JPH04350152A - Production of low yield ratio galvanized steel sheet excellent in high temperature strength property - Google Patents

Abstract

PURPOSE:To manufacture a low yield ratio galvanized steel sheet excellent in high temp. strength properties as well as workability and formability and furthermore excellent in corrosion resistance. CONSTITUTION:This is a method for manufacturing a galvanized steel sheet characterized in that a slab constituted of 0.01 to 0.2% C, <=0.5% Si, <=1.5% Mn, 0.01 to 0.1% Al, 0.005 to 0.15% Ti, 0.0004 to 0.005% B, 0.005 to 0.5% Mo and the balance iron with inevitable impurities is heated to >=1100 deg.C, and after hot rolling or after hot rolling and cold rolling, the steel sheet is heated to the Ac3 or above, is cooled at a cooling rate of >=50 deg.C/S and is thereafter passed in a galvanizing bath. In this way, the galvanized steel sheet excellent in strength properties at high temps. as well as low in yield ratio can be manufactured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a low yield ratio hot-dip galvanized steel sheet having high strength at high temperatures.

0002

2. Description of the Related Art Heat-resistant materials are required to have little strength loss under high temperatures and to have excellent oxidation resistance or corrosion resistance. Examples of heat-resistant materials include high-strength steel plates for high temperature use that contain Cr, Mo, V, or Nb for the purpose of increasing high-temperature strength or oxidation resistance. For example, JP-A-57-3
No. 5623 contains Mo, Cr, V, and Nb,
After hot rolling, the steel structure is changed to ferrite + pearlite or ferrite + pearlite + bainite by slow cooling,
It has been proposed to obtain a steel sheet that has excellent high-temperature strength as hot-rolled and has improved oxidation resistance by containing a large amount of Cr. According to this method, there is an effect that a heat-resistant material having excellent high-temperature strength can be obtained without performing heat treatment after hot rolling. Furthermore, JP-A-60-165366 proposes to obtain a heat-resistant, high-strength aluminized steel sheet that has improved high-temperature strength characteristics by containing Mn and P, and has improved wettability with a plating bath by reducing carbon content. According to this, there is an effect that a heat-resistant material with high temperature and high strength with good corrosion resistance can be obtained.

0003

[Problem to be Solved by the Invention] Each of these is useful, but due to the revision of the Building Standards Act in 1988, greater strength at high temperatures was required for architectural steel materials.
Furthermore, as there is a strong demand for buildings to become taller, it is necessary to further improve their high-temperature strength. Furthermore, since these heat-resistant materials are used for many years as construction materials, it is necessary to improve their corrosion resistance or oxidation resistance. Also, ease of processing is important for producing buildings of various designs. The object of the present invention is to provide a heat-resistant material that exhibits little strength loss at high temperatures, is easy to process and form, and has excellent corrosion resistance.

0004

[Means for Solving the Problems] In order to achieve the above object, the present inventors conducted experiments and research and found that when Ti, Mo, and B are contained in a composite manner, a synergistic effect occurs, resulting in particularly excellent high-temperature strength properties. When a hot-rolled steel plate or cold-rolled steel plate is heated to a temperature of Ac3 or higher, rapidly cooled, and hot-dip galvanized, it has excellent high-temperature strength characteristics, excellent processing/formability, and corrosion resistance, and a low yield ratio. It was discovered that hot-dip galvanized steel sheets can be obtained. The present invention was made based on this knowledge, and the gist thereof is, in weight%, C: more than 0.01 to 0.18%, Si: 0.5% or less, M
n: 1.50% or less, Al: 0.01 to 0.10%, T
i: 0.005-0.18%, B: 0.0003-0.
A steel piece containing 0.006%, Mo: 0.005 to 0.50%, and the remainder consisting of iron and inevitable impurities was heated at 1100°C.
After heating to a temperature above and hot rolling, or after hot rolling and cold rolling, the steel sheet is heated to a temperature of Ac3 or above and then passed through a galvanizing bath to be hot-dip galvanized. The present invention provides a method for producing hot-dip galvanized steel sheets that have excellent high-temperature strength properties and a low yield ratio.

