JP3279335B2 - Manufacturing method of high strength and high toughness structural steel sheet - 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 high strength and high toughness structural steel sheet with high productivity.

[0002]

2. Description of the Related Art In recent years, there has been an increasing demand for material properties of welded structural steels used for large structures such as marine structures, ships, and storage tanks. Due to the size of the anxiety, improvement of the base metal toughness of the steel is demanded.

In order to improve the low-temperature toughness of the base material, it is effective to reduce the ferrite grain size after transformation.
It is known that it is effective to make austenite grains before transformation effective for that purpose. There are many proposals for such a method. For example, there is a method in which heating is performed at a low temperature to increase the processing amount in an unrecrystallized region as described in JP-A-59-47323.

[0004] Conventionally, Japanese Patent Application Laid-Open No.
As disclosed in Japanese Patent Publication No. -19431, Ni and Nb
By using alloy elements such as these, a vTrs value of −50 ° C. to −70 ° C. is obtained in the Charpy impact test for the toughness of the base material.

[0005] Further, there is a technique for reducing the amount of alloying elements to be added to steel in order to improve weldability, increasing the strength with a small amount of components and manufacturing a thick steel plate having good toughness by an accelerated cooling method. The proposed steel plate manufactured by this technology has been applied to structures mainly in fields such as steel plates for shipbuilding or marine structures.

[0006] For example, in Japanese Patent Publication No. 62-130216, "After hot-rolling a Ti-added steel in a low-temperature austenite unrecrystallized region, the steel is immediately cooled at a cooling rate of 1 ° C.
Method for producing a high-strength, high-toughness thick steel plate by accelerated cooling at a rate of at least / sec.
No. 4,075,088, a method of obtaining an extremely thick steel plate by hot-rolling a slab and immediately cooling it at a predetermined water density in consideration of the carbon equivalent.

[0007] Even when the "accelerated cooling method" is applied,
In general, precipitation-type alloying elements such as Nb, V, and Ti are added in a small amount to enhance the strengthening effect by rolling and accelerated cooling, and the strengthening effect of these elements is the amount of solid solution re-dissolved during heating. Therefore, the addition amount of the alloy element is adjusted according to the target strength in consideration of the weldability.

As a method for further improving the accelerated cooling method, Japanese Patent Laid-Open Publication No. Hei 1 (1999) discloses a method for controlling the precipitation during rolling, which precipitates in austenite, and controlling the precipitation during accelerated cooling to increase the strength and toughness of a thick steel plate. -275719.
In this method, the rough rolling is completed and the finishing rolling is performed.
Between 000 ° C and 850 ° C, at least 7
The temperature range over 0 ° C. is cooled at a cooling rate of 0.5 ° C./sec or more, and precipitation control in a temperature range of 850 ° C. to 1000 ° C., which is a precipitation nose due to rolling strain of Nb carbonitride or V carbonitride, etc. And presents an increase effect of about 9 kgf / mm ² in strength.

[0009]

However, each of the above proposals has the following various problems inherent in practical use, and improvements in each of them are awaited. JP-A-59-
As disclosed in Japanese Patent No. 47323, heating is performed at a low temperature, the amount of processing in the non-recrystallized region is increased, and controlled cooling is essential.
In the method of producing fine ferrite and martensite by quenching after rolling, the heating temperature of another slab and the heating temperature of the target slab are different from each other, so that it is necessary to adjust the heating operation conditions before and after this. In addition, a significant decrease in heating efficiency is inevitable, and furthermore, in order to increase the amount of processing in the non-recrystallized region, the temperature waiting time during controlled rolling becomes extremely long,
This leads to a reduction in rolling efficiency, an increase in costs associated with reheating and controlled cooling, and a significant decrease in productivity.

[0010] Further, as described in Japanese Patent Application Laid-Open No. 58-19431, a high-strength steel excellent in arrest characteristics requires, in addition to Ni and Nb, to safely austenite by reheating after rolling. Therefore, an increase in cost and a decrease in productivity due to reheating are inevitable.

Further, Ni and Nb are expensive alloy components, and their addition significantly increases the cost of steel. Nevertheless, vTrs in the Charpy impact test, which indicates the toughness of the base material, is only at the level of -50 ° C to -70 ° C.
Particularly, the addition of a large amount of Nb remarkably lowers the toughness of the HAZ as described later, and is therefore not preferable as a welding steel material.

