JP4853304B2 - High strength hot rolled steel sheet - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet, in particular, a high-strength hot-rolled steel sheet having high ductility suitable for automobile undercarriage parts and the like, and having a tensile strength (TS) of 1000 MPa or more.

  2. Description of the Related Art In recent years, various high-strength hot-rolled steel sheets having high ductility, that is, excellent stretch flangeability and high total elongation, have been studied for automobile undercarriage parts as the strength of automobile steel sheets increases. For example, DP (composite structure) steel sheets mainly containing a ferrite phase (soft phase) and a martensite phase (hard phase) have a high total elongation due to the presence of a soft ferrite phase, but hard in the soft phase. Since the phase is present, the stretch flangeability is inferior. A bainite single-phase steel plate is excellent in stretch flangeability because it consists of only a hard phase, but is inferior in total elongation because it is hard.

Therefore, a high-strength hot-rolled steel sheet that achieves both high ductility and high strength by forming a ferrite single phase that is advantageous for stretch flangeability and total elongation, and uniformly generating nano-order fine precipitates in the ferrite phase. Proposed. For example, Patent Document 1 includes, in mass%, C <0.10%, Ti: 0.03 to 0.10%, Mo: 0.05 to 0.6%, and a fine carbide containing Ti and Mo having a particle size of less than 10 nm in the ferrite phase. A thin steel sheet having a TS of 590 MPa or more dispersed therein is disclosed. Patent Document 2 includes mass%, C: 0.07 to 0.15%, Si: 0.3% or less, Mn: 0.5 to 2.0%, P: 0.06% or less, S: 0.005% or less, Al: 0.06% or less, N: 0.006%, Ti: 0.15-0.35%, Mo: 0.3-0.7% A steel sheet is disclosed.
Japanese Patent Laid-Open No. 2002-322539 Japanese Patent Laid-Open No. 2003-89848

  However, with the thin steel sheet described in Patent Document 1, a TS of 1000 MPa or more cannot be obtained, and with the ultra-high-strength steel sheet of Patent Document 2, when trying to stably secure a high total elongation of 15% or more, TS There is a problem that tends to not exceed 1000MPa.

  An object of the present invention is to provide a high-strength hot-rolled steel sheet having a TS of 1000 MPa or more that can stably ensure a high total elongation of 15% or more.

  In the high-strength hot-rolled steel sheet in which nano-order fine carbides are uniformly generated in the ferrite phase as described in Patent Document 2, the present inventors have stably achieved a high total elongation of 15% or more and 1000 MPa. When the conditions for obtaining the above TS were examined, the following was found.

  A) Even with higher C and higher alloys, precipitation strengthening with fine carbides has limitations due to restrictions in the hot-rolled steel sheet manufacturing process.

  B) If the amount of C is limited to a certain range less than the chemical equivalent in terms of atomic concentration relative to the amount of alloy elements, the formation of coarse carbides is suppressed, and the effect of refining the structure by fine carbides precipitated during transformation can be used, The TS of 1000MPa or more and the total elongation of 15% or more can be secured stably.

The present invention has been made based on such knowledge, in mass%, C: 0.10% or more and 0.12% or less, Si: 0.30% or less, Mn: 1.8% or less, N: 0.003% or less, Ti: 0.10% 0.20% or less, V: 0.15% or more and 0.30% or less, Mo: 0.05% or more and 0.40% or less, the balance is composed of Fe and inevitable impurities, and has a composition that satisfies the following formula (1), Ms defined by the following formula (3) and M ₀ defined by the following formula (4) satisfy the following formula (2), and has a structure composed of ferrite grains having an average grain size of 3 μm or less. A high-strength hot-rolled steel sheet is provided.
0.7 <(C / 12) / [{(Ti-3.42 × N) / 48 + V / 51 + Mo / 96}] <0.9 (1)
4 <Ms / M ₀ ... (2)
Ms = (Ti ₂₀ / Ti _total + V ₂₀ / V _total + Mo ₂₀ / Mo _total ) / 3 (3)
M ₀ = (Ti _sol / Ti + V _sol / V + Mo _sol / Mo) / 3 (4)
However,
In equation (1)
Each element symbol indicates the content (% by mass) of each element,
In equation (3)
Ti ₂₀ , V ₂₀ , Mo ₂₀ is the content of each element in the carbide of Ti, V, Mo with a diameter of less than 20 nm precipitated in the steel sheet (mass% relative to steel),
Ti _total , V _total , and Mo _total are the contents of each element in the total carbide (mass% relative to steel),
In equation (4)
Ti _sol , V _sol , and Mo _sol are the solid solution amount (mass%) of each element in steel.
Ti, V, Mo is the content (mass%) of each element in steel,
To express.

