JP3506023B2 - Method for producing high-strength hot-rolled steel sheet with excellent formability - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high workability and high strength suitable for structural materials such as automobile parts and architectural guard rails.
The present invention relates to a method for manufacturing a hot-rolled steel sheet . 2. Description of the Related Art High-strength hot-rolled steel sheets are required to have stable workability and high workability. However, in conventional high-strength hot-rolled steel sheets, 0.1 mass% or more of C
Was added, which often caused ductility degradation through the formation of a second phase. [0003] In order to suppress the formation of the second phase and improve ductility, a method for securing strength even when the amount of C added is reduced is disclosed in Japanese Patent Application Laid-Open No. 5-5156. A high-strength hot-rolled steel sheet for forming and realizing high ductility with a low yield ratio as well as a complete fixation of C in steel by promoting the precipitation of TiC by adding TiC, and a method for producing the same. However, this technique requires the addition of B for the purpose of improving the brittleness in secondary processing, and therefore unavoidably generates acicular ferrite, resulting in insufficient ductility. Furthermore, since a large amount of Si which causes red scalp is added, good surface properties cannot be obtained. Japanese Patent Application Laid-Open No. 58-39731 discloses a method for producing a high-strength hot-rolled steel sheet by direct rolling. However, in slab reheating rolling, in which the slab is once cooled, the grains are refined by transformation, but in direct-feed rolling, the effect of the solidification structure at the time of casting remains because there is no refining effect due to transformation, and it is difficult to form grains. In addition, since this technique does not regulate the N content at all, the refinement of the solidified structure by Ti nitride may not be achieved, and therefore, the recrystallization of γ grains before rolling is not promoted, and from this aspect, Is also difficult to be sized. Therefore, stable high ductility cannot be obtained by this technique. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art and to produce a high-workability, high-strength hot-rolled steel sheet having high ductility and excellent material stability.
It is to provide a method . Means for Solving the Problems The inventors of the present invention have conducted intensive studies in order to solve the above-mentioned problems, and as a result, have found that a steel having a lower C content than that of the conventional steel and a combined addition of Ti and Mn has been obtained. In a rolled steel sheet, it has been found that the optimum range in which a good strength-ductility balance can be stably obtained exists in the addition amounts of Ti and Mn. Specifically, depending on the range of the Ti addition amount, Ti and Mn
It has been found that by controlling the amount of addition to specific conditions, a high-workability, high-strength hot-rolled steel sheet having a good strength-ductility balance can be obtained. Further, in the hot rolling step when producing a steel sheet having such a component composition, by regulating the material temperature on the side of the finishing rolling stand row by heating or keeping the rough bar in a specific range, It has been found that high ductility can be obtained. The present invention has been made based on such findings, and C: 0.01 mass% or more and less than 0.05 mass%,
Si: 0.1 mass% or less, P: 0.02 mass% or less,
S: 0.004 mass% or less, Sol. Al: 0.01 to
0.1 mass%, N: 0.001 to 0.008 mass%, and 0.02 to 0.13 mass% of Ti and 2.0 mass% or less of Mn are described below according to the range of the Ti content ( Equations (1) and (2) (where Ti: Ti content, M
n: Mn content), when hot rolling a steel having a component composition contained therein, after rough rolling, the rough bar is heated or kept warm to raise the material temperature at the finishing rolling stand row entry side to 1000. This is a method for producing a high-strength hot-rolled steel sheet excellent in formability, characterized in that after finish rolling is performed at a temperature of Ar ₃ or higher, and then finish rolling is performed at a temperature of Ar ₃ or higher, and then winding at 650 ° C. or lower. 1.54 × Ti + 0.9 ≧ Mn ≧ 1.54 × Ti in 0.06 mass%> Ti ≧ 0.02 mass% (1) 2.0 ≧ Mn ≧ 1.54 × Ti in 0.13 mass% ≧ Ti ≧ 0.06 mass% (2) Hereinafter, the present invention will be described in detail and the reasons for limitation. First, the reasons for limiting the chemical components of steel in the present invention will be described. C: 0.0 to reduce the second phase that deteriorates ductility
It was less than 5 mass% . The lower limit is set to 0.01 mass% in order to secure the strength of the steel sheet by precipitation strengthening. Si: Since excessive addition causes deterioration of surface properties due to generation of red scale, the addition amount is set to 0.1 mass% or less . Ti, Mn: These are the most important elements in the present invention, and are added for securing the strength. As described above, the balance of the added amounts is important. In the present invention, the lower limit of the amount of Ti added is set to 0.02 mass% in order to secure the strength of the steel sheet by precipitation strengthening, and when Ti is added in excess of 0.13 mass% , TiC precipitated in ferrite grains increases. Since the ductility deteriorates, the upper limit of the addition amount is set to 0.13 mass% . Further, Mn is necessary for securing the strength, but if added more than necessary, the cost becomes high. Therefore, the upper limit of the added amount is set to 2.0 mass% . Further, depending on the range of the Ti content,
Formulas (1) and (2) (where Ti: Ti content, Mn: M
n content), an excellent strength-ductility balance can be obtained when Ti and Mn are added.
