KR100264258B1 - Cold rolled steel strip and hot dip coated cold rolled steel strip for use as building material and manufacturing method thereof - Google Patents

Abstract

Cold rolled steel strips or hot-dip cold rolled steel strips for use as fire-resistant building members are available in a range of 0.01-0.25% C, up to 1.5% Si, 0.05-2.5% Mn, up to 0.1% At least one selected from P, 0.02% or less S, 0.005-0.1% Al, 0.05-1.0% Mo, optionally 0.005-0.2% Ti Nb, V and W, optionally 0.05-0.6% Cu, 0.05-0.6% by weight Ni, 0.05-3.0% by weight Cr and 0.0003-0.003% by weight of one or more of B, the remainder has a composition which is Fe excluding unavoidable impurities. Cold rolled steel strips are produced by hot rolling, acid washing, cold rolling, followed by annealing at temperatures above their recrystallization temperature but below 950 ° C. Hot-rolled cold rolled steel strips are produced in the same manner except that the cold rolled steel sheet is heated inline at a temperature above its recrystallization temperature but below 950 ° C. and then immersed in the plating bath. Annealed or hot-dipped steel strips can be further cold rolled to low pressures that induce plastic deformation of 1-5%.

Description

Cold rolled steel strips and hot-dip cold rolled steel sheets for use as building materials and manufacturing method thereof

FIELD OF THE INVENTION The present invention relates to cold rolled steel strips and hot dip plated steel strips useful as building materials and also to methods of making these steel strips.

Refractory plating has been applied to building surfaces that require refractory structures to suppress the temperature rise of steel materials during fires. Nowadays, so-called “fireproof steels” that exhibit high strength at elevated temperatures are used so that the building can be kept safe even when steel materials are heated at elevated temperatures near 600 ° C. The use of such refractory steels can reduce or omit refractory plating. In general, high temperature strength is expressed as yield strength at high temperature.

Such refractory steel materials used so far as main structural members for construction are usually relatively thick hot rolled steel sheets, but steel sheets with or without hot-dip steel made of cold rolled steel are also partially used for the purpose.

Building structures also require steel materials for secondary structural members, roofs and walls in addition to the main structural members. Cold rolled steel sheets and hot-dip cold rolled steel sheets are often used as such members. When this kind of steel material is improved in fire resistance by increasing its high temperature strength, the same advantages as that of the main structural member can be expected. In this regard, there is a need to provide a cold rolled steel sheet or a hot rolled cold rolled steel sheet having excellent fire resistance.

Such cold rolled steel sheet is manufactured by performing cold rolling, annealing, hot-dip plating, etc. on a hot rolled steel strip. Sometimes the steel sheet is reformed to high pressure to form the building members needed for practical use. Therefore, the steel sheet must have good formability as well as moderate high temperature strength.

The present invention aims to provide a steel material useful as a fire resistant building member. The steel material may be provided as a cold rolled steel sheet or a hot rolled cold rolled steel sheet having excellent high temperature strength and formability. Excellent high temperature properties are ensured by controlling the alloy composition of the strip and further improved by the introduction of plastic deformation into the strip.

1 is a graph showing the relationship between the cold rolling reduction rate after annealing and the characteristics at room temperature and 600 ° C.

Newly proposed steel strips useful as fireproof building members are 0.01-0.25% C, up to 1.5% Si, 0.05-2.5% Mn, up to 0.1% P, up to 0.02% S, 0.005-0.1 It is composed of Al by weight, 0.05-1.0% by weight of Mo and the rest is Fe excluding unavoidable impurities. The steel strip may comprise at least one of 0.005-0.2 wt% Ti, Nb, V and W and / or 0.05-0.6 wt% Cu, 0.05-0.6 wt% Ni, 0.05-3.0 wt% Cr and 0.0003-0.003 wt% It may contain one or more of B.

Cold rolled steel strips useful as fireproof building members are manufactured as follows. That is, slabs having a specific composition are heated at 1000-1250 ° C., hot rolled at 800-950 ° C., coiled at 400-700 ° C., acid washed, cold rolled at a rolling reduction of 40-90% It is then annealed at a temperature above its recrystallization temperature but below 950 ° C. Either box anneal or continuous annealing can be applied.

Hot-dip cold rolled steel strips useful as fireproof building members are manufactured as follows. That is, the cold rolled steel strip processed in the same manner is heated inline at a temperature above the recrystallization temperature but below 950 ° C. in a continuous hot dip plating apparatus and then immersed in the hot dip plating bath.

