JP3237861B2 - 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 economically producing a high strength and high toughness structural steel sheet with good productivity.

[0002]

2. Description of the Related Art In recent years, a technique has been developed to reduce the amount of alloying elements added to steel to improve weldability, and to produce a structural steel sheet having high strength and high toughness by an accelerated cooling method. It is mainly applied to steel plates for marine and offshore structures.

[0003] 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 and high-toughness thick steel plate by accelerated cooling at a rate of at least / sec. Method of Obtaining Extra-Thick Steel Plate by Accelerated Cooling at Predetermined Water Volume Density ".

By the way, even when the above-mentioned "accelerated cooling method" is applied, a precipitation-type alloy element such as Nb, V, Ti or the like is usually added in a small amount in order to enhance the strengthening effect by rolling-accelerated cooling. Therefore, the strengthening action of these elements is considered to be approximately proportional to the amount of the alloy elements re-dissolved during heating, and the amount of the alloy elements to be added is adjusted in accordance with the target strength in consideration of the adverse effect on the weldability. It has been done.

As a method of further improving the accelerated cooling method, there has been proposed a method of controlling the amount of precipitation in austenite during rolling and controlling the precipitation during accelerated cooling to increase the strength and toughness of a thick steel plate. Japanese Patent Application Laid-Open No. 1-275719.
This method is used for 10 minutes between finishing rolling and finishing rolling.
Between 00 ° C and 850 ° C, at least 70
The temperature range over ℃ is cooled at a cooling rate of 0.5 ℃ / sec or more, and precipitation in the temperature range of 850 ℃ to 1000 ℃ which is precipitation noise due to rolling strain of Nb carbonitride or V carbonitride The control is performed, and the effect of increasing the strength by about 9 kgf / mm ² is described.

[0006]

However, each of the above proposals has the following problems inherent in practical use, and improvement is awaited. In the conventional method of manufacturing a thick steel plate by the accelerated cooling method, Nb or V dissolved as a solid precipitates in austenite during rolling, and no precipitation required during precipitation strengthening is observed. The strengthening effect has not been achieved, and the benefits from the application of the accelerated cooling method have not been fully enjoyed in terms of cost.

In the method described in Japanese Patent Application Laid-Open No. 1-275719, in which the above-mentioned accelerated cooling method is further improved, water cooling for accelerated cooling is limited between rough rolling and finish rolling. For this reason, the alloying element precipitates due to the rolling strain introduced into the steel sheet during the rough rolling, and the precipitation required during precipitation strengthening is not seen, and the strengthening action by the precipitation is not sufficiently achieved.

The present invention has been achieved by the prior art without these problems, that is, without adding a large amount of alloying elements, staying and waiting for temperature adjustment, and further performing reheating rolling in a low temperature range. It is an object of the present invention to provide a method for efficiently and economically producing a structural steel sheet having high strength and toughness equivalent to or higher than that of the above.

[0009]

According to the present invention, in order to achieve the above object, in terms of% by weight, C: 0.20% or less, Si: 0.01 to 1.0%, Mn: 0.3% to 2.0%, Al: 0.001% to 0.20%, N: 0.020% or less. After the solidification of the slab composed of iron and unavoidable impurities is completed, the temperature is reduced to 1000 ° C or more and less than 1200 ° C. In the rolling in which the slab of the heated structural steel is finished at a temperature not lower than the Ar ₃ point temperature, when the rolling is completed , the slab thickness t (mm) between the start of the rolling of the slab and the end of recrystallization. And a cooling rate V (° C./sec) satisfying the relationship of V> (18 / t) ^0.5 between rolling passes , and A after the recrystallization is completed.
The method of producing a high strength and high toughness structural steel sheets and performing temperature range reduction of 50% or more of the pre-recrystallization temperature region rolling with up to r ₃ points temperature as the first means,

[0010] The steel slab in the first means, Ni,
A combination of at least one of Cr, Mo, Cu, V, Nb, Ti, and B
A total of 4.5% by weight or less as a second means,

After the end of rolling, the cooling rate is 5 ° C./sec or more,
The feature is that accelerated cooling is performed to a cooling stop temperature of 650 ° C or less
The first or second means to be referred to as the third means, and the pressure
After the completion of the elongation , the first or second means is characterized in that a quenching and tempering process is continuously performed , and the fourth means is used as the fourth means.

The structural steel to which the present invention is directed is, for example,
As described in JP-B-58-14849, as described below, ordinary welded structural steel is used in order to obtain required materials.
Equivalent functions and effects can be obtained by using the types and amounts of the additional elements determined based on the relation between the functions and effects that have been conventionally confirmed in the field of use in the art. Therefore, the present invention is intended to include structural steels including these. The reason and amount of addition for each of these constituent elements are shown below.

C is added as an effective component for improving the strength of steel. If the content is excessively more than 0.20%, island martensite is precipitated in HAZ, and HAZ is added.
Since the toughness is significantly deteriorated, the content is restricted to 0.20% or less.

