JPS624450B2 - - Google Patents

Description

[Detailed description of the invention]

This invention has a high tensile strength of 70Kg/mm ² or more, and has excellent workability and low-temperature toughness.
This paper relates to a method for producing Ti-added hot-rolled high-strength steel sheets. In recent years, there has been an increasing demand for steel materials with high strength and excellent workability as structural materials for various buildings and industrial machinery, and various steel materials have been developed and used to meet these demands. I'm getting used to it.
Examples include Nb-added steel, V-added steel, and Ti-added steel. Among these, Ti-added steel is attracting attention because it is cheap to manufacture and can provide high strength, but it has the problem that its toughness is inferior to Nb-added steel and V-added steel. . However, recently, due to the deterioration of the energy situation, it has become unavoidable to develop resources in extremely harsh environments.
In the case of the above-mentioned high-strength steel plates, if they are plastically deformed by cold working and used in cold regions, there is a risk of brittle fracture occurring at the plastically deformed parts. In addition to these properties, there was a strong demand for high-strength steel sheets that also had excellent low-temperature toughness so that they could withstand use in cold regions. Therefore, in order to provide a high-strength steel plate that satisfies these demands, a method of hot controlled rolling of Ti-added steel was proposed, as described in Japanese Patent Publication No. 1983-45614. The feature of Ti-added hot-rolled high-strength steel sheet is that it utilizes the precipitation strengthening of TiC, and at the same time replaces MnS, which is an A-based inclusion, with TiS to create a C-based inclusion, which improves workability. In addition to having high strength, it also has excellent cold workability, as it was possible to perform close bending in a JIS standard bending test using specimens with machined edges. It is said that The method described in the above-mentioned Japanese Patent Publication No. 55-45614 further improves the low-temperature toughness by performing controlled rolling during the production of Ti-added hot-rolled high-strength steel sheets having such characteristics. This is an improvement. By the way, in the bending test according to the JIS standard, as mentioned above, a material with a machined end surface is used as a test material, but in actual production of structural members, a material with a shear end surface (the shear cut surface is machined) is used. In most cases, cold-working steel sheets are subjected to cold working as they are. Therefore, from a practical standpoint, cold-working steel sheets are required to have good bending performance in specimens with sheared edges. The Rukoto. However, there are many Ti
It became clear at the subsequent stage of practical application that hot-rolled steel sheets with additives generally had poor bending test performance with shear end faces, and had serious problems such as cracking at the end faces during bending. Ta. From the above-mentioned viewpoints, the present inventor has found that the material not only has high tensile strength of 70 kg/mm ² or more, excellent workability and low-temperature toughness, but also has good workability, especially for materials with shear end faces. As a result of intensive research in order to obtain high-strength steel sheets, we found that a specific composition of
After controlled rolling of Ti-added steel, if it is rolled at a low temperature of approximately 400°C, which breaks conventional wisdom, the tensile strength can be increased to 70°C.
It has been found that a high tensile strength steel plate with a high value of Kg/mm ² or more, improved bending cracking of a steel plate with a shear end face, and excellent low-temperature toughness can be obtained. Based on this, we continued further research, and the following (a)
We have come to the knowledge shown in ~(c). In other words, (a) When producing a Ti-added hot rolled steel sheet with high strength, after hot rolling, the temperature is approximately
When coiled at 600℃, it is true that it is possible to bend closely within the JIS standard bending range, and it has excellent workability.
