JPS5952686B2 - Manufacturing method of non-thermal high tensile strength steel with excellent low temperature toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing non-tempered high tensile strength steel, and more specifically, non-tempered high tensile strength steel with a tensile strength of 60 kg/m! The present invention relates to a method for industrially easily and stably producing high-tensile and high-toughness steel having a strength of L or higher.

Generally, the tensile strength is 60kg/60kg without heat treatment, that is, without heat treatment, it has high strength and excellent low temperature toughness.
- When producing high-strength steel with the above properties, a method is known in which Nb-containing steel is subjected to controlled rolling and then accelerated cooling is performed.

In this method, for example, as shown in JP-A-53-97922, a large pressure is applied in the austenite final recrystallization temperature range, and cooling is started from Ar3 or above.

Usually, the temperature of the austenite non-recrystallized region of Nb-containing steel subjected to such controlled rolling is 950°C to Ar3, preferably 900°C to Ar3, and Ar3 increases due to strain-induced transformation due to rolling. Usually, the temperature is around 770°C, and in fact, a large amount of pressure reduction is applied within a narrow temperature range of 900 to 770°C.

Generally, the lower the rolling temperature, the higher the deformation resistance of steel, and in Nb-containing steels, the deformation resistance increases due to the precipitation of Nb.

For this reason, when rolling at a low temperature of 900 to 770°C, it is necessary to reduce the rolling reduction rate each time due to the limits of the rolling mill's capacity, which inevitably requires an increase in the number of rolling passes. .

On the other hand, as the thickness of the steel plate decreases as rolling progresses, the cooling rate of the steel plate increases, and in this temperature range where the temperature of the steel decreases significantly, conventional methods require many rolling passes, making it difficult for the industry to In the commercial production process, the cooling start temperature tends to fall below Ar3, which causes a partial transformation of austenite to ferrite, resulting in deterioration of strength and toughness.

For example, when cooling using a cooling device such as a roller quench, in the conventional method, the rolling finish temperature is just above Ar3, so even if the leading edge of the steel plate can be cooled from Ar3 or higher, the temperature at the trailing edge of the steel plate will drop below Ar3. , strength and toughness deteriorate, making it difficult to obtain steel sheets of stable quality.

In view of these circumstances, the present inventors have completed the present invention after repeated studies on the steel components, slab heating temperature, rolling conditions, and cooling conditions.

That is, the present invention provides a steel containing Fe, the remainder being substantially Fe, or a steel containing V: 0.01 to 0.01 in addition to these components.
15%, Ni: Q, 1-0.5%, Cr: 0.1-
0.5%, Cu: 0.1-0.5%, Mo: 0.0
5-0.50% or B: 0.0005-0.0
A steel containing one or more of 025% and the remainder substantially consisting of Fe is heated to a temperature range of 1100°C to 1280°C, and in the temperature range from this heating temperature to 1000°C, the cumulative rolling reduction rate R: R =-0, IT + 170% (T: heating temperature) or more, and then repeatedly rolled until the cumulative reduction rate becomes 60% or more in the temperature range of 1000 to 900°C, and then Ar3 or more. Average cooling rate 10℃/s from temperature to below 650℃
It is characterized by cooling at a rate of ec to 100°C/sec.

The structure of the steel sheet produced by the method of the present invention is fine low-carbon bainite, and by reducing the amount of C in particular, it is possible to industrially easily provide a steel sheet with strength and toughness comparable to steel sheets produced by the conventional method. .

The method of the present invention will be sequentially explained below, starting from the reason for limiting the range of steel components.

First, if the C content is less than 0.03%, high strength cannot be obtained, and if it is more than 0.09%, weldability is impaired, and the heating-rolling process in the present invention
Under cooling conditions, coarse low-temperature transformation products are mixed in and the toughness is impaired.

Next, since Si promotes deoxidation of steel and increases its strength, it is added in an amount of at least 0.03%.

However, if too much Si is present, the toughness and weldability will be significantly impaired, so it is limited to a maximum of 0.5%.

Next, Mn increases the strength and toughness of steel, and is actively added in the present invention, at least 0.50% or more.

However, if Mn is added in excess of 2.0%, weld cracking susceptibility is significantly increased, so this is set as the upper limit.

Next, AI acts as a deoxidizer in the steelmaking process,
In addition, nitrides are formed to refine the structure.

However, if the amount is too large, inclusions will increase, and for this reason, Al is added in an amount of 0.05% or less.