[0014] Si is added as a deoxidizing element and an element for increasing the strength of molten steel, but if it is less than 0.01%, the deoxidizing effect is insufficient, and if it exceeds 1.0%, the workability of the steel decreases. However, since the toughness of HAZ decreases, the amount of addition is 0.01 to
Regulate 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 the weldability by an excessive addition, so the upper limit is 2.0%.

Al and N are necessary because the grain size of steel can be reduced by Al nitride. However, if the addition amount is small, there is no effect, and if it is excessive, the toughness of the steel deteriorates. Therefore, the addition amount of Al is restricted to 0.001 to 0.20%, and the unavoidable N content is 0%. 0.020% or less.

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 strength of the base material or improving the toughness of the joint, it is difficult to secure weldability if it is added too much. Therefore, the addition amount of the alloy is Ni, C
At least one of r, Mo, Cu, V, Nb, Ti, and B is regulated to 4.5% by weight or less in total.

When the temperature at the end of rolling is lower than the temperature at the Ar ₃ point, the austenite grains become coarse and the surface toughness deteriorates . In addition, the rolling end temperature in the present invention, which is not lower than the Ar ₃ point temperature, includes the surface temperature.

[0019]

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 studies were repeated using 2 and 9.

It is known that the austenite grain size of this structural steel sheet changes depending on the heating temperature. The present inventors investigated the relationship between the heating temperature and the grain size of the crystal grains in the structural steel by changing the heating temperature of the structural steel slab variously. As a result, the central temperature of the slab was 1000 ° C. or more and less than 1200 ° C. In this case, it was found that the added Nb was sufficiently dissolved to form a solid solution, and that the crystal grains became 100 μm or less as shown in FIG.

Next, the present inventors have recognized that the cooling rate during rolling is 0.4 to 0.5 ° C./sec in order to search for a method for improving the strength and the base material toughness with good productivity and economical efficiency. The relationship between the slab thickness and the cooling rate in normal rolling, which has been performed, 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 FIG.

The present inventors enjoy the effect of controlling the precipitates described above, without adding a large amount of alloying elements, staying / waiting for temperature adjustment, and without performing reheating rolling in a low temperature range. 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, the plate thickness to be rolled is divided into the following areas,
Experimental studies were repeated from three points (1) to (3). : Between 250 mm and 100 mm at the start of rolling (comparison
Target plate temperature is high) : about 100 mm to 10 mm (relatively low plate temperature )
Period) : Between 250mm and 10mm

( 1) The slab residence time is reduced by cooling the slab during rolling to a high temperature range up to the recrystallization ending temperature, the slab during rolling is increased, the recrystallization temperature is increased, and the alloying element for precipitation strengthening is used. Between solid solution maintenance, suppression of grain growth of crystal grains of steel sheet and miniaturization. ( 2) The relationship between the strain accumulated in the austenite before transformation and the grain size of the transformed ferrite under strong pressure in the above-mentioned region, that is, the unrecrystallized region where the cast slab during rolling is from the recrystallization to the Ar ₃ point temperature. . (3) The above-mentioned region, that is, the relationship between the combination of (1) and (2) and the base material toughness . The three application ranges are shown in the upper part of FIG. As is well known, the recrystallization end temperature is not constant due to the relationship between the history temperature of the steel material to be processed and the history processing amount.
Therefore, the sheet thickness of the rolled material corresponding to the illustrated recrystallization end temperature shows an example position.

In this experimental study, a strength of 47 kgf / m
m ² , vTrs in Charpy impact test as base metal toughness
FIG. 2 shows the cooling rate according to the plate thickness of (1) in which the value was −110 ° C.
FIG. It has been found that this curve can be approximated by (18 / t) ^{0.5 when} the thickness of the slab is t. By utilizing this, even when the thickness of the material to be rolled changes during rolling, the cooling rate V (° C./sec) is V> (18 /
t) It was found that the object of the present invention can be achieved by maintaining ^0.5 or more. FIG. 3 shows the relationship between the cooling condition of the steel type 2 and the cooling condition of the steel type 9 = t × V ² [mm · (° C./sec) ^2] , the strength of the steel sheet after rolling, and the 1 / 2t base metal toughness.

According to FIG. 3, a structural steel sheet of steel type 2 obtained by a conventional rolling method showing a strength of 47 kgf / mm ² and a toughness (vTrs value) of −85 ° C. was obtained by the methods (1) and (2) described above. Significantly improved, same strength and toughness (vTrs value)
Was found to improve to -108 ° C.

FIG. 4 shows that the strength is 66 kgf / mm.
^2. Structural steel sheets of steel type 9 obtained by a conventional rolling method exhibiting toughness (vTrs value) -95 ° C are as described in (1) and (2) above.
Greatly improved by the method described above, with a strength of 73 kgf / m
It was found that m ² and toughness (vTrs value) were improved to −123 ° C.

It was also found that when a reduction of 50% or more was applied at a temperature lower than the recrystallization end temperature, the toughness was remarkably improved as shown in FIG.

