JPH0248606B2 - HICHOSHITSUKOCHORYOKUKOJINSEIKOHANNOSEIZOHO - Google Patents

Description

[Detailed description of the invention]

The present invention is a non-tempered high tensile strength steel plate, especially a high
The present invention relates to a method for manufacturing Nb type non-thermal treated high-tensile and high-toughness steel sheets from continuously cast pieces with good surface properties. Conventionally, only strength was required as a characteristic for high-strength steel plates, but as welding is being adopted for steel structures, it is now required to have not only strength but also weldability. In response to this, there is a trend to lower the amount of C, which impairs weldability, and to increase strength and toughness with alloying elements. In particular, recently, requirements for weldability, notch toughness, workability, etc. have become even more stringent, so the strength has been increased by lowering the C content, that is, below 0.06%, and making effective use of trace additive elements such as Nb, V, and Ti. There is a trend to improve toughness and ductility while maintaining the same properties. Such a low C
In steel (C≦0.06%), by increasing Nb,
It is possible to achieve high tensile strength that cannot be seen with current steel (C: 0.09-0.16%). In other words, Figure 1 shows low C
This shows the change in strength when changing the amount of Nb added in a 0.02%C-1.7%Mn steel steel, heated to 1100℃, applied 60% reduction below 900℃, and finished rolling at 790℃. and exceeds 0.10%
The strength increases continuously up to the amount of Nb. This is because due to the low carbon content, Nb dissolves in solid solution during heating up to a wide range of Nb addition amounts, and precipitates during the subsequent cooling process, making it possible to utilize precipitation strengthening. As mentioned above, it is possible to increase the tensile strength of low C steel over a wide range of Nb additions, so it is thought that this steel will be used more and more in the future.
There is one problem in manufacturing Nb steel. When continuous casting technology is applied to low C-high Nb steel, scratches on the surface of the continuously cast slab occur more significantly than with conventional Nb steel (Nb0.04%), and complicated maintenance is required as a pre-rolling process. Therefore, the benefits of continuous casting cannot be fully obtained. It is well known that the occurrence of scratches on the slab surface is generally closely related to a decrease in hot ductility in the γ low temperature range during the post-solidification cooling process. At most, concrete measures have been taken to control the casting temperature, casting speed, and amount of cooling water in the cooling zone. However, these methods have not been a sufficient solution for high Nb steels, and therefore, a decrease in hot ductility is considered to be the fate of high Nb steels, and surface scratches and care during the pre-rolling process are considered to be the fate of high Nb steels. It was considered inevitable. The present invention was created through repeated research in view of the above-mentioned circumstances, and its purpose is to:
The object of the present invention is to provide a method for manufacturing a high Nb type non-thermal treated high tensile strength steel plate with good slab surface properties during continuous casting. In order to achieve this objective, the present inventors studied the phenomenon of hot ductility reduction in the γ low temperature region, especially the phenomenon of hot ductility reduction due to the precipitation of Nb (C, N), and found that the reduction in hot ductility due to high Nb is caused by AlN Precipitation of Nb(C,N)
It was found that the precipitation of the two acts synergistically. Since it is a high-Nb steel, it is not possible to reduce the amount of Nb added, but adding a small amount of Ti as a method to suppress AlN precipitation significantly improves hot ductility. We discovered that increasing the amount of Nb added actually improves hot ductility, and based on these findings, we developed a method for manufacturing high-strength, high-toughness steel sheets with good surface properties by adjusting the chemical composition of steel to be continuously cast. A continuously cast slab is obtained, and this slab is hot-rolled and, if necessary, acceleratedly cooled to obtain a high-Nb type, non-temperature, high-strength, high-toughness steel plate. That is, the characteristics of the present invention are C:
0.005-0.06%, Si: 0.02: 0.50%, Mn: 1.5-2.5
%, S: 0.005% or less, Nb: more than 0.1% and 0.20% or less, SolAl: 0.005 to 0.10% as basic components, TN
amount (total amount and below) below 0.0060% and Ti
The purpose is to hot-roll a slab manufactured by a continuous casting method containing 0.005 to 0.030% of iron and the remainder Fe and unavoidable impurities, and cool it at a cooling rate of not less than air cooling and not more than 50° C./s. In addition, the present invention is characterized by containing the basic components as well as TN: 0.006% or less, Ti: 0.005-0.030%, V: 0.01-0.2%, Cr: 0.5% or less, Cu:
0.5% or less, Mo: 0.5% or less, Ni: 2% or less,
B: Contains one or more of 0.002% or less,
A slab manufactured by a continuous casting method consisting of the remainder Fe and unavoidable impurities is hot rolled and air cooled for more than 50 minutes.
The objective is to cool at a cooling rate of ℃/s or less. Hereinafter, the present invention will be explained in detail based on the accompanying drawings. As mentioned earlier, the occurrence of flaws on the slab surface is closely related to the decrease in hot ductility in the γ low temperature range during the post-solidification cooling process, and naturally the higher the surface flaw occurrence rate, the higher the maintenance rate. Therefore, the present inventors first performed high-temperature tensile tests on continuously cast slabs, examined the relationship between hot ductility and cast hand strain, and determined that It was discovered that there is a relationship between the range (width) (the same applies hereafter) and the casting hand penetration rate as shown in Figure 2. The thermal history used in the high-temperature tensile test is as shown in Figure 3. ) or a case in which stress is applied by rolls is simulated. According to FIG. 2, when the low ductility temperature range is wide, the maintenance rate is high and heavy maintenance is required, so that some slabs become unusable. As the low ductility temperature range decreases, the maintenance rate of the slab decreases, and when the low ductility temperature range is approximately within 100°C, the maintenance rate is 5% or less, and the maintenance method is light. Therefore, the present inventors next investigated conventional Nb steel (0.09
%C-1.5%Mn-0.03%Nb-0.005%N-0.03%
SolAl) and low C-high Nb steel (0.02%C-2.0%Mn-
The hot ductility of 0.10%Nb-0.004%N-0.03%SolAl) was investigated in the temperature range from 700°C to 1000°C. The results are shown in Figure 4, and when comparing the above two steels, the reduction of area (RA) of the low C-high Nb steel is low at all temperatures, and the low ductility temperature range where the reduction of area is 50% or less. However, 0.03%Nb steel
It can be seen that the temperature is wider at 290℃ compared to 150℃. The cause of this is low C-high Nb steel (0.10%Nb
Compared to 0.03% Nb steel, Nb is dissolved in solid solution during heating, and it is thought that the amount of Nb precipitated in the γ low temperature range is large. In other words, in a state where the grain boundaries have become brittle due to Nb(C,N) precipitated at the γ grain boundaries in the γ low temperature range, when a tensile stress exceeding a certain limit is applied near the surface, Nb(C,N) Nucleation of voids occurs in the surrounding area, and these voids agglomerate and connect, leading to final cracking. Moreover, the precipitation of AlN is used synergistically with this. For this reason, the reduction of area becomes low in the case of high Nb steel, and in continuous casting, stress between rolls in the cooling zone or thermal stress due to repeated cooling and reheating can cause tensile stress exceeding the above-mentioned limit value. This results in frequent surface scratches. From the above points, the decrease in hot ductility due to high Nb is
