JPH0138849B2 - - Google Patents

Description

[Detailed description of the invention]

This invention is suitable for use as a hot-rolled high-strength steel plate, especially as a material for line pipes or other structures that require high strength and low-temperature toughness.
This invention relates to a method for manufacturing thick-walled high-strength steel plates with a thickness of 4.5 mm or more. In general, Nb-containing structural steel sheets that are used as rolled are converted to ferrite when cooled after transformation.
Because NbC has the effect of increasing strength due to precipitation, it is used as a high-strength steel plate for line pipes, etc. However, depending on its thermal history, the degree of precipitation hardening may change slightly, and the toughness may change. Various strict rolling conditions and winding conditions are set because they have a large effect on However, with the methods used so far, the upper limit of the strength that can be obtained is low, and the temperature of the heating furnace must be adjusted to a value far different from that in normal work, making it difficult to perform efficient work. There have been various problems such as a steel plate having poor toughness or not being able to obtain a steel plate having good toughness. On the other hand, as a method to solve these drawbacks and stably obtain steel plates with high productivity and superior strength and toughness, for example,
A method such as that seen in Publication No. 25892 was proposed.
This is characterized by hot rolling Nb-containing steel, finishing the hot rolling at ₃ or more Ar points, and then coiling in a temperature range of 680 to 500℃.If the coiling temperature is significantly lower, the upper The lower limit of the winding temperature was set at 500°C, as the inclusion of bainite structure was detrimental to toughness. However, although this method can relatively stably produce hot-rolled high-strength steel sheets with good strength, toughness, and weldability, it is difficult to use a steel material containing a predetermined amount of Nb. is essential, and moreover, rolling must be carried out at relatively high rolling and coiling temperatures, resulting in a problem of increased manufacturing costs. From the above-mentioned viewpoints, the inventors of the present invention have found that in manufacturing, it is possible to lower the rolling temperature and coiling temperature to a much greater extent than before, and even with the reduction of alloying elements, it is possible to achieve good strength, toughness, and As a result of research to obtain hot-rolled high-strength steel sheets that have both good and weldability, we found the following:
The findings shown in (a) and (b) were obtained. That is, (a) After hot rolling, a steel with a predetermined composition is rapidly cooled to a temperature range of 480°C to Ms point at a cooling rate of 3°C/sec or more, and then coiled, a high-strength steel plate can be obtained. To be done. (b) Even in the case of such low-temperature winding, after controlled rolling to refine the austenite grains, Ar ₃
If rolling is completed at a working rate of 30% or more in the temperature range from point to Ar ₁ point, many separations will occur on the shear peace fracture surface, and in this case, as is generally stated, the crack tip By reducing the triaxial stress of the steel, the shearpy fracture transition temperature (low-temperature toughness) never increases (deteriorates) even if a reinforcing structure due to transformation such as bainite structure is mixed in. This fact is also supported by the experimental results shown in FIGS. 1 and 2, for example. 1st
The figure shows Nb-added steel, i.e. 0.17%C-0.30%Si
-1.20%Mn-0.06%Nb-0.03%Al-Fe steel, heating temperature: 1150℃, processing rate in the temperature range of 1100~950℃: 60%, time from completion of rolling to winding: 45 seconds The effects of finish rolling temperature and coiling temperature on the strength and toughness of 11 mm thick steel plates manufactured under these conditions are measured and graphed. As for the coiling temperature, it is clear from FIG. 1 that the highest yield point is reached at 400°C, and this is thought to be due to the inclusion of self-annealed bainite structure. In addition, when winding at 100℃, the yield phenomenon disappears and the yield point becomes low because the winding temperature is below the Ms point.
It can be seen that when rolled at 600°C, the yield point is still low because it becomes a ferrite/pearlite structure. On the other hand, regarding the fracture surface transition temperature of shear peas, it is clear that when coiling at 400°C, separation occurs frequently and a significant improvement in toughness is observed when there is a strong pressure of ₃₀ % or more at Ar 3 points or less. . Note that if the rolling is completed at an Ar point of ₁ or less, the effect of increasing strength due to low-temperature coiling is small because the ferrite-pearlite transformation has been completed by the time the rolling is completed. Therefore, it can be seen that the combination of the rolling conditions and coiling temperature described above is essential in order to obtain a steel with excellent toughness. In addition, Fig. 2 shows ordinary carbon steel,
i.e. 0.18%C-0.25%Si-1.55%Mn-0.05
%Al-Fe steel, heating temperature: 1150℃, 1100~
11mm manufactured under the following conditions: processing rate in the 950℃ temperature range: 60%, time from completion of rolling to winding: 45 seconds
The effects of finish rolling temperature and coiling temperature on the strength and toughness of thick steel plates are measured and graphed. From FIG. 2, it can be confirmed that the same results as Nb-added steel can be obtained even with plain carbon steel. However, in this case, it is thought that the strengthening by low-temperature coiling involves refinement of the inside of the pearlite structure in addition to the inclusion of the bainite structure. Therefore, this invention was made based on the above knowledge, and in weight percent, C: 0.02 to 0.25%, Si: 0.60% or less, Mn: 0.5 to 2.2%, S: 0.010% or less, Sol.Al : 0.10% or less, and if necessary, Nb: 0.10% or less, V: 0.10% or less, Ti: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.50% or less. , B: 0.010% or less, Contains a strength improving component consisting of one or more of the following, and further contains, if necessary, Ca: 0.010% or less, Fe and unavoidable impurities: the remainder, from A steel having a composition of Ar ₃ points ~ _{Ar 1}
