KR101446354B1 - High-strength hot rolled steel plate and manufacturing method thereof - Google Patents

Abstract

The present invention provides a novel high-strength Si-Cr-containing hot-rolled steel sheet having higher strength and excellent workability and a method for producing the steel sheet. The high-strength steel sheet can be obtained by adjusting the particle size of the old austenite to 10 탆 or less and appropriately selecting the coiling temperature. The obtained steel sheet contains 5 to 20% of the retained austenite phase in a volume ratio; 10% or less by volume of the martensitic phase; And a bainite phase in the remaining volume ratio. The particle size of the retained austenite particles is 1 占 퐉 or less, and the retained austenite particles are uniformly dispersed.

Steel plate, rolling, austenite, bainite, martensite, strength

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a high-strength hot-rolled steel sheet,

This application is based on Japanese Patent Application No. 2007-108759 filed on April 17, 2007, the entire contents of which are incorporated herein by reference.

The present invention relates to a high-strength hot-rolled steel sheet having high tensile strength and excellent workability, and also relates to a method for producing the same.

A recent demand for high-strength steel sheets that can exhibit excellent processability will be described, for example, in relation to automobiles. In terms of global environmental protection, it is necessary to reduce the amount of emissions such as CO ₂ in the automotive sector. For this reason, it is absolutely necessary to further reduce the weight of each vehicle body. In order to reduce the weight of the vehicle body, it is required to reduce the thickness of the steel sheet by improving the strength of the steel sheet used for the vehicle body. In addition, the safety of the user in the car must be ensured. For this purpose, the strength of the steel sheet must be further improved.

However, an increase in the strength of the steel sheet may reduce its workability, and a steel sheet having a higher strength may be difficult to apply to cold working such as general press molding.

Hot pressing is a hot pressing process, which typically results in a very small amount of resilient restoration and exhibits desirable shape freezing properties. In addition, due to the curing effect during hot pressing, this method can present parts with significantly higher strength and higher precision. However, this process requires that the steel sheet be heated before the hot press working of the steel sheet, and after the hot press working, the production scale is required to be reduced. Therefore, this process has a possibility of remarkably lowering the processing efficiency. In addition, a shorter lifetime of the mold to be in contact with the heated steel sheet inevitably increases the manufacturing cost.

The elongation of the steel sheet after the hot press working will be reduced so that the hot pressed member can be broken by only a slight deformation caused by some external force, impact or the like. Therefore, steel plates of this type are generally evaluated as having a weak shock absorption capability. Therefore, it is very difficult to use such a hot-pressed portion as a core component for ensuring safety of automobiles and the like.

As a method for improving the strength, there are a reinforcement by solid solution treatment, a reinforcement by precipitation, a reinforcement by grain refinement, and a reinforcement using a low temperature transformation phase. It is impossible to manufacture a steel sheet with remarkably improved strength only by employing a reinforcement mechanism including precipitation requiring a solution treatment or a large amount of alloying addition. Even when the reinforcement by grain refinement is used, the improvement of the strength is limited to some extent. Although reinforcement using low temperature transformation phases is very effective in producing steel sheets in excess of 1200 MPa, this method can not be expected to improve the ductility to balance and improve such strength.

In general, the higher the strength, the lower the ductility can be, which tends to reduce the workability.

Among the high-strength steel sheets, materials known to have improved ductility include dual phase steel plates composed of ferrite and martensitic phase, and transformation induced plasticity consisting of ferrite, bainite and retained austenite phase , TRIP) steel plate.

The two-phase steel sheet is formed by finely dispersing a hard martensite phase in a ferrite phase. Due to the very hard martensitic phase, a significant work hardening is caused in the transformation, and a higher ductility is provided to the steel sheet.

Examples of the TRIP steel sheet are described in Patent Documents 1 and 2. [ This type of steel sheet containing the retained austenite phase exhibits excellent ductility and formability all derived from the processed organic transformation, depending on the amount of retained austenite phase and the resilience against deformation.

However, attempts to obtain a steel sheet having a strength exceeding 1200 MPa may cause delayed fracture. "Delayed fracture" refers to a phenomenon that occurs during the use and assembly of each member without causing cracking and / or fracture. The high-strength steel sheet disclosed in Patent Document 3 has a soft phase such as a ferrite phase as small as possible and a residual austenite phase volume ratio of less than 4% as compared with a low temperature phase transformation phase such as a bainite phase and / or a tempered martensitic phase To thereby provide a more preferable anti-delay fracture property.

Patent Document 1: JP-A-No. 60-43425

Patent Document 2: JP-A-No. 9-104947

Patent Document 3: JP-B-No. 3247908

The two-phase steel sheet and the TRIP steel sheet described above are examples of the steel sheet with improved elongation characteristics in cold working while maintaining a higher strength.

In the above two-phase steel sheet, even when the addition amount of the alloy is smaller, higher strength can be achieved, and more uniform elongation characteristics can be obtained due to work hardening.

