JPS61163213A - Manufacture of steel plate superior in strength and toughness - Google Patents

Abstract

PURPOSE:To manufacture the titled steel plate at a low cost using slab having low Mn content without requiring controlled rolling, by adding suitably combined contents of Mn, C, Si into slab being stock, in hot rolling continuously cast slab. CONSTITUTION:By weight, 0.09-0.18% C, 0.05-0.50% Si, 0.2-0.7% Mn, 0.005-0.1% Al, <0.006% N are added, if necessary, one or >=2 kinds among ranges of <0.1% Ti, Zr, Nb, Ta, V, Ca, <1.0% Ni, Cr, Mo, Cu, <0.003% B, further contents of C, Si, Mn are added so that formulas (1), (2) are satisfied. Consequently, slab of the steel is hot rolled at <=1,200 deg.C by >=50% draft, and hot rolling is ended at <=950 deg.C. This is cooled from >=700 deg.C to <=200 deg.C by >=15 deg.C/sec rate, further if necessary, reheated to <=Ac1 point. Hot rolled titled steel plate is manufactured with low Mn content without controlled rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an economically efficient manufacturing method of a steel plate having a low Mn content and desired strength and toughness without performing controlled rolling.

(Prior Art) One of the conventional methods for manufacturing water-cooled high-strength steel is, for example, as shown in Japanese Patent Publication No. 55-30047.
The rolling end temperature of steel to which wt% or more of Mn is added is 800.
There is a method of obtaining excellent strength and toughness by cooling the steel plate after hot rolling to a temperature of 2 to 10 °C or less. In this manufacturing method, 0.5 wt% or more of M is added in order to obtain excellent toughness.
In addition to adding n, so-called controlled rolling, in which hot rolling is performed at a relatively low temperature, has become an essential requirement, which has disadvantages in industrial production such as restrictions on reducing alloy costs and reduced rolling efficiency. are doing.

Also, as shown in Japanese Patent Application Laid-open No. 55-28318,
The upper limit of 0 is 0.09%, and the steel containing 0.5% or more of Mn is cooled to 500°C or less after normal hot rolling to produce high tensile strength steel of 50 kg class or higher with excellent weldability. There is a manufacturing method. Although this method avoids a decrease in rolling efficiency, it is essential to add Mn in an amount of 0.5% or more and 0.18% or less, which has the drawback of limiting alloy cost reduction.

Furthermore, in Japanese Patent Application No. 58-85588, the lower limit of 00 is set to 0.
．． A method for manufacturing ferrite and pearlite-based copper with 18wt% binding and less than 6wt% of Mn is shown, but what is this? cm value 1,
1 1 1 1 (P Cm
"0 + aoS' +20 Mn+200
u + e o N' + 20Or+πMO+10
It has disadvantages such as increasing V +5 B) and increasing weld hardenability.

In addition, it is impossible to obtain a strength of 50 kyf/ld class with conventional steel sheets manufactured by hot rolling in a non-water-cooled type with an Mn content of about 0.9% or less, and the toughness level is also below vTr.
It has the disadvantage that only low characteristics can be obtained at temperatures of about 20°C.

(Problems to be Solved by the Invention) The present invention solves the alloy cost that conventional water-cooled high-strength steels have in order to obtain high-strength steels with excellent toughness by combining a large amount of Mn addition or controlled rolling. There are economical disadvantages such as increased manufacturing costs due to pressure on production and reduced production efficiency, and the high weld hardening properties of low-Mn based water-cooled materials due to their high O-based content, making them uneasy to use. etc. items.

This method also solves the purchasing problem, and provides a method for manufacturing steel sheets that have excellent strength, toughness, and weldability even when Mni is reduced by an appropriate combination of O and Si contents. This makes it possible to provide steel materials with excellent economic efficiency for ridge applications.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides 0:
More than 0.09Wt tX and less than 0.18wt%, Si: 0.
05 wt% or more and less than 0.50 wt%, Mn: 0.
2 wt% or more and 0.7 wt% or less, Al: 0.00
5 wt% or more and less than 0.1 wt%, N: 0.006
Contains less than wt% and optionally 'rt, zr,
Nb, Ta, v, oa 0.1 wt% or less,
Ni, Cr, MO, (lu not more than 1.0 wt%, B
After continuous casting of steel with one or more types added in a range of 0.003 wt% or less and the remainder consisting of re and unavoidable impurities, rolling is performed at a temperature of 50% or more at a temperature of 1200°C or less, and the rolling end temperature is 950°C or less. and the temperature of the steel plate is 700
A method for manufacturing a steel plate with excellent strength and toughness, characterized by cooling from ℃ or higher to 200℃ or lower at a cooling rate of 15℃/sec or higher, and 0:0.09wt% or more than 0.18
Less than wt%, S': 0.05 wt% or more O05Q
Less than wt'X, Mn: 0.2WtX or more and 0.7wt% or less, Al: 0.005wt% or more and less than 0.1wt%,
N: Contains less than 0°006wt%, and Ti as necessary.
, Zr, Nb, Ta, V, Oa'i0. l
Below wt, Ni.

