JPH04308035A - Production of steel plate for structural use excellent in toughness at low temperature - Google Patents

Abstract

PURPOSE:To economically produce a steel plate for structural use having excellent toughness at low temp. in weld heat-affected zone (HAZ) as well as in base material with superior productivity. CONSTITUTION:In the course of rolling a cast slab of structural steel heated to 1200 deg.C after solidification in the temp. region to the Ar3 point, cooling is done so that the required cooling rate V according to plate thickness (t) satisfies the relation in V>(18/t)<0.5>. By this method, the production of the steel plate having superior toughness in base material and toughness in HAZ is made possible without using the means so far inevitably used in the course of rolling, such as temp. waiting, low temp. heating and rolling, and reheating and rolling, and further, the productivity of the steel plates of this kind can be increased and manufacturing costs can be remarkably reduced.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a highly productive and economical method for producing structural steel plates in which both the base metal and the weld heat affected zone (hereinafter referred to as HAZ) have excellent low-temperature toughness.

0002

[Prior Art] Welded structural steel plates used for large structures such as offshore structures, ships, and storage tanks are not well known in the art due to the magnitude of damage and social unrest caused by the destruction of the structures. Improvements in material properties have been remarkable, and efforts to improve the base material toughness and HAZ toughness of steel materials in particular are continuing. Moreover, as the structures become larger, it is desired that welded structural steel plates, which are the construction materials, be provided at low cost.

There are many proposals for improving the low-temperature toughness of the base material. For example, the proposal of JP-A-59-47323, the proposal of JP-A-58-19431, the proposal of JP-A-60-169516, and the proposal of Japanese Patent Application No. 01-14668 which purports to improve these methods. etc.

0004

[Problems to be Solved by the Invention] However, all of the above-mentioned proposals have inherent problems in practical use, and improvements are awaited in each of them. The proposal in JP-A No. 59-47323 is to heat at a low temperature, apply large processing in the non-recrystallized region, perform controlled cooling, and after finishing rolling, rapidly cool to generate fine ferrite and martensite to form the base material. This method improves toughness, but since the heating temperature of the target slab is different from that of other slabs, it requires time to adjust the operating conditions of the heating furnace between the two, resulting in a significant decrease in heating efficiency. Unavoidably, the amount of processing in the non-recrystallized area is increased, so the temperature waiting time during controlled rolling becomes extremely long, leading to a decrease in rolling efficiency and an increase in costs due to reheating, resulting in a significant decrease in productivity. .

[0005] Furthermore, the high tensile strength steel with excellent arrestability disclosed in JP-A No. 58-19431 uses Ni and Nb and is completely austenitized by reheating after rolling. The toughness of the base material has been improved, and this reheating inevitably increases costs and reduces productivity.

[0006] Furthermore, the method disclosed in Japanese Patent Publication No. 169516/1980 requires heat treatment at a high temperature to ensure the desired toughness of both the base material and the HAZ, and a heat treatment of 800°C after reheating.
It is essential to perform rolling at a temperature below 0.degree. C., and this reheating and rolling significantly reduces productivity and increases costs.

[0007] In addition, a patent application for the purpose of improving these
The method of No. 1-14668 is to heat to a temperature of 1,250°C or higher and then apply a reduction of 50% or more from the recrystallization end temperature to the Ar3 point temperature in the non-recrystallized region in order to achieve segregation and diffusion to ensure HAZ toughness. Because the base material toughness is improved by rolling,
A reduction in economic efficiency due to the temperature waiting time until cooling to a predetermined temperature range and a reduction in rolling t/hr due to the temperature drop is unavoidable, and productivity and economic efficiency are inevitably deteriorated.

[0008] The present invention does not involve these problems and can produce steel sheets having excellent base material toughness and HAZ toughness with good productivity.
The objective is to provide an economically efficient manufacturing method.