When bending a material with a shear end surface, it is difficult to avoid bending cracks due to the brittleness of the ferrite grain boundaries, and the low-temperature toughness also deteriorates due to the precipitation of TiC. However, when the winding temperature is controlled within the range of less than 450℃ to 200℃,
As mentioned above, precipitation of TiC is suppressed and transformation strengthening is performed, suppressing embrittlement of ferrite grain boundaries, and not only does the bending performance of the shear end face improve significantly, but also the shear pie fracture transition temperature is improved. In particular, at temperatures below 900°C, a total of 30
(b) By combining this with controlled rolling in which a reduction of ~90% is performed and rolling is completed at a temperature of 870 to 800°C, a high-strength steel plate with an extremely improved Shalpy fracture transition temperature can be obtained; In addition, if the P content in the Ti-added steel is reduced, each of the properties described in item (a) above will be further improved. (c) One of Ca, B, and Cr may be added to the Ti-added steel. By containing a specific amount of a species or two or more species, further improvement in workability and toughness can be achieved. In order to obtain these findings, the present inventor established a hot rolling simulation experimental method to investigate the effect of coiling temperature on the mechanical properties of Ti-added steel, and repeated various experiments. Needless to say. This hot rolling simulation experimental method involves rapidly cooling the steel material by water spray to a predetermined temperature after rolling, and then putting the rolled material into a furnace whose temperature has been raised to the predetermined temperature and cooling it in the furnace (cooling rate: 20℃/hr)
This is a method of doing this. It was also confirmed that if the "predetermined temperature" at this time was made to match the coiling temperature, a steel plate having the same structure and characteristics as in actual hot rolling and coiling operations could be obtained. Figure 1 shows the results of investigating the effect of coiling temperature on the mechanical properties of Ti-added steel under controlled rolling with a finish of 820°C using such a hot rolling simulation experimental method. Figure 1 shows 0.10%C-0.30%Si-1.65%Mn-
0.002%S - 0.17%Ti - 0.025%Al - 0.0035%N steel (Hereinafter, percentages indicating component composition ratios are weight%)
Apply a 50% reduction at a temperature below 900℃, finishing temperature:
A line showing the effect of the coiling temperature on the mechanical properties of the steel plate when a 6 mm thick hot rolled steel plate is obtained at 820℃ and then cooled at a cooling rate of 10℃/sec, which corresponds to the coiling cooling rate. It is a diagram. From Figure 1, the winding temperature is
When the temperature exceeds 400℃, the deterioration of the bendability of the sheet material with shear edges and the transition temperature of the shear pie fracture surface becomes noticeable, and especially when the temperature exceeds 500℃, the deterioration tendency becomes extreme to the extent that it is not practical. It was found that workability and low-temperature toughness were extremely good when the coiling temperature was in the range of less than 450°C to 200°C, and when the coiling temperature was further lowered to less than 200°C, there was a tendency for these properties to deteriorate again. It is clear that Also,
The P content in steel also affects each of the above properties,
It can also be seen that good results can be obtained if the content is 0.025% or less. On the other hand, Figure 2 shows the optical properties of a conventional Ti-added hot-rolled steel material rolled at 600℃ (Figure 2a) and a material rolled at a lower temperature of 400℃ (Figure 2b). This shows the microscopic structure. Comparing the two, it can be seen that when the 600℃ rolled material is subjected to nital corrosion, uneven ferrite grain boundary corrosion occurs. From these experimental results, the following inferences were made. In other words, when Ti-added steel is coiled at the normal coiling temperature of approximately 600℃, during the slow cooling after coiling, ferrite is deposited in the ground.
TiC precipitation becomes significant, resulting in embrittlement. In addition, uneven ferrite grain boundary corrosion due to nital corrosion occurs within the ferrite grains.