When it reaches the point region, rolling is performed again at a working rate of 30% or more to fully develop the warm worked texture and improve toughness by creating many separations that can be seen on fractured surfaces such as in the Sharpie test. then 3
By rapidly cooling to a temperature range of 480℃ to Ms point at a cooling rate of ℃/sec or higher and then coiling, a self-tempering low-temperature transformation product is created, increasing the strength.
This method is characterized by obtaining a high toughness hot rolled steel sheet. Next, in the method for producing a hot-rolled high-strength steel sheet of the present invention, the reason why the chemical composition, rolling temperature, processing rate, cooling rate, and coiling temperature of the steel are limited as described above will be explained. (a) C The C component has the effect of strengthening the steel by increasing the volume fraction of the bainite structure and fine pearlite structure that occur during low-temperature coiling, and also has the effect of facilitating the melting of steel. If the content is less than 0.02% by weight, the desired effect cannot be obtained;
If the content exceeds 1% by weight, the toughness will deteriorate in the weld heat affected zone.
It was limited to 0.02-0.25% by weight. (b) Si The Si component is necessary to increase strength through solid solution hardening, but if it is included in excess of 0.60% by weight, weldability will deteriorate.
Its content was limited to 0.60% by weight or less. (c) Mn The Mn component has the effect of improving strength through transformation strengthening, pearlite strengthening, etc. However, if its content is less than 0.5% by weight, the desired effect cannot be obtained, while if the content is less than 0.5% by weight, If the content exceeds this, weldability will deteriorate, so the content was limited to 0.5 to 2.2% by weight. (d) SS The S component has the effect of combining with Mn to form A-based inclusions and lowering the lateral Charpy absorption energy. This effect becomes significant when the content exceeds 0.010% by weight.
Its content was limited to 0.010% by weight or less. (e) Sol.Al The Sol.Al component is necessary as a deoxidizing agent, but if it is contained in an amount exceeding 0.10% by weight, it will have a negative effect on toughness, so its content should be controlled.
The content was limited to 0.10% by weight or less. (f) Nb, V, and Ti Nb, V, and Ti components combine with C and have the effect of increasing strength through precipitation hardening, so they may be included as necessary when particularly high strength is required. However, even if the content exceeds 0.10% by weight, no significant improvement in the effect was observed, so in consideration of economic efficiency, the content was limited to 0.10% by weight or less. (g) B The B component has the effect of improving the hardenability of steel and increasing its strength when added in small amounts, so Nb, V,
Like Ti, it is included when high strength is required.
It is an effective component in the steel sheet used in the method of the present invention, which is particularly used for low-temperature coiling, but even if it is contained in an amount exceeding 0.0100% by weight, no improvement in the above-mentioned effects is observed. It was limited to % by weight or less. (h) Cu, Ni, and Cr Cu, Ni, and Cr components include Mn component and
Like the Nb, V, and Ti components, it has the effect of improving strength, but even if its content exceeds 0.50% by weight, there is no significant improvement effect on the above effects, and economic efficiency must be considered. Therefore, the content was limited to 0.50% by weight or less. (i) Ca In the Ca component, MnS-based A-based inclusions and Al ₂ O ₃ -based B-based inclusions are changed to C-based inclusions, resulting in a large improvement in absorbed energy in the lateral direction, which improves toughness. Since it has the effect of improving % or less. (j) Processing rate in the temperature range of 1100 to 950°C Processing rate in the temperature range of 1100 to 950°C is 50%
If it is less than that, the austenite will not become finer, and coarse bainite will be mixed in during low-temperature winding.
Even if the toughness improvement effect of separation is used, there will be a limit to the toughness level of the steel plate, so the processing rate in the temperature range of 1100 to 950℃ is
Limited to 50% or more. (k) Processing rate in the temperature range of Ar ₃ to _{Ar 1} point When the processing rate is less than 30%, the amount of ferrite processed is small, the development of the (100) texture on the plate surface is small, and the separation is poor. As the occurrence of cracking decreases, the effect of improving toughness is reduced. Also
When rolling is completed at a temperature lower than the Ar ₁ point, there is little structural change and no significant improvement in strength is observed. For this reason, the processing rate in the temperature range of Ar ₃ to Ar ₁ was set at 30% or more. (l) Cooling rate and coiling temperature If the cooling rate is slower than 3℃/sec, 480℃
Similar to the high-temperature coiling described above, the transformation is completed in the high temperature range and becomes a ferrite/pearlite structure, and high strength due to the inclusion of hard low-temperature transformed structures such as bainite transformation cannot be obtained, and the coiling temperature below the Ms point In the case of a temperature of 3°C/sec, the yield phenomenon disappears, that is, the self-annealing effect based on slow cooling after coiling becomes insufficiently hardened, and a high yield point cannot be obtained. After rapid cooling at a cooling rate of 480℃ or more
Winding was limited to a temperature range of ~Ms point. In addition, in the method of this invention, rolling is completed at a working rate of 30% or more in the temperature range of ₃ points to ₁ point of Ar (in other words, rolling is performed in the two-phase coexistence region of ferrite + austenite). Therefore, there is no need to particularly limit the upper limit of the cooling rate, but currently the industrially feasible cooling rate is 100℃/
It is up to about sec. Next, the present invention will be explained by examples.