The TRIP steel sheet exhibits higher ductility and has a better deep drawing characteristic. Therefore, this material is suitable for providing a part or member requiring a complicated shape, higher workability and higher strength.

In the TRIP steel sheet disclosed in Patent Document 1, the ferrite phase is caused to appear in the austenite phase by holding the raw material at 450 to 650 ° C. for 4 to 20 seconds in the cooling step after rolling, cooling it to a temperature of less than 350 ° C., And then winding it around.

In Patent Document 2, in order to promote the formation of the ferrite phase in the austenite phase in the cooling step after rolling, the raw material is gradually cooled in Ar3 to Ar1, or approximately 350 to 500 ° C , And wound around the rod material.

The TRIP steel sheet has a structure in which a martensitic phase, a residual austenite phase, and / or a bainite phase are dispersed in a ferrite parent phase, and exhibits excellent strength and elongation characteristics.

However, under the condition of C? 0.20%, which can guarantee the spot welding characteristic, only a tensile strength of about 800 MPa can be obtained, so that further improved workability is desired. Therefore, it is difficult to produce a steel sheet having a remarkably high strength under the above conditions.

Even in the method of progressively cooling the raw material to a temperature of less than 500 ° C without a gentle cooling step in the middle after the rolling, if the rolling completion temperature is set to about Ar 3, the appearance of the fine ferrite phase can be promoted. However, in the quality of the hot-rolled steel sheet material rolled at a temperature of approximately Ar 3, the anisotropy of the material may be undesirably large.

In addition, the hot-rolled steel sheet described in Patent Document 1 has a low rolling workability and has a metal structure in which coarse ferrite particles and residual austenite grains are adjacent to each other due to temporary stop during cooling at approximately A1.

Hydrogen dissolved in a steel plate considered as a cause of delayed fracture is a factor for determining a crystal phase and is preferentially trapped in the retained austenite phase. In particular, the interface between the impacted or treated martensitic phase and the ferrite phase, that is, the processed organic transformation site, is considered to be a very likely hydrogen trapping site.

As the retained austenite grains coarser, the area ratio of the interface between the martensitic phase and the ferrite phase that underwent the processed organic transformation will decrease relative to the volume of the retained austenite grains. As a result, the concentration of captured hydrogen increases, increasing the risk of delayed failure. If the martensite phase and the retained austenite phase coexist adjacent to each other (in the state of M-A), the expansion of fracture can be promoted to further increase the risk of fracture.

The high-strength steel sheet described in Patent Document 3 attempts to improve the anti-delay fracture property by limiting the amount of retained austenite. However, in order to obtain excellent workability while maintaining a higher strength, the use of retained austenite is practically effective. Therefore, it is desirable that the presence of the above-mentioned unrestricted residual austenite does not adversely affect the anti-delayed fracture characteristics.

For this purpose, the present inventors have found that a steel sheet having a bainite phase in which 7 or more residual austenite grains having a grain size of 1 탆 or less are finely dispersed (having a volume ratio within a range of 5% to 20%) per ² 탆 square, Alloy high-strength steel sheet and a method of manufacturing the same, which exhibit not only higher strength but also more preferable workability and robust anti-delay fracture property.

Through a number of experiments, the present inventors have found that a suitable high-strength steel sheet can be obtained by adopting suitable rolling conditions and selecting an appropriate composition of the components for the steel sheet. That is, a slab containing components of appropriate composition is subjected to rough hot rolling under high pressure conditions, finish of second stage high strain rolling under a high temperature condition in a finishing rolling process, The cooling process after air-cooling is started and the processed material is rolled at an appropriate temperature to provide a low-alloy steel sheet with a higher strength and excellent ductility as well as a strong anti-delay fracture property.

The high-strength hot-rolled steel sheet of the present invention contains 5 to 20% by volume of the retained austenite phase; 0% to 10% by volume of the martensitic phase; And a bainite phase in the remaining volume ratio, wherein the particle size of the particles constituting the retained austenite phase is 1 占 퐉 or less. More preferably, the particle size of the old austenite is not more than 10 mu m, and the average aspect ratio of the particles is not more than 2.0.

After the hot rolling, the grain size of the austenite crystal is adjusted to 10 탆 or less (Fig. 3), so that the lath structure on the bainite can be made finer. In addition, a uniform bainite transformation can be completed, and the residual austenite grains having a grain size of 1 탆 or less can be finely and effectively dispersed in the phase at a grain density of 7 or more per 10 탆 ² (Fig. 8).

In this way, excellent anti-delay fracture characteristics can be obtained even in a steel sheet provided with high ductility by using the processed organic calcination by a relatively large amount of retained austenite.

By adjusting the aspect ratio of the old austenite particles to 2.0 or less (Fig. 3), the workability can be improved by reducing the anisotropy of the material extending in both the rolling direction and the direction perpendicular to the rolling direction ).