  According to the present invention, it has become possible to produce a high-strength hot-rolled steel sheet having a TS of 1000 MPa or more that can stably secure a high total elongation of 15% or more. When the high-strength hot-rolled steel sheet of the present invention is applied to undercarriage parts of automobiles, the thickness of the parts can be reduced, so that it is expected that the environmental load of automobiles will be reduced and the impact characteristics will be greatly improved. .

  Hereinafter, the present invention will be specifically described. Unless otherwise specified, “%” in relation to ingredients means “% by mass”.

1) Component composition
C: 0.10% to 0.12%
C is an important element for realizing precipitation strengthening by Ti, Mo, and V carbides described later. In order to obtain a carbide precipitation amount for obtaining TS of 1000 MPa or more, the C amount needs to be 0.10% or more. On the other hand, when the amount of C exceeds 0.12%, the amount of fine carbides less than 20 nm effective for precipitation strengthening sharply decreases due to coarsening of insoluble Ti-based precipitates and strain-induced precipitation during hot rolling. In addition, it promotes the formation of transformation strengthening phases such as pearlite and bainite during the winding or subsequent cooling process, leading to a decrease in ductility.

Si: 0.30% or less
Si is an element effective for increasing the strength by solid solution strengthening, but raises the transformation point of steel as a ferrite stabilizing element. Therefore, in order to perform hot rolling in the austenite region, that is, the finish rolling finish temperature is set to 880 ° C. or more, the upper limit of Si content is set to 0.30%.

Mn: 1.8% or less
Mn is an element effective for increasing the strength by solid solution strengthening, and is an element indispensable for controlling the transformation point of steel as an austenite stabilizing element. In particular, in the present invention, it is extremely important to match the timing of phase transformation from austenite to ferrite after hot rolling and the precipitation of fine carbide used for precipitation strengthening at the time of winding, but the Mn content is 1.8%. If exceeding, the transformation point is lowered, the carbide is coarsened during winding, and the precipitation strengthening ability is remarkably lowered. When the amount of Mn is less than 1.2%, not only the solid solution strengthening ability is lowered, but also the transformation point is raised and the structure after hot rolling becomes coarse, so that a TS of 1000 MPa or more may not be obtained. Therefore, the Mn content is more preferably 1.2% or more and 1.8% or less.

N: 0.003% or less
When the amount of N exceeds 0.003%, a MN type precipitate (here, M represents a metal element) is formed at a high temperature region with a carbonitride-forming element such as Ti described below to form fine carbides. Reduce the amount of Ti, etc.

Ti: 0.10% to 0.20%
Ti is an important element for forming fine carbides and achieving high strength by precipitation strengthening and microstructure refinement. However, if the amount exceeds 0.20%, coarse carbides are formed in the slab manufacturing process, and the amount of Ti necessary for forming fine carbides after hot rolling is reduced. In addition, since Ti forms nitrogen and TiN in steel, the amount of Ti required to fix the amount of N (about 0.003%) contained in normal hot-rolled steel sheets, that is, 0.010% or more, must be fine. Carbide formation becomes difficult.

V: 0.15% to 0.30%
V is an important element for forming fine carbides and achieving high strength by precipitation strengthening and microstructure refinement. However, since carbide forming ability is weaker than Ti, the effect is exhibited only when it is added together with Ti. When V is added alone, the carbide precipitates in the ferrite grains after the austenite → ferrite transformation, and high density precipitation due to precipitation at the phase interface that proceeds almost simultaneously with the transformation phenomenon cannot be expected. The amount is in principle a range satisfying the formula (1), but needs to be 0.15% or more to achieve Ti-V composite fine precipitation. In addition, it is possible to increase the amount of V according to the amount of carbon, but in the case of high carbon, TiC preferentially precipitates at a high temperature range, and the amount of Ti effective to form fine carbide decreases, Complex precipitation of V is difficult to occur, and not only does not greatly contribute to the increase in strength, but also increases the raw material cost. For this reason, the upper limit of the V amount is set to 0.30%.

