KR101360525B1 - Joint of high manganese steel-common steel and method for manufacturing high manganese steel - Google Patents

Abstract

In the method of manufacturing a high manganese steel-general steel joint, which is rolled by joining with the general steel during continuous rolling, the step of welding the filler wire eccentric 0.3 to 0.5mm in the direction of the general steel during welding, and the total reduction ratio 40 to 60% A high manganese steel manufacturing method comprising the step of cold rolling furnace and a high manganese steel-general steel joint having a steepness of 2 mm or less are provided.
According to the present invention, it is possible to provide a high manganese high strength steel sheet excellent in formability and impact characteristics, and can contribute to vehicle body weight reduction and safety improvement.

Description

JOINT OF HIGH MANGANESE STEEL-COMMON STEEL AND METHOD FOR MANUFACTURING HIGH MANGANESE STEEL}

The present invention relates to a high manganese steel-general steel joint and a high manganese steel manufacturing method.

Recently, the use of high-strength steel sheet is gradually increasing in terms of improving fuel efficiency and securing safety, and high-manganese austenitic steel grades having excellent formability and strength have been developed. When applied as a steel grade that can greatly contribute to the weight reduction of the automobile body is very excellent in absorbing collision energy. However, this type of steel is mainly used for high manganese steel. If the structure is austenitic single phase steel and the welded part is connected to other steel types during continuous rolling, the welded structure changes and the breakage of the plate during rolling is not only concerned. Plate breakage often occurs due to misalignment of the rolling load and tension, resulting in a decrease in the error rate.

However, in order to improve the productivity of rolling, it is inevitable to join continuous rolling by joining high manganese steel whose main structure is austenite and ordinary steel having a different structure.

Therefore, when joining with the general steel, defects such as plate breakage may occur at the joints as mentioned above, and the present invention has been devised to solve such problems.

One aspect of the present invention is to propose a method to stably produce a high manganese high strength steel sheet by significantly improving the characteristics of high-manganese steel-general steel joints and drastically reduce the occurrence of sheet breakage during cold rolling.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention is a method of manufacturing a high manganese steel-general steel joints that are rolled by joining with ordinary steel during continuous rolling, the filler wire eccentricity in the direction of the general steel 0.3 to 0.5 It provides a high-manganese steel manufacturing method comprising the step of welding by moving the mm, and cold rolling at a total reduction of 40 to 60%.

Another aspect of the present invention is a high manganese steel-general steel joint, which is rolled by joining with ordinary steel during continuous rolling, and has a steepness of 2 mm or less.

According to an aspect of the present invention, it is possible to provide a high manganese high strength steel sheet excellent in formability and impact characteristics, it can be applied to a variety of automotive steel sheet for contributing to the weight reduction and safety of the vehicle body.

1 is a graph showing the change in roll reduction ratio between the stand and the appropriate ratio reduction ratio distribution according to an embodiment of the present invention.
2 is a photograph showing a joint (welding part) structure according to one embodiment (b) and one comparative example (a) of the present invention.

Hereinafter, a high manganese steel manufacturing method and a high manganese steel-general steel joint of the present invention to be easily implemented by those skilled in the art will be described in detail.

At the time of hot rolling, it is preferable to carry out by joining continuously. However, since the ratio of high manganese steel is not high, it may inevitably be carried out by joining with ordinary steel. At this time, there is no problem when the high manganese steel is bonded to each other, the above-mentioned problem occurs when the high-manganese steel and the joint rolling of the general steel with a different structure structure.

To this end, the present invention proposes a high manganese steel manufacturing method and a high manganese steel-general steel joint designed to be suitable for continuous rolling.

The general steel as used herein generally refers to steel having a carbon content of 0.1% or less (excluding 0%) and a manganese content of 3% or less (excluding 0%), but is not limited thereto.

To this end, one aspect of the present invention, in the method of manufacturing a high manganese steel-general steel joints that are rolled by joining with ordinary steel during continuous rolling, the welding of the filler wire eccentric 0.3-0.5mm in the direction of the general steel during welding It provides a high manganese steel manufacturing method comprising the step, and cold rolling at a total reduction of 40 to 60%.

In other words, cold rolling is performed to match the shape and thickness of the hot rolled steel sheet. If the reduction ratio is less than 40%, the rolling roll and tension control may be inaccurate, resulting in twisting of the plate and exceeding 60%. It is not possible to produce high strength steel under load of rolling rolls. In particular, laser welding using a filler wire is performed to improve the characteristics of high manganese steel-general steel joint in continuous cold rolling.