This seems to be an expression of the grain boundary purification effect, in which carbon present at grain boundaries decreases as TiC precipitates. It is known that steel materials that are prone to such uneven corrosion have weak ferrite grain boundaries, so both the embrittlement of the steel material and the grain boundary embrittlement described above cause the steel sheet to undergo shearing. It is judged that cracks have already formed on the end face, and that this will lead to large cracks due to subsequent bending. However, if low-temperature winding is performed, TiC
Since the precipitation of is appropriately suppressed and transformation strengthening takes place instead of precipitation strengthening, the above-mentioned drawbacks caused by winding at about 600°C can be avoided. In addition, it is thought that the material rolled at a temperature lower than 200° C. has poor self-annealing effect due to slow cooling after winding, resulting in poor bendability and sharpie properties. The reason why the bendability and shear strength properties of steel sheets with shear edges are improved by reducing the P content as much as possible is that the embrittlement of ferrite grain boundaries due to tempering embrittlement promoted by the presence of P This is thought to be because it was suppressed by the decrease. Therefore, the present invention was made based on the above findings, with the aim of improving the workability and low-temperature toughness of Ti-added hot-rolled steel materials with shear end faces, and in which C: 0.05 to 0.20%. , Ti: 0.04-0.20%, Si: 1.2% or less, Mn: 0.5-2.0%, P: 0.025% or less, S: 0.015% or less, Sol.Al: 0.005-0.15%, N: 0.008% or less. Or, in addition, Ca: 0.0100% or less, B: 0.0030% or less, Cr: 1.0% or less, containing one or more of the following, Fe and unavoidable impurities: the remainder, and the killed steel is heated at 900 to 800°C. After hot rolling with a total rolling reduction of 30 to 90% in the temperature range of 870 to 800℃, 5 to 50℃/s
Below 450℃ after rapid cooling at EC cooling rate
By coiling at 200°C, a Ti-added strong hot-rolled high-strength steel sheet with excellent cold workability is obtained. Next, in the method for producing a hot-rolled high-strength steel sheet of the present invention, the reason why the composition range of the steel and the hot-rolling winding conditions are limited as described above will be explained. C The C component has the effect of ensuring the strength of steel,
Tensile strength: It is an essential component to achieve a strength of 70 Kg/mm ² or more, but if its content is less than 0.05%, the desired effect cannot be obtained; on the other hand, 0.20% If the content exceeds 0.05, a high carbon-containing bainite structure will occur in low-temperature winding as employed in this invention, deteriorating bendability and low-temperature toughness. It was set at 0.20%. Ti The Ti component has the effect of strengthening steel by precipitating TiC and improving C bending performance by changing A-based inclusions such as MnS to C-based inclusions such as TiS. If the content is less than 0.04%, not only will it not be possible to impart the desired strength to the steel material, but the control of the shape of inclusions will also be insufficient, resulting in poor C bending performance.On the other hand, if the content exceeds 0.20%, the carbon-containing steel of the present invention ( C: 0.05 to 0.20%) causes significant precipitation hardening, which adversely affects low-temperature toughness;
It was set at 0.20%. Si The Si component has a solid solution strengthening effect and a deoxidizing effect. Although it is preferable to contain about 0.05% or more to increase strength, 1.2%
If the content exceeds 1.2%, the toughness and weldability will deteriorate, so the content was set at 1.2% or less. Mn The Mn component has the effect of toughening steel and is an important component, but if its content is less than 0.5%, the desired effect cannot be obtained.
