JPS6155572B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing high-strength steel with excellent low-temperature toughness and weldability. In particular, the present invention relates to a method for producing high-strength steel that has a three-phase structure of ferrite, bainite, and martensite, and has a high strength level.
The present invention also relates to a method for manufacturing non-temperature high-strength steel that has excellent low-temperature toughness and weldability. Without any refining treatment such as quenching and tempering,
In other words, when manufacturing a non-heat-refined steel with high strength and low-temperature toughness and excellent weldability, a method is conventionally known in which Nb-containing steel is subjected to controlled rolling and then accelerated cooling is performed. The above method sets the stop temperature of accelerated cooling to 500
By forming a fine ferrite-bainite structure at temperatures above ℃, the strength is higher than that of ferrite-pearlite structures,
This method takes advantage of the fact that the toughness does not deteriorate as compared to those with pearlite structure. The object of the present invention is to provide a method for manufacturing high-strength steel that can be made even higher in strength than the conventional method and has excellent low-temperature toughness and weldability, and the invention is based on the following: The above object can be achieved by providing a method. Next, this invention will be explained in detail. The inventors achieved even higher strength than the conventional method by heating and rolling steel with an appropriate composition under appropriate conditions, and then setting the accelerated cooling stop temperature to 450°C or less. This invention was conceived based on the new finding that it is possible to improve the toughness of the steel and that the toughness does not deteriorate. First, in research for this invention, the influence of the stopping temperature of accelerated cooling on strength and toughness was investigated.

[Table] Steel 7 shown in Table 1 above was heated to 1150°C and rolled at a cumulative reduction rate of 61% in the austenite recrystallization region, then rolling was started again from 900°C and rolled at 810°C. The cumulative reduction rate during this period was set to 65%, and the subsequent cooling rate was set to 10%.
Figure 1 shows the influence of the cooling stop temperature on strength and toughness when expressed as °C/sec. Note that the above strength and toughness are values in the direction perpendicular to rolling, that is, the C direction. When the stop temperature of accelerated cooling is set to exceed 450℃ to 300℃, the weight decreases by 10 to 13Kg/mm ² compared to when air cooling is performed after rolling.
There was an increase in TS, and the toughness did not deteriorate, and was actually better than when air cooling was used. Furthermore, the strength is increased by 3 to 7 kg/mm ² compared to when the stop temperature of accelerated cooling is set to 500° C. or higher. This significant increase in strength without deterioration of toughness is due to a detailed study using electron microscopy. It was found that this was due to the extremely refined three-phase structure of ferrite, bainite, and martensite in which sites were mixed. Conventionally, the inclusion of martensite has been avoided as a predisposition to significantly deteriorating toughness, but a few percent of fine martensite mixed in a fine ferrite/bainite structure can be used to increase toughness without significantly deteriorating toughness. It was found that it effectively contributed to strengthening. Based on this knowledge, the present inventors completed the present invention by researching appropriate alloy components and heating-rolling-cooling conditions for producing a fine three-phase structure. As mentioned above, the first essential component of the present invention is the heating-rolling conditions. According to this invention, the slab heating temperature is set to be higher than the temperature at which 0.01% or more of Nb dissolves in solid solution, and lower than 1180°C. The first reason to have Nb in solid solution of 0.01% or more is to
This is because Nb precipitates finely during rolling and delays the recrystallization of austenite, resulting in the expansion of the unrecrystallized region of austenite to the higher temperature side.
This allows rolling at a high cumulative reduction rate in the unrecrystallized region of austenite, increases the density of the deformation zone, and substantially refines the austenite grains before cooling. As will be detailed later, rolling in the non-recrystallized region of austenite is one of the essential components of the present invention. The second reason why 0.01% or more of Nb is dissolved in solid solution is that pearlite transformation is suppressed by utilizing the hardenability improvement effect of the finely precipitated Nb that is still in solid solution during the rolling. This is to obtain a three-phase structure of ferrite, bainite, and martensite, which is a structure to achieve the purpose of. For these two purposes, Nb of 0.01% or more is required.
It is necessary to form a solid solution in C, but the minimum allowable heating temperature for this differs depending on the C content. On the other hand, the upper limit of the heating temperature was set at 1180°C because if it is higher than this, the austenite grain size during heating becomes too large and mixing of austenite grains is unavoidable even during subsequent rolling. This is because coarse bainite and martensite are mixed into the structure and the toughness deteriorates. The slab heated under the above conditions is first rolled repeatedly in the austenite recrystallization region until the cumulative reduction ratio reaches 50% or more. If this cumulative reduction rate is less than 50%, grain refinement and grain size regulation due to repeated processing and recrystallization of austenite will not be sufficient. Therefore, during the subsequent rolling and cooling process, coarse bainite and martensite are mixed into the structure, significantly impairing the toughness. Moreover, the insufficiency of grain refinement and grain size regulation by rolling in this temperature range cannot be compensated for by subsequent rolling in a non-recrystallized region of austenite, so the temperature was limited to 50% or more. Note that the upper limit of the above-mentioned rolling reduction rate is determined by the slab thickness and the amount of reduction in the non-recrystallized area, so it is not intentionally limited. Next, we move on to rolling in the non-recrystallized temperature range of austenite, and according to the present invention, rolling is carried out in the non-recrystallized temperature range of austenite, which is on the low temperature side _.
At least 50% within the temperature range from point to Ar ₃ +150℃
It is necessary to perform rolling at a cumulative reduction rate of . Figure 2 shows Steel 3 shown in Table _{2 ,} which was heated to 1150°C and then rolled at a cumulative reduction rate of 50% in the _austenite recrystallization zone.
This figure shows the change in toughness when rolling is finished at 790°C by changing the cumulative reduction rate within the range of 783 to 933°C (783 to 933°C), and then cooling to 400°C at a rate of 10°C/sec. be. It is understood from this figure that a cumulative reduction of 50% or more is required. This strong pressure at the low temperature side of the non-recrystallized temperature range of austenite is one of the essential conditions for obtaining a structure that achieves the object of the present invention after subsequent cooling, and this condition is not met. Therefore, a fine mixed structure of ferrite, bainite, and martensite cannot be obtained. In other words, due to the cumulative reduction rate of 50% or more in the austenite recrystallization region, the austenite grains that have been refined and sized _are
The cumulative reduction of more than 50% at Ar ₃ +150° C. introduces a high density of deformation zones, which results in an extremely large number of ferrite-forming nuclei. By bringing the austenite into such a state before the start of cooling, a fine ferrite-bainite-martensite structure is formed after accelerated cooling. Further, the upper limit of the rolling reduction rate is not limited because it is naturally determined by the thickness of the slab and the amount of rolling reduction in the recrystallization area.
In addition, Ar ₃ +150 from the austenite recrystallization region
Although the rolling conditions are not limited within the temperature range up to 0.degree.