At this time, it is very preferable that the junction microstructure consists of austenite single phase. When the general welding is carried out, the martensite is formed in the joints, and thus the strength of the joints increases rapidly and acts as a factor of fracture. This is because the possibility of fracture is very high due to martensite formation. For this purpose, when welding using a filler wire, it is preferable to move the wire position by 0.3 to 0.5 mm in the direction of general steel to weld. The reason is that the ferrite structure of ordinary steel is austenitized during welding by using a filler wire having an austenite structure. That is, the movement within 0.3mm has no effect, and when exceeding 0.5mm, the joint austenite single phase cannot be obtained.

Figure 2 shows a junction picture, when comparing the Comparative Example (a) and the invention example (b) it can be seen that in the case of the invention example the junction structure is all made of austenite single phase without martensite formation. Comparative Example (a) and Inventive Example (b) of FIG. 2 are the junction picture photographs of Comparative Example 1 and Inventive Example 1 of Table 2, respectively.

In an exemplary embodiment, the high manganese steel is weight%, C: 0.1 ~ 1.0%, Mn: 8-25%, Al: 3.0% or less, Si: 2.0% or less, P: 0.1% or less, S: 0.02 % Or less, N: 0.04% or less, Ti: 0.01% to 0.2%, Ni: 1.0% or less, B: 0.0005 to 0.006%, balance Fe and other inevitable impurities, but are not limited thereto.

That is, the present invention is optimized to include the austenite stabilizing elements Mn, C, Al, etc. in order to ensure a complete austenite phase at room temperature, the steel sheet is deformed to secure excellent elongation and delayed fracture resistance If appropriate, an appropriate amount of twins was formed. Twin formation is closely related to stacking fault energy (SFE), which depends on the composition of the steel. In the present invention, the lamination defect energy was controlled at a level of 30-40 mJ / m ² , and an appropriate amount of C and Al was added thereto. In addition, Ni and the like were added to prevent the deterioration of the delayed fracture characteristics due to deterioration of the austenite structure due to desorption of surface elements such as decarburization and demanganization in the steel sheet manufacturing process, resulting in transformation into martensite. In the steel sheet manufacturing process, cracking of the slab and hot rolling rolling due to AlN precipitation in steel can be suppressed and N in Ti can be completely precipitated with TiN so that grain boundary embrittlement by P does not occur due to the decrease of solid solution B due to BN precipitation. Ti was added to make it possible.

The component system of this invention based on the said content is demonstrated in detail below. However,% of the component system below means weight%.

C is an element that increases stabilization and stacking defect energy of the austenite phase. When the amount of C added is less than 0.1%, even though the Mn content is increased, stable austenite formation at room temperature is difficult, and since the α 'martensite phase is formed during deformation, cracks occur during processing and ductility is lowered. In order to lower the C, it is necessary to sufficiently add C to 0.1% or more. On the other hand, when the amount of C exceeds 1.0%, the lamination defect energy is excessively increased to 40 mJ / m ² or more, resulting in deformation behavior due to slip deformation rather than twin formation during deformation. Therefore, the amount of C added is limited to 0.1 ~ 1.0%.

Mn is also an essential element for stabilizing the austenite phase, but at less than 8%, the α 'martensite phase is formed which impairs the formability, thereby increasing the strength but decreasing the ductility drastically. On the other hand, when the amount of Mn added exceeds 25%, when added with C, Al, etc., lamination defect energy exceeds 40 mJ / m ² , twin formation is suppressed and slip deformation occurs preferentially, thereby reducing ductility. In addition, as the amount of Mn is increased, internal grain boundary oxidation occurs excessively when the slab is reheated, causing oxide defects on the surface of the steel sheet. In addition to the increase in alloy cost, the upper limit of the amount of Mn is limited to 25% due to inferior surface characteristics of the plated product. .

Al is usually added for deoxidation of steel, but in the present invention, Al is added to control the lamination defect energy and to improve the delayed fracture characteristics. In the present invention, Al increases the stacking fault energy of the steel even at a lower Mn content than the conventional austenitic high manganese steel, thereby suppressing the formation of the ε-martensite phase and improving the ductility by slowly increasing the double precision during deformation. Let's do it. When the content of Al exceeds 3.0%, the ferrite is formed in the structure, the ductility is lowered, and problems such as deterioration of castability during continuous casting, deepening of surface oxidation during hot rolling, etc. may cause a sharp drop in surface quality. Limits Al to 3.0% or less (excluding 0%).

