KR100352587B1 - Method for manufacturing high strength hot rolled steel sheet with excellent elongation - Google Patents

Abstract

PURPOSE: Provided is a method for manufacturing high strength hot rolled steel sheet with excellent elongation for use as vehicle's member and wheel by properly controlling cooling method and coiling temperature. CONSTITUTION: The method includes the steps of hot rolling a steel slab comprising C 0.06 to 0.10 wt.%, 0.25 wt.% or less of Si, Mn 1.4 to 2.0 wt.%, 0.02 wt.% or less of P, 0.01 wt.% or less of S, Al 0.005 to 0.05 wt.%, Ti 0.05 to 0.15 wt.%, Nb 0.02 to 0.04 wt.%, a balance of Fe and incidental impurities, wherein hot rolling is terminated at greater than Ar3 transformation temperature; water cooling the hot rolled steel sheet to a temperature right below Ar3 transformation temperature at which ferrite transformation is initiated; air cooling the cooled steel sheet to the extent that ferrite transformation proceeds up to 70-80%; and water cooling the steel sheet to the temperature range of 4000 to 500°C, followed by coiling.

Description

High strength hot rolled steel sheet with excellent elongation property and manufacturing method

The present invention relates to a hot rolled steel sheet used as a member of a vehicle or a wheel, and a manufacturing method thereof, and more particularly to a high-strength hot rolled steel sheet having a tensile strength of 70kg / mm ² or more excellent in stretch elongation and its manufacturing method will be.

The strength member used for the member of an automobile part, the material for wheels, etc. is employ | adopted the high strength steel plate for reducing the weight of a vehicle body. Such high-strength hot rolled steel sheet for automobiles should have good workability, weldability and fatigue resistance after molding in addition to the required strength. Currently, hot rolled steel sheet used as a chassis part for automobiles is used as a hot-rolled steel sheet having a tensile strength of 50kg / mm ² or less and a precipitation-reinforced or transformed tissue-reinforced hot-rolled steel sheet having a tensile strength of 55-60kg / mm ² . .

However, in order to cope with tightening regulations on the amount of automobile exhaust gas, and to achieve a lighter weight of the vehicle body, it is effective to increase the strength of the existing hot rolled steel sheet. There is a need for a high strength hot rolled steel sheet.

In general, as the strength of the steel sheet increases, the ductility and the elongation and elongation property decrease, and the press formability deteriorates. The machinability of hot rolled steel sheet depends largely on elongation which can be expressed by elongation and elongation which is evaluated by hole enlargement rate, and press forming requires both formability at the same time.

As the strengthening method of 50-60kg / mm ² tensile strength, solid solution strengthening, precipitation strengthening, and transformational tissue strengthening are used, and the range of tensile strength is limited by each strengthening method. However, in the tensile strength of 70-80kg / mm ² grade, the choice of the strengthening method is limited. The reason for this is that it is difficult to secure strength in the case of solid solution strengthening. In the case of precipitation-reinforced type, it is possible in terms of strength, but it is disadvantageous in comparison with other strengthening methods in elongation. In the case of using the bainite structure, the deposition of carbonitride is not easy and inefficient. On the other hand, in the case of the transformed structure reinforcing steel, the tensile strength-elongation balance (TSXEI balance) is high, and there is an advantage of easily obtaining a resistance ratio.

Thus, Japanese Patent Publication No. 61-15128, Japanese Patent Publication JP 91358007, and Japanese Patent Publication Laid-Open are high-strength balance of tensile strength and elongation, which are conventionally proposed as transformation structure reinforcing steels that are easy to obtain a resistance ratio. 3-10049 is representative.

The dual phase steel proposed in Japanese Patent Application Laid-open No. 61-15128 has better strength-elongation balance characteristics than precipitated tempered steel, but is known to be inferior to bainite single phase steel. In addition, in the case of abnormal steel, since a large amount of Si is added, the surface quality of the steel sheet after hot rolling becomes a problem.

Japanese Patent JP 91358007 proposes a high-strength-resistance high-strength hot-rolled steel sheet and a method of manufacturing the ferrite having TiC and / or NbC carbides deposited therein and martensite or retained austenite, unlike conventional abnormal steels. . Also in this method, since 0.5-2.5 wt% of Si is added, the surface defects of hot rolled steel sheets or pickled hot rolled steel sheets, which have been a problem in conventional abnormal steels, cannot be solved fundamentally.

