CN105018875B - A kind of preparation method for the Q＆P steel that carbon partition is completed in hot galvanizing process - Google Patents

Abstract

A method for preparing Q&P steel that completes carbon distribution in the hot-dip galvanizing process, belongs to the field of metallurgy technology, and is carried out according to the following steps: (1) smelting molten steel according to the set composition, and making steel billets through refining and continuous casting; the composition is according to The weight percentage contains C 0.15~0.30%, Si 1.0~2.0%, Mn 1.0~2.5%, Mo 0.1~0.2%, Nb 0.04~0.06%, P≤0.015%, S≤0.005%, N≤0.006%, and the rest is Fe and unavoidable impurities; (2) Rough rolling and finish rolling of the billet after heating, cooling and coiling; (3) Cold rolling after pickling; (4) Continuous annealing; (5) Heating to 450~470°C, Hot-dip galvanizing is carried out at 455~465°C zinc bath, and carbon atoms are enriched to untransformed austenite at the same time during the process, that is, carbon partitioning. The method of the invention shortens the production process of the hot-dip galvanized Q&P steel.

Description

A preparation method of Q&P steel with complete carbon partition in hot-dip galvanizing process

technical field

The invention belongs to the technical field of metallurgy, and in particular relates to a preparation method of Q&P steel in which carbon distribution is completed in a hot-dip galvanizing process.

Background technique

In 2003, Professor J.G.Speer of the Colorado School of Mines proposed the quenching and partitioning (Q&P) heat treatment process for the first time. After the traditional TRIP steel components were treated by this process, they all obtained better strong-plastic bonding.

The main process of the Q&P process is: heating the steel above the temperature of A _c3 or between A _c1 and A _c3 for complete austenitization or partial austenitization, and then rapidly cooling to the start temperature of martensitic transformation after holding for a period of time (M _s ) and a certain quenching temperature between the martensite transformation end temperature (M _f ), at this time the martensite transformation is not complete, and the structure contains a certain amount of untransformed austenite; then at a certain Partition temperature for heat preservation, so that carbon atoms diffuse and enrich from supersaturated martensite to untransformed austenite, so that the carbon-rich austenite is stable enough, no phase transformation will occur in the subsequent cooling process, and finally retained to Below room temperature; when the partition temperature is equal to the quenching temperature, it is usually called a "one-step" carbon partition process; when the partition temperature is higher than the quenching temperature, it is usually called a "two-step" carbon partition process; in addition, it is worth pointing out that , the microstructure of the Q&P steel obtained by complete austenitization followed by quenching-partitioning treatment is mainly martensite + retained austenite; while the microstructure of Q&P steel obtained by partial austenitization and then quenching-partitioning treatment is Martensite + ferrite + retained austenite.

Compared with the traditional quenched-tempered steel, the reason why Q&P steel has excellent strong-plastic combination is that the structure contains a certain amount of retained austenite, and these retained austenites undergo TRIP effect during plastic deformation, that is, Retained austenite transforms into martensite, thereby improving the strength and plasticity of the material; at present, Q&P steel, as one of the third-generation advanced high-strength steels in the field of automotive steel, has received more and more attention.

Hot-dip galvanized steel sheet has good corrosion resistance, so it has been widely used in the field of automobile body plates; hot-dip galvanized process is generally to immerse the strip steel in a pure zinc bath at about 450~460 °C, after a short period of time Dip plating, forming a certain thickness of zinc layer on the surface of the strip; With the increasing demand for energy saving and emission reduction, and the lightweight of automobiles, the proportion of advanced high-strength steel in automobile bodies is increasing, and Q&P steel is the third generation As a representative steel grade of advanced high-strength steel, the production of its cold-rolled hot-dip galvanized products is very necessary; how to combine the design concept of Q&P steel and the production process of hot-dip galvanizing It is of great research value to realize the quenching and carbon partitioning process of the Q&P process.