[0012] In the conventional means for producing a thick steel plate by the accelerated cooling method, Nb and V, which are precipitation strengthening elements re-dissolved during slab heating, precipitate in austenite during rolling, and the precipitation strengthening occurs. Occasionally, necessary precipitation is not seen, the added Nb and V do not contribute sufficiently, and the benefits of the accelerated cooling method are not sufficiently enjoyed both in terms of material and cost.

As a method for further improving the accelerated cooling method, in the method described in Japanese Patent Application Laid-Open No. 1-275719, water cooling is limited between rough rolling and finish rolling. For this reason, precipitates induced by strain introduced during rolling actually precipitate in austenite, and precipitates that can be precipitated during accelerated cooling, which is considered to be effective for strength, decrease, and the precipitation strengthening effect is reduced. Not fully demonstrated.

The present invention does not have these problems, that is, without adding a large amount of alloying elements, staying / waiting for temperature adjustment, and further performing reheating rolling in a low temperature range.
An object of the present invention is to provide a method for efficiently and economically producing a structural steel sheet having high strength and high toughness equivalent to or higher than that obtained by the prior art with good productivity.

[0015]

Means for Solving the Problems To achieve the above object, the present invention has the following features . (1)% by weight
And C: 0.2% or less, Si: 0.01 to 1.0
%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.2
Structural steel containing 0%, N: 0.020% or less, P: 0.015% or less, S: 0.005% or less , the balance being Fe and unavoidable impurities, and heated to Ac ₃ or more after solidification The slab as it is or by applying a pressure, the surface temperature is set to 2 ° C. to a temperature range of Ar ₃ point or less.
At a cooling rate of at least / sec, and then reheated, and rolled at a rolling reduction of 30% or more in a state where the center of the sheet thickness is in a temperature range of recrystallization end temperature to recrystallization end temperature + 150 ° C. thickness center is it to a temperature range of below recrystallization finish temperature
In Tsu state, the average cooling rate V and the thickness t (mm)
(° C./sec), the rolling is performed while cooling to satisfy V> (18 / t) ^0.5 , and the rolling finish temperature is set to Ar
_A method for producing a high-strength, high-toughness structural steel sheet, characterized by _three or more points. (2) Cu, Ni, N
one of b, Cr, V, Mo, B, Ti, Ca, Zr or
It is characterized by containing not less than 4.5% in total of two or more kinds.
The method for producing a high-strength and high-toughness structural steel sheet according to the above (1).

[0016] (3) characterized by performing accelerated cooling to 650 ° C. below the temperature after the end of rolling at 5 ° C. / sec or more cooling rate
The high-strength high-toughness structure according to the above (1) or (2),
For manufacturing steel sheets for automobiles. (4) high strength according to (1) or (2), wherein the performing subsequently quenching and tempering treatment after the end of rolling
Manufacturing method of high toughness structural steel sheet.

The structural steel to which the present invention is directed is, for example,
As described in the above-mentioned JP-B-58-14849, as described below, in order to obtain a required material of a normal welded structural steel, an operation and an effect which have been conventionally confirmed in the field of use in the field of the art. The same action and effect can be obtained by using the type and amount of the additional element determined based on the relationship in the same manner. Therefore,
The present invention is intended to include steels including these.

The reasons and amounts of these components are shown below. C is added as an effective component for improving the strength of steel. However, if the content is excessively more than 0.20%, island-like martensite is precipitated and H is added.
Since the AZ toughness is significantly deteriorated, the content is restricted to 0.20% or less.

Si is added as a deoxidizing element and an element for increasing the strength of molten steel. If it is less than 0.01%, the deoxidizing effect is insufficient, and if it exceeds 1.0%, the toughness of the HAZ decreases. Therefore, the amount of addition is regulated to 0.01 to 1.0%.

Mn is also required as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel is reduced and workability is impaired. Further, it is necessary to add 0.3% or more as a component for improving the strength of the steel material. However, Mn significantly deteriorates weldability due to excessive addition. Therefore, the upper limit is 2.0%.

