

As described above, in the method of manufacturing a high toughness steel sheet using the separation according to the present invention, the primary rolling and the finish rolling are performed under the temperature control, and the reheating treatment is further performed to perform the separation. Since the strength and toughness are balanced by adjusting the index (SI), the Charpy impact property, the NDT temperature in the NRL drop weight test are excellent, and the fracture toughness value (Kca) in the double tensile test is high. Good brittleness It is possible to manufacture a steel sheet having break propagation stopping properties.

[Brief description of drawings]

1] Steel type A is finished into steel sheets under various conditions. For these steel sheets, the relationship between the maximum value of separation index (SImax) at -60 ° C and the impact absorption energy, and
It is a figure showing the relation between the maximum value (SImax) of the separation index and the fracture toughness value at -40 ° C.

Claims (2)

[Claims] 1. C: 0.04 to 0.18%, Si: 0.05 to 0.50%, M
n: 0.70 to 2.0%, P: 0.020% or less, S: 0.010% or less, A
l: 0.010-0.080%, Ti: 0.005-0.030%, N: 0.0015-
A steel slab containing 0.0080% and the balance of Fe and unavoidable impurities is heated to the Ac ₃ transformation point + 100 ° C or higher to finish the rolling within the temperature range of Ar ₃ transformation point to Ar ₃ transformation point + 100 ° C. Primary rolling is carried out by rolling to a thickness 0.5 to 5 mm thicker than the plate thickness, and then (Ar ₃ transformation point −50 ° C.) to 550 ° C.
Air-cooled or forcedly cooled to the temperature range of ₁ ), start rolling again at this temperature and finish rolling, then allow to cool, then reheat to the temperature range of (Ac ₃ transformation point + 50 ° C) to Ac ₁ transformation point. Then, reheat treatment is performed at a cooling rate of 18 ° C / s or less, and the maximum value of the separation index (SIm
ax) is adjusted to 0.08 to 0.35, and a method of manufacturing a high toughness steel sheet utilizing separation. Here, the Charpy impact test piece is JIS Z 2202 No. 4 test piece, and the separation index (SI) is SI = (total length of separation of fracture surface of test piece (mm)) /
(Cross-sectional area of test piece (80 mm ² )). SImax is the maximum value of the separation index (SI) for each manufactured steel sheet. 2. Further, as chemical components, Cu: 0.90% or less, Ni: 1.20% or less, Mo: 0.50% or less, V: 0.080% or less, Nb: 0.005 to 0.050%, B: 0.0004 to 0.0030%, Ca: 0.0
The method for producing a high toughness steel sheet using the separation according to claim 1, characterized in that it contains one or more selected from the range of 005 to 0.0050%.