KR101277874B1 - Hot forming parts having strength distribution and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a hot formed article and a method of manufacturing the same by imparting a temperature difference through partial cooling in the process of heating the blank before hot forming, thereby obtaining different strength and physical properties of the hot formed article.
A first heat treatment step of heating the entire hot forming blank above the Ac3 transformation point;
A local cooling step of cooling one or more portions of the first heat-treated blank below an Ar3 transformation point;
Hot forming the locally cooled blank to produce a molded article; And
It provides a method for producing a hot press molded article having a heterogeneous strength region and a hot molded article having a heterogeneous strength region, the step of cooling the molded article to a martensite transformation start temperature (Ms) or less.

Description

Hot-formed product having heterogeneous strength region and manufacturing method therefor {HOT FORMING PARTS HAVING STRENGTH DISTRIBUTION AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a method for manufacturing a hot molded article used in automobile parts and the like, and more particularly, to a hot molded article having a heterogeneous strength region produced by applying a temperature difference to a blank before hot forming, and a method of manufacturing the same.

Recently, each automobile manufacturer has been increasing the use of high-strength materials in order to respond to the social demands for eco-friendly fuel economy and lightweight in applying parts to automobiles. However, the molding of high-strength material has problems such as spring back and dimensional freezing, and the reality is that its use is limited due to the difficulty of molding.

This molding problem can be solved by molding a material at a high temperature with good moldability and simultaneously quenching it in a mold to manufacture a high strength component. This method is called a hot press molding process. According to such a process, the component which has the intensity | strength of 1500 MPa can be formed normally.

However, through the hot press molding process, parts having only a single strength have a low degree of freedom in terms of design to satisfy the required performance such as crash performance, and to solve this problem, welded blanks (TWB) are frequently used in room temperature molding. Hot press molding technology incorporating) technology has been developed. However, this TWB method has a disadvantage in that a blank welding process is added, and since the integrity of the weld can affect the performance of the part, there are many difficulties in process management.

Therefore, there is a need for a method of manufacturing a hot-formed product that is practically applicable and obtains a desired strength for each region in a complicated shape.

One aspect of the present invention is to provide a hot molded article and a method of manufacturing the same by imparting a temperature difference through local cooling in the process of heating the blank before hot forming, to obtain different strength and physical properties of the hot molded article for each region. .

The present invention comprises a primary heat treatment step of heating the entire hot forming blanks above the Ac3 transformation point;

A local cooling step of cooling one or more portions of the first heat-treated blank below an Ar3 transformation point;

Hot forming the locally cooled blank to produce a molded article; And

It provides a method for producing a hot molded article having a heterogeneous strength region comprising the step of cooling the molded article below the martensite transformation start temperature (Ms).

In addition, the present invention is a hot-formed product manufactured by heating, molding and quenching,

Provided are hot formed articles having heterogeneous strength regions in which the strength of one or more portions of the hot press formed article is different from the other portions.

According to the present invention, during heating of a blank for producing a hot-formed product, by applying a temperature difference to the blank through a heat treatment of heating the entire blank and then cooling a part, a region having different strengths and physical properties is one part It can be applied to light weight of vehicle and optimization of crash performance. In addition, in the case of the hot forming process to which the conventional TWB is applied, the complexity of the process to which the welding process is added and the integrity of the welding part may affect the performance of the part, thereby solving the problems of many difficulties in terms of process management, and the cooling rate. Since it is possible to manufacture a part having a complex shape without the need to control the desired strength has the effect of easy process design.

1 is a graph showing the concept of the method of the present invention.
Figure 2 is an example of a hot molded article having a different intensity distribution for each part produced by the method of the present invention.
3 is a graph showing a change in stress with respect to the amount of deformation of the hot molded article of FIG.
Figure 4 is a graph showing the change in physical properties with the temperature of various local cooling.

Hot forming material is austenitized, then molded and cooled to obtain different physical properties. When cooled at a sufficient cooling rate above the critical cooling rate, martensitic structures are formed to obtain sufficient strength. Under slow cooling rates, ferrite, pearlite, and bainite are formed, so that the strength is relatively low, but the ductility is improved. Can be.

In the case of using such a property, it is not easy to implement different cooling rates after heating to the same temperature in order to secure different strengths and the like in one part, and therefore, the present inventors have a step in heating the blank before hot forming. The present invention has been realized by setting a temperature gradient on the blank and recognizing that it can have a different cooling rate even when cooling at the same time.

First, the manufacturing method of this invention is demonstrated in detail.

In the present invention, first, a hot forming blank is prepared, and a first heat treatment for heating the whole hot forming blank is performed. In the primary heat treatment, it is preferable to heat the temperature of the blank, which is a steel material, above the Ac3 transformation point.

