JPH0353019A - Manufacture of steel having excellent strength and toughness - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Industrial application field> The present invention is directed to C:0. 10~0.20! The lower limit temperature at which austenite (hereinafter abbreviated as γ) can be recrystallized (
(hereinafter abbreviated as ↑p) or more, preferably Tp or more τρ+15
Rolling is started in a temperature range of 0"C or below, and rolling is completed at a temperature above the Arz point temperature, preferably above the Ar2 point temperature and below the Arz point +100°C, and the rolling is continued to 1.5°C continuously. The present invention relates to a method for economically producing steel materials with excellent strength and toughness by obtaining a structure containing fine-grained ferrite and bainite by cooling at a rate of 40° C./sec or more and 40° C./sec or less.

<Prior art> In recent years, a method of directly hot rolling (hereinafter abbreviated as DR) a continuously cast slab immediately after casting has been put into practical use due to its high energy-saving effect in the hot rolling process. On the other hand, the thickness of the continuously cast slab is 50 mm, which is close to the product thickness.
A continuous casting method in which steel is extracted using ~ioolm+ is being put into practical use, and various manufacturing processes are being studied in which a slab cast by the continuous casting method is turned into a steel plate by DR.

In this case, the DR starts rolling from coarse γ grains during solidification of several to ten centimeters, and even at a reduction ratio of 1 to 2, which is much smaller than the conventional reduction ratio applied to slabs with a thickness of 250 to 300 centimeters. It is desired to stably and smoothly provide the desired shape and material. To meet this demand, for example, as shown in Japanese Unexamined Patent Publication No. 61-213322, a cast slab containing TI oxide that has been precipitated finely and in large quantities without recrystallization by rolling is produced. After rolling to the final thickness at a temperature of 900°C or higher during cooling,
A method for producing a strong steel plate having a fine Widmanstätten-like ferrite plate structure, that is, a fine Bohnite & Il texture (hereinafter abbreviated as fine bainite structure) by accelerated cooling. As described above, a high-temperature slab obtained by continuous casting from steel containing Tt is heated directly or after heat retention and heating to the extent that the surface temperature is the same as the center temperature, rolling is started, and Ar
A method for producing a strong thick steel plate, characterized in that hot rolling is completed at a rolling ratio of 4 or more (rolling ratio ≧75z) at 3 points or more and a rolling ratio of 50X or more in the recrystallization region. ■As shown in Japanese Patent Application Laid-open No. 60-75518,
The slab obtained by continuous casting is charged into a heating furnace directly or before the completion of transformation, and a reduction of 5% or more is applied three or more times at a temperature above the recrystallization temperature to make the coarse T grains fine by partial recrystallization. Hot rolling, which is effective for improving low-temperature toughness, is characterized by successively applying a reduction of 15% or more three times or more at a temperature below 1050°C or above the recrystallization temperature, and making the T grains finer by large reduction rolling on the low-temperature side of the recrystallization zone. A rolling method is presented.

<Problems to be Solved by the Invention> In order to ensure the toughness of the steel sheet, the method (2) above transforms Ti oxide, which is finely and abundantly dispersed in the slab, through γ/α (ferrite) transformation to form a fine bainite structure. Therefore, there is a problem in that the cost associated with the addition of Ti increases. In addition, in method (2), in order to ensure the toughness of the steel plate, constraints are set on the rolling ratio, recrystallization zone rolling rate, and finishing temperature, but as shown in the examples, the rolling ratio is 1 to 4
There is a problem that toughness cannot be ensured when In the method (2) above, in order to make the solidified coarse T into fine grains, a reduction of 5% or more in one pass is applied three or more times at a temperature higher than the recrystallization temperature,
Partially recrystallize the solidified coarse T, and then continue to recrystallize at the low temperature side, specifically, as shown in the example, for Nb-added steel, 1 pass in the temperature range of 1050°C or lower and 900°C or higher.
The means of achieving this is to apply forceful pressure of Z or higher.
Rolling must be performed at least 6 times in total,
Rolling efficiency decreases as the number of passes increases. In addition, only Nb-added steel is shown in the examples, and Tp
Si-Mn steel (Hiroshi Sekine,
Tadakatsu Maruyama Tetsu to Hagane 5B (1972), ρ72), or Til, which is considered to have a higher Tρ than Si-Mn steel (
Kazuhisa Kurihara et al.: Tetsu to Hagane 73 (1987), 51399)
is not shown in the examples, and Si-1+I
The specific method for increasing the toughness of N steel and Ti steel is unknown. The present invention has the above-mentioned conventional technology: (1) increased cost due to addition of Ti for precipitated oxides;

? Toughness deterioration when the reduction ratio is 1 to 4.

