JPH0583606B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for manufacturing high-strength steel with excellent low-temperature toughness. Conventional technology High-strength steel with excellent low-temperature toughness is required to have good weldability in cold regions, so P _CM
A method has been taken to reduce C, Mn, Mo, etc. that increase the strength of steel, and compensate for the decrease in strength by lowering the finishing rolling temperature or by accelerated cooling after rolling. However, as the demand for higher strength becomes stronger and stronger, a method has been invented to further reduce C, Mn, Mo, etc. by adding Ti, which is not included in the P _CM formula, and utilizing its precipitation strengthening. I came (Tokuko Akira)
58−28330). Decreasing the P _CM value by adding Ti is an effective manufacturing method for obtaining high-strength steel with excellent low-temperature toughness, but conventionally, TiN and TiC have not been precisely controlled, so it is still difficult to obtain sufficient low-temperature toughness. It has not yet been possible to obtain an excellent high-strength steel, and
According to precipitation strengthening, it is common knowledge that low-temperature toughness deteriorates as strength increases. OBJECTS OF THE INVENTION The present invention aims to improve the drawbacks of the prior art described above, and provides a method for producing Ti-based high-strength steel that has even better low-temperature toughness. Structure of the Invention In order to achieve the above object, the method of manufacturing a Ti-based high-strength steel having excellent low-temperature toughness according to the present invention has the following gist. 1 Weight ratio: C: 0.013-0.30%, Si: 0.80% or less, Mn: 0.40-3.0%, P: 0.020% or less, S: 0.010% or less, Ti: 0.050-0.20%, sol.Al: 0.0040- 0.150%, N: 0.0010~0.0060
% TiN: 45 to 260 ppm, and further contains one or more of Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.150% or less, V: 0.150% or less, and the remainder is Fe and unavoidable TiC is 0.063
After heating to a temperature higher than the temperature at which % or more of solid solution occurs, rolling is performed such that the cumulative reduction rate is 30% or more in the austenite recrystallization temperature range, and then the rolling reduction is 80% in the austenite non-recrystallization temperature range. It is characterized by performing the above rolling and finishing the rolling in a temperature range of Ar3±30°C. 2 In terms of weight ratio, C: 0.013 to 0.30%, Si: 0.80% or less, Mn: 0.40 to 3.0%, P: 0.020% or less, S: 0.010% or less, Ti: 0.050 to 0.20%, sol.Al: 0.0040 to 0.150%, N: 0.0010~0.0060
% TiN: 45 to 260 ppm, further contains one or more of Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.150% or less, V: 0.150% or less, and further Cr: 0.50 % or less, Mo: 0.50% or less, Zr: 0.150
% or less, Ca: 0.050% or less, the remainder is Fe and unavoidable impurities, and TiC is 0.063% or less.
After heating to a temperature higher than the temperature at which % or more of solid solution occurs, rolling is performed such that the cumulative reduction rate is 30% or more in the austenite recrystallization temperature range, and then the rolling reduction is 80% in the austenite non-recrystallization temperature range. It is characterized by performing the above rolling and finishing the rolling in a temperature range of Ar3±30°C. As a result of various research into manufacturing methods for high-strength steel with excellent low-temperature toughness, the present inventors found that in order to make maximum use of the precipitation strengthening of TiC, it is necessary to It was clarified that it is important to heat TiC to a temperature at which TiC is sufficiently dissolved and to uniformly re-precipitate fine TiC of about 50 Å during rolling or during cooling after rolling. However, even though it is possible to increase the strength with only fine TiC precipitation, it is difficult to obtain excellent low-temperature toughness. Through further research, the present inventors have determined that the presence of undissolved TiN is necessary to prevent austenite grains from coarsening during heating during rolling, and that the presence of undissolved TiN can be used to suppress coarsening of austenite grains. By quantitatively understanding the required amount of TiN precipitation, suppressing this grain coarsening, and controlling the recrystallization of austenite grains during rolling, it is possible to manufacture high-strength steel with significantly superior low-temperature toughness. They found that. That is, conventionally, when increasing strength by using precipitation strengthening with NbC, VC, TiC, etc., it was common knowledge that the higher the strength, the more the low-temperature toughness deteriorates.