[0009]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a method for rolling a structural steel slab heated to 1200°C or higher after completion of solidification to a temperature higher than the Ar3 point. From the start of slab rolling to the end of recrystallization, the slab thickness t (mm) and cooling rate V (°C/sec) are V > (1
As a first means, a method for producing a steel plate with excellent low-temperature toughness is characterized by performing cooling that satisfies the relationship of 8/t)0.5,

[0010] In rolling of a structural steel slab heated to 1200° C. or higher after completion of solidification and completed at an Ar3 point temperature or higher, the slab is Thickness t (mm) and cooling rate V (°C/sec) are V>
As a second means, a method for manufacturing a steel plate with excellent low-temperature toughness, which is characterized by performing cooling that satisfies the relationship of (18/t)0.5,

[0011] In rolling a structural steel slab heated to 1200° C. or higher after completion of solidification to a temperature higher than the Ar3 point, the thickness of the slab changes from the start of rolling to the end of rolling. A third method for producing a steel plate with excellent low-temperature toughness is characterized in that cooling is performed such that the relationship between t (mm) and cooling rate V (°C/sec) is V>(18/t)0.5.
as a means of

After rolling, the cooling rate is 5°C/
The first means is characterized by performing accelerated cooling at a cooling stop temperature of 650° C. or less for at least 2 seconds as the fourth means;

[0013] After completion of rolling, the second means is characterized in that accelerated cooling is performed at a cooling rate of 5° C./sec or more and a cooling stop temperature of 650° C. or less;

[0014] After the rolling, the third means is characterized in that accelerated cooling is performed at a cooling rate of 5° C./sec or more and a cooling stop temperature of 650° C. or less;

[0015] The first means is characterized in that a quenching and tempering treatment is subsequently performed after the rolling is completed, and the seventh means is

0
[016] The second means is characterized in that a quenching and tempering treatment is subsequently performed after the rolling is completed, and the eighth means is

001
7) The third means is characterized in that a quenching and tempering treatment is subsequently performed after the rolling is completed, and the ninth means is the third means.

Structural steels to which the present invention is directed include, for example,
As described in Japanese Patent Publication No. 58-14849, as described below, in order to obtain the required material quality of ordinary welded structural steel,
Equivalent actions and effects can be obtained by using the types and amounts of additive elements that have been determined based on the action-effect relationship that has been conventionally confirmed in the field of art. Therefore, the present invention targets structural steels containing these materials. The reason for addition and amount of each of these component elements will be shown below.

C is added as an effective component to improve the strength of steel, but if the content exceeds 0.20%, island martensite will precipitate in the HAZ and the HAZ
Since it significantly deteriorates toughness, it is restricted to 0.20% or less. .

[0020]Si is added as a deoxidizing element and a strength increasing element for molten steel, but if it is less than 0.01%, the deoxidizing effect is insufficient, and if it is added in excess of 1.0%, the workability of the steel decreases. However, since the toughness of HAZ decreases, the amount added is 0.01~
It will be regulated at 1.0%.

Mn is also necessary as a deoxidizing component element, and 0
．． If it is less than 3%, the cleanliness of the steel decreases and the workability is impaired. Further, it is necessary to add 0.3% or more of C as a component to improve the strength of steel materials. However, excessive addition of Mn deteriorates weldability, so the upper limit is set at 2.0%.

[0022] Al and N are necessary because the crystal grain size of steel can be made finer by means of Al nitride. However, if the amount of Al added is small, it will not be effective, and if it is excessive, the toughness of the steel will deteriorate. .020% or less.

The basic components of the structural steel to which the present invention is directed are as described above. Based on this, alloying elements are added depending on the required properties with the aim of increasing base metal strength or joint toughness, but if the amount added is too large, it becomes difficult to ensure weldability. Therefore, the amount of alloy added is Ni, Cr,
Mo, Cu, W, P, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth elements, Y, Ca, Mg, Te, Se,
One or more types of B are regulated within 4.5% in total.

[0024] Furthermore, in order to roll the above-mentioned structural steel slab, first, after the slab has solidified, it is heated to 1200° C. to the center to diffuse segregation. In addition, when the surface of the steel sheet, which is the lowest temperature in the steel sheet, becomes less than the Ar3 point temperature, the austenite grains become coarse and the surface toughness deteriorates. Includes surface temperature.

[0025] In carrying out the present invention, one of the slabs is
The upper limit of heating to 200° C. or higher may be determined from the balance between rolling power consumption and heating fuel consumption and the handling conditions of the slab, and it is usually desirable to set it to 1350° C. or lower.

[0026]

[Operation] In order to solve the problems of the prior art as well as achieve the objects of the present invention, the present inventors have developed the steel types shown in Table 1 of Examples as test steels representative of general structural steels. We repeated various experimental studies using 2.