Mo: 0.05% or more and 0.40% or less
Mo is an important element for forming fine carbides by compounding with Ti and achieving high strength by precipitation strengthening and microstructure refinement. A single addition of Mo or a combined addition with V results in precipitation in a low temperature range, so a sufficient amount of precipitation cannot be secured. Mo is one of elements that enhance hardenability and has an effect of suppressing pearlite transformation. From such a viewpoint, the lower limit of the Mo amount is 0.05%. As with V, it is possible to increase the amount of Mo in accordance with the amount of carbon, but this not only increases segregation and causes material variations, but also increases the raw material cost. For this reason, the upper limit of the Mo amount is set to 0.40%.

  The balance needs to be Fe and inevitable impurities, but Al is preferably 0.5% or less for the following reason. In other words, Al is useful because it is useful as a deoxidizing element during steelmaking and contributes to high strength by dissolving in ferrite, but if its amount exceeds 0.5%, it is coarse. Forms AlN and causes fatigue strength to decrease.

2) 0.7 <(C / 12) / [{(Ti-3.42 × N) / 48 + V / 51 + Mo / 96}] <0.9
(C / 12) / [{(Ti-3.42 × N) / 48 + V / 51 + Mo / 96}] is the actual C chemical equivalent (atomic concentration) depending on the amount of Ti, V and Mo. This represents the ratio to the amount of C. If this value is 1, the amount of C is contained by the chemical equivalent. If this value is 0.9 or more, carbide precipitates and coarsens in the high temperature region during hot rolling, and if it is 0.7 or less, the amount of fine carbide during winding effective for precipitation strengthening cannot be secured. The value must be greater than 0.7 and less than 0.9.

3) Structure The structure of the parent phase needs to be ferrite grains in order to ensure excellent ductility, and the average grain size needs to be 3 μm or less in order to exert the effect of crystal grain refinement on the strength.

4) 4 <Ms / M ₀
In order to achieve high strength by dispersing fine carbide, the amount and size of carbide and the degree of dispersion are extremely important. Carbides in steel have various size distributions depending on the precipitation temperature range, but generally those with a diameter of 20 nm or more that precipitate in the high temperature range of the production process are considered to have little strengthening ability. The present inventors have used hot-rolled steel sheets of various intensity levels, the ratio M ₀ defined by the above formula (3) Ms of the formula defined in (4), i.e., the amount of Ti in the total carbide ( Ti _total) Ti content in the carbide is less than 20nm diameter to (ratio and V of total carbides of Ti ₂₀₎ (V _total) amount of V carbides of a diameter less than 20nm for (V ₂₀₎ of the ratio and total The average value Ms of the ratio of the Mo amount (Mo ₂₀ ) in the carbide with a diameter of less than 20 nm to the Mo amount (Mo _total ) in the carbide, and the ratio of the solid solution Ti amount (Ti _sol ) to the Ti content in the steel amount of solid solution V for V content in the steel the ratio and the solute Mo amount for Mo content in steel (V _sol) ratio to the average value M ₀ of the ratio of _{(Mo sol) (Ms / M} 0) tensile As a result of investigating the relationship with the strength TS, it was found that when Ms / M ₀ exceeds 4, TS of 1000 MPa or more can be obtained reliably as shown in FIG. When Ms / M ₀ is 4 or less, it is considered that a TS of 1000 MPa or more cannot be obtained because there are few fine carbides having a diameter of less than 20 nm.