Either the cold rolled steel strip or the hot dip plated steel sheet can be further cold rolled to a low pressure of 1-5% to induce plastic deformation in the steel strip after annealing or hot dip plating, respectively. Plastic deformation could be induced in the steel strip by the application of stretching loads or leveling, but cold rolling to low pressure is most effective from an industrial point of view. Advantageously further cold rolling increases the high temperature properties of the obtained steel strip.

The inventors investigated and tested the effect of alloying elements on the high temperature strength required for refractory steel members without deterioration of formability.

Hot rolled steel strips designed for use as refractory building members are improved in high temperature properties by the addition of alloying elements such as Mo, W, Ti or Nb dissolved in steel matrix or precipitated as intermetallic compounds. On the other hand, since the cold rolled steel strip lacks formability, the cold rolled steel strip or the cold-plated cold rolled steel sheet is annealed at a temperature exceeding the recrystallization point after cold rolling to improve formability.

Annealing after cold rolling effectively improves high temperature strength due to the addition of Mo, which exhibits the same effect as in conventional hot rolled steel strips designed for use as refractory building members, however, the high temperature strength required for that application is often not realized.

The inventors believe that poor high temperature strength is caused by unexpected precipitation in the annealing step after cold rolling. That is, the cold rolled steel strip or the hot rolled cold rolled steel sheet is in a different metal state from the hot rolled steel sheet because it is processed by cold rolling and annealing. In this respect, it is practical to simply apply the proposed alloy design for hot rolled steel strips to cold rolled steel sheets or hot-rolled cold rolled steel strips if the cold rolled steel sheets or hot-dipped cold rolled steel sheets differ only in thickness from the hot rolled steel sheets. Is not

In view of the metal hysteresis of the cold rolled steel strip or the hot rolled cold rolled steel sheet, the present inventors have found that the addition of Mo is the most effective among the proposed alloy designs, and the dissolution and precipitation of Mo of a specific composition to increase the high temperature properties. An advantageous alloy design was used. Fire resistance and high temperature properties can be further improved by the complex addition of carbide forming elements such as Ti, Nb, V and W. The formability of the steel strip required for use as building members is ensured by controlled heat treatment in the annealing or hot dip plating step.

The high temperature strength is further increased with the introduction of 1-5% plastic deformation into the cold rolled steel strip or the hot rolled cold rolled steel sheet. The introduction of such slight plastic deformation increases the yield strength of the steel strip at high temperatures near 600 ° C., providing its steel material useful as a fire resistant building member due to its excellent fire resistance. Such plastic deformation is applied to the steel strip at low temperature rather than at high temperature. Practically, plastic deformation is applied to steel strips by cold rolling with a small reduction ratio.

The proposed alloy design will become apparent from the following description.

C: 0.01-0.25% by weight

C is an alloying element that provides the steel with the required strength. The effect of C on strength is greater with the addition of C in an amount of at least 0.01% by weight. However, excessive addition of C in excess of 0.25% by weight will cause deterioration of formability or weldability.

Si: up to 1.5% by weight

Si is an alloying element effective for improving strength, but excessive addition of Si in excess of 1.5% by weight will result in hardening of the steel and poor ductility. The addition of Si in an amount greater than 0.3% by weight in the case of the base plate for hot-dip plating leaves unplated surface portions. In this regard the Si content should be less controlled. The defects such as the remaining of the unplated surface portion can be suppressed by electroplating Fe or ferrous alloy on the steel strip and then passing the steel strip through the hot dip plating apparatus, even if the steel sheet contains Si in an amount exceeding 0.3% by weight. Can be. At this point, the steel strip containing up to 1.5% by weight of Si can be processed in the same manner.

Mn: 0.05-2.5 wt%

Mn is also effective in suppressing embrittlement at high temperatures due to S added to the steel as a deoxidizer in the steel manufacturing step and contained as impurities. The Mn effect becomes remarkable when Mn is added in an amount of 0.05% by weight or more. However, excess addition of Mn in excess of 2.5% by weight will result in poor ductility.

P: up to 0.1% by weight

P is an effective element for increasing the strength and also improving the corrosion resistance in combination with Cu, but excessive addition of P in excess of 0.1% by weight will cause embrittlement.

S: 0.02% by weight or less

S is a harmful element contained as an unavoidable impurity. The smaller the content of S, the more suitable for the application. Acceptable S content in the proposed steel is 0.02% by weight or less.

Al: 0.005-0.1 wt%

Al is added to the steel as a deoxidizer and is also effective for stabilizing N as AIN in the steel. This effect is realized by the addition of Al in an amount of at least 0.005% by weight. However, excessive addition of Al in excess of 0.1% by weight will cause deterioration of formability and appearance.