Japanese Patent Application Laid-Open No. 3-10049 proposes a residual austenite steel having a tensile strength of 80-100 kg / mm ² having a higher ductility than the above ideal steel. Although the residual austenite steel has excellent tensile strength-elongation balance, it is difficult to obtain a uniform material in the width direction and the length direction of the steel sheet in manufacturing because the tensile properties are largely dependent on the amount of residual austenite in the structure. It is known.

Thus, the present inventors have solved the problems in the conventional methods described above to conduct research and experiments to produce a precipitation-reinforced ferrite-bainite composite tissue steel having excellent stretch elongation and high tensile strength of 70 kg / mm ² or more. And the present invention was proposed based on the results.

The present invention provides a stretch-extension having ferritic-bainite composite structure which is enhanced by controlling the alloying component of steel and its content, cooling method and winding temperature on the run out table (ROT) after hot rolling. It is an object of the present invention to provide an excellent high strength hot rolled steel sheet and a method of manufacturing the same.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight, C: 0.06-0.10%, Si: 0.25% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05- 0.15%, Nb: 0.02-0.04%, balance: Fe and other unavoidable impurities, the structure is a high-strength hot-rolled steel sheet having excellent stretch elongation, the ferrite-bainite precipitated with Ti and Nb carbide.

In the present invention, by weight%, C: 0.06-0.10%, Si: 0.5% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05- 0.15%, Nb: 0.02-0.04%, Mo: 0.3% or less, remainder: Fe and other unavoidable impurities, and its structure is a stretch-elongation structure of Ti-Mo composite carbide and ferrite-bainite in which Nb carbide is precipitated An excellent high strength hot rolled steel sheet.

In the present invention, by weight, C: 0.06-0.10%, Si: 0.25% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05- 0.15%, Nb: 0.02-0.04%, remainder: When hot-rolling a slab of steel composed of Fe and other unavoidable impurities, finish at a temperature above Ar ₃ transformation point, and then water-cooled and air cooled directly below Ar ₃ , the ferrite transformation initiation temperature. In addition, the present invention relates to a method for producing a high strength hot-rolled steel sheet having excellent stretch elongation, which is cooled by winding again to a temperature range of 400-500 ° C.

In the present invention, by weight%, C: 0.06-0.10%, Si: 0.5% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05- 0.15%, Nb: 0.02-0.04%, Mo: 0.3% or less, Remaining: When hot-rolling a slab of steel composed of Fe and other unavoidable impurities, the steel slab is finished at a temperature above Ar ₃ transformation point, and directly under Ar ₃ , the ferrite transformation temperature. The present invention relates to a method for producing a high strength hot-rolled steel sheet having excellent elongation and stretchability which is cooled by water to air and then cooled to a temperature range of 400 to 500 ° C.

Hereinafter, with reference to the accompanying drawings, the present invention will be described in more detail.

In the present invention, to increase the strength of the precipitated tempered steel based on the addition of the appropriate amount of C and Mn is adjusted, and after the hot rolling during the cooling process on the ROT (Run Out Table) and austenitic / ferrite transformation at the same time with the addition of precipitated element in the ferrite By controlling the cooling history of the hot rolled strip so that precipitation occurs, concentrating the carbon discharged from the wastelite grains into the unaffected austenite grains and winding it at low temperature, the precipitated hardened ferrite phase becomes a base, and as a second phase It was made to be a bainite phase.

The ferritic-bainite composite steel of the present invention manufactured in this way is clearly distinguished from the conventional pearlite structure of the precipitation-reinforced hot-rolled steel sheet in the second phase, and through cooling control despite being cold winding It shows excellent strength-ductility balance because the ferrite shape is equiaxed and the fraction is increased, and at the same time the cementite precipitated at the grain boundary is reduced and extremely fine, which results in extremely excellent elongation and at the same time tensile strength 70kg / It exhibits a high strength characteristic of mm ² or more.

In order for the hot rolled steel sheet of the present invention to have the above characteristics, it is preferable to form an alloy component system of the steel as described above.

C is an element necessary for obtaining the strength of the hot rolled steel sheet. Considering the workability and weldability of the hot-rolled steel sheet for processing at a tensile strength of 70-80kg / mm ^2, 0.07-0.15wt% is usually added. In the present invention, the reduction of grain boundary carbide which is detrimental to stretch elongation and cooling control after hot rolling are performed. In order to promote ferrite transformation, the range was 0.06-0.1 wt%.

