JPS6366367B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is based on Ti, which has a high tensile strength of 70 kg/mm ² or more and has excellent workability and low-temperature toughness.
The present invention relates to a method for producing additive 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 of its low manufacturing cost and high strength.
There was a problem that the toughness was inferior compared to additive steel. On the other hand, in recent years, due to the deterioration of the energy situation, it has become unavoidable to develop resources in extremely harsh environments, and structural materials must also be able to withstand use in such environments. For example, especially in the case of high-strength steel plates with a thickness of 4.5 mm or more, if they are plastically deformed by cold working and used in cold regions, there is a risk of brittle fracture occurring from the plastically deformed parts. In addition to the characteristics of high strength and good workability, there is also a strong demand for high-strength steel sheets that have excellent low-temperature toughness so that they can withstand use in cold regions. Therefore, in order to provide a high tensile strength steel plate that satisfies such demands, we have developed a steel plate as described in Japanese Patent Publication No. 55-45614 and Japanese Patent Publication No. 57-47256.
A controlled hot rolling method and a high temperature coiling method for Ti-added steel have been proposed. By the way, the feature of Ti-added hot-rolled high-strength steel sheet is that it utilizes precipitation strengthening of TiC, and at the same time replaces MnS, which is an A-based inclusion, with TiS.
This method aims to improve processability by forming system inclusions, and has already been published in several papers (e.g.
L. Meyer et al: “Symposium, Low Alloy
High Strength Steels, Nuremberg, May 21
−23, 1970, p.9; M.Korchynsky et al:
“Symposium, Low Alloy High Strength
Steels, Nuremberg, May, 21-23, 1970,
p.17) has also been reported. When manufactured under appropriate conditions, Ti-added high-strength steel sheets have high strength and are extremely superior in that they are capable of close bending in JIS standard bending tests using specimens with machined edges. It is said to have good cold workability. The method described in Japanese Patent Publication No. 55-45614 involves performing controlled rolling during the production of Ti-added hot-rolled high-strength steel sheets having such characteristics, and improving the low-temperature toughness of the steel sheets. This is what we are trying to achieve. Namely, Special Publication No. 55-45614 and Special Publication No. 57
According to the method disclosed in No.-47256, Ti(C,N)
In addition to utilizing the precipitation strengthening effect of
Fine austenite grains are obtained by applying large reduction at least once at a temperature range of 880 to 730°C and 1100 to 980°C with a rolling reduction rate of 28% or more per pass,
Next, in order to prevent the formation of a bainite structure, it is preferable to limit the coiling temperature after hot rolling to 500 to 680°C and obtain a fine ferrite structure after transformation. thus obtained
Ti-added steel sheets not only have high strength, but also have excellent cold workability, as according to JIS standard bending tests using specimens with machined edges, it is possible to bend them in close contact. has been done. By the way, in the bending test according to the JIS standard, as mentioned above, specimens whose end faces have been machined and finished are used, but when actually producing various structural members, shearing ( In most cases, the material with a shear end surface obtained by cutting) (that is, the material with no processing applied to the shear cut surface) is directly subjected to cold working. Therefore, from a practical standpoint, a steel plate for cold working is required to have good bending performance of a test material with a shear end surface as an index of cold workability. However, Ti-added hot-rolled steel sheets, including the steel sheets obtained by the methods described in Japanese Patent Publication No. 55-45614 and Japanese Patent Publication No. 57-47256, are generally subjected to bending tests of materials with sheared edges. At the subsequent stage of practical application, it became clear that the performance was extremely poor and that there was a serious problem of cracking on the end face during bending. From the above-mentioned viewpoints, the present inventor has found that not only does it have high tensile strength of 70 kg/mm ² or more, excellent workability and low-temperature toughness, but also the workability of materials with shear end surfaces. As a result of extensive research in order to obtain good high-strength steel sheets, we have found that if Ti-added steel with a specific chemical composition is rolled and cooled at a low temperature of approximately 400℃, which breaks conventional wisdom,
In addition to exhibiting a high tensile strength of 70Kg/mm2 or ^more , bending cracking of steel plates with shear edges has been improved.
