TW201919788A - Manufacturing ultra-high strength load bearing parts using high strength/low initial yield steels through tubular hydroforming process - Google Patents

Abstract

Rather than using a conventional stamping forming process with steels having high ultimate tensile strength and relatively low initial yield, tubular hydroforming techniques are introduced to synergize with BIW part forming, or forming of other load bearing parts. Such steels can have ultimate tensile strengths of greater than 1000 MPa and initial yields of less than 360 Mpa. In some embodiments, the steels have elongation of at least 40%. Such steels can include retained austenite.

Description

Ultra-high-strength load-bearing parts made of high-strength / low-initial-yield steel by tubular hydroforming

In automotive parts, complex geometric components are conventionally designed as body-in-white (BIW) parts to achieve the desired structural strength and stiffness or to meet packaging constraints. BIW components are generally considered to be upper car bodies, lower car bodies, and / or structural car components. Although designers are looking for lightweight solutions, forming complex geometries with conventional advanced high-strength steels (AHSS) is always challenging due to the limited ductility of these steels.

Steels with high ultimate tensile strength and relatively low initial yield strength, specifically those containing retained austenite, can act as enablers to make ultra-high-strength BIW parts with complex geometries, thereby providing high Ultimate tensile strength (about 1000 MPa or more) and excellent ductility (about 40% elongation or more). However, the relatively low initial yield strength (approximately 360 MPa or less) of such steels can hinder their use in the manufacture of load-bearing structural components.

Instead of using a conventional press forming method using steel with high ultimate tensile strength and relatively low initial yield strength, the present invention introduces tubular hydroforming technology to synergize with BIW component forming. Before undergoing the hydroforming method described herein, although this type of steel may have an ultimate tensile strength of greater than 1000 MPa, preferably greater than 1150 MPa; it has an initial yield of less than 360 MPa. In some embodiments, the steel has an elongation of at least about 40%. Such steels may include retained austenite. An example of such a steel is NXG 1200® steel manufactured by AK Steel Corporation (West Chester, Ohio). The methods described herein can be applied to other steels that exhibit the same or similar mechanical and hardening behavior.

The method for manufacturing hydroforming according to the embodiment of the present invention includes cutting a raw material pipe, pre-bending the pipe, hydroforming (pipe expansion or contraction), and trimming. In some embodiments, pre-forming and intermediate hydroforming are also used to ensure uniform drawing of the steel. In this embodiment, one or both of pre-forming and intermediate hydroforming may be performed between pre-bending and hydroforming.

In the hydroforming step, the hydraulic pressure causes the tube to expand until it matches the female mold. This expansion introduces homogeneous material stretching and thus increases yield strength by means of work hardening of the material. The increase in yield strength is beneficial to the crash performance of the load-bearing structure of the formed part and is made lighter through the application of high ultimate tensile strength / low initial yield material. The closed section of the hydroformed tube also provides stiffness and structural properties.

The hydroforming method of the present invention introduces uniform material stretching through hydraulic pressure-driven expansion and enhances material yield strength by means of work hardening of the material. The increase in yield strength can be controlled by the amount of tube expansion (that is, the diameter of the original tube blank to the diameter of the finished tube) according to the design specifications. The initial tube diameter is determined by a number of factors, such as the initial yield strength of the steel, the target yield strength, and the stress hardening behavior of the steel. In addition to the above factors, consideration should be given to making the initial and final material thickness meet the final component design goals. Each of these factors is known or can be determined by the component designer / manufacturer.

The following equations are known in the industry that can be used to determine the initial and final tube diameter and thickness:

According to Ludwik's stress hardening equation: Where Y ₀ is the initial yield strength, Y is the target strength, k is the strength index, and n is the strain hardening index.

Based on the conservation of volume, then Where D ₀ is the initial diameter, t ₀ is the initial thickness, D is the final diameter, and t is the final thickness. e ₃ is the engineering strain in the thickness direction. The engineering strain is then converted into a true strain, and equation (3) is obtained, The true strain in the thickness direction.