However, according to the present invention, despite the high strength due to the fine precipitation of TiC, it is necessary to perform rolling at a reduction rate of 80% or more in the non-recrystallization temperature range of austenite, and to reduce the non-recrystallization caused by rolling. An extremely large number of transformation nuclei of ferrite transformation are generated at the grain boundaries of the drawn austenite grains and deformation zones within the grains, and an extremely fine fine grain structure is obtained at the end of rolling. FIG. 1 shows the relationship between strength, toughness, and unrecrystallized rolling reduction, and FIG. 2 shows the relationship between toughness, TiC content, and unrecrystallized rolling reduction. As is clear from FIGS. 1 and 2, when the non-recrystallization reduction rate is 80% or more, a fine grain structure is formed and the toughness is significantly improved. This refinement is further promoted by combining the effect of suppressing the coarsening of austenite grains due to undissolved TiN during heating. Furthermore, the effect of suppressing the coarsening of austenite grains by TiN during heating is such that the higher the heating temperature, the larger the difference in grain size from normal steel. FIG. 3 is a diagram showing the relationship between slab heating temperature and austenite grain size in Nb-V steel and Nb-V-Ti steel. However, if TiN increases too much, the low-temperature toughness will significantly deteriorate due to the coarsening of TiN agglomeration. Fourth
The figure shows the relationship between the fracture surface transition temperature of Ti steel and the amount of TiN. On the other hand, when the heating temperature is low, the amount of undissolved TiC increases, and a large amount of coarse TiC exists. In addition, when deposited in a manner consistent with the matrix, the strength increases significantly. In Figure 5
The relationship between the tensile strength of Ti steel and the amount of TiC is shown. In short, the important point of the present invention is that, while conventional production methods have limited the total amount of Ti, the present invention limits the amount of TiN to an appropriate amount, which is substantially effective in suppressing coarsening of austenite grains. , TiC also has a substantial effect on strength.
Quantify the amount, tensile strength 60-65 at 0.063% or more
We invented conditions that make it possible to strengthen by Kg/mm ² or more. However, for low-temperature toughness, it is necessary to take measures during rolling, and with conventional manufacturing methods, the rolling reduction rate in the low-temperature range is low.
Although rolling is limited to 50% or more, in reality it is limited in terms of rolling mill capacity and manufacturing efficiency.
In the present invention, the reduction rate in the low-temperature range of only about 50 to 70% was taken, but in the present invention, it is a method that goes beyond the conventional practical range as a method to prevent deterioration of toughness while making full use of the precipitation strengthening of TiC. In particular, when the rolling reduction is limited to 80% or more, grain refinement and strain-induced precipitation (TiC) in the austenite region are promoted, the toughness is significantly improved, and a steel with high strength and excellent low-temperature toughness can be obtained. . Next, the reason for limiting numerical values of the composition range, heating temperature, and rolling conditions of the steel targeted by the present invention will be described. C: The C component has the effect of ensuring the strength of steel, but if its content is less than 0.013%, the desired effect cannot be obtained in the above function, and high strength cannot be obtained.On the other hand, if the content exceeds 0.30%, If it is included, the toughness of the base metal and welded part will deteriorate, so the content should be reduced to 0.013.