[Table] Next, the second important part of the constituent elements of the present invention is the cooling after the above-mentioned heating and rolling. That is, cooling is started immediately following the above-mentioned rolling, and the cooling rate must be within the range of 2 to 20°C/sec. As mentioned above, in order to achieve the object of this invention, it is necessary to create a structure in which a fine ferrite-bainite structure is mixed with a fine martensitic structure, and in order to obtain such a structure, the cooling rate described above must be adjusted. The range is limited, but the reason is that the cooling rate is 20
If the cooling rate is faster than 2°C/sec, the amount of pro-eutectoid ferrite will be virtually zero, and the three-phase structure that is a feature of this invention will not be formed.On the other hand, if the cooling rate is slower than 2°C/sec, a structure containing pearlite will result, which will reduce the strength and This is because the toughness is also inferior. Immediately after the completion of the rolling, cooling is started at a cooling rate of 2 to 20°C/sec, and cooling is stopped at a temperature below 450°C and above 300°C, followed by air cooling. The reason for this is 450℃
This is because if cooling is stopped at a higher temperature, the structure becomes a two-phase structure of ferrite and bayite, and the effect of increasing strength is not large. On the other hand, if the steel is cooled to a temperature lower than 300°C, the amount of martensite will increase, and the self-tempering effect will be small in the subsequent air-cooling process, resulting in a significant drop in toughness, especially impact absorption energy, making it impossible to achieve the object of the present invention. For these reasons, it is necessary to stop cooling within the range of 450°C to over 300°C and then use air cooling. Note that cooling must be performed immediately after rolling is completed, and specifically, it is desirable to start cooling from ₃ or more Ar points. However, even if it takes time to start cooling and the temperature of the steel plate drops below the Ar ₃ point, Ar ₃ -
If the temperature is up to 40°C, the grain growth of ferrite that precipitates after cutting the Ar ₃ point during air cooling can be virtually ignored, so the purpose of refinement can be achieved. According to the present invention, it is necessary to limit the heating-rolling-cooling conditions as described above, but it is also necessary to limit the chemical composition of the steel, and the reason for this will be explained next. If the C content is less than 0.02%, high strength cannot be obtained, and the weld heat affected zone (hereinafter abbreviated as HAZ) will be greatly softened, and if the C content is 0.15% or more, weldability will be poor. In addition, the heating-rolling-cooling conditions in this invention result in a quenched structure, which impairs toughness and requires a tempering process.
It needs to be 0.02-0.15%. Si promotes deoxidation of steel and increases its strength, so it is added at least 0.03% or more. However, if it is too large, toughness and weldability will be significantly impaired, so it should be kept at a maximum of 0.60%. If Mn is less than 1.0%, the strength and toughness of the steel sheet will decrease, and the HAZ will become more softened, so the lower limit was set at 1.0%. On the other hand, if there is too much Mn, HAZ
The upper limit was set at 2.5% because the toughness of the steel deteriorates. Al needs to be added at a minimum of 0.005% to deoxidize the steel, and on the other hand, if solid solution Al exceeds 0.06%, not only the toughness of the HAZ but also the toughness of the weld metal will deteriorate significantly. Therefore, sol Al was set at 0.005 to 0.060%. If S exceeds 0.008%, the impact absorption energy, especially in the C direction, decreases, which is disadvantageous, so S is
Must be 0.008% or less. According to this invention, as described above, C, Si, Mn and Al are contained within appropriate ranges, and S is 0.008
% or less, it is necessary to further contain Nb. As mentioned above, if Nb is less than 0.01%, the solid solution amount of Nb required during heating cannot be secured, while if it is more than 0.10%, the toughness of the weld metal will deteriorate, so Nb should be in the range of 0.01 to 0.10%. need to be inside. Furthermore, in addition to the basic component system as described above, in order to achieve high tensile strength or other effects,
At least one selected from Ti, Ni, Mo, Cu, V, Cr, B, Ca, and REM can be added. 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. Ti is added for the purpose of improving toughness by refining the γ grains. However, if it exceeds 0.04%, the toughness will deteriorate, so it is set at 0.04% or less. Ni is added because it improves the strength and toughness of the base material without adversely affecting the hardenability and toughness of the HAZ, but it is expensive, so the upper limit was set at 1.0%. Cu not only has almost the same effect as Ni, but also
It also improves corrosion resistance, but if it exceeds 0.50%, hot embrittlement tends to occur and the surface quality of the steel plate deteriorates.
The upper limit is 0.50%. Mo improves strength and toughness by making the austenite grains finer and more regular during rolling, and also generates fine bainite and martensite.
Since it is expensive, the upper limit was set at 0.50%. V is added to improve strength and toughness and to ensure the strength of welded joints, but if it is added in excess of 0.10%, the toughness of the base metal and HAZ will be significantly degraded.
The upper limit is %. Cr generates fine bainite and martensite to improve strength and toughness, but addition of 0.50% or more impairs weldability, so the upper limit was set at 0.50%. B improves strength and toughness because it produces fine bainite and martensite, but it has no effect if added in excess of 0.003%, and hardening of the HAZ is large, so the upper limit was set at 0.003%. Ca and REM control the morphology of MnS and are effective in improving the toughness in the C direction, and are added singly or in combination, but Ca and REM exceed 0.01% and 0.10%, respectively.
Addition of REM in excess of this amount impairs the cleanliness of the steel and causes internal defects, so the upper limits were set at 0.01% and 0.10%, respectively. Next, the present invention will be explained with reference to embodiments. Example 1 First, among the test steel Nos. 1 to 6 melted to the compositions shown in Table 2, Steel No. 1 is a comparative example, and Steel Nos. 2 to 6 are within the composition range of the present invention. It is steel. Next, each of these test steels is made into slabs with the required thickness by ingot formation, blooming rolling, or continuous casting, and these slabs are treated under the heating-rolling-cooling conditions shown in Table 3. did. The strength and toughness of the obtained steel plate were measured and were as shown in Table 3. The final plate thickness was 15 mm and 25 mm, and test pieces were taken in the direction perpendicular to the rolling direction and subjected to a tensile test and a 2 mm V-notch impact test. Numbers 1, 2, on each steel plate
3, 4, 5, and 6 are steel numbers 1 and 6 shown in Table 2, respectively.
This means that No. 2, 3, 4, 5, or 6 steel was used, and the sapphire A, B, C, and D indicate manufacturing conditions. 1A is a comparative example outside the composition range of this invention, 2B is a cooling rate after rolling, 2C
is the cumulative reduction rate in the austenite recrystallization zone,
2D is the solid solution Nb amount during heating, 3B is the cooling stop temperature, 3C
are outside the scope of this invention in terms of the cumulative reduction rate within the temperature range from Ar ₃ to Ar ₃ +150°C, whereas 2A, 3A,
4A, 5A and 6A are steel plates according to 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 3.