And Mn contents are defined in the range of the formulas (1) and (2). 0.04 mass% > 1.54 × Ti + 0.9 ≧ Mn ≧ 1.54 × Ti when Ti ≧ 0.02 mass% (1) 2.0 ≧ Mn ≧ 1.54 × Ti when 0.13 mass% ≧ Ti ≧ 0.06 mass% (2) The experimental results in which the optimum ranges of the Ti and Mn contents were obtained are shown below. In this experiment, C containing 0.01 mass% or more and less than 0.05 mass% , and various amounts of T
After casting a steel containing i and Mn, hot rolling is performed to produce a hot-rolled steel sheet having a thickness of about 2.5 mm, and the thickness of the hot-rolled steel sheet is reduced to 1.6 mm by double-side grinding. A tensile test was performed. FIG. 1 shows the relationship between the strength-ductility balance (tensile strength × total elongation [MPa ·%]) obtained from the tensile test and the amounts of Ti and Mn. According to the figure, the above equation (1) and
In the range of Ti and Mn amounts defined by the equation (2), the strength-
It can be seen that the ductility balance is the highest. Therefore,
If the amounts of Ti and Mn are adjusted within the appropriate ranges, a hot-rolled steel sheet having high ductility can be obtained while securing a predetermined strength level. The reason why an excellent balance between strength and ductility can be obtained by optimizing the amounts of Ti and Mn is not necessarily clear, but the following reasons can be considered. That is, since Ti forms Ti precipitates in the steel and becomes fine precipitates, if the strength is secured by adding a large amount of Ti exceeding 0.13 mass% , the effect of suppressing the grain growth by the fine precipitates is obtained. It is considered that the amount of fine dispersion strengthening of the Ti precipitate increases, and the ductility deteriorates. Also, increased solubility TiC in γ excessively added Mn, and since the fine dispersion strengthening ability of Ti precipitates by lower Ar ₃ transformation point is increased, the amount of addition of Ti up to 0.06 mass% It is considered that the amount of ductility deterioration is greater than the amount of increase in strength. Therefore, in order to ensure high ductility, it is necessary to optimize the amounts of Ti and Mn within the upper limit range. P: Since excessive addition causes grain boundary embrittlement, the addition amount is set to 0.02 mass% or less. S: S in steel forms a low-melting point substance at the grain boundary to reduce hot ductility and deteriorate surface quality.
Effective Ti that combines with i to form precipitates and contributes to strength
It is a harmful element that reduces the amount, and excessive addition deteriorates stretch flangeability, so it is desirable to reduce the amount of addition as much as possible. Therefore, the addition amount is 0.00
4 mass% or less. Sol. Al: An element required as a deoxidizing agent, and for that purpose 0.01 mass% or more is necessary. However, since excessive addition deteriorates ductility, the upper limit of the addition amount is set to 0.1 mass% . N: When the steel sheet of the present invention is manufactured by direct rolling,
It is necessary to prevent γ grains from coarsening before rolling, and it is effective to use fine TiN precipitated during solidification to reduce γ grains before rolling. However, the addition of a small amount of N does not provide the effect of suppressing the coarsening of γ grains as described above, so the lower limit of the addition amount is set to 0.001 mass% . Further, if N is excessively present in the steel, TiN is coarsely formed, so that the effect of suppressing the coarsening of γ grains cannot be obtained. Therefore, the upper limit of the addition amount is set to 0.008 mass% . Further, if necessary, the steel sheet of the present invention may contain 0 to 0.1 mass% of Nb and 0 to 0.1 ma to secure the strength.
ss% V, 0-0.1 mass% Zr, 0-0.01 mass
% Of B, and 0 to 1.0 mass% for improving ductility.
Of Cr, 0 to 0.0 mass% of Ca, etc.
