KR100299443B1 - Method for manufacturing ultra-high strength retained austenite contained steel sheet having superior delayed fracture resistance - Google Patents

Abstract

PURPOSE: A method is provided to improve delayed fracture resistance of a ultra-high strength retained Austenite contained steel sheet by diffusion coating or plating Ni content on the surface thereof when manufacturing the ultra-high strength retained austenite contained steel sheet by reverse transformation or Austempering. CONSTITUTION: In a method for manufacturing a retained Austenite contained steel sheet comprising the processes of manufacturing a hot rolled steel sheet by hot rolling Mn low carbon steel, pickling the hot rolled steel sheet and reverse transforming Martensite into Austenite by reannealing the pickled steel sheet, the method for manufacturing a ultra-high strength retained Austenite contained steel sheet having superior delayed fracture resistance is characterized in that a Ni layer is formed by dipping the steel sheet into an electroless plating solution containing Ni content for 5 to 30 seconds after the pickling process, thereby coating Ni on the surface of the steel sheet, and reannealing the Ni coated steel sheet so that Ni is diffused on the surface layer of the steel sheet. In a method for manufacturing a retained Austenite contained steel sheet comprising the processes of stabilizing Austenite by maintaining Si-Mn medium carbon steel in the Bainite temperature zone, hot rolling the carbon steel, finish rolling and coiling the hot rolled carbon steel, and picking the coiled steel sheet, the method for manufacturing a ultra-high strength retained Austenite contained steel sheet having superior delayed fracture resistance is characterized in that the surface of the steel sheet is electroless plated by dipping the pickled steel sheet into a Ni contained electroless plating solution for 5 to 30 seconds after the pickling process.

Description

[Name of invention]

Manufacturing method of super high strength residual austenite-containing steel sheet with excellent delayed fracture resistance

Detailed description of the invention

[Purpose of invention]

[Technical field to which the invention belongs and the prior art in that field]

The present invention relates to an ultra high strength steel sheet used as an automobile bumper reinforcement material or an automobile door shock absorber, and more particularly, to a method of manufacturing an ultra high strength residual austenite-containing steel sheet having excellent delayed fracture resistance.

Recently, the demand for ultra high strength steel having a tensile strength of 80kg / mm ² or more is increasing for the purpose of lightening the automobile. Ultra-high strength steel, which is usually used as a bumper reinforcement or door shock absorber that requires more than 80kg / mm ^{2 of} tensile strength, has low workability, so it is mainly used for bending, pressing, or pipe-joining with a panel of 1 to 3 rows of hat-shaped sections. have. In recent years, however, many efforts have been made to obtain excellent workability. In particular, many studies have been conducted on steel grades containing a large amount of retained austenite.

Residual austenite-containing steel sheet has excellent uniform ductility because residual austenite is transformed into martensite by processing, and it is very excellent in uniform ductility and necking when residual austenite is transformed into martensite when subjected to local compressive stress such as drawing. Resistance increases rapidly. Due to this, there is a characteristic that drawing processing is possible even if the aggregate structure is not developed, such as the cold rolled steel sheet (222).

Therefore, the application of residual austenite-containing steel having excellent ductility to drawing processing parts will be greatly expanded.

Conventionally, the following two methods are typical of the manufacturing method of the residual austenite-containing steel.

The first method is to maintain the Si-Mn medium carbon steel in the bainite temperature range after continuous cooling, to delay the transformation of austenite to bainite and to stabilize the austenite at the same time.

Secondly, Mn low carbon steel is hot-rolled and reannealed to reverse-transform martensite to austenite.

However, there is a known problem that when drawing residual austenite-containing steel manufactured by the above method, a so-called delayed fracture occurs when a predetermined time elapses (CAMP-ISIJ Vol. 5). (1992), 1841). Delayed fracture occurs frequently in ultra-high strength steels such as 120kg / mm class ² high-tensile bolts or austenitic stainless steels, and hydrogen is diffused and penetrated in the state of high residual stresses to generate cracks.

On the other hand, residual austenite-containing steels contain internal stresses caused by volume expansion caused by transformation of residual austenite into martensite, and lubricating oils or rust-preventing oils during press- ing are intruded by hydrogen generated in contact with low pH solutions. This causes delayed destruction. Particularly, in the mardensite structure, the hydrogen diffusion rate is very high and the solubility is low. Therefore, infiltrated hydrogen easily aggregates at the boundary of the tissue, causing delayed breakthrough.

As a method of reducing the residual austenite-containing steel delayed destruction as described above, there are the following methods, but problems are found in such methods.

1) As a method of suppressing hydrogen infiltration by lubricating oil or the like before and after drawing, it is difficult to completely remove lubricating oil or rust preventive oil during actual molding in this method.

2) As a method of heat treatment to reduce or completely remove residual stress after processing, in this case, there is a problem in that the cost increases due to pickling after annealing.