[Table] First, steel sheets outside the composition range of this invention
1A is not strong enough. Next, both steel plates 2C and 3C, which are outside the scope of this invention in terms of rolling conditions, are steel plates according to this invention.
The toughness is not sufficient compared to 2A and 3A, and the importance of rolling conditions in this invention is clear. Further, the steel sheet 2D, in which the amount of solid solution of Nb during heating is outside the range of the present invention, does not have sufficient strength or toughness. Steel plate 2B made by the conventional method, which was further air-cooled after rolling.
is a steel plate according to the present invention using steel 2 with the same composition.
The strength is extremely low compared to 2A. Further, the steel plate 3B whose accelerated cooling stop temperature is outside the range of the present invention has a lower strength than the steel plate 3A according to the present invention. On the other hand, steel plates 2A, 3A, 4A, 5A according to this invention
and 6A both have excellent strength and toughness. That is, as can be seen from the examples described above, according to the method of the present invention, high-strength steel with sufficient low-temperature toughness and low Ceq as shown in Table 2, that is, extremely excellent weldability, can be produced without heat treatment. could be manufactured. Example 2 In order to produce a steel plate with excellent low temperature toughness and weldability that has a TS of 70Kg/mm2 ^or more without heat treatment, the first
Steels 7 and 8 having the composition shown in the table were melted. After forming a slab, a 16 mm steel plate was produced by applying the heating-rolling-cooling conditions shown in Table 4, and the strength and toughness were measured and found to be as shown in Table 4. That is, 7A and 8A are steel plates according to the present invention,
Steel plates 7B and 8B made by the conventional method of air cooling after rolling were
What used to be a TS of about 60Kg/mm ² becomes a TS of 70Kg/mm ² or more, and has sufficient toughness. Plus 7C
and 8C are outside the scope of this invention in terms of cooling stop temperature.First of all, 7C has a cooling stop temperature of 450°C or higher, so the TS is 70Kg/mm ²
No more than that. On the other hand, 8C was acceleratedly cooled to room temperature, and although it has a remarkable increase in strength, the drop in impact absorption energy is remarkable, and the method of this invention has excellent effects on both strength and toughness. I understand that. As explained above, according to the method of this invention,
It is possible to produce high-strength steel with sufficient low-temperature toughness at a low Ceq, making it ideal as a material for line pipes in cold regions and other welded structures that require low-temperature toughness. Furthermore, as a side effect
Since rolling can be completed at Ar points of ₃ or more, it is effective in reducing separation and increasing rolling efficiency.