Although it is clear that this is due to the synergistic effect of the precipitation of AlN and the precipitation of Nb (C, N), it is not possible to reduce the amount of Nb added as a countermeasure. Therefore, the present invention aims to suppress the precipitation of AlN, which is one of the causes of reduced hot ductility, and as a specific method,
This resulted in a significant improvement in hot ductility without reducing the amount of Nb added. In other words, Figure 5 shows 0.02%C-2.0%Mn
-Nb-0.004% TN steel (Ti-free steel) and 0.02%
This figure shows the change in hot ductility at 800°C and 1000°C when the amount of Nb added is changed in C-2.0%Mn-Nb-0.015%Ti-0.004%TN steel (Ti-added steel). As is clear from FIG. 5, at 1000°C, the hot ductility (RA value) of Ti-free steel decreases rapidly on the high Nb side after reaching 0.03%Nb, reaching around 50%. On the other hand, in Ti-added steel, the reduction of area is almost 100% regardless of the amount of Nb added. 800 again
℃, Ti-free steel shows an extremely low reduction of area of 20% or less regardless of the Nb addition amount, while Ti-added steel has an extremely low reduction of area of around 50% and the Nb addition amount On the contrary, ductility improves at 0.06% or more. The reason for this difference in hot ductility is that Ti fixes N, and in the γ low temperature range,
It is clear that hot ductility was greatly improved by suppressing the precipitation of AlN and grain refinement by TiN, and 0.06%Nb at 800℃
The reason why the hot ductility improves as the Nb content increases is thought to be mainly due to differences in the coarsening process of precipitated Nb (C, N). For these reasons, the present invention fixes N by adding Ti when continuously casting high Nb steel, and utilizes the Nb > 0.1% region where hot ductility improves. Great effects can also be obtained in terms of toughness. In other words, FIG.
0.02%C-2.0%Mn-Nb-0.015%Ti-
70% below 900℃ after heating 0.004% TN steel to 1100℃
This shows the changes in strength and toughness when rolled to 20 mm at 770°C with a cumulative reduction of . This sixth
The figure shows that the strength increases continuously with the amount of Nb added, but the toughness begins to improve after reaching 0.06%, and high toughness of vTs = -100°C or less is obtained.
Furthermore, when the amount of Nb added exceeds 0.10%, the toughness is further improved, and in the region of Nb > 0.10%, vTs
Extremely excellent toughness of = -120 to -140°C was obtained. Therefore, it is desirable to add more than 0.1% of Nb not only from the viewpoint of hot ductility but also from the viewpoint of toughness. The specific component composition for fully exhibiting the above-mentioned characteristics of the present invention is as follows. () C: 0.005-0.06%, Si: 0.02-0.50%,
Mn: 1.5-2.5%, S: 0.005% or less, Nb: 0.1
% more than 0.2%, SolAl: 0.005-0.10% Steel consisting of balance iron and unavoidable impurities, or the above components include V: 0.01-0.2%, Cr: 0.5% or less, Cu: 0.5
% or less, Mo: 0.5% or less, Ni: 2% or less,
B: Using steel containing one or more types of 0.002% or less, () The amount of TN in this steel is 0.006% or less,
Contain Ti in a range of 0.005 to 0.030%. The reasons for limiting the above ingredients are as follows. C is required to be at least 0.005% from the viewpoint of ensuring strength. However, at 0.06% or more, high
In Nb-added systems, most of the added Nb becomes undissolved during heating, and it is not possible to increase the tension even with a high Nb addition amount. Therefore, C was set at 0.005 to 0.06%.
From a practical standpoint, it is within the normal heating range.
When considering the solubility product of Nb (C, N) on the premise of heating from 1050 to 1200°C, it is particularly desirable to reduce C to 0.04% or less in order to effectively utilize 0.10% or more Nb. Si is required as a deoxidizing element at 0.02% or more,
If it exceeds 0.50%, weldability will deteriorate. The minimum Mn content is 1.5% to achieve high tensile strength in a low C system.
The above is necessary, but if it exceeds 2.5%, weldability will deteriorate. S is used to prevent hot ductility from decreasing due to MnS.
Desirably 0.005% or less. As shown in Figure 5, when Nb is added in excess of 0.1%, it improves hot ductility and is effective in preventing scratches on the surface of continuous cast slabs. It is also necessary to add more than 0.1%. Therefore, the lower limit of the amount of Nb added was set to exceed 0.1%. Furthermore, if the amount of Nb added exceeds 0.20%, weldability will be impaired. Therefore, the upper limit was set at 0.20%. SolAl is required as a deoxidizing element in an amount of 0.005% or more, but if it is 0.10% or more, weldability deteriorates. V is required to be 0.01% or more from the viewpoint of precipitation strengthening, and if it exceeds 0.2%, weldability will be impaired. Adding 0.5% or less of Cu, Cr, and Mo as necessary can increase tensile strength, improve toughness, and improve weldability. Ni is effective in terms of both strength and toughness, and an appropriate amount of 2% or less is economical. In order to improve hardenability and achieve high tensile strength, B must be present in an amount of 0.0005% or more, and if it exceeds 0.002%, the weld toughness will be impaired. Next, the reason why the upper limit of the TN content is set to 0.006% is that if it exceeds this, weldability will be impaired and hot ductility will also deteriorate. This is a necessary condition. As mentioned earlier, Ti is an extremely important component in the present invention. If the amount added is too low, N cannot be sufficiently fixed, and AlN precipitates at the γ grain boundaries in the γ low temperature range, reducing ductility.
At least 0.005% or more is required to sufficiently fix N, improve hot ductility, and prevent surface scratches.
Therefore, this is the lower limit. On the other hand, if the amount of Ti added is too large, the excess Ti will become γ as TiC.
It precipitates in a low temperature range and actually reduces hot ductility. Therefore, the upper limit of the amount added is 0.030%, which is 0.005%.
The desired effect can be sufficiently achieved within the range of ~0.030%. Then, all that is left is to adjust the ingredients as described above.
Nb steel is continuously cast, and the continuous casting does not require any special restrictions (such as casting conditions, cooling conditions, etc.) and can be operated according to conventional methods. Then, using the continuous casting slab with good surface quality obtained by the above method, it is hot rolled and then cooled at 50℃/s or less by air cooling or accelerated cooling if necessary. A high Nb type non-tempered steel plate with high tensile strength and high toughness can be obtained.
The reason why the lower limit of the cooling rate is set to be equal to or higher than air cooling is that when slow cooling is performed after rolling, toughness is impaired due to coarsening of crystal grains and coarsening of precipitates.
Also, if the cooling rate exceeds 50℃/s, distortion will occur due to slight variations in the cooling rate within the steel plate, making it difficult to manufacture a flat steel plate.
Up to. Next, specific examples of the present invention will be shown. Example 1 In order to see the transient range of the amount of Ti added in the present invention, the hot ductility at 700 to 1000°C of a low C high Nb steel having the composition shown in Table 1 with the same amount of Nb added was As shown in the figure.