Si is a solid solution strengthening element, the grain size can be reduced by the solid solution effect can increase the yield strength. However, when too much Si is added, Si oxide is formed on the surface of the steel sheet, which is not good for improving the melt plating property. However, in the high manganese-type steel sheet as in the present invention, when a proper amount of Si is added, a thin silicon oxide layer is formed on the surface of the steel sheet, so that the oxidation of Mn can be suppressed and the thick manganese oxide layer formed after rolling in the cold rolled steel sheet can be suppressed. However, excessive Si is concentrated on the surface of the steel sheet during high temperature annealing in the continuous annealing process and the continuous hot dip plating process, which can reduce the wettability of the molten zinc on the surface of the steel sheet during hot dip plating. You can't. In addition, since it may adversely affect weldability, in the present invention, Si is limited to 2.0% or less (excluding 0%).

P is an element that is inevitably contained in the manufacturing process, and the quality of the crack is reduced, and thus the addition range is limited to 0.1% or less.

S is an element that is inevitably contained in manufacturing, and may form coarse manganese sulfide (MnS), which may cause defects such as flange cracks, and it is preferable to suppress the addition amount as much as possible because it reduces the hole expandability of the steel sheet. Therefore, the addition amount is limited to 0.02% or less.

N is an element that acts with aluminum or titanium in the austenite grains to precipitate fine nitrides to promote twin formation. In general, twin formation is formed around the nucleus during deformation, and the nitride formed therein acts as a twin nucleus to help the twins to be generated, thereby improving the elongation. It also contributes to the stabilization of austenite. In this context, when N is added to improve the strength and ductility in forming the steel sheet, when the added amount is excessively more than 0.04%, the nitride may be excessively precipitated and the hot workability and elongation may be reduced, so the amount of nitrogen added is not more than 0.04% Limited to 0%.

Ti is a strong carbide forming element that combines with C to form carbide and functions as an important element in the present invention. The formed carbide is an element effective for suppressing the growth of crystal grains and refining the grain size. In addition, it plays a role of reducing the solid solution N by preferentially forming high temperature precipitates of TiN in the casting structure in the casting process, thereby significantly reducing the occurrence of slab cracks and hot-rolled edge cracks caused by grain boundary AlN and BN precipitation. It can act. In addition, there is a high tendency to precipitate P, which is likely to segregate the grain boundary, into FeTiP, thereby preventing grain boundary embrittlement caused by P. As the Ti content is increased, the above-mentioned effect is increased, but when it exceeds 0.2%, manufacturing problems such as clogging of the playing nozzles occur and the cost of ferroalloy increases, so the content is limited to 0.01 to 0.2%.

Ni is an element that contributes to austenite stabilization and is not only advantageous for improving elongation, but also, among other things, effectively contributing to high temperature ductility. As the added content increases, the effect of preventing delayed fracture and slab cracking is also great, but the production cost is high due to the high material cost, so the content is limited to 1.0% or less (excluding 0%).

B is added in the present invention steel to suppress embrittlement and grain boundary corrosion due to P grain boundary segregation. P is preferentially segregated in the grain boundary and causes grain boundary brittleness. By adding B, it is an element that improves high temperature toughness by suppressing grain boundary segregation of P in casting structure by the effect of site competition. If the content is less than 0.0005%, the effect is almost insignificant, and when the content exceeds 0.006%, the ductility decreases rapidly. Therefore, the content is limited to 0.0005 to 0.006%.

In an exemplary embodiment, the cold rolling is made of a multi-stage continuous rolling, the multi-stage continuous rolling is the first step reduction rate: 20-25%, the second step reduction rate: 13 ~ 19%, the third step after the rolling Rate: It is less than the reduction rate of the previous step, and may be rolled by gradually reducing the reduction rate, the thickness after the second step rolling may have a reduction ratio pattern of 65% or more of the initial thickness, but is not limited thereto.

1 shows an embodiment of the present invention, but consists of six stages, it can be cold rolled through three or more multi-stage continuous rolling.

The cold rolling is applied in the form of shearing and post-weaking in the rolling reduction, because the characteristics of the steel are high in the work hardening rate, so that the breakage and shape are not very good due to the high work hardening rate. According to the analysis results, it is important to limit the initial stage 1 reduction rate to 20-25% and the second stage reduction rate to 13-19%, and to ensure the thickness after the 2nd stage more than 65% of the initial thickness. The reason for setting the first and second reduction ratios is that shear rolling is not performed when rolling out of the respective ranges or rolling out of the maximum rolling load. If the thickness is less than 65% after the second step, shear reduction is not performed, and thus the reduction thickness is limited because the fracture and shape due to the rear drop are not good.