The present invention was achieved by discovering that a high-strength steel plate with excellent low-temperature toughness can be obtained. The findings obtained by the present inventors are summarized as follows: (a) When producing a Ti-added hot rolled steel sheet with high strength, after hot rolling, it is necessary to
When wound at 600°C, it is certainly possible to perform close bending in the JIS standard bending test, and it must be judged that it has excellent workability. but,
When bending a material with a shear end face, it is difficult to avoid bending cracks caused by cracks that already exist on the shear end face, which are induced by the brittleness of the ferrite grain boundaries peculiar to Ti-added hot rolled steel sheets. . Thus, the reason why ferrite grain boundary cracking is likely to occur at the shear end face is that Ti-added hot rolled steel sheets obtained based on normal manufacturing methods mainly utilize the precipitation strengthening of TiC that occurs during transformation to pearlite after rolling. This is because they are doing so. The TiC that precipitates after rolling occurs mainly within the ferrite grains, and relatively little precipitates at the grain boundaries. However, since it is coiled at a high temperature after rolling, cementite precipitates at grain boundaries. For this reason, the relative strength of the grain boundaries with respect to the inside of the grains is reduced, and strain tends to concentrate at the grain boundaries during shearing, causing many microcracks to be generated at the shear end faces. These cracks are then used as starting points to cause ferrite intergranular cracks during bending. Also, it precipitates during pearlite transformation after rolling.
It is generally believed that TiC is compatible with ferrite, that is, it requires the introduction of a large amount of strain, and is strengthened by precipitation through the introduction of such strain. Low-temperature toughness deteriorates due to the precipitation of TiC, which has strong properties. Therefore, it is considered to be very important to suppress the coherent precipitation of TiC on the ferrite ground. Therefore, if the winding temperature is controlled within the range of 500 to 200°C, the precipitation strengthening of TiC that causes embrittlement as described above is suppressed, and the above-mentioned problems are avoided. Moreover,
The expected decrease in strength due to TiC precipitation is due to the fine grain strengthening of ferrite due to such low-temperature winding.
It was found that this was sufficiently compensated by transformation strengthening based on solid solution of Ti. Therefore, fine-grain strengthening of ferrite and transformation strengthening in the second phase occur, and embrittlement of ferrite grain boundaries is suppressed, and a bainite phase is precipitated around the finely precipitated ferrite phase, resulting in the formation of facet units, that is, brittle fracture. It was found that not only the fracture surface unit becomes smaller and the shear pie fracture transition temperature is improved, but also the bending performance of the material with shear end faces is significantly improved. In particular, controlled rolling is performed in which a total reduction of 30% to 90% is performed at a temperature of 900°C or less and rolling is finished at a temperature of 800°C to 900°C, and after rapid cooling to a two-phase coexistence region of ferrite and austenite, slow cooling or After obtaining a ferrite phase with a volume fraction of 10% to 80% by holding, the temperature is further increased to 5°C/sec to 50°C.
By combining this with controlled cooling that performs rapid cooling at a rate of °C/sec, a high-strength steel sheet with extremely improved properties can be obtained; (b) In addition to this, if the P component in the Ti-added steel is reduced, Each of the properties described in item (a) above will be further improved; (c) If the Ti-added steel contains a specific amount of one or more of Ca, B, and Cr, the properties will be even better. Improved workability and toughness. In order to obtain these findings, the present inventor established a hot rolling simulation experimental method to investigate the effects of post-rolling cooling and coiling temperature on the mechanical properties of Ti-added steel. Various experiments were repeated. Here, this hot rolling simulation experimental method means that after rolling a steel material, it is rapidly cooled to a predetermined temperature by water spray to obtain a rapidly cooled rolled material, and then the above-mentioned steel material is placed in a furnace that has been heated to this predetermined temperature in advance. The rapidly cooled rolled material is charged into the furnace and cooled (cooling rate: 20℃/
hr). If the "predetermined temperature" at this time is made to match the coiling temperature, a steel plate having the same structure and characteristics as in actual hot rolling and coiling can be obtained. Through this hot rolling simulation experiment method,
Figure 1 shows the results of an investigation into the effect of coiling temperature on the mechanical properties of Ti-added steel under controlled rolling at 820°C. 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, "%" indicating the composition ratio is "weight%"), 50% at 900℃ or less