■ Low-temperature toughness deterioration where the toughness of vTrs≦-60℃ required for shipbuilding E grade etc. cannot be stably obtained.

■Decrease in rolling efficiency due to an increase in the number of buses.

■It is unclear how to ensure the toughness of Si-Mn steel, Ti steel, etc.

We solve these problems by using manufacturing conditions that were not available with conventional rolling technology, and manufacture steel products such as plumes, slabs, slabs, steel plates, shaped steel, and steel pipes that have the same or better quality than the steel products obtained by this proposal. The present invention provides a method.

Means for Solving the Problems> In order to achieve the above problems, the present invention provides C: 0.10 to 0.
．． 20! After casting the molten steel, rolling is started in the temperature range of Tp or higher while the cough steel is at the Ar point temperature or higher, and rolling is performed to a casting thickness of 30z or more, so that the γ during solidification is at least IOz or more. Recrystallize, Ar 3 points -100℃
Continuously temperature range from above to 100℃ and above at speed 1
．． The basic method is to cool at a rate of 5° (/see or more and 40°C/sec or less), and furthermore, rolling should be started in a temperature range that is greater than or equal to the lower limit temperature at which austenite can be recrystallized + the lower limit temperature at which austenite can be recrystallized + 150°C or less. , ■ Finishing rolling at a temperature range of Ar3 or more and Arl +100°C or less, ■ Tempering at a temperature of Ac, or less, or
Or, the toughness is further improved by the synergistic effect of ■, ■, and ■.Specifically, it is characterized by using the following means. (1) After casting and solidifying the molten steel, while the steel is above the Arz point temperature, it is rolled to 30% or more of the casting thickness in a temperature range above the minimum recrystallization temperature of austenite, and then the Ar3 point -
1.5 for the temperature range from 100℃ or higher to 100℃ or higher
The first means is to cool continuously at a cooling rate of ℃/see or more and 40℃/sec or less, (2) C: 0.10 to o. 2oz S: ≦0.025%S
i: 0.01-0.80t AI:≦O. lZan
:0.20~2.00! After casting and solidifying a certain molten steel from the balance iron and unavoidable components, the steel is rolled at a temperature of 800°C or higher to a thickness of 30% or more of the casting thickness while the steel is at the Ar3 point temperature or higher.
The temperature range from point -100℃ or higher to 100℃ or higher is 1.
．． The second method is to cool continuously at a cooling rate of 5°C/see or more and 40°C/see or less, (3) C: 0.10 to 0.20 at weight 2! S:≦0.0257.
Si: 0.01~0.80Z Al: ≦0.1zM
n: 0.20 ~ 2.00! Nb: 0.002
~0. After casting and solidifying a certain amount of molten steel from the remaining iron and an unavoidable amount containing H, the steel is heated to 900° C. while the steel is above the Arz point temperature.
After rolling 30% or more of the casting thickness in a temperature range of ℃ or higher, the temperature range from 2 points -100℃ to 100℃ or higher is rolled at 1.5℃/sec or higher and 40℃ (: /see
The third method is to cool continuously at the following cooling rate: (4) C: 0.10 to o. 2oz S: ≦0.025%
Si:0.01~0.80zAl:≦0.1zMn:0
．． 20-2. OO! Ti: 0.002 ~ 0.1
After casting and solidifying a certain molten steel from the remaining iron and unavoidable components including
3 points - The fourth means is to continuously cool the temperature range from 100"C or higher to 100℃ or higher at a cooling rate of 1.5° (: /see or higher and 40° (: /sec or lower). 5) C: 0.10 ~o.zoz Al: ≦0.1%S at weight 2
i: 0.01 ~ 0.80% Nb: 0.002 ~ 0.1
%Mn: 0.20-2. OO! Ti:0.002
~0.1! S: After casting and solidifying the molten steel containing ≦0.025”t and the remainder iron and unavoidable components, rolling to 30% or more of the casting thickness in a temperature range of 950°C or higher while the cough steel is at the Ar 3 point temperature or higher. After performing Ar3
The temperature range from point -100℃ or higher to l00℃ or higher is 1.
．． The fifth means is to continuously cool at a cooling rate of 5° (/sec to 40°
Weight 2 and Cu:≦1! Zr:≦0.1zNi:
≦10Z Ca:≦0.008! Cr: ≦
1$ B:≦0.006zHo:≦lzR
EM: ≦0.0lχ ■: ≦0,2z Adding one or more of the following to any of the first to fifth means is the sixth to tenth means; (7) the sixth to tenth means; In any of the 1st to 10th means, the 11th to 20th means include starting rolling in a temperature range from the minimum temperature at which austenite can be recrystallized to the minimum temperature at which austenite can be recrystallized + 150°C; ) In any of the first to tenth means, two or more points of Ar,
(9) After cooling by any of the first to fortieth means, Ac,
The 41st to 80th means include tempering below the point.

