JPS5852423A - Manufacture of unnormalized high tensile boron steel with superior toughness at low temperature and superior weldability - Google Patents

Abstract

PURPOSE:To manufacture a high tensile steel with superior toughness at low temp. and superior weldability by heating and rolling a steel contg. adequate amounts of B and Ti under specified conditions and by carrying out accelerated cooling. CONSTITUTION:A steel slab contg. 0.02-0.12% C, 0.03-0.60% Si, 1.0-2.5% Mn, 0.005-0.01% sol.Al, 0.01-0.10% Nb, 0.0003-0.003% B, 0.005-0.08% Ti and <=0.008% each of N and S as impurities or further contg. a specified small amount of one or more among Ni, Cu, Mo, V, Cr, Ca and a rare earth element is heated to 1,050-1,250 deg.C and rolled at a temp. in a temp. range where austenite is recrystallized and >=50% total draft. It is further rolled at Ar3-Ar3+ 150 deg.C and >=50% total draft and immediately quenched to <=600 deg.C at >=2 deg.C/sec cooling rate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing high-strength steel with excellent low-temperature toughness and weldability. The present invention relates to a method for manufacturing high-strength steel.

When manufacturing steel having high strength and low-temperature toughness without heat treatment such as quenching and tempering, that is, without heat treatment, a method is known in which Wb-containing steel is controlled rolled and then accelerated cooled.

By the way, boron has been used as an element to increase hardenability in tempered steels such as quenching and tempering. Even in non-tempered steel, if boron can be used, the carbon equivalent (0.q
), it is possible to increase the tension without increasing the tension, and there has been a strong desire to establish a method for its use. However, in the case of non-tempered steel, especially steel that is used after being acceleratedly cooled after controlled rolling, the use of boron has not been known in the past. The reason for this is that it has been pointed out that since the heating temperature of non-tempered steel is high, when boron is used, it becomes a boron compound and does not work effectively.

Regarding this point, 0.04% or more of sol and muj are added, and the heating temperature is set to a low temperature (specifically, 1010~/
The only example that has been proposed is that boron is added to non-tempered steel that is subjected to accelerated cooling after rolling (by reducing the rolling temperature to 10°C).
The amount added per 1ole is 0. Since the OA% is high, it not only impairs the toughness of the weld heat affected zone (H[) but also deteriorates the toughness of the weld metal. Regarding the heating temperature, since the heating is done at a low temperature, the peeling off of the scale is poor, and it is inevitable that problems will occur with the surface quality of the spruce and, ultimately, with the product.

The object of the present invention is to provide a method for producing boron-containing non-temperature high-strength steel that has even higher strength, excellent low-temperature toughness, and weldability than the conventional method described above,
The above object can be achieved by providing the method described in the claims.

Next, this invention will be explained in detail.

As a result of repeated research on a method for effectively utilizing boron in non-tempered steel that is subjected to accelerated cooling after rolling, the inventors conducted appropriate heating and rolling on steel containing appropriate amounts of boron and titanium. We have newly discovered that the above problems can be solved by applying accelerated cooling, that is, it is possible to effectively utilize boron by suppressing the defects caused by the various conditions for effectively utilizing boron. Completed the invention.

Next, this invention will be explained based on research data.

First, we investigated the effect of heating temperature before rolling on strength and toughness.

Using steel having the composition range of the present invention shown in Table 1 and conventional steel J, the heating temperature was set to tt100θ~/JjtO℃, and the heating temperature was set to 10°C.
By C, it is rolled until the stacking reduction rate is ji%, and then it is continuously rolled at 100°C or less until the stacking reduction rate is 41%,
The final finishing temperature was 790°C. The finished plate thickness is IT,
It is m. Then reduce the cooling rate by 10℃/- to 300℃
The figure shows the strength and toughness results after cooling to From this figure, @/ in the component range of the present invention is 119f at T and S due to accelerated cooling after rolling, compared to air cooling.
The tension is also higher than /, -. Moreover, the toughness has not deteriorated. Furthermore, increase the heating temperature to 10!0~/,2! It can also be seen that the strength-toughness is stable even when the temperature is changed up to θ°C. On the other hand, steel 3, which is not in the composition range of the present invention, can be made to have a high tensile strength by accelerated cooling, but the slab heating temperature is within a narrow range (1
When the temperature exceeds 1100°C, boron begins to become ineffective and its strength decreases. In other words, by combining titanium and boron and performing appropriate rolling followed by accelerated cooling, boron can be used effectively even in non-thermal treated steel, and the applicable heating temperature range can be increased even though this may cause deterioration of toughness. It has been found that there is no need to increase the mol and mul contents, and the above-mentioned drawbacks can be rationally solved.