Figure 1 shows the change in the roll reduction ratio between the stand according to an embodiment of the present invention shows an appropriate level of the reduction ratio distribution that can be rolled without breaking during ideal rolling. In the first stage, the reduction rate is recorded between 20% and 25%, but it can be seen that the reduction rate is gradually applied.

As a result, the steepness of the joint after cold rolling is limited to 2 mm or less, because when it is exceeded, there is a problem with the shape and causes winding failure. The annealing temperature needs to be carried out at 700 ° C. or higher, because when the annealing temperature is too low, it is difficult to secure sufficient processability and transformation into austenite does not occur sufficiently to maintain the austenite phase at low temperatures.

That is, the hot rolling or ingot slab heating temperature is limited to 1050 ~ 1300 ℃ during hot rolling. This is because if the heating temperature exceeds 1300 ℃ liquid film is formed at the columnar grain boundary of the slab is likely to cause cracks in the slab, and if less than 1050 ℃ the slab is not sufficiently reheated, the inclusion segregation occurs to affect the material.

As described above, any of the above-described high manganese steel can be used as long as it is a hot rolled steel sheet. Accordingly, the hot rolling method of high manganese steel is not particularly limited in the present invention.

However, more advantageous effects can be obtained when hot-manganese steel is hot rolled under the following conditions.

Finishing hot rolling is performed in the range of 850 to 1000 ° C. for the steel slab in which the homogenization treatment is performed by heating. When the finish rolling temperature is lowered below 850 ° C., the rolling load is increased, which may not only cause the rolling mill but also reduce the quality inside the steel sheet. On the other hand, when the rolling finish temperature is excessively high, surface oxidation may occur during rolling, so the rolling finish temperature is limited to 850 to 1000 ° C. If the finish rolling temperature is lowered below 850 ℃ the rolling load is high due to the excessive rolling mill and if the temperature exceeds 1000 ℃ may cause oxidation on the coil surface, so the finish temperature of rolling is limited to 850 ~ 1000 ℃.

After finishing hot rolling, the steel sheet is hot rolled at a temperature of 700 ° C or lower. When the temperature of the hot rolled coil exceeds 700 ℃, since the thick oxide film and internal oxidation may occur on the surface of the hot rolled steel sheet, it is not easy to remove the oxide layer during the pickling process. Therefore, the winding temperature of the hot rolled steel sheet is preferably lowered to 700 ° C or less.

Another aspect of the present invention is a high manganese steel-general steel joint, which is rolled by joining with ordinary steel during continuous rolling, and has a steepness of 2 mm or less.

Limiting the steepness of the joint to 2mm or less is as described above.

Here, the 'stiffness' means the height in the width direction of the coil.

The high manganese steel having the above component system is connected through a joint (eg, a weld) with general steel, and the steepness of the coil is preferably 2 mm or less.

The junction refers to a portion joined by welding or the like to connect the high manganese steel of the present invention and general steel such as TRIP steel or DP steel. The steepness of the coil is limited to 2 mm or less, because if it is exceeded, there is a problem with the shape, causing a winding failure.

In this case, the 'coil steepness' refers to the height of the coil in the width direction.

In an exemplary embodiment, the junction tissue is composed of austenite, and the content may be 98% or more, but is not limited thereto.

At this time, the junction microstructure is more preferably made of austenite single phase. When the general welding is performed, the martensite is formed in the joints, and thus the strength of the joints is rapidly increased and acts as a factor of fracture. In the present invention, the austenite fraction in the joints is secured by 98% or more. If the austenite fraction is less than 98%, the likelihood of breaking due to martensite formation becomes very high.

In an exemplary embodiment, the average size of the grain size of the junction may be 20 μm or less, but is not limited thereto.

The reason for limiting the average grain size of the joint to 20 μm or less is that as the grain size increases, the joint hardness is much lower than the strength of the connecting material (high manganese steel / general steel), thereby increasing the probability of fracture.

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are only for explaining an example of the present invention through experiments, and the scope of the present invention is not limited through the following examples.