Contents of the invention

The purpose of this invention is to provide a kind of preparation method of the Q&P steel that completes carbon partition in the hot-dip galvanizing process, through reasonable alloy composition design, finish the quenching process of Q&P steel in the continuous annealing process, in the subsequent hot-dip galvanizing process Realize the carbon distribution process, and obtain cold-rolled hot-dip galvanized Q&P steel with excellent performance while shortening the production process.

Method of the present invention carries out according to the following steps:

1. Molten steel is smelted according to the set composition, and steel billet is made by refining and continuous casting; its composition contains C 0.15~0.30%, Si 1.0~2.0%, Mn 1.0~2.5%, Mo 0.1~0.2%, Nb 0.04% by weight ~0.06%, P≤0.015%, S≤0.005%, N≤0.006%, the rest is Fe and unavoidable impurities;

2. Heat the steel billet to 1150±50°C, keep it warm for 60~90min, then carry out rough rolling and finish rolling, cool to 550~650°C and coil to obtain hot-rolled sheet; the rough rolling start temperature is 1080~1120°C, and the final The rolling temperature is 980~1020℃, the rough rolling cumulative reduction rate is 50~70%; the finishing rolling start temperature is 940~970℃, the finishing rolling temperature is 910~940℃, and the finishing rolling cumulative reduction ratio is 60~80% %;

3. Pickling the hot-rolled sheet to remove the oxide scale on the surface, and then carrying out cold-rolling. The total reduction rate of cold-rolling is 60-80% to obtain strip steel;

4. Heat the strip steel to 750~850°C in the continuous annealing furnace, hold it for 60~180s, then cool it to 220~270°C at a speed of 30~50°C/s, and complete the quenching process during the continuous annealing process;

5. Raise the temperature of the quenched steel strip to 450-470°C at a rate of 30-50°C/s, and then hot-dip galvanizing with a zinc solution at a temperature of 455-465°C for 5-15s. The enrichment of carbon atoms to untransformed austenite occurs, that is, carbon partitioning; after hot-dip galvanizing is completed, it is cooled to room temperature to obtain Q&P steel with a galvanized layer on the surface.

Cooling at a rate of 30-50° C./s in the above step 4 is achieved by blowing gas.

The composition of the above-mentioned zinc solution contains 0.205~0.235% Al, Fe≤0.03%, Pb≤0.02%, Cd≤0.04% and the rest is Zn by weight percentage.

The yield strength of the above-mentioned Q&P steel is ≥550MPa, the tensile strength is ≥980MPa, and the elongation is ≥25%.

The microstructure of the aforementioned Q&P steel is ferrite + martensite + retained austenite + a small amount of bainite.

The above strip thickness is 1.0~1.5mm.

In the present invention, through reasonable alloy composition design, on the basis of C-Mn steel composition, the precipitation of cementite is suppressed by increasing Si content, the addition of Mo improves hardenability, and the addition of a small amount of Nb refines austenite grains; The quenching process of Q&P steel is completed in the continuous annealing process, and the enrichment of carbon atoms from martensite to austenite is realized in the subsequent hot-dip galvanizing process, that is, the carbon partition process, thereby shortening the production process of hot-dip galvanized Q&P steel process.

Description of drawings

Fig. 1 is the schematic flow sheet of the preparation method of the Q&P steel that finishes carbon distribution in the hot-dip galvanizing process of the present invention;

Fig. 2 is a microstructure diagram of the Q&P steel whose carbon distribution is completed in the hot-dip galvanizing process in Example 1 of the present invention.

detailed description

The components of the zinc solution used in the examples of the present invention contain Al 0.205~0.235%, Fe ≤0.03%, Pb ≤0.02%, Cd ≤0.04%, and the rest is Zn by weight percentage.

The volume fraction of retained austenite in the matrix of the Q&P steel in the embodiment of the present invention is 9.8-13.4%.

In the embodiment of the present invention, cooling at a rate of 30-50° C./s is achieved by blowing gas.

The thickness of the steel strip in the embodiment of the present invention is 1.0-1.5mm.

The performance test in the embodiment of the present invention adopts CMT5105 tensile machine.