Mo: 0.05-1.0 wt%

The additive Mo is dissolved or precipitated as carbide in the steel matrix to increase the high temperature strength. This Mo effect is remarkable by the addition of Mo in an amount of 0.05% by weight or more but is saturated at 1.0% by weight. Excess addition of Mo in excess of 1.0% by weight will rather result in hardening of the steel and poor ductility.

Ti, Nb, V, W: 0.005-0.2% by weight, respectively

These elements are any additives that precipitate as carbides that are effective for high temperature strength as well as tensile at room temperature. Such improvement is realized by the addition of Ti, Nb, V or W in an amount of at least 0.005% by weight. However, this effect is saturated at 0.2% by weight, and excess addition of Ti, Nb, V or W in excess of 0.2% by weight will result in hardening of the steel and poor ductility.

Cu: 0.05-0.6% by weight

Cu is any alloying additive that effectively improves the corrosion resistance of steel in combination with P. Such an effect can be pronounced with the addition of Cu in an amount of 0.05% by weight. However, excessive addition of Cu in excess of 0.6% by weight will rather cause the promotion of hot cracking during hot rolling.

Ni: 0.05-0.6 wt%

Ni is any additive effective for suppressing high temperature embrittlement and corrosion resistance. Such effects can be pronounced with the addition of Ni in an amount of at least 0.05% by weight. However, Ni is an expensive element to increase the forced coarseness, and a beneficial effect on such properties is hardly expected regardless of the increase in Ni content even when Ni is added in an amount exceeding 0.6 wt%.

Cr: 0.05-3.0 wt%

Cr is any additive effective for corrosion resistance and also increases the high temperature strength due to precipitation of carbides. Such effects can be pronounced with the addition of Cr in an amount of at least 0.05% by weight. However, excessive addition of Cr in excess of 3.0% by weight will result in hardening of the steel and poor ductility, but will not further improve corrosion resistance or high temperature strength.

B: 0.0003-0.003 weight%

B is an optional additive that effectively strengthens grain boundaries. The B effect can be pronounced with the addition of B in an amount of at least 0.0003% by weight but is saturated at 0.003% by weight.

A steel material having a specific composition is cast into slabs by a conventional continuous casting process and then hot rolled to a thickness of small diameter.

In the hot rolling step, the slabs are immersed, hot rolled and then wound.

Immersion promotes dissolution of the alloying elements in the steel matrix and brings the slab into a hot rolled state when the slab is heated at a temperature above 1000 ° C. However, excessively high immersion temperatures above 1250 ° C. will cause high temperature embrittlement of the slabs.

Hot rolling is preferably finished at 800-950 ° C. in order to ensure supersaturated dissolution of the alloying elements in the steel matrix without residual ferrite particles produced during operation.

If the finishing temperature is lower than 800 ° C., the alloying elements will partially precipitate in the steel matrix, resulting in poor high temperature strength. However, higher finish temperatures in excess of 950 ° C. will waste heat energy in vain and also impose high pressure on the heating furnace.

Hot rolled steel strips are wound at 400-700 ° C. The controlled winding temperature is effective in maintaining dissolution of the alloying elements without growth of intermetallic compounds or the like during winding. Such dissolution improves the high temperature properties of the steel strip after annealing or in-line heating followed by cold rolling.

After that, the hot rolled steel strip is acid washed before cold rolling.

The steel strip is then cold rolled under conventional conditions. Cold rolling is preferably carried out at a reduction ratio of 40-90% in order to promote recrystallization in the next annealing or continuous hot dip plating step. Such controlled rolling reduction is also effective in inhibiting growth into coarse grains which adversely affects formability.

In the case of manufacturing a cold rolled steel strip without molten plating, the cold rolled steel sheet is immediately transferred to an annealing step. In the annealing step, the steel strip is heated at a temperature above its recrystallization temperature to release deformation due to cold rolling and promote sufficient recrystallization; Otherwise the heat treated steel will be hard and have insufficient formability. On the other hand, overheating at temperatures exceeding 950 ° C causes growth to coarse grains, but the steel strip may soften. Growth to coarse grains will reduce the formability of the heat treated steel strip and result in poor appearance.

In the case of producing a hot-dipped steel strip, the steel strip after cold rolling is heated inline at a temperature above the recrystallization temperature but below 950 ° C in a continuous hot-dip plating apparatus. In-line heating is performed in a reducing atmosphere to activate the surface layer of the strip and to anneal the strip.