[Table] As is clear from Figure 7, the amount of Ti added is 0.004
% (Steel A), the aperture is 50 even at 1000℃.
%, and the low ductility temperature range is extremely wide at 310℃. This is because the amount of Ti added was low, so N could not be fixed sufficiently, and AlN precipitated at the γ grain boundaries in the γ low temperature range, resulting in a decrease in ductility. In addition, when the Ti addition amount is 0.035% (steel B), 900
℃ hot ductility is as low as 35%, and the low ductility temperature range is
It shows a large value of 240℃. This is because the amount of Ti added was too high, contrary to the case of Steel A.
It precipitates as TiC in the γ low temperature range, resulting in a decrease in ductility. On the other hand, the amount of Ti added is within the range of the present invention.
In the case of 0.017% (Steel C), hot ductility is extremely high and the low ductility temperature range is extremely narrow at 80°C.
From this point of view, the amount of Ti added is 0.005 as mentioned above.
It should be in the range of ~0.030%. Example 2 Test steel D was used to examine the limiting effect of each component mentioned above.
~H was manufactured by continuous casting and heated to 1100℃ and 900℃.
Figure 2 shows the chemical composition, strength, toughness, and hot ductility data when a cumulative reduction of 70% was applied and rolling was completed at 750°C. The finished plate thickness is 20 mm.