In the embodiment of the present invention, FEI Quanta 600 scanning electron microscope was used for tissue detection.

Example 1

Smelt molten steel according to the set composition, and make steel slabs through refining and continuous casting; its composition contains C 0.15%, Si 2.0%, Mn 1.0%, Mo 0.2%, Nb 0.04%, P 0.014%, S 0.005% by weight percentage, N 0.003%, the rest is Fe and unavoidable impurities;

Heat the billet to 1150±50°C, keep it warm for 60 minutes, then carry out rough rolling and finish rolling, cool to 550°C and coil to obtain a hot-rolled sheet; the rough rolling start temperature is 1080°C, the final rolling temperature is 980°C, and the rough rolling temperature is 980°C. The cumulative reduction rate is 50%; the finish rolling start temperature is 940°C, the finish rolling temperature is 910°C, and the finish rolling cumulative reduction rate is 60%;

The hot-rolled sheet is pickled to remove surface oxide scale, and then cold-rolled with a total reduction rate of 80% to obtain strip steel;

The steel strip is heated to 850°C in a continuous annealing furnace, held for 60s, then cooled to 270°C at a rate of 30°C/s, and the quenching process is completed during the continuous annealing process;

The quenched steel strip is heated up to 470°C at a rate of 50°C/s, and then hot-dip galvanized with a zinc solution at a temperature of 455°C for 15s. Set, that is, carbon distribution; after hot-dip galvanizing, cool to room temperature to obtain Q&P steel with a galvanized layer on the surface, with a yield strength of 556MPa, a tensile strength of 992MPa, an elongation of 30.2%, a strong plastic product of 30.0GPa%, and a microstructure of Ferrite + martensite + retained austenite + a small amount of bainite, as shown in Figure 2, where the volume fraction of retained austenite is 9.8%.

Example 2

Method is with embodiment 1, and difference is:

(1) The composition of the billet contains C 0.20%, Si 1.8%, Mn 1.8%, Mo 0.16%, Nb 0.05%, P0.015%, S 0.003%, N 0.004% by weight percentage, and the rest is Fe and unavoidable impurities ;

(2) Heating the billet to 1150±50°C and holding it for 70 minutes; cooling to 550°C for coiling; the starting temperature of rough rolling is 1100°C, the temperature of finishing rolling is 990°C, and the cumulative reduction rate of rough rolling is 55%; The rolling start temperature is 950°C, the final rolling temperature is 920°C, and the cumulative reduction rate of finish rolling is 65%;

(3) The total reduction rate of cold rolling is 75%;

(4) Heat the steel strip to 820°C in a continuous annealing furnace, hold it for 90s, and then cool it to 250°C at a rate of 35°C/s;

(5) Raise the quenched steel strip to 460°C at a rate of 40°C/s, and then hot-dip galvanizing with a zinc bath at a temperature of 460°C for 10s;

(6) The yield strength of Q&P steel is 560MPa, the tensile strength is 1016MPa, the elongation is 27.2%, the strong-plastic product is 27.7GPa%, and the volume fraction of retained austenite is 12.5%.

Example 3

Method is with embodiment 1, and difference is:

(1) The billet composition contains C 0.25%, Si 1.4%, Mn 2.5%, Mo 0.12%, Nb 0.05%, P0.012%, S 0.002%, N 0.004% by weight percentage, and the rest is Fe and unavoidable impurities ;

(2) Heat the billet to 1150±50°C, keep it warm for 80 minutes; cool to 600°C for coiling; the rough rolling start temperature is 1110°C, the final rolling temperature is 1000°C, and the rough rolling cumulative reduction rate is 65%; The rolling start temperature is 960°C, the finish rolling temperature is 930°C, and the cumulative reduction rate of finish rolling is 70%;

(3) The total reduction rate of cold rolling is 70%;

(4) Heat the steel strip to 800°C in a continuous annealing furnace, hold it for 120s, and then cool it to 230°C at a rate of 40°C/s;

(5) Heat the quenched steel strip to 460°C at a rate of 40°C/s, and then hot-dip galvanize with a zinc solution at a temperature of 465°C for 10s;

(6) The yield strength of Q&P steel is 606MPa, the tensile strength is 1057MPa, the elongation is 27.1%, the strong-plastic product is 28.6GPa%, and the volume fraction of retained austenite is 12.7%.