After obtaining a hot-rolled steel plate with a thickness of 6 mm at a finishing temperature of 820℃, two types of cooling patterns were applied: water cooling at 20℃/second to 650℃, air cooling for 10 seconds, and further cooling. When the temperature is lowered to each winding temperature by water cooling at 20℃/second (cooling pattern),
It is a graph showing the influence of the winding temperature on each mechanical property when the temperature is lowered from the finishing rolling temperature to each winding temperature simply by water cooling at 10° C./second (cooling pattern). From Figure 1,
When the winding temperature exceeds 400℃, the bendability of the material with shear end faces and the shear pie fracture transition temperature begin to deteriorate.In particular, when the winding temperature exceeds 500℃, the deterioration reaches a level that is not practical. The tendency becomes more pronounced, but when the winding temperature is 500
It was found that workability and low-temperature toughness were extremely good in the range of ~200°C, and when the winding temperature was further lowered to less than 200°C, there was a tendency for these properties to deteriorate again. It's obvious. In addition, the cooling pattern has better properties than the cooling pattern, and the P content in steel also affects each of the above properties, and if the content is 0.025% or less, , it can also be seen that good results are obtained. In the figure, black circles indicate the case of 0.025% P, and open circles indicate the case of 0.006% P. On the other hand, Figure 2 shows a conventional hot-rolled Ti-added steel material rolled at 600℃ (Figure 2c) and a material rolled at a lower temperature of 400℃ with a cooling pattern (Figure 2c). a) and the cooling pattern as expected.
This shows the optical microstructure of the material rolled up at 400℃ (Figure 2b), and when these three are compared,
It can be seen that when the 600℃ rolled material is subjected to nital corrosion, uneven ferrite intergranular corrosion occurs. In other words, (1) Ti-added steel is rolled at approximately 600°C, which is the normal coiling temperature.
If the material is rolled up, TiC, which has consistency with the ferrite ground, will significantly precipitate in the ferrite ground during slow cooling after winding, resulting in embrittlement. Further, since such TiC precipitation mainly occurs within the ferrite grains, the strength of the ferrite grain boundaries is relatively reduced, and strain tends to concentrate at the grain boundaries in response to external forces. Furthermore, the unevenness of ferrite grain boundary corrosion due to nital corrosion is thought to be an expression of the grain boundary purification effect, in which carbon present at the grain boundaries decreases as TiC precipitates within the ferrite grains. It is known that steel materials that are prone to such uneven corrosion have weak ferrite grain boundaries. Due to the above-mentioned embrittlement of the steel material and relative embrittlement of the grain boundaries to the inside of the grains, microscopic cracking occurs at the ferrite grain boundaries on the end face of the steel sheet, which undergoes severe processing during shearing. It is considered that this will lead to large cracks during the subsequent bending process. However, if low-temperature winding is performed, TiC precipitation is appropriately suppressed and transformation strengthening occurs instead of precipitation strengthening, so the above-mentioned drawbacks caused by winding at about 600°C can be avoided. (2) Regarding the cooling pattern after rolling, the amount of ferrite is higher in the cooling pattern than in the cooling pattern, and the bainite structure is clearly divided. This seems to be the reason why the cooling pattern achieved better characteristics. Note that air cooling may be performed instead of water cooling in the previous stage. (3) In addition, the material to be rolled at a temperature lower than 200℃ is
It is thought that because the self-annealing effect due to slow cooling after winding was small, both bendability and sharpie properties were poor. (4) 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 due to the embrittlement of ferrite grain boundaries due to tempering embrittlement promoted by the presence of P. This is thought to be due to the fact that this was suppressed by the decrease in P. Thus, the present invention has been made based on the above findings, with a particular aim of improving the workability and low-temperature toughness of a Ti-added hot-rolled steel material with a shear end face, and its gist is as follows: C: 0.05-0.20%, Si: 1.2 or less, Mn: 0.5-2.0%, Ti: 0.04-0.20%, P: 0.025% or less, S: 0.015 or less, sol.Al: 0.005-0.15%, N: 0.0080% Below, if necessary, Ca: 0.0100% or less, B:
Killed steel containing one or more of 0.0030% or less and Cr: 1.0% or less (more than 1% by weight), the balance being Fe and unavoidable impurities, with a total rolling reduction of 30% in the temperature range of 900 to 800℃. % to 90%, and after finishing the rolling at 800℃ to 900℃, air cooling or rapid cooling to a two-phase coexistence region of ferrite and austenite, and then slow cooling or holding.