(Including the case of no addition). Further, elements such as 0 to 1.0 mass% of Mo, 0 to 1.0 mass% of Ni, and 0 to 1.0 mass% of Cu are added to improve corrosion resistance. Including). The steel sheet of the present invention substantially comprises the above components and Fe, and therefore does not prevent other elements such as unavoidable impurities from being contained in trace amounts as long as the effects of the present invention are not impaired. Next, the manufacturing conditions of the present invention will be described. In the production method of the present invention, when hot rolling the steel having the above-described composition, the material temperature, the finishing temperature, and the winding temperature of the coarse bar on the side of the finishing rolling stand row are defined as follows. Material temperature on the finishing roll stand row entry side: Ti
In order to use the precipitates effectively for grain refinement, it is necessary to stably precipitate the Ti precipitates during rolling. To do this, heat or heat the coarse bar obtained by rough rolling,
It is effective to set the material temperature (coarse bar temperature) on the finishing rolling stand row entry side to 1000 ° C. or more. In the conventional rolling, there is a variation in the material temperature at the entrance of the finishing rolling stand row, and particularly when the finishing rolling is started at a low temperature, precipitates start to precipitate at the beginning of rolling, and this precipitates in the grains during recrystallization. Crystal grains obtained by being taken in also become coarse. On the other hand, by setting the rough bar temperature at the entrance of the finishing rolling stand row to a certain value or more as described above, the amount of fine precipitation during rolling is stabilized, and more stable refinement of crystal grains is achieved. Therefore, higher ductility can be achieved. In particular, in the case of direct-feed rolling, since the material temperature on the entrance side of the finishing rolling stand row tends to vary on the low-temperature side, heating or heat retention of the coarse bar is particularly effective. In order to obtain the above-mentioned functions and effects, it is necessary to raise or lower the temperature of the material on the side of the row of the finishing rolling stands by heating or keeping the rough bar at 1000 ° C.
If the temperature is lower than 00 ° C., a sufficient effect cannot be obtained. On the other hand, if the material temperature on the side of the row of the finishing rolling stands is too high, the γ grains before rolling become coarse, and the scale becomes thicker, causing scale defects. Therefore, the upper limit temperature is set to 1100 ° C. The heating or heat retention of the coarse bar can be performed by any method such as induction heating, direct current heating, and burner heating. In the heat retention of the coarse bar, the coarse bar is wound up in a coil box for heat retention, or the heat is retained by using a tunnel furnace installed between the rough rolling stand row and the finish rolling stand row. May be. Further, in order to prevent a variation in the material in the longitudinal direction of the coil, it is desirable that the variation in the material temperature in the longitudinal direction of the rough bar be within ± 30 ° C. Finishing Temperature (FT): If the finish rolling is performed in the ferrite region, coarse α-extended grains are formed and ductility is deteriorated.
Score ₃ or more. However, if the finishing temperature is too high, the γ grains recrystallize and become coarse before transformation, so the finishing temperature is preferably 900 ° C. or less. Winding temperature (CT): If the winding temperature is too high, the precipitates become coarse and the crystal grains become coarse, so the upper limit was set to 650 ° C. Further, if the winding temperature is too low, the shape of the steel sheet is deteriorated. The steel sheet of the present invention can be produced by a normal slab reheating rolling method (a method in which a cast slab is cooled to room temperature and then reheated and rolled). Alternatively, if the slab temperature after casting is not less than Ar ₃ points, if it is manufactured by a hot strip reheating rolling method of reheating and rolling in a heating furnace, all elements acting as precipitation strengthening are rolled in a solid solution state Is, because all the precipitates can contribute to fine precipitation and fine graining,
Particularly, a high-strength hot-rolled steel sheet excellent in strength-ductility balance can be stably manufactured. Therefore, the effect of the present invention is most remarkable in a steel sheet manufactured by a hot strip reheating rolling method in which a slab temperature after casting or a slab temperature after casting does not become less than Ar ₃ points and reheating and rolling is performed in a heating furnace. Be demonstrated. The hot-rolled steel sheet to which the present invention is applied includes, in addition to the black-scaled hot-rolled steel sheet, various types of plating (for example, zinc plating, tin plating, etc.) and chemical conversion treatment, etc. Surface treated steel sheet is included. Further, the melting of the steel as a raw material may be performed by either a converter or an electric furnace, and the production using a thin slab is also possible. In that case, the rough rolling can be omitted. Furthermore, the effect of the hot-rolled steel sheet obtained by the present invention is not lost even if the skin pass is applied, but the elongation in that case is preferably 5% or less. EXAMPLES [Example 1] Steel types (1) to 1 shown in Table 1
After smelting steel having the chemical composition of (9) and casting it into a slab, the slab is directly rolled without cooling, or the slab once cooled to room temperature is cooled to 1250 ° C.