The equivalent strain can be defined as among them and The main strains are tangential and axial. Assuming uniform expansion conditions, the strain in the axial direction is close to zero. Based on the incompressible plasticity principle, the following relationship can be obtained

Subsequent substitution And in equations (5) to (6), the equivalent strain can be expressed as, Where k 'is the coefficient of equivalent strain. Then replacing ε in equation (1) with the above equation, we can get

Finally, replace Eq. (4) with Eq. (4) , The diameter relationship can be obtained as follows,

Therefore, to estimate the required diameter change in a simplified form, the following equation 9 can be used: (9) where D ₀ is the initial diameter, D is the nominal diameter of the final part, Y ₀ is the initial yield strength of the material, Y is the target yield strength, n is the strain hardening index, and K is determined by the material's stress hardening behavior and stress conditions Stress coefficient.

This unique solution allows the use of a single material to form complex geometric parts with the expected high strength and structural hardness. Other types of advanced high-strength steel ("AHSS") and conventional stamping cannot easily obtain such complex geometric parts.

The manufacturing method for hydroforming the steel of this application includes the following steps:

Raw tube cut (Figure la): Choose a raw tube diameter sufficient to allow stretching in a later hydroforming step, thereby achieving the required yield strength and maintaining thinning of the induced material within the fault limits and design tolerances.

Pre-bend (Figure lb): The raw tube is then loaded into a pipe bender to produce a smooth curvature to achieve more uniform deformation in a later step. The goal is to form a smooth curve without wrinkles or large local stress gradients.

Pre-bending is a standardized procedure for hydroforming and this requirement is a common requirement in this step.

Hydroforming (Figure 1c): The bend is placed in a hydroforming press, where the tube is filled with pressurized hydraulic fluid. Increasing pressure gradually expands the tube until it reaches the mold and gives the part its final shape and appearance. In this step, the material is subjected to relatively uniform stretching, which induces an increase in yield strength via strain hardening. The distribution of enhanced yield strength can be controlled by the amount of material stretch in the case of selecting various initial tube diameters.

Dressing (Figure 1d): Place the shaped part into a cutting machine for a specific dressing process.

The method of the present invention allows the use of steels with high tensile strength and ductility to make ultra-high-strength BIW parts. It further provides effective lightweight solutions and uniformly enhances the material yield strength of components formed from the steels described herein, including NXG 1200 steel. This solution can promote the application of steels with high ultimate tensile strength but low initial yield strength in bearing BIW components or other bearing components. It also provides design flexibility for shaped parts with complex geometric components and expected structural strength. The material yield strength distribution can also be controlled by the amount of steel material tensile.

Example 1 A steel containing retained austenite is used for manufacturing a front pipe for an automobile, as shown in FIG. 2. Between processing, it has an ultimate tensile strength of 1150 MPa and an initial yield strength of 360 MPa. The finished part has an outer diameter of 20 mm. The initial tube blank has an outer diameter of 16 mm and a wall thickness of 2.0 mm.

The initial tube blank is produced by tube cutting, which is then subjected to prebending, then subjected to hydroforming, and then subjected to trimming.

Pre-bending pressure is applied as shown in FIG. 3. The blank is bent in four steps, as shown in Fig. 4a, where the inner bend produces a bend with a radius of 109 mm and the outer bend produces a radius of 200 mm, as shown in Fig. 4b.

The tube blank is hydroformed at a pressure of 500 MPa. As indicated above, the initial tube blank has an outer diameter of 16 mm and a wall thickness of 2.0 mm, as shown in FIG. 5. The hydroformed part has an outer diameter of 20 mm and a wall thickness of 1.45 mm, and a wall thickness of 1.76 mm and an average wall thickness of 1.59 mm at the internal bends, as shown in FIG. 6.

No predictable formability issues. The average wall thinning is about 20%, and ranges from a minimum thinning of 12% in the concave curved region to a maximum thinning of 27% in the convex curved region, as shown in FIG. 7.

Plastic strain between 0.24 (which produces a true hardening stress of 1200 MPa at the concave bending area) to 0.27 (which produces a hardening stress of 1400 MPa in a flat area) to 0.37 (which produces a hardening stress of 1600 MPa in a convex bending area) ), All are shown in Figure 8.

Example 2 A second tube blank was processed according to the method of Example 1. Steel holds retained austenite. Before processing, it has an ultimate tensile strength of 1150 MPa and an initial yield strength of 360 MPa. The tube blank has an outer diameter of 16 mm and a wall thickness of 2.5 mm, as shown in FIG. 9.