Next, in the method of the present invention, as described above, C1Si, M
In addition to adding n and AI within appropriate ranges, Nb is also added.

Nb is dissolved in steel by high-temperature heating, and precipitates as carbonitrides during rolling.

Therefore, the recrystallization of austenite grains is significantly delayed, and the austenite grains become finer.

Therefore, in the method of the present invention, under the rolling conditions described below, N
There is one feature in exhibiting the above effects of b.
In order to exhibit this effect significantly, Nb is added in an amount of at least 0.01%.

In addition, when the amount of Nb increases, the toughness of the welded part decreases during welding, and for this reason, in the method of the present invention, Nb is 0.10%
is the upper limit.

In addition to the above basic components, in the present invention, V: 0.01
~0.05%, Ni: 0.1~0.5%, Cr: 0.1
~0.5%, Cu: 0.1~0.5%, Mo: 0.
05-0.50% or B: 0.0005-0.
0025% of one or more of them can be added and contained.

Ni can improve the strength and toughness of the base material without adversely affecting the hardenability and toughness of the heat affected zone (HAZ).

However, Ni is expensive, and adding more than 0.5% increases manufacturing costs;
Since there is no effect if it is less than 0.1 to 0.5%.

Cu has almost the same effect as Ni and also improves corrosion resistance, but if it exceeds 0.5%, cracks are likely to occur during hot rolling and the surface quality of the steel sheet deteriorates.

Moreover, since there is no effect if it is less than 0.1%, it is set to 0.1 to 0.5.

Cr is an element that increases strength, but if it exceeds 0.5%, it impairs the toughness of the weld zone, and if it is less than 0.1%, there is no effect, so it is set at 0.1 to 0.5%.

(2) is added to improve the base material strength and toughness of the steel plate according to the present invention, but if it is less than 0.01%, it is not effective, and if it is less than 0.01%,
If added in excess of 5%, the toughness of the base metal and weld zone will be significantly deteriorated, so the content should be 0.01 to 0.15%.

Since Mo makes the τ grains regular during rolling and also produces fine bainite, it is preferable to add Mo from above in order to improve strength and toughness.

However, adding more than 0.5% will increase manufacturing costs, and less than 0.05% will have no effect.
The content was set at 5% to 0.5%.

B also generates bainite, which particularly increases strength, but if it is added in an amount exceeding 0.0025%, it will lead to deterioration of toughness, and if it is less than 0.0005%, it will have no effect, so B should be added in an amount of 0.0005 to 0.0025%. And so.

Now, the steel melted with the above-mentioned components is usually heated to a temperature of 1100° C. to 1280° C. in the form of a slab.

The reason why the heating temperature was limited to 1100°C to 1280°C is as follows.
First, regarding the lower limit of 1100°C, Nb is dissolved in solid solution to exhibit the above-mentioned effect of Nb, and at the same time, as described later, the purpose is to ensure the cumulative reduction rate required at temperatures of 1000°C or higher. Furthermore, heating above the upper limit of 1280°C will cause the γ grain size to become extremely coarse, so in the method of the present invention, the γ grain size can also be made finer by adding Nb as described above, and therefore the toughness will deteriorate. , the upper limit of heating temperature is 1280℃
I made it.

As described above, the steel is heated to 1100 DEG C. to 1280 DEG C., and the steel is repeatedly rolled in the range from this heating temperature to 1000 DEG C. or higher until the cumulative rolling reduction (R) reaches (1)2 or higher.

In this case, rolling at 1000°C or higher constitutes one of the gist of the present invention, and in particular, rolling at 1000°C or above is performed in the austenite final recrystallization region in the conventional example.
It can be said that the method of the present invention was established by noting that high strength and high toughness could be ensured despite rolling in a high temperature range of ~900°C, and finding an appropriate cumulative reduction rate from this.

That is, Fig. 1 is a graph showing the relationship between the heating temperature and the cumulative rolling reduction rate at 1000°C or higher in steel with the compositions 0.05%C-1,2%Mn-〇, 03%Nb-0,03%AI. It is.

In Figure 1, the cumulative reduction rate below 1000℃ is 75%.
under the conventional conditions, i.e. 950°C ~ 75% at Ar3.
Comparing the case of rolling and the case of rolling by 75% at 1000°C to 900°C, the cases where the same toughness is obtained with the latter rolling are marked with ○, and the cases where the toughness deteriorates with the latter rolling and the same Those in which toughness could not be obtained are indicated with an X mark.