Next, the present invention will be explained in detail. First, we will discuss the reason for limiting the steel components. When the content of C decreases, it becomes difficult to generate carbides that allow the added alloying elements to work effectively, so 0.0
It needs to be over 1%. On the other hand, as the content increases, the strength at room temperature increases and the formability deteriorates, so 0.18%
The following shall apply. If Si content increases, it deteriorates the adhesion of zinc plating, so the content is set to 0.5% or less. Mn is contained in order to prevent hot embrittlement due to the unavoidably contained S, but if the amount is too large, the room temperature strength becomes too high for architectural steel materials and the formability deteriorates, so the content is limited to 1.50% or less. Al is contained in an amount of 0.01% or more in order to deoxidize molten steel and improve the yield of added alloying elements, but on the other hand, when the content increases, it causes an increase in oxide inclusions, and the above-mentioned effect becomes saturated. Therefore, it should be 0.10% or less. In the present invention, Ti, B, and Mo are contained in a composite manner in order to reduce the decrease in strength at high temperatures and obtain high-temperature strength characteristics. Even if these elements are contained alone, the increase in strength is small, but when they are contained in combination, the high-temperature strength properties are excellent, and high strength can be obtained even at high temperatures, for example, 600° C. or higher. Figure 1 shows 0.03%C steel with Ti, Mo,
A steel piece with B added alone and a steel piece with Ti, Mo, and B added in combination were heated to 1180°C, hot rolled, descaled, heated to 940°C and held for 60 seconds,
The results of measuring the tensile strength at room temperature and at 600°C of a steel plate cooled at an average cooling rate of 70°C and subsequently hot-dip galvanized are shown. As can be seen from this figure, the steel sheet to which Ti, Mo, and B are added in combination has a higher high-temperature strength than the steel sheet to which Ti, Mo, and B are added in a combination, and the strength decreases less when the temperature rises than that in which Ti, Mo, and B are added alone. To obtain this effect, Ti needs to be at least 0.005%, Mo needs to be at least 0.005%, and B needs to be at least 0.0004%. On the other hand, when these contents increase, the strength at room temperature becomes too high,
In addition, the effect of improving high temperature strength properties is saturated and the cost increases, so Ti is 0.18%, Mo is 1.5%, and B is 0.006%.
% respectively. The effect of the composite inclusion of Ti, B, and Mo is that Mo and B work together as elements for refining the structure, refining the structure, and the refining of the structure is caused by the refining of Ti.
It is thought that the fine precipitation of C contributes to large precipitation strengthening, and furthermore, the fine TiC, solid solution Mo, and B suppress the movement of dislocations and grain boundaries in the steel and prevent softening due to high temperatures.

[0006] In the steel sheet of the present invention, a steel billet having the above-mentioned composition with the remainder being iron and unavoidable impurities is heated prior to hot rolling, and in order to ensure high-temperature strength, fine TiC and microstructural refinement are added. heat to a temperature of 1100°C or higher. Although there is no particular need for hot rolling after heating, it is preferable to set the finishing temperature to Ar3 or higher in order to obtain good formability after galvanizing. The winding temperature in hot rolling is also not required to be specific and is arbitrary. Hot-rolled coils are passed through a galvanizing process after pickling or after pickling and cold rolling, but in order to obtain a low yield ratio by changing the structure of the steel plate to bainite after plating, it is immersed in a galvanizing bath. Heat to a temperature of Ac3 or higher in the reduction furnace before heating to 50℃/S
After cooling in the transformation temperature range at the above cooling rate, plating is performed under normal conditions. Usually, the fine precipitates obtained after hot rolling are removed by subsequent high temperature treatment, e.g. Ac3
Heat treatment above the temperature causes the growth to coarsen and its strengthening ability is significantly reduced, but in this component system, even at high temperatures of Ac3 or higher, the strengthening ability does not decrease due to the effect of composite addition and it is possible to ensure good high-temperature strength. becomes. Further, this effect can be similarly obtained when a hot rolled steel sheet is passed through a galvanizing process after cold rolling.

[0007]

[Example] Next, an example will be described. Table 1 shows the steel composition of the test materials, and Table 2 shows the manufacturing conditions. The tensile strength (TS) of the obtained steel plate at room temperature (30°C) and 600°C,
Table 3 shows the results of measuring yield strength (PS). As seen from this example, the present invention has high strength at 600°C, excellent high-temperature strength properties, and yield ratio (PS/T
An excellent hot-dip galvanized steel sheet with low S) and good workability and no unplated parts can be obtained.

[0008]

[Table 1]

[0009]

[Table 2]

0010

[Table 3]

[0011]

[Effects of the Invention] As described above, hot-dip galvanizing due to the synergistic effect of the combined addition of Ti, Mo, and B and the heat treatment conditions after hot rolling or hot rolling and cold rolling has extremely excellent high-temperature properties. In addition, a hot-dip galvanized steel sheet with a low yield ratio and excellent processability, formability, and corrosion resistance was obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the results of an experiment of the present invention.

Claims (2)

[Claims] Claim 1: In weight percent, C: more than 0.01 to 0.18% Si: 0.5% or less Mn: 1.50% or less Al: 0.01 to 0.10% Ti: 0.005 to A steel piece containing 0.18% B: 0.0003 to 0.006% Mo: 0.005 to 0.50%, with the balance consisting of iron and unavoidable impurities is heated to a temperature of 1100°C or higher, and After rolling for a while,
High-temperature strength characterized by heating a steel plate to a temperature of Ac3 or higher, cooling it in the transformation temperature range at a cooling rate of 50°C/sec or higher, and then passing it through a galvanizing bath for hot-dip galvanizing. A method for manufacturing hot-dip galvanized steel sheets with excellent properties and a low yield ratio. 2. In weight percent, C: more than 0.01 to 0.18% Si: 0.5% or less Mn: 1.50% or less Al: 0.01 to 0.10% Ti: 0.005 to A steel piece containing 0.18% B: 0.0003 to 0.006% Mo: 0.005 to 0.50%, with the balance consisting of iron and unavoidable impurities is heated to a temperature of 1100°C or higher, and After rolling and cold rolling, the steel plate is heated to a temperature of Ac3 or higher, and
Hot-dip galvanizing with excellent high-temperature strength properties and a low yield ratio, characterized by cooling in the transformation temperature range at a cooling rate of ℃/second or higher, followed by passing through a galvanizing bath for hot-dip galvanizing. Method of manufacturing steel plates.