The temperature for inline heating is set above the recrystallization temperature: otherwise the release of deformation due to cold rolling and recrystallization will be insufficient to soften the steel strip. Such low temperature heating results in insufficient activity of the steel strip, leaving unplated surface portions after hot dip plating. On the contrary, overheating at temperatures exceeding 950 ° C causes the growth of coarse grains or surface defects.

The inline heated steel strip is then immersed in the molten plating bath in a continuous hot dip plating apparatus. The hot dip bath may be Zn, Al or Zn-Al. In this way, the steel plate plated with the Zn, Al, or Zn-Al plating layer is obtained.

The cold rolled steel sheet or hot-dip cold rolled steel sheet produced by the above method may be further cold rolled under the condition that plastic deformation of 1-5% is applied to the steel sheet. The introduction of plastic deformation effectively improves the high temperature strength of the steel strip.

The effect of plastic deformation on high temperature strength is newly discovered by the inventors. The high temperature strength is increased with the degree of plastic deformation, which can be represented by the reduction rate in such cold rolling step at the low pressure described above.

Figure 1 consists of 0.09 wt% C, 0.05 wt% Si, 0.55 wt% Mn, 0.012 wt% P, 0.006 wt% S, 0.035 wt% Al, 0.31 wt% Mo, 0.07 wt% V and the rest are unavoidable impurities. The effect of cold rolling reduction on mechanical properties at room temperature and 600 ° C., when hot rolled, cold rolled, annealed at 800 ° C. for 1 minute and further cold rolled, is shown.

It can be seen from FIG. 1 that the yield strength at 600 ° C. increases with increasing cold rolling reduction rate. Such improvement in mechanical properties is clearly recognized at rolling rates in excess of 1%. Although high yield strength at 600 ° C. is obtained in the case of a reduction ratio of more than 5%, elongation at room temperature is undesirably reduced. Elongation reduction implies poor moldability of cold rolled steel sheets or hot rolled cold rolled steel sheets. From these results, the reduction ratio during cold rolling after annealing is preferably controlled in the range of 1-5%.

EXAMPLE

Example 1

Each steel having the composition shown in Table 1 was melted and cast into slabs. The slabs were forged and hot rolled with a steel strip of 4.0 mm thickness. The hot rolled steel strip was then cold rolled to a thickness of 1.0 mm and annealed under the various conditions shown in Table 2.

Test pieces were cut at each steel strip produced by this method and subjected to tensile test at room temperature and 600 ° C. The results are shown in Table 2 together with the annealing temperature.

It can be seen from Table 2 that the steel sheet having a specific composition and annealed at 650-950 ° C. has sufficient ductility at room temperature and higher yield strength at 600 ° C. compared to the comparative test specimen. Therefore, it is recognized that the steel strip within the scope of the present invention is useful as a fire resistant building member having excellent high temperature characteristics.

Example 2

Each slab having the composition shown in Table 3 was produced by a continuous casting process.

The slabs were immersed at 1150-1200 ° C., hot rolled to a thickness of 2.3-3.0 mm at a finishing temperature of 850-870 ° C. and wound at 550-580 ° C. Next, the hot rolled steel strip was cold rolled to a thickness of 0.8-1.2 mm.

A group of cold rolled steel strips was transported in a continuous annealing line, while the remaining steel strips were transported in a continuous hot dip plating line. In a continuous annealing line, each strip was heated at 820 ° C. for 40 seconds and then cooled to room temperature with or without overaging. In the continuous hot dip plating line, each steel strip was inline annealed at 800 ° C. for 35 seconds, cooled to 500 ° C. near the temperature of the plating bath, and then immersed in the molten Zn or Zn-5% Al bath.

Tensile tests were performed at room temperature and 600 ° C. with test pieces cut from each cold rolled steel sheet and hot-dipped steel sheet. The results are shown in Table 4.

Either cold rolled or hot rolled steel sheets annealed in specific temperature ranges with specific adjustments from Table 4 are useful as fire-resistant building members due to their high ductility at 600 ° C as well as good ductility at room temperature compared to comparative steel strips. It is admitted.

Example 3

Each steel having the composition shown in Table 5 was melted, cast, forged and hot rolled into a steel strip of 4.Omm thickness. Next, the hot rolled steel strip was cold rolled to a thickness of 1.0 mm. The cold rolled steel strip was annealed by heating at 800 ° C. for 1 minute and cooling in an open atmosphere. A group of annealed steel strips was further cold rolled to low pressure to induce plastic deformation.