Therefore, the first essential component of the present invention is the limitation of the steel components, and first the reason for the limitation of each alloy component will be explained in detail below.

0 means that if the content is less than 0.0.2-, high strength cannot be obtained and the weld heat affected zone (hereinafter abbreviated as HAZ) will be greatly softened; In the case of 0.01 to 0.0, the weldability is impaired, and the heating-rolling-cooling conditions in this invention result in a quenched structure, which impairs the toughness, making a tempering process necessary. It is necessary to satisfy /ko≦.

Sl is added in an amount of at least 0.03% because it promotes the hardening of steel and increases its strength. However, if the content is too large, the toughness and weldability will be significantly impaired, so it is limited to a maximum of 0.0%.

If 111L is less than 10%, the strength and toughness of the steel plate will decrease, and the softening of the steel plate will increase, so the lower limit was set at 80%. On the other hand, if Kn is too large, the toughness of (2) deteriorates, so the upper limit was set at -1j%.

In order to deoxidize the steel, it is necessary to add at least 0.00 - to Mu 1. On the other hand, if 5ale Mu I exceeds 0.04 -, not only the toughness of H Mu 2 but also the toughness of the weld metal will deteriorate significantly. .

For this reason, -on and muj were set to o, ooz to 0.04θ.

If S is more than o, oot%, the impact absorption energy, especially C
S is 0.00t% because it is disadvantageous as it decreases in the direction.
It is necessary to do the following.

If N is contained in excess of 0.00t%, A in the scope of this claim
With the amount of Ti, B becomes boron nitride and the hardenability of B becomes invalid, so the upper limit was set at 0.00 t %.

According to this invention, as described above
n It is necessary to let it go.

If the Wb content is less than 0.1%, it will not be possible to secure the solid solution and amount of Hb required during heating, as will be described later.
If it is more than 0%, it will deteriorate the toughness of the weld metal, so N)
must be within the range of 0.0/~0.10% O T1 is added to effectively utilize B and to improve toughness through the effect of refining the austenan 44 grains during heating and rolling. However, it has no effect at less than o.ooz%, while at 0.0oz%.
Adding more than 0 t % would actually deteriorate the toughness, so the content was set at 0.01% or less.

Needless to say, B is an essential component of the present invention, but 0.0
Addition of less than 0.002% has no effect, while addition of more than 0.003% deteriorates toughness.

Furthermore, in addition to the basic component system as described above, N is added as necessary to achieve high tension or other effects.
i e No e Cu e V e Or + Qa
+ At least one selected from rare earth elements (RleM) can be added and contained. Even if these elements are added, none of the features of the present invention will be lost, and the addition of the above-mentioned elements is effective because it can increase the tensile strength appropriately and achieve the following effects.

Next, the purpose of adding the above components and the reason for limiting the amount added will be explained.

N1 is added because it improves the strength and toughness of the base metal without adversely affecting the hardenability and toughness of HAZ.
Since it is expensive, the upper limit was set at 80%.

On not only has almost the same effect as N1, but also improves corrosion resistance, but if it exceeds 0.20%, hot embrittlement tends to occur and the surface quality of the steel sheet deteriorates, so the upper limit is set at 0.20%.

MO makes the austenite grains fine and regular during rolling,
Moreover, with fine bainite! Although it improves strength and toughness by producing a It was set as sO%.

If this happens, the toughness of the base material and Hmu2 will be significantly deteriorated, so 0.1
The upper limit is 0%.

(3r produces fine bainite and martensite and improves strength and toughness, but addition of 0.rO4 or more impairs weldability, so the upper limit was set as o and ro%.

C& and iM control the morphology of Mllg and are effective in improving the toughness in the C direction, and either one type or a combination of both can be added, respectively. , Oa exceeding oi% and □,i.

Since adding more Rmii than is rumored would impair the cleanliness of the steel and cause internal defects, the upper limits were set to □, 0/%, and o, io, respectively.