3) According to the methods as a way to distribute the tissue of the soft at the interface of the transformation of martensite as much as possible to absorb the volume expansion to relieve internal stress and residual way to limit the amount of austenite 80kg / mm ² or more residual austenite Production of knight-containing steel is almost impossible, and as a result, drawing processing of ultra high strength has a difficult problem.

4) A method of mitigating delayed fracture susceptibility through the control of chemical components. In this method, by lowering C and P or adding Al, the strength of martensite is reduced and Si increases toughness so that delayed fracture is prevented. (Steel and Steel, Vol. 63 (1977), p.659).

[Technical problem to be achieved]

In order to solve the problems of the above-described methods, the present inventors have repeatedly studied and experimented and proposed the present invention based on the results, and the present invention contains ultra-high strength residual austenite by reverse transformation or ostampering. In the manufacture of steel sheet, the purpose is to improve the delayed fracture resistance of the ultra-high strength residual austenite-containing steel sheet by blocking the penetration of hydrogen gas from the outside of the steel sheet after drawing processing by spreading or plating the Ni component on the surface.

[Configuration and Function of Invention]

The present invention relates to a method for producing a retained austenite-containing steel sheet, including a step of hot rolling Mn low carbon steel to produce a hot rolled steel sheet, a process of pickling and re-annealing to reverse transformation of martensite into austenite. After the pickling process, the steel sheet was deposited on the Ni-containing electroplating solution for 5-30 seconds to apply Ni to the surface of the steel sheet, and re-annealed to diffuse Ni to the surface of the steel sheet to form a Ni layer. The present invention relates to a method for producing a super high strength residual austenite-containing steel sheet.

In addition, the present invention is to produce a retained austenite-containing steel sheet including a step of stabilizing austenite by maintaining the Si-Mn medium carbon steel in the bainite temperature range, hot rolling, finishing rolling and winding process and pickling process In the method, the steel sheet is immersed in the Ni-containing electroless plating solution for 5-30 seconds after the pickling process to produce an ultra-high strength residual austenite-containing steel sheet excellent in delayed fracture characteristics by electroless plating Ni on the surface of the steel sheet It is about.

In addition, the present invention is a retained austenite including the process of stabilizing austenite by maintaining the Si-Mn medium carbon steel in the bainite temperature range, the process of hot rolling, the process of manufacturing cold rolled cold steel sheet, and the continuous annealing process In the method for producing a steel sheet containing the ultra-high strength residual austenite having excellent delayed fracture characteristics by electroless plating Ni on the surface of the steel sheet by immersing the steel sheet in Ni-containing electroless plating solution after the continuous annealing process for 5-30 seconds. It relates to a method for producing a steel sheet.

Hereinafter, the present invention will be described in more detail.

Ni component is excellent in absorbing power of hydrogen, so it is possible to prevent hydrogen gas from lubricating oil or rust preventive oil from invading steel plate and to effectively block the movement of hydrogen gas to martensite interface when it is diffused in steel. Suppresses the delayed fracture of knight-containing steel.

Starting from this point of view, the present invention is to prevent delayed fracture occurring after drawing processing of an ultra high strength steel sheet having a tensile strength of 80 kg / mm ² or more containing a large amount of retained austenite, and thus producing residual austenite-containing steel by reverse transformation method. In the case of pickling, after application of Ni, reverse transformation annealing is carried out, and in the case of the ostempering method, Ni is plated after cooling to room temperature to form a Ni layer on the surface of the steel sheet to produce super high strength residual austenite-containing steel having excellent delayed fracture characteristics. It is about.

First, the production of residual austenite-containing steel by the inverse transformation method includes a process of hot rolling Mn low carbon steel into a hot rolled steel sheet and a process of pickling, followed by re-annealing to reverse transformation of martensite into austenite. In the present invention, Ni is applied after the pickling process. In other words, in the case of the reverse transformation method, Ni is applied in the intermediate process of pickling and reverse transformation annealing, and diffused to the surface layer of the steel sheet during the annealing process.

Next, in the case of manufacturing the steel containing the residual austenite by the retained austenite-containing hot rolled steel sheet in the austempering method, the process of stabilizing austenite by maintaining Si-Mn medium carbon steel in the bainite temperature range, hot rolling and The process of winding up and pickling are performed. In the present invention, Ni is electroless plated after the pickling process.

Next, in the case of manufacturing the retained austenite-containing steel by the retained austenite-containing cold rolled steel sheet in the austempering method, the process of stabilizing austenite by maintaining Si-Mn medium carbon steel in the bainite temperature range, hot rolling, cold rolling And, and undergoes a continuous annealing process, in the present invention, after the continuous annealing, Ni is electroless plated.

That is, in the case of the ostempering method, Ni is electroless plated immediately after the steel sheet is manufactured.