Al is necessary because the grain size of steel can be reduced by Al nitride. However, when the addition amount is small, the effect is not obtained. When the addition amount is excessive, an oxide-based inclusion is formed by bonding with oxygen in the steel to reduce the cleanliness of the steel. Restrict to 0.20%.

N is an element inevitably contained,
Excessive addition promotes the formation of island martensite, so the upper limit is 0.020%. P is 0.015 to prevent deterioration of the toughness and crack resistance of HAZ due to micro segregation.
% Is the upper limit. S is defined as an upper limit of 0.005% in order to form coarse A-based inclusions and prevent deterioration of the toughness and anisotropy (difference in physical properties between the rolling direction and the direction perpendicular to the rolling direction) of the base material. The basic components of the structural steel targeted by the present invention are as described above. On the basis of this, when an alloy element is added in accordance with the required properties for the purpose of increasing the base material strength or improving the joint toughness, if it is added too much, it becomes difficult to secure weldability. Therefore, the addition amount of the alloy is Ni, C
r, Mo, Cu, W, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
One or more types of B can be added, but the total amount is regulated within 4.5%.

In the present invention, the upper limit of the heating temperature of the slab is 1200 ° C. in order to prevent austenite coarsening, and the lower limit temperature is preferably 900 ° C. or higher in consideration of the rolling operation. Further, a steel material containing an Nb element requires heating at 1100 ° C. or more to completely dissolve Nb. If the end temperature of rolling is lower than the Ar ₃ point temperature, ferrite transformed from austenite is processed and the toughness of the surface layer deteriorates. Therefore, the rolling end temperature in the present invention was set to the Ar ₃ point temperature or higher.

[0024]

In order to solve the problems of the prior art and to achieve the object of the present invention, the present inventors have proposed a steel type shown in Table 1 in Examples as a test steel representative of general structural steel. Various experimental examinations were repeated using 2, steel type 9.

In order to establish a method for improving the strength and the base material toughness economically with good productivity, the relationship between the temperature and time at the center of the sheet thickness where the rolling temperature does not easily decrease, and the cooling rate during rolling is set to 0.1. The relationship between the slab thickness and the cooling rate in normal rolling recognized as 4 ° C to 0.5 ° C / sec was investigated. As a result, the actual condition of the cooling rate corresponding to the thickness of the material to be rolled, which is not used at all in the conventional rolling technology, has been found. The actual situation is shown by a curve A in FIGS.

The present inventors, while enjoying the high strength effect of the precipitates, add a large amount of alloy elements, stay for adjusting the temperature,
In order to establish a method of manufacturing a structural steel sheet having high strength and toughness equivalent to or higher than that obtained by the prior art without performing standby and further reheating rolling in a low temperature range, the following three points are required. The experimental study was repeated.

The slab during rolling is cooled in a high temperature range close to the recrystallization end temperature, thereby reducing the slab residence time, lowering the rolling temperature range, dissolving the alloying elements for precipitation strengthening, and toughness in the center of the sheet thickness. And the relationship between grain refinement. The relationship between the accumulation of strain in austenite grains before ferrite transformation and the transformed ferrite grain size by cooling while rolling in the temperature range up to the Ar _three- point temperature after the recrystallization of the slab during rolling. And strength and toughness of base metal.

As is well known, if the temperature history and the processing amount of the steel material to be processed change, the temperature at which the recrystallization is completed changes. Therefore, the illustrated recrystallization end temperature and the corresponding sheet thickness of the rolled material are examples.

In this experimental study, the strength was 47 kgf / mm ² and the vTrs in the Charpy impact test as the base material toughness was −10.
The temperature history at the center of the sheet thickness up to the recrystallization end temperature of the steel material showing 8 ° C., and the cooling rate for each thickness from the end of recrystallization to the end of rolling are shown by curves B in FIGS.

The present inventors paid attention to the temperature at the center of the sheet thickness and investigated the relationship between the rolling temperature range in the recrystallization region and the austenite grain size after the completion of the recrystallization. The result is shown in FIG.
From the figure, it was found that the range of the recrystallization end temperature to the recrystallization end temperature + 150 ° C. was effective in reducing the austenite grain size after recrystallization. FIG. 4 shows the result of investigating the influence of the rolling reduction in the above temperature range. From the figure, the rolling reduction is 30%.
It turned out to be necessary.