On the other hand, if the content exceeds 2.0%, A-based inclusions tend to occur and the C bending performance deteriorates, so the content was set at 0.5 to 2.0%. P The P component tends to segregate at ferrite grain boundaries during slow cooling after winding and cause grain boundary embrittlement. Therefore,
Since it causes deterioration in the bending performance of the material with the shear end surface, it is better to have as little as possible, but from an economic point of view, the content is set at 0.025% or less as an allowable range. however,
The content is preferably 0.010% or less. S S is an impurity element that tends to form A-based inclusions in steel, and even in Ti-added steel, if its content exceeds 0.015%, it will combine with Mn to form A-based inclusions and cause bending. Since this would degrade performance, its content was set at 0.015% or less. Sol.Al The Sol.Al component has the effect of ensuring the effectiveness of added Ti, but if the content is less than 0.005%, the effect of Ti addition will not be fully exhibited, while if the content exceeds 0.15% If it is included, the amount of nonmetallic inclusions will increase and the steel will become brittle, so the content was set at 0.005 to 0.15%. N N is an impurity element that generates TiN in steel and reduces the amount of TiasTiC, which is effective for precipitation hardening, or TiasTiS, which is effective for spheroidizing nonmetallic inclusions, so it is better to minimize it as much as possible. However, the upper limit of its content has been set at 0.008% as an allowable range considering economic efficiency. However, it is preferably 0.0050% or less. Ca The Ca component has the effect of combining with Al ₂ O ₃ -based B-based inclusions and converting them into C-based inclusions to improve workability. In other words, since Ti can reduce A-based inclusions and Ca can also reduce B-based inclusions, the addition of Ca in Ti-added steel is very favorable for controlling the shape of inclusions, so it is particularly effective for improving workability. If further improvement is required, it is desirable to contain 0.0008% or more. However, if the content exceeds 0.0100%, the number of inclusions will increase beyond the allowable range, so the content was set at 0.0100% or less. B The B component has the effect of improving the hardenability of steel and imparting toughness, especially under the condition that Ti-added steel is coiled at a low temperature as in the high-strength steel plate manufacturing method of this invention. The effect of improving the hardenability of steel by adding a small amount of B is very large. Therefore, when higher toughness is required, it is desirable to contain 0.0001% or more. However, even if the content exceeds 0.0030%, no further improvement effect can be obtained, so the content was set at 0.0030% or less. Cr Similar to Mn, the Cr component has the effect of toughening steel, and when it is necessary to further improve the toughness of steel, it is desirable to add 0.1% or more, but it is not recommended to add more than 1.0%. Since no further improvement effect could be obtained even if the content was reduced to 1.0% or less, the content was set at 1.0% or less. Hot rolling/coiling conditions (iv) Hot rolling conditions In Ti-added steel, precipitation hardening of TiC and coarse
Since the presence of TiN deteriorates low-temperature toughness, as a countermeasure, the total
It is necessary to carry out controlled rolling in which rolling is completed at 870 to 800°C with a reduction of 90%. In this case, if the finishing temperature of rolling exceeds 870°C or the total rolling reduction rate is less than 30 in the temperature range of 900 to 800°C, the desired fine grain structure cannot be obtained, and the low temperature required as a structural material cannot be obtained. It becomes difficult to ensure toughness, so the higher the reduction rate, the better, but 90%
When the rolling reduction exceeds , the rolling capacity becomes constrained. On the other hand, if rolling is finished at a temperature below 800℃, the texture will develop and anisotropy will occur, as well as deterioration of C bendability, so the total The rolling reduction rate is 30-90%, and the rolling end temperature is
The temperature was set at 870-800℃. (ii) Cooling rate after hot rolling until coiling The cooling rate after controlled rolling until coiling is 5
If the temperature is lower than ℃/sec, almost no transformation strengthening effect occurs, making it difficult to obtain the desired high strength. Therefore, the faster the cooling rate, the better; however, since a cooling rate exceeding 50°C/sec would be difficult in actual operation, the cooling rate was set at 5 to 50°C/sec. (iii) Coiling temperature As mentioned above, when the coiling temperature is 450℃ or higher, the bendability of the material with shear end faces and the shear pie fracture surface transition temperature deteriorate significantly, whereas when the coiling temperature is lower than 200℃. In this case, the characteristics also tend to deteriorate, so the winding temperature was set at less than 450°C to 200°C. Next, the present invention will be explained using examples and comparing with comparative examples. Example First, the chemical composition range of steel used in the method of the present invention having the chemical composition shown in Table 1