Preferably, the high-strength hot-rolled steel sheet of the present invention comprises: C (0.13-0.21 wt%), Si (0.5-2.0 wt%), Mn (0.2-1.0 wt%), Cr (1.0-4.0 wt% , Ni (0.02-1.0 wt.%), Mo (0.05-0.4 wt.%), P (0-0.010 wt.%), S (0-0.003 wt.%), N (0.005-0.015 wt.%) Fe and other unavoidable impurities.

The chemical composition comprising the selected type and amount of the above-selected components in the appropriate type and amount can facilitate the formation of a high-strength steel sheet comprising the above-mentioned phases and exhibiting the desired mechanical properties.

Since the above alloying elements can constitute the desired steel sheet structure of the present invention in the step of cooling after hot rolling and the step of winding the cooled material, Cr and Si, which have a great influence on bainite transformation, are included as main elements do. By controlling the amounts of these elements, the bainite transformation can be promoted, the formation of the martensite phase can be suppressed, and the whole phase having the desired strength can be controlled.

The effect of each component is described below.

The high-strength hot-rolled steel sheet preferably has the above-mentioned structure, and the thickness of the plate is 1.0 to 3.0 mm, the tensile strength (TS) is 1200 MPa or more, and the elongation is 13% or more (JIS. 5 specimen).

That is, this steel sheet can have the above structure and exhibits higher strength and better elongation property.

The method for manufacturing a high-strength hot-rolled steel sheet according to the present invention includes the following steps:

(1) C (0.13-0.21 wt%), Si (0.5-2.0 wt%), Mn (0.2-1.0 wt%), Cr (1.0-4.0 wt%), Ni (0.02-1.0 wt% 0.05 to 0.4% by weight), P (0 to 0.010% by weight), S (0 to 0.003% by weight), N (0.005 to 0.015% by weight) and the balance of Fe and other unavoidable impurities Rolling material);

(2) the extraction temperature of the reheating furnace is 1250 DEG C or higher; The outlet-side temperature of the rough rolling mill is at least 1030 占 폚; And subjecting the steel material to a rough rolling under the condition that the reduction ratio of each of the roughing passes in the last three passes is 30% or more;

(3) the outlet-side temperature of the finishing mill is at least 950 ° C; The first through third rolling mills in the front-stage of the finishing process have a rolling reduction of not less than 40% (this is the case where six rolling mills are used and in the case of using seven rolling mills, the first to fourth rolling mills are used ) And finishing the steel material under the condition that the accumulated deformation in the state pressed by three mills in the rear-stage of finishing is 0.5 or more; And

(4) cooling the steel material by air-cooling for 2 to 6 seconds, followed by water-cooling, and winding the steel material at a coiling temperature of 550 to 650 ° C.

In order to improve the strength by obtaining a low-alloy bainite phase by employing a temperature history (Fig. 1) for maintaining the temperature, rapid cooling after hot rolling using a hot strip mill, and cooling the material at a predetermined temperature condition In the winding step, a homogeneous phase of bainite in which martensite and retained austenite are finely dispersed can be obtained by adding Cr and Si as main alloy elements and a composition containing less Mn and Ni (see Fig. 7 b)).

The addition of Si can control the precipitation of carbide, form a more uniform bainite phase, and retain a large amount of austenite having a carbon density of 0.8% or more. In this way, a steel sheet having improved strength and better processability can be obtained (Fig. 11).

By adjusting the hot rolling finishing temperature to 950 DEG C or higher, the aspect ratio of the old austenite particles can be adjusted to 2.0 or less (FIG. 3).

In order to prevent biting failure of the topmost portion of the rolled material in the roll, it is desirable that the finish rolling be carried out in the first to fifth rolling mills (as in the case of using a finishing mill of six stages, It is preferable to reduce the maximum reduction amount of the steel material in comparison with the expected reduction amount (or the originally set reduction amount for the predetermined rolling) by using the first to sixth rolling mills when the finishing mill is used. The amount is increased to less than 10% of the expected amount in each rolling mill. It is also preferable that the length rolled with an increased reduction amount is within 5 m as measured from the uppermost cutting position of the rolled material.

To prevent slippage between the rolled material and the roll during the rolling process, it is also preferable to use a special high-grip roll as the work roll for each of the first through third mills, including the final mill.

The test at the time of manufacture as described below proves that the above-mentioned high-strength steel sheet can be easily obtained by adopting the conditions as described above.

And of the present invention in strength steel sheet, the retained austenite is feed at a weight ratio of 5% to 20% in the bainite phase is finely dispersed in 10㎛ ² more than 7 per particle density. Therefore, both the strength and the workability which are mutually opposed to each other can be provided to the steel sheet, and excellent anti-delay fracture characteristics can be also provided.

According to the method of the present invention for producing a high-strength steel sheet, the above-mentioned high-strength steel sheet can be manufactured easily and robustly.