The reasons for limiting the amount of each element added in each of the above methods are described below. C is because a steel material with excellent strength and toughness can be obtained economically by obtaining a structure consisting of ferrite and bainite by performing the cooling described in the present invention, and both Si and Mn have properties that improve toughness and weldability in addition to strength. From the viewpoint of performance, AI is limited to the above range from the viewpoint of economy, and S is limited to the above range from the viewpoint of toughness.

In addition, since Cu prevents hot cracking of weld metal, N
i is to ensure economic efficiency, Ti is not for precipitated oxides to refine the crystal grains, but to ensure the toughness of the weld heat affected zone, and Cr, No, V, and Nb are for heat affected zones. B improves hardenability and prevents hot cracking during the transformation process. Zr and Ca improve ductility and notch toughness, respectively. In terms of prevention and cleanliness maintenance, REM is limited to the above ranges for the same reasons as Ca. The purpose of addition of each element and its effects as described above are within the range of the purpose of addition and expected effects commonly used in this field of application. <Function> The present inventors used the test steel shown in Table 1 in order to solve the above-mentioned problems and establish a method for manufacturing high-toughness steel materials with excellent thermoeconomic efficiency and high practicality. We conducted many experiments and studies.

table l Figure 1 shows the minimum toughness values at each rolling reduction.

From Figure 1, the inventors have determined that the casting thickness is 30! in the temperature range above tp. When the above-mentioned rolling is performed and subsequently cooled within the range of cooling conditions described in the present invention, the structure becomes a certain structure consisting of ferrite grains of 30μ1 or less and bainite, and a large size on the low temperature side of the recrystallization region (lower than Tp-.-7ρ+150℃) is formed. There is no need to repeat reduction rolling.By ensuring a rolling reduction of 30% or more, vTrs≦− as shown in FIG. 1(a).
It was discovered that toughness of 60°C can be obtained smoothly and stably.

Furthermore, when the rolling reduction ratio is less than 30χ of the casting thickness, the ferrite grains do not become less than 30//II even if the cooling of the present invention is performed, and as shown in Fig. 1(b), when vTrs≦-60℃, They discovered that toughness cannot be obtained. Further, as shown in FIG. 2 (1) with a rope, rolling to a casting thickness of 30z or more is started in a temperature range of Tp or higher, and rolling is finished in a temperature range of Ar3 or higher, and then the present invention is continued. When cooled, the structure becomes a fine-grained structure consisting of ferrite grains of less than 30 μm and bainite, and the toughness level at that time is vTrs
It was found that it is stable at ≦-60°C. Further, the method described in the present invention is performed after either: ■ rolling is started in a temperature range of Tp or higher and Tp + 150°C or lower, or ■ rolling is finished in a temperature range of Arz point or higher and Ar3 point + lOO°C or lower, or both are performed. When cooled, the M weave becomes 25μ1
The fine-grained structure consists of the following ferrite grains and bainite, and the toughness level is as shown by hatching in Figure 2 (2) for ■, and as shown by hatching in Figure 2 (3) for ■. They found that it is stable when vTrs≦-70"C or less, and the desired purpose can be achieved. If the finishing temperature is below the Arz point, processed ferrite remains, and when vTrs≦-60℃, it is stable I can't.

Furthermore, after rolling that satisfies the conditions of the present invention, as shown in FIGS. 3 and 4, Ar 3 points -100° C.
Temperature range from 0℃ to 1.5℃/see to 40℃/
When cooled below sec, fine ferrite and bechinite are generated, and the strength of TS≧50kgf/am” and vTr
A toughness of s≦-60°C was obtained.

When the cooling start temperature is Ar. When the temperature drops below -100°C, coarse grain boundary ferrite grows; when the cooling stop temperature falls below 100°C, when the cooling rate exceeds 40°C/sec, bainite increases or martensite grows; together v
Trs did not satisfy -60℃. The present invention was made based on the above knowledge, and by implementing the present invention,
The objective is to reform the manufacturing method of steel materials by DR and HCR, which is inevitably carried out, and to dramatically improve the practicality of these methods in terms of thermo-economic efficiency, productivity, etc.