In structural steel sheets obtained from slabs produced by continuous casting, it is theoretically possible that the segregation of C and Mn concentrations at the center of the sheet thickness will diffuse in accordance with the heating temperature, improving HAZ toughness. It has been confirmed experimentally. The present inventors investigated the relationship between the heating temperature and HAZ toughness of the structural steel by varying the heating temperature of the structural steel slab, and found that FIG.
As shown in Figure 2, it was confirmed that when the center temperature of the slab was in the region of 1200°C or higher, a good value for the toughness of HAZ 1mm was obtained. On the other hand, 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 grain size of the structural steel by varying the heating temperature of the structural steel slab, and found that the center temperature of the slab was as shown in Figure 2. It was confirmed that when the temperature exceeds 1100°C, the crystal grains become coarser and, contrary to the above-mentioned improvement in HAZ toughness, deterioration in base material toughness is unavoidable.

[0028] In order to search for a method of improving the HAZ toughness by improving the above-mentioned C and Mn segregation while also improving the base metal toughness with good productivity and economically, the present inventors decided to increase the cooling rate during rolling. The relationship between slab thickness and cooling rate during normal rolling, which is recognized to be 0.4 to 0.5°C/sec, was investigated. The slab used at this time was a slab that had been heated to 1200°C or higher to diffuse center segregation after solidification. As a result, the actual state of the cooling rate that corresponds to the thickness of the rolled material, which has not been utilized at all with conventional rolling technology, was revealed. The actual situation is shown by curve A in FIG.

[0029] The present inventors took advantage of this fact to solve the problem of extremely low-temperature heating of slabs and rolling temperature adjustment, which are common causes of productivity decline and deterioration of economic efficiency in the prior art. In order to establish a method for manufacturing steel sheets that has base metal toughness and HAZ toughness equivalent to or better than those obtained with conventional technology without using holding, waiting, or reheating rolling at low temperatures, the following steps were taken. Experimental studies were conducted from three points.

■ By cooling the slab during rolling in a high temperature range up to the recrystallization completion temperature, the residence time of the slab is reduced, the recrystallization completion temperature is increased, grain growth is suppressed, and the crystal grains of the steel sheet are made finer. relationship. ■ Relationship between the accumulation of strain in the austenite grains before ferrite transformation due to cooling of the slab during rolling after completion of recrystallization and the ferrite grain size of the steel sheet after transformation. ■ Relationship between the combination of ■ and ■ and base material toughness. These three applicable ranges are shown in the upper part of Figure 3. As is well known, the recrystallization end temperature, which is the boundary between (1) and (2) in this applicable range, is not constant due to the relationship between the hysteresis temperature of the steel to be processed and the hysteresis processing amount. Therefore, the illustrated recrystallization end temperature and the corresponding plate thickness of the rolled material indicate an example position.

[0031] In this experimental study, the vTrs in the Charpy impact test was -89°C as the base material toughness.■, -85
Curve B in FIG. 3 shows the cooling rate for each thickness in ■, which indicates ℃, and -100℃. It was found that this curve can be approximated by (18/t)0.5, where t is the thickness of the slab. As a result, it was found that even if the thickness of the rolled material changes during rolling, the objects of the present invention can be achieved as long as the cooling rate V (°C/sec) satisfies (18/t) 0.5 or more. . Figure 4 shows the cooling conditions t×V2 [mm・(℃/sec)2]
The relationship between and the 1/2t base material toughness of the steel plate after rolling is shown.

Further, in order to improve the strength of the structural steel plate obtained as described above by accelerated cooling, after the rolling is completed, a cooling rate of 5° C. is applied using any coolant such as water, steam, or a mixture of steam and water.
It is sufficient to carry out accelerated cooling at a cooling stop temperature of 650° C. or more for at least 650° C., and if the structural steel sheet of the present invention obtained as described above is subjected to quenching and tempering after rolling, the strength and strength can be improved without impairing the effects of the present invention. It has been found that toughness can be improved. The present invention has been made based on the above knowledge.