The present inventors have found that the productivity is good and the strength is economical.
And to find ways to improve base metal toughness,
Slab thickness in case of rolling without water cooling during rolling)
The result of adjusting the relationship between the temperature and the temperature drop rate is shown by the curve A in FIG.
Shown in

[0029]

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 and the cooling conditions during rolling, and Table 4 shows the obtained materials together with the conventional example.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

The steel numbers 1 to 3 of the test steels shown in Table 1 are 40 kg class steels, the steel numbers 4 to 7 are 50 kg class steels, and the steel numbers 8 to 10 are 60 kg.
It is a kilo-grade steel. Alloy elements are added to each as needed. As shown in Table 4 , no. A1 ¹ to A10 ⁹
According to the present invention, the structural steel sheet having the same strength or toughness as or more than the strength and toughness of the base material obtained by the prior art in both the designated pass cooling and the zone cooling can be manufactured with high productivity and economy.

On the other hand, no. In the conventional examples B1 to B10, since the required cooling rate and unrecrystallized zone rolling are not applied, there is a problem in strength, toughness, productivity, and economy, and a structural steel sheet satisfying the above-mentioned problems cannot be obtained. Was. In addition, B1 and B4 whose heating temperatures were 1200 ° C. or more had low base material toughness of −30 ° C. and −40 ° C. B10 having a heating temperature of less than 1000 ° C. is 60 kgf / mm ²
Regardless of the component, Nb was not sufficiently dissolved and the strength was only 57 kgf / mm ² . B6, B7
Although the sample was subjected to accelerated cooling after rolling, the finishing temperature was high, and the toughness was inferior to those of A5 and A6 of the present invention which did not undergo accelerated cooling.
Further, prior art B8 that by adding Nb element has lower toughness than invention sample A 6 without the addition of Nb element, even when compared with those without the addition of Nb element, by the accelerated cooling after rolling Conventional example B2 was inferior in toughness to inventive example A1 without the accelerated cooling.

[0037]

According to the present invention, as is apparent from the above description,
Since the above action generated by the above means is utilized,
In addition to reducing costs by reducing the composition, the joint toughness and base metal toughness of the weld zone have been improved, and further, the structural steel sheet whose base metal strength has been increased to the point where it is possible to omit accelerated cooling after rolling can be produced with high productivity This enables smooth, stable and economical production, and has an extremely large effect on the application field of the present invention and a ripple effect on related fields.

[Brief description of the drawings]

FIG. 1 shows the relationship between the heating temperature of a structural steel slab and the austenite grain size.

FIG. 2 shows the relationship between the thickness of a slab during rolling and the average cooling rate in the thickness direction according to the thickness.

FIG. 3 shows the relationship between the cooling conditions of the present invention, the strength at 1/2 thickness, and the base material toughness.

FIG. 4 shows the relationship between the cooling conditions of the present invention, the strength at 1/2 thickness, and the base material toughness.

FIG. 5 shows the relationship between the rolling reduction and the toughness of the base metal at a thickness of 1/2 in the range from the end of recrystallization to the temperature of Ar ₃ point.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Hiroshi Takezawa 1 Nishinosu, Oita, Nippon Steel Corporation Inside Nippon Steel Works (72) Inventor Yoshikika Kawashima 1 Nishinosu, Oita, Nippon Steel Corporation Nippon Steel Corporation Inside the Oita Works (72) Inventor Hiroshi Yoshikawa 1 Nishinosu, Oita-shi, Nippon Steel Corporation Inside Nippon Steel Corporation (72) Inventor Masakazu Shishido 1 Nishinosu, Oita-shi, Oaza, Nippon Steel Corporation Nippon Steel Corporation Oita Works (56) References JP-A-59-20421 (JP, A) JP-A-2-254120 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8 / 00-8/10 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. In weight%, C: 0.20% or less, Si: 0.01 to 1.0%, Mn: 0.3% to 2.0%, Al: 0.001 to 0.20% , N: at containing 0.020% or less, after made slab solidification completion and the balance iron and unavoidable impurities, the slab of structural steel heated to below 1000 ° C. or higher 1200 ° C. Ar ₃ point temperature or higher When the rolling is completed, the slab thickness t (mm) and the cooling rate V (° C./sec) between the start of the rolling of the slab and the end of the recrystallization are V>
(18 / t) Cooling that satisfies the relationship of ^0.5 is performed between rolling passes, and rolling is performed in a non-recrystallization temperature range with a rolling reduction of 50% or more in a temperature range from the completion of recrystallization to the Ar ₃ point temperature. A method for producing a high-strength, high-toughness structural steel sheet, comprising: 2. Ni, Cr, Mo, Cu, V, Nb, T
The method for producing a high-strength and high-toughness structural steel sheet according to claim 1, wherein one or more of i and B are added in a total amount of 4.5% by weight or less. 3. After completion of rolling, the cooling rate 5 ° C. / sec or more, the production of high strength and high toughness structural steel sheet according to claim 1 or 2, characterized in that accelerated cooling to a cooling stop temperature 650 ° C. or less Method. 4. After completion of rolling, the method of producing a high strength and high toughness structural steel sheet according to claim 1 or 2 subsequently characterized by performing quenching and tempering treatment.