Mn is an effective element for solid solution strengthening. According to the basic experimental results related to the present invention, the expected strength is not obtained at less than 14 wt%, and when it exceeds 2.0 wt%, the strength is obtained but the elongation is drastically reduced. In other words, when the Mn content is increased, the ferrite transformation start temperature during continuous cooling decreases, the ferrite transformation suppression and the bainite fraction increase during cooling, and the precipitation strengthening effect is reduced by inhibiting the precipitation of TiC and the like. It was limited in the range of -2.0 wt%.

Si has the effect of promoting the formation of the wastelite phase during the cooling after hot rolling and the C thickening to the unmodified austenite, but it essentially causes the scaled surface bonding of the hot rolled steel sheet after hot rolling. Therefore, in the present invention steel, the addition range of Si is made 0.25wt% or less in consideration of impurities which are inevitably added for the shape control of the emulsion in the steel manufacturing process. In addition, the Mo addition steel was added in an amount of 0.5 wt% or less in order to compensate for the strength and promote the waste light transformation in the case of the addition amount.

Nb is known to be precipitated during hot rolling to be effective for grain refinement of austenite and to be effective for strengthening the precipitation of ferrite. Therefore, Nb is added 0.02% or more. However, in the present invention, in the production of coaxial ferrite-bainite composite tissue steel by low temperature winding, Nb is limited to 0.04 wt% or less because it is effective in inhibiting ferrite transformation and equalization of ferrite.

Ti is an important element in the steel of the present invention, because it is precipitated as a carbide in the ferrite structure in addition to the transformation during austenate / ferrite transformation after hot rolling to contribute to the strengthening of ferrite. The change in tensile properties according to the amount of Ti added is shown in FIG. When the content of Ti is less than 0.05%, the precipitation strengthening effect cannot be sufficiently expected. On the contrary, when the addition amount is more than 0.15%, the amount of C required to form the second phase is lowered. In addition, as shown in FIG. 1, it was confirmed that the strength increase due to precipitation strengthening was saturated at 0.19 wt% Ti addition. Therefore, the Ti addition range was set to 0.05-0.15 wt%.

In the present invention, an appropriate amount of Mo can be added in addition to the above components. Mo increases the bainite fraction while hardly affecting the ferrite transformation, and thus Mo increases the strength without deteriorating the ductility. Moreover, also in the weldability of a hot rolled sheet steel, there exists an effect which suppresses softening of a heat affected zone. The Mo titration addition amount at this time is 0.3 wt% or less.

Al is an element added for grain size refinement and deoxidation of the steel, and the content thereof is preferably in the range of 0.005-0.05 wt%, which is usually added.

It is important to manage P and S as low as possible as impurities in the steel, and it is desirable to keep the content at 0.02 wt% and 0.01 wt% or less, respectively.

In the present invention, after forming the steel to satisfy the alloy component system as described above, it is hot-rolled, it is preferable to cool three stages without using the conventional shear cooling method as shown in Figure 2, the reason is as follows same.

2 schematically shows a conventional shear cooling method and a three-stage cooling method of the present invention. The three-stage cooling method includes a primary water cooling station 15 and a secondary water cooling station 8, and the temperature therebetween, The temperature and time in the temperature range 6 affect the ferrite transformation and the precipitation of carbonitrides.

C: 0.06-0.08wt%, Si: 0.15wt%, Mn: 1.4-1.8wt%, P: 0.01wt%, S: 0.04wt%, Al: 0.005% -0.05%, Nb: 0.03- When hot-rolling a steel slab of 0.04 wt% and / or Ti: 0.1 wt%, finishing at a temperature above the Ar ₃ transformation point, followed by three stage cooling and the intermediate temperature in three stage cooling when the coiling temperature is 450 ° C. The effect of ((primary cooling end temperature + secondary cooling start temperature) / 2) on tensile properties and stretch elongation is shown in comparison with the conventional shear cooling method.

As can be seen in Figure 3, in the case of the three-stage cooling of the present invention compared with the shear cooling method, the strength is hardly changed, but the elongation and elongation is improved. As the austenite / ferrite transformation progresses due to the maintenance at the intermediate temperature range, Ti carbide in the ferrite is uniformly precipitated, the shape of the ferrite is uniformly condensed, and the fraction of the ferrite phase is increased compared to conventional shear cooling, It is thought that this is because bainite is produced by the difference cooling.