Cr, MO+Cu 1. Qwt% or less, B is 0.0
0.03 After continuous casting of steel consisting of one or more kinds added in a range of 0.03 wt% or less and the balance being Fe and unavoidable impurities, 1.
After rolling 50% or more at a temperature of 200°C or lower to bring the rolling end temperature to 950°C or lower, and cooling the steel plate from 700°C or higher to 200°C or lower at a cooling rate of 15°C/sec or higher, Ac, point A method for manufacturing steel plates with excellent strength and toughness is adopted, which involves reheating the steel plate below the plate. In each of the above methods, it is preferable to add O, Si, and Mn within a range that satisfies the following formulas 0 and [2].

Mn (wt%)≧0.099TS-11.200-2.
731Si-2.379...■ Mn(wB, 0≧-0.061vTrs+2.870-
7.92Si-0.687...■ (Here, TS is tensile strength (kyf/mA), v'l"r
s represents the fracture surface transition temperature (°C). ) Each limitation of the constituent elements of the method of the present invention was determined based on the following reasons.

C is an effective element for increasing strength, but too much C increases the Pcrn value and impairs weldability, so the upper limit is set at 15 wt.
limited to less than %. In addition, if the C content is less than 0.09 wt%, the strength will be insufficient, so the range of C added should be reduced to 0.09 wt% or less.
．． It was set to less than 18 wt%.

In addition to deoxidizing and strengthening the base steel, 3i is effective in improving toughness by adding 0.05 wt% or more, so the lower limit is set to 0.0.
5WtXK limited. Also, if the amount is too high, weldability and HA
Since it is harmful to the Z part toughness, the upper limit is set to 0. It was set to less than 5 wt%.

Mn is an element that increases toughness, but if used in large quantities, it reduces the alloy reduction benefits of water-cooled high-strength steel, so the upper limit is set to 0.7 wt% or less, and Mn is 0.2 wt%.
If the Mn content is less than 0.2 wt% or more, the toughness may deteriorate or hot cracking may occur in the slab.
The following was made.

Al is necessary for deoxidation and grain refinement, and is limited to 0.005 wt% or more and less than 0.1 wt% as a sufficient amount for this purpose.

N is 0.0 to maintain good weldability and toughness of the male part.
0.6 wt% or less.

When manufacturing steel with the above composition range), cast slabs before heating (
It is important to prevent the solidification structure of the slab (hereinafter referred to as slab) and the coarsening of precipitates of Al and other trace elements, and to make the austenite grains finer during heating of the slab, making a continuous casting process an essential requirement.

The rolling temperature of the slab is preferably low from the point of view of refining austenite grains, but it is set to a certain degree of temperature in consideration of the temperature drop during rolling. When rolling the slab in continuous casting, or when heating the slab to a desired temperature, if the slab is rolled at a temperature higher than the limit for grain refinement, the toughness targeted by the present invention cannot be obtained, so the rolling start temperature of the slab is the upper limit of 120
The temperature was 0°C.

Next, since the rolling reduction during rolling has a large effect on the grain refinement of austenite, a total rolling reduction of 50% or more was specified.

The upper limit of the rolling end temperature was set at 950° C., which could ensure m properties without impeding productivity.

It is preferable that the time after rolling until the start of cooling is as short as possible. Leaving it for a long time after rolling will cause coarsening of crystal grains and temperature unevenness, and furthermore, if the temperature drops, ferrite transformation will start, so it should be kept for a maximum of 10 minutes. It is necessary to start cooling from 700°C or higher.