Test pieces were cut at each strip and subjected to tensile tests at room temperature and 600 ° C. The results are shown in Table 6. It can be seen from Table 6 that the steel strip having a specific composition and given plastic deformation of 1-5% is useful as a fireproof member due to its high yield strength at 600 ° C as well as good ductility at room temperature as compared with the comparative steel strip.

Example 4

Each slab having the composition shown in Table 7 was prepared by a continuous casting process. The slabs were immersed at 1180-1210 ° C., hot rolled with a steel strip of thickness 2.3-3.Omm at a finishing temperature of 840-870 ° C. and then wound up at 530-580 ° C. The hot rolled steel strip was cold rolled to a thickness of 0.6-1.0 mm.

The group of cold rolled steel strips produced by this method was transported to the continuous annealing line, while the steel strips of the remaining group were transported to the hot dip plating line. In the annealing line, each steel strip was heated at 800 ° C. for 40 seconds and then cooled to room temperature with or without overaging. In the hot-dip plating line, each steel strip was heated in-line at 800 degreeC for 35 second, cooled to 500 degreeC near the temperature of the plating bath, and then immersed in the plating bath. Melted Zn or Zn-5% Al fuol was used as the plating bath.

Each steel strip was cold rolled to low pressure after annealing or hot dip plating to induce plastic deformation.

The test pieces were cut in each cold rolled steel strip and the molten plated steel strip and subjected to a tensile test at room temperature and 600 ° C. The results are shown in Table 8. It can be seen from Table 8 that the steel strip having a specific composition and given a plastic deformation of 1-5% is useful as a fireproof member due to its high yield strength at 600 ° C as well as good ductility at room temperature compared to the comparative steel strip.

The cold rolled steel strip or hot-dip cold rolled steel sheet according to the present invention is excellent in high temperature properties and formability required for use as a fireproof building member. The steel strip is advantageously manufactured from an industrial point of view because no specific change in manufacturing method is required from the steel manufacturing step to the annealing or hot dip plating step. The fire resistance of the steel sheet is further improved by cold rolling the steel sheet to a low pressure to induce 1-5% plastic deformation after annealing or hot dip plating.

Claims (3)

In the manufacturing method of cold rolled pole for use as a fire-resistant building member, 0.01-0.25% by weight of C, Si up to 1.5% by weight, 0.05-2.5% by weight Mn, P up to 0.1% by weight, 0.02% by weight or less S, 0.005-0.1 wt% Al, 0.05-1.0 wt% Mo, optionally 0.005-0.2 wt% Ti, Nb, V and W at least one selected from, optionally 0.05-0.6 wt% Cu, 0.05-0.6 Preparing a slab having a composition consisting of at least one of wt% Ni, 0.05-3.0 wt% Cr, and 0.0003-0.003 wt% B, with the remainder being Fe except for unavoidable impurities; Heating the slab at 1000-1250 ° C .: hot rolling the heated slab at a finishing temperature of 800-950 ° C .; Winding the hot rolled steel strip at 400-700 ° C .; Acid washing the hot rolled steel strip; Cold rolling the washed steel strip at a reduction ratio of 40-90%; Annealing the cold rolled steel strip at a temperature above its recrystallization temperature but below 950 ° C .; And cold rolling the annealed pole to a low pressure such that a plastic strain of 1-5% is induced in the steel strip. The method according to claim 1, wherein the cold rolled steel strip is box-annealed or continuously annealed. A method for producing a hot-dip cold rolled steel strip for use as a fire-resistant building member, comprising: 0.01-0.25 wt% C, up to 1.5 wt% Si, 0.05-2.5 wt% Mn, up to 0.1 wt% P, 0.02 Up to% S, 0.005-0.1% Al, 0.05-1.0% Mo, optionally at least one selected from 0.005-0.2% Ti, Nb, V and W, optionally 0.05-0.6% Cu, Preparing a slab having a composition consisting of at least one of 0.05-0.6% by weight of Ni, 0.05-3.0% by weight of Cr and 0.0003-0.003% by weight of B, except for the inevitable impurities; Heating the slab at 1000-1250 ° C .; Hot rolling the heated slab at a finishing temperature of 800-950 ° C .; Winding the hot rolled steel strip at 400-700 ° C .; Acid washing the hot rolled steel strip; Cold rolling the washed steel strip at a reduction ratio of 40-90%; In-line heating the cold rolled steel strip at a temperature above its recrystallization temperature but below 950 ° C. in a continuous hot dip plating apparatus; Immersing the steel strip heated inline in a hot dip bath; And cold-rolling the hot-dip steel strip to a low pressure enough to induce plastic deformation of 1-5% to the steel strip.