After obtaining a ferrite phase with a volume fraction of 10% to 80%, quenching is performed again at a rate of 5°C/sec to 50°C/sec.
The present invention provides a method for producing a Ti-added hot-rolled high-strength steel sheet with excellent cold workability, which is characterized by winding at 500 to 200°C. The reason why the chemical composition of the steel and the hot rolling, cooling and winding conditions are limited as described above in the method for manufacturing a hot rolled high tensile strength steel sheet according to the present invention will be explained below. Carbon (C): Carbon has the effect of ensuring the strength of steel, and is an essential component in order to achieve tensile strength of 70 kg/mm ² or more, but its content is
If the content is less than 0.05%, it will not be possible to obtain the desired effect on the above-mentioned action, while if the content exceeds 0.20%, the bainite-like structure partially generated during low-temperature winding as employed in the present invention will have a high carbon content. The content was determined to be 0.05 to 0.20% because it becomes a bainite structure and deteriorates bendability and low-temperature toughness, and also causes deterioration of welding performance. In particular, the effect is remarkable at 0.08-0.20%. Silicon (Si): The silicon component has a solid solution strengthening effect and a deoxidizing effect. In order to increase the strength, it is preferable that the content be about 0.05% or more, but if the content exceeds 1.2%, toughness and weldability will deteriorate, so the content is set at 1.2% or less. Ta. Manganese (Mn): Manganese has the effect of toughening steel.
Although it is an important component, if the content is less than 0.5%, the desired effect cannot be obtained in the above action, and on the other hand, if the content exceeds 2.0%, A-based inclusions are likely to occur, and C Since the bending performance deteriorates, the content was set at 0.5 to 2.0%. Titanium (Ti): The titanium component has transformation strengthening with solid solution Ti,
In addition to strengthening the steel through the precipitation of TiC, it has the effect of changing the A-based inclusions made of MnS to the C-based inclusions made of TiS to improve the C bending performance, but if the content is less than 0.04%, the steel Not only is it not possible to impart the desired strength to the steel, but the shape of the inclusions is also insufficiently controlled, resulting in poor C-bending performance, and, on the other hand,
When the content exceeds 0.20%, C according to the present invention:
In steel with a concentration of 0.05-0.20%, significant precipitation hardening will adversely affect low-temperature toughness, so the content should be reduced to 0.04-0.20%, preferably 0.08%.
-0.20%. Phosphorus (P): The phosphorus component segregates at ferrite grain boundaries during slow cooling after winding, and tends to cause grain boundary embrittlement. Therefore, since it causes deterioration in the bending performance of the material with a shear end surface, it is better to reduce the amount as much as possible, but from an economic point of view, the content is set at 0.025% or less as an allowable range. However, 0.010%
The following are preferred. Sulfur (S): Sulfur component is an impurity element that easily combines with Mn in steel to form A-based inclusions.
Even with additive steel, if the content exceeds 0.015%, there is a high possibility that it will combine with Mn and form A-based inclusions, deteriorating bending performance, so the content is set at 0.015% or less. Ta. Preferably 0.005%
It is as follows. sol.Al: The sol.Al component has the effect of ensuring the effectiveness of the added Ti, but if the content is less than 0.005%, the effect of Ti addition will not be fully exhibited;
If the content exceeds 0.005% to 0.15%, the amount of nonmetallic inclusions increases and the steel becomes brittle, so the content was set at 0.005 to 0.15%. Nitrogen (N): Since the nitrogen component easily forms TiN in steel, the amount of Ti in the form of TiC, which is effective for precipitation hardening, or in the form of TiS, which is effective in spheroidizing nonmetallic inclusions, is determined. Therefore, it is better to reduce the amount of impurities as much as possible, but the upper limit of the content is set at 0.0080% as an allowable range considering economic efficiency.
It was determined that However, it is preferably 0.0050% or less. Calcium (Ca): The calcium component has the effect of combining with Al-O-based B-based inclusions and converting them into C-based inclusions to improve processability. 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, and is particularly effective in improving workability. When it is necessary to further improve the content, it is desirable to contain it in an amount of 0.0008% or more. However, if the content exceeds 0.0100%, the number of inclusions will increase beyond the allowable range.