[ Example ]

Table 1 shows the composition of the invention steel, and the hot-rolled steel sheet was manufactured by maintaining the ingot of each composition in a heating furnace at 1200 ° C. for one hour and then hot rolling finish temperature at 940 ° C. and winding temperature at 550 ° C.

division C Mn Al Si P S N Ti Ni B Invention river 0.72 17.0 1.9 0.02 0.008 0.008 0.007 0.11 0.03 0.0015

After the pickling treatment of the prepared hot-rolled steel sheet was subjected to cold rolling by applying a cold reduction rate of about 50% using the conditions as shown in Table 2 for the welding conditions and distribution of the dissimilar connector. The cold rolled steel sheet was annealed to analyze the material.

division Annealed Steel Sheet Filler wire hetero welding
(General Steel / High Manganese Steel) Reduction rate (%) Steepness (mm)
Standard: 2 mm or less
And characteristics TS
(MPa) T-El
(%) Eccentric
(General steel direction mm) Austenitic fraction
(%) Grain
(탆) One
step 2
step 3 ~ 6
step Inventory 1 990 65 0.3 99 18 22 15 13 1.5 normal Inventive Example 2 985 65 0.3 99 17 23 14 13 1.6 normal Inventory 3 987 58 0.3 99 18 21 16 13 1.8 normal Comparative Example 1 985 63 0.3 99 18 * 5 * 12 33 * 5 Fracture Comparative Example 2 988 61 0.4 98 19 * 7 * 12 30 * 6 Fracture Comparative Example 3 996 61 0.4 98 19 * 12 * 12 26 *4 Fracture Honorable 4 998 62 0.4 98 18 23 18 9 1.8 normal Comparative Example 4 997 63 0.2 * 76 19 20 15 15 * 6 Fracture Comparative Example 5 1002 56 0.5 * 72 19 21 15 14 * 6 Fracture

In Table 2, those that do not meet the criteria of the present invention are marked with * and the items corresponding to the invention examples do not cause fracture during rolling, and the shape and steepness meet the criteria of the present invention. In the comparative example, when welding dissimilar connectors, the degree of eccentricity does not match toward the general steel, or when the reduction ratio is inappropriate, a problem of fracture or poor shape is generated.

(In Table 2, TS represents tensile strength, T-El represents elongation, A represents austenite, and M represents martensite.)

Claims (8)

In the method of manufacturing a high manganese steel-general steel joint which is rolled by joining with ordinary steel during continuous rolling,
Welding by moving the filler wire eccentric 0.3-0.5mm in the direction of the general steel during bonding; And
A method for manufacturing high manganese steel, comprising the step of cold rolling at a total reduction ratio of 40 to 60%. The method of claim 1,
The high manganese steel is weight%, C: 0.1 ~ 1.0%, Mn: 8-25%, Al: 3.0% or less (excluding 0%), Si: 2.0% or less (excluding 0%), P: 0.1% S: 0.02% or less, N: 0.04% or less (excluding 0%), Ti: 0.01% to 0.2%, Ni: 1.0% or less (excluding 0%), B: 0.0005 to 0.006%, balance Fe and others High manganese steel production method, which is made of inevitable impurities. The method of claim 1,
The cold rolling is made of a multi-stage continuous rolling,
The multi-stage continuous rolling is the first step reduction rate: 20 ~ 25%, the second step reduction rate: 13 ~ 19%, the third step reduction rate: less than the previous step reduction rate, rolling reduction by gradually reducing the reduction rate,
The manufacturing method of high manganese steel which has a rolling reduction pattern whose thickness after a 2nd step rolling becomes 65% or more of initial thickness. The method of claim 1,
The steepness of the joined part after cold rolling is 2 mm or less, The manufacturing method of high manganese steel. High manganese steel-general steel joint that is rolled by joining with ordinary steel during continuous rolling
High manganese-steel joint with steepness of less than 2mm. 6. The method of claim 5,
The junction structure is composed of austenitic, the content of which is more than 98%, high manganese steel-general steel junction. 6. The method of claim 5,
A high manganese steel-general steel joint, wherein the average grain size of the joint is 20 μm or less. 6. The method of claim 5,
The high manganese steel is weight%, C: 0.1 ~ 1.0%, Mn: 8-25%, Al: 3.0% or less (excluding 0%), Si: 2.0% or less (excluding 0%), P: 0.1% S: 0.02% or less, N: 0.04% or less (excluding 0%), Ti: 0.01% to 0.2%, Ni: 1.0% or less (excluding 0%), B: 0.0005 to 0.006%, balance Fe and others High manganese steel-general steel joint, consisting of inevitable impurities.

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