motore passo passo unipolare non funzionate con libreria

I'm trying to run a stepper motor with this program

int freq =3;
void setup (){
pinMode (3,OUTPUT);
pinMode (4,OUTPUT);
pinMode (5,OUTPUT);
pinMode (6,OUTPUT);

Serial.begin(9600);
}

void loop () {

//passo A
digitalWrite (6,LOW);
digitalWrite (3,HIGH);
digitalWrite (4,LOW);
digitalWrite (5,LOW);

delay (freq);
//passo B
digitalWrite (3,LOW);
digitalWrite (4,HIGH);
digitalWrite (5,LOW);
digitalWrite (6,LOW);
delay (freq);
//passo C
digitalWrite (3,LOW);
digitalWrite (4,LOW);
digitalWrite (5,HIGH);
digitalWrite (6,LOW);
delay (freq);
//passo D
digitalWrite (3,LOW);
digitalWrite (4,LOW);
digitalWrite (5,LOW);
digitalWrite (6,HIGH);
delay (freq);
the engine is running properly as soon as I try to use the library stepper.h the motor starts to vibrate, and n turns ...
someone can tell me why?