Its content was set at 0.0100% or less. Boron (B): The boron component has the effect of improving the hardenability of steel and imparting toughness. In particular, as in the method for producing high-strength steel sheets according to the present invention, some parts are When utilizing the strengthening mechanism due to the generated bainitic structure, the effect of improving the hardenability of steel by adding a small amount of B is very effective. 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.
It is set at 0.0030% or less. Chromium (Cr): Similar to Mn, chromium 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 1.0 If the content exceeds 1.0%, no further improvement effect can be obtained, and the welding performance deteriorates, so the content was reduced to 1.0%.
It was determined as follows. Hot rolling/coiling conditions (i) 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 for this, in the present invention, a total reduction of 30% to 90% is performed at a temperature of 900°C or less, and a total reduction of 30% to 90% is performed at a temperature of 800°C to
Controlled rolling is performed in which rolling is finished at ℃. In this case, if the rolling end temperature is higher than 900°C or the rolling reduction is less than 30%, the desired sufficient fine grain structure cannot be obtained, making it difficult to secure the low-temperature toughness required as a structural material. The higher the total rolling reduction, the better, since it is easier to obtain fine-grained ferrite, but for practical purposes, the upper limit is set at 90%. On the other hand, if rolling is finished at a temperature lower than 800°C, not only will the texture develop and anisotropy occur, but the C-bending performance will also deteriorate.
Therefore, in the present invention, as mentioned above, the total rolling reduction in the temperature range of 900 to 800°C is 30
%~90%, and the finishing temperature is
The condition was that hot rolling be performed at a temperature of 800°C to 900°C. (ii) Cooling rate after hot rolling until coiling: In the present invention, after the above-mentioned controlled rolling, air cooling or rapid cooling to a two-phase coexistence region of ferrite and austenite is performed, followed by gradual cooling or cooling to that temperature. By holding 10%~80% in volume ratio
ferrite phase is generated. It is difficult to generate more than 80% ferrite phase from the chemical composition specified in the present invention, and when the ferrite volume fraction increases, strength decreases. After controlled rolling in the austenite region, as mentioned above, the microcrystalline grains are brought in as they are because they are air-cooled or rapidly cooled to the two-phase coexistence region of ferrite and austenite, and the subsequent slow cooling or temperature The ferrite phase obtained by holding at (causing high-temperature transformation) becomes extremely fine, and the structure is strengthened by the fine ferrite grain structure. The volume fraction of the produced ferrite phase is 10% to 80%, preferably 10 to 50%. Then, after rapid cooling, a bainitic structure is obtained by maintaining the coiling temperature as described below.
During the rapid cooling, if the rapid cooling is performed at 5°C/sec to 50°C/sec without generating 10% or more of the ferrite phase as described above, the volume fraction of low-temperature transformed structures such as bainite becomes significantly high. Desired characteristics cannot be obtained. On the other hand, slow cooling at a speed of less than 5°C/second hardly produces any transformation strengthening effect due to the formation of a bainitic structure, making it difficult to obtain the desired high strength and low-temperature toughness. I decided to do so. The higher the cooling rate, the more dense the bainitic structure can be obtained, which is preferable from the viewpoints of toughness and workability, but for practical purposes, the upper limit is 50°C/sec. (iii) Winding temperature: As mentioned above, if the winding temperature exceeds 500℃, the bendability of the material with shear end faces and the shear pie fracture transition temperature will deteriorate significantly.
On the other hand, even when the temperature is lower than 200°C, the characteristics tend to deteriorate, so in the present invention, the winding temperature is set at 500 to 200°C. Preferably it is 400-200°C. The invention will now be described in connection with examples, which are given by way of illustration only and include:
It is not intended to control the present invention. Example 1 Various steels having the chemical compositions shown in Table 1 were prepared by melting and casting using a high frequency furnace. Steel types A to H in the same table have steel compositions within the range specified in the method according to the present invention, and on the other hand,
Steel types I to S are for comparison outside the scope of the present invention. Components outside the composition range defined in the present invention are marked with *. Next, these steel materials were hot rolled under the conditions shown in Table 2 to produce hot rolled steel sheets having a thickness of 6 mm. In Table 2 as well, conditions outside the scope of the present invention are marked with *. The mechanical properties of the hot rolled steel sheets thus obtained were tested, and the results obtained are also shown in Table 2. As is clear from the results shown in Table 2, the hot rolled steel sheets of test numbers 1 to 9, in which the chemical composition range of the steel materials used and the hot rolling/coiling conditions were within the ranges specified by the present invention, all had high It is clear that it has high strength, excellent low-temperature toughness, and excellent bending performance in materials with sheared edges.