The cooling rate during cooling was set to 15° C./sec or more, which is effective for improving the strength and toughness of steel. Also, the temperature at which cooling is completed,
That is, the lower the cooling stop temperature, the greater the cooling effect, and the upper limit required to form a ferrite bainite structure in the above component range was limited to 200° C. or less.

In addition to the numerical limitations of the above-mentioned specifications, the present invention further includes Ti,
Zl', Nb, Ta, V, Oa at 0. l
, wt% or less, Ni.

Cr, Mo, Cu f 1. Owt% or less, B is 0
．． In the case of steel in which one or more types are added in the range of 003 wt% or less, 50K has even better strength and toughness.
It is possible to produce f/- treated copper, and when it is further cooled to 200° C. or lower and then reheated to a temperature of Ac1 or lower, a thick steel plate with even better mechanical properties can be obtained.

(Function) Conventionally, in order to improve the strength and toughness of thick steel plates with low carbon and low alloy components, it has been necessary to use approximately 0.7W regardless of water-cooled or non-water-cooled
Addition of t% or more of Mn is said to be effective, and controlled rolling is essential. In the present invention, the present inventors have considered the effect of Mn on improving toughness, and have determined that even if Mn is lowered, Sl can be increased from 0.05 wt% to 0.05 wt%. This was based on the discovery that adding an appropriate amount of 5 wt% makes it possible to produce thick steel plates with excellent strength and toughness without using controlled rolling, which reduces rolling efficiency due to controlled cooling after rolling. Tsumashi: The present invention reduces the Mn element, which is a feature of water-cooled high-strength steel, without impeding productivity by reducing Si to 0.05 to 0.
．． 5wt%, and provides an economical manufacturing method for steel materials with excellent strength and toughness as well as weldability, especially similar to coarse upper bainite, which was conventionally thought to cause deterioration of toughness. Despite exhibiting a bainite structure, the addition of an appropriate amount of Si creates a structure that does not cause deterioration in toughness, that is, a low-Mn ferrite-9 bainite structure. This point will be discussed in detail below.

1 to 6 show the results of experiments conducted by the present inventors for the above study.

Figures 1 and 2 show C-shape of 0.09.0.13 in steel containing S' f 0.25 wt% and Mn 0.6 wt%.
．． and [2], with three levels of 17 wt%, and a slab of 1
The strength (TS) and toughness (vTrs) of a steel plate heated to 050°C, hot rolled to a thickness of 301 m, and then cooled to 200°C or less at a cooling rate of 15°C/sec are shown. As is clear from FIG. 1, the strength (TS) increases with the amount of O.

Furthermore, as is clear from FIG. 2, the toughness (vTrs) deteriorates somewhat.

In Figures 3 and 4, 0 is 0.15 wt%, Mn is 0
．． 5W'
．． Strength (
TS), toughness (vTrS). As is clear from FIG. 3, the intensity (TS) increases with 5ili. Furthermore, as is clear from FIG. 4, the toughness (v'l'rs) is also improved.

In Figures 5 and 6, C is 0. L 3 wt%, in steel containing 0.25 wt% Si, the amount of Mn was set to 0.15.0.
35 and 0. 7 'Q wt%, and the above 2
Strength (TS) of steel plate obtained by the same method as in the example case
, indicates toughness (v'l'rs). As is clear from FIG. 5, the strength (TS) increases with the amount of Mn. Furthermore, as is clear from FIG. 6, the toughness (vTrs) is also improved.

As is clear from the above results, the strength (TS) is C15
In the case of both elements i and Mn, the toughness (vTrs) increases as the amount added increases, and the addition of Si and Mn improves the toughness (vTrs), while the addition of C deteriorates the toughness (vTrs).

The structure obtained under the above conditions consists of relatively coarse ferrite and upper bainite, as shown in FIG. 9, for example. It has been said that such a structure deteriorates toughness, and it is usually fine ferrite. Pearlite structure, fine ferrite to bainite structure or lower bainite structure, tempered martensitic structure, etc. are said to be preferable for toughness. However, Si from 0.05
When adding about 0.5wt%, the fi and M are different from the conventional wholesale.
n amount is 0. Even with a low Mn content of about 7 wt% or less, not only the strength but also the toughness improves, although the structure is a relatively coarse ferrite/bainite structure as described above.
A new fact has been discovered that controlled rolling is not required.

Figure 7 shows the effect of C1 on the strength (TS) of the above results.
Figure 7 shows the correlation between the estimated TS value based on the prototype of Equation 0 and Equation 90 obtained from the multiple regression results of the effects of Si and Mn contents, and the actual measured value. This can be said to show that estimation is possible with an accuracy that does not pose any practical problems.