Hereinafter, an embodiment of a thin plate steel used for producing a sheet steel by processing a sheet steel and a manufacturing method of the thin plate steel, which requires good workability and anti-delay breaking property while maintaining a tensile strength of 1200 MPa or more .

The steel sheet has a composition containing the following components: C (0.13-0.21 wt%), Si (0.5-2.0 wt%), Mn (0.2-1.0 wt%), Cr (1.0-4.0 wt% 0 to 0.003% by weight), N (0.005 to 0.015% by weight), and the remaining components include Fe and others Inevitable impurities are included.

In the present specification, the term "thin plate" means a steel plate having a thickness of 1.0 mm to 3.0 mm. The steel sheet produced under the conditions of the above composition can be mainly used as parts of automobiles, consumer electronic products, electronic devices, etc., which require higher workability and strength. The steel sheet may also be applied as a steel pipe material.

First, the components of the steel sheet will be described.

The amount of carbon (C) should be in the range of 0.13 to 0.21%.

C is the most important component to stabilize the retained austenite. If the amount of C is less than 0.13%, sufficient stability can not be obtained. Therefore, the amount of C should be 0.13% or more. However, if the amount exceeds 0.21%, the welded portion becomes too hard and can break. Such a situation somewhat restricts the use of formed sheet steel. Therefore, the upper limit as described above for the amount of C is presented. That is, it was confirmed that by setting the amount of C within the range of 0.13 to 0.21%, a composite structure meeting the intention of the present invention can be obtained.

The amount of silicon (Si) should be in the range of 0.5 to 2.0%. Si also acts to stabilize the retained austenite. In addition, Si improves the strength obtained by solution strengthening. When the amount of Si is 0.5% or more, a desirable composite structure and material quality can be obtained. The greater the amount of Si, the more the retained austenite can be increased and the stability can be improved. However, if the amount of Si exceeds 2.0%, the upper limit of the Si amount should be set to 2.0% from the viewpoint of cost reduction, since the characteristics satisfying both strength and ductility will be excessively satisfied.

The amount of chromium (Cr) should be in the range of 1.0 to 4.0%. Cr can form a bainite phase, and the strength of the formed steel sheet can be improved.

If the amount of Cr is less than 1.0%, the amount of ferrite increases excessively, which makes it difficult to obtain a steel sheet having a preferable strength. Therefore, the amount of Cr should be 1.0% or more. However, when the amount exceeds 4.0%, a martensitic phase can be generated, and the strength of the steel sheet becomes too strong and the anti-delayed fracture characteristic becomes insufficient. Therefore, 4.0% is set as the upper limit.

The amount of manganese (Mn) should be in the range of 0.2 to 1.0%. If the amount of Mn is less than 0.2%, the production of the steel sheet will become difficult. Therefore, the amount should be at least 0.2%.

In order to achieve higher strength, it is preferable to add Mn as much as possible. However, when it is added in an excess amount, a martensitic phase can be produced, making it impossible to obtain the desired structure of the present invention. Therefore, the upper limit of the Mn content should be set to 1.0%.

The amount of nickel (Ni) should be in the range of 0.02 to 1.0%. Ni can improve the strength of the steel sheet by solution strengthening. However, if the amount of Ni is increased too much, a martensitic phase can be produced. Inadvertent additions will also result in an increase in production costs. Therefore, the upper limit should be set to 1.0%.

Molybdenum (Mo) can form a bainite phase like Cr and can also improve the strength of the formed steel sheet. In addition, the hydrogen trapping effect due to Mo carbide is useful for providing anti-delay properties to the steel sheet. However, inadvertent addition will not only increase the cost, but will also cause excessive recrystallization. Therefore, the amount of Mo should be set within the range of 0.05 to 0.40%.

In order to improve the weldability, it is necessary to reduce the amount of phosphorus (P) as much as possible. Therefore, the upper limit of this element should be set to 0.010%.

In addition, in order to improve the weldability, it is necessary to reduce the amount of sulfur S as much as possible. Therefore, the upper limit of this element should be set to 0.003%.

The amount of nitrogen (N) should be within the range of 0.005 to 0.015%. Similar to carbon, nitrogen is also useful for stabilizing the austenite phase. However, if it exists in excess, it will deteriorate the weldability. Therefore, the range of the amount should be set to about 0.005 to 0.015%.

The slabs produced in the above-mentioned composition are then hot-rolled after reheating, or hot-rolled immediately after casting.

Figure 1 is a graph schematically illustrating the temperature history of hot rolling in the manufacturing process of one embodiment of the present invention, which also shows the particle size of the prior austenite. The horizontal axis represents elapsed time, and the vertical axis represents temperature.