Further, the present inventors have conducted various studies for the purpose of controlling the temperature at the center portion of the sheet thickness. As a result, before or during rolling, the surface temperature was cooled to ₃ points or less of Ar. It has been found that it is effective to wait for a predetermined reheating so that the center of the thickness is rolled in the above-mentioned temperature range.
From the viewpoint of austenite grain refinement, the surface is cooled at a cooling rate of 2 ° C./sec or more to an Ar ₃ point or less as a cooling condition of the surface to quickly reach the temperature at the center of the sheet thickness to the above-mentioned optimum temperature range. It is clear from the results of the heat conduction analysis in the thickness direction that this is effective.

The curve in FIG. 1 indicates that the thickness of the slab is t,
(18 / t) It was found that approximation was possible with ^0.5 . Thus, even if the material to be rolled changes its thickness during rolling during rolling,
It has been found that the object of the present invention can be achieved when the cooling rate V (° C./sec) satisfies (18 / t) ^0.5 or more. FIG.
FIG. 6 shows the relationship between the cooling condition t × V ² [mm · (° C./sec) ² ] and the t / 2 base metal toughness of the rolled steel sheet.

According to FIG. 5, the strength is 47 kgf / mm ² and the toughness (v
A steel sheet for structural use of steel type 2 obtained by a conventional rolling method exhibiting a Trs value of −85 ° C. is significantly improved by the method of the present invention, and the toughness (vTrs value) is improved to −108 ° C. with the same strength. There was found. The manufacturing conditions in FIG. 5 are as follows. Heating temperature: 1050 ° C Finishing temperature: 770 ° C Slab thickness: 150mm Product thickness: 25mm Steel type: Table 1 2 Recrystallization end temperature-Recrystallization end temperature + rolling reduction at 150 ° C: 35% Reduction in the non-recrystallization region Rate: 65%

According to FIG. 6, the strength is 66 kgf / mm ² and the toughness (v
A steel sheet for structural steel of type 9 obtained by a conventional rolling method exhibiting a Trs value of −95 ° C. is greatly improved by the method of the present invention, and has a strength of 73 kgf / mm ² and a toughness (vTrs value) of −123.
° C. The manufacturing conditions in FIG. 6 are as follows. Heating temperature: 1180 ° C Finishing temperature: 756 ° C Slab thickness: 260mm Product thickness: 17mm Steel type: 9 in Table 1 Recrystallization end temperature-Recrystallization end temperature + reduction at 150 ° C: 40% Reduction in unrecrystallized area Rate: 70%

In order to improve the strength of the structural steel sheet obtained as described above by accelerated cooling, a cooling rate of 5 ° C./cm.
It is only necessary to perform accelerated cooling at a cooling stop temperature of 650 ° C. or less for at least seconds, and when the structural steel sheet of the present invention obtained as described above is quenched and tempered after the above rolling, the strength and toughness are maintained without impairing the effects of the present invention. It has been found that can be improved. The present invention has been made based on the above findings.

[0036]

The components of the test steel of the present invention may be in any combination as long as they are the same as the elements of the above-mentioned general structural steel, but the present steel is used as a representative structural steel having different strength levels. Table 1 shows the chemical composition of the steel used in the examples, and Table 2 shows the manufacturing conditions.
Table 3 shows the rolling pass schedule used at that time and the cooling conditions during rolling, and Table 4 shows the obtained materials together with the conventional example.

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

[Table 6]

[0043]

[Table 7]

The steel types 1 to 3 of the test steels shown in Table 1 are 40 kg class steels, the steel types 4 to 7 are 50 kg class steels, and the steel types 8 to 10 are 60 kg.
It is a kilo-grade steel. Alloy elements are added to each as needed. As shown in Table 2, No. 1 of the present invention example using steel types 1 to 10 was used. A1 to A10 exhibited the required strength, and the toughness was a favorable value of -107 to -133 ° C in vTrs. Further, it was possible to produce a steel sheet having excellent strength and toughness in both the designated pass cooling and the zone cooling with high productivity and economy.