FIG. 8 similarly affects v'1'rs. , Si,
Figure 8 shows the correlation between the vTrSO value estimated based on the prototype of Equation 0 and Equation 90 (based on the multiple regression results of the influence of Mn amount) and the actual measured value. It can be said that this shows that it is possible to estimate with an accuracy of 0.

1.1 'rs=24+113 (0+';781 +n7Mn
)...■V'rrS = -11,2+ 46.80
-129.1'S 1-16.3 Mn , , ■ In addition,
The above formulas (1) and (2) are obtained by converting each of the above formulas (2) and (2) into formulas for determining the amount of Mn necessary to obtain desired strength and toughness.

(Example) Table 1 shows the hot rolling conditions and cooling conditions for each steel with each component listed in the table, and continuous cast material with a thickness of 180 degrees Celsius at 1050°C.
After heating, hot rolling is carried out, and the rolling end temperature is 950°C or less, and the plate thickness is 30cm, and the cooling rate is 15cm from 820°C or higher.
It was cooled down to 100°C or less at a rate of 100°C/sec, and the mechanical properties are the results of a tensile test and an impact test.

Steel A-E contains approximately 0.3wt% Si and 0.65w Mn.
(Steel containing about t%, C content is 0.073 to 0.191W
t
In contrast, Comparative Steel A had good toughness but a strength (TS) of 50 kgf/- or less, clearly demonstrating the strength increasing effect of the present invention's zero addition. Although Steel E is a comparative steel and has sufficient strength and toughness, it has the disadvantage of impairing weldability because it has a high Pα (0.23) component system. I can't say it's a thing.

Steels F to J contain approximately 0.15 wt% of C and Mn'! 1l-
Steel containing approximately 0.65 wt%, with an Si content of 0.01 to 0.
Inventive steel G, H, I
In contrast, the comparative steel F has a TS of 50 or more and vTrs of -25°C to -65°C.
When the TS was 501 qf/- or less, the VT r 13 also deteriorated to -10°C, and the strength and toughness improving effect of the Si of the present invention was clearly recognized. Further, although Steel J is a comparative steel and has sufficient strength and toughness, it cannot be said to be practically sufficient in terms of weldability and HAZ toughness.

-0 for steel is approximately 0.15 wt% O and 0.25 wt% Si.
The amount of Mn is 0.15 to 0.89 w with components containing about wt%
t%, the steels of the present invention, M, and N all show good values of TS of 50 kgf/- or higher and vTrs of -40°C or lower, whereas the comparative steel has a high strength (TS). is 5
This is an insufficient value of 0 kqf/- or less, and the Mn of the present invention
The effect of improving strength and toughness is recognized. Furthermore, although Steel O has sufficient strength and toughness as a comparative steel, the use of a large amount of Mn reduces the alloy reduction benefits of water-cooled high-strength steel, so it was excluded from the scope of the present invention.

Table 2 shows C at 0°169 wt% and f9i at 0.15
The effect of the manufacturing conditions shown in the table was investigated on steel containing 0.62 wt% of Mn, and the results of a tensile test and an impact test are shown as its mechanical properties.

n Kaisho Gl-163213 (8) Steel P is a comparative steel with a rolling start temperature of over 12,000°C, and when compared with the steel with a rolling start temperature of 1,200°C, the toughness lzher is inferior by about 35°C in terms of v'I'rs, and this steel The effect of improving toughness by ensuring the rolling start temperature of the invention is clearly recognized.

The steel box, S, has an influence on the rolling end temperature, and the v'1"rs of the present invention @S, where the rolling end temperature is 942°C, is -35°C.
On the other hand, the comparison steel strip with a rolling finish temperature of 976°C had an inferior vTr s of 118°C, which clearly shows the toughness improvement effect achieved by securing the rolling finish temperature of the present invention.

Steel T has a low cooling start temperature of 685°C, and when compared with Invention Steel Q, which has a cooling start temperature of 837°C, its strength is lower.
Both toughness and toughness are poor, and it is clear that securing the cooling start temperature of the present invention is effective in improving strength and toughness.

Steel U has a high cooling stop temperature of 350°C, and is inferior in both strength and toughness when compared to the steel of the present invention, which has a cooling stop temperature of 100°C or lower. It is clear that it is effective.