At the time of hot rolling, the extraction temperature of the reheating furnace was set at 1250 ° C. Such high temperature conditions will result in the inevitable growth of some austenite grains in the reheating furnace, but this temperature is chosen to desirably ensure a surface temperature of 950 占 폚 after finishing. However, the size or diameter of the austenite particles will be reduced in the next rolling process. Therefore, it is necessary to reduce the size of the old austenite grains as finely as possible before passing through the finishing mill. Therefore, in the rough rolling step, the crystal grain size is reduced to 35 μm or less in advance by setting the respective reduction rates of the final three passes during rough rolling at an outlet side temperature of 1030 ° C. or more on the outlet side of the roughing mill to 30% or more . Fig. 2 shows the size of the prior austenite grains after rough rolling, where the worked material is cut into a pre-finish crop shearing machine.

For the first to third mills of finishing (using six finishers, using seven finishers when using a finisher, the first to fourth mills are used), the reduction rate per mill is at least 40% Setting. For the latter three rolling mills of finishing, the accumulated deformation in the pressed state was set to 0.5 or more, and the outlet side temperature of the finishing mill was reliably set to 950 DEG C or higher, so that the size of the austenite grains was equal to or smaller than 10 mu m Small. It is also air-cooled for 2 to 6 seconds after finish rolling, followed by water cooling. The coiling temperature is set to 550 to 650 占 폚. In the air cooling step, the size of the austenite particles is also adjusted. That is to say, prior to starting the post-hot-rolling hot run cooling during the hot rolling step to remove the processing strain by adjusting the size of the old austenite grains, The size is adjusted to 10 μm or less.

3 shows the results of observation of the prior austenite grains of the steel sheet according to the present invention by SEM observation. The average particle size of the prior austenite particles is 9.3 占 퐉 and exhibits a uniformly granulated structure. The average aspect ratio of the major axis / minor axis is 1.7.

When a low temperature rolling with a rolling completion temperature of 950 占 폚 or less is used and the cumulative deformation of the finishing three rolling mills is set at 0.5 or less, the austenite grains gradually increase (10 占 퐉 or more) The shape may become flat due to the rolling, resulting in an increase in anisotropy. Fig. 4 shows the relationship between the temperature at the outlet side (FDT) of the finish rolling mill and the anisotropy of elongation. As can be seen from Fig. 4, the anisotropy of the elongation is shown when the FDT is 950 DEG C or lower. Here, anisotropy is defined by the following formula: C-L | / (C + L) / 2 where L is the elongation in the rolling direction and C is the elongation in the direction perpendicular to the rolling direction. The smaller the value obtained by this formula, the lower the anisotropy.

In the present specification, the term "modified"

_{ε = (h 0 -h 1)} / {(h 0 + h 1) / 2}

, And the difference between the thickness h ₀ of the inlet side steel plate and the thickness h ₁ of the outlet side steel plate at each point (each step or each pass at the time of rough rolling) is defined as an average thickness of both thicknesses Share it.

In the present specification, the term " accumulated strain "

? _c =? _n +? _n-1 /2 +? _n-2 /4

The mean values, expressed as ε _c, the deformation of the three points the second half of the finish of each stage (each pass), considering the strength of each effect is granted on the metal, was calculated using a weighted measurement value from, where The strains caused by the final stage (final pass), the propagation (pre-pass), and the pre-propagation (pre-pass) are expressed as ε _n , ε _n-1 , and ε _n-2 , respectively.

In order to carry out the hot-finish rolling, a temperature raising process for the steel sheet by employing the heat radiated by the rolling operation was adopted. For this reason, it is important to provide a high-strain rolling schedule for each post-rolling machine and to set the rolling reduction of the front point to 40% or more. As can be seen in FIG. 5, there is a difference in the discharge thickness in the roughing mill, but with the same rolling size in relation to the reduction rate, depending on the type of steel, the surface temperature after finishing may vary up to 80 ° C Can be confirmed.

Hot rolling is completed at a temperature of 950 占 폚 or higher, and then the material is air-cooled for 2 to 6 seconds without hot-rolling cooling after hot-rolling to reduce the dislocation density in the crystal grains. Figure 6 shows the changes in austenite grain size and dislocation density, where these data are calculated over the period from the finish F1 rolling mill to the start of hot working cooling, when the rolling temperature is changed for the same type of steel . It can be seen from the figure that the dislocation density is significantly affected by the rolling temperature. It can also be seen that under high pressure rolling conditions, the austenite grain size will be reduced at lower temperature conditions, which indicates that the processing temperature is equal to or higher than that required for Ar3 transformation. However, under such low temperature conditions, the dislocation density will be higher, so that a material with an increased anisotropy is provided. Further, it can be seen that effective disposition can be obtained by employing the cooling time within 6 seconds while the dislocation density is remarkably reduced due to the hot air cooling after the rolling. In this simulation model, setting the aspect ratio to less than 2.0 can be interpreted as adjusting the dislocation density to at least 2.50E + 10 (p / cm2) (this is because the actual data and the simulation model are compared Confirmed). However, the reduction of the dislocation density causes an increase in the size of the prior austenite grain. The above-mentioned rolling conditions (rolling temperature: 950 DEG C or more, cooling time: 2 to 6 seconds) are required to further reduce the above-mentioned numerical value of the dislocation density while controlling the size of the austenite particles to 10 mu m or less.