_{Here, Ti total, V total, Mo} total, Ti 20, V 20, Mo 20, Tisol, V sol, Mo sol was determined as follows.
Ti _total , V _total , Mo _total : determined by an extraction residue method using 10% AA (acetylacetone electrolytic solution) which is a conventional extraction residue method. That is, Ti _total is a value determined as the amount of precipitated Ti (mass%) in steel, V _total is the amount of precipitated V (mass%) in steel, and Mo _total is the amount of precipitated Mo in steel (mass%).
Ti ₂₀ , V ₂₀ , Mo ₂₀ : Among carbides observed in TEM (transmission electron microscope) images, carbides with a diameter of less than 20 nm are extracted by image analysis, and the volume fraction of these carbides is obtained from the diameter. In total, the volume fraction of carbides below 20 nm in the steel was calculated. As a result of EDX analysis, most of the carbides having a diameter of less than 20 nm were Ti—V—Mo composite carbides, and the composition ratio of Ti, V, and Mo was approximately 1: 1: 1. From these results, it is assumed that all precipitates of less than 20 nm are Ti-V-Mo composite carbides (TiVMoC ₃ ), and the lattice constants of Ti-V-Mo composite carbides are assumed under the following i) to iii). The density (6.264 g / cm ³ ) was calculated as 4.337 mm, and contained in the precipitates of less than 20 nm in the steel using the volume ratio and Fe density (7.86 g / cm ³ ) determined above. Ti ₂₀ , V ₂₀ , and Mo ₂₀ that are Ti, V, and Mo were determined.
i) Ti-V-Mo-C is NaCl-type crystal structure
ii) Based on VC (4.16mm), TiC (4.38mm), and MoC (4.47mm), the lattice constant is calculated by the ratio of Ti, V, and Mo (Because MoC has no literature value, substitute the value of NbC. )
iii) Composition ratio Ti: V: Mo = 1: 1: 1
_{Ti sol, V sol, Mo sol} : Ti sol is dissolved Ti content in steel is determined as the difference between Ti _total obtained as Ti content and the in the steel (= Ti-Ti _total) It was. V _sol and Mo _sol were determined in the same manner.

  The diameter of the carbide containing Ti, V, and Mo is observed by TEM, and the carbide containing Ti, V, and Mo is analyzed by the energy dispersive X-ray spectrometer (EDX) equipped in TEM. After confirmation, image analysis was performed using image processing software “Particle Analysis II Ver. 2.0” manufactured by Sumitomo Metal Technology Co., Ltd., and the diameter of each carbide was determined by spherical approximation. At this time, the thickness of the sample for TEM observation was about 50 nm, the observation magnification was 100,000 times, and the number of observation fields was five. The diameter of the carbide containing Ti, V, and Mo thus obtained was averaged to obtain the diameter of the carbide.

5) Manufacturing method The high-strength hot-rolled steel sheet of the present invention is, for example, a steel slab having the above-described component composition, for the following reasons, finish rolling end temperature: 880 to 940 ° C, coiling temperature: 600 to 670 ° C. It can manufacture by performing hot rolling on conditions.

Finishing rolling finish temperature: 880-940 ° C
The finish rolling finish temperature is important for securing stretch flangeability and total elongation and reducing rolling load. When the finish rolling finish temperature is less than 880 ° C., strain accumulation during rolling is large, Ti-based strain-induced precipitation is likely to occur during hot rolling, and a relative reduction in the amount of fine carbides effective for strengthening is caused. On the other hand, since the ferrite transformation is promoted (the structure becomes coarse), the ductility is improved, but the structure strengthening of the matrix is insufficient. When the finish rolling finish temperature is 880 ° C or higher, the rolling load decreases and the manufacturability is stabilized as the rolling temperature rises, and strain-induced precipitation is less likely to occur during hot rolling. While the amount is secured, the transition is made to the transformation, the microstructure is refined by the simultaneous progress of transformation and precipitation, and the matrix strength is increased. However, it is difficult to secure a finish rolling finish temperature of 940 ° C. or higher in a general production line. Therefore, the finish rolling finish temperature is preferably 880 to 940 ° C.

Winding temperature: 600-670 ° C
In order to obtain a ferrite structure, to ensure sufficient precipitation of carbides, and to reduce the amount of water injected on the run-out table and to run a thin steel plate stably, the coiling temperature is 600 ° C or higher. Moreover, in order to suppress the formation of pearlite, the temperature is preferably 670 ° C or lower.

  In addition, steel slabs such as steel slabs may be cooled and then reheated to a predetermined temperature before hot rolling, or reheated before the steel slab becomes lower than the predetermined temperature. You may hot-roll without it. The reheating temperature of the steel slab is preferably about 1150 ° C to 1300 ° C in order to re-dissolve the carbide (or not to cause precipitation growth), and in this temperature range for 30 min in order not to leave undissolved Ti-based carbide. It is desirable to hold the above.