Example 4

Method is with embodiment 1, and difference is:

(1) The billet composition contains C 0.30%, Si 1.0%, Mn 1.0%, Mo 0.1%, Nb 0.06%, P0.014%, S 0.004%, N 0.005% by weight percentage, and the rest is Fe and unavoidable impurities ;

(2) Heating the steel billet to 1150±50°C and keeping it warm for 90 minutes; cooling to 650°C for coiling; the rough rolling start temperature is 1120°C, the finish rolling temperature is 1020°C, and the rough rolling cumulative reduction rate is 70%; The rolling start temperature is 970°C, the final rolling temperature is 940°C, and the cumulative reduction rate of finish rolling is 60%;

(3) The total reduction rate of cold rolling is 60%;

(4) Heat the steel strip to 750°C in a continuous annealing furnace, hold it for 180s, and then cool it to 220°C at a rate of 50°C/s;

(5) Raise the quenched steel strip to 450°C at a rate of 30°C/s, and then hot-dip galvanizing with a zinc solution at a temperature of 465°C for 5s;

(6) The yield strength of Q&P steel is 670MPa, the tensile strength is 1073MPa, the elongation is 26.2%, the strong-plastic product is 28.1GPa%, and the volume fraction of retained austenite is 13.4%.

Claims (2)

1. a kind of preparation method of the Q&P steel that finishes carbon distribution in hot-dip galvanizing process, it is characterized in that carry out according to the following steps: (1) Molten steel is smelted according to the set composition, and steel billet is made by refining and continuous casting; its composition contains C 0.15~0.30%, Si 1.0~2.0%, Mn 1.0~2.5%, Mo 0.1~0.2%, Nb by weight percentage 0.04~0.06%, P≤0.015%, S≤0.005%, N≤0.006%, the rest is Fe and unavoidable impurities; (2) Heat the billet to 1150±50°C, keep it warm for 60~90min, then carry out rough rolling and finish rolling, cool to 550~650°C and coil to obtain a hot-rolled sheet; the starting temperature of rough rolling is 1080~1120°C, The final rolling temperature is 980~1020℃, the rough rolling cumulative reduction is 50~70%; the finish rolling start temperature is 940~970℃, the final rolling temperature is 910~940℃, and the finishing rolling cumulative reduction is 60~ 80%; (3) pickling the hot-rolled sheet to remove surface oxide scale, and then cold-rolling, the total reduction rate of cold-rolling is 60-80%, to obtain strip steel; the thickness of the strip steel is 1.0-1.5mm; (4) Heat the steel strip to 750~850°C in a continuous annealing furnace, keep it warm for 60~180s, then cool it by blowing gas, and cool it to 220~270°C at a speed of 30~50°C/s. The quenching process is completed during the annealing process; (5) Heat the quenched steel strip to 450-470°C at a rate of 30-50°C/s, and then hot-dip galvanize with a zinc solution at a temperature of 455-465°C for 5-15s. At the same time, carbon atoms are enriched to untransformed austenite, that is, carbon partitioning; after hot-dip galvanizing, cool to room temperature to obtain Q&P steel with a galvanized layer on the surface; the yield strength of the Q&P steel is ≥550MPa, and the tensile strength ≥980MPa, elongation ≥25%, microstructure is ferrite + martensite + retained austenite + a small amount of bainite. 2. a kind of preparation method of the Q&P steel that finishes carbon partition in hot-dip galvanizing process according to claim 1, it is characterized in that described zinc liquid composition contains Al 0.205～0.235% by weight percentage, Fe≤0.03% , Pb ≤0.02%, Cd ≤0.04%, and the rest is Zn.