After pre-bending and hydroforming in the method described in Example 1, the outer diameter of the hydroformed tube is 20 mm, and the wall thickness varies from 1.80 mm to 2.18 mm, and the average thickness is 1.98 mm, as shown in Figure 10. As shown.

No predictable formability issues. The average wall thinning is about 21%, with a minimum thinning of 12% in the concave curved region and a maximum thinning of 27% in the convex curved region, as shown in FIG. 11.

Plastic strain between 0.25 (which produces a true hardening stress of 1270 MPa at the concave bending area) to 0.28 (which produces a hardening stress of 1400 MPa in a flat area) to 0.38 (which produces a hardening stress of 1600 MPa in a convex bending area) ), All are shown in FIG. 12.

Example 3 A steel with high ultimate tensile strength and low initial yield is formed by the following steps:

By selecting an initial tube stock with a raw tube diameter sufficient to allow the steel to be stretched to a predetermined yield strength in a later hydroforming step while keeping the induced material within predetermined failure limits and predetermined design tolerances Thinning and forming an initial tube blank conforming to the diameter;

Pre-bending the initial tube blank to produce a smooth curvature in the tube;

Hydroforming the elbow in the mold by filling the tube with pressurized liquid until the wall of the tube contacts the mold;

Trim formed parts.

Example 4 The method of Example 3 or any of the following examples, wherein the steel contains retained austenite.

Example 5 The method of any one of Examples 3 or 4 or below, wherein the steel has a ultimate tensile strength of greater than 1000 MPa and an initial yield strength of less than 360 MPa before hydroforming.

Example 6 The method of any of Examples 3, 4 or 5, or any of the following examples, wherein the steel has an ultimate tensile strength of greater than 1150 MPa and an initial yield strength of less than 360 MPa prior to hydroforming.

Example 7 The method of Example 3, 4, 5, or 6, wherein the tube stock is subjected to at least one of preforming or intermediate hydroforming after prebending and before hydroforming.

Figures 1a-1d show the deformation of the tube after each manufacturing step in one embodiment of the method of the invention.

FIG. 2 is an exemplary part made by one embodiment of a hydroforming method.

FIG. 3 is a diagram showing a pre-bending pressure-load relationship applied to an initial tube blank to form an exemplary component.

Figures 4a-4b show an initial tube blank for an exemplary component after pre-bending.

Figure 5 shows the initial tube outside diameter and wall thickness before pre-bending.

Figure 6 shows the outside diameter and wall thickness of the tube after it has been hydroformed.

FIG. 7 shows wall thinning after hydroforming a tube blank to form the exemplary component shown in FIG. 2.

FIG. 8 shows true hardening stress in the finished exemplary component of FIG. 2.

FIG. 9 shows the initial tube outside diameter and wall thickness before pre-bending of the second exemplary component.

Figure 10 shows the outside diameter and wall thickness of the tube after it has been hydroformed.

FIG. 11 illustrates wall thinning after hydroforming a tube blank to form the exemplary component shown in FIG. 9.

FIG. 12 shows the hardening stress in the finished exemplary component of FIG. 9.

Claims (5)

A method for forming a steel with high ultimate tensile strength and low initial yield, comprising the following steps: a. By selecting a raw material tube sufficient to allow the steel to be stretched to a predetermined yield strength in a later hydroforming step Diameter to form the initial tube blank while keeping the induced material thin within the predetermined fault limit and predetermined design tolerance and forming an initial tube blank that conforms to the diameter; b. Pre-bending the initial tube blank to produce a smooth curvature in the bend C. Hydroforming the elbow to form a part in a mold by filling the elbow with a pressurized liquid until the wall of the elbow contacts the mold; d. Trimming the formed part. The method of claim 1, wherein the steel contains retained austenite. The method of claim 1, wherein before the hydroforming, the steel has an ultimate tensile strength of more than 1000 MPa and an initial yield strength of less than 360 MPa. The method of claim 3, wherein the steel has an ultimate tensile strength greater than 1150 MPa before hydroforming. The method of claim 1, wherein the tube blank is subjected to at least one of preforming or intermediate hydroforming after prebending and before hydroforming.