It can be seen that the hot-rolled steel sheets of comparative test numbers 10 to 23, in which the winding conditions were outside the range defined by the present invention, were inferior in low-temperature toughness and bending performance for materials with sheared edges. In particular, if only low-temperature winding is performed without controlled rolling or controlled cooling, as in the comparative examples of test numbers 10 and 11, the structure of the resulting steel sheet will be as shown in Figure 2a above. It was also found that the mixed structure of fine ferrite and fine bainite structure with a volume fraction of 10% or more did not result, but instead became an almost coarse bainite structure, resulting in a significant deterioration of toughness. The experimental results when the winding temperature was outside the range of the present invention were not good, as shown in the graphs in FIG. 1 above. Furthermore, it was confirmed that the bending performance of D steel, F steel, G steel, and H steel in Table 1 to which Ca was added was significantly improved. In this way, by subjecting Ti-added steel with a specific chemical composition to a combination of predetermined controlled rolling, controlled cooling, and low-temperature winding, a mixture of fine-grained ferrite and fine bainite structure with a volume fraction of 10% or more is achieved. A structure (in which fine-grained ferrite and fine bainite structure are separated) is obtained, which ensures not only toughness but also excellent workability. As described 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 that can be machined and, in addition, extremely excellent low-temperature toughness, can be obtained by relatively simple means, and are suitable for buildings used in cold regions and other areas. By applying it to the structural materials of industrial machines, etc., it can be expected to achieve better results than ever before, and it will bring about industrially useful effects.

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【table】 [Brief explanation of drawings]

Figure 1 is a graph showing the effect of coiling temperature on the mechanical properties of Ti-added steel; Figure 2a is a graph showing the effect of coiling temperature on the mechanical properties of Ti-added steel; Figure 2-a shows Ti-added steel that was controlled rolled, cooled in a cooling pattern, and then rolled at 400°C. Optical micrograph of the structure of a steel plate due to nital corrosion; Figure 2b shows Ti-added steel that is controlled rolled and then cooled using a cooling pattern.
Then, an optical micrograph of the structure of a steel plate rolled at 400° C. due to nital corrosion; and FIG. 2c is an optical micrograph of the structure of a steel plate rolled at 600° C. after controlled rolling of Ti-added steel.

Claims (1)

[Claims] 1 C: 0.05-0.20%, Si: 1.2 or less, Mn: 0.5-2.0%, Ti: 0.04-0.20%, P: 0.025% or less, S: 0.015 or less, sol.Al: 0.005- 0.15%, N: 0.0080% or less (weight%), and the balance is Fe and unavoidable impurities. After hot rolling and finishing rolling at 800°C to 900°C, air cooling or rapid cooling to a two-phase coexistence region of ferrite and austenite, and then gradual cooling or holding to reduce the volume fraction.
After obtaining 10% to 80% ferrite phase, again 5 more
℃/sec~50℃/sec after rapid cooling to 500~200℃
A method for producing a Ti-added hot-rolled high-strength steel sheet with excellent cold workability, which is characterized by winding the steel sheet. 2 C: 0.05-0.20%, Si: 1.2 or less, Mn: 0.5-2.0%, Ti: 0.04-0.20%, P: 0.025% or less, S: 0.015 or less, sol.Al: 0.005-0.15%, N: 0.0080 % or less and one or more of Ca: 0.0100% or less, B: 0.0030% or less, and Cr: 1.0% or less (weight% or more), and the balance is Fe and unavoidable impurities. After hot rolling with a total rolling reduction of 30% to 90% in the temperature range of 800°C to 900°C, air cooling or quenching to the two-phase coexistence region of ferrite and austenite. , by slow cooling or holding, by volume percentage.
After obtaining 10% to 80% ferrite phase, again 5 more
℃/sec~50℃/sec after rapid cooling to 500~200℃
A method for producing a Ti-added hot-rolled high-strength steel sheet with excellent cold workability, which is characterized by winding the steel sheet.