The heating above the Ac3 transformation point is to obtain a high strength molded article by fully utilizing the martensite transformation from austenite in subsequent hot forming and mold cooling. Thereafter, as described later, the locally cooled portion can obtain low strength. In more detail, the hot-formed portion is strengthened by martensite transformation after the high temperature is maintained in the blank heated above the Ac3 transformation point, and the locally cooled portion is ferrite and perlite, depending on any temperature below Ar3, or A composite structure containing ferrite and pearlite and bainite, or a mixture of ferrite and martensite is formed, so that relatively low strength or desired strength can be formed as compared with the region where martensite tissue is formed.

The primary heating method is not particularly limited, and various methods such as heating by an induction furnace and induction heating may be applied. For example, heating by a heating furnace loads and heats a blank in the heating furnace heated more than Ac3 transformation point. At this time, in consideration of economical efficiency, it is preferable to heat within 180 seconds, and the average heating rate at this time is preferably 5 ° C / s or more.

It is more preferable to maintain the heated blank for a certain time. The reason is to ensure uniformity of the temperature in the blank, to promote the diffusion of carbon (C) in the blank material, and to secure the increase in strength and uniformity when forming and cooling after some cooling. The holding time varies depending on the steel grade of the blank material and the size of the blank. A preferred example of the holding time is 30 seconds or more, and when the holding time is more than 10 minutes, it is not preferable in terms of performance deterioration due to growth of austenite grains and in terms of efficiency of equipment and energy of the process. Do not exceed

Local cooling is performed to cool one or two or more portions of the primary heated blank below the Ar3 transformation point. The primary heated blank is divided into a plurality of parts according to the required physical properties, and local cooling is performed on one or two or more of them. The compartment depends on the design of the final molded article, which may be before or after the primary heating. This cooled portion is sufficiently transformed and then cooled after hot forming to include a softer structure than the martensite structure, for example, ferrite, pearlite, bainite or a mixture thereof, so that relatively low tensile strength Will have. Ductility has the property of improving.

It does not specifically limit the said method of local cooling, It is preferable to perform the said cooling rate at the cooling rate of 40-100 degreeC / s. When the cooling rate is cooled at a cooling rate of less than 40 ℃ / s, the temperature transition region is too wide between the portion to be cooled and the portion where the high temperature is maintained, thereby increasing the possibility of causing unnecessary supercooling at the portion where the high temperature is maintained. This is because if the cooling exceeds 100 ° C / s, the cooling temperature control becomes unstable and the subcooling area is widened in some cases, so the temperature range is preferably 40 to 100 ° C / s.

The local cooling is preferably cooled to a temperature range below the Ar3 transformation point and above the martensite transformation start temperature (Ms). The reason for cooling below Ar3 is that if the temperature above Ar3 is cooled, the austenite structure is maintained so that martensite transformation can proceed during subsequent hot forming and mold cabinets, and it is impossible to form products with different strength distributions. When cooled, the martensite structure is formed prior to hot forming, thereby reducing the ductility and adversely affecting the formability, and cannot form different strength distributions in the final product.

On the other hand, it is preferable to hold the locally cooled blank for 25 seconds or longer. The blank at this time includes both the portion having Ar 3 or more and the portion having an arbitrary temperature in the range of less than Ar 3 to Ms depending on the local cooling amount. The reason for maintaining the above 25 seconds is not preferable because it is sufficient for more than 25 seconds to cause a sufficient transformation in the cooled portion, and if it does not exceed this, subsequent hot forming is not performed to obtain a low strength structure.

The heat treatment for the local cooling may use a separate device. The apparatus preferably includes a portion of the first heat-treated blank that maintains the first heat treatment temperature, that is, cooling means capable of cooling the temperature of the portion to be cooled and the temperature of the portion to be cooled.

The whole of the locally cooled blank is hot formed and cooled. Through the hot forming it is possible to secure the shape of the desired part, the portion at which the temperature of more than Ar3 is maintained in the blank has a martensite structure through the cooling after hot forming to secure a high strength, the locally cooled portion is ferrite The strength is relatively low, including pearlite, bainite or mixed structure thereof, but high ductility can be ensured.

1 is a conceptual diagram showing an example of the method proposed by the present invention as a graph of temperature and time. That is, the entire blank is heated to Ac3 or more, maintained for a predetermined time, and then partially cooled B below Ar3 to an arbitrary temperature between Ms temperatures, hot forming the blank thus heat-treated, and simultaneously performing mold cooling. do. An example of a part manufactured accordingly is shown in FIG. 2, where part A maintained at a temperature above Ar3 has a high tensile strength TS, and some cooled parts B have a slightly lower tensile strength, but a higher It has an elongation (EL).

Hereinafter, the hot-molded article of the present invention will be described in detail.

The hot formed article of the present invention is a hot formed article produced through heating, molding, and quenching, and is a hot formed article in which one or two or more portions of the hot formed article have different strengths from other portions. Hot-rolled pickled steel sheet, cold-rolled steel sheet, Al or Al alloy plated steel sheet, Zn or Zn alloy plated steel sheet, etc. may be used to manufacture the molded article.