【table】

【table】

[Table] First, steel having the composition shown in Table 1 was melted and cast into slabs, and then subjected to hot rolling simulation experiment rolling to obtain a hot rolled coil with a thickness of 11 mm. The hot rolling conditions were as shown in Table 2. Among these, test numbers 1 to 3 used the same steel type A, but in test number 1, which is a comparative example, the rolling reduction rate in the temperature range of 1100 to 950°C was lowered to 20%. In Test No. 2, another comparative example, the cooling rate from the end of rolling to winding was slowed to 1.0° C./sec. Test pieces from the hot-rolled coil thus obtained (JIS No. 5 and 2 mm V-notch JIS No. 4 sharp pieces)
was cut out and its mechanical properties were measured. The results are also listed in Table 2. From this result, 1100~950℃
The steel plate manufactured by the method of Test No. 1, which has a low processing rate outside the scope of this invention, has significantly poor toughness, and the cooling rate from the end of rolling to coiling is slow, which is also outside the scope of this invention. It is clear that the steel plate manufactured by the method of Test No. 2 had low tensile strength and yield point, and was inferior in strength.
On the other hand, it can be seen that the steel plates manufactured by the method of the present invention (test numbers 3 to 9) are excellent in both strength and toughness. Further, when the weldability of the steel plate manufactured by the method of the present invention was tested, it was found that good results were obtained. As described above, according to the present invention, a thick-walled hot-rolled steel sheet that does not require large amounts of expensive alloying elements and has good strength, toughness, and weldability even at low rolling and coiling temperatures. A tensile steel plate is obtained,
This brings about industrially useful effects such as the ability to drastically reduce the cost of steel plates.