【table】

[Table] As is clear from Table 2, in the case of test steel D, the amount of TN and the amount of Ti added are appropriate;
Since the amount of Nb added is 0.04%, which is outside the scope of the present invention,
The strength is lower by about 6 kg/mm ² than steels E and F produced by the method of the present invention, and the toughness is also impaired by about 20 to 30°C compared to the method of the present invention. On the other hand, the steels E and F of the present invention have high strength and toughness, and the low ductility temperature range is extremely narrow, within 80°C, so the casting rate is less than 3% and almost no maintenance is required. It was unnecessary. In addition, in sample steel G, the amount of Nb and Ti added is appropriate, but the amount of TN is high at 0.0070%, and in sample steel H, the amount of Nb and TN added is appropriate, but the amount of Ti added is 0.002%. low. Therefore, in both cases, hot ductility is reduced due to the presence of free N, and the low ductility temperature range is as wide as 280 to 300°C. Therefore, the casting hand retention rate is also 30 ~
At 33%, it was extremely high, and due to heavy maintenance, it was close to being unusable. Example 3 Table 3 shows that in addition to the basic components, V, Cr, Cu, Mo,
Using test steels I, J, K, M and N containing one or more of Ni, B, heating at 1100℃ and 900℃
If a cumulative reduction of 65% is applied below, rolling is finished at 720℃, and then air cooled, or when heating to 1100℃ and a temperature range of 900℃ or less, a cumulative reduction of 65% is applied and rolling is finished at 750℃, and rolling is completed. 10℃/sec up to 600℃
It shows the chemical composition, strength, toughness, hot ductility, and continuous casting hand handling rate when accelerated cooling is performed.