It is preferred that the one or two or more portions of the hot formed article have a tensile strength of less than 1500 MPa, and the other portion is preferably at least 1500 MPa. That is, the one or more portions of the low-temperature strength region are formed by the above-described manufacturing method, and include ferrite, pearlite, bainite, or a mixed structure thereof, and thus have a somewhat lower strength than other portions of the high-temperature intensity region. Will have The high temperature strength region is transformed into martensite structure through quenching after molding at a high temperature of Ar3 or higher. On the other hand, in the hot-molded article of the present invention, it is possible to form portions having various strength regions at less than 1500 MPa.

After all, depending on the cooling temperature in hot forming, the one or two or more portions preferably have a tensile strength of 500 ~ 1500MPa or less.

Hereinafter, the Example of this invention is described in detail. The following examples are for the understanding of the present invention, and the present invention is not limited thereto.

(Example 1)

After preparing cold-rolled hot forming specimens to which boron with 1 mm width, 4 mm length and 10 mm height was added, the specimens were heated to a temperature of 10 ° C. per second to 900 ° C. or higher at Ac 3 and maintained at a primary heat treatment for 60 seconds. Some of the specimens were subjected to transformation induction through local cooling in a region of less than Ar 3 and up to 600 ° C., which is Ms or more.

In the above method, the specimens partially heat-treated at 900 ° C. and 600 ° C. were maintained for 30 seconds, and transferred to a mold to perform hot forming and cooling to manufacture parts.

Physical properties of the parts were measured, and the results are shown in FIG. 3. That is, in one part, it can be seen that the portion maintained at a temperature of 900 ℃ has a high stress against the strain, has a high strength, the portion maintained at 600 ℃ can secure excellent strain even if the strength is low It can be seen that excellent ductility can be obtained.

Therefore, through the method of the present invention, it was possible to produce one component having heterogeneous physical properties through heat treatment before hot forming.

(Example 2)

For the same specimen as in Example 1, the first heating treatment by heating to 900 ℃ (Ac3 or more) at a temperature increase rate of 10 ℃ per second, and maintained for 60 seconds, after a variety of temperatures less than Ar3 (700) , 650, 600, 550, 500, 450 ℃) was subjected to transformation induction through local cooling.

In the above method, the locally cooled specimen was maintained at 900 ° C. and various temperatures for 30 seconds, transferred to a mold, and hot formed and cooled to prepare a molded article. The strain-stress of each part was measured in the manufactured molded article and shown in FIG. 4. As shown in Figure 4, it can be seen that a combination of various physical properties can be obtained according to the temperature conditions of the local cooling.

For example, the portion where the local cooling is performed in the 550 ° C. temperature range can obtain a slightly higher strength than the 600 ° C. temperature range, but the strain that can be secured is slightly lowered.

Through the results of Example 2, by changing the process conditions, that is, the local temperature range of the local cooling according to the physical properties that meet the design requirements, it is possible to design a flexible product.

Claims (10)

A first heat treatment step of heating the entire hot forming blank above the Ac3 transformation point;
A local cooling step of cooling one or more portions of the first heat-treated blank below an Ar3 transformation point;
Hot forming the locally cooled blank to produce a molded article; And
Cooling the molded product to a martensite transformation start temperature (Ms) or less;
Method for producing a hot molded article having a heterogeneous strength region comprising a.
The method according to claim 1,
The step of local cooling is a method for producing a hot molded article having a heterogeneous strength region that is cooled to a temperature range above the martensite transformation start temperature (Ms) below the Ar3 transformation point.
The method according to claim 1,
The method for producing a hot molded article having a heterogeneous strength region in which the average heating rate is 5 ° C./s or more during the first heat treatment.
The method according to claim 1,
After the first heat treatment, the method of manufacturing a hot molded article having a heterogeneous strength region further comprising the step of maintaining the blank temperature for more than 30 seconds, less than 10 minutes
The method according to claim 1,
The method for producing a hot molded article having a heterogeneous strength region wherein the local cooling is performed at a cooling rate of 40 to 100 ° C./s.
The method according to claim 1,
After the local cooling, the method of manufacturing a hot molded article having a heterogeneous strength region further comprising maintaining the blank temperature for 25 seconds or more.
A hot formed article manufactured by heating, forming and quenching,
A hot formed article having a heterogeneous strength region in which the strength of one or two or more portions of the hot press formed article is different from the other portions.
The method of claim 7,
The hot formed article is a hot molded article having a heterogeneous strength region produced by any one of the methods according to claims 1 to 6.
The method of claim 6,
Wherein at least one portion has a heterogeneous strength region comprising ferrite, pearlite, bainite or a mixed structure thereof.
The method of claim 6,
The one or two or more portions of the hot molded article having a heterogeneous strength region having a tensile strength of 500 ~ 1500MPa.

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