[0033]

[Example] The composition of the test steel of the present invention may be any combination of the elements and addition amounts of the general structural steels mentioned above, but this test was carried out as a typical structural steel with different strength levels. Table 1 shows the chemical composition of the steel used in the example. Further, the manufacturing conditions of this example are shown in Table 2, the rolling pass schedule used at that time and the cooling conditions during rolling are shown in Tables 3 and 4, and the obtained material is shown in Table 5 along with the conventional example.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

[Table 5]

[0039] Of the test steels shown in Table 1, steel numbers 1 to 3 are 40 kg class steel, steel numbers 4 to 7 are 50 kg class steel, and steel numbers 8 to 10 are 60 kg class steel.
It is kilo class steel. Alloying elements are added to each as necessary. As shown in Table 2, No. A1-1 to A10
Example -3 of the present invention was able to produce a steel plate with good productivity and economy, with both designated pass cooling and zone cooling having the same or higher base material toughness and HAZ toughness than those of the prior art.

On the other hand, No. In all of the conventional examples B1 to B10, although temperature waiting was performed during rolling,
Since the cooling rate of curve A in FIG. 3 was used, only the indicated base material toughness and HAZ toughness could be obtained. FL shown in Table 2 indicates a solid-liquid boundary line, and HAZ1mm indicates a position 1mm away from the FL, hereinafter HAZ3mm and HAZ5mm indicate positions 3mm and 5mm away, respectively.

[0041] No. 1 with a heating temperature of less than 1200°C. B3,
Conventional examples of B5 to B10 have FL and H at each test temperature.
The toughness of AZ1 mm was 2.1 to 7.8 kgf·m, and the absorbed energy was lower than that of the present invention example using the same heating temperature.

[0042] In addition, B1, B2, B4, and B9, in which the center of the slab was also heated to 1200°C or higher, have a FL and HAZ of 1 mm.
The HAZ toughness of 11.7 to 23.1 kgf・m showed good absorption energy, but
mm is 5.9 to 7.7 kg due to the low base material toughness.
f・m and absorbed energy were low.

[0043]

[Effects of the Invention] As is clear from the above description, the present invention has the following features:
By utilizing the above-mentioned effects generated by the above-mentioned means, we have established a technology to smoothly, stably, and economically manufacture structural steel plates with excellent low-temperature toughness in both the base material and the HAZ with high productivity. Therefore, the effect brought about in the field of application of the present invention and the ripple effect on related fields are extremely large.

[Brief explanation of the drawing]

[Figure 1] Heating temperature of structural steel slab and HAZ1mm of steel plate
shows the relationship between toughness.

FIG. 2 shows the relationship between heating temperature and austenite grain size of structural steel slabs.

FIG. 3 shows the relationship between the thickness of a slab during rolling and the average cooling rate in the thickness direction at that thickness.

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

Claims (9)

[Claims] Claim 1: After completion of solidification, rolling of a structural steel slab heated to 1200° C. or higher is completed at an Ar3 point temperature or higher, from the start of rolling to the end of recrystallization. The thickness t (mm) and the cooling rate V (°C/sec) are V>
A method for producing a steel plate with excellent low-temperature toughness, characterized by performing cooling that satisfies the relationship of (18/t)0.5. [Claim 2] In rolling a structural steel slab heated to 1200° C. or higher after completion of solidification at an Ar3 point temperature or higher, from the time after recrystallization of the slab until the end of the rolling, The slab thickness t (mm) and the cooling rate V (°C/sec) are V>
A method for producing a steel plate with excellent low-temperature toughness, characterized by performing cooling that satisfies the relationship of (18/t)0.5. 3. After completion of solidification, rolling of a structural steel slab heated to 1200° C. or higher is completed at an Ar3 point temperature or higher, from the start of rolling of the slab to the end of rolling. The thickness t (mm) and the cooling rate V (°C/sec) are V>
A method for producing a steel plate with excellent low-temperature toughness, characterized by performing cooling that satisfies the relationship of (18/t)0.5. [Claim 4] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 1, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. [Claim 5] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 2, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. [Claim 6] After finishing rolling, the cooling rate is 5°C/sec or more,
The method according to claim 3, characterized in that accelerated cooling is performed at a cooling stop temperature of 650° C. or lower. 7. The method according to claim 1, further comprising performing quenching and tempering treatment after the rolling is completed. 8. The method according to claim 2, further comprising performing quenching and tempering treatment after the rolling is completed. 9. The method according to claim 3, further comprising performing quenching and tempering treatment after the rolling is completed.