<Example> (Example 1) Table 2 shows the chemical composition of the test steel used in each of the invention examples 1 to 20 and each comparative example, and Tables 3 and 4 show the chemical composition of each steel plate. The manufacturing conditions and the obtained material are shown. As is clear from the table, all of the steel numbers 1 to 30 of the invention examples showed vTrs of -60°C or less, and steel materials having the desired material properties were obtained. In contrast to these examples of the present invention, steel numbers 31, 32, 34, 35, 37, 38, 40, and comparative examples in which any of the rolling reduction, rolling start temperature, and finishing temperature do not satisfy the range of the present invention, 4
2, 43, 45, 47, 49, 50, 53, 54, 56
, 57.58, the vTrs did not reach -60 °C because the ferrite grains did not become finer than 30μ. In addition, steel numbers 33 and 36 of comparative examples in which any of the cooling start temperature, cooling stop temperature, and cooling rate do not satisfy the range of the present invention
．． 39, 41, 44, 46.4B, 51, 52, 55,
In case of 59.60, the vTrs did not reach -60°C because the ferrite grains did not become finer than 30.17L. (Example 2) Table 2 shows the chemical composition of the test steels used in the 21st to 40th invention examples and their comparative examples, and Tables 5 and 6 show the manufacturing conditions and yields of each steel plate. Indicates the material used.

As is clear from the table, all of the steel numbers 61 to 90 of the invention examples showed vTrs of -70°C or less, and steel materials having the desired material properties were obtained.

In contrast to these inventive examples, steel numbers 91, 92, 94, 95, 97.98, and 100 were comparative examples in which any of the rolling reduction, rolling start temperature, and finishing temperature did not satisfy the range of the present invention.
,102, 103, 105, 107, 109,
110, 113, 114, 116, 117,
No. 118 has ferrite grains that do not become finer than 25 μm,
vTrs did not even reach -60°C, let alone -70°C. In addition, steel number 93.96 of a comparative example in which any of the cooling start temperature, cooling stop temperature, and cooling rate does not satisfy the range of the present invention,
99, iot, 104, 106, 108,
111, 112, 115, 119, 120
The ferrite grains do not become finer than 25μ1, and vTr
s did not even reach -60°C, let alone -70°C. (
Example 3) Table 2 shows the chemical fractions of the test steels used in each of the invention examples 41st to 80th and their comparative examples, and Tables 7 and 8 show the manufacturing conditions of each steel plate. and the resulting material.

As is clear from the table, steel numbers 121 to 150 of the invention examples all exhibited vTrs of -70°C or less, and steel materials having the desired material properties were obtained. In contrast to these examples of the present invention, steel numbers 151, 152, 156, 158, 160, and 16 of comparative examples in which any of the rolling reduction, rolling start temperature, and finishing temperature do not satisfy the range of the present invention
1162, 166, 167, 168, 173,
In Nos. 174 and 177, the ferrite grains were not refined to less than 25 μm, and vTrs did not reach -60°C let alone -70°C. Comparative example Steel No. 153,1 in which any of the cooling start temperature, cooling stop temperature, and cooling rate does not satisfy the range of the present invention
57, 163, 164, 169, 170, 17
1, 175, 178. In the case of No. 179, the ferrite grains did not become finer than 25 μm, and vTrs did not reach -60°C let alone -70°C. Also, does the tempering temperature not meet the range of the present invention? Comparative steel numbers 154, 155,
159, 165, 172, 176, and 180, the ferrite grains were not refined to 25 μ■ or less, and vTr
s did not even reach -60°C, let alone 70°C. <Effects of the Invention> The present invention provides c:o. After casting and solidifying molten steel from to~0.20χ, solidify γ to 30% of the casting thickness in a temperature range of Tp or higher.
The above rolling is performed to recrystallize the γ during solidification to at least 10χ, and then the temperature is increased from Ar3-too℃ to 100℃.
Temperature range up to 1.5”C/sec or above 40℃
By continuously cooling at a cooling rate of /sec or less, a certain structure is obtained from fine ferrite and bainite, which increases the strength and
By improving toughness at the same time, the problems of the conventional technology are basically solved, and furthermore, rolling can be started in the temperature range of Tp or higher and τρ+150 or lower, or finish rolling can be started at a temperature range of Ar■ point or higher and Ar3
The toughness is further improved by tempering the cast steel at a low temperature of +100°C or lower, and then tempering the cast steel at a temperature lower than the Ac+ point. In the case of OR, which is used for pushing through, and in the case of HCH, where the billet is reheated from the point at which it is in the Ar3 temperature range or higher and then subjected to rolling, steel materials with excellent toughness manufactured by each of these are used with good thermoeconomic efficiency. It has great effects in this field, such as making it possible to manufacture products with low productivity.