On the other hand, no. None of the conventional examples B1 to B10 satisfy the conditions described in the present invention, and each has a problem. That is, recrystallization end temperature-recrystallization end temperature +
No. of the comparative example whose rolling reduction at 150 ° C. was less than 30%. B
Sample Nos. 5 and B9 had lower base material toughness than A5 and A9 of the present invention manufactured using the same test steel. Comparative Example No. No cooling was performed in the recrystallization region. B3, B
4, B7, B8, and B9 had deteriorated base metal toughness as compared with A3, A4, A7, A8, and A9 of the present invention manufactured using the same test steel.

Comparative Example No. No cooling was performed during rolling in the unrecrystallized region. B1, B2, B5, B6, B7, B
Sample Nos. 8 and B9 had deteriorated base metal toughness as compared with A1, A2, A5, A6, A7, A8 and A9 of the present invention manufactured using the same test steel. In addition, in Comparative Example No. Nb-added steel having a heating temperature as low as 985 ° C. B10 had excellent toughness, but the strength did not reach the required strength.

As shown in the examples, in the case of the Si-Mn-based steel containing no precipitation element such as Nb, although the strength is not changed by the method disclosed in the present invention, the toughness is greatly improved. In addition, the steel material containing a precipitation element such as Nb is greatly improved in strength toughness by the method of the present invention.

[0048]

According to the present invention, as is apparent from the above description,
By utilizing the above-mentioned action generated by the above-mentioned means, the strength and toughness of the base material have both been established and a technique for economically manufacturing them has been established. The ripple effect of the present invention on the application field is extremely large. .

[Brief description of the drawings]

FIG. 1 is a chart showing the relationship between time and temperature from heat extraction.

FIG. 2 is a table showing a relationship between a slab thickness during rolling and an average cooling rate in the thickness direction.

FIG. 3 is a table showing a relationship between a rolling temperature range in a recrystallization region and an average γ particle size after completion of recrystallization.

FIG. 4 is a table showing a relationship between a rolling reduction in an optimum recrystallization temperature region and an average γ particle size after completion of recrystallization.

FIG. 5 is a table showing cooling conditions, strength at t / 2 parts, and base metal toughness.

FIG. 6 is a table showing cooling conditions, strength at t / 2 parts, and base metal toughness.

Continuation of the front page (72) Inventor Yuji Funatsu 1 Nishinosu, Oita, Nippon Steel Corporation Oita Works (56) References JP-A-4-311521 (JP, A) JP-A-3-310400 (JP, A) JP-A-1-275719 (JP, A) JP-A-1-268818 (JP, A) JP-A-2-217416 (JP, A) JP-A-63-243220 (JP, A) 63-179020 (JP, A) JP-A-3-13524 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02 B21B 1/22 B21B 37/76

Claims (4)

(57) [Claims] C .: 0.2% or less by weight , Si: 0.01 to 1.0 % by weight.
%, Mn: 0.3 to 2.0%, Al: 0.001 to 0.2
Structural steel containing 0%, N: 0.020% or less, P: 0.015% or less, S: 0.005% or less , the balance being Fe and unavoidable impurities, and heated to Ac ₃ or more after solidification The slab as it is or by applying a pressure, the surface temperature is set to 2 ° C. to a temperature range of Ar ₃ point or less.
At a cooling rate of at least / sec, and then reheated, and rolled at a rolling reduction of 30% or more in a state where the center of the sheet thickness is in a temperature range of recrystallization end temperature to recrystallization end temperature + 150 ° C. thickness center is it to a temperature range of below recrystallization finish temperature
In Tsu state, the average cooling rate V and the thickness t (mm)
(° C./sec), the rolling is performed while cooling to satisfy V> (18 / t) ^0.5 , and the rolling finish temperature is set to Ar
_A method for producing a high-strength, high-toughness structural steel sheet, characterized by _three or more points. 2. The steel composition further comprises Cu, N
1 of i, Nb, Cr, V, Mo, B, Ti, Ca, Zr
It is a special feature that it contains 4.5% or less in total
2. Production of a high-strength and high-toughness structural steel sheet according to claim 1.
Law. 3. A process for producing a high strength and high toughness structural steel sheet according to claim 1 or 2, characterized by performing rolling after the end of the accelerated cooling at 5 ° C. / sec or more cooling rate to a temperature of 650 ° C. or less . 4. A process for producing a high strength and high toughness structural steel sheet according to claim 1 or 2, characterized by performing subsequently quenching and tempering treatment after the end of rolling.