After rolling for 3 hours, rolling was performed. In this hot rolling, finishing temperature: 880 ° C., winding temperature: 600 ° C.
Then, a hot-rolled steel sheet having a thickness of 2.5 mm was manufactured. In this example, hot rolling was performed without heating or keeping heat of the coarse bar on the side of the row of finishing rolling stands.
From the hot-rolled steel sheet manufactured in this way, JIS5
A tensile test specimen was sampled and subjected to a tensile test to evaluate the strength-ductility balance of each of the obtained hot-rolled steel sheets. Table 2 shows the results. According to Table 2, the steels Nos. (1), (2), (4) to (7) and (9) satisfying the component conditions of the present invention were used. 1, No. 2, No. 4-No. 8, No. 1
0-No. No. 12 (all reference examples) show high ductility even when manufactured by either direct-feed rolling or slab reheating rolling, and it can be seen that they have an excellent strength-ductility balance. On the other hand, No. 3 using steel types (3) and (8) in which the addition amount balance of Ti and Mn was out of the component conditions of the present invention. 3, no. Comparative Example 9 cannot ensure high ductility and is inferior in strength-ductility balance. Note that N
o. 12 and No. Comparative Example 13 uses steel types (1) and (5) satisfying the component conditions of the present invention. However, since the finishing temperature or the winding temperature is not appropriate, sufficient high ductility cannot be ensured, and the strength is low. -Ductile balance is reduced. [ Table 1] [ Table 2] Example 2 Steel types (1), (2), (4) to (7) satisfying the component conditions of the present invention shown in Table 1
After smelting steel having the chemical composition of (9) and casting it into a slab, the slab is directly rolled without cooling, or the slab once cooled to room temperature is cooled to 1250 ° C.
After rolling for 3 hours, rolling was performed. During the hot rolling, the rough bar after the rough rolling is heated or heat-retained, and the rough bar temperature on the side of the finishing rolling stand row is set to 1000 ° C. to 1100 ° C. which is within the range of the production method of the present invention. After that, finish rolling was performed to produce a hot-rolled steel sheet. In addition,
Other hot rolling conditions were the same as in Example 1. From the hot rolled steel sheet thus manufactured, a JIS No. 5 tensile test piece was sampled in the L direction and a tensile test was performed.
The strength-ductility balance of each of the obtained hot-rolled steel sheets was evaluated.
Table 3 shows the results. According to the same table, by controlling the coarse bar temperature on the entrance side of the finishing rolling stand row by performing heating or heat retention of the coarse bar which is a production condition of the present invention, a heat having a particularly excellent strength-ductility balance. It turns out that a rolled steel plate can be manufactured. [Table 3] As described above, the production method of the present invention
The resulting high-strength hot-rolled steel sheet has high ductility, and is excellent in strength-ductility balance and material stability. For this reason
The high-strength hot-rolled steel sheet is particularly useful as a structural material for automobile parts, architectural guardrails and the like .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the amounts of Mn and Ti added to a steel sheet and the strength-ductility balance.

Continuation of the front page (56) References JP-A-5-98355 (JP, A) JP-A-10-219351 (JP, A) JP-A-7-150237 (JP, A) JP-A-7-197124 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00 C21D 9/46

Claims (1)

(57) [Claims] [Claim 1] C: 0.01 mass% or more and 0.05 mass%
Less than, Si: 0.1 mass% or less, P: 0.02 mass% or less, S: 0.004 mass% or less, Sol. Al: 0.0
1 to 0.1 mass%, N: 0.001 to 0.008 mass%
In addition, Ti of 0.02 to 0.13 mass% and Mn of 2.0 mass% or less are added to the following formulas (1) and (2) (where Ti: Ti content Quantity, M
n: Mn content), when hot rolling a steel having a component composition contained therein, after rough rolling, the rough bar is heated or kept warm to raise the material temperature at the finishing rolling stand row entry side to 1000. ° C. or higher and 1100 ° C. or less, after the finish rolling at Ar ₃ temperature above process for producing a high-strength hot-rolled steel sheet excellent in formability, characterized in that winding at 650 ° C. or less. 1.54 × Ti + 0.9 ≧ Mn ≧ 1.54 × Ti in 0.06 mass%> Ti ≧ 0.02 mass% (1) 2.0 ≧ Mn ≧ 1.54 × Ti in 0.13 mass% ≧ Ti ≧ 0.06 mass% (2)