4 shows a schematic diagram of a cooling system of ROT forcing water-cooling from the finishing temperature (1) after the hot rolling to the winding temperature (2) in the hot rolling process and the cooling thermal history of the hot rolled steel sheet. After hot rolling, the cooling on the ROT (3) depends on the length of the stand and the plate speed, and the residence time in the stand is determined, and the cooling temperature at this time depends on its cooling capacity. In the three-stage cooling method as in the present invention, any one of the cooling zones 4 composed of a plurality of water cooling banks on the ROT is designated as the pivot bank 7 so that the primary water cooling zone 5 and the second water cooling zone ( Water cooling is performed in 8). As a result, the holding time at the intermediate temperature and its temperature range is determined by the setting of the pivot bank position and the intermediate temperature.

In the present invention, the three-stage cooling method implemented as described above, when applied, hot-rolled steel slab to finish at a temperature above the Ar ₃ transformation point, and then starts the first water cooling in the water-cooled bank on the run-out table to start the waste light transformation. Cool to just below the temperature of Ar ₃ transformation point. Subsequently, it is preferable to start the secondary water cooling after winding the waste light transformation to proceed 70-80%, and to wind it in the temperature range of 400-500 ° C. The reason is as follows.

If the primary water cooling end temperature is above the Ar ₃ transformation point, the amount of untransformed austenite increases and the amount of bainite in the final structure increases.

Subsequently, air cooling is performed. The time (6) from the temperature directly below the Ar ₃ transformation point, which is the first water cooling completion temperature, to the temperature of the second water cooling start varies depending on the plate speed of the hot rolled strip. Preference is given to 4-8 seconds running 70-80%. If the holding time in this intermediate temperature range is shortened below, ferrite transformation becomes insufficient, and precipitation of Ti or Mo in the transformed ferrite grains and C enrichment of untransformed onstenite become insufficient.

After the air cooling is completed, the second water cooling is started. At this time, the second water cooling should be started when the above-mentioned waste light transformation is 70-80% completed. Otherwise, the shape of the ferrite becomes needle and the ductility decreases. As a result, a target equiaxed ferrite-bainite structure cannot be obtained.

After performing the second water cooling, it is preferable to odor in the temperature range of 400-500 ° C. The reason for this is that when the coiling temperature is higher than 500 ° C, precipitation and grain boundary cementite become coarse, so the elongation elongation is deteriorated. In the temperature range of 400 ° C. or less, hard phase low-temperature transformation products including martensite structure are formed to increase strength but decrease elongation and yield.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Hot rolled slabs of the inventive steels (2, 4, 5) and comparative steels (1, 3, 6), as shown in Table 5, finished at 840 ° C, and then up to 700 ° C, which is directly below the start temperature of ferrite transformation. After performing water cooling, after performing air cooling for 4-8 seconds, secondary water cooling was started at 650 degreeC, and it wound up at the temperature like Table 2 below.

Tensile characteristics and stretch elongation characteristics of the hot rolled steel sheets thus obtained were investigated, and the results are shown in Table 2 below.

At this time, the tensile test was performed by collecting a JIS No. 5 tensile test piece in the C direction. The hole enlargement rate is 120mm x 120mm, and the punched hole with the diameter of 18mmφ is processed and pushed up with a shoe punch with a diameter of 50mm at the bottom to measure the diameter Df when the crack penetrates the thickness of the steel sheet. Stretch elongation was evaluated by a calculation formula.

Hole Magnification (%) = (Do-Df) / Do * 100

Table 1

TABLE 2

In the invention in Table 2, the tensile strength is basically 70kg / mm ² or more, the tensile strength-elongation balance is 1350kg / mm ^2- % or more (O mark), the tensile strength-elongation balance is 8050kg / mm ^2- % It is a case where these three conditions are satisfied simultaneously by the above (O display).

As can be seen in Table 2, in the case of the invention satisfying the scope of the present invention (7, 16, 19-21), any of the tensile strength is 70kg / mm ² or more and elongation is 19% or more, elongation elongation More than 110%. In the invention examples (16) and the invention examples (19-21) in which a small amount of Mo was added, the tensile strength-elongation and the tensile strength-extension elongation balance were satisfied, and the hardness softening of the CO ₂ gas weld was MO. It was confirmed that it was small compared with no addition steel (for example, invention example (7)). In addition, as can be seen in Figure 5 showing a typical precipitate distribution of the invention, it was confirmed that Ti or Ti-Mo carbide was uniformly deposited on the ferrite matrix, and the second phase was composed of bainite structure.