In both cases, cooling is performed at a cooling rate of 6 or higher from Ar3 or higher.
The temperature was increased to room temperature at 0°C/sec.

In addition, in Figure 1, the symbol A is R=-0, IT+170 (%
) is shown.

As is clear from Figure 1, if the cumulative reduction rate (R) of rolling at 1000°C or higher is reduced under the relationship R> -0,1T+170, then the rolling reduction at 1000°C or lower is 1000°C.
Even if it is rolled at a temperature of 950°C to 900°C, as in the conventional example,
Toughness equivalent to that obtained by rolling at r3 can be obtained.

Subsequently, after rolling at <1000°C or higher as described above, 100°C
Rolling is repeated in a temperature range of 0° C. to 900° C. until a cumulative reduction ratio of 60% or more is achieved, and the rolling is completed to obtain a finished plate thickness.

Needless to say, the finish rolling temperature at this time is 900°C or higher, but this rolling further refines the austenite grains that were refined by rolling at 1000°C or higher.
Some of the grains elongate without recrystallization, and deformation bands are introduced within the grains.

In this case, if rolling is terminated at a cumulative reduction ratio of less than 60%, the austenite will not be sufficiently refined, and coarse low-temperature transformation products will be mixed in with subsequent cooling, resulting in deterioration of toughness.

Therefore, the cumulative reduction rate at 1000°C to 900°C is 60°C.
% or more and it is necessary to roll repeatedly.

After finishing the rolling at 900° C. or higher, cooling of the steel plate is started from Ar3 or higher, where ferrite does not precipitate, and is cooled at an average cooling rate of 10° C./sec or higher.

At this time, there is sufficient time from the end of rolling to the start of cooling, so operations can be carried out extremely stably.
Unlike the conventional example, cooling is not started below the Ar3 point, and the necessary strength and toughness can be ensured.

At this time, cooling at a cooling rate of 10°C/sec or more after rolling is because a cooling rate of 10°C/sec or less does not result in a low-carbon bainite structure, but in the above chemical composition range, a ferrite-pearlite structure occurs, and the strength is sufficient. This is because it does not become.

Note that it is preferable that the cooling rate be fast in order not to form a ferrite-pearlite structure, but if it exceeds 100° C./sec, martensite will be mixed in and the toughness will deteriorate, so it is necessary to keep it below this range.

Further, the cooling is stopped at a temperature below 650°C, because if the cooling is stopped at a temperature above 650°C, the amount of low carbon bainite produced decreases, making it difficult to achieve the object of the present invention.

For this reason, it is cooled to 650°C or less at a cooling rate of 10°C/sec to 100°C/sec.

First, the steel samples were melted to have the composition shown in Figure 1, and among these test steels, perfume 1 had a composition outside the range of the present invention, and perfumes 2 to 6 had a composition within the composition range of the present invention.

Comparative Example Perfume 1 only has C not as low as the present invention, but the other components are within the scope of the present invention.

Next, each of these test steels is made into slabs having the required thickness by ingot-forming, blooming rolling, or continuous casting. Rolling was carried out repeatedly at a temperature of 1000° C. or higher, and then the temperature was varied as shown in Table 2 for the method of the present invention, the comparative method, and the conventional method, and then cooled.

The heating temperature at this time, the cumulative rolling reduction rate during rolling from the heating temperature to 1000°C, the cumulative rolling reduction rate during rolling below 1000°C, and the cooling conditions are as shown in Table 2. The strength and toughness of the steel plate were measured and were as shown in Table 2.

The final plate thickness was 15 mm or 35 mm, the test pieces were taken in the direction perpendicular to the rolling direction, the tensile test pieces were 6 mmφ, and the impact test pieces were 2 mm V notch full size.

The number 1.2.3.4.5.6 on each steel plate means that the steel of fragrance 1.2.3.4.5.6 shown in Table 1 was used, and the suffix A. ...F indicates manufacturing conditions.

IA is a steel plate manufactured in a comparative example, and 2B and 3B are steel plates whose compositions are within the scope of the present invention but were manufactured using a conventional method.

Similarly, 2D and 3C are cumulative reduction rates of 1000℃ or more, 2E is heating temperature, 2F is cooling rate, 3D is cooling start temperature, 3E is cooling stop temperature, 3F is 1000 to 900
The cumulative reduction ratio of 2A and 2C13A is outside the scope of the present invention.
, 4A, 5A and 6A are steel plates within the scope of the present invention.