【table】

【table】

[Table] Steels A to H that satisfy the following, and comparative steels I to S in which the composition range of any chemical component deviates from that of the steel subject to the present invention (components that are outside the composition range are marked with *) ) was made using the usual melting and casting method. Next, these steel materials were hot-rolled under the conditions shown in Table 2, and wound to produce hot-rolled steel sheets having a thickness of 6 mm. In addition, the * mark in Table 2 is
All of these conditions are outside the range defined in the present invention. Furthermore, the mechanical properties of the hot rolled steel sheet thus obtained were measured, and the obtained results are also listed in Table 2. From the results shown in Table 2, the hot-rolled steel sheets obtained by the methods of test numbers 1 to 13, in which the chemical composition range of the steel used and the hot-rolling/coiling conditions are within the range specified by the present invention, are It is clear that all of them have high strength, excellent low-temperature toughness and shear end face bending performance, and on the other hand, the chemical composition of the steel material and the conditions of hot rolling and winding are Test number 14 ~ which is outside the specified range
It can be seen that the hot-rolled steel sheet obtained by method 27 is inferior in low-temperature toughness and shear end face bending performance. In particular, when only low-temperature coiling is performed without controlled rolling, as in the comparative example of test number 14, the structure of the obtained steel sheet is as shown in Figure 2b, with a small amount of fine-grained ferrite and fine particles. It was also found that the structure did not become a mixed structure of bainite structure, but instead became a coarse bainite structure, resulting in a significant deterioration of toughness. In addition,
Needless to say, the experimental results when the winding temperature was outside the range of the present invention were not good, as shown in FIG. 1 above. In addition, D steel, F steel in Table 1 with Ca added,
It was confirmed that the bending performance of G steel and H steel was extremely improved. In this way, by subjecting Ti-added steel with a specific chemical composition to a combination of predetermined controlled rolling and low-temperature coiling, a mixed structure of a small amount of fine-grained ferrite and a fine bainite structure (a bainite structure due to fine-grained ferrite) Therefore, not only can toughness be ensured, but also excellent workability can be obtained. As mentioned above, according to the present invention, it is possible to achieve high tensile strength of 70 kg/mm ² or more without any special post-treatment, and to cool well without cracking even when the material has a shear end surface. Hot-rolled high-strength steel sheets that have excellent workability and extremely low-temperature toughness can be obtained by relatively simple means, making them ideal for buildings used in cold regions and other areas. By applying it to the structural materials of industrial machinery, etc., it can be expected to achieve better results than ever before, and will bring about industrially useful effects.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the effect of coiling temperature on the mechanical properties of Ti-added steel, and Figure 2-a is an optical microscope showing nital corrosion of a steel plate rolled at 600℃ after controlled rolling of Ti-added steel. Organization chart, Figure 2b is 400
FIG. 2 is an optical microscopic microstructure diagram of a similar steel plate coiled at ℃ due to nital corrosion.

Claims (1)

[Claims] 1 C: 0.05-0.2%, Ti: 0.04-0.2%, Si: 1.2% or less, Mn: 0.5-2%, P: 0.025% or less, S: 0.015% or less, Sol.Al: 0.005 to 0.15%, N: 0.008% or less, and the remainder consists of Fe and other unavoidable impurities (weight%), the total rolling reduction rate in the temperature range of 900 to 800℃ is 30~90
% conditions, and after finishing the rolling at a temperature of 870 to 800°C, it is rapidly cooled at a cooling rate of 5 to 50°C/sec, and coiled at a temperature of less than 450°C to 200°C. A method for producing Ti-added strong hot-rolled high-strength steel sheets with excellent cold workability. 2 C: 0.05-0.2%, Ti: 0.04-0.2%, Si: 1.2% or less, Mn: 0.5-2%, P: 0.025% or less, S: 0.015% or less, Sol.Al: 0.005-0.15%, N : 0.008% or less, and further contains one or more of the following: Ca: 0.01% or less, B: 0.003% or less, Cr: 1% or less, with the remainder consisting of Fe and other unavoidable impurities. (more than weight%), the total rolling reduction in the temperature range of 900-800℃ is 30-90
% conditions, and after finishing the rolling at a temperature of 870 to 800°C, it is rapidly cooled at a cooling rate of 5 to 50°C/sec, and coiled at a temperature of less than 450°C to 200°C. A method for producing Ti-added strong hot-rolled high-strength steel sheets with excellent cold workability.