In the present invention, in applying or plating Ni in the above process sequence, the steel sheet is deposited for 5-30 seconds in an electroless plating solution. The reason is that in order to improve the delayed fracture by applying or plating Ni on the surface of steel sheet, the coating amount should be at least 100mg / m ² , and the minimum deposition time is 5 seconds. Although delayed-destructive effect is increased, since it becomes almost constant in 20 g / m <^2> or more, it will limit the 30 second which is required at this time to an upper limit.

The electroless plating solution to be applied to the present invention can be used as long as it is a normal one, and examples thereof include a mixed solution of nickel sulfate, sodium sulfate and sodium hypophosphite.

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

EXAMPLE

In order to manufacture ultra-high strength residual austenite steel having excellent delayed fracture characteristics according to Ni coating, it was prepared by dissolving the reverse transformation residual austenite-containing steel having the chemical composition shown in Table 1 below and the austempering residual austenite-containing steel. .

In case of inverse transformation type, after pickling using steel No. 1 in Table 1 below, the solution was deposited on the solution containing Ni component, a mixed solution of nickel sulfate, sodium sulfate, and sodium hypophosphite, for 5-30 seconds, and then applied at 5% at 620 ° C. The coated Ni was annealed over time to form a Ni layer by forming a Ni layer. The cold rolled steel sheet was annealed at 600 ° C. for 24 hours after cold rolling.

In addition, except that the Ni coating is not carried out, a hot rolled steel sheet subjected to the same conditions as described above was also produced.

In the case of the ostampering type, the hot rolled steel sheet is rolled at 850 ° C. using the No. 2 steel in Table 1 and wound up at 350 ° C., and then pickled and electroless plated in a solution containing the Ni component for 5-30 seconds. After annealing at 820 ° C. for 5 minutes using steel grade 3, overaging was completed, and Ni electroless plating was performed under the same conditions. In addition, except that the Ni electroless plating was not carried out, a hot rolled steel sheet subjected to the same conditions as described above was also produced.

Steel sheets having the same manufacturing conditions as described above were prepared in the thickness shown in Table 2 below. Obtained mechanical properties for each steel grade and its thickness are shown in Table 2 below.

In the meantime, the delayed fracture characteristic evaluation was carried out by processing a 95mm diameter disk, drawing a cup-shaped head with a flat punch of 45mm diameter, and then immersing it in ethyl alcohol for 3 days and 7 days to investigate the average length of cracks. The mechanical properties and delayed fracture characteristics were shown before processing.

In the present invention material coated with Ni in Table 2, cracks did not occur when immersed for 3 days, but the comparative material that did not perform Ni coating caused delayed fracture in all steels. On the other hand, during the immersion for 7 days, the case of the reverse transformation type in which Ni was diffused during the annealing did not cause cracking, but a slight crack occurred in the ostempering type electroless plated only at the surface at room temperature. This is because Ni not only blocks hydrogen gas invading from the outside as Ni is applied to the surface layer or diffuses directly under the surface layer, but also suppresses the aggregation of hydrogen gas inside.

[Effects of the Invention]

As described above, the present invention improves the delayed fracture characteristics by forming a Ni layer on the surface of the steel sheet in the retained austenite-containing steel, and the steel sheet according to the present invention has excellent ultrahigh strength residual austenite having excellent drawing and delayed fracture characteristics. As a steel-containing steel sheet, it can be used not only for bending reinforcement of automobile reinforcement and shock absorber, but also for general drawing, so that the safety of the body when the steel sheet with the tensile strength of 40-60kg / mm ² is used instead of some parts. And weight reduction.

Claims (3)

A method for producing a residual austenite-containing steel sheet, including a step of hot rolling Mn low carbon steel to produce a hot rolled steel sheet, a step of pickling, and a step of re-annealing to inversely transform martensite into austenite. The steel sheet was then deposited on the Ni-containing electroless plating solution for 5-30 seconds to apply Ni to the surface of the steel sheet, and Ni was diffused to the surface of the steel sheet by re-annealing to form a Ni layer. Excellent method for producing super high strength residual austenite-containing steel sheet. A method of manufacturing a retained austenite-containing steel sheet, including a step of stabilizing austenite by maintaining Si-Mn medium carbon steel at a bainite temperature range, hot rolling, finishing rolling and winding, and pickling, After the pickling process, Ni-containing electroless plating solution is deposited for 5-30 seconds, Ni is electroless plated on the surface of the steel sheet, characterized in that the super high strength residual austenite-containing steel sheet having excellent fracture delay characteristics. Maintaining Si-Mn medium carbon steel at bainite temperature range to stabilize austenite, hot rolling, cold rolling to cold rolled steel sheet, and continuous annealing to produce steel sheet containing residual austenite In the method, after the continuous annealing process, the steel sheet is immersed in the Ni component-containing electroless plating solution for 5-30 seconds to electrolessly plate Ni on the surface of the steel sheet. Method of manufacturing steel sheet.