On the other hand, in Comparative Example Comparative Example (1-4) is a case where the Ti addition amount is low, the tensile strength is not secured, Comparative Example (11-13) meets the above balance condition, but 0.1 wt% Ti addition steel In contrast, when the increase in strength is saturated, the upper limit of Ti addition is shown. In the comparative example, the CT upper limit and the lower limit are out of the elongation or elongation balance range due to the microstructure and mechanical properties obtained at the winding temperatures of the respective conditions. In other words, the upper CT is a ferrite-perilite steel, and the elongation is reduced by coarse cementite precipitated at the grain boundary, and the lower CT is lowered in the elongation balance due to a decrease in elongation due to an increase in strength. In addition, Comparative Example (24-27) is a case of ferritic and martensitic composite steels precipitated and strengthened by TiC, but the tensile strength-elongation balance is excellent, but the tensile strength-elongation balance is significantly lower than that of the present invention steel. You can see that.

As described above, the present invention provides a high-strength hot-rolled steel sheet having a tensile strength of 70kg / mm ² or more combines the characteristics of the conventional precipitation-reinforced steel and transformation tissue-reinforced steel by appropriately controlling the steel component and its cooling pattern during hot rolling. can do. In addition, the hot rolled steel sheet obtained by the method of the present invention has high strength and good tensile strength-extension elongation balance, and excellent elongation property evaluated by hole expansion ratio, which is suitable for using chassis parts and strength members such as wheel discs of automobiles. It works.

1 is a graph showing the change of mechanical properties according to the change of Ti content

2 is a conceptual diagram of the conventional shear cooling method and the three-stage cooling method of the present invention

3 is a graph comparing the tensile properties of the three-stage cooling method of the present invention and the conventional shear cooling method

4 is a cooling thermal history curve in the three-stage cooling method to which the method of the present invention is applied.

5 is a photograph showing the distribution of Ti oxide in the ferrite structure in the ferrite-bainite composite steel according to the present invention.

Explanation of symbols on the main parts of the drawings

1 ‥‥‥ Hot Rolling Finishing Temperature

2 ‥‥‥ Winding temperature

3 ‥‥‥ Run out table (ROT)

4 ‥‥‥ Cooling table consisting of multiple water cooling banks

5 ‥‥‥ Primary water cooling zone in the three-stage cooling of the present invention

6 ‥‥‥ Intermediate temperature range / holding time in three-stage cooling of the present invention

7 ‥‥‥ Pivot Bank in 3-stage cooling of the present invention

8 ‥‥‥ Secondary water cooling zone in the three-stage cooling of the present invention

Claims (6)

By weight%, C: 0.06-0.10%, Si: 0.25% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05-0.15%, Nb: 0.02-0.04%, balance: Fe and other unavoidable impurities, high-strength hot-rolled steel sheet having excellent elongation property, which is a ferrite-bainite precipitated with Ti and Nb carbides By weight%, C: 0.06-0.10%, Si: 0.5% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05-0.15%, Nb: 0.02-0.04%, Mo: 0.3% or less, balance: Fe and other unavoidable impurities, and its structure is a high strength with excellent elongation property, which is a ferrite-bainite in which Ti-Mo composite carbide and Nb carbide are precipitated Hot rolled steel sheet By weight%, C: 0.06-0.10%, Si: 0.25% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05-0.15%, Nb: 0.02-0.04%, remainder: In hot rolling a slab of steel composed of Fe and other unavoidable impurities, finishing at a temperature above the Ar ₃ transformation point, and then water-cooled directly below Ar ₃ , the ferrite transformation initiation temperature, and the ferrite transformation is 70-80. Method of producing a high strength hot rolled steel sheet having excellent elongation property after cooling by air to proceed by%, then water-cooled to 400-500 ° C. again The method of claim 3, The air cooling is a method for producing a high strength hot rolled steel sheet having excellent stretch elongation, characterized in that performed for 4-8 seconds By weight%, C: 0.06-0.10%, Si: 0.5% or less, Mn: 1.4-2.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.005-0.05%, Ti: 0.05-0.15%, Nb: 0.02-0.04%, Mo: 0.3% or less, remainder: When hot-rolling a slab of steel composed of Fe and other unavoidable impurities, the steel slab is finished at a temperature above the Ar ₃ transformation point, and then water-cooled directly below Ar ₃ , the ferrite transformation start temperature. Method for producing a high strength hot-rolled steel sheet having excellent elongation property, which is cooled by air to allow the ferrite transformation to progress 70-80%, and then cooled by water to 400-500 ° C. The method of claim 5, The air cooling is a method for producing a high strength hot rolled steel sheet having excellent stretch elongation, characterized in that performed for 4-8 seconds