[Brief explanation of drawings]

Figure l shows the rolling reduction ratio and v of the steel material cooled in the present invention.
Figures 2 (1) to (3) are diagrams illustrating the relationship between vTrs and the rolling start temperature and finishing temperature of steel materials that have been cooled in the present invention. Figure 3 is a diagram illustrating the relationship between the cooling rate after rolling of the steel material rolled in the present invention and vTrs, TS, and Figure 4 is a diagram showing the cooling start temperature and cooling after rolling of the steel material rolled in the present invention. It is a diagram showing the relationship between stop temperature and vTrs.

Claims (9)

[Claims] (1) After casting and solidifying the molten steel, while the steel is at the Ar_3 point temperature or higher, it is rolled to 30% or more of the casting thickness in a temperature range of at least the lower limit temperature at which austenite can be recrystallized, and then at the Ar3 point -100°C. 1. Temperature range from above to 100℃ or above.
A method for manufacturing a steel material with excellent strength and toughness, characterized by continuously cooling at a cooling rate of 5° C./sec or more and 40° C./sec or less. (2) C: 0.10-0.20% S: ≦0.025% Si: 0 in weight%
．． 01~0.80% Al: ≦0.1% Mn: 0.20~
After casting and solidifying molten steel containing 2.00% iron and the balance consisting of iron and unavoidable components, the steel is rolled at a temperature of 800°C or higher to a thickness of 30% or more of the casting thickness while the steel is at a temperature of Ar_3 or higher. , Ar
_3 points - Production of steel materials with excellent strength and toughness characterized by continuous cooling in the temperature range from 100°C or higher to 100°C or higher at a cooling rate of 1.5°C/sec to 40°C/sec. Method. (3) C: 0.10-0.20% S: ≦0.025% Si:
0.01-0.80% Al:≦0.1%Mn:0.2
After casting and solidifying molten steel containing 0-2.00% Nb: 0.002-0.1% and the balance iron and unavoidable components, it is cast in a temperature range of 900°C or higher while the steel is at Ar_3 point temperature or higher. After rolling 30% or more of the casting thickness, Ar_3 points-
1.5 for the temperature range from 100℃ or higher to 100℃ or higher
A method for manufacturing a steel material with excellent strength and toughness, characterized by continuously cooling at a cooling rate of ℃/sec or more and 4℃/sec or less. (4) C: 0.10-0.20% S: ≦0.025% Si:
0.01-0.80% Al:≦0.1%Mn:0.2
After casting and solidifying molten steel containing 0-2.00% Ti and the remainder iron and unavoidable components, it is cast in a temperature range of 950°C or higher while the steel is at Ar_3 point temperature or higher. After rolling 30% or more of the casting thickness, Ar_3 points-
1.5 for the temperature range from 100℃ or higher to 100℃ or higher
A method for manufacturing a steel material with excellent strength and toughness, characterized by continuously cooling at a cooling rate of ℃/sec or more and 40℃/sec or less. (5) C: 0.10-0.20% Al: ≦0.1% Si: 0.
01~0.80%Ti:0.002~0.1%Mn:0
．． 20~2.00%Nb:0.002~0.1%S:≦
After casting and solidifying molten steel containing 0.025% iron and the balance consisting of iron and unavoidable components, the steel is rolled at a temperature of 950°C or higher to a thickness of 30% or more of the casting thickness while the steel is at a temperature of Ar_3 or higher. , Ar
_3 points - Production of steel materials with excellent strength and toughness characterized by continuous cooling in the temperature range from 100°C or higher to 100°C or higher at a cooling rate of 1.5°C/sec to 40°C/sec. Method. (6) In weight% Cu: ≦1% Zr: ≦0.1% Ni: ≦10% Ca: ≦0.008% Cr: ≦1% B: ≦0.006% Mo: ≦1% REM: ≦ Production of a steel material with excellent strength and toughness according to any one of claims 1 to 5, characterized in that it contains one or more of 0.01% V:≦0.2%. Method. (7) In the method according to any one of claims 1 to 6, rolling is started in a temperature range from the minimum temperature at which austenite can be recrystallized to the minimum temperature at which austenite can be recrystallized + 150°C or less. A manufacturing method for steel with excellent strength and toughness. (8) A method for manufacturing a steel material with excellent strength and toughness, characterized in that rolling is completed at a temperature of Ar_3 points or more and Ar_3 points + 100°C or less in the method according to any one of claims 1 to 7. . (9) A method for manufacturing a steel material with excellent strength and toughness, which comprises cooling by the method of claims 1 to 8 and then tempering at a temperature of Ac_1 point or lower.