  The thickness after hot rolling is preferably about 1.4 to 5.0 mm as a hot rolled steel sheet for automobiles. Conventionally, it has been difficult to produce a high strength hot rolled steel sheet with a thin plate thickness due to an increase in rolling load, but the steel sheet of the present invention has a fine carbide in the winding process after the end of rolling in order to increase the strength. Since it can be produced by precipitation, a high-strength hot-rolled steel sheet having a thickness of 2.5 mm or less can also be produced.

  The high-strength hot-rolled steel sheet of the present invention includes a surface subjected to surface treatment or surface coating treatment. In particular, the steel sheet of the present invention is suitable as a base steel sheet for a surface-treated steel sheet on which a hot dip galvanized coating film is formed.

  A steel slab having the composition shown in Table 1 is heated to 1250 ° C. and finished at a finish rolling end temperature of 840 to 920 ° C. by a normal hot rolling process, and a hot rolled steel sheet having a thickness of 3.5 mm, and a winding temperature of 580 to 685 It was wound up at ° C. After pickling the obtained steel plate, the thin film sample created by collecting from the 1/4 thickness position of the steel plate is observed with TEM, and quantitative analysis of the solid solution amount of fine carbides and carbide forming elements as described above is performed did.

  Since the structure of the high-strength hot-rolled steel sheet of the present invention is finer than that of a general steel sheet, the ferrite grain size was evaluated using a scanning electron microscope (hereinafter referred to as SEM). The sample was mirror-polished on the L cross section parallel to the rolling direction, and then the structure was revealed by electrolytic polishing, and SEM observation was carried out at a thickness of 1/4. Accelerating voltage was 15kV, and 3 fields of view were taken 2000 times by backscattered electron image that easily recognized the grain structure. From these backscattered electron images, the average crystal grain size was calculated for each sample by a cutting method that is generally carried out with an optical micrograph. If there is a needle-like or lath-shaped structure (bainite or a structure generally called acicular ferrite) clearly different from the equiaxed ferrite phase in the matrix structure, the average particle size is not calculated. It was.

  Further, JIS No. 5 tensile test specimens were collected from the rolling direction of the obtained steel sheet, and TS and total elongation (El) were obtained.

The results are shown in Table 2. It can be seen that Sample Nos. 4 to 7, and 9, 10 as examples of the present invention can obtain TS of 1000 MPa or more and El of 15% or more. On the other hand, component range is within the scope the present invention, manufacturing conditions change, the sample No.8,11,12 Ms / M ₀ value outside the range of the present invention, although the ductility is ensured, over 1000MPa TS is not obtained. In the case of sample No. 3 that does not satisfy the formula (1), the amount of C is insufficient with respect to the metal elements (Ti, V and Mo in the present invention). Even if it is high, the absolute amount of fine precipitates decreases, and TS of 1000 MPa or more cannot be obtained.

It is a diagram showing the relationship of Ms / M ₀ and TS.

Claims (1)

In mass%, C: 0.10% or more and 0.12% or less, Si: 0.30% or less, Mn: 1.8% or less, N: 0.003% or less, Ti: 0.10% or more and 0.20% or less, V: 0.15% or more and 0.30% or less, Mo : 0.05% or more and 0.40% or less, the balance is composed of Fe and inevitable impurities, has a component composition satisfying the following formula (1), and has a structure composed of ferrite grains having an average grain size of 3 μm or less And Ms defined by the following formula (3) and M ₀ defined by the following formula (4) satisfy the following formula (2):
0.7 <(C / 12) / [{(Ti-3.42 × N) / 48 + V / 51 + Mo / 96}] <0.9 (1)
4 <Ms / M ₀ ... (2)
Ms = (Ti ₂₀ / Ti _total + V ₂₀ / V _total + Mo ₂₀ / Mo _total ) / 3 (3)
M ₀ = (Ti _sol / Ti + V _sol / V + Mo _sol / Mo) / 3 (4)
However,
In equation (1)
Each element symbol indicates the content (% by mass) of each element,
In equation (3)
Ti ₂₀ , V ₂₀ , Mo ₂₀ is the content of each element in the carbide of Ti, V, Mo with a diameter of less than 20 nm precipitated in the steel sheet (mass% relative to steel),
Ti _total , V _total , and Mo _total are the contents of each element in the total carbide (mass% relative to steel),
In equation (4)
Ti _sol , V _sol , and Mo _sol are the solid solution amount (mass%) of each element in steel.
Ti, V, Mo is the content (mass%) of each element in steel,
To express.

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