【table】 [Brief explanation of the drawing]

Figure 1 shows the effect of the cooling stop temperature of a steel plate after hot rolling on its strength and toughness, and Figure 2 shows the effect of the cumulative reduction rate of the steel plate between Ar ₃ and _{Ar 3} +150℃ on the impact value. FIG.

Claims (1)

[Claims] 1 C0.02-0.15%, Si0.03-0.60%, Mn1.0-2.5
%, Sol Al0.005~0.060%, Nb0.01~0.10%,
After heating a slab containing 0.008% or less S and the remainder substantially Fe at a temperature at which 0.01% or more Nb in the slab becomes a solid solution and 1180℃ or less, it accumulates in the austenite recrystallization temperature range. 50 in rolling reduction rate
% or more, and then continue rolling until Ar ₃ ~ Ar ₃ +150
℃ temperature range until the cumulative reduction rate is 50% or more, then immediately cooled at a cooling rate of 2 to 20℃/sec to a temperature of 450℃ to over 300℃, and then air cooled to form ferrite and bainite. A method for producing high-strength steel with excellent low-temperature toughness and weldability, characterized by having a three-phase structure of martensite and martensite. 2 C0.02~0.15%, Si0.03~0.06%, Mn1.0~2.5
%, Sol Al0.005~0.060%, Nb0.01~0.10%,
Contains S0.008% or less, and Ti0.04% or less,
Ni1.0% or less, Cu0.5% or less, Mo0.4% or less, V0.1
% or less, Cr0.5% or less, B0.003% or less, Ca0.01%
The following contains one or more selected from REM0.10 or less, and the remainder is substantially Fe.
After heating the slab made of
Rolling is carried out in the austenite recrystallization temperature range until the cumulative reduction ratio is 50% or more, and then Ar ₃ ~
Roll within the temperature range of Ar ₃ +150℃ until the cumulative reduction rate is 50% or more, and then immediately roll at 2 to 20℃/
A high-strength steel with excellent low-temperature toughness and weldability, which is cooled to a temperature of 450℃ to over 300℃ at a cooling rate of sec, and then air-cooled to form a three-phase structure of ferrite, bainite, and martensite. Production method.