[Brief explanation of drawings]

Figure 1 is a diagram showing the effects of finish rolling temperature and coiling temperature on the strength and toughness of Nb-added steel sheets, and Figure 2 is a diagram showing the effects of finish rolling temperature and coiling temperature on the strength and toughness of ordinary carbon steel plates. FIG.

Claims (1)

[Claims] 1 C: 0.02 to 0.25%, Si: 0.60% or less, Mn:
A steel containing 0.5 to 2.2%, S: 0.010% or less, sol.Al: 0.10% or less, and the remainder consisting of Fe and unavoidable impurities (weight% above) is heated at 1100 to 950℃.
After rolling at a working rate of 50% or more in the temperature _{range of} A method for producing thick hot-rolled high-strength steel sheets with a thickness of 4.5 mm or more that has excellent low-temperature toughness and is characterized by rapid cooling to a temperature range of 480°C to the Ms point and coiling. 2 C: 0.02-0.25%, Si: 0.60% or less, Mn:
Contains 0.5 to 2.2%, S: 0.010% or less, Sol.Al: 0.10% or less, and further Nb: 0.10% or less, V: 0.10%
Below, Ti: 0.10% or less, Cu: 0.50% or less, Ni:
0.50% or less, Cr: 0.50% or less, and B: 0.010%
Steel containing one or more of the following strength-improving components, with the remainder consisting of Fe and unavoidable impurities (more than 1100% by weight)
Rolled at a processing rate of 50% or more in a temperature range of 950℃,
Furthermore, after rolling again at a processing rate of 30% or more in the temperature range of Ar ₃ to Ar ₁ point, quenching at a cooling rate of 3°C/sec or more to a temperature range of 480°C to Ms point, and coiling. A method for producing thick-walled hot-rolled high-strength steel plates with a thickness of 4.5 mm or more that have excellent low-temperature toughness. 3 C: 0.02-0.25%, Si: 0.60% or less, Mn:
Contains 0.5 to 2.2%, S: 0.010% or less, Sol.Al: 0.10% or less, and Ca: 0.010% or less,
Steel having a composition (weight %) with the remainder consisting of Fe and unavoidable impurities is rolled at a processing rate of 50% or more in a temperature range of 1100 to 950°C, and further has a composition of Ar ₃ to _{Ar 1}
After rolling again at a processing rate of 30% or more in the temperature range of point, 480 ~ Ms at a cooling rate of 3℃/sec or more
A method for producing thick hot-rolled high-strength steel sheets with a thickness of 4.5 mm or more that has excellent low-temperature toughness and is characterized by rapid cooling to a point temperature range and then coiling. 4 C: 0.02-0.25%, Si: 0.60% or less, Mn:
Contains 0.5 to 2.2%, S: 0.010% or less, sol.Al: 0.10% or less, and further Nb: 0.10% or less, V: 0.10%
Below, Ti: 0.10% or less, Cu: 0.50% or less, Ni:
0.50% or less, Cr: 0.50% or less, and B: 0.010%
A composition containing one or more of the following strength-improving components, and Ca: 0.010% or less, with the remainder consisting of Fe and unavoidable impurities (wt%)
50% in the temperature range of 1100 to 950℃
After rolling with the above processing rate and further rolling again with a processing rate of 30% or more in the temperature range of Ar ₃ to Ar ₁ point, the temperature range of 480 to Ms point with a cooling rate of 3℃/sec or more A method for manufacturing thick-walled hot-rolled high-strength steel sheets with a thickness of 4.5 mm or more that has excellent low-temperature toughness and is characterized by rapid cooling to a maximum temperature and then winding.