Next, the first essential component of this invention is the heating-rolling conditions. In the present invention, boron effectively affects the hardenability, so if the rolling conditions are inappropriate, coarse bainite and martensite will be mixed into the structure of the steel sheet after cooling, and the toughness will likely deteriorate. Therefore, it is necessary to contain Nb, roll it under appropriate rolling conditions described below, and distribute fine bainite and martensite in the structure after cooling. The slab heating temperature is limited for this reason, but the upper limit is set at 1Jzo'c because if it is higher than this, the austenite grains will become too large during heating, and mixing of austenite grains will be unavoidable even during rolling. This is because coarse bainite and martensite are mixed into the structure after cooling, resulting in deterioration of toughness. On the other hand, the lower limit is related to the amount of W dissolved in Nlt. In other words, when heating the N-juku slab! I
It is necessary to melt the austenite and cause it to precipitate finely during rolling to promote recrystallization of austenite, and as a result, to expand the unrecrystallized region of austenite to a higher temperature side. This enables rolling with a high cumulative yield rate in the non-recrystallized region of austenite, increases the density of the deformation zone, substantially refines the austenite grains before cooling, and provides the steel sheet with sufficient low-temperature toughness after cooling. show. The amount of solid solution of Wb during heating that brings about these effects may be 0.0/≦ or more, and the minimum heating temperature for this varies depending on the amount of 0 and Njl,
In the ingredient range of the present invention, it is 10! The temperature may be set to θ°C or higher.

For the above reasons, the slab heating temperature is toso~l-30℃
And so.

The slab heated under the above conditions is first repeatedly rolled in the austenite recrystallization region until the cumulative rolling reduction reaches m% or more. If this cumulative rolling reduction ratio is less than 50%, grain refinement and grain size regulation due to repeated austenite recrystallization will not be sufficient. Therefore, subsequent rolling-
Cooling causes coarse bainite and martensite to be mixed into the structure, significantly impairing toughness. Moreover, the insufficient grain refinement and grain size grading by rolling in this temperature range,
Since it cannot be compensated for by subsequent rolling in the non-recrystallized region of austenite, it is limited to 104 or more.

Next, we move on to rolling in the austenite non-recrystallization temperature range, and according to the present invention, the Ar3+730°C column v, which is on the low temperature side even in the austenite non-recrystallization temperature range.
At least well between sA. It is necessary to perform rolling at a cumulative reduction rate of %. In the composition range of the present invention, B has a sufficient effect on hardenability as described above, so if rolling within this range is inappropriate, coarse bainite and martensite will be mixed into the structure after accelerated cooling, resulting in a significant deterioration of toughness. . In addition, Ar3 + /! from austenite reconsolidation & area! Although the rolling conditions are not limited in the temperature range up to 0''C, the purpose of the present invention is not hindered even if rolling is carried out in this range.

Accelerated cooling is performed after the above-mentioned rolling, but the cooling rate is set to λ° C./− or more because the accelerated cooling has no effect if it is less than buff m.

This accelerated cooling at a cooling rate of 0.degree. C./g or higher is continued until an arbitrary temperature of 600.degree. C. or lower, but if it is stopped at 600.degree. C. or higher, there is almost no effect of increasing strength due to accelerated cooling, so the upper limit is set at 600.degree. On the other hand, although no lower limit is specified, in order to effectively utilize the precipitation hardening element, cooling may be stopped at around 0° C. and thereafter air-cooled. For these reasons, Co'C/
Accelerated cooling is performed to 60O<0>C or less at a cooling rate of s or more.

Note that it is necessary to perform cooling immediately after the completion of rolling, and specifically, it is desirable to start cooling from the 3 or more points of unevenness. However, even if it takes time to start cooling and the temperature of the steel plate falls below the Mr3 point, if it is still within Mr3-v℃, the grain growth of ferrite that precipitates after the Mr3 point is reached during air cooling will not occur. Since it can be virtually ignored, the purpose of miniaturization can be achieved.

Next, the present invention will be described with reference to embodiments.

Example 1 Samples of 1 to 6 grams of incense melted to the compositions shown in Table 1 are examples of steel within the composition range of the present invention.

Each of these test steels was ingot-formed and then subjected to blooming rolling or continuous casting to form slabs having the required thickness.1 These slabs were treated under the heating-rolling-cooling conditions shown in Table 1, respectively. The strength and toughness of the obtained steel plate were measured and were as shown in Table 1.

The final plate thickness was set to /1-, test pieces were taken in the direction perpendicular to rolling, and a tensile comber notch impact test was conducted. Number 1 on each steel plate. λ and J mean that the subnumber/1JvJ steel shown in Table 1 was used, and the alphabetic characters in the suffix indicate manufacturing conditions.