The above simulation model is described in Yanagimoto, Morimoto, et al., "Iron and Steel ", vol. 88 (2002). 11, the coefficients of each numerical value have been reviewed in this application.

The coiling temperature is set within the range of 550 to 650 DEG C, but a temperature range lower than 550 DEG C tends to increase the martensite phase, thereby increasing the possibility of delayed fracture. On the other hand, a temperature range higher than 650 ° C will produce more ferrite and pearlite, making it difficult to obtain higher strength. 7 shows a cross-sectional view of a 3-type high-strength steel sheet.

7 (a) shows a structure rich in martensite, Fig. 7 (b) shows a structure having a relatively small martensite and a relatively fine structure, and Fig. 7 ) Shows a ferrite-containing structure. Fig. 7 (b) shows the structure obtained according to the present invention.

In the bainitic-based structure, austenite remains not only at each interface between the prior austenite particles, but also within each packet boundary and each block boundary, i.e., the prior austenite particles themselves. By employing a bainite phase as the parent phase and setting the size of the prior austenite particles before transformation to 10 m or less, the retained austenite is densely and uniformly dispersed in the mother phase, and a very fine particle size The retained austenite particles may be dispersed in an amount of 7 or more per 10 탆 ² . Fig. 8 is a photograph of each cross-sectional structure obtained by the EBSP method for the steel sheet according to the present invention. The bainite phase on the body-centered cubic and the austenite phase on the face-centered cubic are distinguished by color. The retained austenite phase represented by a bright color constitutes a structure in which 7 or more fine austenite grains having a grain size of 1 탆 or less per 10 탆 ² are uniformly dispersed.

The bainite phase can be obtained by the above-mentioned control for the hot rolling, and the residual austenite particles are finely and uniformly dispersed therein.

When high-strength laminate materials (thickness less than 2 mm) are rolled under high-strain and high-reduction conditions, slippage between rolls and rolled material may occur during cutting failures and / . From the rolling results, it was confirmed that when the TS material of less than 1000 MPa was used at a rolling reduction of 40 to 50% for each mill, the binding properties at the top of the rolled material were not problematic. On the other hand, the binding failure at the top of the rolled material is believed to occur frequently (incidence: 50%) in the first to second mills of the final mill and full mill when using TS materials higher than 1000 MPa. In order to solve this problem, we have increased the rolling grinding finishing roughness Ra to 1 μm (usually 0.5 μm) to increase the rolling friction coefficient, obtain a friction coefficient (μ) of 0.4 (usually 0.3) during rolling Respectively. In addition, we have reduced the flow rate of the rolling cooling water so that the temperature at the top of the rolled material is not excessively reduced. However, no definitive and effective results were obtained. Therefore, as shown in Fig. 9, we attempted to make the top of the rolled material thinner at a position within 5 m from the exit side of the rolling mill, so that the plate thickness became thinner (10% of the finally expected thickness) Till). Thus, a slope up to the expected plate thickness was provided to the plate material.

As a result, the binding failure was significantly reduced (incidence: 0%). Further, the reduction amount used in the range from the finish rolling mill to the rolling mill located before the final rolling mill of finishing was set to a value obtained by adding 10% or less of the set value expected to be obtained. The pressing time is set within 2 seconds from the uppermost cutting position of the plate material in the rolling mill.

For the slip between the rolling mill and the rolled material during the rolling process, when the TS material larger than 1000 MPa is rolled under the high temperature and high pressure conditions to have a final plate thickness set to less than 2 mm, the rolling mill located immediately before the final rolling mill and the final rolling mill Slip may occur. As a result of this situation, metallic noise is generated during the rolling process and the rolling load of the rolling mill is significantly reduced by 50% in the event of slip. Then, the roll starts to revolve, and the rolled plate can not advance. At this time, when the rolling roughness Ra of the roll is measured by pulling the roll from each rolling mill, it is measured to be less than 0.1 mu m, which shows that the rolled material and the roll are slipped with respect to each other. To solve this situation, we adopted a special high-grip roll. As a result, the occurrence of slip was completely prevented. The rolls are each formed by uniformly dispersing micro-carbide particles (particle size: less than 1 mu m) on the entire surface of the roll. These carbide particles can be used as spikes and can be supported by a hard base material. In addition, even if the micro-carbide particles are to be worn on the surface, the micro-oxide particles will appear continuously from below, thereby maintaining a stable coefficient of friction and preventing the occurrence of slip. As shown in Fig. 10, the change in the coefficient of friction due to the rolling process is maintained within a more suitable range (about 0.3) as compared with generally known rolls.