Steel V has a slow cooling rate of 8°C/sec, and is inferior in strength and toughness when compared to the cooling rate of Inventive Steel Q of 15°C/sec, indicating that securing the cooling rate of the present invention is effective for improving strength and toughness. is clear.

(Effect of the invention) As explained above, the present invention provides 0.05 wt% of 81.
By adding less than 5wt% of Mn, the amount of Mn added can be increased by 0.2 to 0. 7 wt%, while TS 5
A steel material with high strength, high toughness, and excellent weldability (C) of vTrs -25°C to -65°C at 0 o f / - or more can be manufactured without using controlled rolling that reduces productivity, This has great industrial effects, such as making it possible to supply high-quality and inexpensive steel materials to this type of application field.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of 0 on strength, Figure 2 is a graph showing the influence of 0 on toughness, Figure 3 is a graph showing the influence of 0 on toughness, Figure 3 is a graph showing the influence of Sl on strength, Figure 5 is strength. Fig. 6 is a graph showing the influence of Mn on toughness, Fig. 7 is a graph showing the relationship between the estimated value of TS by Equation 0 and the actual measured value, and Fig. 8 is the graph showing the estimated value of vTr s by Equation 0 and the actual measurement. A graph showing the relationship between the values, and FIG. 9 is a photograph of the microstructure of the steel of the present invention. Agent Patent Attorney Masamitsu Aki Sawa and 2 others C(wt'1) -A'1
t%) -A'3 Figure intensity t; influence of 5L C(wt'l) r2 [Zl htl-i tJ+rsC(7)
Kage '4' power, 4-figure potato blade, f raw 1chi attack 5-Kage VTS
Jf! 5tII (K3/mm') Mrr (wt
'/,)

Claims (4)

[Claims] (1) C: more than 0.09wt% and less than 0.18wt%, Si
:0.05wt% or more and less than 0.50wt%, Mn:0.
2wt% or more and 0.7wt% or less, Al: 0.005wt
% or more and less than 0.1 wt%, N: less than 0.006 wt%, and optionally contains Ti, Zr, Nb, Ta, V,
Ca: 0.1 wt% or less, Ni, Cr, Mo, Cu: 1
．． 0 wt% or less, one or more types of B are added in the range of 0.003 wt% or less, and the balance is Fe and unavoidable impurities.
Perform the above rolling to reduce the rolling end temperature to 950°C or less,
A method for producing a steel plate with excellent strength and toughness, characterized in that the temperature of the steel plate is cooled from 700°C or higher to 200°C or lower at a cooling rate of 15°C/sec or higher. (2) Within the range that satisfies the following formulas [1] and [2], C
, Si, and Mn are added. 1. The method for producing a steel plate with excellent strength and toughness according to claim 1. Mn (wt%)≧0.099TS-11.20C-2.
731Si-2.379...[1] Mn (wt%)≧-0.061vTrs+2.87C-
7.92Si-0.687...[2] Here, TS is tensile strength (kg^f/mm^2), vTr
s represents the fracture surface transition temperature (°C). (3) C: more than 0.09wt% and less than 0.18wt%, Si
:0.05wt% or more and less than 0.50wt%, Mn:0.
2wt% or more and 0.7wt% or less, Al: 0.005wt
% or more and less than 0.1 wt%, N: less than 0.006 wt%, and optionally contains Ti, Zr, Nb, Ta, V,
Ca: 0.1 wt% or less, Ni, Cr, Mo, Cu: 1
．． 0 wt% or less, one or more types of B are added in the range of 0.003 wt% or less, and the balance is Fe and unavoidable impurities.
Perform the above rolling to reduce the rolling end temperature to 950°C or less,
A method for manufacturing a steel plate with excellent strength and toughness, characterized in that the temperature of the steel plate is cooled from 700°C or higher to 200°C or lower at a cooling rate of 15°C/sec or higher, and then reheated to Ac_1 point or lower. (4) C within the range that satisfies the following formulas [1] and [2],
The method for producing a steel plate with excellent strength and toughness according to claim 3, wherein Si and Mn are added. Mn (wt%)≧0.099TS-11.20C-2.
731Si-2.379...[1] Mn (wt%)≧-0.061vTrs+2.87C-
7.92Si-0.687...[2] Here, TS is tensile strength (kg^f/mm^2), vTr
s represents the fracture surface transition temperature (°C).