In order to make the above relationship easier to understand, conditions outside the scope of the present invention are underlined in Table 2.

In the results in Table 2, steel plate 2 manufactured by the conventional method
Compared to B and 3B, steel plates 2A and 3A manufactured by the method of the present invention have a temperature of 1000°C in the manufacturing method.
The following rolling methods were different, and the other conditions were exactly the same.

More specifically, while the conventional method rolls at 900 to Ar3, the method of the present invention rolls at 1000 to 900°C, but as is clear from the strength and toughness measurement results shown in Table 2, It can be seen that the results obtained are comparable to those obtained by the method.

That is, according to the present invention, rolling can be completed at 900° C. or higher, and subsequent cooling can be carried out easily and stably.

Further, the steel plate IA that is outside the composition range of the present invention does not have sufficient toughness, and even if the steel composition range is within the range of the present invention, the steel plate 2F that is simply air-cooled after rolling does not have sufficient strength.

Further, the steel plates 2D and 3C, in which the cumulative reduction rate of 1000° C. or more is outside the range of the present invention, do not have sufficient toughness.

Moreover, the steel plate 3F in which the cumulative reduction rate of 1000° C. to 900° C. is outside the range of the present invention has sufficient strength, but extremely poor toughness.

Furthermore, steel plate 2E whose heating temperature is outside the range of the present invention
In addition, both 3D and 3E, whose cooling start temperature and cooling stop temperature are outside the range of the present invention, do not have sufficient strength and toughness.

In addition, as shown in Table 1, 4A, 5A, and 6A contain MOIB, Cr, Cu, Ni, ■, etc. in addition to the basic components, so they have sufficient strength even though they are thick. It can be seen that the material exhibits excellent strength and toughness.

As described above, according to the present invention, it is possible to obtain high-strength steel with sufficient low-temperature toughness and tensile strength of 60 kg/- or more with Ceq < 0.37%, and it is possible to obtain a high-strength steel with a tensile strength of 60 kg/- or more, which can be used as a material for line pipes for cold regions. Of course, it is also ideal as a steel for other welded structures that require low-temperature toughness.

As explained above, according to the present invention, it is possible to easily and stably produce high-strength, high-toughness steel that is non-thermal treated, has a tensile strength of 60 kg/- or more, and has excellent low-temperature toughness and weldability, and has industrial effects. This is a great invention.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between heating temperature and cumulative rolling reduction rate of 1000° C. or higher.

Claims (1)

[Claims] IC: 0.03 to 0.09%, Si: 0.0
3-0.5%, Mn: 0.5-2.0%, Al5ol
: 0.05% or less, and Nb: 0.01 to 0.10
%, with the balance consisting of iron and unavoidable impurities, is heated to a temperature in the range of 1100°C to 1280°C, and in the temperature range from this heating temperature to 1000°C, the cumulative reduction rate (R
) is rolled repeatedly until R = −0, IT + 170% (T: heating temperature) or more, and then rolled to 1000 to 9
Rolling is repeated until the cumulative reduction rate is 60% or more in the temperature range of 00°C, and then the average cooling rate is 10°C/sec to 100°C from a temperature of Ar3 or higher to 650°C or lower.
A method for producing non-thermal high tensile strength steel with excellent low-temperature toughness, characterized by cooling at a rate of °C/sec. 2C: 0.03-0.09%, Si: 0.0
3-0.5%, Mn: 0.5-2.0%, Al5o1.
: 0.05% or less and Nb: 0.01-0.10%
Contains V: 0.01-0.15%, Ni:
0.1-0.5%, Cr: 0.1-0.5%, C
u: 0.1-0.5%, Mo: 0.05-0.
50% or B: 0, 0005-0.0025% 1
The steel containing the species or two or more species is heated at 1100°C to 128°C.
Heating to a temperature in the range of 0°C and 1000°C from this heating temperature.
In the temperature range of ℃, the cumulative reduction rate (R) is R knee〇, IT
+ 170% (T: heating temperature) or higher, and then rolled repeatedly in the temperature range of 1000 to 900°C until the cumulative reduction rate is 60% or higher, and then rolled from a temperature of Ar3 or higher to 650°C. A method for producing non-thermal high tensile strength steel having excellent low-temperature toughness, characterized in that cooling is performed at an average cooling rate of 10°C/sec to 100°C/sec to below 0°C.