11 is a graph showing the relationship between the volume ratio (Vy) of the retained austenite in the hot-rolled steel sheet produced by the manufacturing process shown in Fig. 1 and the data obtained by the tensile test. 11 (a) shows the relationship between the volume ratio Vy and (tensile strength x elongation). 11 (b) shows the relationship between the volume ratio Vy and elongation. As can be seen from the figure, in the range of 5 to 20% of the retained austenite volume ratio, the data of tensile strength x elongation and elongation alone are improved as the volume ratio Vy increases. The metal phase corresponding to the above data can be considered as a martensite-like fine bainite phase as shown in Fig. 7 (b).

The present invention has been completed on the basis of the above experiments.

As described above, the high-strength steel sheet according to the present invention exhibits high strength and high ductility characteristics in a low alloy composition and is suitable for use as a component for manufacturing an automobile structure.

Hereinafter, embodiments of the present invention will be described.

The slab material (rolled material) was produced from molten steel having the chemical compositions shown in Table 1 by using a forging method or a continuous casting method. Subsequently, these slab materials were heated again and then hot-rolled to obtain respective hot-rolled steel sheets. Table 2 shows each hot rolling condition and material characteristics.

In the steel types shown in Table 1, A, B and C represent steel sheets produced according to the present invention, and D, E, F, G and H are shown as comparative examples.

As a comparative example, steel type D contains significantly less Si and Ni is too abundant and deviates from the preferred range of the present invention.

Since the steel type E contains a considerable amount of Si, it is also outside the range defined by the present invention.

Steel types F and G contain less C and deviate from the preferred range of the present invention. Steel type I contains an excessive amount of C and is also outside the preferred range of the present invention. The steel type H is excessively rich in Cr and deviates from the preferred range of the present invention.

Nos. 1 to 6 in Table 2 are examples in which the steel types A, B, and C of Table 1, which satisfy the preferred ranges of the present invention, respectively, were rolled under various conditions.

No. 1 was produced by using a steel type A containing 0.51% of Si and employing a hot rolling coiling temperature of 655 캜. In this case, TS (tensile strength) x EL (elongation) data is very preferable, but is not preferable because the tensile strength is as low as 778 MPa.

No. 2 was produced by employing a hot rolled coiling temperature of 630 캜 using a steel type A. This example shows a tensile strength of 1200 MPa and an elongation of 13%, showing excellent properties.

No. 3 was produced by using a steel type B containing 1.00% of Si and employing a hot rolling coiling temperature of 595 캜. As can be seen in Table 2, this example is excellent in both strength and elongation. In addition, this example exhibits improved properties in both strength and elongation as compared with the No. 2 embodiment.

Nos. 4 and 5 were produced under unsatisfactory rolling reduction conditions during hot rolling, and these examples show negative retardation failure while satisfying strength and elongation.

No. 6 was produced by employing a hot rolled coiling temperature of 610 캜 using a steel type C containing 1.44% of Si. This example exhibits excellent properties in both strength and elongation, and is superior to the No. 3 embodiment in all aspects of strength and elongation.

Nos. 7 to 12 are each produced by hot rolling, using the steel type of the comparative example deviating from the preferred range of the composition used in the present invention.

No. 7 was produced by rolling using a steel type D containing a small amount of Si and a large amount of Ni. This comparative example is insufficient not only in terms of the spot welding characteristic (S / W characteristic) but also in the delayed fracture characteristic.

No. 8 was produced by using a steel type E containing a small amount of Si, and the strength was insufficient and the strength / ductility imbalance was exhibited.

No. 9 and No. 10 were produced using steel types F and G each containing less C, respectively, and the strength was very low, indicating an unbalance in strength / ductility.

Nos. 11 and 12 were produced using steel types H and I both containing a very large amount of C and exhibited a good balance of adequately high strength and strength / ductility. However, these comparative examples were insufficient in terms of not only the spot weldability but also the delayed fracture characteristics.

The volume ratio of the ferrite particles was measured by observing with an optical microscope after abrading the cut surface along the rolling direction of each steel sheet and corroding the worn surface with nital. A commercially available image analyzer was used for the measurement.

The volume ratio of martensite was measured by abrading the cut surface along the rolling direction of each steel sheet and etching the worn surface with a 1: 1 mixed solution of 4% picric acid-alcohol and 2% sodium pyrophosphate, / 4 spot was observed by optical microscopy, and the white martensite phase was measured in the image analysis process.

The measurement of the retained austenite was carried out by adopting X-ray diffraction using Kα line of Cu. The volume ratio was obtained by electrolytically polishing a 1 / 2t point in the plate thickness direction and then measuring the integral intensity of the (200), (220) and (311) planes of the austenite phase and the integral intensity of the (200) And the average of the retained austenite volume ratios calculated from the combination of the data obtained by each measurement.

The tensile properties (tensile strength (TS) and elongation (EL)) were measured by tensile test on each sample formed in the shape according to JIS No. 5 test piece.