【table】

[Table] As is clear from this Table 3, B in high Nb system
Steel added with Mo (I, J), Steel added with Mo (K),
The present invention applies to any of steel added with Cu, Ni and B (O), steel added with Mo and B (P), steel added with Ni (Q), and steel added with Cr and B (R). Since the requirements are met, it has high-speed high toughness both when air-cooled after hot rolling and when accelerated cooling is performed. In addition, the low ductility temperature range in which hot ductility is 50% or less in these steels is narrow at 80°C or less, and the continuously cast slabs also have good surface properties. On the other hand, although low Nb steel (M, N) is not much different from the present invention in terms of hot ductility and cast penetration rate, compared to the high Nb steel according to the present invention,
In both cases of air cooling or accelerated cooling after hot rolling, the strength and toughness are significantly deteriorated. According to the method of the present invention explained above, the effect of suppressing AlN precipitation can be obtained by adding a small amount of Ti, and
By increasing the amount of Nb added, the hot ductility improvement region can be effectively utilized, and furthermore, by adding Ti, the structure can be refined to improve hot ductility. From this, according to the present invention, low C-high
It is possible to accurately prevent the decrease in hot ductility and the occurrence of scratches on the surface of continuously cast pieces, which have been a problem in continuous casting of Nb steel, and easily produce high Nb steel sheets with good surface properties and high tensile strength and high toughness. You can get excellent results.

[Brief explanation of drawings]

Figure 1 is a graph showing the change in strength when changing the amount of Nb added in 0.02%C-1.7%Mn low C steel, and Figure 2 is a graph showing the results of a high temperature tensile test on low C-high Nb steel. Figure 3 is an explanatory diagram of the thermal history used in the high-temperature tensile test in Figure 2, and Figure 4 is a graph showing the relationship between the low ductility temperature range and the cast hand strain rate. Hot ductility of Nb steel is 700
A graph showing the change in aperture due to change in deformation temperature when examined in the temperature range of ~1000℃, Figure 5 is 0.02
%C-2.0%Mn-Nb-0.004%N steel and 0.02%
A graph showing the change in hot ductility when changing the amount of Nb added in C-2.0%Mn-Nb-0.015%Ti-0.004%N steel at 800°C and 1000°C, respectively.
Figure 6 shows the strength of the Ti-added steel shown in Figure 5 as a function of the amount of Nb added when cumulative reduction and rolling are applied.
Figure 7 is a graph showing changes in toughness, and shows changes in hot ductility at 700 to 1000°C for three types of low C-high Nb steels with different amounts of Ti added.

Claims (1)

[Claims] 1 C: 0.005 to 0.06%, Si: 0.02 to 0.50%,
Mn: 1.5-2.5%, S: 0.005% or less, Nb: 0.1%
Exceeding 0.20% or less, SolAl: 0.005-0.10%, TN:
Contains 0.006% or less, Ti: 0.005-0.030%, the balance
A slab manufactured by continuous casting method consisting of Fe and unavoidable impurities is hot rolled and air cooled at 50℃/s or more.
A method for producing a non-thermal high tensile and high toughness steel plate characterized by cooling at the following cooling rate. 2 C: 0.005-0.06%, Si: 0.02-0.50%,
Mn: 1.5-2.5%, S: 0.005% or less, Nb: 0.1%
Exceeding 0.20% or less, SolAl: 0.005-0.10%, TN:
0.006% or less, Ti: 0.005-0.030%, further V: 0.01-0.2%, Cr: 0.5% or less, Cu: 0.5% or less, Mo: 0.5% or less, Ni: 2% or less, B: 0.002
% or less, and the balance is Fe and unavoidable impurities, a slab manufactured by a continuous casting method is hot rolled and cooled at a cooling rate of not less than air cooling and not more than 50°C/s. A method for manufacturing non-tempered high-tensile and high-toughness steel sheets.