No. 2 and 3 are comparative examples outside the composition range of this invention, /B is the cumulative reduction at 0°C from MUr3 to MUr3+/j, and IC is the cumulative reduction in the austenite recrystallization region. , /D is the slab heating temperature, iE is the cooling stop temperature of accelerated cooling after rolling, and 7F is the cooling rate after rolling, which are outside the scope of this invention, and for these, /A#/G #/H is a steel plate according to this invention. In order to make the above relationship easier to understand, conditions outside the scope of the present invention are underlined in Table 2.

First, steel plate-A, which does not have the composition range of the present invention, ie, does not have T1 and B added, does not have sufficient strength.

Conventional steel sheet JA (
Comparing the steel sheets of the present invention (high aluminum J-B-Wb steel), the heating strength is 1110°C, which is the lower limit even with the normally used heating strength.
In heating, the steel sheet of the present invention has higher strength even though the cell is the same.

Next, regarding the rolling conditions, the steel plates /B and 10, which are outside the scope of the present invention, have insufficient toughness compared to the steel plate TSU according to the present invention, which clearly shows the importance of the rolling conditions in the present invention.

Also, steel plate/D whose heating temperature is outside the range of this invention
The strength is not sufficient compared to the steel plate of this invention. Furthermore, when comparing the steel plate /1 produced by the conventional method, which was air-cooled after rolling, and the steel plate /A, which was subjected to accelerated cooling, the strength of TSUMU was greatly exceeded, and the toughness was also the same, demonstrating the effect of the present invention. There is.

Further, steel plate /E, whose cooling stop temperature is outside the range of the present invention, has almost no effect of accelerated cooling.

On the other hand, all of the steel sheet Tsums*/Gy/H according to the present invention are
It has excellent strength and toughness.

That is, according to the present invention, as can be seen from the examples described above, boron could be effectively used to increase the strength even in non-tempered steel subjected to accelerated cooling after rolling. Therefore, low 0
@11, that is, high-strength steel with excellent weldability and sufficient low-temperature toughness can be produced.

Example 2 A steel having the chemical composition shown in Table 1 for the purpose of manufacturing a steel plate having a thickness of J to 3 cm with excellent low-temperature toughness and weldability, having T and S of 701 qt/mm or more without heat refining. Das! After melting and making a slab, a steel plate with a thickness of 3.2 mm was manufactured by applying the heating-rolling-cooling conditions shown in Table 3, and the strength-toughness was measured. It was as shown.

Steel plate #B'*jB* made by the conventional method of air cooling after rolling
4B does not reach the target T and B, but according to the present invention, all have T and S of 70J9f/sll&2 or more, and also have excellent low-temperature toughness.

Furthermore, as can be seen from the Oeq shown in Table 1, 06g
-~O, da J% Ceq j~31am thickness +7)'
[', S, 〉70 bit/m2 steel can be produced without heat refining, which is revolutionary from the common sense in the industrial world.

As explained above, according to the method of the present invention, high-strength steel with sufficient low-temperature toughness can be manufactured at low C@q, and can be used as material for line pipes for cold regions and other materials requiring low-temperature toughness. It is ideal as a steel for welded structures. Furthermore, as a secondary effect, rolling can be completed at the unevenness point of 3 or more, which is effective in reducing separation and increasing rolling efficiency.

[Brief explanation of the drawing]

The figure shows the influence of the heating temperature of sup on the strength and toughness of a steel plate. Patent Applicant Kawasaki Steel Co., Ltd. Representative Patent Attorney Mura 1) Politics 1 → Fu”zo Wax Publishing - Katana L ('n)

Claims (1)

[Claims] 1, oo, oko ~ 0.11% t Sl 0.0
3~0-&0% eM! L/-0~J-j%
E! Io! , AI 0.00! r ~0.04%
eMla O,0/ ~0.10% snow B O,0003
~0-002%. TL O,00k -0,01%, if necessary ML /,01 or less t On O-k % or less e Mo
O-'l % or lessvVO-/-r Or O,3
% or less, c & 0.0/% or less, contains any 11m or more selected from 0.10% or less of rare earth elements, and the remainder consists of 1. and unavoidable impurities; After heating the step in which N and S are o and oot% or less, respectively, to iozθ~1aso''c, the step is rolled in the austenite recrystallization temperature range until the cumulative reduction rate is go% or more, and then the mulch r3 to mulr3
+/! Low-temperature toughness characterized by rolling within the temperature range of rO"C until the cumulative reduction ratio is 10% or more, and then immediately cooling to any temperature below 100°C at a cooling rate of a°C/- or more. A method for manufacturing boron-containing non-thermal high tensile strength steel with excellent weldability.