It was set at ~0.30%. Si: The Si component is an effective component as a deoxidizing agent for steel, but if its content exceeds 0.8%, inclusions will increase and weldability will deteriorate, so the content should be reduced to 0.8% or less. And so. Mn: The Mn component has the effect of improving the strength and toughness of the base metal and welded part, but its content is
If the content is less than 0.4%, the desired effect cannot be obtained; on the other hand, if the content exceeds 3.0%, the formation and increase of low temperature transformation products such as paynite, deterioration of toughness, and furthermore, an increase in the segregation of Mn. The content was set at 0.4 to 3.0% because it causes deterioration of hydrogen-induced cracking resistance. P, S: Reducing P and S is extremely effective in toughening steel, but in order to stably exhibit the features of the present invention, P must be 0.020% or less and S must be 0.010% or less. . Preferably, P and S should be as small as possible. Ti: The Ti component has the effect of suppressing the coarsening of γ grains during slab heating and refining the weld structure, but if the content is less than 0.05%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.2%, the toughness of the weld heat affected zone deteriorates, so the content was limited to 0.05 to 0.2%. sol.Al: The sol.Al component has a deoxidizing effect on steel, but
If the content is less than 0.004%, the desired effect cannot be obtained, and if the content exceeds 0.15%, the toughness of the weld heat affected zone will deteriorate, so the content should be reduced to 0.004 to 0.15%. It was determined that N: The N component suppresses the coarsening of austenite grains during reheating of the slab by producing nitrides, but if it is less than 0.0010%, sufficient TiN will not be produced to suppress grain growth and the toughness will deteriorate. . but
If it exceeds 0.0060%, the toughness deteriorates due to the coarsening of TiN, so the content was set to 0.0010 to 0.0060%. TiN: TiN suppresses the coarsening of austenite grains during slab reheating and is effective in refining the structure of the base metal and weld zone, but if its content is less than 45 ppm, the desired effect cannot be obtained. On the other hand
If the content exceeds 260 ppm, the low temperature toughness will be significantly deteriorated due to agglomeration and coarsening of TiN, so the content was limited to 45 to 260 ppm. Cu, Ni, Nb, V: These components are effective in improving the strength and toughness of steel, but if the Cu content exceeds 0.5%, it tends to cause hot cracking in the slab, so it should not exceed 0.5%. The Ni content is 1.0% because if it exceeds 1.0%, the toughness of the base metal and weld will deteriorate.
The content of both Nb and V was limited to 0.15% or less since the toughness of the welded part would deteriorate if it exceeded 0.15%. Cr, Mo, Zr, Ca: These components are effective in improving the strength and toughness of steel, but Cr,
If Mo exceeds 0.50%, the toughness of the base metal and weld will deteriorate, so it should be 0.5% or less, and Zr should be 0.5% or less.
If it exceeds 0.15%, the toughness of the base metal and weld zone deteriorates, so it is set to 0.15% or less, and if it exceeds 0.05%, the hot workability of the steel deteriorates, so it was limited to 0.05% or less. Heating temperature: In order to fully utilize the precipitation strengthening of TiC, the heating temperature during rolling needs to be high enough to completely dissolve TiC into solid solution.
It is necessary to make it a solid solution of 0.063% or more and to finely re-precipitate it during or after rolling. dissolve again
At a heating temperature where the amount of TiC is less than 0.063%, sufficient strength of the above effect cannot be obtained, so the heating temperature should be adjusted.
The temperature was set as the temperature at which 0.063% or more of TiC becomes a solid solution. Rolling conditions: Rolling in the austenite recrystallization temperature range is effective in refining the austenite grains by recrystallization, which can improve the toughness of the product, but the cumulative reduction ratio is less than 30%. In this case, grain refinement was insufficient and sufficient toughness of the product could not be obtained, so the cumulative reduction rate in the austenite recrystallization region was set at 30% or more. Rolling in the non-recrystallization temperature range stretches the austenite grains, increases the grain boundary area, and introduces defects such as deformation bands into the grains, thereby increasing the number of ferrite nuclei during ferrite transformation. As a result, it is effective to improve toughness by making the ferrite grains finer, but if the total reduction rate in this temperature range is less than 80%, a sufficient fine grain structure cannot be obtained and the toughness deteriorates. Therefore, the reduction rate in the non-recrystallization temperature range of austenite was set at 80% or more. Lowering the rolling finishing temperature is effective in improving strength and toughness, but temperatures higher than Ar ₃ +30°C cause a decrease in strength or deterioration of low-temperature toughness, while temperatures lower than Ar ₃ -30°C Since the ductility decreases significantly, the rolling finishing temperature is set to Ar ₃ ±30℃.