The delayed fracture characteristics were evaluated by observing each sample immersed in 1N hydrochloric acid for a predetermined time after forming a ψ10 mm punch hole with a gap of 12.2% in a central region loaded with deformation test of 8% or more by tensile test.

As mentioned above, the high-strength steel sheet obtained in the examples exhibits high strength and high ductility characteristics in a low alloy composition and is suitable for use as a component for manufacturing an automobile structure.

For example, the high-strength steel sheet according to the present invention has sufficient tensile strength to support automobile doors and prevent deformation upon impact, bendability for press molding, deep drawability, Such as a center filler for automobiles, which requires very good properties including hole expandability to form mounting holes, and weldability to weld the material to other automotive parts.

Although the invention has been described in some detail in a preferred embodiment, obviously many variations and modifications are possible therein. Accordingly, the present invention may be carried out by other methods than those described above within the scope and spirit of the present invention.

The above and other objects, features and advantages of the present invention will become more apparent from the accompanying drawings and the following description thereof,

1 is a graph schematically showing the temperature history of hot rolling in the manufacturing process of one embodiment of the present invention.

Fig. 2 is a photograph of the austenite grains of the crop at the finishing run.

Figure 3 is a photograph of the prior austenite particles.

4 is a graph showing the relationship between the finish rolling temperature and anisotropy of elongation.

5 is a graph showing the relationship between the rolling schedule and the rolling temperature.

6 is a graph showing the relationship between the dislocation density and the particle size of the prior austenite particles.

Figure 7 is a photograph of a typical top view of cross sections.

Fig. 8 is a photograph of each section obtained by the EBSP method on the steel sheet produced under the composition and rolling conditions according to the present invention, and the bright or light color region shows the retained austenite.

9 is a graph showing the deviation of plate thickness at the end of the rolled material.

10 is a graph showing the relationship between the friction coefficient and the reduction rate according to the type of roll.

11 is a graph showing a balance between strength and ductility and a relationship between ductility and retained austenite amount.

Claims (7)

(0.1 to 0.21 wt%), Si (0.5 to 2.0 wt%), Mn (0.2 to 1.0 wt%), Cr (1.0 to 4.0 wt%), Ni (0.02 to 1.0 wt% (0 to 0.03 wt.%), S (0 to 0.003 wt.%), N (0.005 to 0.015 wt.%) And the balance Fe and other unavoidable impurities, 5% to 20% residual austenite phase by volume; 0% to 10% martensitic phase in a volume ratio; And a residual volume ratio of bainite phase, wherein the residual austenite particles have a particle size of 1 탆 or less and the retained austenite particles are dispersed at 7 or more particle densities per 10 탆 ² High - strength hot - rolled steel sheet. The high-strength hot-rolled steel sheet according to claim 1, wherein the particle size of the old austenite particles is 10 μm or less and the average aspect ratio of the old austenite particles is 2.0 or less. delete The high-strength hot-rolled steel sheet according to claim 1 or 2, wherein the steel sheet has a thickness of 1.0 to 3.0 mm and a tensile strength of 1200 MPa or more. (0.1 to 0.21 wt%), Si (0.5 to 2.0 wt%), Mn (0.2 to 1.0 wt%), Cr (1.0 to 4.0 wt%), Ni (0.02 to 1.0 wt% (0 to 0.010% by weight), S (0 to 0.003% by weight), N (0.005 to 0.015% by weight) and the balance of Fe and other unavoidable impurities ; Rolling the steel material under a condition that an extraction temperature of the reheating furnace is not lower than 1250 DEG C, an outlet-side temperature of the roughing mill is not lower than 1030 DEG C, and a reduction rate of each of the roughing passes in the last three passes is 30% or more; Finishing the steel material under the condition that the outlet-side temperature of the finishing mill is 950 DEG C or higher, the reduction ratio of the finishing mill is 40% or more, and the accumulated deformation is 0.5 or more due to the rolling by the three rolling mills of finishing Rolling; And And cooling the steel material by air cooling for 2 to 6 seconds followed by water cooling; and winding the steel material at a coiling temperature of 550 to 650 占 폚. The rolling mill according to any one of claims 5 to 6, characterized in that, at the time of finishing rolling, the highest rolling reduction amount of the steel material in one or more rolling mills except for the rolling mill at the final stage is set to be larger than an expected rolling reduction amount, The length of the rolled steel sheet is set to a value that is increased to less than 10% of the amount of the rolled steel sheet, and the length rolled with the increased rolling reduction is within 5 m measured from the uppermost cutting position of the steel material. By weight of the steel sheet. Process according to claim 5 or 6, characterized in that as a finishing roll for finishing rolls comprising a final mill, a high-grip roll with micro-carbide particles dispersed on the surface of the high-grip roll is used A method for producing a high - strength hot - rolled steel sheet.

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