It was determined that Examples Example 1 Chemical composition: C: 0.09, Si: 0.47%, Mn: 1.58
%, P: 0.020%, S: 0.010%, Ti: 0.086%,
sol.Al: 0.036%, N: 0.0026%, TiN: 114ppm
A 20 mm steel plate was manufactured from a steel slab cut from a continuously cast steel ingot consisting of the balance Fe and other impurities. The manufacturing conditions and mechanical properties are shown in Table 1. As is clear from Table 1, No. 5 of the method of the present invention
- No. 9 obtained better performance in terms of strength, toughness, and ductility than Comparative Example Method Nos. 1 to 4.

[Table] Example 2 Cumulative reduction ratio in the recrystallization zone after heating the inventive steel and comparative steel produced by air and vacuum melting to 1250°C
40%, rolling reduction in non-recrystallized area 85%, finishing temperature 770℃
The material was rolled to a thickness of 20 mm, and air cooled after rolling. The chemical composition and mechanical properties of the steel are shown in Table 2. As is clear from Table 2, the steels of the present invention No. 1 to
No. 10 has superior performance in terms of strength, toughness, and ductility compared to comparative steels No. 11 to No. 17.

[Table] Effects of the Invention As described in the examples above, the manufacturing method of the present invention produces steel with extremely excellent low-temperature toughness by strictly regulating the composition range, heating temperature, and rolling conditions of the steel. This allows tensile steel to be manufactured at low cost.

[Brief explanation of the drawing]

Figure 1 shows the relationship between strength, toughness, and unrecrystallized reduction; Figure 2 shows the relationship between toughness, TiC content, and unrecrystallized reduction; and Figure 3 shows the relationship between Nb-V steel and Nb−
Figure 4 shows the relationship between slab heating temperature and austenite grain size in V-Ti steel, Figure 4 shows the relationship between fracture surface transition temperature and TiN content in Ti steel, Figure 5 shows the relationship between fracture surface transition temperature and TiN content in Ti steel.
FIG. 3 is a diagram showing the relationship between the tensile strength of Ti steel and the amount of TiC.

Claims (1)

[Claims] 1. In weight ratio, C: 0.013 to 0.30%, Si: 0.80% or less, Mn: 0.40 to 3.0%, P: 0.020% or less, S: 0.010% or less, Ti: 0.050 to 0.20%, sol.Al: 0.0040~0.150%, N: 0.0010~0.0060
% TiN: 45 to 260 ppm, and further contains one or more of Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.150% or less, V: 0.150% or less, and the remainder is Fe and unavoidable TiC is 0.063
After heating to a temperature higher than the temperature at which % or more of solid solution occurs, rolling is performed such that the cumulative reduction rate is 30% or more in the austenite recrystallization temperature range, and then the rolling reduction is 80% in the austenite non-recrystallization temperature range. A method for producing Ti-based high-strength steel with excellent low-temperature toughness, which comprises performing the above rolling and finishing the rolling in a temperature range of Ar3±30°C. 2 In terms of weight ratio, C: 0.013 to 0.30%, Si: 0.80% or less, Mn: 0.40 to 3.0%, P: 0.020% or less, S: 0.010% or less, Ti: 0.050 to 0.20%, sol.Al: 0.0040 to 0.150%, N: 0.0010~0.0060
% TiN: 45 to 260 ppm, further contains one or more of Cu: 0.50% or less, Ni: 1.0% or less, Nb: 0.150% or less, V: 0.150% or less, and further Cr: 0.50 % or less, Mo: 0.50% or less, Zr: 0.150
% or less, Ca: 0.050% or less, the remainder is Fe and unavoidable impurities, and TiC is 0.063% or less.
After heating to a temperature higher than the temperature at which % or more of solid solution occurs, rolling is performed such that the cumulative reduction rate is 30% or more in the austenite recrystallization temperature range, and then the rolling reduction is 80% in the austenite non-recrystallization temperature range. A method for producing Ti-based high-strength steel with excellent low-temperature toughness, which comprises performing the above rolling and finishing the rolling in a temperature range of Ar3±30°C.